FOREST STAND DYNAMICS AND ECOLOGICAL FACTORS IN RELATION TO DWARF 1
MISTLETOE SPREAD, IMPACT, AND CONTROL 2
J. R. Parmeter, Jr. Abstract: Dwarf mistletoes are markedly host specific, perennial, obligate parasites. The success of mistletoe pop- ulations is tied not only to the suitability of the environ- ment, but also to the availability and conditions of the hosts they infect. Thus, the dynamics of forest stand de- velopment and change exert major influences on the capacity of dwarf mistletoes to spread, intensify, and cause damage to host trees. Silvicultural control of dwarf mistletoe dam- age rests mainly with manipulation of stand dynamics to min- imize spread and intensification. Thorough understanding of dwarf mistletoe/forest stand interactions is therefore crucial to the development and application of controls. In North America there are some two dozen species of dwarf mistletoes associated with numerous host species growing in a variety of forest and habitat types distributed over dif- ferent terrains under widely divergent cli- matic conditions. Detailed treatment of stand dynamics in each of these associations is beyond the scope of this discussion. Ra-
ther, I have attempted to highlight some stand and ecological relationships that appear to be general to dwarf mistletoes, to indicate how these relationships provide bases for dwarf mistletoe control, and to suggest where increased information could inprove control application. The literature on dwarf mistle- toes is voluminous (Scharpf, et al. 1976). Citations were selected to confirm or to il- lustrate principles, but no attempt was made to cite all papers dealing with the material discussed here. number, size, quality, or longevity of trees, and such reductions can affect the productiv- ity and value of trees or stands. Unfortu-
nately, a standard terminology to express dis- ease effects has not evolved. Such terms as effect, damage, loss, and impact are often used interchangeably to designate mistletoe- induced changes in trees or stands. It has been suggested that the term impact be re- stricted to "...the cumulative net effects of a given pest or pest complex on the productiv- ity, usefulness and value of a tree species or forest type with respect to different resource uses and values (timber production, watershed protection, wildlife cover, recreation, etc.) and the management objectives . . . " ('Vaters,
1976). By this definition, various types of damage (growth reduction, quality reduction, mortality, etc.) may or may not have impact, depending upon whether such damage reduces the productivity or value of a stand. DAMAGE, IMPACT, AND DWARF MISTLETOE POPULATIONS Investigators have seldom dealt with im- pact. Most available data involve various measures of damage either to individual trees or to stands. The following discussion there- fore deals mainly with damage, with the rec- ognition that land managers must ultimately convert damage data to impact data and that this conversion is dependent on the amount of damage to individual trees and the numbers and distribution of damaged trees in a stand. These data in turn are dependent on the rates at which mistletoes spread through stands and Land managers are concerned about dwarf mistletoes because mistletoes can reduce the Ñ~resente at Symposium on Dwarf Mistletoe Control Through Forest Management, Berkeley, Calif. April 11-13, 1978. Ñ'~rofessor Dept. of Plant Pathology, Univ. of Calif. Berkeley, California. i n t e n s i f y within t r e e s .
G e n e r a l l y , t h e amount o f damage i s r e l a t e d t o t h e numbers o f i n f e c t i o n s and t h e i r
d i s t r i b u t i o n w i t h i n t r e e crowns (Baranyay
1970; Baranyay and S a f r a n y i k 1970; C h i l d s and
Wilcox 1966; Dooling 1974; Dooling and Brown
1976; Hawksworth 1961a; Hawksworth and Lusher
1956; K o r s t i a n and Long 1922; P i e r c e 1960;
Shea 1963; Shea and O r r 1963; Shea and B e l l u s c h i 1965; Smith 1 9 6 9 ) . A number o f systems
have been d e v i s e d t o r a t e i n t e n s i t i e s o f
dwarf m i s t l e t o e i n f e c t i o n (Hawksworth 1977;
Dooling, t h i s v o l . ) and t o c o r r e l a t e damage
with i n t e n s i t i e s i n individual t r e e s o r with
general conditions of stands.
n o t been i n v e s t i g a t e d t h o r o u g h l y . P i e r c e
(1960) s u g g e s t e d t h a t t h e growth o f t r e e s
w i t h brooms i n t h e lower crown was comparable
t o t r e e s w i t h h e a l t h y crowns o f a l e n g t h e q u a l
t o t h e unbroomed p o r t i o n o f i n f e c t e d t r e e s .
Hawksworth ( l 9 6 l a ) , however, showed t h a t
growth r e d u c t i o n was g r e a t e r i n broomed t r e e s
t h a n i n non-broomed t r e e s o f t h e same i n f e c t i o n c l a s s . F u r t h e r , L i g h t l e and Hawksworth
(1973) i l l u s t r a t e d r a m a t i c improvement i n
h o s t v i g o r f o l l o w i n g p r u n i n g o f brooms from
t h e lower crown. S i n c e t h e numbers o f brooms
increase with increasing i n t e n s i t i e s o f infect i o n (Hawskworth 1 9 6 1 a ) , t h e amount o f damage
t o t r e e s can b e e x p e c t e d t o i n c r e a s e w i t h i n creasing levels of infection.
Damage on a s t a n d b a s i s i s u s u a l l y e s t i mated by comparing volumes, b a s a l a r e a s , o r
m o r t a l i t y r a t e s i n s t a n d s without m i s t l e t o e
t o t h a t i n s t a n d s w i t h v a r i o u s amounts o f
m i s t l e t o e . While a b s o l u t e volume, b a s a l a r e a
o r m o r t a l i t y may d i f f e r w i t h d i f f e r e n t h o s t /
m i s t l e t o e c o m b i n a t i o n s and w i t h a v a r i e t y o f
s i t e and s t a n d f a c t o r s , t h e a s s o c i a t i o n o f
increasing l o s s with increasing mistletoe int e n s i t y i s w e l l e s t a b l i s h e d (Andrews and
D a n i e l s 1960; Baranyay 1970; Graham 1960a;
Graham and F r a z i e r 1962; Haglund and Dooling
1972; Hawksworth 1958a, 1968a; Hawksworth and
Hinds 1964; Hawksworth and Lusher 1956;
L i g h t l e 1966; Richardson and van d e r Kamp
1972; Wagener 1 9 6 1 ) . I n g e n e r a l , s t a n d
l o s s e s below some c r i t i c a l l e v e l o f m i s t l e t o e
i n t e n s i t y a r e n e g l i g i b l e (Baranyay 1970;
Baranyay and S a f r a n y i k 1970; Dooling 1974;
Richardson and van d e r Kamp 1972; Shea and
Orr 1 9 6 3 ) . T h i s l e v e l i s a f u n c t i o n o f numbers of infected t r e e s i n a stand, t h e intens i t i e s o f i n f e c t i o n i n t h e s e t r e e s , and t h e
amount o f damage r e s u l t i n g from d i f f e r e n t i n tensities.
R a t e s o f i n c r e a s e i n dwarf m i s t l e t o e
p o p u l a t i o n l e v e l s u n d o u b t e d l y a r e a f f e c t e d by
a number o f h o s t , s t a n d , and e n v i r o n m e n t a l
f a c t o r s . Few d a t a a r e a v a i l a b l e on measured
r a t e s o f i n c r e a s e . Hawksworth (1969) found
t h a t t h e numbers o f i n f e c t i o n s on i n o c u l a t e d
d i g g e r p i n e s i n c r e a s e d from 4 t o 240 p e r t r e e
w i t h i n 16 y e a r s a f t e r i n o c u l a t i o n , i n d i c a t i n g
a population doubling r a t e o f about 2.5 years.
Doubling r a t e s were e s t i m a t e d t o b e 1 . 2 5
y e a r s i n l o d g e p o l e p i n e (Muir 1972) and 4
y e a r s i n hemlock (Richardson and van d e r Kamp
1 9 7 2 ) . S c h a r p f and P a r m e t e r (1976) found
t h a t population increases i n inoculated red
and w h i t e f i r s were h i g h l y v a r i a b l e . I n two
groups o f t r e e s , p o p u l a t i o n s doubled w i t h i n
12-15 y e a r s from i n o c u l a t i o n ; i n a t h i r d
group t h e p o p u l a t i o n t r i p l e d ; i n a f o u r t h
group t h e p o p u l a t i o n i n c r e a s e d 35 t i m e s ( a
d o u b l i n g r a t e o f a b o u t 3 y e a r s ) , and i n one
group t h e p o p u l a t i o n s c a r c e l y i n c r e a s e d .
These extreme v a r i a t i o n s a r e p r e s e n t l y unexplained, but they suggest t h a t t h e c o l l e c t i o n
o f p o p u l a t i o n d a t a i n one a r e a may have l i m i t e d relevance t o o t h e r areas.
