ecosystem impacts of disturbance in a dry tropical forest in … · 2010-06-11 · ecosystem...

ECOHYDROLOGYEcohydrol. 1, 149–160 (2008)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/eco.14

Ecosystem impacts of disturbance in a dry tropical forestin southern India

Vishal K. Mehta,1* Patrick J. Sullivan,2 M. Todd Walter,3 Jagdish Krishnaswamy4

and Stephen D. DeGloria1

1 Department of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853, USA2 Department of Natural Resources, Cornell University, Ithaca, NY 14853, USA

3 Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA4 Ashoka Trust for Research in Ecology and the Environment (ATREE), Bangalore-560 024, India

ABSTRACT

Indian forests provide a multitude of services to vast populations. Common human activities including livestock grazing,fuelwood extraction and burning have the potential to impact forest ecosystem structure and function. The effects of theseactivities on vegetation, ecology and soil properties were investigated in Bandipur National Park (BNP) in southern India.Data were collected from 200 sites in four watersheds within the park. Sample sites spanned a degradation gradient measuredby a field disturbance index (FDI). This paper focusses on the impacts on vegetation structure, diversity and composition, andintegrates impacts on soil. Shrub and tree species were inventoried and evaluated in plots 10-m in diameter. The tree layerwas dominated by Anogeissus–Emblica–Tectona species. The understory was dominated by invasives Chromolaena odorataand Lantana camara, and native Gymnosporia emarginata. Vegetation plot heights, canopy cover and tree diameters werenegatively correlated with field disturbance resulting in stunted forest stature in degraded sites. Vegetation composition indegraded watersheds was dominated by small woody tree species and a greater diversity of shrub species. Ordination analysiswas used to integrate soil data with vegetation and disturbance, revealing that deciduous forest in the park is degrading to scrubforest along with negative impacts on soil characteristics. Consequences of services currently enjoyed by local populationsare discussed. Copyright 2008 John Wiley & Sons, Ltd.

KEY WORDS anthropogenic disturbance; ecosystem impacts; tropical dry forest; vegetation and soils; ordination; India

Received 15 December 2007; Accepted 6 April 2008

INTRODUCTION

Indian forests have been used by communities overmillennia for a variety of uses and practices (Lele andHegde, 1997; Shankar et al., 1998a). Of these, livestockgrazing, fuelwood, fodder extraction, and burning topromote grasses for fodder are common historic andcontinuing uses of the forest (Banerjee, 1995; Bhat et al.,2001; Saha, 2002; Kodandapani et al., 2004; FAO, 2006).These activities represent substantial pressures on theforest resource base. The anthropogenic pressures fromcommunities residing within and on the fringes of thesereserves continue to be enormous. Over five millionpeople have been estimated to reside within a country-wide network of 593 wildlife reserves and national parks(Kothari et al., 1995); (Madhusudan, 2005).

Consequently, livestock grazing, fuelwood, fodderextraction and burning are recognized as a ‘chronic dis-turbance’ (Singh, 1998) that can have substantial impactson the entire forest ecosystem (Tilman and Lehman,2001) including impacts on vegetation, soil and waterresources, fauna and micro-climate. However, quanti-tative studies on the impacts of forest disturbance inIndia were relatively few until recently (Shahabuddin and

* Correspondence to: Dr. Vishal K. Mehta, 133 D St. Suite F, Davis CA,95616, USA. E-mail: [email protected]

Prasad, 2004). A majority of these studies has separatelyfocussed on impacts on either vegetation (Shankar et al.,1998b; Madhusudan, 2000; Sagar et al., 2003; e.g. Kumarand Shahabuddin, 2005), soils (e.g. Sahani and Behera,2001) or wildlife (e.g. Madhusudan, 2004). Few stud-ies have attempted an integrated ecosystem functionsapproach to the impacts of forest disturbance, one thatelucidates the linkages and feedbacks between, for exam-ple, changes in vegetation structure and composition, soilimpacts, nutrient cycling and hydrology. Research thataddresses the ‘multi-functionality’ of ecosystems (Hectorand Bagchi, 2007) and the linkages of ecosystem func-tioning to services they underpin (Balvanera et al., 2006)is especially important given that ecological change canalter the ecosystem services currently enjoyed, and canincrease the vulnerability of people and ecosystems tofurther changes (Carpenter et al., 2006a).

To address this critical issue, a research campaignwas conducted in a deciduous forest in BandipurNational Park,(BNP) India, focussing on forest distur-bance impacts on three aspects of forest ecosystemfunction, namely, vegetation ecology (this paper), soils(Mehta et al., 2008, this issue) and watershed-scalehydrology (Krishnaswamy et al., in preparation). In thispaper, we (i) investigate the impact of disturbance onvegetation structure, diversity and composition in BNP;

Copyright 2008 John Wiley & Sons, Ltd.

150 V. K. MEHTA ET AL.

(ii) integrate impacts on vegetation with impacts on soilprocesses presented earlier (Mehta et al., 2008, this issue)by means of ordination analysis; and (iii) based on (i) and(ii) discuss the feedbacks and implications for servicescurrently enjoyed by local populations.

