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17,~:i...,*<,,',d ~.~</l~l,oD.,./al 1;.:,.{). \ ' d :12.X() I. pp 319 4?.. 1992
01)91{ i~172 99 7o0() i I).(ll)
( 1!197])(i~illIl()II lilt kb l.ld
Pihlh'd hi (llMII I{tihliu.
M O R P H O L O G I C A L CAUSES FOR THE R E T E N T I O N O1"
1)RECIPITATION IN THE C R O W N S OF A I 2 I N E PLANTS
RUSSELL K. MONSON,*t MICHAEL C. GRANT,* CHARLES H. JAEGER* and
ANNA W. SCHOETTLE~
* l)cpartmcnt ()f Environmental. Population and ()rzanismic Biology, (]ampus Box 334,
[_llli\t'l'Sit\: of(iol()l'ado, Boulder, (',() bi()309, I,!.,%.A. dlld 4.+Ro<kv ~Iotllllain I"orcsl and Range
l']xl)i'rhncnt Station. l_:nitcd Slates l'orcsi S('r\'icc. 20 \V. Pi'(isp('ct Slr('t't, [:ori (',ollins.
(',() gO0<_)(i,[7.S..\.
I Re~ei~,cd 10 ,]a,la(r 1992; aa'@td iszi:,al ,,'&~'d./Dr,~ 31 .l,¢,>t 19{)91
Mt)xs<)x R. K., (;RANT ~[. (:., ,JAEGER(:. H. and S(:H()ETTLE A. \\'. 31oJphol,<,,ical cau.~e~./br/De
rc:a~li,, ,?lpseci/filalio, in the cr0a't#.r o/a&i,e p/ohM. ENVIRONMENTAl.ANI) EXPERIMI~;N'I'AI.Bo'rAN'.~
32, 319 327. 199L Studi('s \~,ert' c()ndut'tcd on 27 species of all)in(' plants to if'st In(' h.~poth('sis
l ] l a I s t r t l c l u r a l characteristics ()llcav('s ha\c a predictable hlllucncc ()n liic amouiit o f nl()isturc
i'ctained I)y a plaili (TOWll tbllowiug a sinltllatcd raiil i'VClIt. The rt'icniion (11"precipitation ili
CI'O:VilS has I)t'Cll I l r c \ i o u s l . \
(IClliOIIStI';llcd
{is ()lit' [~i¢'lOl" p o t t ' n i i a l l }
contributing
to t i w dir('ct
('tt'ccts of acid I'aill O11 alpin(" p l a n t s . 'l'h(' r c s u h s o f this Sttld\ dt'lllOllSll'di(' Ihal :1 signifi('alll s h a f t '
o f the a n l o u n t ( ) t ' w a t c r r e t a i n e d c o u l d ])(' c x p l a i u c d I)y g e n e r a l s t r u c i l l r a l f 0 a t u r c s o f leaves a n d
t l m x c r s ('OlllniOll [() all t h c divers(' l a x a s t t i d i c d . \\7at('r r c i a h l ( ' d p c r u n i t M i f al't'il "~%{is I)('st
explained by ])UI)osCCllC(' r a l l k , lltll/qbcr of leaves, pc\it)Iv I)asc width, tlowcr l'flaSS, tlox:cr w ( ' t l al)ilitv and petit)h" bas(' aligle. ~i\'atcr r('taincd p('r unil I//ds~; :',;its t)('si CxlJlained I)} pUbt'St'Cll('('
rallkhlo, llUilli)cr of h'axcs, pet;oh' I)asc widih, leaf al'C{I, flower :~clial)ilii\. ttowcr i/l{iSs alld
l U l l l / I ) c r ()f flm~ cl's.
INTRODUCTION
THE i n t e r c e p t i o n ot'acidic rain I)y plants has been
i m p l i c a t e d as a m a j o r cause of productix'ity losses
7,13
in both a g r o n o m i c and n a t u r a l ecosystems.
C()ntact with acid rain is knoxvn to cause lesions
on v e g e t a t i v e tissues -'" and r e d u c t i o n s in reprod u c t i v c success. -':) P l a n t m o r p h o l o g i c a l tbatures
can hlfluence die d i s t r i b u t i o r i o f acid rain ;is it
a('ctinlulates on aerial surlaces, in Soille cases
e x a c e r b a t h i g the p o t e n t i a l lot d a m a g e . FUNK and
BONI)E ~{ obserx'ed d a m a g e to a x i l l a r \ [loral m e r is\eros in iin a l p i n e p l a n t species when acidit' solu;ions I)eclime pooled in the basal re,gion o1 the
leaves. Pooled acidic solutions h a v e been s h o w n
l() cause e p i d e r m a l d a m a g e to tissues in o t h e r
si)ecies as well. :,(~ ])espite a g e n e r a l r e c o g n i t i o n
of the damag(" that can be caused l)v contact
with acid rain, v e r y little is k n o w n a b o u t the
m()rphoh)gk:al [i'atures of plants that influence
acid rain i n t e r c e p t i o n or su[)sequent d i s t r i l m t b n
across aerial surfaces.
