Impacts of human disturbance in tropical dry forests increase with soil moisture stress

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1 28 (217) Impacts of human disturbance in tropical dry forests increase with soil moisture stress R. K. Chaturvedi, A. S. Raghubanshi, K.W. Tomlinson& J. S. Singh Keywords Browsing; Drought; Harvesting; Mortality; Recruitment; Tropical dry forest Abbreviations AMI = annual mortality index; ARI = annual recruitment index; D = stem diameter (cm); H = plant height (m); SMC = soil moisture content (%); TDF = tropical dry forest. Nomenclature Duthie ( ) Received 21 October 214 Accepted 6 April 217 Co-ordinating Editor: Leandro Duarte Chaturvedi, R.K. (corresponding author, ravikantchaturvedi1@gmail.com; ravi@xtbg.ac.cn) 1, Raghubanshi, A.S. (raghubansh@gmail.com) 2, Tomlinson, K. (kyle.tomlinson@xtbg.org.cn) 1, Singh, J.S. (singh.js1@gmail.com) 3 1 Centre for Integrative Conservation, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun 66633,Yunnan, China; 2 Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi 2215, India; 3 Ecosystems Analysis Laboratory, Department of Botany, Banaras Hindu University, Varanasi 2215, India Abstract Questions: (1) Which disturbances (harvesting, browsing, drought and fire) are important for tree mortality in a tropical dry forest (TDF), and do these change with size class (juvenile, sapling, adult)? (2) How is mortality and recruitment in a TDF related to soil moisture content (SMC) and does the relationship change for different size classes? (3) Is disturbance-related mortality selectively removing particular species from the TDF? Location: Sonebhadra and Mirzapur districts, Uttar Pradesh, India. Methods: We analysed the structure of a TDF in central India in terms of the tree composition of juveniles, saplings and adults at five distinct sites located along a gradient of SMC, and recorded the numbers of individuals in each size class killed by the four disturbance types over 2 yr. We also recorded total stem density and recruitment at each site. We compared annual mortality index (AMI) and its four disturbance components (harvesting, browsing, drought and fire) and annual recruitment index (ARI) against the mean SMC of each site using GLMM. Results: The impact of all disturbances on total AMI decreased as SMC increased whereas ARI increased as SMC increased. Mortality due to harvesting was substantially higher than other disturbances for adult and sapling trees, whereas both harvesting and browsing were important drivers of mortality for juveniles. There was little evidence that particular species were being deliberately selected for harvesting across sites. Conclusions: Tree saplings and adults in this TDF were mainly killed by harvesting, indicating that anthropogenic impacts on tree mortality are more important than non-anthropogenic impacts in the TDF, and impacts of all disturbances become more severe with increasing water stress. Thus changes in TDF structure due to harvesting are likely to be more rapid in more arid environments. Introduction Determining the causes of the tropical forest deforestation and degradation is a prerequisite for understanding how to manage them better (Salvini et al. 214; Morales-Barquero et al. 215). The global forest change quantification from 2 to 212 showed that among the four climate domains (tropical, subtropical, temperate and boreal), the tropical domain exhibits the highest ratio of loss to gain of total forest (3.6 for >5% of tree cover; Hansen et al. 213). Moreover, only the tropics exhibited a statistically significant trend in annual forest loss, at the rate of 211 km 2 yr 1 (Hansen et al. 213). In South and Southeast Asia, the rate of net forest loss was 25% higher between 21 and 215 compared to that in 199s (Keenan et al. 215). It has been reported that disturbances in tropical forests, mostly due to human activities, are responsible for the major land use-derived atmospheric CO 2 additions across the globe (Houghton 23; Lambin et al. 23; Elmqvist et al. 27; Grace et al. 214). To prioritize conservation of tropical forests it is critical to understand how sensitive they are to human impacts. Tropical dry forests (TDF) are subject to severe anthropogenic pressure (Miles et al. 26) and have experienced the third Doi: /jvs International Association for Vegetation Science 997

