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Sankaran, M.

Publications and source records attributed to Sankaran, M..

18 recordsLinked to original sources
bioRxiv2025

Bottom-up effects of a megaherbivore alter plant growth and competition regimes, promoting vegetation heterogeneity

O_LIMegaherbivores are known to strongly influence multiple ecological processes, but their bottom-up impacts on vegetation via nutrient redistribution remain poorly understood, particularly in mesic ecosystems. Here, we investigated how woody plant communities are influenced by large nutrient inputs in the form of dung deposited by Asias largest herbivore, the Asian elephant (Elephas maximus), in the tropical forests of southern India. C_LIO_LIWe conducted field and mesocosm experiments on woody saplings to examine three mechanisms through which dung deposition by elephants can alter plant community assembly. Specifically, we tested if elephant dung input 1) creates hotspots of plant growth, and shapes plant communities by altering 2) the negative density-dependent effects of neighborhood competition, and 3) interference competition between plant functional types differing in nutrient limitation, namely nitrogen-fixers and non-nitrogen-fixers. C_LIO_LIOur findings show that dung deposition by elephants can generate fine-scale spatial differences in woody sapling communities by creating local growth hotspots and altering competitive interactions. We analyzed relative growth rate and final sapling size in the field experiment, and found that average-sized and large saplings receiving dung inputs were buffered against the negative density-dependent effects of neighborhood competition. In the mesocosm experiment, non-nitrogen-fixing species (which are nitrogen-limited) outcompeted nitrogen-fixers in accumulating biomass when supplied with elephant dung. These outcomes were associated with changes in their relative competitive strength which was stronger for non-nitrogen fixers under dung treatment and for nitrogen-fixers under control. Such bottom-up effects on plant growth and competition can be of substantially large magnitude, as we estimated that elephants in these forests create a total of 11000 such nutrient-rich sites /km2/year, with each elephant redistributing [~]130 kg Nitrogen/year through this pathway and each site receiving [~]20 g Nitrogen. C_LI SynthesisOur findings on the outcomes of sapling competition highlight the role of nutrient redistribution by megaherbivores as an underappreciated driver of species interactions that can alter plant communities at fine scales, effects that are widespread across megaherbivore habitats. Such bottom-up effects of megaherbivores, along with their top-down effects, have important conservation implications and can help in restoring species interactions and spatial heterogeneity in plant communities in defaunated habitats.

ecology↗
bioRxiv2025

Insect herbivory on restored rainforest seedlings weakened by neighbours but unaffected by invasive coffee

Restoration of degraded tropical forests is often impeded by invasive species. For regenerating native seedlings, the presence of invasives in the neighbourhood can alter insect herbivory patterns, ultimately shaping restoration trajectories; however, such indirect effects are rarely examined. Here, we investigated the effect of robusta coffee (Coffea canephora) - a shade-tolerant invasive species under closed-canopy secondary forests in the Western Ghats - on the incidence of herbivory (proportion of leaves with any sign of damage) and the extent of leaf damage (percentage leaf area consumed) in seedlings of 10 rainforest species in plots from which coffee plants were either weeded out or left intact. We further examined whether local neighbourhood densities of coffee and other saplings, and species leaf traits, explained herbivory patterns. Removal of invasive coffee did not influence herbivory incidence or the extent of damage across our focal species. However, the incidence of herbivory declined with increasing neighbourhood plant density, suggesting that neighbourhood plants provide a resource dilution effect. Both incidence and extent of herbivore damage were strongly species-specific and partly explained by leaf traits: greater leaf carbon content was correlated with lower herbivory incidence. Contrary to expectations, plants with resource-acquisitive traits (high leaf nitrogen and specific leaf area) experienced lower incidence of herbivory and extent of damage. Our findings suggest that in this system, the indirect effects via herbivores are perhaps not as important in influencing restoration as the more direct effects of invasive species, such as competition for resources.

ecology↗
bioRxiv2025

Distribution patterns of two widespread, co-dominant savanna tree species of the Indian subcontinent under current and future climates

