Seediscussions,stats,andauthorprofilesforthispublicationat:https://www.researchgate.net/publication/234072474
DifferencesinPlantandSoilWaterRelationsin
andAroundaForestGapinWestAfricaduring
theDrySeasonmayInfluence...
ArticleinJournalofEcology·February1996
DOI:10.2307/2261702
CITATIONS
READS
104
89
6authors,including:
ElmarVeenendaal
MDSwaine
WageningenUniversity&Research
UniversityofAberdeen
126PUBLICATIONS3,309CITATIONS
104PUBLICATIONS5,113CITATIONS
SEEPROFILE
SEEPROFILE
AllcontentfollowingthispagewasuploadedbyMDSwaineon18December2016.
Theuserhasrequestedenhancementofthedownloadedfile.Allin-textreferencesunderlinedinblueareaddedtotheoriginaldocument
andarelinkedtopublicationsonResearchGate,lettingyouaccessandreadthemimmediately.
Differences in Plant and Soil Water Relations in and Around a Forest Gap in West Africa
during the Dry Season may Influence Seedling Establishment and Survival
Author(s): E. M. Veenendaal, M. D. Swaine, V. K. Agyeman, D. Blay, I. K. Abebrese, C. E.
Mullins
Source: Journal of Ecology, Vol. 84, No. 1 (Feb., 1996), pp. 83-90
Published by: British Ecological Society
Stable URL: http://www.jstor.org/stable/2261702 .
Accessed: 01/09/2011 05:14
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .
http://www.jstor.org/page/info/about/policies/terms.jsp
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of
content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms
of scholarship. For more information about JSTOR, please contact [email protected].
British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of
Ecology.
http://www.jstor.org
Journalof
Ecology1995,
83, 83-90
in plantand soil waterrelationsin and
Differences
arounda forestgap in WestAfricaduringthedryseason
mayinfluence
seedlingestablishment
and survival
E. M. VEENENDAAL,
M. D. SWAINE,
I. K. ABEBRESE*
and C. E. MULLINS
V. K. AGYEMAN,*
D. BLAY,*
Department
ofPlant and Soil Science,University
ofAberdeen,Cruickshank
Building,St. Machar Drive,
AberdeenAB9 2UD, UK and *Forestry
ResearchInstitute,
Ghana University,
PO Box 63, Kumasi,Ghana
Summary
1 Soil and plant-waterrelationswerestudiedin a canopy openingand surrounding
forestshade in a moisttropicalforestin Ghana usingseedlingsof two treespecies
withdifferent
regenerationstrategies,the pioneerTerminaliasuperbaand the nonutile.
pioneerlightdemanderEntandrophragma
2 During two consecutivedryseasons,soil matricpotentialvariedfrom-30 kPa at
theend oftherainyseason,to below-2.5 MPa in themiddleofthedryseason.During
part of the firstdryseason soil matricpotentialwas higherin the gap than in the
wereobserved.
forest.In theseconddryseason no differences
surrounding
3 Leaf waterpotentialvariedfromabout -750 kPa at dawn forunstressedseedlings
wilted.For both
of both species to <-2.5 MPa when seedlingswere permanently
species,themaximumstomatalconductanceof unstressedseedlingsvariedfrom300
to 450 mmolm-2 s-' forthosein thegap centreto 150mmolm-2 s-' forthoseunder
forestshade. Maximumconductancedecreasedto between10 and 30 mmolm-2 s-1 in
seedlingsduringthe middle of the dry season. The two species
drought-stressed
respondedsimilarlyto a dryingsoil profilebut the decreasein leaf waterpotential
and stomatalconductancewas morerapidunderforestcanopythanin thecentreof
thegap.
utile
4 Only 7% of the seedlingsof Terminaliasuperbaand 4% of Entandrophragma
In
and
78%, respectively,
survivedthedryseason underforestshade. contrast,85%
survivedof thosegrowingin thecentreof the forestgap. At all positions,seedlings
of T. superbahad largerrelativeheightgrowthratesduringthe wet season than E.
utile(29.6, 21.4 and 8.4% month-'comparedwith5.2, 20.0 and 1.6% month-',for
gap centre,marginand forestshade,respectively).
5 The observedchangesin leafwaterpotentialand stomatalconductanceindicated
thatdespitethereducedirradiance,shadedseedlingsexperiencedmoredroughtstress
duringthedryseason than seedlingsgrowingin the forestgap. The reasonsforthis
in leafphenologyand regeneration
and possibleconsequencesfortreeswhichdiffer
strategyare discussed.
Keywords:forestgaps, leafwaterpotential,seasonal droughts,soil matricpotential,
stomatalconductance
JournalofEcology(1996) 84, 83-90
Introduction
ofseedlings
in
andmortality
Moststudieson growth
on theeffect
of
tropicalforestshave concentrated
? 1996British
Ecological Society
Correspondence:E. M. Veenendaal. Tel.: 01224272692.