2 Importance of human disturbance in dry forests R. K. Chaturvedi et al. highest loss in forest cover, after boreal forests and humid tropical forests (Kalacska et al. 24; Sanchez-Azofeifa et al. 25; Hansen et al. 21; Griscom & Ashton 211). Human settlements are commonly found in TDF because they occur in favourable climatic conditions and lands are easy to clear (Murphy & Lugo 1986; Ewel 1999). In India TDF account for 38.2% of the total forest area (Ministry of Environment and Forests, India reports 1999) and support 81% 1% of household fuel needs and 8% 95% of fodder needs of the livestock of the country (Singh & Singh 211). Thus these forests provide crucial and substantial resources to local communities and need to be preserved. An important process that can contribute to forest degradation is alteration to the disturbance regime of forests. A wide range of disturbances of variable frequency, intensity and duration impact tropical forests (Chazdon 23), and impacts differ according to tree size. Small-sized trees are thought to be more sensitive to environmental changes than mature trees (Jarvis 1995), and even under ideal conditions, mortality rates of smaller trees are much higher than those of mature trees (Grogan et al. 211). Disturbances are often selective and therefore can cause shifts in species composition of forests (Kennard et al. 22), while excessive disturbance may lead to loss of forest cover (Leiva & Fernandez-Ales 23; Quezel & Medail 23; Plieninger et al. 24; Dufour-Dror 27). Natural disturbances in tropical dry forests include drought, fire and herbivory. TDF are subject to long dry seasons each year of 5 8 mo with little or no rain (Pan et al. 213) and experience dry spells during the wet season. Drought can cause mortality of trees, especially juveniles (Vieira & Scariot 26). The long dry seasons also dry out the herbaceous vegetation, which makes it flammable and, consequently, fires occur frequently in TDF (Ratnam et al. 216). Although local tree species are adapted to fire (Vieira & Scariot 26; Markesteijn & Poorter 29), mortality under fire is greater among small trees than large trees (Swaine 1992; Eriksson et al. 23) because defences such as bark thickness increase with plant size (Backeus et al. 1994; Eriksson et al. 23). Finally, TDF are frequented by large mammalian herbivores (Chaturvedi et al. 212; Chaturvedi et al. 217), which may impose substantial and selective herbivory pressure on juvenile and sapling trees (Chaturvedi et al. 212; Chaturvedi et al. 217). Human activities alter natural disturbance regimes and impose additional types of disturbance that can cause tree juvenile mortality. Humans increase fire frequencies (Pausas & Ribeiro 213; Knorr et al. 214) and retain livestock at very high densities, and thus increase herbivory pressure above natural levels (Chaturvedi et al. 212). Additionally, humans actively harvest trees, either for fuel or building materials (low quality hardwood species) or for commercial gain (high quality hardwood species; Singh & Singh 211). This process is also selective and can presumably alter long-term species composition in forests (Zida et al. 27). In contrast to the above-mentioned non-anthropogenic disturbances, which may have more severe effects on smaller size classes, harvesting is likely to be targeted to larger size classes. The sensitivity of forests to disturbance impacts may differ as a function of other environmental variables. One environmental parameter which may alter the impact of disturbance is soil moisture content (SMC). Where SMC is high, plants may be able to regenerate quickly following disturbance and forests may sustain higher numbers of individuals that re-occupy available space more quickly than at lower SMC where growth rates are slower and plant density is lower. Under low SMC, the lower density of individual stems may lead to greater impacts of human disturbance. This all means that forests growing on drier soils are likely to be more sensitive to the impacts of natural and human disturbance than those on moister soils. In this study we evaluated how the severity of four disturbance types (harvesting, browsing, fire and drought) change in tropical dry forest fragments in central India arrayed across a soil moisture gradient. We asked the following questions: (1) Which disturbances (harvesting, browsing, drought and fire) are important for tree mortality in the TDF, and do these change with size class (juvenile, sapling, adult)? (2) How is mortality and recruitment in a tropical dry forest (TDF) related to SMC, and does the relationship change for different size classes? (3) Is disturbance-related mortality selectively removing particular species from the TDF? We expected the relative importance of different disturbances to change with tree size class such that harvesting would be more severe in larger size classes. We expected that disturbance impacts would be more severe at the drier end of the soil moisture spectrum. Methods Study area The study was conducted in the Vindhyan tropical range of India. The climate is tropical monsoon, with three seasons in a year, viz. dry hot (April to mid-june), wet hot (mid- June to September) and dry cool (November to February). Thus there is an extended dry period of about 9 mo (October to mid-june) in the annual cycle (Chaturvedi & Raghubanshi 214). The mean maximum monthly temperature ranges from 2 C in January to 46 C injune, and the mean minimum monthly temperature ranges 998 Doi: /jvs International Association for Vegetation Science