Whether damage i s e x p r e s s e d a s d e c r e a s e d
r a d i a l , h e i g h t , o r volume growth o r a s i n c r e a s e d m o r t a l i t y , s i g n i f i c a n t damage i s s e l dom o b s e r v e d u n t i l a t l e a s t t h e lower 1/3-1/2
o f a t r e e crown i s h e a v i l y i n f e c t e d (Baranyay
1970; Baranyay and S a f r a n y i k 1970; Dooling
1974; Dooling and Brown 1976; Hawksworth
1961a; Hawksworth and Lusher 1956; P i e r c e
1960; Shea 1963; Shea and O r r 1963; Shea and
B e l l u s c h i 1965; Smith 1 9 6 9 ) . L i g h t i n f e c t i o n
t h r o u g h o u t t h e crown o r heavy i n f e c t i o n i n
t h e lower crown u s u a l l y c a u s e l i t t l e o r no
damage,
e x c e p t where stem i n f e c t i o n s may
l e a d t o h e a r t r o t ( E t h e r i d g e 19731, where stem
i n f e c t i o n s o c c u r i n s e e d l i n g s and s m a l l s a p l i n g s (Knutson and Toevs 1972; Roth 1971;
Weir 1 9 1 8 ) , o r p e r h a p s where l a r g e brooms a r e
produced.
Generalized estimates o f population inc r e a s e r a t e s have been made by r a t i n g t h e
m i s t l e t o e i n t e n s i t y a c c o r d i n g t o t h e system
o f Hawksworth (1977) i n s t a n d s o f d i f f e r e n t
ages o r i n stands i n f e c t e d f o r d i f f e r e n t peri o d s o f t i m e . Hawksworth and Hinds (1964)
e s t i m a t e d t h a t t h e a v e r a g e r a t i n g (on a 0-6
s c a l e f i n l o d g e p o l e p i n e s t a n d s i n c r e a s e d 0.80 . 9 p e r decade f o r t h e f i r s t 4 d e c a d e s o f i n f e c t i o n , w i t h i n c r e a s e s o f 0 . 6 and 0 . 3 f o r
t h e n e x t 2 d e c a d e s . T h i s i n d i c a t e s an a v e r a g e
i n c r e a s e o f 1 c l a s s each 14 y e a r s . F l o r a
(1966) i n d i c a t e d t h a t i t t a k e s a b o u t 70 y e a r s
f o r ponderosa p i n e s t o go from a 1 . 5 t o a 5 . 5
r a t i n g , an i n c r e a s e o f 1 c l a s s each 1 7 . 5
y e a r s . Wass (1976) s u g g e s t s t h a t s h o r e p i n e
i n f e c t i o n i n t e n s i t i e s i n c r e a s e d by 1 c l a s s
each 15 y e a r s . Myers, et_g.(1976) developed
an e q u a t i o n f o r c a l c u l a t i n g i n c r e a s e s i n i n f e c t i o n i n t e n s i t y f o r m i s t l e t o e i n southwestern
The e f f e c t s o f brooms on h o s t growth h a s
p o n d e r o s a p i n e . By t h i s e q u a t i o n , r a t e s o f
i n c r e a s e vary according t o time o f i n f e c t i o n ,
s t a n d d e n s i t y , and s i t e i n d e x .
I t i s obvious t h a t e q u a t i o n s o r e s t i m a t e s
f o r p o p u l a t i o n o r r a t i n g i n c r e a s e s can a p p l y
only t o s p e c i f i e d time periods, host/mistletoe
c o m b i n a t i o n s , and s t a n d / s i t e c o n d i t i o n s . With
e s t i m a t e d d o u b l i n g t i m e s o f 1.25-4 y e a r s ,
dwarf m i s t l e t o e p o p u l a t i o n s would r e a c h i m p o s s i b l e numbers w i t h i n normal r o t a t i o n p e r i o d s f o r most h o s t s p e c i e s . On a s t a n d b a s i s ,
i f i n f e c t i o n r a t i n g s i n c r e a s e d by 1 e v e r y 1 5
y e a r s ( a l l o w i n g 10 y e a r s f o r e s t a b l i s h m e n t ) ,
young i n f e c t e d s t a n d s would r e a c h damaging
3-4 l e v e l s o f i n f e c t i o n w i t h i n 55-70 y e a r s ,
and we would b e h a r d p r e s s e d t o j u s t i f y r o t a t i o n s beyond t h i s t i m e . However, f o r each
5 - y e a r i n c r e a s e i n t h e number o f y e a r s i t
t a k e s f o r r a t i n g s t o i n c r e a s e by 1, t h e r o t a t i o n i s e x t e n d e d by 30 y e a r s ( F i g . 1 ) . Whethe r increase r a t e s are s t r a i g h t - l i n e functions
o r w h e t h e r t h e y change w i t h s t a n d age and
d u r a t i o n o f i n f e c t i o n ( a s t h e d a t a o f Hawksworth and Hinds, 1964, s u g g e s t ) , r e d u c t i o n i n
t h e r a t e o f i n c r e a s e may be c r i t i c a l t o sound
management.
(3) h o s t age, (4) s t a n d s t r u c t u r e , and (5)
s t a n d composition. These v a r i o u s e l e m e n t s o f
s t a n d dynamics a r e i n e x t r i c a b l y interwoven
and d i f f i c u l t t o d i s c u s s i n i s o l a t i o n . For
convenience, however, t h e y w i l l b e d i s c u s s e d
individually.
ELEMENTS OF STAND DYNAMICS AFFECTING SPREAD,
INTENSIFICATION, AND DAMAGE
S i t e Q u a l i t y and Host Vigor
Reports o f t h e r e l a t i o n s h i p o f dwarf m i s t l e t o e p r e v a l e n c e , i n t e n s i t y o r damage t o s i t e
quality are difficult t o interpret.
I n pond e r o s a p i n e , i n c r e a s e d m i s t l e t o e h a s been a s s o c i a t e d w i t h poor s i t e q u a l i t y (Daubenmire
1961, 1969; K o r s t i a n and Long 1 9 2 2 ) , w i t h i n t e r m e d i a t e s i t e q u a l i t y (Larson e t a l . 1970),
o r showed no s i g n i f i c a n t r e l a t i o n s h i p t o s i t e
( C h i l d s and Edgren 1967; Hawksworth 1961a,
1961b, 1 9 6 8 a ) . In l o d g e p o l e p i n e , Alexander
(1975) r e p o r t e d h e a v i e r i n f e c t i o n on p o o r
s i t e s , Hawksworth and Graham (1963) found a
higher percentage of infected t r e e s i n reprod u c t i o n on good s i t e s , and H a d f i e l d (1977)
found no r e l a t i o n s h i p o f s i t e q u a l i t y t o p e r c e n t o f t r e e s i n f e c t e d . Baranyay and S a f r a n y i k (1970) found g r e a t e r growth l o s s on d r y
t h a n on wet s i t e s . I t h a s been s u g g e s t e d t h a t
because m i s t l e t o e s a f f e c t h o s t growth, h e a v i l y
i n f e c t e d s t a n d s may g i v e t h e a p p e a r a n c e o f low
s i t e q u a l i t y ( C h i l d s and Edgren 1967; Hawksworth l 9 6 l a ) .
S i n c e s i t e q u a l i t y i s measured by t r e e
growth, and growth may b e a f f e c t e d by a number
o f f a c t o r s ( s o i l t y p e and d e p t h , n u t r i e n t s ,
a v a i l a b l e w a t e r , d r a i n a g e , l e n g t h o f growing
season, e t c . ) , it i s not s u r p r i s i n g t h a t d a t a
a r e d i f f i c u l t t o i n t e r p r e t . While s i t e q u a l i t y p e r s e might have some d i r e c t e f f e c t s on
dwarf m i s t l e t o e a c t i v i t y , i t i s l i k e l y t h a t
fundamental h o s t / s t a n d / p a r a s i t e i n t e r a c t i o n s
a f f e c t e d by s i t e f a c t o r s a r e more i m p o r t a n t
i n d e t e r m i n i n g m i s t l e t o e s p r e a d and i n t e n s i fication.
F i g u r e I--Ages a t which s t a n d dwarf m i s t l e t o e
r a t i n g s would r e a c h l e v e l s o f 3 o r more
w i t h i n f e c t i o n r a t i n g s i n c r e a s i n g by l e v e r y
10 y r (A), 1 5 y r @ I , 20 y r (C), 25 y r (Dl,
o r 30 y r ( E ) , assuming i n f e c t i o n i s e s t a b l i s h e d a t about 10 y r .