Future climate is expected to alter the dynamics and distribution of vegetation. Here, we use ecological niche modelling to investigate the current and predicted future distributions of two dominant tree species that characterize the ecology and dynamics of the widespread tropical dry biomes of the Indian subcontinent- Terminalia anogeissiana (formerly and henceforth Anogeissus latifolia) and Terminalia elliptica (both Combretaceae). Using current occurrence data and six ecologically relevant climate variables, we find that, as expected, T. elliptica and A. latifolia share highly similar climatic niches with substantial overlap ([~]84%). Mean annual precipitation (MAP), mean temperature of the driest quarter (MTDQ), and temperature annual range (TAR) emerge as the key environmental variables explaining the distribution of both species, but their relative influence differs. T. elliptica appears to occur across a broader climatic niche that includes wetter areas with cooler dry seasons (lower MTDQ) and lower temperature annual ranges (TAR) relative to A. latifolia. Currently, the area of modelled suitable habitat for both species are vast (A. latifolia: 416,057 km{superscript 2}; T. elliptica: 401,544 km{superscript 2}). However, under future climate scenarios, both species are projected to incur dramatic losses of suitable habitat (A. latifolia: [~]63% by 2041-2060 and [~]84% by 2081-2100; T. elliptica: [~]57.8% by 2041-2060 and [~]73% by 2081-2100). By 2100, the Western Ghats are projected to remain as the last key refugia for both species, while vast other regions are likely to become increasingly unsuitable. Contrary to current understanding, our results underscore the high vulnerability of currently dominant tree species of the tropical dry biomes, through potential losses of suitable habitats under future climate regimes.

ecology↗
bioRxiv2025

Patterns of grass (Poaceae) species distribution and richness across India

Tropical grassy biomes (TGBs) cover a substantial portion of Earths land surface area and support a rich diversity of fauna and flora. Yet, despite their importance for biodiversity and human livelihoods, the TGBs of South Asia, including India, have historically been undervalued, and as result, understudied. Here, we address this gap with the first subcontinental-scale analyses of grass species richness and distribution across the Indian region. Using data on species occurrences collated from multiple regional floras, we describe the distribution of families, functional groups and species of Poaecea across this region, and their variation as a function of climate and geomorphology. India hosts >1100 grass species, comprising [~]10% of global grass richness. Over half of these were perennial C4 grasses, with Panicoideae being the most speciose subfamily in the country. Consistent with patterns observed globally, the PACMAD clade consisting of both C3 and C4 species was more speciose in warmer regions, while the C3 subfamilies of the BEP clade, Pooideae and Bambusoideae, were more abundant in cooler regions of the country. Both total and C4 species richness were greatest in warmer districts with high moisture availability, while cooler and wetter districts with low precipitation seasonality supported greater C3 species richness. Our results spotlight previously unrecognized areas of high grass species richness in the arid and semi-arid areas of North-West India and the Deccan Peninsula in India, and call for conservation attention to neglected regions that are outside known biodiversity hotspots and protected areas.

ecology↗
bioRxiv2025

Consistency in floral volatiles impacts plant-pollinator interactions in the face of environmental change in the Eastern Himalayas

Floral scents are an important mediator of many plant-pollinator interactions. However, environmental and ecological change could affect the synthesis and emission of volatile chemicals that constitute floral scents, potentially influencing pollinator perception of scents. We investigated the effects of elevation and warming on the floral scents of multiple co-occurring alpine meadow plant species along an elevational gradient in the Eastern Himalayas (3000 - 4000 masl). We also investigated pollinator responses in the same locations using 3-D printed floral models that replicated the differing volatile constituents. Finally, we assessed how pollinator preferences changed with variations in elevation, flowering season and location. We found that elevation and in-situ warming using Open Top Chambers (OTC) led to significant intraspecific variation in the composition of floral scents, both in terms of quality and quantity of volatiles released. Nevertheless, conspecific floral scents across elevations were more similar to each other than to other species. Cafeteria assays using artificial floral models revealed that pollinator preferences were driven by a small number of volatiles (p-cymene, 2-pentylfuran and -pinene), but these preferences were abolished in a novel plant-pollinator community where the same floral volatiles were not detected. Our results suggest that the local odourscape established by the floral community present plays a key role in plant-pollinator interactions. This emphasizes the need to incorporate ecological impacts such as changes in community into climate change analyses to understand the long-term impacts of human activity on co-evolutionary relationships like pollination. Significance StatementFloral scents are important for most plant-pollinator interactions, and can be affected by environmental factors like elevation and temperature. Our study shows that elevation and simulated in situ warming change the composition of floral scents, yet scents from the same species were more similar to each other across elevations and temperatures than to other species. Further, pollinator preferences to these scents replicated in artificial models persisted in spite of variation in temperature and elevation, but not in a novel plant-pollinator community containing floral species not releasing those volatiles. This emphasizes the need to incorporate factors such as floral community composition into our understanding of the long-term impacts of climate change and human activity on co-evolutionary relationships like pollination.