In tile current study we tested the hypothesis
that structural characteristics of shol)ts hay(' a
p r e d i c t a b l e influence on tile alFlOUnt of m o i s t u r e
r e t a i n e d t)v a plant cr( wn h i l t ) w i n g a sinlulated
r a i n e v e l l t . ~IV(" r e a s o l l e d t h a t if a g i v e n ilqass ()[
shoot tissues wet(' s t r u c t u r e d such thai it had
-t-To wh(m~ all ( orrcspondencc should hc addressed.
319
320
R . K . MONSON el al.
numerous small leaves with broad leaf or petiole
bases, it would retain more water c o m p a r e d to
tissue with fbwer, large leaves with narrow bases
or petioles. Small leaves with t)road bases would
retain water in the fi'equent, close spaces between
leaves and in the broad intersections between leaf
and stem. A d d i t i o n a l l y , we tested the hypothesis
that because of the more highly divided structure
of [hn'al tissues, they too might retain significant
all'lotlnts o f l l l o i s t t n e due to their s l r t l c t t l r a l
tbatures. In this study we were not interested in
assessing d a m a g e bv acid rain per se. Rather, we
wished to address the potential tbr p r e d i c t a b l e
patterns in rain retention, one possil)lc tactor contributing to acid rain-induced d a m a g e (or indeed,
(lamage from any water-soluble contponent of
rain) in plants. It is hoped that su<'h intbrmation
will prove usethl to future eflbrts aimed at understanding species-spccilic differences in the susceptibility to acidic dalnagc.
MATERIALS AND METHODS
,S'ludy area
T h e studies were conducted in the Glacier
l,akes area of the Snowy R a n g e near L a r a m i e ,
\V.xoming (106 W , 4 1 ' N ) . T h e hydrology,
geolog} and ecology of the (;lacier I,akes Ecosystem Experiment Site have bccn described in it
recent rep<)rt, a~ Twenty-seven plant species were
examined in their n a t u r a l habitats. T h e habitats
included it rocky liqltield and a dry m e a d o w sit(:
which were established along the easterly topographic rise fi'om east Glacier lmke a! distances
a p p r o x i m a t e l y 200 and 100 m fiom the eastern
edge of the lake, respectively. T h e tbllticld site was
located on a broad, flat shelf with ti'equent large
boulders. T h e dry m e a d o w site was located on
gently sloping ground with fkwcr rocks than the
tbllfiCd sit(:. A wet m e a d o w site was established
at the lake margin. T h e soil at this latter site was
sltturatcd with water d u r i n g J u n e , but became
drier d u r i n g the middle of the summer. Finally,
one species, Eo'lhronium grandiflorum (glacier lily),
growing in a snnwbed site: was examined. T h e
snowbed lasted until mid-July. After snowmelt,
the site' was characterized I)y coarse soil surlace,
with sparse vegetation, d o m i n a t e d i)y E. grandi/Iorum. Most precipitation that occurs d u r i n g the
s u m m e r is in the tbrm o[" heavy thundershowers.
Thus, our studies were concentrated on the effects
of wet, heavy precipitation (as opposed to snow,
mist and lbg), using simulated rain ev(:nts of a
m a g n i t u d e that would induce the m a x i m u m
water-holding capacity o f a plant crown.
Simulated rain lrealmenLs
S i m u l a t e d rain events were conducted d u r i n g
7-12 J u l y and 15 1 7 A u g u s t 1988. W e m e a s u r e d
how much loose water wits held by a crown tbllowing a simulated rain event, as a function of
19 different morphological characteristics. After
spraying w a t e , on a crown (or part of a crown)
the q u a n t i t ) of'lnosely 1)ound water was ol)tained
by inverting the excavated crown (or part of lhe
CI'O'WI'I) and gently t a p p i n g it while inserted into
a l)olyethylene bag. T h e t a p p i n g was continued
until all pnoled water was fieed, even thal held
its tightly a d h e r e d droplets.
Details of the spraying p r o t o c C were its tbllows.
Prior to spraying, the experimental plant was
measured tbr crown height and diameter. After
m a k i n g these measurements, i t t h e crown (or part
<)fttle crown) being s p r a ) e d was rooted in the
ground, it was excaxatcd with a hand trowel.
:\f~er excavation, the phmt was placed in its natural orientation back into the excavated region of
the soil and sprayed. Crowns we,e sprayed within
15 scc n f e x c a v a t i o n t() avoid problems with crown
wilting,. T h e ex,cavatcd crt)wn was then gently
lifted from the soil, inverted into the plastic bag
and gently tapped. T h e bag containing the water
was sealed and returned to the l a b o r a t o r y fbr
weighing. In the ('its{: of cnnifi:rous trees the
sprayed b r a n c h was severed liom the tree and the
water tapl)cd into the bag. At'to? weighing, the
bags were cut at three sides, opened to the
air, and left to dry to constant mass on the lab
bench. After 48 60 hr of drying, the bags were
reweighed. This p r o c e d u r e allowed us to quantit}
the t r a p p e d waler and a c c n u n l Ii)t" It/(' ltlass o f
any soil or loose plant debris that might have been
knocked into tll(: ])ag d u r i n g t a p p i n g of'the crown.