F o r t u n a t e l y , r a t e s o f dwarf m i s t l e t o e
s p r e a d and i n c r e a s e a r e s u b j e c t t o a v a r i e t y
o f l i m i t a t i o n s t h a t c a n be manipulated by
l a n d managers o r t h a t he can t a k e advantage
o f . These i n v o l v e t h e i n f l u e n c e s o f : (1) s i t e
q u a l i t y and h o s t v i g o r , (2) s t a n d d e n s i t y ,
Host v i g o r , which i s i n p a r t a r e f l e c t i o n
of s i t e quality, a f f e c t s mistletoes i n several
i m p o r t a n t ways. Vigorous t r e e s a r e g e n e r a l l y
l a r g e r , have f u l l e r crowns, g r e a t e r n e e d l e
complements, and l o n g e r n e e d l e s t h a n do unt h r i f t y o r s u p p r e s s e d t r e e s . I t h a s long
been r e c o g n i z e d (Weir 1916) and r e p e a t e d l y
confirmed ( C h i l d s 1963; C h i l d s and Edgren
1967; G i l l 1957; Hawksworth 1961a, 1965;
Hawksworth and Graham 1963; Roth 1959; Wicker
1967; Wicker and Shaw 1967) t h a t v i g o r o u s ,
f u l l - c r o w n e d t r e e s i n t e r c e p t more s e e d s and
a r e t h e r e f o r e more l i a b l e t o i n f e c t i o n . I t
appears well e s t a b l i s h e d t h a t f o r a given
l e v e l o f inoculum, t h e r a t e o f i n f e c t i o n i n creases with host vigor.
Host v i g o r a l s o promotes dwarf m i s t l e t o e
v i g o r . The e n d o p h y t i c system i n v a d e s v i g o r ous h o s t t i s s u e s more r a p i d l y (Hawskworth
1960a; S c h a r p f 1962; Parmeter u n p u b l . ) , and
more r o b u s t s h o o t s and l a r g e r amounts o f s e e d
develop on v i g o r o u s h o s t s (Baranyay and Smith
1972; C h i l d s 1963; Dooling 1974; Hawksworth
1961a; Roth 1 9 5 3 ) .
w i t h h o s t v i g o r . Vigorous crowns, b e c a u s e o f
t h e i r f u l l n e s s , w i l l impede v e r t i c a l s p r e a d ,
even though t h e r a t e o f p o p u l a t i o n i n c r e a s e
i n t h e lower crown may be h i g h . I n c o n t r a s t ,
u n t h r i f t y o r suppressed t r e e s , because o f t h i n
crowns, may p r o v i d e l e s s b a r r i e r t o v e r t i c a l
s p r e a d . For each m i s t l e t o e / h o s t c o m b i n a t i o n ,
it may b e p o s s i b l e t o c o n s t r u c t h e i g h t - g r o w t h /
v e r t i c a l - s p r e a d curves t h a t w i l l allow pred i c t i o n o f p r o b a b l e damage f o r any s e t o f v i g d s i t e conditions (Fig. 2).
I t h a s a l s o been r e p o r t e d t h a t v i g o r o u s
l o d g e p o l e p i n e s a r e more l i a b l e t o damage.
Statements regarding t h e e f f e c t s o f host vigo r on m i s t l e t o e development a r e n o t a p t t o
have much s i g n i f i c a n c e u n l e s s t h e s t a n d and
s i t e c h a r a c t e r i s t i c s a r e s p e c i f i e d . While a
v i g o r o u s t r e e i n one s t a n d may b e more l i a b l e
t o damage, a s i m i l a r t r e e i n a n o t h e r s t a n d
may b e l e s s l i a b l e t o damage.
S i n c e t r e e s o f most s p e c i e s seldom show
s i g n i f i c a n t growth l o s s e s when m i s t l e t o e s a r e
c o n f i n e d t o t h e lower crown, f a c t o r s t h a t a f f e c t t h e r a t e o f v e r t i c a l spread through t r e e
crowns are. i m p o r t a n t . Alexander (1975) i n d i c a t e d t h a t where t h e s i t e i n d e x exceeded 70,
u s u a l l y o n l y t h e middle and lower crowns o f
dominant and codominant l o d g e p o l e p i n e s were
i n f e c t e d . Below i n d e x 70, a l l crown c l a s s e s
were s u s c e p t i b l e t o heavy i n f e c t i o n througho u t t h e crown. Graham (1967) s u g g e s t e d t h a t
v e r t i c a l s p r e a d i n ponderosa p i n e was about
4 i n c h e s p e r y e a r , and S t r a n d (1973) e s t i mated 3 i n c h e s p e r y e a r . Hawksworth (1969)
found t h a t v e r t i c a l s p r e a d i n d i g g e r p i n e s
averaged 2 f e e t p e r y e a r . Richardson and van
d e r Kamp (1972) e s t i m a t e d v e r t i c a l s p r e a d t o
b e 65 cm ( a b o u t 2 f e e t ) p e r y e a r i n open
grown hemlock and 30 cm ( a b o u t 1 f o o t ) p e r
y e a r i n d e n s e hemlock. S c h a r p f and Parmeter
(1976) found t h e a v e r a g e v e r t i c a l s p r e a d i n
r e d and w h i t e f i r s was about 3 i n c h e s p e r
y e a r . C h i l d s (19641, Richardson and van d e r
Kamp (1972), and S c h a r p f and Parmeter (1976)
have s u g g e s t e d t h a t most t r e e s w i l l e s c a p e
s e r i o u s damage i f t h e r a t e o f h e i g h t growth
exceeds t h e r a t e o f v e r t i c a l s p r e a d s u f f i c i e n t l y t o c o n f i n e m i s t l e t o e t o t h e lower
crown and t h a t , i n t h e a b s e n c e o f o v e r s t o r y
seed s o u r c e s , damaging i n v a s i o n o f upper
crowns w i l l n o t o c c u r u n t i l h o s t growth slows
with age.
The r a t e o f v e r t i c a l s p r e a d i s p r o b a b l y
governed mainly by crown c h a r a c t e r i s t i c s .
S p e c i e s w i t h open crowns, such a s d i g g e r p i n e ,
p e r m i t r a p i d v e r t i c a l s p r e a d , whereas s p e c i e s
w i t h dense crowns, such a s t r u e f i r s , g r e a t l y
impede v e r t i c a l s p r e a d . Among t r e e s w i t h i n
a s p e c i e s , crown c h a r a c t e r i s t i c s can change
Figure 2--Hypothetical r e l a t i o n s h i p o f v e r t i c a l
s p r e a d r a t e s o f 3 , 6 , 1 2 , o r 24 i n c h e s p e r
y e a r t o p o s i t i o n o f dwarf m i s t l e t o e i n crowns
o f t r e e s a t d i f f e r e n t a g e s on h i g h and low
s i t e s . I n f e c t i o n o f young t r e e s js e s t i m a t e d
t o o c c u r s l i g h t l y e a r l i e r on h i g h s i t e s .
In addition to rates of population build- up within crowns, site and vigor might also affect rate of tree-to-tree spread through a stand. Trees of low vigor on poor sites have thin crowns and might therefore provide less lscreeningn for lateral spread. Such effects, if any, would likely be small in comparison to the effects of stand density, structure, and composition. Stand Density Stand density can affect spread and in- tensification either directly or through changes in host vigor and growth. In general, mistletoes spread more rapidly in open stands than in dense stands (Hawksworth 1958a, 1961a). Hawksworth (l96la) found that average spread in open stands of ponderosa pine reproduction was 1.5 times as fast (1.7 feet/year) in open stands as in dense stands (1.2 feet/year). Data on spread in lodgepole pine were similar (Hawskworth 1958a) but spread rates were slightly less (average 1.2 feet/year vs. 1.5 feet/year). Density may affect spread in sev- eral ways. Beyond some critical distance that varies with tree size and mistletoe species, the chances of seed shooting from one tree to another decrease until trees are so far apart that ballistic spread is very unlikely (Fig. 31. As spacing decreases, the likelihood of seed reaching adjacent trees increases until densities are sufficiently high that trees in- terfere with spread by "screening" out seeds, thus "protecting" the trees beyond. Where densities are sufficiently high as to reduce host growth and vigor, seed production may be reduced, thus decreasing the opportunities for spread. The generalized relationship of density to spread presented by Hawksworth (l961a) undoubtedly can be applied to most mistletoe/host combinations, but the curve would have to be adjusted according to species and to the sizes of trees involved. The rate of mistletoe increase and subse- quent host damage is also related to stand density, but the relationship is not entirely clear. Some workers have reported an inverse relationship of density to prevalence or dam- age (Graham 1960b; Richardson and van der Kamp 1972; Williams et al. 1972). Williams et al. (1972) suggested, however, that infection in limber pine was directly related to density. Since density relationships are relative and may vary according to a variety of site and stand factors, generalizations are feasible only within specific contexts; e.g.,similar densities on high and low sites may not have similar effects on dwarf mistletoes. Density can affect host vigor, as has al- ready been discussed. Where the relationship of height growth to vertical spread of mistle- toe is changed by crowding and suppression, the rate of crown involvement might be expect- ed to change, but the nature of the change may depend on specific densities. Childs (1964) has discussed in some detail the relationship of crown closure to mistletoe population in- crease and vertical spread. Baranyay and Smith (1972), Childs (1963), Richardson and van der Kamp (1972) and Scharpf and Parmeter (1976) have also suggested that crown closure and subsequent "pruning" of lower branches may stabilize or reduce population increases in tree crowns. If the rate of crown closure and branch pruning is greater than the rate of vertical spread, populations of dwarf mis- tletoes might virtually disappear from trees not exposed to overstory seed sources (Fig. 2).