Fax: 01224272703. E-mail:[email protected].
attentionhas also
variationin irradiance,butrecently
beendrawnto theimportanceof soil water.Seedlings
in forestgaps have been reportedto have a lower
mortality
duringthedryseason thanseedlingsin the
understorey,while irrigationin the understorey
enhanced survival(Fisher et al. 1991). Higher soil
matricpotentialshave been observedin gaps (Becker
84
Plantand soil
waterrelationsina
forestgap
et al. 1988) and Wright(1992) and Grubb (1995)
have proposedthat,in comparisonto plantsin gaps,
understorey
plantswill be moreaffectedby seasonal
droughts.
It is however,unclearwhetherhighersoil matric
potentials in gaps would automatically lead to
reduceddroughtstress,sinceincreasedcanopyopening also leads to increasedirradiance,highertemperaturesand greatervapour pressuredeficitson the
forestfloorand a more rapidlydryingsoil surface
(Fetcheret al. 1985;Ashton1992;Brown1993).Also
Robichaux et al. (1984) reportlower predawnleaf
waterpotentialsin shrubsgrowingin gaps compared
to theunderstorey,
implyingincreaseddroughtstress
ingaps. Thereis thusa needforfieldstudiesintropical
rain foreststhat combinemeasurementson the soil
waterregimewithplant-waterrelations.
Tree species differin theirregenerationstrategy
with regardto irradiancerequirements.In Ghana,
specieshave been dividedintoguilds(Hall & Swaine
1981; Hawthorne 1993). Some species regenerate
exclusivelyin open habitatswhileothersalso regenerate under the forestcanopy. On the basis of the
reporteddifferences
in soil waterregimebetweengaps
and theunderstorey,
itmaybe hypothesized
thatduring periods of drought,the way in which water
relationsvarybetweenplantsgrowingin gaps, compared to the understorey
will depend on the species'
regeneration
strategy.
In thispaper we reportobservations,made in and
around a large gap in a West Africanmoist semideciduousforest,on thesoil waterregimeand plantwaterrelationsof treeseedlingsof two specieswhich
differ
in regeneration
We testthehypothesis
strategy.
thattreeseedlingsregenerating
in a forestgap have
an enhanced plant-waterstatus compared to tree
in shade.
seedlingsregenerating
Materials and methods
PLANTING
? 1996British
Ecological Society,
JournalofEcology,
84, 83-90
EXPERIMENT
The study site was located in Tinte Bepo Forest
Reserve(7?04'N, 2?06'W) a moistsemideciduousforest in Ghana. Rainfallis concentratedin themonths
May to Juneand Septemberto Octoberwiththemain
dryseason betweenDecemberand February(Fig. 1).
Soils at thesitewereacid (pH 5.5-6.5),reddish-brown
sandyclays,witha free-draining
profileon a slope of
0-3%.
A large gap ' 26 m in diameterwas created by
removingshrubsand trees until photosynthetically
activeradiation(PAR) inthecentreofthegap reached
60% of ambientdaily irradiance,as measuredover
PAR sensors
weeklyperiodswithDidcot integrating
(DRP 02) (Agyeman1994).
Three-month-old
seedlings
of Terminalia superba
Engl.andDielsandEntandrophragma utile (Dawe
and Sprague) Sprague were used in the study. T.
300-
250E
E
200D
150-
CIO10050J F M A M J JA S ON
Month
D
Fig. 1 Averagemonthlyrainfallat Bechem(7?7'N, 2?3'W;
1970-93;long-term
meanannualrainfall
is 1288mm;Error
baris 1standard
deviation.
Source:Department
ofMeteorologicalServices,
Accra.)
superbabelongsto thepioneerguildand requireshigh
irradiancethroughoutits life,whereas E. utile is a
nonpioneerlight-demanding
speciesand althoughit
is foundas a seedling(< 5 cm d.b.h.) undertheforest
canopy, it requiresgaps to develop further(Hawthorne1993).
Plots (4 m wide) wereestablishedperpendicular
to
thegap boundaryto thenorth,south,east and west.
The plots stretchedacross the canopy marginand
weredividedinto threesections(1) gap (at least 5m
intothegap fromthemargin)(2) gap margin(at most
3m into the gap or into the forest)and (3) forest
shade (undertheforestcanopy,at least 5 m fromgap
margin).In June1992 treeseedlingswereplantedin
thesectionsat a minimumspacingof 1m, withseven
plantsforeach speciesin each of the gap centresectionsand eightplantsin each gap marginand forest
shade section.All plantswererandomlyallocated to
theirposition.Heightand diameterwererecordedat
plantingtime,againaftertheend oftherainfallseason
(December 1992) and at the startof the nextrainfall
season (May 1993) and used to calculate relative
growthrate of heightand diameter(Beadle 1993).
Growthdata at theend ofthewetseason and survival
at theend ofthewetand thedryseasonwereanalysed
withGames & Howell's approximatetestof equality
ofmeansbecauseofheterogeneity
ofvariances(Sokal
& Rohlf1981,pp. 408-412; notethatstrictly
speaking
samplesare biaseddue to mortality).
Duringtherainy
seasons,vegetationaround seedlingsin the gap was
cut down everytwo months.
SOIL
WATER
REGIME
Soil matricpotentialwas measuredusing the filter
papertechnique(Deka etal. 1995). Random soil samples weretakenwithan augerfromthecentreof the
gap, thegap marginand undertreecanopy. Samples
weretakenfromtwoor threepitsineach siteat depths
of 20, 40 and 60cm. Sampleswereequilibratedwith
filterpapers fromindividuallycalibratedbatches,in
an insulatedbox at room temperature
for7-10 days.
