The Influence of Island Area on Ecosystem Properties - Ecology and ...

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REPORTSisland-area gradients for evaluating processesand factors that operate at the ecosystem-level<strong>of</strong> resolution (5).We selected an island archipelago inthe northern boreal forest zone <strong>of</strong> Sweden,located within two adjacent lakes—LakeHornavan and Lake Uddjaure (65°55–66°09N; 17°43–17°55E). We chose 50islands, ranging in area from 0.02 to 15.0ha (and with these areas being distributedlognormally) which were formed on morainicdeposits created by the retreat <strong>of</strong>land ice 9000 years ago. Our island systemis ideal for testing hypotheses relating toisland area effects because all the islandsare <strong>of</strong> the same age and origin, and becausethey have all been subjected to comparativelyminor human interference (6).<strong>The</strong> main disturbance regime on the islandsis wildfire by lightning strike; lightning(and therefore wildfire) appears tostrike larger islands more frequently thansmaller ones, presumably because theyhave a larger area to intercept. This isreflected in both the vegetation composition(7) and fire history data (8) collectedfrom each <strong>of</strong> these islands (Table 1). <strong>The</strong>vegetation data demonstrated that largerislands are dominated by earlier-successionalplant species which dominate in thepresence <strong>of</strong> regular wildfire, for example,Pinus sylvestris and Vaccinium myrtillus,whereas smaller islands show a greaterabundance <strong>of</strong> late successional specieswhich occur in the prolonged absence <strong>of</strong>fire, for example Picea abies and Empetrumhermaphroditum (9). Further, evaluation <strong>of</strong>fire history with both fire-scar data and14 C dating <strong>of</strong> charcoal particles in humuspr<strong>of</strong>iles reveals that the larger islands, ingeneral, have burned much more recentlythan the smaller ones and have a greaterfire frequency (Table 1).Those late successional plant speciesthat dominate on the smaller islands typicallyproduce foliage and litter <strong>of</strong> poorquality with high levels <strong>of</strong> secondary metabolites,principally phenolics (10), andthese compounds have the potential toreduce the ability <strong>of</strong> the soil micr<strong>of</strong>lora todecompose substrates and mineralize nutrients(11). Measurements <strong>of</strong> soil chemicaland biological properties (12) wereconsistent with this—we found higherconcentrations <strong>of</strong> water-soluble phenolics,reduced microbial biomass, and reducedmicrobial activity in the humus <strong>of</strong> thesmaller islands (Fig. 1). This retardation <strong>of</strong>microbial activity was also reflected in areduction <strong>of</strong> the rates <strong>of</strong> decompositionand N mineralization <strong>of</strong> V. myrtillus litterplaced on the smaller islands (13). Further,the N concentration <strong>of</strong> both thehumus and the added litter was highest onthe smaller islands, suggesting that onthese islands organic N becomes bound inprotein-phenolic complexes, which arenotoriously resistant to microbial attack(14). <strong>The</strong> inhibition <strong>of</strong> soil biotic processeson the smaller islands probably contributesto the substantial accumulation <strong>of</strong>Fig. 1. Humus chemical and biological characteristics in relation to island area. (A through F) Chemicaland biological properties are shown for humus samples collected from each island. Concentrations <strong>of</strong>phenolics are expressed as microgram <strong>of</strong> gallic acid equivalent per gram <strong>of</strong> humus. (G through J)Properties are shown for leaf litter <strong>of</strong> V. myrtillus placed in litter bags and left to decompose for 1 year.(K and L) Total mass <strong>of</strong> humus and humus N was determined on an areal basis.Fig. 2. Proportion <strong>of</strong> terrestrial C present in humus and plant pools in relation to island area. Total Cvalues are mean SD.www.sciencemag.org SCIENCE VOL. 277 29 AUGUST 1997 1297
