Structure and function in two tropical gallery forest communities: implications for forest conservation in fragmented systems

Journal ofApplied Ecology0887\ 24\084Ð195

Þ 0887 BritishEcological Society

084

Structure and function in two tropical gallery forest

communities] implications for forest conservation in

fragmented systems

MARTIN KELLMAN\ ROSANNE TACKABERRY and LESLEY RIGGDepartment of Geography\ York University\ Toronto\ Ontario\ Canada M2J 0P2

Summary

0[ Composition\ growth and turnover of trees in two species!rich tropical galleryforests were examined to evaluate what community reorganization may be needed totransform recently created tropical forest fragments into stable refugia for regionalforest biotas[1[ Rates of tree growth and turnover over a 4!year interval were comparable to thoserecorded in continuous forests and in both communities there had been some treespecies turnover in the measured stem size classes during the 4!year interval[2[ The more abundant tree species in both communities formed three functionalgroups along gradients between streams and forest edges] edge!concentrators\ core!concentrators and generalists[3[ Soil fertility showed no consistent increase close to streams and neither tree growthnor recruitment rates were increased in this zone[ In contrast\ forest edge zonesexhibited increased rates of tree growth and recruitment indicating that growth pro!cesses in these forests are light!limited rather than soil!limited\ and that forest edgezones are generally favourable habitats for tree populations[4[ Both communities showed signs of past _re incursions\ and the tendency of a subsetof tree species to concentrate in the more growth!limited core habitats is attributedto their _re sensitivity[5[ Rapid development of an edge zone of _re!insensitive tree species is considered tobe essential to the survival of forest community fragments in the _re!prone landscapesof the tropics\ and the edges of gallery forests are recommended as potential sourcesof species with which to fashion these protective ecotones[6[ Preservation of a diverse forest biota in the _re!protected interiors of fragmentswill require natural or arti_cially enhanced immigration rates that are su.cient too}set local extinctions[

Key!words] ecotone\ fragmentation\ riparian habitat\ savanna _re[

Journal of Applied Ecology "0887# 24\ 084Ð195

Introduction

Fragmentation of tropical forest due to human activi!ties is a dominant process in many contemporary trop!ical landscapes "Laurance + Bierregaard 0886#[ If sig!ni_cant proportions of tropical forest biotas are tosurvive the present epoch of deforestation and beavailable for future forest restoration\ much of thispreservation will have to take place in small com!munity fragments "Schelhas + Greenberg 0885^Turner + Corlett 0885#[ However\ most tropical forestfragmentation has been so recent that it is di.cult toevaluate the long!term consequences of the process

and how e}ective community fragments can be asforest refugia[ The composition and dynamics ofrecent fragments are likely to be dominated by rem!nant conditions derived from the original continuousforest and by short!term processes of re!adjustmentto the fragmented state[ Under these circumstances\naturally isolated tropical forest fragments are ofvalue as indicators of the steady!state conditions thatcan potentially develop after prolonged frag!mentation[ Development of these conditions in recentfragments may need to be facilitated by managementinterventions[

Gallery forests in savannas represent one of the

085

Tropical galleryforest structure

Þ 0887 BritishEcological Society\Journal of AppliedEcology\ 24\084Ð195

few examples of naturally fragmented tropical forest[They are generally rich in woody plant species typicalof continuous forests and are postulated to have pro!vided refugia for tropical forest species in areas defor!ested during Pleistocene drought "Kellman et al[ 0883^Meave + Kellman 0883#[ While recently created trop!ical forest fragments occupy a variety of habitats\ asigni_cant proportion of these occur on the steepslopes adjacent to streams and in headwater basinsthat are typical gallery forest habitats[ For these recentfragments\ gallery forests are likely to provide usefulmodels of the community composition and dynamicsthat can be expected to develop after prolonged frag!mentation\ and of the restructuring that may need tobe induced to ensure that fragments persist and pro!vide refugia for forest biotas[

In this paper we report on the spatial organizationand dynamics of tree populations in two gallery foreststhat have been mapped and re!censused after a 4!yearinterval[ We pay special attention to how structureand function of tree populations change along gradi!ents between community edges and cores[ Most gal!lery forests and many recent forest fragments are cen!tred on streams\ and changes in soils and soil moistureconditions between more elevated forest peripheriesand core areas along streams represent a major poten!tial edaphic gradient[ Moreover\ peripheral zones rep!resent areas most subject to intrusions of externalatmospheric conditions or various forms of externallyderived disturbances "{edge e}ects|#\ while core areasare less subject to these intrusions[ Consequently\ thespatial gradient between community cores and edgesrepresents a complex gradient in environmental anddisturbance conditions that may be expected to a}ectthe distribution and dynamics of tree populations[Finally\ we discuss the implications that the patternsfound in gallery forests hold for the persistence andfunctioning of recently created forest fragments andthe conservation of forest biotas in these[