ecology↗
bioRxiv2024

Should elephants graze or browse? The nutritional and functional consequences of dietary variation in a mixed-feeding megaherbivore

Unlike specialist browsers and grazers, the diets of mixed-feeding megaherbivores like elephants are complex and broad, comprising numerous plant species of variable nutritional quality. Here, we revisit an unresolved debate on whether browse is more nutritious than grasses for elephants, as browse is thought to have greater crude protein content. We first analyzed carbon isotopes in 102 fecal samples of Asian elephants to quantify the contribution of browsing and grazing to their diet in Nagarahole National Park, southern India. We show that elephants are predominantly browsers in the grass-scarce Nagarahole forests, in contrast with predominant grazing reported in the nearby grass-abundant savannas of the Nilgiri Biosphere Reserve. We then compared the forage quality of high-browsing and high-grazing diets by analyzing these samples for two proxies of crude protein content (CP): nitrogen content (N%) and Carbon-to-Nitrogen ratio (C:N). Interestingly, high-browsing diets did not have higher N% or lower C:N (proxies for high CP) than low-browsing (high-grazing) diets. To explore the generality of this finding, we analyzed nutritional differences (i.e., CP and fibre content values) between browse and grass across 141 plant species consumed by Asian elephants. We show that woody browse and non-legume browse, which are major components of elephant browse, do not have appreciably higher CP than grasses. Our findings suggest that browsing and grazing broadly have similar nutritional value for such bulk-feeding mixed feeders. Finally, based on the observed habitat-wide variation in browsing, we provide a new framework for assessing how elephants shape woody vegetation across the forests and savannas of Asia, with important implications for conservation and carbon cycling.

ecology↗
bioRxiv2024

Harmonising distributed tree inventory datasets across India can fill critical gaps in tropical ecology

AO_SCPLOWBSTRACTC_SCPLOWO_LIGlobal analyses of tree diversity and function are strongly biased geographically, with poor representation from forests of the Indian subcontinent. Even though data from India - representing two-thirds of the subcontinent and spanning a wide range of tree-based biomes - exists, a barrier to syntheses is the absence of accessible and standardised data. Further, with increasing human footprint across ecosystems, data from Indian landscapes, with their long history of human-nature interactions is a key link to understand the future of tropical forested landscapes. Given the long history of human-nature interactions and high human footprint in the region, accessible and standardized data from the subcontinent can enable understanding the future of tropical landscapes under increasing human footprints globally. C_LIO_LICombining literature searches with manual data retrieval, we assembled the INdia Tree Inventory dataset, INvenTree. INvenTree is the largest meta-dataset of peer-reviewed publications (n = 465) from 1991-2023 on geolocated plot-based tree inventories of multispecies communities from Indian ecosystems, in aggregate covering 4653.64 ha and all of its woody biomes. C_LIO_LIUsing INvenTree, we show extensive sampling across tropical moist and dry forests, the dominant ecosystem types in the country. However, most studies have low sampling effort (median sampled area = 2 ha) and data across studies is not openly accessible (73.33 % of studies representing 83.43% of the sampled area), potentially hindering inclusion into regional or global syntheses. C_LIO_LISignificantly, we show majority authorship from within the country; 82.8% of corresponding authors were from India and 73.33% of the studies had all authors affiliated with Indian institutions. We also identify ecological and conservation sampling priority regions based on forest cover and forest loss and set a blueprint for future sampling efforts in the country. C_LIO_LIBased on extensive Indian scholarship in forest ecology showcased through the INvenTree dataset, we see opportunity for regional collaboration to create scientific inferences that are larger and scalable, while prioritising data and knowledge equity. Harmonising these existing datasets, and synthesising historic and grey literature, will contribute enormously to understanding the human dimension of tropical ecology as well as informing regional management and conservation. C_LI