The above observations suggest that for each site/host/mistletoe combination there is an optimum density that promotes good height growth and rapid crown closure to minimize population buildup and vertical spread of mis- tletoe. At densities above optimum, crown thinning and reduced height growth might lead to increased vertical spread, or with densities below optimum, increased branch longevity, in- creased mistletoe inoculum, and increased op- portunities for "stepwise" vertical spread be- tween trees might also lead to increase dam- age. The relation of site and density to ver- tical spread and population buildup warrants research emphasis. Stand Age and Size Structure Figure 3--Rate of dwarf mistletoe spread in re- lationto standdensity (afterHawksworth l96la) Trees of any age or size are susceptible to dwarf mistletoe infections. Seedlings one- year-old or less are readily infected under greenhouse conditions (Knutson and Toevs 1972; Knutson 1974), and infection of trees 2.5 in- ches tall have been observed in nature (Hawks- worth and Graham 1963). Infection of very young or very small trees is, however, rela- tively rare. Data from many sources indicate that numbers of trees infected, intensity of infection, and degree of damage consistently increase with the age of trees or stands and, in the case of reproduction exposed to over- story seed sources, with size of trees (Bar- anyay 1970; Childs 1963; Graham 1960a, 1964; Hadfield 1977; Hawksworth 1958a, 1961a; Hawksworth and Graham 1963; Hawksworth and Hinds 1964; Muir 1972; Scharpf 1969a; Stewart 1976). The escape of or reduced infection in young trees involves the relatively small tar- get area, the short duration of exposure to inoculum, and the short period for inoculum production and spread within crowns. Loss of seeds under snow may also reduce infection of small trees (Wicker 1967). Because young trees tend fo escape infec- tion, reproduction following opening of infest- ed. stands by logging, fire, insects or other agencies is ordinarily lightly infected until it reaches some critical age or size. Jones (1974). Shea and Stewart (19721, and Scharpf (1969a) have provided guidelines for timing of overstory removal. If overstory sources of mistletoe seed are removed, subsequent buildup and damage in the released stand de- pend upon the site and density factors already discussed and upon silvicultural manipulation of the young stand, as will be discussed in later sections of this symposium. oculum from larger trees. Under these circum- stances, the upper crowns of understory trees can rarely remain free of increasing mistletoe populations, and reductions in growth with further increase in mistletoe is almost cer- tain. The impact of mistletoe in such stands can render them virtually non-productive. The rate of spread through two-storied or multistoried stands is more rapid than through single-storied stands. Small trees provide little "screening" of mistletoe seeds, there- fore tall seed sources provide for greater un- impeded seed trajectories. Hawksworth (1961a) calculated trajectories for mistletoe seeds discharged at different angles (Fig. 4). From these calculations, it is evident that, in the example shown, a nearby tree less than 40 feet tall would not impede flight from the 50 foot seed source at most discharge angles above the horizontal. A nearby 60 foot tree would, how- ever, intercept seeds and markedly reduce the distance of flight. At the opposite end of stand development, all trees eventually cease appreciable height growth. In such trees, vertical spread can be expected to involve the entire crown, and the intensity of infection can be expected to increase because of the long duration of ex- posure to inoculum. Mortality rates are apt to be high in old stands in which heavy in- fection levels have been reached. Roth (1953) suggested that less mistle- toe seed is produced in old, declining trees than in young trees. Although seed production may be reduced, appreciable numbers of seeds have been trapped under old, heavily infected and broomed trees (Parmeter and Scharpf 1972). It is probably safest to consider all such trees as potential sources of inoculum. Perhaps the most firmly established of all stand influences on dwarf mistletoes is that of stand structure (Gill and Hawksworth 1954; Graham and Frazier 1962; Graham 1959a; Hawksworth 1961a; Kimmey and Graham 1960; Kuiyt 1955; Roth 1953; Scharpf and Hawksworth 1968, Shea 1963; Shea and Stewart 1972; and many others). In stands comprised of suscep- tible trees of different sizes, the crowns of smaller trees are continually exposed to in- HORIZONTAL
DISTANCE
(FEET)
Figure 4--Calculated trajectories of dwarf mistletoe seeds expelled at four vertical angles from a point 50 feet above ground level (after Hawksworth 1961a). Average distances of maximum spread,based on seed trapping or on infection of reproduc- tion, range from about 40-60 feet in southwest- ern ponderosa pine (Gill 1954; Gill and Hawks- worth 1954; Hawksworth 1961a, 1961b), 25-60 feet in lodgepole pine (Gill and Hawksworth 1964; Hawksworth and Graham 1963; Hawksworth 1973; Muir 1970), and 15-50 feet for western hemlock (Shea and Stewart 1972; Smith 1973). These data correspond well with trajectories calculated by Hawksworth. Schwandt 1977) Wind may, however, increase the distance of spread. Seed from tall overstory ponderosa or Jeffrey pines can be deposited on smaller trees over distances of 100 feet or more (Roth 1953; Scharpf and Parmeter 1967, 1971; Parmeter and Scharpf 1972) and could infect nearly an acre of reproduction if so dis- persed in all directions. Muir found that spread from isolated overstory trees was greater (avg. 45 feet) than from stand margins (avg. 28 feet). The above data indicate that greatest distances of spread are associated with tall, isolated overstory trees exposed to wind. Least distances of spread are as- sociated with relatively even-aged stands. Discussion of dwarf mistletoes and stand dynamics would not be complete without consid- eration of special events in the history of stand development that affect the character- istics of stands and the incidence of dwarf mistletoes. Chief among these are logging, fire, forest succession, insect activity, and possibly the evolution of resistance mechan- isms. These past events probably have had major influences on the mistletoe situations we observe today. Stand Composition The geographic ranges of many species of dwarf mistletoes overlap (Hawksworth and Wiens 1972); however, the occurrence of two species of mistletoe in the same place is not common. While a variety of cross overs" among host species have been observed (Gill and Hawks- worth 1961; Hawksworth 1974; Hawksworth and Wiens 1972; Muir 1973; Smith 1974), in gener- al, one or more species in a mixed stand will be free of mistletoe (Kjuit 1955). The effects of stand composition on any one host species have not been well document- ed. In some areas, mistletoe is more fre- quent in stands where the host species pre- dominates (Graham 1964). In other areas, mistletoe is as abundant or more so in stands where the host species is a lesser component (Acciavatti and Weiss 1974; Graham 1959b, 1960b). In stands undergoing conversion', mistletoe damage is often reduced (Baranyay and Smith 1972; Jones 1974; Hawksworth 1973), and selective pressure on one host may pro- mote stand conversion (Parmeter and Scharpf 1963). While quantitative data are limited, it can be argued a priori that mistletoe impact will be reduced in mixed stands because a percentage of the trees will not be damaged. Spread through mixed stands may also be re- duced if nonsusceptible trees are sufficient- ly large and sufficiently numerous as to im- pede flight of mistletoe seeds between sus- ceptible trees. Manipulation of stand com- position to reduce damage has been recommend- ed (Baranyay and Smith 1972; Jones 1974; Kimmey 1957; Kimmey and Graham 1960; Parmeter and Scharpf 1963; Scharpf 1964, 1969a; Stand History Logging and other stand manipulations will be discussed in following sections of this symposium and will not be dealt with in detail here. Obviously, past cutting prac- tices that have modified vigor, density, age, structure, and composition of stands have in- fluenced mistletoe activity as previously in- dicated. In a sense we have inherited many of our mistletoe problems from past forest practices. High-grading, selective cutting, and preferential removal of certain species have left many stands open, multistoried, re- duced in species complexity, and in a condi- tion to favor rapid buildup and damage by mistletoe. Conversely, extensive clearcutting in some timber types has resulted in large areas of essentially mistletoe-free second growth. Along with logging, fire has undoubtedly had a major influence on present mistletoe conditions in many areas. The relationship of fire to mistletoe activity has been thor- oughly discussed in the excellent reviews of Alexander and Hawksworth (1975) and Wicker and Leaphart (1976) and will be discussed by Muraro in a later section of this symposium. Fires may have positive or negative effects on dwarf mistletoes, depending on the extent and intensity of the burn and on the charac- teristics of the stand and the terrain. Where large areas of forest have been completely destroyed by intense wildfire, the replacement stand will be mistletoe free except