Materials and methods

STUDY SITES

The two forests examined are part of a system ofgallery forests found along streams in the MountainPine Ridge Savanna of Belize "06>N 77>W#[ The sav!anna is located on a granite plateau and adjacentmeta!sedimentary terrain at an elevation of ¼499 ma[s[l[ The area receives 0459 mm of rainfall per yearwith a pronounced dry season between February andApril "Walker 0862#[ Soils of the savanna are intenselyweathered ultisols\ low in cation exchange capacity"CEC#\ base saturation\ available P and total N "Kell!man + Sanmugadas 0874^ Kellman 0878#[ The sav!anna appears to be at least 00 999 years old and maybe a remnant of a more widespread xerophytic com!munity that developed during Pleistocene drought"Leyden 0873^ Kellman 0878#[ It is thought to have

resisted forest reinvasion due to high _re frequencyinteracting synergistically with low soil fertility "Kell!man 0873#[ Arti_cial _re suppression over the past49 years has resulted in the expansion of previouslyscattered populations of Pinus caribaea Morelet toform an open pine woodland[ However\ an under!storey of savanna grasses and sedges persists beneaththis tree stratum\ intermingled with a shrub layerdominated by Miconia albicans "Swartz# Triana\ andperiodic _res still occur within the savanna[ Usingforestry _re maps for the 0848Ð71 period\ Kellman +Meave "0886# have estimated an average savanna _rerecurrence interval of 49Ð59 years[

Within the savanna\ gallery forests occur as dis!continuous strips of closed!canopy woodland alongthe main streams and in some headwater basins"Meave + Kellman 0883#[ One of the communitiesenumerated "{Site 16|# is a small isolated forest patch9=78 ha in area located on a headwater of MahoganyCreek in the south!western part of the savanna[ Thepatch consists of a shallow basin _lled by alluviumand colluvium plus a more steeply sloped gully"Fig[ 0#[ The outer forest boundary coincides partiallywith a slope shoulder of modest convexity[ A geo!logical boundary between granite and meta!sedi!mentary rocks bisects the patch along the axis formedby the lower reach of the stream and its main tributary\with granite present SW of the axis "Fig[ 0#[ Thesecond community enumerated "{Slate Creek|# was a199!m long segment of a 0=4!km strip of gallery forestthat occupies a deeply entrenched tributary of SlateCreek in meta!sedimentary terrain in the north!west!ern part of the savanna[ This gallery forest is ¼099 mwide and the total area enumerated was 1=03 ha"Fig[ 0#[ An extension of the forest along a small tribu!tary stream was not enumerated[ Most slopes are stee!per than those at Site 16 and the outer forest boundarycoincides closely with a distinct slope convexity to~atter terrain in the surrounding savanna[ Within theforest\ ~at terrain is con_ned to small alluvial terracesthat alternate along the main stream channel[

Since 0848\ when _re records began\ the savannaadjacent to Slate Creek has experienced _res in 0851\0854\ 0869\ 0866\ 0871 and 0877[ The 0877 _re\ whichwas observed during _eld work\ burned to the for!est:savanna boundary on the north side of the creek\but did not enter the forest[ At Site 16\ only one _rehas been recorded in the same interval[ During a sev!ere _re season in 0860\ the savanna on all sides of thepatch is reported to have been burned[

FIELD METHODS

In each community a grid of pegs was established at4!m intervals and surveyed by Abney level for prep!aration of a topographic map[ The outer forest bound!ary was de_ned by the presence of either Tripsacumlatifolium Hitchcock or Dicranopteris pectinata"Willd[# Underw[\ two savanna species that are uni!

086

M[ Kellman\R[ Tackaberry +L[ Rigg


Fig[ 0[ Topographic maps of the two gallery forest communities studied\ showing locations of _re!scarred trees and changesof the forest boundary over 4 years[

versal at forest:savanna boundaries in this area wherethey form mono!speci_c thickets\ and whose presencecoincides with an open or absent tree canopy[ Allstems ×4 cm d[b[h[ "Site 16# or ×09 cm d[b[h[ "SlateCreek# were mapped to within 0 m\ identi_ed to spec!

ies and marked with an aluminium tag hung on agalvanized steel nail[ Site 16 was mapped\ and enu!merated in May and June 0878\ with a small peripheralzone added 0 year later after clari_cation of theboundary de_nition[ Slate Creek was mapped in May

087



and June 0880[ Site 16 and Slate Creek were re!enu!merated in June 0883 and May 0885\ respectively[Changes in forest boundaries were noted on the ori!ginal maps\ tree d[b[h[ was re!measured\ dead treesrecorded\ and new trees entering the minimum diam!eter class measured and mapped[

Soil pits were dug and soil samples taken at 9Ð4!\09Ð04!\ 19Ð14! and 34Ð49!cm depths at three topo!graphic positions on the north and south sides of SlateCreek] creek!bank terraces\ mid!slope\ and upperslopes within 09 m of the forest edge[ Comparable soilsamples were also taken in a pit dug in the savannabeyond the forest boundary[ At Site 16 soil sampleswere taken at similar depths from two pits located inmid!slope positions on granite and meta!sedimentarybedrocks and in two pits dug in the savanna beyondthe forest boundary on these bedrocks[ All sampleswere analysed for pH "0]1 soil]water slurry#\ CEC "Na!acetate saturation followed by NH3!acetate extractionat pH 6#\ extractable cations "NH3!acetate extractionat pH 6# and Truog|s P "0]199 soil]extractant ratio#[