Rain wits simulated by spraying plants from a
height of 1 m tiom the top of the c a n o p y using it
pressurized herl)icide sprayer. T h e s p r a y e , was
calibrated at t h e study site to d e t e r m i n e the prcssurizatinn required t<) deliver between 350 and
450 cm :~ ot'x~atcr in a .30 scc burst. At ;.t height of.
1 m the water was delivered in a roughly circular
RETENTION OF P R E C I P I T A T I O N IN ALPINE CROWNS
p a t t e r n with a d i a m e t e r of a p p r o x i m a t e l y 24 cm.
T h e a m o u n t of w a t e r used tbr the spray t r e a t m e n t
(350 450 cm ~) was chosen because it represents
twice the a p p r o x i m a t e volume of a cylindrically
shaped crown of 5 cm d i a m e t e r a n d 7 cm height.
These dimensions represented the average size of
the plants (or portions of plants)
that were
studied. S p r a y i n g with twice the a p p r o x i m a t e
crown wTlume ensured that enough sinmlated
rain was applied to saturate the crowns, such that
excess water d r i p p e d tieely at the end of the spray
treatment.
T h e total n u m b e r of plants analyzed in this
study was 183. These plants were liom 27 ditt'erent species distrilmted across tbur different alpine
habitats. Typically, tbur plants ti'om each species
in each h a b i t a t t~pe were measured, a h h o u g h in
some cases tire to eight replicate plants were used.
This was true tbr Acoma.rl~,lis fossil in the t~lltield
site (eight replicates) and Bislorla bislorloides in the
d r y m e a d o w sit(: (five replicates). Six species were
sampled that occurred in both the tEllfield and dry
m e a d o w h a b i t a t types. T w o species were sampled
that occurred in both the wet m e a d o w and dry
m e a d o w h a b i t a t types. Nine (7t"the species were
sampled d u r i n g J u l y and August with the remaining species sampled in only one of the months. For"
the m u h i p l e regression analyses described below,
sample sizes were restricted to tour tot each species
and to one s a m p l i n g period only. As described
above, tbr the largest plants only portions of the
plant were sampled. Those species tbr which only
a porti(m of the p l a n t was studied are: Silene
acauli.~, Arcloslaphylos uva-m.si, Phlox pulvinata, Tri-
/ali.,,+ dasyp&'llum, Erilrichium areloides, Picea ep~gelmamtii and t~irtu,~,/texili,s. T h e tirst live species tbrm
extensive mats {so only a portion of the mat was
sampled), while the last two species tbrm trees (so
only the terminal 40 cm of the branch tips was
sampled).
Morphological measurements
After t a p p i n g the loose w a t e r from the crowns,
the e x c a v a t e d plants were placed into plastic
bags, stored on ice, and returned to the l a b o r a t o r y
tbr analysis of m o r p h o l o g i c a l traits. T h e traits
that were chosen represent those that a p p e a r e d
most likely t() influence the retention of water on
leaf surfaces, in the space between the leaf/petiole
base and the stem, or by capillarity in the spaces
321
between finely divided leaves or leaflets. To test
tile hypothesis concerning flower water retention,
flower wettability was measured a n d c o m p a r e d
to leafwettability. M e a s u r e m e n t techniques were
as {bllows:
1. Pubescence. I t was hypothesized that irmreased
put)escence would resuh in increased water
retention due to the capillarity of holding water
droplets between the leaf hairs. T h e degree of
leaf pubescence was estimated using a relative
ranking between 0 and 3, with 0 being the least
pubescent and 3 being the most pubescent. Examinations of h'af surlaces were m a d e with a h a n d
lens. In order to provide some perspective of tire
relative ranking, the h'ast two pubescent species
were t's_rchrophila [eplo,~elpa and E~_vlhro~dum gra,~di/torum (puhescence ranking 0). T h e two most
pubescent species ~xere Anlennaria umbirtella and
Hymel~ovr.~ grm'~diflora (pubescence ranking 3 y.
2. LeajTpelio/e ba,~e angle in cross-.secliom It was
hyt)othesized that the greater the cross-sectional
angle of the leaf or petiole base, the greater the
volume of the lea(/stem junction, and the greater
the c a p a c i t y to hold pools of water that might
collect there. T h e c,oss-sectional angle ol'dte petiole or leaf base at tire point of insertion was measured with a modilied protractor. T h e de'+ice had
two pieces of thread originating from the c e m r a l
point of the p r o t r a c t o r and extending to the angle
scale at the perimeler. A leaf base or petiole was
stood on cross-sectional end at the cemral point
of the p r o t r a c t o r and the threads were used to
delineate the cross-sectional angle and extend
that angle to the scale at the protractor's perimeter.