at the margins. Since dwarf mistletoes reinvade such new stands at an average of only 1-2 feet per year following initial infection from the margins (Hawksworth 1958a, 1960b, 1961a; Shea and Stewart 1972; Wagener 1965), large areas of replacement stands remain mistletoe free. Owing to patchy fuel accumulations, rock outcrops, and other irregularities, even large fires often leave groups or individual live trees within burned areas. When infect- ed, these can serve as sources of inoculum to reinfest the replacement stand. In some re- gions, the prevalence of dwarf mistletoes is markedly associated with areas where large, complete burns are rare because of terrain or fuel conditions (Baranyay 1970). Available evidence suggests that frequent low-intensity ground fires were characteristic of many forest types (Biswell 1967; Weaver 1974). Because heavy dwarf mistletoe infec- tion leads to accumulation of dead trees, witches' brooms, resinous stems and branches, and other fuels, low intensity fires may flare up and consume most of the trees in lo- calized infection centers. In addition, heavy brooming provides concentrated fuel for "torching" and destruction of infected trees, where normal tree crowns are too open to carry fire up the tree. Low intensity fires may also kill infected lower branches, thus reducing mistletoe populations in lower crowns (Roth 1974). These fire effects have probably reduced mistletoes in many areas, but not all fire effects have been beneficial. In the long view, fires have also served to perpetuate seral fire types in which dwarf mistletoes may be a perpetual problem. In addition, owing to accumulation of fuels in stands with mistletoe levels sufficient to apply selection pressures for resistance, locally intense fires may have greatly re- duced opportunities for the evolution of re- sistance, since any resistant trees within such stands would be consumed along with in- fected trees (Roth 1966). Resistance to dwarf mistletoes is infre- quently observed and poorly understood. Some trees within stands are essentially free of mistletoe, even when exposed to abundant in- oculum (Hawksworth 1961a, Roth 1953, 1971; Wagener 1965). In other cases,, stands over large areas may show differing susceptibili- ties. Hawksworth (l961b) found that inocu- lation of trees in infested stands produced 10 times the rate of infection obtained from inoculations in uninfested stands, apparent- ly because of differences in the survival of germinated seed. Infection of ponderosa pine by A. americanum is uncommon in stands infested by A. vaginatum and vice versa
(Hawksworth 1968b1. Scharpf and Parmeter (1967) provided limited evidence that Jef- frey pines from high elevation seed sources were more susceptible than trees from low elevation sources. Smith and Wass (1976) found that A. tsugensis includes forms that
preferentially attack shore pine in some areas and hemlock in others. Hawksworth and Wiens (1972) discussed the "exclusion prin- ciple", a frequently observed phenomenon in which other species of mistletoes seldom in- fect trees in stands infested by a principal dwarf mistletoe of the host. While genetic interactions among hosts and parasites undoubtedly account for some of the above observations and some bases for re- sistance have been suggested (Roth 1966; Smith 1974), environmental limitations and evolution of ecotypes might also account for different host/parasite interactions in dif- ferent stands. These phenomena deserve fur- ther investigation, since prediction of mis- tletoe damage in different stands may rest with understanding of the bases for differing host/parasite interactions. Forest succession has also influenced the present distribution and intensity of dwarf mistletoes. As has been pointed out by Hawks- worth (1973), mistletoe damage is often re- duced when stands of seral species are re- placed by "climax" species. It can be argued that in stands where succession from one spe- cies to another is possible, dwarf mistletoe will apply pressure against the susceptible host, thus hastening stand succession (Par- meter and Scharpf 1963). However, if mistle- toe species attacking the "climax" species are present in stands, reduction of mistletoe dam- age by succession may be only temporary. Wass (1976) indicated that hemlock mis- tletoe on shore pine is mainly a problem where shore pine tends to be climax. In California, it appears that heavy mistletoe damage is of- ten associated with such timber types as eastside Jeffrey pine, upper elevation red fir, and poor-site lodgepole pine in which the host species are self perpetuating and opportunities for succession to other species or to mixtures are minimal. Perpetuation of "climax" types may eventually lead to heavy mistletoe damage because such types tend, in the absence of catastrophic removal, to de- velop uneven-aged structures, a condition favoring dwarf mistletoe intensification. It is difficult to reconstruct past suc- cessional changes that have affected present mistletoe conditions. It seems safe to as- sume, however, that frequent changes in tim- ber type resulting from the interplay of ca- tastrophe and succession have helped to re- duce mistletoe incidence. And conversely, perpetuation of the same type, either seral fire types or "climax" types has tended to increase mistletoe damage. Knowledge of such trends can aid land managers in long-range planning. While logging, fire, and succession are probably the main factors that have shaped the character of stands and the distribution of mistletoes within stands, other events have likely had significant influence. Epidemic insect activity can change the composition and structure of forest stands (Schmid and Hinds 1974). Few data are available to as- sess possible effects of insect activity on dwarf mistletoe populations, and both plus and minus effects can be postulated. Infect-
ed trees are often attacked by bark beetles (Hawksworth 1973), and each killed, infected tree represents a temporary reduction in in- oculum. However, random killing of indivi- dual trees may lead to open, uneven-aged stands in which mistletoes are ultimately more damaging. Extensive killing, as can re- sult from repeated defoliation of lodgepole pine by needle miners (Koerber and Struble 19711, might have effects similar to fire in that replacement stands would tend to be even- aged and surviving overstory trees would provide inoculum for infection of reproduc- tion. from certain habitat types (Acciavatti and Weiss 1974; Hadfield 1977; Hawksworth 1973; Hawksworth and Wiens 1972), and absence or re- duced activity in relation to slope, aspect, and other topographic features (Andrews and Daniels 1960; Baranyay 1970; Gill 1957; Gill and Hawksworth 1961, 1964; Hawksworth 1958b, 1961a, 1968a; Larson et al. 1970; Roth 1954; Williams et al. 1972). Dwarf mistletoes may also influence in- sect activity. Trees weakened by dwarf mis- tletoes are often susceptible to bark beetle attack (Hawksworth 1973; Miller and Keen 1960; Struble 1965), although heavily infect- ed trees may provide poor substrate for brood development (Roe and Amman 1970'). Details of possible relationships between dwarf mistle- toes and population dynamics of destructive forest insects are poorly understood and should provide a fruitful area for further study. Environmental limitations may also be re- sponsible for some of the observed patterns of dwarf mistletoe distribution. Seed survi- val, germination, and radicle development are sensitive to both high and low temperature extremes (Beckman and Roth 1968; Knutson 1971; Scharpf 1969b, Wicker 1974a). Gill (1957) recognized that the upper elevational limit of A. americanum in the Rocky Mountains coin- cides with the 30 F mean annual isotherm. Baranyay and Smith (1974) showed that freez- ing temperatures before or during seed dis- persal permanently damaged fruits and greatly reduced seed discharge of A. americanum and A. tsugense.