DATA ANALYSIS

Distance to the nearest upslope forest:savanna bound!ary was used as a measure of a tree|s exposure tovarious edge e}ects\ and height above the stream ofthe rooting point of the tree was used as a measure ofits position on the edaphic gradient[ These two valueswere recorded for each tree in the two communities\with the exception of those lying between the arms ofthe tributary stream at Slate Creek\ whose nearestforest:savanna boundary was unde_ned[ These twovariables were strongly negatively correlated in thetwo communities "Slate Creek\ r � −9=84\ n � 0502\P ³ 9=990^ Site 16\ r � −9=89\ n � 0404\ P ³ 9=990#and in each the two were collapsed into a single gradi!ent using principal components analysis "PCA#[

Segregation of the more abundant species in eachcommunity along the core!edge gradient was assessedby x1 analyses of stem frequencies classi_ed into threegroups "{edge|\ {intermediate| and {core|# de_ned byPCA score groups of equal size[ In each community\the proportion of total stem numbers in each of thethree categories was used to de_ne expected fre!quencies in the x1 analyses[ All species with stem num!bers ×04 were included in the analysis[

Tree growth was expressed as proportional annualdiameter increment ") of original d[b[h[# to com!pensate for size!dependency in absolute increments"cf[ Ashton + Hall 0881#[ To evaluate the e}ects offorest edge and stream proximity on tree growth rates\proportional annual diameter increments were log!transformed and regressed upon distance to the forestedge and height above the stream for all stems andfor each species with stem numbers ×09 in each com!munity[

Results

SOIL CONDITIONS

At Slate Creek there was little variability in soilproperties of deeper horizons either with slope pos!ition or between forest and savanna cover "Table 0#[In the surface soil only P increased consistentlydownslope\ while CEC and extractable Ca showed thereverse trend\ and Mg and K showed no consistentpattern[ Consequently\ while there was some evidenceof topographic di}erentiation in soil properties\ thiswas not uniformly towards more fertile soils in stream!side locations[ At Site 16\ soils derived from graniteshowed greater concentrations of Ca\ but there wereno other consistent di}erences with geology or veg!etation cover "Table 0#[

FOREST COMPOSITION

At the _rst census\ 55 tree species with stem diameters×09 cm were recorded at Slate Creek and 38 at Site16[ The number of tree species at the latter site roseto 59 when stems 4Ð09 cm diameter were included[Forty!one species were common to the two sites[Dominance was more pronounced at Site 16\ wherethe _ve most common species "×09 cm d[b[h[# madeup 40=5) of all stems^ at Slate Creek the same numberof species comprised only 22=4) of all stems[

Tree densities "×09 cm d[b[h[# were comparablyhigh at the two sites "Slate Creek\ 604 ha−0^ Site 16\530 ha−0#\ and basal areas comparably low "SlateCreek\ 08=60 m1 ha−0^ Site 16\ 08=61 m1 ha−0#[ Treediameter class distributions for all species combinedwere {reverse J!shaped| in both communities\ sug!gesting continuous recruitment "Fig[ 1#[ Diameter dis!tributions of most of the more common species were ofsimilar form\ but at Slate Creek three species possessedfew small!diameter stems\ suggesting episodic recruit!ment] Didymopanax morototonii "Aubl[# Dene +Planch\ Podocarpus guatemalensis Standl[ and an asyet unidenti_ed tree "RT8#[

4!YEAR CHANGE IN FOREST COMPOSITION

The most obvious change at the 4!year re!census wasa 2=7) increase in the area covered by forest at Site16 "Fig[ 0#[ In contrast\ changes in forest edge positionat Slate Creek were negligible over the same timeinterval[ Total stem densities in the original mappedareas had undergone little change at Slate Creek\ andshowed only a slight increase at Site 16 "Table 1#[However\ in both communities there had been a sub!stantial increase in total basal area in the originalmapped areas "Table 1#[ Much of the increase in thetwo communities came from an increased number ofstems in the intermediate diameter classes "04Ð14 cmat Site 16 and 19Ð39 cm at Slate Creek^ Fig[ 1#[ Atboth sites there had been a corresponding decrease in

088



Table 0 Soil chemical properties at "a# Slate Creek and "b# Site 16[ Values for 09Ð49 cm represent the mean of values at 09Ð04\19Ð14 and 34Ð49 cm depths

"a# Slate Creek

South side North sidePropertyand depth "cm# Lower Middle Upper Lower Middle Upper Savanna

