THE SEED SOURCE QUESTION FOR PONDEROSA PINE Roy R. Silen Forestry Sciences Laboratory Pacific Northwest Forest and Range Experiment Station Forest Service, U.S. Department of Agriculture Corvallis, Oregon Reprinted from
REGENERATION OF PONDEROSA PINE
Symposium held September 11-12,
1969
Compiled and edited by
Hermann Associate Professor of Ecology R. K.
December 1970 School of Forestry Oregon State University COPIES OF THESE PROCEEDINGS
are available for $5.00 each from the
School of Forestry
Oregon State University
Corvallis, Oregon 97331
PURCHASED BY THE FOREST SERVICE, U.S.
OF AGRICULTURE, FOR OFFICIAL USE.
DEPARTMENT
THE SEED SOURCE QUESTION FOR PONDEROSA PINE Roy R. Silen Forestry Sciences Laboratory Pacific Northwest Forest and Range Experiment Station Forest Service, U.S. Department of Agriculture Corvallis, Oregon SUPPOSE, FOR A MOMENT, THERE WERE N Q RESEARCH ON SEED-SOURCE DIFFERENCES of
ponderosa pine. Would there be no basis at all on which foresters could decide whether local or nonlocal seed would
give the best returns over a rotation? Long before any research, the pattern of variation in the vast natural stands of
ponderosa pine provided good information for inferences about the question of seed source. Although no one can
separate what is genetic and what is environmental by observation alone, anyone can see that the species is markedly
different over its range from west to east, from south to north, and from desert edge to high elevations on almost any
major mountain range of the West.
ON THE DESERT EDGE, PONDEROSA PINE is
slow-growing, hardly different from the juniper with
which it successfully competes for limited moisture,
although the juniper successfully competes with it in
growth rate. Further up the slope) with more moisture
assured, the ponderosa pine gains in height, but finally
loses out to various more rapidly growing species. At its
moist upper elevational limit, it grows much like the
associated firs and pines. It successfully competes with
them for light as they, in turn, successfully compete
with it for the available moisture. That species are
similar in growth rates in areas where their ranges
overlap is a common observation. This elevational
pattern of growth rate for ponderosa pine is repeated
horizontally on a grand scale from the moist western
side of the species' range, where its height sometimes
approaches 250 feet, to the droughty eastern side,
where it is a slow-growing tree.
One could speculate that the extreme differences
between the moisture-loving and the drought-tolerant
types, with all the gradations between, simply reflect
the reaction of one genetic kind of seed to a good or
poor environment. Obviously, a large part of the
difference is environmental, because we can observe
great variations between rocky and deep soil on the
same acre. But, modem genetic research shows that
scores of other organisms, including man, exhibit wide
variability of large numbers of traits. Detailed studies of
these organisms show that there is an intense selection
from the available genetic pool of variability for the
genetic type that gives the species an advantage in each
local environment. Presumably, local ponderosa pine has
the same advantage. One would no more expect seed
from the droughty eastern side of the Rockies to grow
successfully in the moist Cascades than expect
transported Pygmies to live successfully under natural
conditions in the Arctic. With the question of seed
source in ponderosa pine, the forester is usually not
22
dealing with such extremes, but rather with how fine a
gradation in environment between these extremes may
have developed a genetic difference of importance. Is
any movement of seed tolerable? Is the movement
limited to the seed's flight distance? To a 1 ,000-foot
elevational zone? To a 2,000-foot zone? To a 5,000-foot
zone? How far north or south in a zone? These are the
typical questions that presently have no exact answers
from research.
We should consider two more observations about
the natural stand of ponderosa pine before we examine
the limited information from seed-source studies.
The first observation concerns climatic stability.
Climate in western North America has been essentially
the same for perhaps 5,000 years. One good indication
of this stability is that individual bristlecone pine trees
have Jived the last 4,000 years on some western
mountains. This time span represents ample generations
of ponderosa pine for the gene frequencies that
characterize a race to essentially stabilize in local seed.
The second observation concerns pollen. Local
stands display great diversity. But, in addition,
individual trees in any ponderosa pine stand become
receptive to pollination at different times over a range
of several weeks. Although most pollination must come
from neighboring trees or stands, there is no barrier to
considerable pollination of early flowers from lower or
more southerly stands, and of late flowers from higher
or more northerly stands. Thus, each generation would
seem not only to represent a stability of type from long
selection for a local environment but also to incorporate
whatever genes are brought in from other fastergrowing
or more drought-tolerant stands from many miles on all
sides.
Without research in genetics, then, we can
reasonably conclude that the local seed source must
have .adapted its genetic options into a sensitive balance
with the local environment, and that each generation
23
The Seed Source Question for Ponderosa Pine
has had ample resupply of diverse genes to adjust to
favorable or unfavorable climatic cycles.