-
OTHER ECOLOGICAL INFLUENCES Available evidence suggests that a var- iety of factors other than stand dynamics per se affect the occurrence and intensity of dwarf mistletoe populations. Many of these factors are not well understood and evidence for their influences is often circumstantial. Mistletoes are higher plants and we can ex- pect that they are subject to many of the same environmental limitations that govern the distribution of other higher plants. De-
tailed discussion of these ecological factors is beyond the scope of this paper, but men- tion should be made of their existence and possible implications in land management. The distribution of dwarf mistletoes is not continuous over the ranges of their host species. These discontinuities have been discussed by a number of workers (Baranyay 2nd Smith 1972; Baranyay 1970; Hawksworth and Wiens 1972; Smith and Baranyay 1970; Wicker 1974b). They include the absence of mistletoes over large geographic areas (Hawksworth and Wiens 1972); the absence The absence of dwarf mistletoes on some hosts over major portions of the host range may involve large scale shifts in ranges as- sociated with advance and retreat of ice sheets during glacial epochs (Hawksworth and Wiens 1972). Other large-scale discontinui- ties may be due to geographical barriers or discontinuities in the distribution of host species (Wicker 1974b). On a smaller scale, absence of mistletoe can involve such catas- trophes as volcanic devastation (Hawskworth 1960b). In some regions, mistletoes may be re- stricted by inadequate moisture during the period of germination and infection (Bonga 1969, Hawksworth 1967). Moisture is required for seed gemination, and unsuitable moisture conditions may reduce viability, germination, or survival (Bonga 1969; Knutson 1971; Wicker 1974a). Our present state of knowledge about re- quirements and limitations is inadequate to permit firm correlation of distribution pat- terns with environmental limitations in most cases, but it seems reasonably certain that such limitations exist. Future studies may help to distinguish between areas that are mistletoe free because mistletoes haven't suc- cessfully spread into them and areas that are mistletoe free because mistletoes can't suc- cessfully spread into them. In either case, to- day's land managers must deal with mistletoes where they are, and prudent land managers will take steps to minimize the possibility of their spread or introduction into areas presently free of mistletoes on important hosts. CONCLUSIONS Mistletoe damage and impact in forest stands are mainly functions of the numbers of infected trees and the distribution and num- bers of infections within tree crowns. These parameters are in turn functions of the rates at which mistletoes spread through stands, the rates at which they spread vertically within crowns, and the rates at which popula- tions increase. These rates are governed in large part by stand characteristics that can be manipulated by land managers. Basic principles of tree/stand/mistletoe interactions are well established and provide broad guidelines for stand management. Many refinements of our information base will be necessary to develop precision in the manage- ment of the many timber types and host/mis- tletoe combinations under different geograph- ic, topographic, climatic, and site condi- tions. Ultimately, predictive models of varying precision, as have been explored by Myers, et al. (l976), Strand and Roth (l976), and Edminster (this vol.), should become available to managers. We need not wait for such models, however. The broad principles already developed can and should be applied to the management of stands in which dwarf mistletoes are an actual or potential prob- lem. Ideally, as these principles are ap- plied, post-treatment research and monitoring on a long-term basis will: (1) provide need- ed refinement of our data base, (2) confirm or dismiss presently uncertain but suspected relationships, and (3) lead to the develop- ment of new concepts and approaches to the control of dwarf mistletoe impact. Baranyay, J. A. 1970. Lodgepole pine dwarf mistletoe in Al- berta. Can. For. Serv. Publ. 1286. 22 p. Baranyay, J. A., and L. Safranyik. 1970. Effect of dwarf mistletoe on growth and mortality of lodgepole pine in Alberta. Can. For. Serv. Publ. 1285. 19 p. Baranyay, J. A., and R. B. Smith 1972. Dwarf mistletoes in British Columbia and recommendations for their control. Can. For. Serv. BC-X-72. 18 p. Baranyay, J. A., and R. B. Smith 1974. Low temperature damage to dwarf mis- tletoe fruit. Can. J. For. Res. 4:361-365. Beckman, K. M., and L. F. Roth 1968. The influence of temperature on lon- gevity and germination of seed of western dwarf mistletoe. Phytopathology 58:147- 150. Biswell, H. H. 1967. Forest fire in perspective. Proc. Tall Timbers Fire Ecology Conf.:43-64. Bonga, J. M. 1969. Growth potential of seeds and site requirements of eastern dwarf mistletoe. Can. Dep. Fish and For., Bi-monthly Res. Notes 25(2) :lo-11. Childs, T. W. 1963. Dwarfmistletoe control opportunities in ponderosa pine reproduction. USDA For. Serv., Pac. Northw. For. Range Exp. Stn. Ofc. Rep. 20 p. LITERATURE CITED Acciavatti, R. E., and M. J. Weiss. 1974. Evaluation of dwarf mistletoe on En- gelmann spruce, Fort Apache Indian Reser- vation, Arizona. Plant Dis. Rep. 58:34-35. Alexander, R. R. 1975. Partial cutting in old-growth lodge- pole pine. USDA For. Serv. Res. Pap. RM136. 17 p. Alexander, M. E. and F. G. Hawksworth. 1975. Wildlands fires and dwarf mistletoes: a literature review of ecology and pre- scribed burning. USDA For. Serv. Gen. Tech. Rep. RM-14. 12 p. Andrews, S. R. and J. P. Daniels. 1960. A survey of dwarfmistletoes in Ari- zona and New Mexico. USDA For. Serv., Rocky Mtn. For. Range Exp. Stn. Pap. 49. Childs, T. W., and J. W. Edgren 1967. Dwarfmistletoe effects on ponderosa pine growth and trunk form. For. Sci. 13: 167-174. Childs, T. W., and E. R. Wilcox 1966. Dwarfmistletoe effects in mature pon- derosa pine forests in south-central Or- egon. J. For. 64:246-250. Daubenmire, R, 1961. Vegetative indicators of rate of height growth in ponderosa pine. For. Sci. 7~24-34. Daubenmire, R. 1969. Ecological plant geography of the Pa- cific Northwest. Madroffo 20:lll-128. Dooling, 0. J. 1974. Dwarf mistletoe control--why and what? USDA For. Serv.
16. 11 p. - Northern Reg. Rep. 74-
Dooling, 0. J., and D. H. Brown 1976. Guidelines for dwarf mistletoe control in lodgepole pine in the northern and cen- tral Rocky Mountains. USDA For. Serv., Northern Reg., For. Environ. Prof. Rep. 76- 14. 10 p. Graham, D. P. 1964. Dwarfmistletoe survey in western Mon- tana. USDA For. Serv. Res. Note INT-14. 7 P-
Graham, D. P. 1967. A training aid on dwarf mistletoe and its control. USDA For. Serv., Pac. Northw. Reg., Portland, Oregon. 40 p. Etheridge, D. E. 1973. Wound parasites causing tree decay in British Columbia. Can. Dep. Environ. For. Serv., Pac. For. Res. Cent. For. Pest Leafl. 62. 15 p. Graham, D. P., and W. E. Frazier 1962. Dwarfmistletoe survey in northeastern Washington. USDA For. Serv., Interm. For. Range Exp. Stn. Res. Note 103. 8 p. Flora, D. F. 1966. A method of forecasting returns from ponderosa pine dwarfmistletoe control. USDA For. Serv. Res. Pap. PNW-32. 17 p. Hadfield, J. S. 1977. A disease survey of lodgepole pine stands in Oregon and Washington. USDA For. Serv., Pac. Northw. Region, For. Insect Disease Management. 13 p. Gill, L. S. 1954. Dwarfmistletoe of ponderosa pine in the Southwest. USDA For. Serv., Rocky Mtn. For. Range Ex?. Stn., Stn. Paper 14. 9 p. Gill, L. S. 1957. Dwarfmistletoe of lodgepole pine. USDA For. Serv., For. Pest Leafl. 18. 7 p.
Gill, L. S., and F. G. Hawksworth 1954. Dwarfmistletoe control in southwestern ponderosa pine forests under management. J. Forestry 52:347-353. Gill, L. S. and F. G. Hawksworth 1961. The mistletoes, a literature review. USDA Tech. Bull. 1242. 87 p .
Gill, L. S., and F. G. Hawksworth 1964. Dwarfmistletoe of lodgepole pine. USDA For. Serv. For. Pest Leafl. 18. 7 p. Graham, D. P. 1959a. Dwarfmistletoe survey in Kootenai National Forest. USDA For. Serv. Intern. For. Range Exp. Stn. Res. Note 67. 5 p. Graham, D. P. 1959b. Dwarfmistletoe survey in Coeur D' Alene National Forest. USDA For. Serv., Interm. For. Range Exp. Stn. Res. Note 68. 5 P. Graham, D. P. 1960a. Dwarfmistletoe survey in Nezperce National Forest. USDA For. Serv., Interm. For. Range Exp. Stn. Res. Note 75. 7 p. Graham, D. P. 1960b. Dwarfmistletoe survey in Kaniksu National Forest. USDA For. Serv., Interm. For. Range Exp. Stn. Res. Note 74. 6 p. Haglund, S. A., and 0. J. Dooling 1972. Observations on the impact of dwarf mistletoe on Douglas-fir in western Mon- tana. USDA For. Serv., Northwest Reg. Rep. D-72-1. Hawksworth, F. G. 1958a. Rate of spread and intensification of dwarfmistletoe in young lodgepole pine stands. J. For. 56:404-407. Hawksworth, F. G. 1958b. Survey of lodgepole pine dwarfmis- tletoe on the Roosevelt, Medicine Bow, and Bighorn National Forests. USDA For. Serv., Rocky Htn. For. Range Exp. Stn. Pap. 35. 13 p. Hawksworth, F. G. 1960a. Growth rate of dwarfmistletoe infec- tions in relation to the crown class of the host. USDA For. Serv. Rocky Mtn. For. Range Exp. Stn. Res. Note 41. 4 p. Hawksworth, F. G. 1960b. Distribution of ponderosa pine dwarfmistletoe in the vicinity of an Arizona volcano. Ecology 41:799-800. Hawksworth, F. G. 1961a. Dwarfmistletoe of ponderosa pine in the Southwest. USDA For. Serv. Tech. Bull. 1246. 112 p. Hawksworth, F. G. 1961b. Dwarfmistletoes of ponderosa pine. Pages 1537-1541
Recent Advances in Botany. Univ. Toronto Press, Toronto, Can. Hawksworth, F. G. 1965. Life tables for two species of dwarf mistletoe. 1. Seed dispersal, intercep- tion and movement. For. Sci. 11:142-151. Hawksworth, F. G. 1967. Distribution of ponderosa pine dwarf mistletoe on the south rim of Grand Can- yon, Arizona. Plant Dis. Rep. 12:1049- 1051. Hawksworth, F. G. 1968a. Ponderosa pine dwarf mistletoe in relation to topography and soils on the Manitou Experimental Forest, Colorado. USDA For. Serv. Res. Note RM-107. 4 p. Hawksworth, F. G. 1968b. Lodgepole pine dwarf mistletoe on ponderosa pine. Plant Dis. Rep. 52:125- 127. Hawksworth, F. G. \
1969. Rapid intensification and upward spread of dwarf mistletoe in inoculated digger pines. Plant Dis. Rep. 53:615-616. Hawksworth, F. G. 1973. Dwarf mistletoe and its role in lodge- pole pine ecosystems. Pages 342-358 1.n Proc. Manage. Lodgepole Pine ~ c o s ~ s t e m s Sym-p. D. M. Baumgartner, ed. Wash. State
Univ., Pullman. Hawksworth, F. G. 1974. Mistletoes on introduced trees of the world. USDA For. Serv. Agric. Handb. 469. 49 p. Hawksworth, F. G. 1977. The 6-class dwarf mistletoe rating system. USDA For. Serv. Gen. Tech. Rep. RM-48. 7 p. Hawksworth, F. G., and D. P. Graham 1963. Spread and intensification of dwarf- mistletoe in lodgepole pine reproduction. J. For. 61:587-591. Hawksworth, F. G. and T. E. Hinds 1964. Effects of dwarfmistletoe on immature lodgepole pine stands in Colorado. J. For. 54 : 384-390.