3 R. K. Chaturvedi et al. Importance of human disturbance in dry forests from 12 C in January to 31 C in May.About 85%of the annual rainfall occurs during the wet hot season from the south-west monsoon (Chaturvedi et al. 211a). We selected five tropical forest fragments in the Vindhyan highlands ( N, E). Among the five forest fragments, Kotwa is in Mirzapur district and Ranitali, Harnakachar, Gaighat and Hathinala sites are situated in Sonebhadra district of Uttar Pradesh, India (Fig. 1). The land areas occupied by these forest fragments are 199, 2118, 157, 394 and 2555 ha, respectively. The selected sites had different soil water regimes (Chaturvedi & Raghubanshi 214). According to data collected from the meteorological stations of the state forest department for , mean annual rainfall ranges from 865 mm (Kotwa site) to 1196 mm (Hathinala) (Chaturvedi et al. 211a). These forests have been traditionally managed through selective felling, i.e. harvesting of individuals above a certain diameter, which varies from species to species, and leaving a few mother trees for regeneration (Singh & Singh 211). Soil physico-chemical characteristics of the five sites are reported in Chaturvedi et al. (211a). The annual average SMC (% by volume) of the five sites, as ordered from lowest to highest, are: Kotwa: 6.4%; Ranithali: 7.8%; Harnakachar: 1.2%; Gaighat: 11.1% and Hathinala: 13.%. The forests on all the five sites have experienced disturbance in the form of grazing and illegal harvesting of trees by local people, even below the acceptable diameter (Singh & Singh 211). Low intensity ground fire occurs every 2 3 yr at all sites (Singh & Singh 211). Study design At each of the five sites, we randomly chose nine coordinates and established nine plots, each of 1 m 2 (5 m 9 2 m), for sampling the trees. The plot size was decided on the basis of species area curves following the minimal area concept (Mueller-Dombois & Ellenberg 1974). For individuals <3 m height (H), stem diameter (D) was measured 1 cm above the ground and away from stem Fig. 1. Map showing the location of study areas (reproduced from Chaturvedi et al. 211a). [Colour figure can be viewed at wileyonlinelibrary.com] Doi: /jvs International Association for Vegetation Science 999

4 Importance of human disturbance in dry forests R. K. Chaturvedi et al. irregularities, and for others having 3 m H, D was measured at 1.37 m above the ground. In this study, all woody species having D.5 to <3.2 cm were considered as juveniles. Saplings were considered as having D 3.2 to <9.6 cm and the individuals having D 9.6 cm were considered as adults. In each plot, all individuals were counted, measured for stem diameter (D) and marked with permanent paint at the point of D measurement. In each site, we measured D for all the individuals four times per year for 2 yr at an interval of 3 mo from Jul 25 to Jun 27. Measurement of individuals <4 cmd was done with the help of Vernier callipers to the nearest.1 mm, and others having 4 cm D were measured using metal dendrometer bands fitted at the point of D measurement (Cattelino et al. 1986; Keeland & Sharitz 1993). To measure H, we used a 15 m graduated measuring pole (Clark & Clark 21) one person holding the pole, a second acting as spotter to assess the mark on the pole which reached the level of the top of the crown. Stems that had died were assigned to four disturbance categories: (1) harvested by humans, as shown by stumps; (2) heavily browsed, indicated by physical damage to twigs and leaves; (3) potentially killed by drought with no apparent physical damage to the aerial parts; and (4) died in fire. With regard to the third cause of mortality, drought, it is possible that these seedlings might have died due to other causes such as root pathogens, but we think that given that water stress is major part of these systems, drought is likely to have been involved in these deaths. The trees which died after fire might also have been previously heavily browsed or killed by drought, but were considered to have died in fire if the occurrence of fire was observed. During each measurement period, the number of trees in each of the four disturbance categories was recorded in each plot. Numbers of new individuals growing into each size class were also counted during each measurement period in each plot. These individuals were marked with permanent marker and considered as new individuals for subsequent measurements. Annual recruitment index (ARI) in a given size class was calculated as the proportion of new individuals entering the size class over a given year. Annual mortality index (AMI) was calculated as the proportion of individuals killed during a given year. Mortality due to harvesting, browsing, drought and fire were similarly calculated