STUDY AREA

The study area is the BNP (Longitudes 76°120 –76°530 E,Latitudes 11°350 –11°580 N), in the southern state of Kar-nataka. The park is approximately 874 km2 in area and islocated leeward of the Western Ghats mountain range andbiodiversity hotspot (Figure 1 in Mehta et al., 2008, thisissue). It is home to 17 critically endangered, endangeredand vulnerable plant and animal species (2001 IUCNRed List, www.redlist.org), and is one of the ProjectTiger conservation areas in the country. BNP lies inthe sub-humid transitional zone between the humid high-elevation backslopes of the Western Ghats and the semi-arid Deccan plateau interiors (Bourgeon, 1989 p37–56).Elevations in BNP range from a low of 700 m on thewestern border, to a maximum of 1450 m at GopalswamyBetta in the central portion of the park. Topographyis gently undulating. The climate is tropical savanna,hot, seasonally dry (IMD, 1984). Average annual rainfallranges from 900 to 1200 mm within the park boundaries(Pascal, 1982). Rainfall is trimodal, occurring as convec-tional pre-monsoon storms (March–May), the southwestmonsoon (June–September) and the northeast monsoon(October–December), with the highest rainfall in Octo-ber (Devidas and Puyravaud, 1995). Mean minimum andmaximum daily temperatures are 15 and 35 °C respec-tively. There is a prolonged dry season from January to

May (Pascal, 1982). BNP falls in the granite–gneiss com-plex of the Archaean (Peninsular gneiss) group, the chiefrocks being gneisses, granites and charnockites. Soilsin the research watersheds are haplic alfisols of mod-erate depth with ustic soil moisture regime and are welldrained (Bourgeon, 1989; Shiva Prasad et al., 1998). Sur-face soils are generally sandy clay loam with underlyingargilic horizons of high to medium-base saturation, andcoloured deep red with iron oxides. Soils are moderatelyacidic (pHKCl ¾ 5Ð3), with a cation exchange capacity(CEC) of approximately 20 cmol/kg, and base saturationabove 70% (Ferry, 1994 p190–193). Mehta et al. (2008,this issue) report the negative impacts of forest distur-bance on CEC, through reductions in soil organic carbon(SOC) and soil clay content. Soil hydraulics are also neg-atively impacted by reduced available water capacity, anda likelihood of increased Hortonian overland flow due tohigher cattle trail density in degraded forest areas.

Vegetation in BNP is classified as dry deciduouswoodland to savanna woodland forests of the Anogeis-sus latifolia — Tectona grandis — Terminalia tomentosatype (Pascal, 1982). Most researchers believe that thepresent vegetation represents degraded stages of suc-cession, varying from a savanna woodland to low dis-continuous thickets. These ‘clump thickets’ and stuntedscrub forests can be found extensively along the northernand eastern boundaries of BNP. Grasses belong predomi-nantly to the Cymbopogon and Themeda spp (Prasad andSharatchandra, 1984). Grass heights and cover are notice-ably low and sparse in the degraded northern borders ofthe park. Vegetation is prone to annual, low-intensity firesat the end of the dry season (Devidas and Puyravaud,1995).

Figure 1. Research watersheds: D1—Hediyala; D2—Muntipur; P3—Hebhalla; P4—Soreda.

Copyright 2008 John Wiley & Sons, Ltd. Ecohydrol. 1, 149–160 (2008)DOI: 10.1002/eco

ECOSYSTEM IMPACTS OF FOREST DISTURBANCE 151

Unlike many other protected areas in the countrythere are no human settlements within the bound-aries of BNP (Madhusudan, 2005). All anthropogenicbiomass pressures—primarily livestock-grazing and fuel-wood removal—on the park originate from the villagesalong the reserve’s northern flank. Over 100 000 cattlefrom nearly 180 villages graze in the buffer zone ofBandipur’s northern fringes (Lal et al., 1994). The currentdensity of livestock is estimated at 236 animals per squarekilometre (Madhusudan, 2005). Livestock and wild her-bivores compete for grazing resources in BNP; recov-ery of wild herbivores following livestock decline hasbeen documented by Madhusudan (2004). Cattle grazingand fuelwood collection occurs within protected forestareas, while small-scale farming is practised outside parkboundaries. Until recently, this use of the forest could bequalified as subsistence use, with local village communi-ties as consumers. However, in a study on the impact ofgrazing on vegetation in BNP, Madhusudan (2005) doc-umented the intensification of grazing practices since the1990s, linked to the export of cattle dung as manure toa neighbouring coffee growing district. This demand fordung from a non-local source, in turn driven by globalcoffee markets, has converted cattle dung from a localsubsistence agriculture resource to a commercial exportto neighbouring coffee estates. As a result, the pressureon the northern boundaries of the park has substantiallyincreased in the last decade.