ecology↗
bioRxiv2024

Soil respiration is correlated with rainfall and soil moisture at multiple temporal scales in a seasonal wet tropical forest.

Soil respiration is the second largest natural flux of carbon (C) between terrestrial ecosystems and the atmosphere, with tropical forests amongst the largest contributors of such soil-derived carbon efflux. With climate change expected to drive changes in both temperature and rainfall, our ability to predict responses of the carbon cycle in the future hinges upon an understanding of how these factors influence soil respiration (Rs). However, these relationships remain poorly characterised across the seasonal tropics, especially south Asia. Here, for two seasonal tropical sites in the Western Ghats of India, we characterised annual, seasonal and temporal variation in Rs and assessed its temperature and moisture sensitivity over 6 years. At both sites, Rs was positively correlated with temperature at the instantaneous scale, but showed no relationship with temperature at seasonal or annual scales. In contrast, Rs showed significant relationships with rainfall and soil moisture at all temporal scales. At the annual scale, respiration was negatively correlated with total annual rainfall. At the seasonal scale, wet season Rs was significantly lower than in the dry season. At the instantaneous scale, Rs showed a parabolic relationship with soil moisture, where soil respiration increased with soil moisture up to [~]21% and decreased beyond that point. Our results suggest that future responses of soil respiration in this seasonal tropical forest will largely be driven by changes in the Indian Summer Monsoon, not only mean annual precipitation, but also the frequency and intensity of extreme rainfall events.

ecology↗
bioRxiv2023

Effects of sampling methodology on phenology indices: insights from sites across India and modelling

Plant phenology is the study of timing and extent of leaf, flower, and fruit production. Phenology data are used to study drivers of cyclicity and seasonality of plant life-history stages, interactions with organisms such as pollinators, and effects of global change factors. Indices such as timing of phenological events, proportion of individuals in a particular phenophase, seasonality, and synchrony have often been used to summarise plant phenology data. However, these indices have specific utilities and limitations and may be sensitive to sampling methodology, making cross-site comparisons challenging, particularly when data collection methods vary in terms of sample size, observation frequency, and the resolution at which phenophase intensity scores/values are recorded. We use fruiting phenology data from tropical trees across five sites in India to study the effects of sampling methodology on two indices: an index of population-level synchrony (overlap), and an index of seasonality. We supplement these results with simulations of fast- and slow-changing phenologies to test for the effects of sampling methodology on these indices. We found that the overlap index is sensitive to the phenophase intensity measurement resolution--with coarser intensity measures leading to overestimation of the overlap index. The seasonality index, on the other hand, was not affected by intensity resolution. Simulations indicated that finer intensity resolution is more important than frequency of observation to accurately estimate population synchrony and seasonality for fast- and slow-changing phenophases. Based on our findings, we provide recommendations for study design of future tropical tree phenology research, particularly for long-term or cross-site studies.

ecology↗
bioRxiv2023

Structure and dynamics of secondary and mature rainforests: insights from South Asian long-term monitoring plots