as the proportions of individuals in a given size class that were killed by that disturbance agent during the given year. We averaged all values across the 2 yr of measurement. In all plots at all study sites, soil moisture content (SMC) was measured every month at ten random locations as percentage by volume at 1-cm depth using a theta probe (type ML 1; Delta-T Devices, Cambridge, UK). For each plot, all SMC values measured across a year were averaged to obtain SMC per plot and the SMC vlaues across plots at a site were averaged to obtain SMC per site. Data analysis Dominant species in the three size classes were calculated on the basis of importance value index (IVI; Curtis & McIntosh 1951). All response data was fractional and therefore analysed using a binomial model in a GLM framework. Interactions and main effects were tested using likelihood ratio tests (LRTs). Parameter estimates were evaluated using Wald tests. As data were nested (groups inside plots inside sites) all regression analyses were conducted using the glmer() function ( lme4 package) in R (R Foundation for Statistical Computing, Vienna, AT). To understand which disturbances were most important for our TDF and how these changed with size class (Question 1), we compared mortality rates due to each disturbance across the five sites using a mixed model, where disturbance type and size class were treated as interacting fixed effects nested within plots and nested within forest sites (thus plots and sites were random effects; n = 54). We graphed the chosen model to visualize the patterns. To understand whether AMI, ARI and the mortality rates for each of the four disturbance types were changing across the SMC gradient and if these were different for different size classes (Question 2), we modelled each of these responses as a function of site SMC and size class (n = 135) nested within plots and within forest sites. To visualize these effects, we graphed their observed and predicted values using ggplot2 in R (Figs. 3, 4). To understand which species are being selectively removed by harvesting (Question 3), we plotted total stem number per hectare and harvesting mortality indices for all species recorded in each size class at a given site and visually assessed which species were being selectively removed by disturbance. Results Dominant species differed across drier to wetter sites. Acacia catechu, Lannea coromandelica and Nycthanthes arbor-tristis are dominant at the drier sites, whereas Shorea robusta is dominant at the wetter sites (Table 1). However, at the juvenile stage, Diospyros melanoxylon is dominant across most sites (Table 1). Relative importance of different disturbance types for tree mortality There was a significant interaction between size class and causes of mortality (LRT comparing interaction 1 Doi: /jvs International Association for Vegetation Science

5 R. K. Chaturvedi et al. Importance of human disturbance in dry forests Table 1. Tree densities and dominant species in each size class and quadratic mean diameter (QMD) of adult trees in the study sites. Site Parameter Adult Sapling Juvenile Hathinala Tree density (ha 1 ) 467(145) 1383 (425) (3913) Dominant species Shorea robusta Shorea robusta Diospyros melanoxylon QMD (cm) 23. (2.19) Gaighat Tree density (ha 1 ) 335(67) 762 (14) 9178 (1451) Dominant species Shorea robusta Shorea robusta Diospyros melanoxylon QMD (cm) 2.3 (1.59) Harnakachar Tree density (ha 1 ) 339(83) 662 (162) (2349) Dominant species Shorea robusta Shorea robusta Diospyros melanoxylon QMD (cm) 21.3 (1.67) Ranitali Tree density (ha 1 ) 22(49) 524 (132) 78 (166) Dominant species Acacia catechu Acacia catechu Acacia catechu QMD (cm) 18.7 (1.15) Kotwa Tree density (ha 1 ) 13(42) 33 (65) 2767 (52) Dominant species Lannea coromandelica Nycthanthes arbor-tristis Nycthanthes arbor-tristis QMD (cm) 14.7 (1.6) and additive models: P <.1, marginal R 2 =.879 for interaction model; Fig. 2). Mortality of adults and saplings was mostly caused by harvesting whereas mortality of juveniles was mostly due to browsing or harvesting. Deaths due to harvesting were much higher for adults and saplings than for juveniles. Mortality due to browsing increased significantly from large to small size classes. Mortality due to drought and fire were very low for all size classes, but mortality due to drought was higher for saplings than for either adults or juveniles. Mortality and recruitment across the soil moisture gradient There was a significant interaction between size classes and SMC on AMI (LRT comparing interaction and additive models: P <.1, marginal R 2 =.85 for interaction model; Fig. 3). Overall AMI decreased as SMC increased (z-value: P <.1) but the slope of the relationship was steepest for adults, then saplings, then juveniles. There was no significant interaction between size classes and SMC on ARI (marginal R 2 =.95 for additive