MATERIALS AND METHODS

Field sampling

Sample locations were nested within four watersheds inBNP that had been previously identified for a study onthe impacts of forest degradation on rainfall/stream-flowresponse (Krishnaswamy et al., 2006). A 1 km ð 1 kmgrid was laid over the entire park, and five grids wererandomly selected within each of the four watersheds.Soil and vegetation sampling were conducted at ten loca-tions within each grid for a total of 200 sample locations(sites), between November 2005 and March 2006. Usinga topographical sheet [Survey of India (SOI); 1 : 50 000scale] of the area, and a GPS unit, a four-wheel-drivejeep was used to drive to each selected grid on exist-ing trails. Each site was selected at random distances oneither side of the trails that intersected any part of theselected grid. Among other wildlife, elephants or signs ofelephants were encountered at 150 of the 200 locations,and a tiger was spotted on two occasions. Considerationsof safety prevented us from venturing more than 500 maway from the access trails within the forest. Figure 1shows the Digital Elevation Model (DEM) of the researchwatersheds, with the selected grids and sites overlaid.The DEM (90-m resolution) source for this dataset wasthe global land cover facility (http://www.landcover.org).Research watersheds spanned the elevation and sloperanges typical of the entire park, and within each water-shed selected grids spanned low and high elevations

(Figure 1). Details on watershed characteristics are avail-able elsewhere (Mehta et al., 2008, this issue).

At each of the 200 sites, a 10-m-diameter plotwas marked out. Shrub and tree vegetation of aboveone foot in height was inventoried within each plot.Species counts and heights were noted. Individual treediameter at breast height (DBH) was measured whengreater than 10 cm, and noted as below 10 cm other-wise. Average plot tree height, understory height andcanopy cover were recorded. Grasses were not inven-toried completely—only the dominant grass species andheight in each plot were noted. Dominant grasses wereeither Cymbopogon spp. or Themeda spp. Grass observa-tions were coded as a factor variable with four levels foruse in ordination as follows: G1 (Themeda spp, <3 fthigh); G2 (Themeda spp, 3–5 ft); G3 (Cymbopogonspp, <5 ft; and G4 (Cymbopogon spp, 5–7 ft). Vegeta-tion heights were measured using a Suunto PM-5 opticalheight metre (Suunti Finland, Vantaa Finland). Plot slopeswere measured using an Abney level (Lawrence & Mayo,Bangalore India). Tree canopy cover was measured usinga densiometer (Forestry Suppliers, Inc., Jackson, MI,USA). Observations of the field disturbance in the plotwere recorded and combined into a field disturbanceindex (FDI). FDI comprised of presence/absence obser-vations of five indicator variables: T—Trails (cattle orjeep); C—cut and/or broken stems; D—livestock dung;P—People; and F—Fire. For each plot, FDI was cal-culated as FDI D T C C C D C P C F. Indicator P wasassigned a value of 1 if people were sighted while sam-pling. The choice of indicator variables was informedby literature concerning anthropogenic impacts on veg-etation ecology in Indian forests (Shankar et al., 1998b;Kumar and Shahabuddin, 2005).

Data analysis

All analyses were conducted using the open-source Rstatistical software (R Development Core Team, 2006).Ordination was run in R using the VEGAN communityecology package (Dixon, 2003; Oksanen et al., 2007).

Vegetation structure. Average tree and understoryheights were calculated for each sampled vegetation plot.Density for each vegetation plot was calculated as thesum of all individuals per species per plot. Frequencywas the number of sites in which each species occurred.The mean density of each species (MD1), when it occurs,was calculated by dividing the species density by the fre-quency. A mean density (MD2) was also calculated overall 200 plots. To study the relationship of tree diame-ter with disturbance, individual tree species in each plotwere classified into ‘tree’ and ‘saplings’ if their DBHwas above or below 10 cm, respectively. This allowedtree density and sapling density estimation for each plot.Additionally, DBH frequency distributions for three dom-inant tree species were tested for differences betweendegraded and protected watersheds using Chi-square testson contingency tables. Effects of disturbance on vegeta-tion structure were investigated by testing the significance



of Pearson’s correlation coefficient (r) between grid-mean structure variables and grid-mean FDI. Grid-meanFDI was used as a continuous variable after ensuring thatit was approximately normally distributed.

Vegetation composition and diversity. Diversity (H0)was estimated using the Shannon index, which combinesthe number of species (richness) and evenness (Legendreand Legendre, 1998);

H0 D �i pi log2 pi; �1�

where pi is the proportional abundance of species i.Since only counts, but not the individual DBH of indi-

viduals <10 cm DBH was measured, we used frequencyof occurrence as a measure of the dominant species.

Effects of disturbance on diversity and compositionwere analysed in two steps. Sites were first clusteredusing K-means clustering using the FDI indicator vari-ables, into two groups (‘degraded’ and ‘protected’).K-means clustering is a non-hierarchical method that pro-duces a single partition optimizing within group homo-geneity (Legendre and Legendre, 1998). Differences indiversity between the two groups were analysed usingtwo sample, unequal, variance t-tests. Community com-position differences were assessed by calculating the rel-ative frequencies of the most dominant species withineach cluster group, supported by ordination.