3. Leq//peliole ba.~e ze,idlh. It was hypothesized
that the wider the leaf or petiole base, the greater
the volume awfilable at the leaf/stem j u n c t i o n tbr
water pooling. T h e width of the petiole or leaf
base at the point of insertion was measured t(7 the
nearest 0.5 mm using precision calipers.
4. Leaj+angle. It was hypothesized that a leaf
angle of 4 5 with respect to the vertical stem
would p r o d u c e a larger volume c a p a b l e of supporting water pooling at the leatTstem junction.
I,eaf angle from the horizontal was measured in
.~ilu for the third through to the sixth nodes t>om
the apex. T h e p r o t r a c t o r was used tbr these
measurements. The leaf angles were averaged to
17rovide one value tbr each phmt.
322
R.K.
MONSON el al.
5. LeaJnumber and leaJarea. It was hypothesized
that the greater the leaf n u m b e r and the smaller
the leaf area, the more highly divided tire structure of the shoot, and the greater the potential tiw
water to be held by capillary fbrce in the spaces
between the leaves. In the l a b o r a t o r y , all leaves
were counted on each plant (or portion of plant)
and the total projected leaf area was measured
with a leaf area meter (modC A M S , Delta-T,
Cambridge, U.K.).
6. Area ojfimclional leaf unit. This index was
developed to account tot the tiict that some species
had a large, non-dissected leaf lamina, whereas
others had a small, highly dissected lamina. W e
hypothesized thai these different types of laminae
would c a p t u r e precipitation diflierently. T h e
thnctional unit was defined as tire entire l a m i n a
in the case of non-dissected leaves, and the leaflet
or lobe in tire case of dissected leaves.
7. Lea.f wellabililr and flower wellabilil~'. L e a f and
flower wettabilities were measured in the laboratory. I n d i v i d u a l leaves or flowers were
weighed, then immersed in water to ensure complete exposure of the organ to the water. T h e
leaf or flower was then removed from the water,
allowed to drip tor 15 20 sec (until all d r i p p i n g
had stopped), then reweighed. Finally, the leaf or
flower was dried and the d r y mass determined.
Wettabilities were expressed as a m o u n t of water
retained tbllowing immersion per unit of leaf or
flower area.
8. Leqf dry mass,flower dr)' mass, and lolal aboveground dry mass. T h e masses of leaves, flowers, and
total a b o v e g r o u n d portions of the plant were
measured after oven d r y i n g tbr 48 hr at 70°C.
Data analysis
In o r d e r to explore the relationships a m o n g
the various m o r p h o l o g i c a l t~atures, the tbllowing
questions were asked: W h i c h structural t~atures of
the crown most influence precipitation retention
and w h a t are the q u a n t i t a t i v e relationships of
these features with respect to precipitation retention? To address these questions, we c o n d u c t e d a
multiple-regression analysis of the morphological
traits in relation to the a m o u n t of water retained
and to tire a m o u n t of water retained per unit of
crown biomass as well as per unit of crowm leaf
area.
S t a n d a r d stepwise multiple regression analyses
were used in a heuristic t~tshion to examine the
ability of various combinations of morphological
traits to predict water retention per unit of leaf
area or a b o v e g r o u n d biomass. T h e results
reported below were essentially stable indep e n d e n t of the stepwise method (tbrward or backward) and essentially i n d e p e n d e n t of the
inclusion or exclusion of the structural traits not
listed in tire final set of p r e d i c t o r variahles. Strong
statistical association or statistical predictive
power does not, in and of itselt; d e m o n s t r a t e a
direct causal relationship. In multiple regression
analysis, unmeasured or so-called latent variables
highly correlated with one or more measured
variables can cause misidentification of the true
source of causal wtriation.
RESULTS
Stepwise multiple regression analysis, all
species combined, of w a t e r retention vs m o r p h o logical variables revealed that leaf" dry mass was
the single most i m p o r t a n t predictive character; it
explained 4701, of the variability in the volume
of loosely b o u n d water retained by entire plant
crowns ( d a t a not shown). Thus, the larger a
crown, the more water it will hold. Flower mass
c o n t r i b u t e d an a d d i t i o n a l 11~Ii, of the variability.
Most of the water retained by a crown, theretbre,
is retained because of structural t~atures (including size) associated with the leaves. T h e a m o u n t
of water retained per unit leaf area by entire
canopies ranged over tbur orders of m a g n i t u d e
tbr the 27 species Ihat were exalnined (Table 1).
A p p r o x i m a t e l y 2 2 - 3 2 % of the v a r i a b i l i t y in the
a m o u n t of water retained per unit leaf area could
be explained by structural difl>rences a m o n g
crowns (Table 2). Pubescence rank and flower
mass accounted for the largest share of water
retention per unit leaf area while leaf n u m b e r and
the width of the petiole base accounted tbr the
most wtriation in water retention per unit of functional leaf area. Pubescence rank and leaf n u m b e r
were also the most i m p o r t a n t factors explaining
variability in the a m o u n t of water retained per
unit of biomass (Table 2). O t h e r i m p o r t a n t t~tctots included flower wettability, leaf angle, flower
mass and flower number.