We assessed tree community and carbon dynamics over 5 years in two 1-ha long-term ecosystem monitoring plots, one each in mature tropical rainforests (MR) and 10-year post-agroforestry secondary rainforests (SR) in Indias Western Ghats mountains. Both plots were established in 2017 and monitored annually six times. We expected (1) higher tree diversity, distinct species composition, and greater carbon stock in MR; (2) higher carbon sequestration rates in SR; and (3) carbon dynamics shaped by growth and mortality in SR and MR, respectively. The SR plot had fewer species (67 vs. 84), stored substantially less carbon (76 vs. 193 Mg), and comprised a distinct community with fewer late-successional species than MR. SR gained 5.8 Mg carbon, due to tree growth exceeding losses from mortality, while in MR mortality exceeded growth and recruitment resulting in a 3.3 Mg carbon decline over five years. While MR had higher tree diversity, carbon stocks and relatively intact composition, the high rates of biodiversity and carbon accrual in SR highlight the conservation and climate significance of post-agroforestry secondary forests. Moderate carbon losses noted here in MR, as in other mature South Asian tropical forests, is a cause for concern under ongoing climate change.

ecology↗
bioRxiv2022

Prioritizing landscapes to reconcile biodiversity conservation, ecosystem services, and human well-being in India

Biodiversity conservation and human well-being are tightly interlinked; yet mismatches in the scale at which both priorities are planned and implemented have exacerbated biodiversity loss, erosion of ecosystem services, and declining human quality of life. India houses the second largest human population on the planet, while <5% of the countrys land area is effectively protected for conservation. This warrants landscape-level conservation planning through a judicious mix of land-sharing and land-sparing approaches, and co-production of ecosystem services. Through a multi-faceted assessment, we prioritize spatial extents of land parcels that, in the face of anthropogenic threats, can safeguard conservation landscapes across Indias biogeographic zones. We find that only a fraction (~15%) of such priority areas identified here are encompassed under Indias extant PA network, and several landscapes of high importance were omitted in all previous global-scale assessments. We then examined the spatial congruence of priority areas with administrative units earmarked for economic development by the Indian government, and propose management-zoning through state-driven and participatory approaches. Our spatially explicit insights can help meet the twin goals of biodiversity conservation and sustainable development in India and other countries across the Global South.

ecology↗
bioRxiv2022

Biodiversity effects on seedling biomass growth are modulated by light environment across functional groups

O_LITree biodiversity has the potential to ensure consistency in the functioning of forest ecosystems, not just over space, but over long-timescales by maintaining composition through recruitment. However, for continued buffering in the face of global environmental change, the sensitivity of biodiversity-ecosystem functioning relationships to heterogeneous environments needs to be understood. C_LIO_LISeedling recruitment in carbon-rich tropical forests is a result of biotic and abiotic drivers but their combined outcomes at the community-level remain poorly understood. Although biodiversity in seedling communities can potentially increase their growth and biomass accumulation, abiotic drivers like light can alter this effect through divergent effects on constituent species and functional groups. In forests with high baseline heterogeneity in microclimates, these processes can enhance or constrain regeneration. C_LIO_LIWe tested the effects and interactions between species richness and canopy cover on the growth of seedling communities consisting of tropical broad-leaved evergreen and deciduous forest species using a fully crossed manipulated experiment in the Andaman Islands, India and compared these with field observations from a long-term forest plot in the same landscape. C_LIO_LIWe show that in the critical seedling establishment phase, species richness and light increase community biomass independently. Accounting for variation across species, individual species on average accumulated more biomass in communities with both higher light and higher diversity. C_LIO_LIWe also show that overyielding in species rich communities fits expectations from a model of complementarity with non-random overyielding than selection or spatial insurance effects. C_LIO_LISynthesis Taken together, our results show that the potential for biodiversity to increase ecosystem functioning in seedling communities is modulated by light. Further understanding on the interaction of biodiversity with multiple abiotic drivers and their effect on regeneration dynamics is crucial for predicting future ecosystem functioning. C_LI

ecology↗
bioRxiv2020

Response of mycorrhizae to herbivory and soil moisture in a semiarid grazing ecosystem