Ordination. Partial canonical correspondence analysis,(pCCA), (Ter Braak, 1986; Palmer, 1993) was used tofurther elucidate the relationship of vegetation sites andspecies composition with disturbance (FDI) and soil gra-dients. Additional variables were projected post factoonto the ordination space to elucidate soil–vegetationlinkages with disturbance. CCA is a method of direct gra-dient analysis that allows a direct comparison betweencommunity and environmental data matrices (Legendreand Legendre, 1998). The ordination plot was constrainedby FDI and soil CEC after conditioning out possibleeffects of elevation. Among the soil data gathered forthis research, CEC was chosen in this analysis as it isa key property describing nutrient availability for plantgrowth, and was shown to be negatively influenced bydisturbance [Mehta et al. (2008, this issue)]. Significanceof the CCA model, each CCA axis and constraint terms(FDI and CEC) were assessed separately using permu-tation tests. Three variables—tree heights, soil clay andgrass cover—were projected post facto onto the ordi-nation space to assess their possible associations withcommunity data. The pCCA was performed on the meanvalues of community and environmental data, i.e. basedon grid-means of all species and environmental variables.Additionally, presence–absence transformation was per-formed, and all species that had a mean occurrence of<5 were removed in order to better interpret the majorstructure of the dataset.

RESULTS

Field disturbance index

Cut and/or broken stems(C) were most frequentlyobserved in 55% of the plots. Jeep or cattle trails (T) wereencountered in 40% of the plots. Signs of fire (F) wereobserved in 20Ð5% of the plots, cattle or cattle dung (D) in15Ð5% and people sighted (P) in only four of the plots.The combined disturbance variable FDI ranged from 0to 4—there were no plots in which all five indicatorswere observed. The livestock indicator (D) was proba-bly under-counted, because the dung deposited by live-stock inside the forest boundaries is removed by herdersand sold to coffee estates in the neighbouring district(Madhusudan, 2005). Similarly, people sightings (indica-tor P) were also probably under-counted because of theevasive behaviour of people who are illegally within thepark. The results were not affected by exclusion of thisvariable from the combined FDI disturbance index.

The distribution of FDI from the 50 samples ineach of the four watersheds indicates that the twonorthern watersheds (D1 and D2, mean FDI of 2Ð26and 1Ð98 respectively) are more disturbed than the twosouthern watersheds (P3 and P4, mean FDI of 0Ð42 and0Ð66 respectively). This was confirmed with K-meansclustering performed on FDI. Cluster 1 (total D 113)contained 91 out of the 100 sites sampled in the twonorthern watersheds (D1 and D2). Cluster 2 (total D87) contained 78 of the 100 sites sampled in the twosouthern watersheds (P3 and P4). As a result, the twonorthern watersheds were grouped together as ‘degraded’,and the two southern watersheds as ‘protected’. Thisbinary classification is used below to assess diversity andcomposition differences.

Species inventory, frequency and density

In all, 47 species (excluding grasses) were recorded,including 35 tree species. Themeda triandra and Cymbo-pogon citratus were the dominant grass species. As alsodocumented by Prasad and Hegde (1986), most speciesoccur at low frequencies. Only 5 species occur in morethan 50 plots, and 40 of the 47 species occur in less than30 plots. The observed species richness (no. of non-grassspecies) per plot ranged from 1 to 8 with a mean of4Ð7. The complete vegetation inventory along with localuses of each species is listed in (Mehta, 2007, AppendixB). Except for the invasives and two other species, allspecies are used locally for at least one, and most oftenfor several purposes.

Table I below lists the 12 species, including 8 treespecies, that occurred in more than 10% of the 200plots. The invasives, Chromolaena odorata and Lan-tana Camara occurred frequently throughout the forestunderstory. Among the tree species, A. latifolia, Emblicaofficinalis and T. grandis most commonly made up thetree layer. Few species occurred at high densities withina plot. The widespread tree species listed in Table I occurat low densities (MD1), whereas the weeds, C. odorataand L. camara, are both widespread as well as abundant.



Table I. Species with frequency greater than 10%.

ID Species Frequency (%) MD1 MD2

Understory (non-tree) speciessp1 Chromolaena odorata 135 (67Ð5) 9Ð1 6Ð12sp3 Canthium parviflorum 87 (43Ð5) 4Ð1 1Ð78sp2 Lantana camara 72 (36Ð0) 5Ð2 1Ð87sp5 Gymnosporia emarginata 27 (13Ð5) 3Ð1 0Ð42

Tree speciest1 Anogeissus latifolia 147 (73Ð5) 2Ð95 2Ð17t36 Emblica officinalis 77 (38Ð5) 2Ð04 0Ð79t3 Tectona grandis 45 (22Ð5) 1Ð62 0Ð37t4 Terminalia crenulata 37 (18Ð5) 2Ð22 0Ð41t2 Terminalia alata 26 (13Ð0) 2Ð08 0Ð27t6 Acacia chundra 26 (13Ð0) 1Ð89 0Ð25t23 Careya arborea 24 (12Ð0) 1Ð21 0Ð15t8 Dalbergia latifolia 22 (11Ð0) 1Ð5 0Ð17

The denser the vegetation in a plot, the more the num-ber of species (species richness) in the plot (Figure 2).Exceptions were plots 12 and 122, where only one(non-grass) species was found at high density, corre-sponding to clumped distributions of Schrebera spp. andPhoenix spp., respectively, found at these sites. Threetree species—t5 (Eucalyptus spp.), t11 (Schrebera spp.)and t33 (Phoenix spp.)—showed a particularly clumpeddistribution, occurring at low frequency but high density.One of the randomly sampled grids overlay a Eucalyptusplantation planted by the Forest Department, accountingfor its clumped distribution and the presence of this non-native tree species in the dataset.

