Publications by authors named "Sumanta Bagchi"

8 Publications

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Greening of the earth does not compensate for rising soil heterotrophic respiration under climate change.

Glob Chang Biol 2021 May 9;27(10):2029-2038. Epub 2021 Feb 9.

Centre for Ecological Sciences, Indian Institute of Science, Bangalore, India.

Stability of the soil carbon (C) pool under decadal scale variability in temperature and precipitation is an important source of uncertainty in our understanding of land-atmosphere climate feedbacks. This depends on how two opposing C-fluxes-influx from net primary production (NPP) and efflux from heterotrophic soil respiration (R )-respond to covariation in temperature and precipitation. There is scant evidence to judge whether field experiments which manipulate both temperature and precipitation align with Earth System Models, or not. As a result, even though the world is generally greening, whether the resultant gains in NPP can offset climate change impacts on R , where, and by how much, remains uncertain. Here, we use decadal-scale global time-series datasets on NPP, R , temperature, and precipitation to estimate the two opposing C-fluxes and address whether one can outpace the other. We implement machine-learning tools on recent (2001-2019) and near-future climate scenarios (2020-2040) to assess the response of both C-fluxes to temperature and precipitation variation. We find that changes in C-influx may not compensate for C-efflux, particularly in wetter and warmer conditions. Soil-C loss can occur in both tropics and at high latitudes since C-influx from NPP can fall behind C-efflux from R . Precipitation emerges as the key determinant of soil-C vulnerability in a warmer world, implying that hotspots for soil-C loss/gain can shift rapidly and highlighting that soil-C is vulnerable to climate change despite widespread greening of the world. The direction of covariation between change in temperature and precipitation, rather than their magnitude, can help conceptualize highly variable patterns in C-fluxes to guide soil-C stewardship.
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http://dx.doi.org/10.1111/gcb.15531DOI Listing
May 2021

Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands.

Glob Chang Biol 2020 Feb 3. Epub 2020 Feb 3.

Department of Terrestrial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, The Netherlands.

Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature - herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local-scale herbivory, and its interaction with nutrient enrichment and climate, within global-scale models to better predict land-atmosphere interactions under future climate change.
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http://dx.doi.org/10.1111/gcb.15023DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7155038PMC
February 2020

Change in dominance determines herbivore effects on plant biodiversity.

Nat Ecol Evol 2018 12 29;2(12):1925-1932. Epub 2018 Oct 29.

Institut Polytechnique Rural/Institut de Formation et de Recherche Appliquee, Katibougou, Mali.

Herbivores alter plant biodiversity (species richness) in many of the world's ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis-that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally.
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http://dx.doi.org/10.1038/s41559-018-0696-yDOI Listing
December 2018

Quantifying long-term plant community dynamics with movement models: implications for ecological resilience.

Ecol Appl 2017 07 30;27(5):1514-1528. Epub 2017 May 30.

Centre for Ecological Sciences, Indian Institute of Science, Bangalore, 560012, India.