model). Inversely to mortality patterns, proportional tree recruitment increased as soil moisture increased (z-value: P <.1) and proportional recruitment was significantly higher for saplings than adults (z-value: P <.1), and higher for adults than juveniles (z-value: P <.1). Among individuals killed by harvesting only, there was a significant interaction between SMC and size class (LRT: P <.1, marginal R 2 =.158 for interaction model; Fig. 4). Harvest mortality fraction of all size classes decreased as SMC increased, but the slope of the relationship was steeper for adults than for saplings or juveniles. Because mortality due to browsing was very low and mortality due to both drought and fire were very rare for all Fig. 2. Annual mortality indices assigned to four different disturbances in three tree size classes averaged across five tropical dry forest fragments in India. Results were derived from a model testing percentage mortality as a function size class by disturbance type. Mean values and 95% CI are based on the raw data. Same letters indicate no significant difference between groups under the tested linear model. Doi: /jvs International Association for Vegetation Science 11

6 Importance of human disturbance in dry forests R. K. Chaturvedi et al. 75 AMI (%) ARI (%) Fig. 3. Annual recruitment index (ARI) and annual mortality index (AMI) in three tree size classes across annual average soil moisture content (SMC). Raw data points and 95% CI around fitted regression lines are shown. [Colour figure can be viewed at wileyonlinelibrary.com] groups, the proportions being estimated were very small and the data contained many zeros for these groups. Thus, the glmer() algorithm was unable to produce reliable effect estimates for browse, fire and drought groups, and so no analysis is presented for these disturbances. Species being selectively harvested across the TDF There was little evidence that harvesting was killing particular species more frequently than others across sites (Appendices S1 S5), although usually some species suffered >2% mortality among adult stems and sapling stems and many species exceeded 1% mortality during the 2 yr of measurement. Among adult and sapling trees the species that had >2% mortality rates were usually unique to each site. Only Phyllanthus emblica had >2% stem mortality at two locations for adult trees, only Holeoptelea integrifolia had >2% stem mortality at two locations for sapling trees. Very few juvenile trees had >1% mortality due to harvesting at any site. The results suggest that stem harvesting is mostly proportional to abundance rather than selective. Discussion Disturbance effects at different size classes Trees generally experience severe disturbance by biotic and abiotic environmental factors at every stage of the life cycle (Clark & Clark 1991), although there is a dramatic decrease in mortality rates beyond the seedling stage (e.g. Welden et al. 1991; De Steven 1994). In our study, we observed differences in mortality threats at the juvenile, sapling and adult tree stages at all five study sites. Juveniles died mostly due to browsing, whereas harvesting was the most important factor for the death of saplings and adults. Impacts of harvesting were generally higher for largersized trees (Tredennick & Hanan 215), which simply reflects that trees are harvested for structural material and fire, both of which require larger stems. Browsing reduces the survival and growth of juveniles because browsers can easily access trees in this size class (<1 m).browsingalso results in biomass loss and delayed maturity, reducing reproductive potential of trees (Moncrieff et al. 211; Staver & Bond 214). Direct effects of drought and fire on mortality were negligible for all size classes considered. Although this result was surprising, TDF species possess functional adaptations for coping with drought and fire. With respect to drought, many species in the TDF are deciduous (Chaturvedi et al. 211b), which allows them to avoid water stress during the dry season (Markesteijn & Poorter 29). With respect to fire, TDF species possess numerous traits for coping with fire, including thick bark, underground storage and basal resprouting ability (Ratnam et al. 216). This means that they are only likely to be killed by very severe fires, which 12 Doi: /jvs International Association for Vegetation Science