Disturbance and structure

Tree heights, diameter and canopy cover. Table II sum-marizes the average plot heights and canopy coverover all 200 plots. Tree heights were on average9Ð5 m (C/� 3Ð9 m sd). Tree heights are significantlynegatively correlated with degradation (r D �0Ð7, p <0Ð005). Figure 3(a) shows the grid-means of tree heights

Figure 2. Species richness and total density. Dotted line is a lowesssmooth.

Table II. Summary of plot characteristics (n D 200 plots).

Variable Mean (1sd) Range CV (%)

Tree height (m) 9Ð5 (3Ð9) 2Ð7–19Ð5 41Ð0Understory height (m) 1Ð7 (0Ð48) 0Ð0–3Ð0 28Ð1Canopy cover 12Ð8 (9Ð9) 0Ð0–48Ð9 77Ð3CEC (cmol/kg)a 17Ð6 (7Ð87) 3Ð20–39Ð40 44Ð6% Soil organic carbona 2Ð44 (1Ð04) 0Ð08–4Ð92 42Ð6% Claya 22Ð0 (6Ð94) 6Ð60–38Ð40 31Ð5a Only soil properties used in Figure 7 are included.

against FDI. Understory heights showed no relation-ship with degradation. The understory in most degradedsites was a mix of thorny shrubs and regenerating trees<10 cm DBH of short stature, whereas in protected sitesthe understory was comprised of either only grass orweeds at low abundance. Figure 3(c) shows the positiverelationship between tree heights and canopy cover for all200 plots. As expected, tree heights are positively cor-related to canopy cover. Disturbance reduces the canopycover by replacing tall stature with short-stature foresttypes with lower canopy cover (Figure 3). Figure 4(a)and (b) compares tree densities (DBH >10 cm) and per-centage saplings (percentage of tree individuals <10 cm),respectively against FDI. Grid-means of tree densities andpercentage saplings are plotted against FDI, respectively.Both correlations (r D �0Ð77 and r D 0Ð80 respectively)are significant at the 5% level. Degraded sites have lowertree (DBH >10 cm) densities, and are dominated bysaplings (juvenile or regenerating individuals) when treespecies are present. Across all tree species, the stature ofthe plots, both in tree height and diameter, is reduced tomore degraded plots. There was no significant relation-ship between understory density and disturbance.

Size class distribution of dominant tree species. TableIII and Figure 5 display the DBH distribution for thethree tree species that were most dominant–A. latifolia,E. officinalis and T. grandis— grouping them accordingto degraded (D1 and D2) and protected (P3 and P4) for-est classes. A. latifolia is the most abundant tree speciesacross the gradient. Further, T. grandis (teak), a highlyvalued timber species, is more abundant in the protectedforest (62 individuals) than in the degraded forest (11individuals). E. officinalis is a valued non-timber forestproduct (NTFP) for its fruit. Its greater relative abundancein the protected forest likely reflects more frequent inter-ference to its growth in the degraded forest by lopping ofits branches for fruit. Among these three tree species, thefrequency distribution of DBH classes was significantlydifferent at the 5% level between the degraded and pro-tected watersheds in the case of A. latifolia (�2 D 95,df D 3; three DBH classes used : <10 cm, 10–20 cm,20–40 cm). In the case of E. officinalis and T. grandis,although the total abundance was greater in protectedwatersheds, the size class distribution, (proportional tothe total number of trees within each group) was not sig-nificantly different between the groups at the 5% level.



Figure 3. Tree heights and canopy cover. Error bars are 1SE (colour on-line).

Figure 4. Tree density and sapling density against FDI (colour on-line).

Table III. Size class distribution of dominant tree species.

DBH (cm) Class 1—degraded Class 2—protected

A. latifolia E. officinalis T.grandis A. latifolia E. officinalis T. grandis

<10 242 56 6 86 70 2810–20 15 7 0 74 16 520–30 0 0 1 16 7 1430–40 0 0 2 1 0 1140–50 0 0 1 0 0 250–65 0 0 1 0 1 2More 0 0 0 0 0 0Total 257 63 11 177 94 62



(a)

(b)

Figure 5. DBH classes for three dominant tree species. (a) Northern watersheds; (b) southern watersheds.