This proceduregives accurate resultsdown to -2.5
85
E.M. Veenendaal
et al.
MPa below which the methodmay lead to underestimation(Deka et al. 1995). Afterarcsine transofvariance,water
to improvehomogeneity
formation
contentsof the filterpapers were used fortwo-way
analysisofvariance.Daily rainfallwas measuredwith
a manual rain-gaugeon the forestedge, 3 km from
thesite.
PLANT-WATER
Results
1992-93
Soil waterregime
? 1996British
Ecological Society,
JournalofEcology,
84, 83-90
(a)
50
E
E
40
30
._
or
O3
20
10
i
-10
(b) 20 cm
RELATIONS
Leaf waterpotentialwas measuredusingthepressure
bomb technique(Tyree& Hammel 1972). Measurements were made at dawn (between 06.30 and
08.00hours) and at midday (12.30-14.00hours) on
plantsselectedfromthe gap centre,gap marginand
under forestshade. One plant of each species was
selectedrandomlyfromeach section in the north,
south,east and west blocks. In the second year,diweretakenfromthreeplantsof
urnalmeasurements
each species remainingfromthe previousyear,but
only in the centreof the gap. Water potentialswere
measuredin the fullyexpanded matureleaves from
thetop oftheplantsof T. superbaand on distalleaflets
of E. utile.Leaf waterpotentialdata werecompared
usinganalysisof variance.
Stomatal conductance,leaf temperature,photosyntheticphoton fluxdensityand relativehumidity
diffusionporomwere measuredwitha transit-time
eter(modelA.P.4, Delta T. Devices,Cambridge,UK)
(Beadle et al. 1993). Plantswererandomlyselectedin
thesame way as fortheleafwaterpotentialmeasurements.Stomatal conductancewas measuredin the
plantsfromthoseused forleaf
firstyearon different
in the second year
Measurements
water potential.
of theporometer
same
Use
on
the
weremade
plants.
to measure leaf temperatureled on occasion to an
of up to
of theactual leaftemperature
underestimate
1 ?C forleavesgrowingin highirradiance.New sensor
were used in
heads calibratedby the manufacturer
was recordedwitha thereach year.Air temperature
sensorand a Squirreldata logmocoupletemperature
Vapour pressuredeficitwas
ger(GrantInstruments).
calculatedas in Jones(1993, Appendix4).
DRY SEASON
60
In 1992 the last rains of the rainyseason fellon 12
November(Fig. 2) and apart fromfourlightshowers
inJanuaryand February,totalling11.5mm,no further
rainyseasonon
rainfelluntilthestartofthefollowing
matric
of
soil
March.
Measurements
potentialwere
5
startedon 21 November.Matric potentialsdeclined
over a period of 7 weeks to about -2 MPa. Matric
weregenerally
potentialsshoweda largevariability,but
highestin thegap and lowestin thegap margin.Stat-
-1 00
-
-1000-1 0000
-1 0
*
(c) 40 cm
-100
0
a.
-1000
-1 0
(d) 60 cm
-10000
-1 0000
-1 0000
1
1
----------I
7 14 21 28
NOVEMBER'92
I
I
I
5 12
19 26
DECEMBER
I
I
I
2 9 16 23
JANUARY'93
(n = 2) at
potential
(a) andsoilmatric
Fig.2 Dailyrainfall
depthsof(b) 20,(c) 40 and(d) 60cminTinteBepoForest
thedryseason
during
forest)
Reserve
(moistsemi-deciduous
halfclosedsymbols,
gapcentre;
(opensymbols,
1992/1993
shade.The brokenline
forest
closedsymbols,
gapmargin;
belowwhichvaluesarelessreliable.
indicates
thepotential
Errorbar is + 1 standarderror.If not shown,theyare
thanthesymbol).
smaller
differences
(P < 0.01) werefound
isticallysignificant
betweenboth date and site at depthsof 20cm and
60 cm (Appendix1). At thesedepthssoilmatricpotential values werehighestin the gap. By mid-January,
matricpotentialsat all depthsdecreasedto lessthanaccuracyof the
2.5 MPa, below whichexperimental
filter
papertechniquebecomeslimiting.
Plant-waterrelations
Observationson leafwaterpotential(T) and stomatal
conductancewere made in the beginning(21 November),duringthemiddle(12 December)and towards
theend (22 January)of thedryseason.
Initially'Pdawn variedbetween-0.75 and -1.0 MPa
while Pmidday varied between-1.45 and -1.65 MPa
86
Plantand soil
waterrelationsina
forestgap
(Fig. 3). Differencesbetweentime of day were significant,but not between the two species or the
locationof thetrees.
ThreeweekslaterTdawn had droppedto -0.9 MPa
in the gap and -2.6 MPa in the shade, withthe gap
margin
intermediate.
between
-2.3MPa
Tmidday varied
in the gap and -3.0 MPa in the shade. Differences
betweentime of day and canopy shadingwere significant,
butnot betweenthetwo species.T valuesin
themorningweresignificantly
higherin thegap than
theotherlocations.