Extr[ Ca "meq 099 g−0#9Ð4 9=11 9=02 1=18 9=80 0=92 1=20 0=07

09Ð49 9=94 9=92 9=98 9=97 9=94 9=04 9=93

Extr[ Mg "meq 099 g−0#9Ð4 9=30 9=58 1=00 0=59 9=50 0=47 9=11

09Ð49 9=10 9=21 9=52 9=41 9=12 9=13 9=97

Extr[ K "meq 099 g−0#9Ð4 9=02 9=20 9=21 9=22 9=15 9=22 9=97

09Ð49 9=09 9=01 9=00 9=04 9=09 9=00 9=96

Truog|s P "ug g−0#9Ð4 4=19 2=39 0=25 1=42 1=52 1=13 9=14

09Ð49 3=38 0=92 9=48 9=49 9=42 9=34 9=97

CEC "meq 099 g−0#9Ð4 06=09 29=70 27=54 15=30 24=69 31=44 13=99

09Ð49 00=85 10=60 04=46 04=42 13=06 10=90 09=13

pH9Ð4 3=33 3=23 4=24 3=03 3=35 3=54 3=36

09Ð49 3=72 3=73 3=82 3=44 3=47 3=74 4=94

"b# Site 16

Meta!sedimentaryProperty Graniteand depth "cm# Forest Savanna Forest Savanna

Extr[ Ca "meq 099 g−0#9Ð4 2=67 9=23 9=20 9=06

09Ð49 9=76 9=09 9=95 9=97

Extr[ Mg "meq 099 g−0#9Ð4 9=35 9=43 9=84 9=25

09Ð49 9=44 9=25 9=33 9=14

Extr[ K "meq 099 g−0#9Ð4 9=94 9=98 9=02 9=01

09Ð49 9=03 9=92 9=01 9=00

Truog|s P "ug g−0#9Ð4 0=59 ND 0=19 ND

09Ð49 9=24 ND 9=26 ND

CEC "meq 099 g−0#9Ð4 12=6 ND 10=0 ND

09Ð49 02=5 ND 04=5 ND

pH9Ð4 4=32 4=90 4=45 3=64

09Ð49 4=20 4=49 4=11 4=25

stem numbers ³04 cm d[b[h[ over the 4!year interval"Fig[ 1#[

At Slate Creek two species had disappeared fromthe enumerated tree population due to tree death and

one new species had entered the ×09!cm diameterclass[ Both species which disappeared had been pre!sent only as single stems in the original enumeration[At Site 16\ two species had disappeared over the 4!

199



Fig[ 1[ Tree stem diameter class distributions in the two gallery forest communities at the _rst and second enumerations\ andfor dead stems at the second enumeration[

Table 1[ Changes in aggregate community characteristics in two gallery forests over a 4!year interval[ Data for stems ×09 cmd[b[h[ in originally enumerated areas

Original census 4 year re!census Change ")#

Stem density "stems ha−0#Slate Creek 603=4 602=5 −9=02Site 16 539=6 547=4 ¦1=67

Basal area "m1 ha−0#Slate Creek 08=60 10=20 ¦7=01Site 16 08=61 10=24 ¦7=16

year interval and two new species had entered the ×4!cm diameter class\ one due to edge expansion[ Forstems ×09 cm diameter at this site\ three species haddisappeared and no new species had appeared[ Onespecies which disappeared had occurred as only asingle stem "Hyeronima alchorneoides Allemao#\ buttwo other extinctions involved multiple stems] Vismia

ferruginea Kunth\ _ve stems^ Miconia dodecandra"Desr[# Cogn[\ two stems[

Average annual tree mortality rates at Site 16\expressed as a percentage of trees present at the _rstenumeration\ were 1=15) and 0=60) for stems ×4and ×09 cm\ respectively[ At Slate Creek\ the mor!tality rate for trees ×09 cm d[b[h[ was 0=49)[ The

190



diameter frequency distribution of dead stems at thesecond census at Slate Creek was not signi_cantlydi}erent from that of live stems at the _rst census"x1 � 4=536\ d[f[ � 4\ P × 9=94#[ However\ at Site 16\death over the 4!year interval was signi_cantly moreconcentrated in the smaller size classes of the originalpopulation than expected\ both for stems ×4 and×09 cm d[b[h[ "x1 � 15=012\ d[f[ � 5^ x1 � 08=373\d[f[ � 4\ respectively\ P ³ 9=94^ Fig[ 1#[

The mean distances to forest edges and heightsabove the streams of dead stems were not signi_cantlydi}erent from those of live stems at the _rst censusin either community\ indicating that mortality wasspread throughout both communities[ However\ inboth communities\ new recruits were signi_cantlycloser to the forest edge and further from the streamthan live stems at the _rst census "Table 2#[

TREE GROWTH

Proportional annual diameter increment of all ori!ginally censused trees that remained alive 4 years laterwere identical in the two communities for trees×09 cm d[b[h[ "0=10)# and was 0=59) at Site 16 fortrees ×4 cm d[b[h[ Proportional diameter incrementfor individual species varied widely\ and includedmuch within!species variability[ However\ there was aweak positive correlation in mean growth rates amongspecies common to the two sites "r � 9=316\ n � 22\P ³ 9=94#[

There was no signi_cant correlation between overalltree growth rates at Slate Creek and either distance ofthe tree to the forest edge\ or its height above thestream "Table 3#[ However\ a small proportion of the29 species whose growth rate correlations were evalu!ated separately showed signi_cant correlations withthese two variables when stems were disaggregated byspecies[ Growth rates of Photinia sp[ "JM0950# andPodocarpus guatemalensis increased with proximity to

Table 2[ Mean distances to forest:savanna boundaries and heights above streams of live stems "_rst census#\ dead stems "secondcensus#\ and new recruits in the originally censused areas "mean ¦ 0SD#[ Results of comparisons "by t!test# between meansfor live stems and those for dead stems or new recruits also shown

Slate CreekSite 16

×09 cm d[b[h[ ×4 cm d[b[h[ ×09 cm d[b[h[

Distance to boundaryLive stems 07=93 2 01=46 09=58 2 8=09 00=98 2 8=14Dead stems 08=68 2 01=54NS 8=46 2 7=33NS 8=66 2 6=11N

Recruits 03=37 2 01=37�� 5=32 2 6=19�� 5=33 2 5=69��

Height above streamLive stems 00=01 2 4=52 2=13 2 1=23 2=00 2 1=26Dead stems 09=52 2 4=93NS 2=28 2 1=41NS 1=77 2 1=38NS