Disturbance and diversity

Species diversity H0 across the 200 sites ranged from 0to 2Ð836 with a mean of 1Ð75 and was approximatelynormally distributed overall. These estimates are compa-rable to H0 estimated in previous studies in other parts ofBNP (Elouard and Krishnan, 1999; Madhusudan, 2000).Significant differences emerged between degraded andprotected watersheds. Figure 6 shows the boxplots ofH0 by degradation class. Using a two-sample, unequal,

variance t-test, diversity H0 was significantly higher inthe degraded watersheds than in the protected watersheds(t D 4Ð53, p < 0Ð001, n D 100 plots each). In Figure 6(a)all 47 species were included. Further insight is obtainedby computing H0 separately for tree and non-tree (under-story) species. Figure 6(b) and (c) shows boxplots of H0

for tree and understory species respectively. There was nosignificant difference in diversity of tree species betweenthe two watersheds. However, in the case of understory

Figure 6. Boxplots of Diversity H0. 1—degraded; 2—protected (a) All 47 species; (b) only tree species; (c) only understory species.



species, species diversity was significantly higher in thedegraded watersheds (class 1) than the protected water-sheds (w D 6698, p < 0Ð001, Wilcoxon rank-sum test).The greater overall diversity in degraded forest is a con-sequence of greater understory diversity compared to theprotected forest.

Disturbance and vegetation composition

Table IV presents a two-way table of relative frequenciesof species present in degraded and protected watersheds.Only those species that had a frequency of at least 10%in either watershed class are included. These relativefrequencies are combined with the pCCA results givenbelow to elucidate composition differences.

CCA results. Figure 7 shows the pCCA ordination plot.Since mean data (means calculated from 10 samples pergrid) and species with frequency >5 were used, there are20 sites (corresponding to the 20 sample grids) and 21species in the ordination plot. The 20 sites are colour-coded by watershed as in previous figures. Directions ofthe biplot vectors (FDI and CEC) allow interpretationof the CCA axes. CCA1 is interpreted as a degradation

Table IV. Relative frequencies of species within degradationclass.

IDa Species 1b 2b

sp1 Chromolaena odorata 0Ð51 0Ð84sp2 Lantana camara 0Ð54 0Ð18sp3 Canthium parviflorum 0Ð56 0Ð31sp5 Gymnosporia emarginata 0Ð27 0Ð0sp7 Ziziphus oenoplia 0Ð12 0Ð01sp11 Phyllanthus reticulatus 0Ð02 0Ð15t1 Anogeissus latifolia 0Ð71 0Ð76t2 Terminalia alata 0Ð1 0Ð16t3 Tectona grandis 0Ð09 0Ð36t4 Terminalia crenulata 0Ð15 0Ð22t5 Eucalyptus spp. 0Ð1 0Ð0t6 Acacia chundra 0Ð26 0Ð0t7 Ziziphus xylopyrus 0Ð1 0Ð03t8 Dalbergia latifolia 0Ð05 0Ð17t11 Schrebera swietenioides 0Ð0 0Ð18t16 Premna tomentosa 0Ð04 0Ð1t17 Grewia tiliifolia 0Ð1 0Ð03t21 Chloroxylon swietenia 0Ð14 0Ð0t23 Careya arborea 0Ð16 0Ð08t34 Erythroxylon monogynum 0Ð15 0Ð0t36 Emblica officinalis 0Ð31 0Ð46

a prefix ‘sp’—understory species; ‘t’—tree species.b 1—degraded; 2—protected.

Figure 7. pCCA ordination plot. Grid-mean data with symbols coded by watershed. Symbols—sites; italic text—species; Solid arrows—constraintvectors; dashed arrows—fitted vectors; G1 to G4—centroids of grass factor variable (colour on-line).



gradient (increasing in the negative CCA1 direction).CCA2 is correlated with soil CEC. Table V reports themodel summary and significance tests based on 1000permutations. The constrained axes (CCA and CCA2)together account for 23% of the total variability (inertia ormean square contingency coefficient) after conditioningout the effect of elevation. The overall model testedusing permutation tests for the eigenvalues is significant(Table V part b). On the basis of permutations testsfor significance of individual axes and terms, CCA1 ishighly significant. However CCA2 (correlated to CEC)is not significant at the 10% level. This means that afterconditioning out the elevation effect, CEC is a weakgradient compared to disturbance (FDI) in explaining themajor variation in the community data. Nevertheless, itis retained in the ordination plot because CEC was foundto be highly significant when all 200 plots were used inthe pCCA (not shown). Only the grid-mean data resultsare reported in Table V.

The ordination plot confirms that degradation level(FDI or CCA1) is a very good discriminator betweensites that are in the northern degraded watersheds andsites in the southern protected watersheds. All grids inthe degraded watersheds (black and red open symbols)are located in the left half of Figure 7, while all gridsin the protected watersheds are located in the righthalf. Projection of sites (species), onto any vector inthe plot, allows a comparison between sites (species)along that vector. In the case of the degraded watersheds,projection of sites onto CEC, clay and tree height vectorsallows further discrimination within the degraded sites.Sites in watershed D1 have less soil CEC, averagetree heights and soil clay than those in watershed D2;

Table V. pCCA results.

pCCA model resultsa. Model summary

Inertia Total (%)Total 0Ð8307 100Conditional 0Ð1242 15Ð0Constrained 0Ð1926 23Ð2Unconstrained 0Ð5139 61Ð9Eigenvalues for constrained axes