On 22 Januaryonly plantsin the gap centrestill
had greenleaves.Tdawn and Tmidday did not differ
significantly
and had a mean value of-2.8 MPa.
Early morningstomatalconductance(g,) on the
firstday of measurements(Fig. 4a) reached 300450 mmolm-2 s- in bothspeciesin thegap and in the
gap margin,and 150mmolm-2 s- underforestshade
at a timewhen photosynthetic
photon fluxdensity
(PPFD) was below 100lumol M-2 s-' in the gap and
below 15,mol m-2 s-' in theshade. By midday,when
PPFD had reached980,mol m-2 s- in gap centreand
0-
(a)
o
- S
0
-1000-
600-
(a)
500C\j
400-
e
300
E
E
200
i
-
100
-
7
9
.
11
13
15
600_
17
(b)
500400-
E
300
E
E
200-
co
10007
9
11
13
15
17
Time of the day (h)
Fig.4 Diurnalvariation
instomatal
conductance
(ga)intree
seedlings
during
(a) theearlyand(b) themiddledryseason
in different
locations(opensymbols,
gap;closedsymbols,
gap margin;
no symbols
forest
shade;solidlines,measurements
onE. utile;broken
lines,measurements
on T. superba.
n = 4. Errorbaris + 1 standard
error.If notshown,they
aresmaller
thanthesymbol.
-1 500-2000-2500-
rsd
-3000-35000
0
(b)
d-3500
-1000
.i
500M
4-1
0
-2000
CD
CU
-2500-
-3000-
-3500
I0 E. utile
U T. superba
-1000-
MIDDAY
-1500i
-2000
-2500-
GAP0
DA
E. utile
U
T. superba
AWGN
FORES
-3000-3500*
? 1996 British
Ecological Society,
JournalofEcology,
84, 83-90
GAP
GAP MARGIN
FOREST
Fig.3 Leaf water potential at dawn (06.30-08.30hours)
and at midday(12.30-14.30hours)during(a) the early(21
November 1992), (b) middle (12 December 1992) and
(c) late (22 January1993) dry season in tree seedlingsof
utileand T. superbagrowingin
locations(n = 4.
F.
Errorbar is 1 standarderror). different
25,mol m-2s-' underforestshade, some depression
ing9was presentin bothspeciesin mostsites.Vapour
pressuredeficit(VPD) varied between0.4 MPa at
08.30hours to 1.4 MPa at 21.00hours and air temperaturebetween23?C at 8.30hours and 29?C at
midday. Midday leaf temperatures
in plants in the
gap centrewere2-4?C and in the shade 1 ?C higher
thanair temperature.
T. superbaplantsin thecentre
of thegap showedvisiblesignsof wiltingby midday,
butrecoveredin theafternoon.
Threeweekslaterthedailypatternof gs was very
different
(Fig. 4b). In the earlymorning,g9in plants
in the gap centrereachedvalues similarto those in
Novemberwhilein thegap margings was reducedto
150mmolm-2s-' and underforestshade to 30 mmol
m-2 s-'. After 09.00hours stomatal conductance
reducedstrongly
in bothgap and gap marginplants.
By midday,gs of plantsgrowingunderforestshade
had fallenbelow O mmolm-2s-' and measurements
on theseplantswerediscontinued.It was overcastso
that by middaymaximumPPFD in the gap centre
was no morethan250 ktmolm-2 S-' in thegap centre,
100ktmolm-2s- in thegap marginand 20 ktmolm-2
s- underforestshade. Air temperature
was on average 1 C lower than the previousday of measurements.VPD variedbetween0.7 and 1.8MPa at midday and differences
betweenleafand air temperature
were as before. In the gap, T. superba seedlings
showedsignsofwiltingby 10.00hoursand wereheavily wiltedby 14.00hours but by 17.30hours plants
87
E.M. Veenendaal
et al.
showed signs of recovery.In the shade, T. superba
seedlingswerewiltedthroughout
theday,whileplants
in the gap marginshowed an intermediate
statusof
wilting.In E. utile,a similarbutless severepatternof
wiltingwas observed.
As the dryseason progressed,diurnalpatternsof
wiltingbecamemoresevereand bytheendofJanuary
leaves of plants in the forestshade and gap margin
had driedoutand had startedto changecolour.Plants
in thecentreof thegap stillmaintainedgreenleaves,
butshowedheavywiltingwithearlymorning
g, below
30 mmolm-2s- .
Seedlinggrowthand survival
Analysisof varianceof relativegrowthrateof height
(RGRH) and diameter(RGRD) did not show significantdifferences
between east, west, north and
south blocks and the level block was omittedfrom
further
comparisons.
In surviving
plantsRGRD and RGRH werelarger
for T. superbathan forE. utile(Table 1). For both
species,largestvalues for RGRH and RGRD were
obtainedduringthewetseason in thegap centreand
gap margin but these were only significantly
(P < 0.05) higherthan in the forestshade for the
pioneer T. superba.During the dry season average
RGRD was close to zero,butnegativein bothspecies,
withtheexceptionof T. superbain thegap centre.
Most seedlingssurvivedthroughtherainyseason,
butbothspeciessurvivedleastwellin theshade (91%
survivalforE. utileand 78% forT. superba,Table 2).