Hawksworth, F. G., and A. A. Lusher. 1956. Dwarfmistletoe survey and control on the Mescalero Apache Reservation, New Mexico. J. For. 54:384-390. conifers and aspen: the status of our knowledge. USDA For. Serv. Res. Pap. RM- 122. 44 p. Kimey, J. W. 1957. Dwarfmistletoes of California and their control. USDA Calif. For. Range Exp. Stn. Tech. Pap. 19. 12 p. Kimmey, J. W., and D. P. Graham 1960. Dwarfmistletoes of the Intermountain and northern Rocky Mountain regions and suggestions for control. USDA For. Serv., Interm. For. Range Exp. Stn. Res. Pap. 60. 19 p. Knutson, D. M.
1971. Dwarf mistletoe seed storage best at low temperature and high relative humid- ity. USDA For. Serv. Res. Note PNW-145. 8 P. Knutson, D. Pi. 1974. Infection techniques and seedling re- sponse to dwarf mistletoe. Plant Dis. Rep. 58:235-238. Knutson, D. M . , and W . J. Toevs
1972. Dwarf mistletoe reduces root growth of ponderosa pine seedlings. For. Sci. 18~323-324. Koerber, T. W., and G. R. Struble 1971. Lodgepole needle miner. USDA For. Serv., For. Pest Leafl. 22. 8 p. Korstian, C. F., and W. H. Long 1922. The western yellow pine mistletoe. U.S. Dep. Agric., Agric. Bull. 1112. 35 p. Kuijt, J. 1955. Dwarf mistletoes. Botan. Rev. 21:569- 620. Larson, F. R., P. F. Folliott, and W. P. Clary 1970. Distribution of dwarf mistletoe in ponderosa pine stands on the Beaver Creek watershed, Arizona. USDA For. Serv. Res. Note R ? M 7 5 . 4 p. Lightle, P. C. 1966. Dwarf mistletoe reduces basal area growth in ponderosa pine in the South- west. (Abstr.) Phytopathology 56:886-887. Hawksworth, F. G., and D. Wiens 1972. Biology and classification of dwarf mistletoes (Arceuthobium). USDA For. Serv. Agric. Handb. 401. 234 p .
Lightle, P. C., and F. G. Hawksworth 1973. Control of dwarf mistletoe in a heavily used ponderosa pine recreation forest: Grand Canyon. Arizona. USDA For. Serv. Res. Pap. RM-106. 22 p. Jones, J. R. 1974. Silviculture of southwestern mixed Miller, J. M., and F. P. Keen
1960. Biology and control of the western p i n e b e e t l e . USDA Misc. P u b l . 800. 381 p
Muir, J . A.
1970. Dwarf m i s t l e t o e s p r e a d i n young lodgepole pine stands i n r e l a t i o n t o density
o f i n f e c t i o n s o u r c e s . Dep. F i s h For. B i mon. Res. Notes 26:49.
I'.luir, J . A.
1972. I n c r e a s e o f dwarf m i s t l e t o e i n f e c t i o n s
on young l o d g e p o l e p i n e s . Can. J . For. Res
2~413-416.
Muir, J . A.
1973. Lodgepole p i n e dwarf m i s t l e t o e on
Douglas f i r i n A l b e r t a . Can. For. S e r v .
Bi-mon. Res. Notes 29:25-26.
Myers, C . A . , C. B . E d n i n s t e r , and F. G .
Hawksworth
1976. SWYLD2: Y i e l d t a b l e s f o r even-agedand
two s t o r i e d s t a n d s o f Southwestern pond e r o s a p i n e , i n c l u d i n g e f f e c t s o f dwarf
m i s t l e t o e s . USDA For. S e r v . Res. Pap. RM163. 25 p .
P a r m e t e r , J. R . , J r . , and R . F. S c h a r p f
1963. Dwarf m i s t l e t o e on r e d f i r and w h i t e
f i r i n C a l i f o r n i a . J . F o r . 61:371-374.
P a r m e t e r , J . R . , J r . , and R. F. S c h a r p f
1972. Spread o f dwarf m i s t l e t o e from d i s c r e t e s e e d s o u r c e s i n t o young s t a n d s o f
p o n d e r o s a and J e f f r e y p i n e s . USDA F o r .
S e r v . Res. Note PSW-269. 5 p .
Pierce, H. R.
1960. Dwarf m i s t l e t o e and i t s e f f e c t upon
t h e l a r c h and Douglas f i r o f w e s t e r n
Montana. Mont. S t . Univ. Sch. F o r . B u l l
1 0 . 38 p .
R i c h a r d s o n , K. S . , and B . J . van d e r Kamp
1972. The r a t e o f upward advance a n d ' i n t e n s i f i c a t i o n o f dwarf m i s t l e t o e on immature
w e s t e r n hemlock. Can. J . For. Res. 2:313316.
Roe, A. L . , and G . D . Amman
1970. The mountain p i n e b e e t l e i n l o d g e p o l e
p i n e f o r e s t s . USDA F o r . S e r v . I n t e r m .
F o r . Range Exp. S t n . Res. Pap. INT-71.
23 p .
p a t h o l o g y 44 :504.
Roth, L. F.
1959. N a t u r a l emplacement o f dwarf m i s t l e t o e s e e d on ponderosa p i n e . For. S c i . 5 :
365-369.
Roth, L. F.
1966. F o l i a r h a b i t o f ponderosa p i n e a s a
h e r i t a b l e b a s i s f o r r e s i s t a n c e t o dwarf
m i s t l e t o e . Pages 221-228
Breeding PestR e s i s t a n t T r e e s (H. D . G e r o l d , R. E . McDermott, E . J . S c h r e i n e r , and J . A. Wini e s k i , e d ~ . ) . Pergamon P r e s s , New York.
505 p .
Roth, L . F.
1971. Dwarf m i s t l e t o e damage t o s m a l l pond e r o s a p i n e s . For. S c i . 17:373-380.
Roth, L. F.
1974. J u v e n i l e s u s c e p t i b i l i t y o f p o n d e r o s a
p i n e t o dwarf m i s t l e t o e . P h y t o p a t h o l o g y
64:689-692.
S c h a r p f , R. F.
1962. Growth r a t e o f t h e e n d o p h y t i c s y s t e m
o f t h e dwarf m i s t l e t o e on d i g g e r p i n e .
USDA F o r . S e r v . , PSW Res. Note 193. 5 p .
S c h a r p f , R . F.
1964. Dwarf m i s t l e t o e on t r u e f i r s i n C a l i f o r n i a . USDA For. P e s t L e a f l . 89. 7 p .
S c h a r p f , R . F.
1969a. Dwarf m i s t e l t o e on r e d f i r , i n f e c t i o n
and c o n t r o l i n u n d e r s t o r y s t a n d s . USDA
For. S e r v . Res. Pap. PSW-50. 8 p .