CCA1 CCA20Ð1408 0Ð0518

b. Permutation tests for significance (1000 permutations)df �2 F Pr > F

1. Overall 2 0Ð1926 2Ð99 <0Ð0052. Individual terms

FDI 1 0Ð141 4Ð38 <0Ð005CEC 1 0Ð052 1Ð62 0Ð109

3. CCA axesCCA1 1 0Ð141 4Ð38 <0Ð005CCA2 1 0Ð052 1Ð62 0Ð131

4. Correlation of fitted vectors with CCA axesCCA1 CCA2 r2 Pr > r

Clay 0Ð656 �0Ð754 0Ð5019 <0Ð005Tree height 0Ð85 �0Ð526 0Ð6647 <0Ð005G: G1 �0Ð922 0Ð2803 0Ð299 0Ð044G2 �0Ð4048 �0Ð3721G3 0Ð4685 0Ð0895G4 1Ð3243 �0Ð2137

hence D1 can be rated as more degraded than D2.This internal discrimination does not appear betweenthe two protected watersheds P3 and P4. The directionsof (post facto fitted) clay, tree height and SOC vectorsshow them to be closely related to each other andto CEC. Projections of the sites onto these vectorsconfirm earlier findings that the protected sites alsoexhibit greater average tree heights (this paper), soilclay contents and organic carbon (Mehta et al., 2008,this issue) compared to the degraded sites. Reducedclay content in disturbed sites is attributed to increasedvulnerability of soils to erosion upon reduction of canopycover with increasing disturbance (Figure 3(c)), withconsequent selective removal of finer clay particles.

CCA1 being a degradation gradient, species composi-tion differences emerge on either side of the CCA1 originwith degraded sites to the left (CCA1 <0) more popu-lated by species that are also to the left. Hence, as alsoshown in Table IV earlier, the degraded sites are morecommonly populated by tree species t17 and t6 than theprotected sites, with tree species t3 and t8 more preva-lent in protected sites. As for shrub species, sp5 and sp7are more prevalent in degraded sites compared to pro-tected sites, whereas sp11 is more prevalent in protectedsites. Those species that do not show discrimination alongthe degradation gradient appear closest to the origin ofthe pCCA plot. Projecting the levels of the grass factorvariable, it is evident that the most degraded sites haveshort Themeda spp. cover (G1); moderately degradedsites have tall Themeda spp. cover (G2); moderately pro-tected sites have short Cymbopogon spp. cover (G3) andwell protected sites have tall Cymbopogon spp. cover(G4).

On the basis of the pCCA results and Table IV,the following community characteristics and differencesbetween protected and watersheds are summarized. Thedominant trees in degraded watersheds are A. latifolia,E. officinalis, Acacia chundra and Terminalia spp. Theunderstory is dominated by the weeds C. odorata, L.camara and native Canthium parviflorum; which arealmost equally dominant.

The dominant trees in protected watersheds are A. lat-ifolia, E. officinalis, T. grandis and Terminalia spp. Theunderstory is dominated by C. odorata, C. parviflorumand L. camara, with C. odorata more widespread thanthe others. The contrasts between degraded and protectedforests are as follows. In tree species, (i) degradedforests had less widespread teak, (ii) degraded forestshad several small woody tree species in moderatefrequency—Chloroxylon swietenia , Careya arborea,Erythroxylon monogynum , Ziziphus xylopyrus —compared to protected forests in which these specieswere almost absent, (iii) protected forests had a mod-erate frequency of large woody trees—Dalbergia latifo-lia and Schrebera swietenioides —compared to degradedforest which had practically none. In understory species,(i) Gymnosporia emarginata and Ziziphus oenoplia weremoderately widespread in degraded forests, compared to



almost no presence in protected forests; (ii) the herba-ceous Phyllanthus reticulatus , was more frequent in pro-tected forests compared to degraded forests.

DISCUSSION AND CONCLUSIONS

In BNP, disturbance quantified as a combined impactof grazing, fuelwood/fodder extraction and fire are sig-nificantly related to forest structure, composition anddiversity. Tree heights, densities and diameters wereless in more degraded sites. There was no significantrelationship in understory heights and shrub densitieswith disturbance. A reduced recruitment of teak and anincreased mortality of A. latifolia in bigger size classesare evident in degraded watersheds. Species composi-tion and diversity differences exist. Degraded forestshad greater diversity, attributed to greater diversity inshrub species. Species assemblages in degraded forestsshowed a greater presence and density of small woodytree species, and greater numbers of understory species.The increasing dominance and diversity of small woodyspecies, along with shrubaceous species that bear thornystructures (e.g. G. emarginata, Z. oenoplia) has beenreported in other dry tropical forests in India (Pandeyand Singh, 1991; Shankar et al., 1998b; Kumar and Sha-habuddin, 2005; Madhusudan, 2005). Invasives, C. odor-ata and L. camara, are widespread and abundant through-out the forest, indicating that ‘protected’ and ‘degraded’classifications are relative—‘protected’ southern forestedareas are also disturbed, but less so than northern areas.A. latifolia (family Combretaceae) is widespread through-out the forest, and teak individuals as well as Terminaliaspp. are also present in degraded areas (albeit not abun-dantly). This supports the published vegetation maps weused as baseline information as well as local and forestdepartment accounts that the entire study area was oncecovered with dry deciduous woodland to savanna wood-land forests of the A. latifolia–T. grandis–Terminaliaspp. type.