Recruits 01=68 2 4=47�� 3=00 2 1=12�� 3=46 2 1=19��

NS P × 9=94^ �� P ³ 9=90^ �� P ³ 9=990[

the forest edge\ while that of Cyathea multi~ora J[ E[Smith decreased[ Growth rates of Sloanea tuerck!heimii Donn Sm[ and Hyeronima oblonga "Tul[#Muell[!Arg[ decreased with proximity to the stream\while no species showed the reverse trend[

Overall growth rate of trees at Site 16 was positivelycorrelated with proximity to the forest edge\ bothfor trees ×09 cm d[b[h[ and for trees ×4 cm d[b[h["Table 3#[ At this site\ a larger proportion of tree spec!ies showed accelerated growth near the forest edge] 01of 14 for stems ×4 cm d[b[h[\ and seven of 10 for stems×09 cm d[b[h[ "Table 3#[ There were no examples ofdecreased growth rate close to the forest edge at thissite[ Overall tree growth rates at Site 16 showed asigni_cant decrease with vertical proximity to thestream both for stems ×4 cm d[b[h[ and for stems×09 cm d[b[h[ "Table 3#[ A small number of individualspecies showed the same pattern^ four of 17 species"×4 cm d[b[h[# and two of 10 species "×09 cm d[b[h[#"Table 3#[ There were no examples at this site of accel!erated growth rates closer to the stream[

SPATIAL ORGANIZATION OF COMMUNITIES

At Slate Creek\ 08 of 16 tree species whose dis!tributions were analysed along the core!edge gradientshowed signi_cant segregation "Table 4#[ Ten speciesshowed signi_cant monotonic increases from edge tocore\ while a further seven showed the reverse trend[Only one species\ Matayba oppositifolia "A[Rich[#Britt[\ showed unequivocal concentration in the inter!mediate zone\ while Myrsine coriaceae "Sw[# R[Br[\was most abundant in edge and intermediate zones\but virtually absent in core areas[ The remaining eightspecies showed no signi_cant tendency to segregatealong the edge!core gradient and can be regarded asgeneralists[

The same analysis at Site 16 showed 06 of 16 speciesto be signi_cantly segregated along the core!edge

191



Table 3 Correlations between log!transformed proportional annual diameter increment and distance to the forest:savannaboundary or height above the stream of all tree stems and of species with stem numbers ×09 in two gallery forests[ Specieswithout any signi_cant "P ³ 9=94# correlations are not shown

Site 16

Slate Creek Dist[ to Ht[ abv[Boundary Stream

Dist[ to Ht[ abv[Boundary Stream ×4 cm ×09 cm ×4 cm ×09 cm

All tree stems NS NS −9=175 9=107 9=055 9=134Calophyllum brasiliense NS NS −9=792 −9=653 9=598 9=637Clethra hondurensis −9=164 NS −9=141 NS NS NSCyathea multi~ora 9=491 NS NS NS NS NSHenrietta succosa NS NS −9=324 Ð 9=237 ÐHirtella racemosa Ð Ð −9=325 Ð NS ÐHyeronima oblonga NS 9=131 Ð Ð Ð ÐLacistema aggregatum NS NS −9=444 −9=507 NS NSMatayba oppositifolia NS NS −9=394 −9=461 NS 9=475Myrsine coriaceae NS NS −9=552 −9=522 NS NSPera barbellata NS NS −9=368 NS NS NSPhotinia sp[ −9=439 NS Ð Ð Ð ÐPodocarpus guatemalensis −9=562 9=476 Ð Ð Ð ÐSloanea tuerckheimii NS 9=221 −9=298 NS NS NSSymplocos martinicensis NS NS NS −9=663 9=351 NSVochysia hondurensis NS NS −9=189 NS 9=119 NSXylopia frutescens NS NS −9=339 −9=157 9=260 NSZinowiewa pallida NS NS −9=377 NS NS NS

NS\ insigni_cant correlation^ Ð\ correlation not determined due to small sample size[

gradient and 09 species to be generalists "Table 4#[ Asat Slate Creek\ most of these species were either core!or edge!concentrated\ with only one species unequivo!cally concentrated in the intermediate zone "Miconiamicrocarpa DC[ vel sp[ a}[#[ Two other species showedgreatest concentration either in the intermediate ¦core zones "Cyathea multi~ora# or intermediate ¦edge zones "Henrietta succosa "Aubl[# "DC#[

There were 06 species common to the segregationanalyses at the two sites[ Of these\ only six speciesshowed a common pattern] Licania hypoleuca Benth[\L[ sparsipilis Blake and Sloanea tuerckheimii DonnSm[ concentrated in both core zones^ Clethra hon!durensis Britt[ and Pinus caribaea concentrated at bothedges^ and Calophylum brasiliense Camb[ was a gen!eralist at both sites[ The most common exceptionswere species that were edge!concentrated at SlateCreek\ but generalists at Site 16 "Ilex guianensis"Aubl[# O[ Kuntze\ Schippia concolour Burret\ Xylopiafrutescens Aubl[ and Zinowiewa pallida Lundell[ How!ever\ there were no examples of a switch between core!concentrated and edge!concentrated status by anyspecies[ Species that showed accelerated growth nearthe forest edge were found in all groups "Table 4#[