Arbuscular mycorrhizal fungal (AMF) symbioses with plants can be influenced by top-down forces such as grazing, and also by bottom-up forces such as soil resource availability, both of which are being altered by anthropogenic and global change drivers. While the influence of each of these factors on AMF symbioses has been widely studied, explicit tests of the relative strengths of top-down versus bottom-up influences on these ubiquitous plant root symbioses are few. We studied AMF colonization responses of four species of graminoids (3 grasses Elymus longae-aristatus, Leymus secalinus and Stipa orientalis, and a sedge Carex melanantha) common to semiarid high-altitude rangelands of the Spiti region, Trans-Himalaya, to changes in a top-down driver, grazing intensity (through short-term clipping and long-term grazer exclusion experiments), and a bottom-up driver, water availability (using irrigation treatments, and by evaluating responses to annual precipitation levels across years). Over three years, AMF colonization in all four host species was influenced by precipitation, with the highest and lowest AMF colonization levels corresponding to years with the lowest and highest rainfall, respectively. However, responses to long-term grazer exclusion differed among host species, and across years: while some species showed decreases in AMF colonization levels under grazing, others showed increases from ungrazed control levels, and these responses changed, even reversed, across years. Responses to short-term clipping and irrigation treatments also differed among hosts, with some species responding to irrigation alone, some to clipping and irrigation combined, and others showing no changes in AMF colonization from control levels in any of the treatments. In our study, long-term changes in water availability influenced AMF colonization levels, while short-term responses were host specific. Responses to above-ground tissue loss, however, differed among host species both in the long- and short-term. Overall, this study demonstrates that while AMF colonization levels correspond to annual precipitation levels in this semiarid ecosystem, host species also play a role in influencing plant-AMF interactions in these rangelands, with colonization levels and responses to abiotic factors changing with host species.

ecology↗
bioRxiv2019

Understanding the variation in wood densities of trees and its implications for carbon assessments

Wood density is a key functional trait used to estimate aboveground biomass (AGB) and carbon stocks. A common practice in forest AGB and carbon estimation is to substitute genus averages (across species with known wood densities) in cases where wood densities of particular species are unknown. However, the extent to which genus-level averages are reflective of species wood densities across tree genera is uncertain, and understanding this is critical for estimating the accuracy of carbon stock estimates. Using primary field data from India and secondary data from a published global dataset, we quantified the extent to which wood density varied among individuals within species (intraspecific variation) at the regional scale and among species within genera (interspecific variation) at regional to global scales. We used a published global database with wood density data for 7743 species belonging to 1741 genera. Linear models were used to compare the species values with the genera averages and the individual values with the species averages, respectively. To estimate the error associated with using genus-level averages for carbon stocks estimation, we compared genus values averaged at the global, old world and continental scales with species values from actually measured data. We also ran a simulation using vegetation data from a published database to calculate the estimation errors in a 1 hectare plot level when genera-averaged wood densities are used. Intraspecific variation was significantly lower than interspecific variation. Continental level genera averages led to estimates closer to the species values for the 10 genera for which most data on species was available. This was also evident from a comparison of genera averages at these three spatial scales with species values from our data. Species within certain hypervarying genera showed relatively high levels of variation, irrespective of the spatial scale of the dataset used. The error in estimation of AGB when genera-averaged values were used for species wood densities was 0.35, 0.71 and 2.43% when 0, 10 and 25% of the girth of the trees in the simulated plot were from hypervariable genera. Our findings indicate that species values provide the most accurate estimates for individuals. Genus average wood density values at the continental scale provided more reliable estimates than those at larger spatial scales. The aboveground biomass estimation error when species wood densities were approximated to the genera-average values was 1.4 to 3.7 tonnes per ha when 10% and 25%, respectively, of the girth of trees was from species from hypervariable genera. Our findings indicate that regional or continental scale genera averages provide more reliable estimates than global data and we propose a method to identify hypervariable genera, for which species values rather than genera averages can provide better estimates of carbon stocks.

ecology↗
bioRxiv2018

Climate and vegetation collectively drive soil respiration in montane forest-grassland landscapes of the southern Western Ghats, India