In dry tropical forests of the Western Ghats, intenseovergrazing and lopping has been theorized to convertthe tree savanna into ‘clump thickets’, with the selec-tion of thorny and unpalatable species (Pascal, 1988p291–293). This study provides field evidence in sup-port of this theory in BNP. The removal of dominant treespecies for timber/fuelwood opens the canopy allowingmore species to compete. Following continual biomassextraction and further aridification of the soils and micro-climate, unpalatable, small, woody and thorny speciesbecome more dominant.

Feedbacks and implications for services

Field evidence shows that forest degradation in BNP hassubstantial impact on the entire forest ecosystem, throughits impact on forest structure, diversity and composition(this paper); and soil physical, nutrient and hydrauliccharacteristics Mehta et al. (2008, this issue). Nutrient

and water availability are key resources for plant produc-tivity and quality (Tilman and Lehman, 2001; Sankaranand McNaughton, 2005). The negative impacts on bothrepresent a negative feedback on regeneration. With theconsequent (possibly irreversible) change in vegetationcomposition towards unpalatable species and a reduc-tion in grass cover, the feedback towards continuationof grazing and forage services currently enjoyed by localpopulations is also negative. In their related study, Mehtaet al. (2008, this issue) discuss the negative impacts ofdisturbance on hydrological services to downstream agri-culture. Evidence points to increased vulnerability to boththe ecosystem as well as people (Carpenter et al., 2006a).We note that greater diversity in degraded forests illus-trates that anthropogenic disturbance can increase biodi-versity, but still impair ecosystem functioning. Therefore,from a management perspective, the use of a diversityindex alone as an indicator of ecosystem health is notadvisable. Further, diversity indices are rarely applied tothe whole community and do not address functional dif-ferences among species (Krebs, 1999).

Research results point towards potential impacts onother aspects of ecosystem function. Reduction in canopycover by replacement of tall stature forest by openshrubaceous vegetation also impacts boundary-layer cli-matology and rainfall partitioning (Giambelluca, 2002).Reduced soil carbon in degraded sites suggests losses tosoil carbon sequestration services.

Dry tropical forests account for some 60% of the Indianforest cover (WRI, 1996) and more than 70% of the West-ern Ghats forests (Kodandapani et al., 2004). In the pastdecade, several studies on floristic structure, composi-tion and impacts of forest disturbance have begun fillingthe gap in ecological knowledge that existed previously.However, inter-disciplinary studies on ecosystem func-tioning that include energy, water and nutrient cyclingare necessary for a comprehensive understanding on localand regional ecosystem change impacts in a region thatforms the major watershed for southern India and is expe-riencing considerable forest cover loss and fragmentation(Menon and Bawa, 1998; Jha et al., 2000; Amarnathet al., 2003). Coordinated ecological experimental pro-grammes (e.g. BIODEPTH in European grasslands (Hec-tor and Bagchi, 2007) ), will have to be conducted, alongwith a network of long-term environmental monitoringstations. Efforts are under way to set up a coordinatednetwork of terrestrial, atmospheric and marine monitor-ing stations for India called INDOFLUX, along the linesof FLUXNET (Sundareshwar et al., 2007), that will beginaddressing the environmental data gap. Simultaneously,ecological and socioeconomic research will need to beintegrated to effectively inform forest management policy(Carpenter et al., 2006b). Forest management response touses and impacts described here call for options that meetlocal energy and livelihood needs while preserving forestecosystem health. Alternative sources of local energy inthe form of options including biogas and social forestry,and attempts by the Forest Department in Bandipur to



distribute cooking gas cylinders to villages are conceiv-ably viable options. However, the linkage of grazing andmanure collection to export-oriented coffee markets ischanging the traditional outlook of local forest use aslocal subsistence practice. Although fuelwood extractionmay be addressed through alternative energy schemes,the response to grazing pressures under the existing sce-nario may well involve a combination of forest protectionand the provision of alternative economic opportunity inthe long term. At the time of writing this, researcherswith a partner institution in India (Lele, pers. comm.) areanalysing the socioeconomic aspects of ecosystem changein the Bandipur region.

ACKNOWLEDGEMENTS

This research was financially supported by grants fromthe International Foundation for Science (IFS, Sweden);Ford Foundation with UNESCO; Cornell University’sCenter for the Environment and Bradfield Awards. Wethank the Karnataka Forest Department for researchpermissions, and the Ashoka Trust for Research inEcology and the Environment (ATREE) Bangalore, forresearch and logistic support. International travel supportwas provided by Cornell University’s Einaudi Centerfor International Studies and the South Asia Program.We are grateful for assistance in field logistics fromN. Samba Kumar and the Centre for Wildlife Studies,Bangalore. Special thanks to local field assistant, MrDasappa, and to Kiran M.C., Kiran Yadav, Rakesh K.N.and Dr Pradeep Joshi for research assistance. We aregrateful to Dr R. Ganesan for help with taxonomy andlocal uses of vegetation inventoried. We express thanks toDr Hugh Gauch Jr for ordination advice and constructivecomment on the manuscript.

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