humus that occurs on them (Fig. 1); thesmallest islands contain up to 10 timesmore humus per unit area than do thelargest ones. <strong>The</strong>refore, our results demonstratethat island area is critical in regulatingkey ecological processes (and thereforeecosystem function) and that this ismost likely attributable to plant specieseffects.Carbon partitioning is clearly regulatedby island area. With increasing island size,there is a distinct trend <strong>of</strong> an increasingTable 1. Vegetation and fire history characteristics in relation to island area. Values presented aremeans SE.Species/measurement0.1 ha(n 14)<strong>Area</strong> class0.1–1.0 ha(n 24)1.0 ha(n 12)Tree stem density (N o /ha)Pinus sylvestris 206 82 222 48 410 78Betula pubescens 1233 229 1707 186 982 214Picea abies 278 87 211 48 45 22Total 1720 198 2140 169 1438 201Tree biomass (kg/m 2 )Pinus sylvestris 1.697 0.627 2.627 0.563 6.160 0.864Betula pubescens 0.844 0.198 1.287 0.211 0.887 0.199Picea abies 1.329 0.323 1.462 0.289 0.332 0.169Total 3.869 0.624 5.376 0.466 7.385 0.826Dwarf shrub biomass (kg/m 2 )Vaccinium myrtillus 0.069 0.014 0.083 0.011 0.119 0.017Vaccinium vitis-idaea 0.164 0.030 0.215 0.021 0.153 0.025Empetrum hermaphroditum 0.129 0.024 0.075 0.010 0.060 0.018Total 0.365 0.022 0.383 0.021 0.332 0.033Bryophyte biomass (kg/m 2 )Pleurozium schreberi 0.152 0.038 0.143 0.030 0.155 0.030Hylocomium splendens 0.121 0.021 0.061 0.016 0.077 0.021Total 0.283 0.033 0.217 0.027 0.261 0.035Additional parametersTotal vegetation biomass 4.517 0.616 5.977 0.455 7.979 0.817(kg/m 2 )Shannon-Weiner species 1.784 0.131 1.465 0.065 1.178 0.134diversity index (H / )*Number <strong>of</strong> fires in past 250 0.143 0.097 0.208 0.085 0.667 0.256yearsAge <strong>of</strong> charcoal in years( 14 C)†2984 340 2081 428 1106 495*Index calculated by use <strong>of</strong> biomass values for each plant species. †Obtained by 14 C radiocarbon dating <strong>of</strong>uppermost charcoal layer in humus pr<strong>of</strong>ile; n 5 islands used for each area class.Fig. 3. Total N concentration<strong>of</strong> selected organismsin relation to islandarea. <strong>The</strong> relationship<strong>of</strong> humus N concentrationto island areais included for comparativepurposes. <strong>The</strong> Nconcentration <strong>of</strong> leavesfrom V. myrtillus and V.vitis-idaea did not showa statistically significantrelationship with islandarea.proportion <strong>of</strong> organic C that is bound inliving organisms, especially trees (Fig. 2).Because smaller islands contain much higherterrestrial C levels on an areal basis thando larger ones, our data indicates that wildfireis <strong>of</strong> critical importance (either directlyor indirectly) in reversing C lock-up inboreal forest ecosystems. This finding suggeststhat deliberate anthropogenic supression<strong>of</strong> fires in boreal forests over the pastcentury has the potential to lead to retardation<strong>of</strong> soil biological processes and substantialterrestrial C sequestration (15),which is likely to be <strong>of</strong> global significancegiven the role <strong>of</strong> boreal forests in the globalC cycle (16).<strong>The</strong>re is evidence that N partitioning isalso likely to be affected by island area.Although the humus N content was negativelyrelated to island area, the N concentrations<strong>of</strong> leaves <strong>of</strong> E. hermaphroditum,shoots <strong>of</strong> the moss Pleurozium schreberi,and the soil microbial biomass were allpositively related to island area (Fig. 3).This is indicative <strong>of</strong> enhanced acquisition<strong>of</strong> N by these organisms, and means thatthe N present in the humus <strong>of</strong> the smallerislands is clearly less available to livingorganisms than that <strong>of</strong> the larger islands.This again supports the concept that higherlevels <strong>of</strong> phenolics on the smaller islandscontribute to reduced N availability.<strong>The</strong> lower plant tissue N content <strong>of</strong> plantson smaller islands would be expected toresult in subsequently produced plant litterwith a lower C:N ratio, and thus, areduced rate <strong>of</strong> decomposition (17), ultimatelyresulting in further organic matteraccumulation.<strong>Island</strong> area studies have potential forinvestigation <strong>of</strong> relationships between speciesdiversity and ecosystem-level properties(5). In our study, plant species diversitywas highest on the smallest islands(Table 1), meaning that ecosystem processrates were lowest on those islands with thegreatest diversity. This finding is in directcontrast to other studies which haveshown elevated process rates in more diversecommunities (18). However, wefound that the plant biomass <strong>of</strong> the largerislands was dominated by a single earlysuccessional competitive tree species, P.sylvestris, which has comparatively favorablelitter quality but which contributes toa reduced species diversity. <strong>The</strong> greaterecological stresses on the smaller islands(due to higher phenolics, lower pH, andreduced N availability) presumably preventeddominance by a single competitivespecies, resulting in a greater diversity(19), but with a greater abundance <strong>of</strong>those plant species with traits likely tocontribute to retarding ecosystem-levelprocesses. Other possible explanations for1298SCIENCE VOL. 277 29 AUGUST 1997 www.sciencemag.org
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