Land-use conversion to non-native species plantations not only affects biodiversity but also alters important ecosystem functions including above- and below-ground carbon sequestration, and CO2 release rates from soils via soil respiration. Though the role of soil temperature and moisture on soil respiration is well recognized, little is known about how their effects vary across different land-use types. This study looked at the effects of land-cover change on temporal patterns of soil respiration in a montane forest-grassland-plantation matrix, a highly diverse but climatically sensitive ecosystem in the tropical Western Ghats of India. Among native vegetation types, soil respiration rates were higher in grassland compared to forest patches. Invasion of grassland by an exotic tree species - wattle (Acacia mearnsii) reduced soil respiration rates to levels similar to that of forests. However, conversion of native grasslands to non-native pine (Pinus patula) plantations led to the largest declines in soil respiration rates. In addition, the sensitivity of soil respiration to changes in temperature and moisture differed between different vegetation types. Across all vegetation types, respiration was largely insensitive to changes in soil temperature when moisture levels were low. However, when soil moisture levels were high, respiration increased with temperature in grassland and wattle patches, decreased in the case of pine plantations, and remained largely unchanged in shola forests. Our results suggest that changes in aboveground vegetation type can significantly affect soil C cycling even in the absence of any underlying differences in soil type.

ecology↗
bioRxiv2018

Nutrient deposition enhances post-fire survival in non-N-fixing savanna tree seedlings

Nutrient deposition can modify plant growth rates and potentially alter the susceptibility of plants to disturbance events, while also influencing properties of disturbance regimes. In mixed tree-grass ecosystems, such as savannas and tropical dry forests, tree seedling growth rates strongly influence the ability of seedlings to survive fire (i.e. post-fire seedling survival), and hence, vegetation structure and tree community composition. However the effects of nutrient deposition on the susceptibility of recruiting trees to fire are poorly quantified. In a field experiment, seedlings of multiple N-fixing and non-N-fixing tropical dry forest tree species were exposed to nitrogen (N) and phosphorus (P) fertilisation, and fire. We quantified nutrient-mediated changes in a) mean seedling growth rates; b) growth rates of the fastest growing individuals and c) post-fire seedling survival. N-fixers had substantially higher baseline post-fire seedling survival, that was unaffected by nutrient addition. Fertilisation, especially with N, increased post-fire survival probabilities in non-N-fixers by increasing the growth rates of the fastest growing individuals. These results suggest that fertilisation can lead to an increase in the relative abundance of non-N-fixers in the resprout community, and thereby, alter the community composition of tropical savanna and dry forest tree communities in the long-term.

ecology↗
bioRxiv2017

Effects of increased N and P availability on biomass allocation and root carbohydrate reserves differ between N-fixing and non-N-fixing savanna tree seedlings

In mixed tree-grass ecosystems, tree recruitment is limited by demographic bottlenecks to seedling establishment arising from inter- and intra-life form competition, and disturbances such as fire. Enhanced nutrient availability resulting from anthropogenic nitrogen (N) and phosphorus (P) deposition can alter the nature of these bottlenecks by changing seedling growth and biomass allocation patterns, and lead to longer-term shifts in tree community composition if different plant functional groups respond differently to increased nutrient availability. However, the extent to which tree functional types characteristic of savannas differ in their responses to increased N and P availability remains unclear. We quantified differences in above- and belowground biomass, and root carbohydrate contents - parameters known to influence the ability of plants to compete, as well as survive and recover from fires - in seedlings of multiple N-fixing and non-N-fixing tree species characteristic of Indian savanna and dry-forest ecosystems to experimental N and P additions. N-fixers in our study were co-limited by N and P availability, while non-N-fixers were N limited. Although both functional groups increased biomass production following fertilisation, non-N-fixers were more responsive and showed greater relative increases in biomass with fertilisation than N-fixers. N-fixers had greater baseline investment in belowground resources and root carbohydrate stocks, and while fertilisation reduced root:shoot ratios in both functional groups, root carbohydrate content only reduced with fertilisation in non-N-fixers. Our results indicate that, even within a given system, plants belonging to different functional groups can be limited by, and respond differentially to, different nutrients, suggesting that long-term consequences of nutrient deposition are likely to vary across savannas contingent on the relative amounts of N and P being deposited in sites.

ecology↗