I can illustrate this hypothesized structure for a
stand grading from mountaintop to desert, such as we
view here on the east side of the Cascades. Locally, the
sharp moisture gradient overrides the smaII changes in
temperature with elevation in the ponderosa pine
elevational zone. We could expect the seed mix of many
trees at any distinct locality along the gradient to
represent a stabilization of the optimum average rate of
growth that can be sustained in nature against long-term
weather extremes, particularly moisture stresses. For a
locality along the gradient somewhat higher and
westward, where moisture is ample, the seed mix should
have higher portions of genes for faster growth and
lower portions for drought tolerance. Conversely, lower
and eastward, where moisture is less, the seed mix
should have lower portions of genes for fast growth and
more for drought tolerance. As the local race is always
receiving pollen from stands on all sides, presumably it
should have the variability to shift considerably toward
faster growth should the climatic cycle shift to more
moisture. On the other hand, should a droughty cycle
prevail, an ample proportion of drought-tolerant types
should also be in the seed mix to fully stock the stand
after the beetles have had. their day.
If these observations of the natural stand lead us to
the correct genetic conclusions, we would select local
seed as the only reasonable choice for a long-rotation
tree crop. Natural selection pressures on any nonlocal
seed should be toward restoring the balance of genes as
they occur in local seed. Higher proportions of
unadapted trees would be expected than with local seed,
hence, heavier losses would be expected during adverse
periods.
Now, let us see whether these conclusions are
altered by research on seed sources. If we regard studies
under 20 years of age as having a limited interpretation,
then there are only seven studies that provide data on
inherent growth and survival in ponderosa pine.
They include the following plantings: Twenty-two
races in Idaho, begun in 1 91 1 (3, 1 3 , 15); several races
Table 1.
Re1 ation
at Flagstaff, Ariz., started about 1 91 1 (4, 9, 10); ten
races planted at five sites in Oregon and Washington,
started in 1926 (7, 8, 13 ); New Zealand plantings of
seven races, started in 1931 ( 6, 1 3 ) ; local variation in 81
families, begun in 1933 at Placerville, California (I);
variation of progeny from 89 seed trees that originated
along an altitudinal transect in the Sierra Nevada,
planted at three elevations, started in 1 93 9 (2, 5); and
arboretum plantings of many species at Wind River,
Washington ( 1 1 , 1 2).
These studies tend to confirm several of the genetic
inferences I have made from observations of the natural
stand (Table I). My first inference is that inherent rates
of growth correlate moderately to strongly with the
yearly climatic pattern of available moisture and with
favorable springtime temperatures. Races with the
highest inherent rate of growth developed in the mild
climate west of the Cascade and Sierra Nevada ranges
where springtime soil moisture is ample and assured.
The lowest rates of growth developed east of the
Rockies and in Arizona in areas of deficient springtime
moisture
and where long drought cycles
are
experienced. Between these extremes, growth rates
grade from west to east clinally, but also respond to
differences in moisture and temperature from locality to
locality. The evidence for this statement comes from
three studies in Idaho, Oregon, Washington, and New
Zealand, through 25 to 55 years of testing. About half
the variation in racial growth on the plots is accounted
for by climate at the source.
Evidence for a similar gradation of growth rates on
a single mountain range, such as on the east slope of the
CasCade or Sierra Nevada, is convincing. Though not
deliberately tested in the same three studies, several
examples of reasonably paired seed sources from moist
or dry sites in the same mountain range can be cited
from data at 25, 40, and 55 years (Table I ). The "rain
shadow" effect on inherent growth is observed to occur
on the east side of several other western mountain
masses in these studies.
Estimates were made of the genetic and
environmental proportions of variation in average height
.
o f seed sourc e to HelQ:Iht
High precip it a tion zoi;ie
-
Mountain
Seed source,
National Forest
range
I
Trees
0f
L ow
-
rn Plan ta t"ions.
p recioit a tion z one
Seed source,
Height
National Forest
1
Percent
I
Hei ht
Percent
74
Sierra Nevada
Sierra
141
Lassen
Sierra Nevada
Eldora do
138
Lassen
Cascade
Rogue
126
Deschutes
118
N. Rocky Mt.
Bitterroot
118
Helena
108
1
Based
on average height of all trees in th e plantation.