Quantification of rates and patterns of community dynamics is central for understanding the organization and function of ecosystems. These insights may support a greater empirical understanding of ecological resilience, and the application of resilience concepts toward ecosystem management. Distinct types of dynamics in natural communities can be used to interpret and apply resilience concepts, but quantitative methods that can systematically distinguish among them are needed. We develop a quantitative method to analyze long-term records of plant community dynamics using principles of movement ecology. We analyzed dissimilarity of species composition through time with linear and nonlinear statistical models to assign community change to four classes of movement trajectories. Compositional change in each sampled plot through time was classified into four classes, stability, abrupt nonlinear change, transient reversible change, and gradual linear drift, each representing a different aspect of ecological resilience. These competing models were evaluated based on estimated coefficients, goodness of fit, and parsimony. We tested our method's accuracy and robustness through simulations, or the ability to distinguish among trajectories and classify them correctly. We simulated 16,000 trajectories of four types, of which 94-100% were correctly classified. Next, we analyzed 13 long-term vegetation records from North American grasslands (annual grasslands with warm-season and cool-season communities, shortgrass, mixedgrass, and tallgrass prairies, and sagebrush steppe), and a record of primary succession at Mt. St. Helens volcano. Collectively, we analyzed 14,647 observations from 775 plots, between 1915 and 2012. Dynamics could be reliably assigned for 705 plots (91%), and overall statistical fit was high (goodness of fit, 0.77 ± 0.15 SD). Among the perennial grasslands, stability was most common (44% of all plots), followed by gradual linear (22%), abrupt nonlinear (17%), and reversible (6%) change. Among annual grasslands, abrupt nonlinear shifts (33%) were more common in the warm-season community than in the cool-season (20%). As expected, abrupt nonlinear change was common during primary succession (51%) while reversible change was rare (3%). Generally, reversible dynamics often required 2-3 decades. Analysis of long-term community change, or trajectories, with principles of movement ecology provides a quantitative basis to compare and interpret ecological resilience within and among ecosystems.
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http://dx.doi.org/10.1002/eap.1544DOI Listing
July 2017

Functional response and body size in consumer-resource interactions: Unimodality favors facilitation.

Theor Popul Biol 2016 08 16;110:25-35. Epub 2016 Apr 16.

Centre for Ecological Sciences, Indian Institute of Science, CV Raman Ave, Bangalore 560012, India. Electronic address:

Theoretical models suggest that competitive coexistence of consumers over shared resources can occur only under very restrictive conditions. Yet, in apparent defiance of the competitive exclusion principle, large numbers of species form natural communities while sharing a small number of limiting resources. Consumers not only coexist, but also show positive facilitative interactions among themselves. Since body size and functional responses may play important roles in these interactions, we investigate their joint effects on two consumers over a single resource. We find that two consumers with unimodal Type IV functional response can facilitate each other by increasing each other's intake rates. But, this facilitation does not necessarily impact their co-existence. When the consumers differ in their body sizes, the larger consumer receives greater absolute benefits, and the smaller consumer gains more relative benefits. These results are consistent with empirical observations, and do not require any additional assumptions over the parameters governing dynamics of resources to explain net positive interactions between consumers.
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http://dx.doi.org/10.1016/j.tpb.2016.04.001DOI Listing
August 2016

Empirical assessment of state-and-transition models with a long-term vegetation record from the Sonoran Desert.

Ecol Appl 2012 Mar;22(2):400-11

Department of Ecosystem Science and Management, Texas A&M University, 2138 TAMU, College Station, Texas 77843, USA.

Resilience-based frameworks, including state-and-transition models (STM), are being increasingly called upon to inform policy and guide ecosystem management, particularly in rangelands. Yet, multiple challenges impede their effective implementation: (1) paucity of empirical tests of resilience concepts, such as alternative states and thresholds, and (2) heavy reliance on expert models, which are seldom tested against empirical data. We developed an analytical protocol to identify unique plant communities and their transitions, and applied it to a long-term vegetation record from the Sonoran Desert (1953-2009). We assessed whether empirical trends were consistent with resilience concepts, and evaluated how they may inform the construction and interpretation of expert STMs. Seven statistically distinct plant communities were identified based on the cover of 22 plant species in 68 permanent transects. We recorded 253 instances of community transitions, associated with changes in species composition between successive samplings. Expectedly, transitions were more frequent among proximate communities with similar species pools than among distant communities. But unexpectedly, communities and transitions were not strongly constrained by soil type and topography. Only 18 transitions featured disproportionately large compositional turnover (species dissimilarity ranged between 0.54 and 0.68), and these were closely associated with communities that were dominated by the common shrub (burroweed, Haplopappus tenuisecta); indicating that only some, and not all, communities may be prone to large compositional change. Temporal dynamics in individual transects illustrated four general trajectories: stability, nondirectional drift, reversibility, and directional shifts that were not reversed even after 2-3 decades. The frequency of transitions and the accompanying species dissimilarity were both positively correlated with fluctuation in precipitation, indicating that climatic drivers require more attention in STMs. Many features of the expert models, including the number of communities and participant species, were consistent with empirical trends, but expert models underrepresented recent increases in cacti while overemphasizing the introduced Lehmann's lovegrass (Eragrostis lehmanniana). Quantification of communities and transitions within long-term vegetation records presents several quantitative metrics such as transition frequency, magnitude of accompanying compositional change, presence of unidirectional trajectories, and lack of reversibility within various timescales, which can clarify resilience concepts and inform the construction and interpretation of STMs.
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http://dx.doi.org/10.1890/11-0704.1DOI Listing
March 2012