7 R. K. Chaturvedi et al. Importance of human disturbance in dry forests 6 Harvest (%) Browse (%) Drought (%) Fire (%) 8 4 Fig. 4. Annual mortality indices due to harvesting, browsing, drought and fire in three tree size classes across annual average soil moisture content (SMC). 95% CI around the fitted regression lines are shown for the harvest data. Regression models could not be resolved for the browse, drought and fire data due to the large number of zeros and low observed proportions (see text for further details). [Colour figure can be viewed at wileyonlinelibrary.com] only occur when dead grass and woody biomass build up during long no-fire intervals (Stott 199). However, at our study sites, fires pass through at least every 3 yr, so are unlikely to possess sufficient fuel to sustain high-intensity fires. Effects of drought and fire are likely to be severe on very young plants (<1 yr old; Bond 28) because these do not possess deep roots to access deep soil water or carbohydrate reserves to support resprouting following decapitation (Hoffmann et al. 24). Annual low intensity fires have been shown to select for a reduced set of species in TDF because of this selection constraint (Saha & Howe 23); however our study did not incorporate this group. Indirect effects of soil moisture on mortality and recruitment There was clear evidence that total mortality rate of trees in all size classes (Fig. 3), mortality rates due to harvesting Doi: /jvs International Association for Vegetation Science 13

8 Importance of human disturbance in dry forests R. K. Chaturvedi et al. in all size classes, and mortality rate due to browsing in the juvenile size class (Fig. 4) all increased as mean SMC decreased across the sites. Conversely, recruitment in all size classes increased as mean SMC increased across the sites. There are multiple explanations for this. It is possible that mortality after disturbance is exacerbated by water stress, leading to higher mortality per tree harvested. Many TDF species are resprouters and so should be able to survive some pole damage. In turn, it can be speculated that mortality following disturbance during dry years will be significantly higher than during wet years. Murphy & Lugo (1986) and Borchert (1998) also emphasized the common occurrence of tree mortality during dry years, due to higher levels of water stress, and suggested that the extreme years significantly influence the structure, composition and functional properties compared to the average years in dry forests. Increased intensity and frequency of drought events could potentially reduce and decrease the drought-sensitive species leading to significant species composition shifts and reduction of diversity in tropical forests (Comita & Engelbrecht 29). Several studies (e.g. Slik 24; Engelbrecht et al. 27) have reported changes in species composition and diversity of tropical forests due to drought-induced differential mortality among tree species and size classes. In our study region, Acacia catechu, Boswellia serrata, Diospyros melanoxylon, Gardenia latifolia, Hardwickia binata, Holarrhena antidysenterica, Lagerstroemia parviflora, Nycthanthes arbortristis and Sterculia urens have been identified as drought hardy species (Troup 1921); therefore, these species can better survive under severe drought events compared to other species. Among the size classes, we observed higher mortality due to drought for juveniles compared to the mature trees at all study sites. Yeakley et al. (1998) observed that the temporal variation in soil moisture in deeper soil layers (3 9-cm depth) is much less compared in shallower soil layers ( 3-cm depth). Since mature trees depend on the groundwater and juveniles and saplings mainly depend on precipitation or stream water for around 1 13 yr (Dawson & Ehleringer 1991), mortality due to drought in juveniles and saplings is higher compared to that of mature trees. However, due to harvesting and browsing, mortality of even the drought-tolerant species has been commonly observed. For example, A. catechu and H. binata remain green during the drought period, therefore people commonly harvest these species for feeding domestic cattle. At the start of the dry period (May June), all mature new leaves of D. melanoxylon are plucked off to make bidis, an indigenous leaf-rolled cigarette. After plucking leaves, the base of the tree is scorched with fire to improve the quantity and quality of leaves for the next season. This practice, along with drought, may kill the tree and also the neighbouring trees. In the dry season, when grasses dry and no fodder is left, a large number of domestic cattles regularly go to the forest, they trample, pull and debark juveniles and saplings, leading to mortality of a large number of small-sized trees every year. In our study, we recorded increased mortality and less recruitment in drier sites compared to wetter sites. In the tropics, Chazdon et al. (27) suggested that survival and regeneration of small-sized trees is less in dry forests compared to wetter forests. Drier areas have lower densities of trees which, with equal harvesting intensity across sites, would lead to higher mortality in drier sites since proportionally more trees would be cut. Presently, we cannot disentangle these effects because we only recorded tree deaths and not all trees that had been harvested. However, we can conclude that mortality appears