For T. superba,survivalin theshadewas significantly
less thanat theotherlocations.However,manyseedlings had died by the end of the dry season, with
greatestsurvivalin the gap followedby gap margin
and leastunderforestshade. The trendwas thesame
forbothspecies.
DRY SEASON
1993/1994
Soil waterregime
In 1993 the last rains of the rainyseason fellon 25
Novemberand the nextrainfallstartedon 12 Feb-
ruary 1994 (Fig. 5). Soil matricpotentialmeasurementsin the gap and the surroundingforestwere
startedon 5 November (Fig. 5). Matric potentials
declinedto values below -2.5 MPa within5-7 weeks
fromthe end of the last rains dependingon profile
depth.Althoughpotentialswerevariable,theywere
similarin all threelocations. Soil matricpotentials
tended to be greaterat greaterdepth. Analysis of
varianceindicatedstatistically
significant
differences
betweendatesbutnotbetweenlocations(Appendix1).
Plant-waterrelations
Duringtheseconddryseason observationswereonly
made in the gap on plantsof both species,thathad
establishedas seedlingsin the previous
successfully
year.The 1.5-year-oldsaplingshad by now reached
heightsof 120-200cm inE. utileand 120cm to 300cm
in T. superbaand werebigenoughto takeleafsamples
fordiurnal 11eafpatternswithoutdisturbing
theplant.
At the beginningof November,with ample water
supply, 11eafdecreasedin both speciesfrombetween
-0.5 MPa beforedawn to -1.9 MPa forE. utileand
-2.1 MPa for T. superbaat midday(Fig. 6). By late
afternoon 11eafincreasedagain to predawnlevels.
By 14 December 'pre-dawn decreased to below
-0.5 MPa. Middayvaluesforbothspecieswerebelow
-2.0MPa. Late afternoonvalues did not returnto
predawnvalues,but increasedto -1.5 MPa forboth
species.
By 22 January'pre-dawn had decreasedto less than
-2.0 MPa in T. superba,but was higherin E. utile.
In both species Tmidday reached -3.0 MPa and did
not recoverduringtherestof theday. All plantssurvivedthisdryseason.
Stomatalconductancesduringthewetseason were
similarto thoseobservedin gap plantsin theprevious
year except that maximumg, observed in E. utile
was somewhatlowerat 250 mmolm-2 s-'. As thedry
season progressed,g, was highestin mid-December
during the early morning reaching maxima of
300mmolm-2 s-' in T. superbaand 200 mmolm-2 s-'
in E. utile. T. superbastartedleaf abscissionat the
beginningof January,while E. utilekept its leaves
Table 1 Relativeheightgrowthrate(RGRH) and relativediametergrowthrate(RGRD) of survivingseedlingsof E. utileand
T. superbain different
locationsduringthewet(June-December)and dry(December-May)season. Decembervalues sharing
a superscript
do notdiffer
significantly
(P < 0.05) accordingto Games & Howell's testforapproximateequalityof means
Location/species
Gap
RGRD (% month-')
December
December
May
May
E. utile
5.2 + 0.9a
29.6 + 1.4b
4.4 + 1.0
10.4 + 2.3
E. utile
2.0 + o.9a
21.4 + 1.5b
2.4 + 1,2
4.5 + 2.3
3.4 + o.9a
15.0 + 1.5b
-1.2 + 0.6
-0.4 + 0.7
1.6 + 0.8a
8.4 + 1.7a
-0.7 + 2.6
7.8 + 5.0
2.4 + 0.6a
3.5 + 0.9a
-0.5 + 2.4
-4.4 + 1.8
T. superba
Gap margin
?) 1996 British
Ecological Society,
JournalofEcology,
84, 83-90
RGRH (% month-')
T. superba
Forestshade
E. utile
T. superba
6.5 + 1.oa -0.3 + 0.7
22.6 + 1.8b
4.7 + 1.2
88
Plantand soil
waterrelationsina
forestgap
Table 2 Numbersof survivingof seedlings(withpercentage
survivalinparentheses)ofE. utileand T. superbaindiffereni
locationsduringwet(June-December)and dry(DecemberMay) seasons. Values in each column sharinga commor
superscript
do notdiffer
significantly
P < 0.05 accordingtc
Fisher'sexact test
0
-500-
-2000 -2500-3000 -3500-4000 -
Seedlingssurviving
Location/species
JuneDecember
Initial
DecemberMay
cis
E. t ile
T. superba
Gap margin
E. utile
T. superba
Forestshade
E. utile
T. superba
28
28
27a
27a
(96)
(96)
32
32
31a
32a
32
32
21a
23a
(78)
(85)
(97)
12bc
(100)
18b
(39)
(56)
29ab
(91)
4d
25b
(78)
7cd
(14)
(28)
0-
L
e
Gap
(a)
-1000-1500
0
0.