Herbivore effects on above- and belowground plant production and soil nitrogen availability in the Trans-Himalayan shrub-steppes.

Oecologia 2010 Dec 29;164(4):1075-82. Epub 2010 Jun 29.

Department of Biology, Syracuse University, Syracuse, NY 13244, USA.

Large mammalian herbivores may have positive, neutral, or negative effects on annual net aboveground plant production (NAP) in different ecosystems, depending on their indirect effects on availability of key nutrients such as soil N. In comparison, less is known about the corresponding influence of grazers, and nutrient dynamics, over annual net belowground plant production (NBP). In natural multi-species plant communities, it remains uncertain how grazing influences relative allocation in the above- and belowground compartments in relation to its effects on plant nutrients. We evaluated grazer impacts on NAP, NBP, and relative investment in the above- and belowground compartments, alongside their indirect effects on soil N availability in the multiple-use Trans-Himalayan grazing ecosystem with native grazers and livestock. Data show that a prevailing grazing intensity of 51% increases NAP (+61%), but reduces NBP (-35%). Grazing also reduced C:N ratio in shoots (-16%) and litter (-50%), but not in roots, and these changes coincided with increased plant-available inorganic soil N (+23%). Areas used by livestock and native grazers showed qualitatively similar responses since NAP was promoted, and NBP was reduced, in both cases. The preferential investment in the aboveground fraction, at the expense of the belowground fraction, was correlated positively with grazing intensity and with improvement in litter quality. These results are consistent with hypothesized herbivore-mediated positive feedbacks between soil nutrients and relative investment in above- and belowground compartments. Since potentially overlapping mechanisms, such as N mineralization rate, plant N uptake, compositional turnover, and soil microbial activity, may contribute towards these feedbacks, further studies may be able to discern their respective contributions.
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http://dx.doi.org/10.1007/s00442-010-1690-5DOI Listing
December 2010

Introduced grazers can restrict potential soil carbon sequestration through impacts on plant community composition.

Ecol Lett 2010 Aug 12;13(8):959-68. Epub 2010 May 12.

Department of Biology, Syracuse University, Life Sciences Complex, 107 College Place, Syracuse, NY 13244, USA.

Grazing occurs over a third of the earth's land surface and may potentially influence the storage of 10(9) Mg year(-1) of greenhouse gases as soil C. Displacement of native herbivores by high densities of livestock has often led to overgrazing and soil C loss. However, it remains unknown whether matching livestock densities to those of native herbivores can yield equivalent soil C sequestration. In the Trans-Himalayas we found that, despite comparable grazing intensities, watersheds converted to pastoralism had 49% lower soil C than watersheds which retain native herbivores. Experimental grazer-exclusion within each watershed type, show that this difference appears to be driven by indirect effects of livestock diet selection, leading to vegetation shifts that lower plant production and reduce likely soil C inputs from vegetation by c. 25 gC m(-2) year(-1). Our results suggest that while accounting for direct impacts (stocking density) is a major step, managing indirect impacts on vegetation composition are equally important in influencing soil C sequestration in grazing ecosystems.
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http://dx.doi.org/10.1111/j.1461-0248.2010.01486.xDOI Listing
August 2010