70
1
Regeneration of Ponderosa Pine
24
of races observed on the plots in two of the studies. In
Experiment Station, Berkeley, California. The data at
both, the estimates were similar, with about two-thirds
29 years now show that medium-elevation sources
of the variation ascribed to environment a· nd one-third
exlribit
to seed source. For many reasons, one could expect a
elevations,
superiority
and
in
height
high
growth
sources
at
at
medium
lrigh-elevation
lower portion of the observed variation between average
p l a n t ations.
heights of natural stands to be genetic, but the major
somewhat taller at low elevations. Jn earlier years, the
Medium-elevation
sources
are
still
ones are the widely differing races being compared in
middle-elevation sources were distinctly superior at all
these studies and research plots deliberately located to
elevations,
minimize
normally occurs in the Sierra Nevada Mountains. This
environmental
differences.
Undoubtedly,
which
reflected where
best
tree growth
considerable genetic variation within each race was not
middle-elevation group of seed -sources, however, now
estimated from the studies.
appears
to
be losing
to the
locations with only moderate moisture stress, hence,
elevations,
with
reactions to long-term climatic extremes have been slow
superior height. Differences in survival have developed
plots
of existing
studies
are
poorly
mid-elevation
defined
lrigher
but
the
are
at
elevations,
of
trends
local races
in
Most
sources
at
low
maintaining
in expression. At the test site at Flagstaff, Arizona,
only weakly at 29 years in this study. The data nicely
however, with irregular springtime moisture and drought
confirm
cycles over various periods of years, the expression was
differences of seed sources with elevation and indicate
the
concept
of
gradually
changing
genetic
rapid (4). The surviving trees are almost entirely from a
that
few races that fit the local dry-springtime moisture
measurable genetic difference in a seed mix-in this
pattern-that is, races from Arizona, New Mexico, and
instance, in inherent height growth.
small
differences
in
elevation can produce
a
the east slope of the Rocky Mountains. All races from
Other important patterns have developed in the
moister climates further west and north have few or no
older plantations. Needle diseases can be a serious pest
survivors. Many seedlings from the westerly races died in
of offsite trees. For example, a generalization seen in
the nursery from frosts. Early growth of those that
the Wind River Arboretum, as well as in the test sites
survived through the nursery period was good, which
west of the Cascades, is that ponderosa pine from drier
indicated a capacity to grow well as long as suitable
climates and, incidentally, lodgepole pine, western larch,
conditions prevailed.
and Douglas-fir have sustained repeated crippling or
At the next most severe site, near Bend, Oregon,
killing attacks of needle diseases. Damage from snow
the local Deschutes race is showing distinct superiority
and ice is often severe to some offsite races. Trees from
in survival, yet survivors of several other races still show
high-elevation sources burst buds early at low elevations
better growth in height. Superiority in survival requires
and are often killed by frost. These same factors damage
time for expression on less severe sites.
or kill individuals in the local population, and usually
For example, in the oldest study at Priest River,
only the difference in severity favors local races.
A lthough
Idaho, where 22 races have been growing for 55 years,
the latest data (14) show low survival for races beyond
research
200 miles from Priest River. The local Kaniksu race is
ponderosa pine,
gaining over
perplexing
other
races in
both
growth
and plot
plots
generally
support
inferences of genetic variability in the natural stands of
they
also present
exceptions.
We
do
not
a smattering of
have
adequate
volume, although individual trees of a near local race
explanations
(Lolo) show superior height but lower survival, and a
majority of the limited data in the seven studies seems
race in a very similar elevation and climatic situation
to confirm inferences from the natura1 stand.
for
these
exceptions.
Yet,
the
great
(Umatilla) shows superior plot volume. The research
Since all of life is a balance of gains and risks, what
evidence, then, indicates that on the most severe plot,
do the foregoing observations and data suggest as an
only races adapted for extreme conditions survived.
answer to the seed-source question?
Under less severe conditions, the local or best-adapted
race
may
take
half a
century
to show
a
distinct
Most. important is to appreciate that the local seed
source is something special. It appears to be the kind of
superiority in survival and plot volume. Although other
seed combination that incorporates the best gain and
races som.etimes grow more rapidly, the higher survival
least risk our present knowledge can provide. Unie·ss
of the local race, coupled with reasonably good growth,
climates change appreciably from the last 5 ,000 years,
appears to produce superior volume in the Jong run.
the local source will guarantee a good stand.
Genetic differences are expressed over rather small
Under natural conditions, any departure from the
changes in elevation in a remarkably complete sampling
local seed source carries unknown risk. If the concept is
along a transect in the Sierra Nevada Mountains near
correct that each stand carries a balanced seed mix of
Placerville, according to data contributed by Dr. Tom
fast-growing and drought-tolerant types attuned to the
Conkle,
local
Pacific
Southwest
Forest
and
Range
environment, then nonlocal types selected for
The Seed Source Question for Ponderosa Pine
faster growth would probably sustain greater losses than
the local stand in adverse climatic cycles. Observations
of both the natural stands and the oldest research plots
point to a survival trend in this direction.