Discussion

Despite possessing stem densities that are higher thanaverage\ and basal areas that are lower than average\for tropical forests generally\ both gallery forests show

many other compositional and functional similaritiesto this community type[ At local scales "¼0 ha# num!bers of tree species are comparable to those that havebeen recorded in continuous forests of the region[ Forexample\ eastern and western hectare blocks at theSlate Creek site contained 49 and 48 species ×09 cmd[b[h[\ respectively\ while three 0!ha samples taken inthe Rio Bravo area of continuous forest in northernBelize contained 49\ 47 and 48 tree species "N[Brokaw\ personal communication#[ Annual tree mor!tality rates of 0=60 and 0=49) for trees ×09 cm d[b[h[lie within the range of those reported for other tropicalforests "9=85Ð1=82)\ median 0=5)] Swaine\ Lieb!erman + Putz 0876^ de Rankin!Merona\ Hutchings +Lovejoy 0889^ Lugo + Scatena 0885#[ Proportionaldiameter increments of trees in the two gallery forests"0=10) year−0 in both communities# also fall withinthe rather broad range of increment values reportedfor a variety of other tropical forests "9=29 to ¼4=9)year−0] Ashton + Hall 0881^ Condit\ Hubbell + Fos!ter 0881^ Herwitz + Young 0883#[ Similarly\ whilesome of the species losses recorded in these forest sitesover 4 years may have been pseudo!extinctions\ withthe species persisting as juveniles below measurablesize or as a seed bank\ the compositional turnoverobserved suggests that the less abundant tree speciesin the forests may be subject to high extinction prob!abilities[ Comparable local extinctions have beendocumented in permanent plots in continuous tropicalforests "e[g[ Hubbell + Foster 0881#[

192



Table 4[ Functional groups of species found in two galleryforests\ classi_ed according to their pattern of distributionon core!edge gradients as determined by PCA

Slate Creek Site 16

Core!concentratedAlchornea latifolia� Hirtella racemosa�Didymopanax morototonii Lacistema aggregatum�Henrietta succosa Licania hypoleucaLuehea sp[ "JM0083# Licania sparsipilisHyeronima oblonga Pera barbellata�Inga cocleensis Pouteria estoriensisLicania hypoleuca Sloanea tuerckheimii�Licania sparsipilis Symphonia globuliferaSloanea tuerckheimiiIndeterminate "RT8#

Edge!concentratedClethra hondurensis� Clethra hondurensis�Photinia sp[ "JM0950#� Conostegia icosandraPinus caribaea Miconia dodecandraSchippia concolour Myrsine coriaceae�Ilex guianensis Pinus caribaeaXylopia frutescens Vochysia hondurensis�Zinowiewa pallida

Concentrated in intermediate zonesMatayba oppositifolia� Cyathea multi~oraMyrsine coriaceae Henrietta succosa�Miconia microcarpa

Generalists without any signi_cant pattern of segregationCalophyllum brasiliense Alchornea latifoliaCyathea multi~ora Calophyllum brasiliense�Dendropanax arboreus Cassipourea ellipticaPera barbellata Ilex guianensisQuercus sp[ "JM0044# Matayba oppositifolia�Lacistema aggregatum Miconia holosericeaTerminalia amazonia Schippia concolourVochysia hondurensis Symplocos martinicensis�

Xylopia frutescens�Zinowiewa pallida�

� Species whose proportional annual diameter incrementincreased signi_cantly with proximity to the forest edge[

Data on the dynamics of other gallery forests donot exist with which to evaluate how typical were thetwo communities studied\ but more extensive sam!pling of tree diversity in a variety of gallery forestselsewhere has shown that high local diversities arecharacteristic of this vegetation type in well!drainedhabitats "Meave et al[ 0880^ Kellman et al[ 0883^Meave + Kellman 0883#[ Within the Mountain PineRidge gallery forest system\ the two communities rep!resent opposite extremes of the fragmentation spec!trum\ as well as possessing certain habitat di}erences[Slate Creek remains connected to continuous forestand occupies steeply sloping terrain with abrupt upperslope convexities that probably provide partial pro!tection from _res and some light attenuation[ In con!trast\ Site 16 is small\ completely isolated\ with lesstopographic protection and thus is more vulnerable

to _re incursions[ While the latter community showssome signs of the e}ects of these special characteristics"greater species dominance\ more successional dyna!mism\ edge species becoming generalists#\ and the exis!tence of some species may be more tenuous here thanat Slate Creek\ it continues to preserve a remarkabledegree of tree diversity[

Despite their habitat di}erences\ both communitiesappear to be resilient to a variety of disturbances[ Thefrequency of _re!scarred trees "Fig[ 0# indicates thepresence of periodic _re incursions\ with Site 16 hav!ing being especially widely a}ected[ Unfortunately\the absence of annual growth rings precludes docu!mentation of the chronology of _re incursions[However\ both communities have survived theseevents\ and at Site 16 the forest edge is expanding[Furthermore\ the increase in basal area in both com!munities suggests recovery from some common dis!turbance\ the most likely source being a 0850 hur!ricane which severely a}ected most forests in the area"Wol}sohn 0856#[ Kellman + Tackaberry "0882#found a high frequency of atypically orientated tree!throw mounds on the upper slope shoulders at SlateCreek\ and tentatively attributed this to hurricanedamage[ The abundance of certain light!demandingspecies in these forests also suggests a relatively recentmajor disturbance[ At Slate Creek the pioneer treeDidymopanax morototonii is present only as largestems\ while Matayba oppositifolia is positively associ!ated with the tree!throw mounds on upper slopeshoulders "Kellman + Tackaberry 0882#[ This wouldalso explain the latter species| unique {intermediatezone| concentration in this forest "Table 4#[