W h e n individual leaves were removed ti'om the
plant crown and examined to determine the
RETENTION
O1: P R E C I P H ' A T I O N
Table 1. Spc~ie,~ ranked according to average total water
retained per unit of leaf area
Species
g water retained/
m :~ Jca["area
Polcnti/la ni+,ca
,S?dium lam'eota/unl
. lchillea lanulo~a
l'¢denlilla di~'erii/blia
Clemen/~ia rho&mlha
l'edicMari.~ :4rocnlaltdica
l'hlo~ pulcinala
"l;me.~/u.~p_):{maea~
l'5i~r;on melanoc@halu~
Pinu.~ ./h'x ili~
75 i/i*liam da.~rphyllum
Eritrichiam mv/ioide~
.lcoma,~lr/i~ r,,,.,ii
l~,l~.monium ~.i~co~um
.lnt~'nna~ia umbine/la
l:'~i2eron pimmli.seclu.*
HrmctuJ v> 2*and(flora
.lrcto~tapt{rlo., ava-urJ
Pi(ca en~clmanii
(.'are~ n(~,,rican~
I'cdk'ulari.s pa~rii
l'>,chrophila lcplo,sepala
]]islorta hi.~loHol'(]~ ~
. ]~ro/!l~,m ~c~ihm'ri
Sileac a~auli~
Er vlh roaiam 2 ~and(florum
(,'ar:~ nardina
49.89
23.44
4.63
3.7!)
3.72
1.7',3
1.71
1.55
1.33
I. 22
1.04
O.78
0.71
O. 7 l
0.62
0.56
0.51
0.40
0.38
0.35
0.33
O. 15
0.()9
IN .'\I~PINE C R O W N S
a l n o u n t o f w a t e r r e t e n t i o n on the e x t e r n a l sur[~lces after i m m e r s i o n ( w e l t a b i l i t y ) , r a b i e s ~ e r e
o b t a i n e d that w e r e g e n e r a l l y one to three orders
()I'maglfitude l()wcr tha.l the wat(w r e t e n t i o n of
whol(" c r o w n s ¢ c o m p a r e values in Tat)les 1 and
3;. l h e s e results i n d i c a t e that most of the w a t e r h o l d i n g c a p a c i t y of c r o w n s is due to e n l e r g c n l
properties at the c r o w n level o f o r g a n i z a t i o m i.e.
properties that c a l l n o l be p r e d i c t e d on the basis
of isolated h'at" properties alone. A l t h o u g h the
;.II/IOLIIII o f flower biomass had only a small inlluUII('C ()ll \V~I[CI" r e l c n t i o n bY the entire c r o w n , each
flower of mosl species had a thMy high c a p a c i t y
Io retain water. In d e t e r m i n i n g the c a p a c i t y lbr
a c m : of leaf ilrea to c a p t u r e w a t e r vs a ('Ill" O["
flower re'ca, in lwarlv e v e r y species flowers can
c a p t u r e m a r c ( T a b l e 33.
"l'hc d e g r e e o f l i l w a r association a m o n g all pairs
o f l n o r p h o l o g i c a l traits is s h o w n in T a b l e 4. T h e
strollgcs[ t o n c l a r i o n s s h o w e d up a m o n g the \ a r i ous m e a s u r e s o1" size such as shoot weight, leaf
n u m b e r , leaf area. etc. T h e vast majorit.~ of
c h a r m t c r pairs, hm~ c \ e r , show no signiiicant c o l
relation i n d i c a t i n g a high level o t i n d c p v n d c m
v a r i a t i o n in those characters.
DISCUSSION
0.09
0.05
O.02
0.00
Plants of dw salne spccics that occul in more than
tmc habitat tXpc were combined.
323
T h e resuhs of'this study s u p p o r t the hypothesis
that the structlu'al [~aturcs of leaves and flowers
h a v e a p r e d i c t a b h ' influence on the a m o u n t of
m o i s t u r e r e t a i n e d bv a plant c r o w n tbllowing a
s i m u l a t e d rain event. T h e results presented in
Table 2. Multiple reg~e,~sion ana&sif q/ the ,~tali~licallv mo,~l important leaJ and/tower morphological Oail,~ li,~ted in ord~'* ¢,q"
their abililr to predict the amoant of loose!r bound water retained &, all plant,~ acro~ all tava.fidlowing a simulated precipitation
gU('II[
1)epcndem variable
\Valor retained/unit leaf area
Water retained/unit fimcdonal leaf
\Vat('r rctaincd/unit l('afnmss
\\:atcr retained/unit plant mass
lndcpendclH variables
Pubcsccncc rank, flower mass, flowcr wcttabililv,
leaf angle, petiole basc width
lx'af numbcr, pedolc base width, flower mass,
petiole base angle, flower wcttabilitv
Pubescence rank, petiole base width.
flower wettability, flower mass
l,eafnumber, leaf area, pubcscencc rank,
flower lltlmber
Each dcpcndcnl variable ratio was translbrmed to natural logs to i m p r o w linearity.