_
D
:
CD
-500-1000-1500-2000-2500-3000-
(b)
-3500-4000-
-J
I
0
-500-
\
-1000-1500-2000-2500-
(a)
60-
50E
-4000
40=
30-
-------0
5
,
7
-
* -
9
11
13
15
17
Time of the day (h)
20-
cc
t^s
-3000-3500-
Fig.6 Diurnal changesin leafwaterpotentialin E. utileand
T. superbasaplings growingin the centreof a large gap
during(a) the early (21 November 1993), (b) middle (12
December 1993) and (c) late (22 January1994) dryseason
(solidlines,measurements
on E. utile;brokenlines,measurementson T. superba.n = 3. Errorbar is + 1 standarderror.
If not shown,theyare smallerthanthesymbol).
100
(b) 20 cm
-100-
-1000.;S- -
'
-1 000
-1 0 -
1-10000
-
I
-
- -
I
I
I
I
I
I
-
I
I
-
-
-
-
I
I
I
I
(c) 40 cm
throughout
thedryseason. By 21 January,T. superba
was almost leafless.At this time,g, in the morning
was reducedto less than 100mmol m-2 s in both
species.
-100
Discussion
E-i000
IIIII
d
-1 0(d) 60 cm
-100
-1000-
-10000
1 7 14 21 28
NOVEMBER'93
?
1996 British
Ecological Society,
JournalofEcology,
84, 83-90
5 12 19 26
DECEMBER
2 9 16 23 30
JANUARY'94
-T=
6 13 20 27
FEBRUARY
Fig.5 Daily rainfall(a) and soil matricpotential(n = 3) at
depthsof (b) 20, (c) 40 and (d) 60cm iibTinte Bepo forest
Reserve(moistsemideciduousforest)duringthedryseason
1993-94 (open symbols,gap centre;half-closedsymbols,
gap margin;closed symbols,forestshade. Errorbar is + 1
standard error.If not shown, they are smallerthan the
symbol).
In the firstdryseason, soil matricpotentialsin the
gap werehigherthanin the gap marginor in forest
shade. In the second year,no significant
differences
wereobserved.Root densityin newlyformedgaps is
initiallydecreased(Sanford1990;Silver& Vogt 1993)
and this,togetherwiththeloss of transpiring
leaves,
will contributeto a slower depletionof soil water
in the centreof new gaps. However, as vegetation
regrows,microclimatic
conditionsingaps changerapidly (Fetcher et al. 1985). Our data confirmthat
differences
in soil waterregimebetweengaps and surroundingforestdecreaseover time.Because we discouraged re-growthby cuttingdown plants, soil
matricpotentialdifferences
may normallybe smaller
thanfoundin thisstudy.
In dryingsoils, plants growingin the centreof a
gap mayhave access to smalladditionalquantitiesof
waterthroughupwardmovementof waterdrivenby
89
E.M. Veenendaal
et al.
? 1996British
Ecological Society,
Journalof Ecology,
84, 83-90
in day and nightsoil surfacetemthe differences
perature(Marshall & Holmes 1988). The soil surface
temperatureduringthe day in gaps is greaterthan
underforestshade (Lawson etal. 1970;Ashton1992).
Thoughsmallin absoluteterms,thismovementcould
factor,whereseedlingsurvivalis
be a contributing
concerned.
seedlingsof both E. utile
In non-drought-stressed
and T. superba,earlymorningg9reachedmaximaof
300-450mmolm-2 s-' in thegap and thegap margin,
and 150mmol m-2 s-1 in the shade whilePPFD was
m-2 S-' inthegapand < 10ktmol
stilllow( < 100ktmol
m-2 s-' in theshade). Althoughthisseemswastefulin
termsof wateruse, it resultsin a decreasein stomatal
(Pearcy
duringsunflecks
limitationof photosynthesis
1990). The midday depression of stomatal conas
ductanceof seedlingsin thegap is not surprising,
watervapour pressuredeficitsof morethan 1.7MPa
are similarto thoseexperiencedbyleavesin theupper
canopy of rain foresttrees(Aylett1985; Koch et al.
1994).
As the dry season progressed,stomatal conductance showed the sharpestreductionin shaded
plantsfollowedby plantsin the gap marginand the
least reductionin plants growingin the gap centre
(Fig. 4). This and theassociateddecreasein leafwater
duringtheearlymornpotentials(Fig. 3), particularly
ing in gap marginand shade, suggeststhatplantsin
the shade, despitelowerwatervapour pressuredeficits,are subjectedto greaterdroughtstressthanthose
in the gap. Part of the explanationis attributable
in soil matricpotential,but other
to the differences
aspects should also be considered.Deep shade conditionswithless than 1% of ambientirradiancewill
resultin morphologicaland physiologicalchangesin
leavessuchas an increasein specificleafarea,reduced
wax formation,reduced stomataldensityand thinningofthecuticularmembraneoftheleaves(Givnish
1988; Kerstiens1994). This makesshade leaves more
vulnerableto waterloss and could account for the
decreasein leafwaterpotentialand reducedstomatal
conductance. Leaf temperatureswere not high
damaging,eitherin the
enoughto be physiologically
gap or underforestshade.