Despite possessing many similarities to continuousforests\ the two gallery forests show a distinctive spa!tial structure\ with many tree species segregating alongthe edge!core gradient "Table 4#[ However\ neither thesoil data "Table 0# nor the patterns of tree growth"Table 3# suggest that this patterning is driven by anunderlying edaphic gradient involving increased habi!tat favourability as streams are approached[ Rather\the forest edge habitat appears to be the most favour!able for tree growth[ Tree recruitment was greatercloser to the forest edge in both communities and thegrowth of many species\ especially at Site 16\ wasaccelerated in this zone[ Moreover\ species showingaccelerated growth near forest edges included mem!bers of all functional groups] core!\ edge! and inter!mediate!zone concentrators and generalists "Table 4#[This general enhancement of tree growth near forestedges suggests that these forest communities are light!limited rather than soil!limited\ and contradicts thecommon asssumption that edge zones are lethal habi!tats for all forest organisms[ MacDougall + Kellman"0881# have shown that increased light levels at theforest ~oor occur within 04 m of the forest edges inthese communities[ The present data on tree growthsuggest that increased light near forest edges alsoexists at higher levels in the forest canopy and prob!

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ably extends farther into the forest than that measuredat ground level[ Such a persistent zone of increasedillumination represents a habitat type that is absentfrom continuous forest communities where increasedillumination is a transient phenomenon associatedwith temporary canopy gaps[

While the tendency of many tree species to segregateat opposite ends of a core!edge forest light gradientcould\ in theory\ be ascribed to di}ering shade tol!erances\ a variety of circumstantial evidences pointsto di}erential _re tolerance as the more importantin~uence on this pattern[ Gallery forest edges are sub!ject to periodic _re incursions "Kellman + Meave0886# and signs of _re decrease in frequency mono!tonically between forest edges and streams "Meave0880#[ Tropical forest trees are generally intolerant of_re "Uhl + Kau}man 0889# and it seems probablethat the core!concentrating group is comprised pri!marily of _re!sensitive forest species[ In contrast\ spec!ies present at boundaries must be tolerant of _re or beable to develop populations opportunistically between_res[ Fires intruding into gallery forests create a var!iety of levels of disturbance\ but most frequentlydestroy litter\ seedling and sapling layers\ while leav!ing trees _re!scarred\ but with intact canopies "Kell!man + Meave 0886#[ This creates a specialized seed!bed but prevents the entry of grasses and sedges thatwould facilitate further _re incursions[

Several of the edge!concentrated species docu!mented in this study\ and others observed at othergallery forest boundaries in the area\ occur also infrequently burned savanna ðe[g[ Byrsonima crassifolia"L[# H[B[K[\ Clethra hondurensis\ Pinus caribaea\Quercus oleoides Schlecht[ + Cham\ Q[ peduncularisNee\ Schippia concolourŁ[ Several other members ofthe group "Myrsine coriaceae\ Photinia sp[ and Xylopiafrutescens# have been found to regenerate pref!erentially at sites of _re incursions into forests "Kell!man + Meave 0886#\ while seedlings of Clethra hon!durensis and Xylopia frutescens have been found toconcentrate in the better!illuminated zones near forestedges "MacDougall + Kellman 0881#[ Some\ such asIlex guianensis and Xylopia frutescens are capable ofestablishing seedlings in savannas during _re!freeintervals[ The edge!concentrating species thus appearto comprise a functional group that share one or moreof the following characteristics] tolerance of _re\ anability to establish opportunistically after _re incur!sions\ an ability to invade savanna and sensitivity toshade[ Species belonging to the generalist group areapparently able to establish and grow throughout therange of light levels and _re disturbance regimes thatare found in gallery forests\ although they do notinvade savanna "M[ Kellman\ unpublished data#[ Thisgroup contains some of the most abundant speciesfound in these forests\ such as Calophyllum brasiliense\Terminalia amazonia "Gmel[# Exell[ and Vochysia hon!durensis Sprague[

While at local scales the size of the three main func!

tional groups "edge!concentrators\ core!concen!trators\ generalists# is approximately equal "Table 4#\Tackaberry "0881# has found that new speciesaccumulate primarily in core areas at larger spatialscales[ In sampling transects dispersed throughout the0=4!km length of Slate Creek\ new species accumulatedprimarily in core areas rather than at edges\ and asimilar pattern was found along a 04!km stretch ofgallery forest sampled in the Apure savannas of Vene!zuela[ This suggests that the gallery forests consist ofprotective ecotonal communities of limited diversitythat enclose core areas in which most regional diver!sity is likely to reside[