P value
~
<~0.0001
22",
40.0001
28°,,
~<().0005
21",,
~<0.0001
23",,
324
R. K. MONSON et al.
Table 3. Individual lea,I and,flower external waler-holding capacilie,s ( wellabilities ) o/ several
alpine .~])ecie,~
Species
Polemonium viacosum
Erilrhhium arelioide,~
Erigeron pinnali.sectu.~
Achillea lanulo.~a
Phlox pulvinala
Silene acauli~
T~me~lus/o,gmaeu,~
Polenlilla nicea
:lcomo.~lrlis ros.~ii
Hymeno.~,.~ grad?~tora
,bclo,~lapt!~,los uva-ur,li
l'ediculari,~ parrii
Clemenlaia rhodanlha
Anlemmria umbinella
Pedicularia groenlandica
Psychrophila leplo.*epla
Sedum lanceolalum
Care, nardina
Polentilla diversijblia
Er~,eron melanoc@kalus
Bi,~lorla bL~lorloide,s
A~ropyron ,tcribneri
Pinu,~flexili.~
7)(/blium dasy[d!~,llum
(,'aper m~ricam
Picea engelmanii
Eo'lhronium grand!)qorum
Leaf wetmbility
(g H~O/m ! leaf area)
Hower wettability
(g H~()/m ~ flower area)
0.071 ± 0.047
0.059 + 0.015
0.053 ± 0.015
(I.046 ± 0.008
0.042 ± 0.008
0.040 ± 0.001
0.033 ± 0.004
0.032 _+0.007
0.025 ± 0.005
0.024 + 0.004
0.018 ± 0.001
0.018 + 0.002
0.016 ± 0.003
0.015 ± 0.003
0.012 ± (}.003
0.012 _+0.001
0.010 ± 0.002
0.009 + 0.001
0.009 ± 0.001
0.008 ± 0.001
0.005 ± 0.001
0.005 ± 0.001
0.004 ± 0.001
0.004 ± 0,002
0.004 ± {}.001
0.003 + 0,001
0.001 ± 0,001
0.049 (n = 1)
0.043 ± 0.015
0.063 ± 0.008
0.065 ± 0.008
0.064 ± 0.028
0.055 ± 0.004
0.055 ± 0.008
0.067 ± 0.008
0.032 ± 0.004
0.032 ± 0.003
0.053 ± 0.004
0.026 ± 0.004
0.025 ± 0.001
0.094 ± 0.037
0.056 ± 0.018
0.051 ± 0.013
0.057 ± 0.004
0.022 ± 0.003
0.004 ± 0.001
0.017 + 0.003
0.043 ± 0.006
0.043 ± 0.001
0.075 ± 0.007
Flower wettability is expressed per unit of total, exposed floral area, measured on
flowers at thll anthesis by summing the areas of all dissected parts. Values are
mean±S.E. (n - 2 4, unless otherwise indicated). Missing values arc for plants which
lacked flowers during our measurements.
T a b l e 2 indicate that tile most i m p o r t a n t determ i n a n t s measured of how effective a gram of
shoot tissue will be in c a p t u r i n g wet precipitation
are its surfhce area and the degree to which that
surthce area is divided into small leaves, leaflets
or flowers. T h e large importance of factors such
as leaf n u m b e r and leaf area as well as flower
n u m b e r reflect these influences of area division on
a plant's capacity to capture water. I n contrast,
the ability of a gram of leaf tissue to capture water
is best explained by a somewhat different set of
characters. I,eafpubescence proved to be the most
i m p o r t a n t property of water retention per grant
of leaf tissue highlighting the functional sig-
nificance of water trapping capabilities of" these
surthce hairs. Similarly, flower wettability played
a major role along this scale of measurement
although it was slightly less i m p o r t a n t than the
dimension of the petiole base. Leaf pubescence
presumably contributes to water capture by
increasing the roughness of the surface and thus
the n u m b e r of interstitial spaces in which adhering water droplets cart be trapped. Larger petiole
base widths presumably contribute to water capture by providing troughs at tile j u n c t i o n of the
leaves and stems. Flower mass typically indicates
a rather highly dissected type of biomass contribution which presumably functions much in
RF/I'I':NTI()N O1: I ) R E C I P I T A T I ( ) N
1N A I A ) I N E ( ; R ( ) W N S
325
Tnble 4. Pear,on cmrelalion co{,tficienI.LJbr all pa#wi.w comhmali,,~ q/all ~tmctural lrails
2
2.
',I.
-t.
5.
(L
7.
II.
9.