Plant size and architecture
mayalso have playeda
of
role. Relativegrowthrates heightand diameterin
largerin plantsgrowing
T. superbaweresignificantly
in thegap and gap margincomparedto thosegrowing
differunder the canopy. Similar,but insignificant
Plants
utile
for
E.
ences were observed
(Table 2).
growingin shade are likelyto show a reducedroot:
shoot ratioas a responseto low irradiance,possibly
reducingrootingdepth,whichmay be importantfor
the survivalof understoreyseedlingsin dry forest
com(Mulkeyet al. 1991). Growthand architecture
binedwillreducethesoil volumethatcan be exploited
by the roots of plants growingunder shade. In
addition, shade plants will encountermore comroots
petitionfromthedensernetworkofsurrounding
compared to plants establishingin gaps (Sanford
1990; Silver& Vogt 1993).
The morerapiddecreaseofleafwaterpotentialand
stomatalconductanceindicatethatplantsgrowingin
forestshade mayexperiencedroughtstress3-4 weeks
longerthan plants growingin the gap. Plants specialized to shaded habitatswill have to be adapted
to these drierconditions.Drought stresshas been
reportedbeforeas a factorin the mortalityof seedlings (Turner 1990) and our findingssupport the
suggestionthatdroughtis an importantstressfactor
for understoreyvegetationin drierforests(Wright
1992;Grubb 1995).
In thefirstseason, seedlingsof T. superbaand E.
utilefollowedan evergreenstrategyuntilthe leaves
died of desiccation.In thesecond season, thespecies
stronglyin leaf phenology.T. superbaseeddiffered
lingswere decidupusand shed leaves by the end of
and was
JanuarywhereasE. utileremainedevergreen
able to takeup waterto retainsometurgorduringthe
eveninthemiddleofthedry
earlypartofthemorning,
evergreen,
season. Shade tolerantspeciesare typically
which allows a longerpay-back time for leaf constructioncosts (Williams et al. 1989). Deciduous
behaviour on the other hand is associated with a
greater instantaneous nitrogen and water use
efficiency
(Sobrado 1991)and mayhelpto avoid cavitationin the xylemvessels(Jackson& Grace 1994).
to notethatmanyWestAfricanunderIt is interesting
everstoreyspeciesindrierforestshavesclerophyllous
green leaves with a thickcuticularlayer (personal
thattheleaves
observation)and itcan be hypothesized
of theseplantsmaybe adaptednot onlyto shade but
floor.
on theforest
experienced
alsotothedryconditions
Acknowledgements
acknowledgetheadvice of Dr
The authorsgratefully
S. A. Coke, and thehelp of Ms D. D. Teye,and Ms
M. S. Boafo fromtheInstituteof RenewableNatural
Resources,Kumasi, Ghana. They also thankMr P.
Amoakoh,Mr J. S. Nahyi,Mr M. Mensah and Mrs
R. T. Lecha fortheirassistancewithdata collection.
Dr J. Thompson and two anonymousrefereesare
thankedforcommentson a firstdraft.The workwas
supportedby theODA/OFI FRP Grantno R4740
References
on tropical
Agyeman,V.K. (1994) Environmental
influences
ofAberdeen.
PhD thesis,University
treeseedlinggrowth.
Ashton,P.M.S. (1992) Some measurementsof the microclimatewithina Sri Lankan tropicalrainforest.Agri59, 217-235.
culturaland ForestMeteorology,
leaf conducAylett,G.P. (1985) Irradiance,interception,
in Jamaicanuppermontane
tance,and photosynthesis
19, 323-337.
rainforesttrees.Photosynthetica,
and
Beadle, C.L. (1993) GrowthAnalysis.Photosynthesis
(eds D. 0. Hall,
Productionin a ChangingEnvironment
R. C.
J. M. 0. Scurlock,H. R. Bolhar-Nordenkampf,
Leegood & S. P. Long), pp. 36-46. Chapman & Hall,
London.
90
Plantand soil
waterrelationsina
forestgap
Beadle,C.L., Ludlow,M.M. & Honeysett,J.L.(1993) Water
relations.Photosynthesis
and Productionin a Changing
Environment
(eds D. 0. Hall, J. M. 0. Scurlock,H. R.
R. C. Leegood & S. P. Long),
Bolhar-Nordenkampf,
pp. 103-109.Chapman & Hall, London.
Becker,P., Rabenold,P.E., Idol, J.R. & Smith,A.P. (1988)
Water potentialgradientsfor gaps and slopes in an
Panamaniantropicalmoistforest'sdryseason. Journal
of TropicalEcology,4, 173-184.
Brown, N. (1994) The implicationsof climate and gap
microclimate
forseedlinggrowthin a Borneanlowland
rainforest.Journalof TropicalEcology,9, 153-168.
Deka, R.N., Wairu, M., Mtakwa, P.W., Mullins, C.E.,
Veenendaal,E.M. & Townend,J. (1995) Use and accuracy of the filterpaper techniquefor measurementof
soil matricpotential.EuropeanJournalof Soil Science,
46, 233-238.
Fetcher,N., Oberbauer,S.F. & Strain,B.R. (1985) Vegetationeffects
on microclimate
in lowlandtropicalforest
in Costa Rica. International
Journalof Biometeorology,
29, 145-155.
Fisher,B.L., Howe, H.F. & Wright,S.J.(1990) Survivaland
growthof Virola surinamensisyearlings.Water augmentation
ingap and understorey.
Oecologia,86,297-297.