S c h a r p f , R. F.
1969b. Temperature a f f e c t s p e n e t r a t i o n and
i n f e c t i o n o f p i n e s by dwarf m i s t l e t o e s .
For. S c i . 15:149-151.
S c h a r p f , 9 . F. and F. G . Hawksworth
1968. Dwarf m i s t l e t o e on s u g a r p i n e . USDA
For. S e r v . , For. P e s t L e a f l . 113. 4 p .
S c h a r p f , R . F . , F. G . Hawksworth, and B . J .
Erickson
1976. H i s t l e t o e l i t e r a t u r e o f t h e w o r l d : a
u s e r ' s g u i d e t o a FAMULUS r e t r i e v a l s y s tem. USDA For. S e r v . Gen. Tech, Rep. RM30. 5 p .
Roth, L. F .
1953. P i n e dwarf m i s t l e t o e on t h e P r i n g l e
F a l l s Experimental F o r e s t . USDA For.
S e r v . , Pac. Northw. For. Range Exp. S t n .
Note 91. 3 p .
S c h a r p f , R . F . , and J . R . P a r m e t e r
1967. Spread o f d w a r f m i s t l e t o e i n t o J e f f r e y
p i n e p l a n t a t i o n . USDA F o r . S e r v . Res. Note
PSW-141. 6 p .
Roth, L. F.
1954. D i s t r i b u t i o n , s p r e a d and i n t e n s i t y o f
dwarf m i s t l e t o e on ponderosa p i n e . Phyto-
S c h a r p f , R . F. and J . R. P a r m e t e r , J r .
1971. Seed p r o d u c t i o n and d i s p e r s a l by
dwarf m i s t l e t o e i n o v e r s t o r y J e f f r e y p i n e s
i n C a l i f o r n i a . USDA For. S e r v . Res
PSW-247. 5 p .
.
Note
u a l hemlock on c u t o v e r s t a n d s . USDA For.
S e r v . Res. Note PNW-278. 3 p .
S c h a r p f , R. F . , and J . R . P a r m e t e r , J r .
1976. P o p u l a t i o n b u i l d u p and v e r t i c a l
s p r e a d o f dwarf m i s t l e t o e on young r e d
and w h i t e f i r s i n C a l i f o r n i a . USDA For
S e r v . Res. Pap. PSW-122. 9 p .
S t r a n d , M. A.
1973. S i m u l a t i o n o f p o p u l a t i o n changes o f
w e s t e r n dwarf m i s t l e t o e on p o n d e r o s a p i n e .
Ph.D. T h e s i s , Oregon S t a t e U n i v e r s i t y ,
Corvallis.
Schmid, J . M., and T. E . Hinds
1974. Development o f s p r u c e - f i r s t a n d s f o l lowing s p r u c e b e e t l e o u t b r e a k s . USDA For.
S e r v . Res. Pap. RI1-131. 16 p.
S t r a n d , M. A . , and L . F. Roth
1976. S i m u l a t i o n model f o r s p r e a d and i n t e n s i f i c a t i o n of w e s t e r n dwarf m i s t l e t o e i n
t h i n n e d s t a n d s o f ponderosa p i n e s a p l i n g s .
Phytopathology 66:888-895.
Schwandt, J . W.
1977. Dwarf m i s t l e t o e c o n t r o l g u i d e l i n e s
f o r Idaho f o r e s t s , Idaho Dept. o f Lands,
Div. For. Res., Bur. P r i v a t e F o r . , Rep.
77-2. 1 3 p .
Shea, K . R .
1963. Diameter i n c r e m e n t i n old-growth
Douglas f i r i n f e c t e d by dwarf m i s t l e t o e .
Weyerhauser Co. For. Res. Note 50. 11 p .
Shea, K . R . , and P. G . B e l l u s c h i
1965. E f f e c t s o f d w a r f m i s t l e t o e on d i a m e t e r
i n c r e m e n t o f immature p o n d e r o s a p i n e bef o r e and a f t e r p a r t i a l l o g g i n g . Weyerh a u s e r For. Res. C e n t e r Pap. 4 . 7 p .
Shea, K . R . , and T. J . Orr
1963. A s u r v e y o f d w a r f m i s t l e t o e o f pondero s a p i n e i n s o u t h - c e n t r a l Oregon. J . For.
61: 138-141.
Shea, K. R., and J . L. S t e w a r t
1972. Hemlock dwarf m i s t l e t o e . USDA For.
S e r v . , For. P e s t L e a f l . 135. 6 p .
Smith, R . B .
1969. A s s e s s i n g dwarf m i s t l e t o e on w e s t e r n
hemlock. For. S c i . 15:277-285.
S t r u b l e , G . R.
1965. A t t a c k p a t t e r n o f mountain p i n e b e e t l e
i n s u g a r p i n e s t a n d s . USDA For. S e r v . Res.
Note PSW-60. 7 p .
Wagener, W. W .
1961. F o r t y y e a r s o f d w a r f m i s t l e t o e on a
ponderosa p i n e p l o t n e a r Quincy, C a l i f o r n i a . USDA F o r . S e r v . , PSW For. D i s . Res.
4600 ( f i l e ) . 15 p .
Wagener, W . W.
1965. D w a r f m i s t l e t o e removal and r e i n v a s i o n
i n J e f f r e y and ponderosa p i n e , n o r t h e a s t e r n C a l i f o r n i a . USDA For. S e r v . Res. Note
PSW-73. 8 p .
Wass, E. F.
1976. Ecology o f s h o r e p i n e s t a n d s i n f e s t e d
w i t h dwarf m i s t l e t o e . Can. F o r . S e r v . BCX-142. 33 p .
Waters, W . E .
1976. E v a l u a t i o n o f i n s e c t i m p a c t s on f o r e s t
p r o d u c t i v i t y and v a l u e s .
The Bases f o r
I n t e g r a t e d P e s t Management. P r o c . X V I IUFRO
World Congr., Group 6 , Oslo, Norway. 28 p .
Smith, R. B . , and J . A. Baranyay
1970. Dwarf m i s t l e t o e s i n B r i t i s h Columbia.
Canad. For. S e r v . , F o r . Res. Lab., Misc.
P u b l . BC-M3-71. 8 p .
Weaver, H .
1974. E f f e c t s o f f i r e on t'emperate f o r e s t s :
Western United S t a t e s , p . 279-319. I n
and
F i r e and Ecosystems. Kozlowski, T .
C . E . Ahlgren, e d s . Academic P r e s s , New
York. 542 p .
Smith, R. B .
1973. F a c t o r s a f f e c t i n g d i s p e r s a l o f dwarf
m i s t l e t o e s on p l a n t a t i o n grown t r e e s i n
B r i t i s h Columbia. Can. For. S e r v , Rep.
BC-X-97. 21 p .
Weir, J . R .
1916. Larch mist1etoe:some economic c o n s i d e r a t i o n s o f i t s i n j u r i o u s e f f e c t s . USDA
B u l l . 317. 25 p .
Smith, R . B . , and E . F. Wass
1976. F i e l d e v a l u a t i o n o f e c o l o g i c a l d i f f e r e n t i a t i o n o f dwarf m i s t l e t o e on s h o r e
p i n e and w e s t e r n hemlock. Can. J . F o r .
Res. 6:225-228.
S t e w a r t , J . L.
1976. Dwarf m i s t l e t o e i n f e c t i o n from r e s i d -
Weir, J . R.
1918. E f f e c t s o f m i s t l e t o e on young c o n i f e r s
J . A g r i c . Res. 12:715-718.
Wicker, E . F.
1967. Seed d e n s i t y a s a k l e n d u s i c f a c t o r . o f
i n f e c t i o n and i t s impact upon p r o p a g a t i o n
o f Arceuthobium s p p . Phytopathology 57:
1164-1168.
Wicker, E. F. 1974a. Ecology of dwarf mistletoe seed. USDA For. Serv. Res. Pap. INT-154. 28 p. Wicker, E. F. 1974b. Arceuthobium doug1asii:its absence from the flora of the Puget-Willamette lowlands. Northw. Sci. 48:239-244. Wicker, E. F., and C. D. Leaphart 1976. Fire and dwarf mistletoe (Arceuthobium spp.) relationships in the northern Rocky Mountains. Proc. Montant Tall Timbers Fire Ecology Conference and Fire and Land Man- agement Symposium, No. 14, Missoula, Montana. 1974. p. 279-298. cker, E. F., and C. G. Shaw 1967. Target area as a klendusic factor in dwarf mistletoe infections. Phytopathol- ogy 57:1161-1163. Williams, W. T., F. Fortier, and J. Osborn 1972. Distribution of three species of dwarf mistletoe on their principal pine hosts in the Colorado Front Range. Plant Dis. Rep. 46:223-226.