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l,cal'anRh'
l,caf llUllll)('r
|,cat" arcn
l'un{'li<)nal lcal'arca
1
3
-.-{1.21
1
4
5
li
7
-0.26
-0.19
0.05
0.8~
0.04
- 0.13
0.00
0.12
l
0.t3
1
-0.1t
-().03
1
0.79
0.20
0.13
I
l/
9
(!.74
0.33
0.16
0.(i0
- 0.1(i
|).66
1
- 0.15
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1
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1 (I
(I.33
-0.'3(I
{).9t
0.08
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It.ll-t
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[
11
12
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16
17
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0.03
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- 0.()t)
-0. I 1
-(i.07
l
0.Zq
0.2(7
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0.08
0.~0
0.19
0.12
0.21
0.47
0.04
0.20
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-0.23
0.0l
0.22
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0.11
0.10
0.06
0.17
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0.07
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().(){~
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--0.02
0.Ot
().12
0.11
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0.00
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().()6
0.10
0.01
0.'32
0.07
0.22
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0.09
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0.72
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0.70
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')
3
t
5
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7
3
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I II
0.29
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0.72
O. lii
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0.2ti
0.11
0.14
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-0.10
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0.32
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0.0'2
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(i.12
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-().()1
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0.73
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-0.21
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0.40
I).<38
().i7
().1()
- I).02
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0.18
i).7(7
I).Z¢
0.',;(7
11
12
13
17
20
21
12. Sp~'cilic I('al'x~l
13. l:h)w('r lltlllll)t'l"
(i.0t
I
-().11
l
l:r. Fh)v¢(,l" ;i i'('ii
15). How('l" IIIIISS
0.01
().()(i
2.
;g.
[.
5.
(i.
7.
II.
{).
1().
I I.
Sh()ol x~l
Pill)Cst'CllC(' rallk
Pciioh' i)as(' illl~i('
l)clioh ' I)asc w i d d i
l,cafanglc
],l'at'l/tllill)l'l"
l :'al" aica
l:tln('iiolial leaf area
l,l'nl'lllnss
I :'al'wcii<llfilit\
12. Spccith" icat'~xl
11. l:h)wvr llllll/t)Cl"
I L l:l()wci'nr(':l
15. l"hiwcr llli(s ~;
16. l"h)~cr weual)ilitv
17. 'l'()ta] v, nl('r i'('lain[.'(I
20. t l c i / h l
21. l ) i a n w t v r
16.
17.
70.
91.
l"h>w('r w c l i a l J i l i t \
T o l a l \tal(T l'('{;:lill('(I
Hcighl
l)iani('tcr
0.0(i
0.0(i
-I).()ll
0.09
0.04
(i.(i8
0.23
0.10
O.lli
0.07
0.17
().1)1
0.12
().~0
0.01
().(l(,)
l).()3
-0.22
0.01
0.01
-0.18.
14
15
0.l)7
(I.20
0.08
().15
0.{/9
[
l
0.1,()
0.27l
0.0(}
0.10
0.13
().56
0.01
0.30
16
--0.21
0. t, 1
0.14
0.13
1
().22
0.07
0.25
0.23
0.00
1
O.l(i
().1,-)
0./)l
().01
().l)l
().0l
l).4(I
0.40
l).2<3
0.09
-0.1<2
0.f)5
(i.14
0.17
().()2
0.38
1
21
(/.39
- ().l(I
().11
0.12
I).18
0.3[)
0.59
0.39
(i.'g6
().i) I
0.13
().09
0.0ll
0.()3
0.03
0.15
0.59
I
Namplc size is n
123 tbr all pairs a n d all correlations reflect the original scah' of m c a s u r c m c n l tiw all viuiabh's.
'l'hv c h a r a c i c r n u m b c r s , s h o w n with their n a m c s a l o n g the left h a n d c o l u m n , signit~" thc sanic traits w h e n used
as cl)lunln In'adings. C o r r c l a t i o n s with al)sohltc v a h i c s l a v / c r t h a n O. 18 arc significant at P ~< 0.05.
326
R . K . MONSON el al.
the same m a n n e r as do highly divided leafsurthce
areas. W h e n taken together, the results of these
studies reveal that the crown architectures of
different species are structured in ways that result
in significantly dift~rent amounts of water capture. At the same time, these resuhs show that
some xery general structural attributes, applicable to a b r o a d range o f t a x a , prove to be highly
uselhl in predicting water retention chm'actcristics.
T h e influence of morphological traits on water
retention and " p o o l i n f ' patterns on the shoot
could have i m p o r t a n t implications t()r speciesspeciIic diflierences in d a m a g e bv acid rain. Ft:NK
and BONDF II fbund that tloral buds of the alpine
roselle p l a n t ,lcomasl~'lis ro,s,sii were a b o r t e d tbllowing exposure to pooled solutions of sulturic
acid (pH - 2.5 3.5) in the leaf axils. In plants
from other ecosystems pooled acidic solutions
have also been shown to cause a variety of injuries
to plants. '~~;' T h e results of this study indicate that
the influence of leatTpetiole base structure on the
pooling of water in alpine plants is widespread.
Visual observations m a d e d u r i n g the spraying
indicated that large droplet pools were c o m m o n
in the leafaxils of alpine plants. Given the common expression of the rosette growth-tbrm in
alpine plants, i with tloral buds positioned at the
base of leaves and petioles, the potential exists {br
the type of d a m a g e t)roposed l)y FUNK and BONDE
to be widespread in alpine plants.