to be exacerbated in drier environments and that, given opposing trends in recruitment and mortality across the SMC gradient, tree loss and degradation at the drier end is proceeding more rapidly than at the wetter end. Thus TDF in drier environments appear to be more susceptible to the deleterious impacts of tree harvesting. No selectivity among trees harvested We found little evidence that particular tree species were being more heavily selected than others, although there was a clear seasonal pattern to tree harvesting. Local people harvest the highest number of trees and store them for firewood in June, before the onset of the monsoon, for use in the rainy season. The demand for harvested treesinjunealsoincreasesduetotheiruseinfencing agricultural fields for protection against browsing animals. The fact that we did not observe differences in species selection suggests that local harvesters do not select between species for fire or fencing. The problem becomes even more serious at the drier forests where species richness and stem density is lower. Therefore, in the current situation, harvesting may lead to a rapid change in the structure of TDF in India. The lack of any clear preferences for species may reflect the fact that the human population living close to our study region utilizes most tree species for some purpose. Almost all tree species in the study region have been reported to have medicinal importance (Singh et al. 22). Mortality of such species is commonly observed when their roots are entirely harvested and tree trunks are circularly de-barked for medicinal use. For superior wood quality, height and diameter, local people generally harvest Anogeissus latifolia, Hardwickia binata, Lagerstroemia parviflora, Shorea robusta, Soymida febrifuga and Terminalia tomentosa. Most preferred species for browsing animals are A. catechu, H. binata, S. robusta and T. tomentosa. At the most dry site, Kotwa, the effect of browsing can be clearly seen 14 Doi: /jvs International Association for Vegetation Science

9 R. K. Chaturvedi et al. Importance of human disturbance in dry forests where the commonly dominant species of the dry region, A. catechu, was replaced by Nycthanthes arbor-tristis as the dominant species at juvenile and sapling stages. N. arbortristis is not readily browsed, even by goats, and succeeds in establishing itself in grazed habitats (Troup 1921). Holarrhena antidysenterica, Lagerstroemia parviflora, Lannea coromandelica, S. robusta and Soymida febrifuga are least affected by fire (Troup 1921), however other species have been observed to be killed due to fire. Thus, although tree species in our study region are adapted to harmful effects of drought and fire, they are regularly harvested by the local people and extensively browsed by domestic cattle. The harvesting is non-selective, more severe than other forms of disturbance, and more severe in drier than wetter areas of this TDF. Conclusion We observed differences in mortality threats at the juvenile, sapling and adult tree stages at all five study sites. Impact of harvesting was higher for saplings and adult trees, whereas juveniles died mostly due to browsing. Since the tree species in our study region are well adapted to the harmful effects of seasonal drought and fire, the direct effects of mortality due to drought and fire were negligible. The evidence that total mortality rate of trees in all size classes increased as mean SMC decreased across the sites was very clear in our study. We also observed a decrease in tree recruitment at the dry sites compared to the moist sites. We found little evidence for the selectivity of tree species for harvesting, although there was a clear seasonal pattern to tree harvesting. The anthropogenic impacts on tree mortality are more important than nonanthropogenic impacts in the TDF, and impacts of all disturbances are more severe with increasing water stress. Thus changes in TDF structure due to harvesting are likely to be more rapid in more arid environments. Acknowledgements We thank the Ministry of Environment and Forests, Govt. of India, and the Chinese Academy of Sciences, China, for the financial support. RKC is supported under a CSIR Research Associate scheme and JSS is supported under a INSA senior scientist scheme. Two anonymous reviewers are also thanked for comments and suggestions. References Backeus, I., Rulangaranga, Z.K. & Skoglund, J Vegetation changes on formerly overgrazed hill slopes in semi-arid central Tanzania. 5: Bond, W.J. 28. What Limits Trees in C 4 Grasslands and Savannas? Annual Review of Ecology, Evolution, and Systematics 39: Borchert, R Response of tropical trees to rainfall seasonality and its long-term changes. Climate Change 39: Cattelino, P.J., Becker, C.A. & Fuller, L.G Construction and installation of homemade dendrometer bands. 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