The risk is related to the length of rotation. For a
short-rotation crop such as Christmas trees, the risk of
damage from climatic extremes during the rotation is
low. For a I 00-year rotation, however, who \fOUld take
a chance on the weather'?
The risk is also related to the site. The outcome of
a nonlocal seed source on a severe site is already known
from many experiences. One would face less risk on a
mild, protected site because the intervals between
climatic extremes are longer.
If one wishes to risk a nonlocal, fast-growing race,
he can usually find it in areas of assured springtime
moisture and mild springtime temperatures.
But, in closing, we must add that our present effort
to improve growth rate through genetics is based upon
finding well-adapted parents in local stands capable of
better growth rates in the local environment.
Eliminating the rigors that beset seedlings in early life
may change the picture. We also expect to see
improvements to the local environment, such as
thinning and control of competition, that will assure
more moisture to the individual tree. Considerable work
is planned or underway to understand the question
"How local is local?" But with our limited present
knowledge, local seed appears to be the only reasonable
choice for most of us, regardless of how high early rates
of growth of nonlocal trees may seem to be on our site.
An old Chinese proverb says, "Forester who drops
his local genes may also lose shirt."
25
Serv., Rocky Mountain Forest & Range Expt. Sta., Res.
Note RM-73, 7 p. 1966.
5. MIROV, N. T., J. W. DUFFIELD, and A. R.
LIDDICOET. "Altitudinal Races of Pinus ponderosa-a
1 2-Year Progress Report." J. Forestry 50:825-831 .
1952.
6. MOORE, A. M. "Pinus ponderosa Douglas,
Comparison of Various Types Grown Experimentally on
Kaingaroa State Forest." New Zealand J. Fo estry
55:208-210. 194.
7. MUNGER, T. T. Progress Report on a Study of
Regional Races of Ponderosa Pine. Unpublished report
on file at Pacific Northwest Forest & Range Expt. Sta.,
Portland, Oregon. 1 5 p. 1941.
8. MUNGER, T. T . "Growth of Ten Regional Races of
Ponderosa Pine in Six Plantations." U.S. Dept. of Agric.,
Forest Serv., Pac. N.W. For. & Range Expt. Sta., Res.
Note 39, 4 p. 1 947.
9. PEARSON, G. A. "Forest Types in the Southwest as
Determined by Climate and Soil." U.S. Dept. of Agric.,
Tech. Bull. 247, 1 43 p. 1 931.
10. PEARSON, G. A. "Management of Ponderosa Pine in
the Southwest." U.S. Dept. of Agric. Monogr. 6, 21 p.
1950.
1 1. SI LEN, R. R. "A Pioneer Search for Better Kinds of
Trees for the Northwest." U.S. Dept. of_ Agric., Forest
Serv., Pac. N.W. For. and Range Expt. Sta., 15 p.
(unnumbered). 1 967.
LITERATURE CITED
I. CALLAHAM, R. Z., and A. A. HASEL. "Ponderosa
Pine Height Growth of Wind Pollinated Progenies."
Silvae Genet. 10(2):33-42. 1 96 1 .
2 . CALLAHAM, R . F . and A . R . LIDDICOET.
"Altitudinal Variation at 20 Years in Ponderosa and
-Jeffrey-Pines. "-J.--Forestry-59(+1-):-814"8-20:-1961;
3. KEMPF, G. "Non-Indigenous Western Yellow Pine
Plantations in Northern Idaho." Northwest Sci.
2(2):54-58. 1928.
4. LARSON, M. M. 'Racial Variation in Ponderosa Pine
at Fort Valley, Arizona." U.S. Dept. of Agric., Forest
1 2. SILEN, R. R., and L. R. WOIKE. "The Wind River
Arboretum from 1 9 1 2 to 1956." U.S. Dept. of Agric.,
Forest Serv., Pac. N.W. For. and Range Expt. Sta., Res.
Paper 33, 50 p. 1 959.
13. SQUILLACE, A. E., and R. R. SILEN. "Racial
Variation in Ponderosa Pine." Forest Sci. Monogr. 2, 27
p. 1 962.
14. STEINHOFF, R. J. "Northern Idaho Ponderosa Pine
Racial Variation Study-50 Year Results." U.S. Dept. of
Agric., Forest Serv., Intermountain Forest and Range
Expt. Sta., Res. Note (in press). 1 969.
1 5. WEIDMAN, R. N. "Evidences of Racial Influence in
a 25-Year Test of Ponderosa Pine." J. Agr. Res.
59(12):855-887. 1939.
'