Givnish,T.J. (1988) Adaptationto sun and shade: a whole
plant perspective.AustralianJournalof Plant Physiology,15, 63-92.
Grubb,P.J.(1995) Rain forestdynamics,The need fornew
paradigms. Tropical Rain Forest Research: Current
Issues (eds S. C. Choy,D. S. Edwards& W. E. Booth),
Kluwer.The Hague.
Hall, J.B. & Swaine,M.D. (1981) Distribution
and Ecology
of VascularPlantsina TropicalRain Forest.Dr W. Junk
Publishers,The Hague.
Hawthorne,W.D. (1993) Forestregeneration
afterlogging.
O.D.A. ForestrySeriesno. D.A./N.R.I. Chatham.
Jackson,G. & Grace, J. (1994) Cavitationand watertransportin plants.Endeavour,18, 50-54.
Jones,H.G. (1992) Plants and Microclimate.Cambridge
University
Press,Cambridge.
Kerstiens,G. (1994) Effectsof low irradianceintensity
and
highair humidityon morphologyand permeability
of
plantcuticles,withspecialrespectto plantsculturedin
vitro.Physiology,
Growthand Developmentof Plants in
Culture(eds P. J. Lumsde, J. R. Nicholas & W. J.
Davies), pp. 132-142.Kluwer.The Hague.
Koch, G.W., Amthor,J.S.& Goulden,M.L. (1994) Diurnal
patternsof leafphotosynthesis,
conductanceand water
potentialat the top of a lowland rainforest
canopy in
Cameroon:measurements
fromtheRadeau des Chimes.
Treephysiology,
14, 347-360.
Lawson, G.W., Armstrong-Mensah,
K.O. & Hall, J.B.
(1970) A catena in moist semi-deciduousforestnear
Kade, Ghana. JournalofEcology,58, 371-398.
Marshall,T.J.& Holmes,J.W.(1988) Soil Physics,2ndedn.
CambridgeUniversity
Press,Cambridge.
Mulkey, S.S., Smith,A.P. &. Wright,S.J. (1991) Comparativelifehistoryand physiologyof two understorey
tropicalherbs.Oecologia,88, 163-273.
Pearcy,R.W. (1990) Sunflecksand photosynthesis
in plant
canopies.AnnualReviewofPlant Physiology,
421-453.
Robichaux,R.H., Rundel,P.W., Stemmerman,
L., Canfield,
J.E., Morse, S.R. & Friedmann,W.E. (1984) Tissue
waterdeficitsand plantgrowthin wettropicalenvironments.Physiological
EcologyofPlans oftheWetTropics
(eds E. Medina,H. A. Mooney & C. Velasquez-Yanes),
pp. 99-112. Dr W. JunkPublishers,The Hague.
Sanford,R.L. (1990) Fine root biomass under irradiance
gap openingsin an Amazonian rain forest.Oecologia,
83, 541-545.
Silver,W.L. & Vogt, K.A. (1993) Fine root dynamicsfollowingsingleand multipledisturbancesin a subtropical
wetforestecosystem.JournalofEcology,81, 729-738.
Sobrado, M.A. (1991) Cost-benefit
in decidurelationships
ous and evergreenleaves of tropicaldryforestspecies.
FunctionalEcology,5, 608-616.
Sokal, R.R. & Rohlf,F.J. (1981) Biometry,
2nd edn. W. H.
Freeman& Co., New York.
Turner,I.M. (1990) The seedlingsurvivorship
and growth
of three Shorea species in a Malaysian tropical rain
forest.Journalof TropicalEcology,6, 469-478.
Tyree,M.T. & Hammel,H.T. (1972) The measurement
of
turgorpressureand the water relationsof plants by
the pressurebomb technique.Journalof Experimental
Botany,23, 267-282.
Williams,K., Field,C.B. & Mooney,H.A. (1989) Relationships among leaf constructioncost, leaf longevityand
irradianceenvironment
in rain-forest
plantsofthegenus
Piper. TheAmericannaturalist,
133, 198-211.
Wright,S.J. (1992) Seasonal drought,soil fertility
and the
species densityof tropical forestplant communities.
Trendsin Ecologyand Evolution,7, 260-263.
Received27 February1995
revisedversionaccepted3 July1995
Appendix1
Analysisof varianceof watercontentsof filterpapersequilibratedagainstsoil samplesobtainedfromdifferent
depths
Depth
20 cm
? 1996 British
Ecological Society,
JournalofEcology,
84, 83-90
View publication stats
40 cm
60 cm
Source of variation
d.f.
mean squares
d.f.
mean squares
d.f.
mean squares
1992/1993
Date
Location
Date x location
Error
5
2
10
16
0.138****
0.029**
0.015*
0.005
5
2
10
18
0.14****
0.033NS
0.01INS
0.010
5
2
10
18
0.082****
0.037***
0.016***
0.003
1993/1994
Date
Location
Date x location
Error
6
2
12
35
0.287****
0.006NS
0. lINS
0.181
6
2
12
36
0.114****
0.013NS
0.014NS
0.024
6
2
12
35
0.287****
0.003NS
0.009NS
0.005
*P
<
0.0
.*
<
0.0
.*
<
0.05
_*
<
0.00.._