An additional morphological trait that could
also have a large influence on t)lant susceptibility
to acidic precipitation is flower wettability
' T a b l e s 2, 3). Due to the delicate, timthery nature
of m a n y floral parts, flowers tend to retain large
quantities of water. High flower wettabilities
could have p r o l b u n d implications tbr reduction
of fitness of alpine plants if alpine acid deposition
were to reach excessive levels. Acid deposition
titlling on flowers vxould bring acidic solutions into
direct contact with the organs that are activel}
involved in the production of pollen and eggs, as
well as those that tbster seed m a t u r a t i o n . Several
past studies have d e m o n s t r a t e d the high sensitivity of plant reproductive processes to acidic
solutions. Simulated acid rain applied to stigmatic surthces at the time of pollination has been
shown to inhibit pollen grain germination. +'-':+c_,.~,
Acid rain applied to corn stigmas after pollination
has been shown to inhibit seed production,
p r e s u m a b l y through inhibition of pollen tube
growth. '~ Even if applied to stigmatic surtiwes
prior to pollination, acid rain can exert a residual
inhibition of pollen germination." t:~
Recent studies have d e m o n s t r a t e d that precipitation in Rocky M o u n t a i n alpine environments is clearl,~ acidic. ~'t< Several research eflbrts
are currently u n d e r w a y to assess the potential
impact of such acidic deposition on alpine plant
growth and vigor. In the current study we have
d e m o n s t r a t e d that morphological ti+atures of
alpine plants can intluence tim retention o[" precipitation in predictable ways. F u r t h e r studies
i,tvolving the a p p l i c a t i o n of simulated acid rain
to plants hz ,silu are required to determine if
structural timtures tbreshadow differential susceptibilities o[" alpine plants to acid rain. Such
studies could contribute to a more comprehensive
u n d e r s t a n d i n g of the relationship between acidic
d a m a g e and plant structure.
Ack~*owledgments These studies were supported by contract #28-K7-418 with the United Slates Forest Service Rocky Mountain Forest and Range Station and
in part by NSF grant BSR-9011658.
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2.
3.
4.
5.
6.
7.
BLiss 1,. C. (1985),3 Alpine. Pages 41 65 in B. F.
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DUBAV D . T .
989) Direct effects of simulated
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I1.
9.
().
I.
()WEN E. M. (1989) Yields oftield-growll soyl)cans
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FUNK D. ~g. and Boxm.; E. K. (19891 Abortion of
floral buds in .h:oma.+!r& ro,++ii plants cxposcd to
artificial acid mists in alpine ttmdra. +lm. ,]. 1]ol.
76, 878 ~183.
(;R:,X'r Xl. (:. and l+Ewxs \V. M., ,JR i1!)821
(',heroical loading rates from precipitation in the
(:ohwad. Rockies. Tel/,,+ 34, 71+ 88.
l,v,wIS \V. M.,.JR and (;RANT M. (:. +1980i Acid
precipitation in the western (!nitcd Slaws. ,S'ctvltte
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Nlt+ssl~:Liax R. C. cod.) :1990) The (;lacier £a/,c~
lccor>lem Etperimet*l +S'ilcs (;LEE,S': a ~il+'dc,wriptio,~,
(;cm'ral 'l'cclmical report RM. Rocky Mountain
l:orcst and Range l",xpcrimcnt Station, Fort
('.ollins, (',(), U.S.A.
327
12. Sn)nu S. S. :1983~ Eft'cots ofsinmlatcd acid rain
on pollen gmmination and pollen tube grox~th ot
white spruce ! t'i+,, Sara'a). Capz. ,]. 11o:. 61, 3095
309!).
13+ LTIII,MANN ~,~'., ALTNEI( H., S(:,trLzv: E.-I). and
l+Ax(n,: O. 1~. ,19{:~95 The prol)lem of for('sl decline
and Ib+c l'~axa+riall I:orest foxh'ologx R.escard~
(;ruup. Pae/cs 1 7 m E.-1). ,'St:nvt+zi< O. I~. l,\xc;~.:
and R. ()RFN. ('ds P;~re,sl dc+li,e a , d ,i~ p,/I,:i,,. .1
~ludl q/ spruce !Picca ]bits', u, acid ~oils. Sprine, cr,
Berlin.
14. VAN RvN I)..NI.. ,],\uol~sox ,]. S. and l,Assou:.].
P. 1986; Ellbcts ot'aciditv on h+ ~i/~0 gcrmin,tfion
and tttJ)c clon/ation it] Ibm" hardwood spccic,<. (.'.:+.
.]. l+'+,. Re+. 16, 397 400.
13. \\'I, RTm,:IM 1:..";. and (*+RAKER l++ E. (t987, .\cid
rain atud p<>llvn ~crmination in c~wn. l+hl+'ir, l+,/]ul.
48, t67~ t 7'.2>.