The literature on recently created tropical forestfragments has been preoccupied with potential lossesof species from these areas\ and losses of plant specieshave been con_rmed in many instances "Turner 0885^Turner et al[ 0885#[ However\ the persistence of spec!ies!rich gallery forests indicates that ~oristic depau!perization is not an inevitable consequence of frag!mentation[ It also raises the question as to how recentfragments may best be restructured to achieve com!parably diverse steady!state conditions "cf[ Kellman0885#[ Most recently created fragments have beguntheir existence as {samples| of the original continuousforests[ The instantaneous excision of these from theoriginal forest matrix precludes the gradual com!munity re!organization that probably accompaniedslow shrinkage of continuous forest to systems of gal!lery forests in response to historic changes in climateand _re regimes[ Consequently\ it is unlikely that\without management intervention\ recent fragmentswill undergo spontaneous restructuring with su.cientrapidity to prevent signi_cant species loss[

We suggest that\ for purposes of species conser!vation\ the most bene_cial short!term restructuring ofrecently created forest fragments will be the estab!lishment at outer edges of protective ecotonal com!munities comparable to those that exist in galleryforests[ Short!term responses to edge formation inrecent fragments often involves some tree mortality"Lovejoy et al[ 0875#\ but also accelerated growth ofunderstorey plants\ leading to partial closure of theopened edge "Williams!Linera 0889#[ While this clos!ure leads to an attenuation of atmospheric edge e}ects"Kapos et al[ 0886#\ well!developed lateral canopiesare unlikely to persist in tropical landscapes that arecharacteristically subject to high _re frequencies "Uhl+ Buschbacher 0874^ Goldammer 0889#[ However\gallery forest ecotonal communities are manifestlycapable of persisting in a matrix subject to high _refrequency and provide a reservoir of species withwhich to fashion suitably protective ecotonal com!munities at new fragment edges[

Unfortunately\ little information exists on the aut!ecology of these edge species as most are of littleeconomic value and\ being generally common\ areregarded as of little conservation signi_cance[However\ our results suggest that these species may

194



have an important conservation role to play in futurefragmented landscapes of the tropics and that researchon their management is a matter of some urgency[They are also likely to have a potentially importantrole to play in forest restoration e}orts as many actas _re!tolerant nutrient accumulators within savannas"Kellman 0868#\ and can facilitate the establishment ofmore demanding forest species\ such as the generalistCalophyllum brasiliense "Kellman 0874#[

Fewer obvious prescriptions exist for the restruc!turing of forest fragment interiors\ where mostregional ~oristic diversity is likely to be preserved[ Thespecies turnover recorded in the two gallery forestcommunities indicate that local species extinction canbe expected in recent fragments\ especially among rarespecies[ In gallery forests\ extinctions appear to becompensated by suitably high immigration rates\which are probably facilitated by the inter!connectedness of gallery forest systems\ and their exis!tence in a savanna matrix that is not completely hostileto animal seed!dispersers[ However\ frequent seed dis!persal is likely to be more problematical among recentfragments\ which may be less interconnected and areoften embedded in an agricultural matrix that is inhos!pitable to seed!dispersers[ This suggests that a pro!gramme of interfragment transfer of seed not dis!persed by volant animals may be necessary to plantspecies conservation in these fragmented systems[Because preservation of a regional ~ora in a fragmentsystem of limited area will require not only high localspecies richness\ but also much interfragment com!positional turnover\ there appears to be considerablescope for the development of seed transfer pro!grammes that are planned to maximize intrafragmentdiversity and interfragment compositional di}erences[

Conclusions

The two gallery forests studied have preserved adiverse forest tree ~ora despite showing signs of bothhurricane and _re damage[ The forests are not micro!cosms of continuous forest but rather are re!organizedcommunities whose structure re~ects the complexresource:disturbance gradient that exists between for!est edges and streams[ Near!edge forest zones are boththe most favourable habitats for tree establishmentand growth\ and zones that are lethal to _re!sensitiveforest plants[

A sub!community of trees that are tolerant of _rehas developed in the edge zone\ preserving an intactforest canopy through _re incursions and preventingthe development of a ~ammable graminoid under!storey[ Some of these species establish preferentiallyafter _re incursions and are capable of expanding intosurrounding savanna[ This forest edge communityusually protects the interior zone from _re\ thus pro!viding a micro!refugium in which a _re!sensitive forest~ora can survive[ Preservation of a diverse regionalforest ~ora in a system of recently created forest frag!

ments will require the development of a protectiveedge community comparable to that found in galleryforests\ and natural or arti_cially enhanced immi!gration rates that are su.cient to o}set the extinctionsthat appear to occur in individual fragments[

Acknowledgements

We thank the Belize Forestry Department for per!mission to work in the Mountain Pine Ridge ForestReserve and for logistical support in the _eld[ Fieldassistance has been provided by Augustine Howe\Marco Howe\ Moses Howe and Trudy Kavanagh\and Linnaea Chapman and Gregg McNeil assistedwith data analysis[ Assistance with specimen identi!_cation has been provided by Drs Gerritt Davidse andJorge Meave[ We thank Nick Brokaw for providingtree diversity data for the Rio Bravo area\ and AlanHill and two anonymous referees for commenting onan earlier draft of this paper[ The research was sup!ported by grants from the National GeographicSociety and the Natural Sciences and EngineeringResearch Council of Canada to M[K[

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Received 18 May 0886^ revision received 07 September 0886

Documents

Structure and function in two tropical gallery forest communities: implications for forest conservation in fragmented systems