Biology subjects

Ratnam, J.

Publications and source records attributed to Ratnam, J..

9 recordsLinked to original sources

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↗

bioRxiv2024

Taking stock of selective logging in the Andaman Islands, India: recent & legacy effects of timber extraction, assisted natural regeneration and a revamped working plan

Forest management is an evolving balance between biodiversity conservation and economic needs. A shift in Andaman Islands Working Plan mandate in 2000s reflects this evolution. Our study independently assesses the impact of said policy change on post-logging recovery of forests in Baratang and Middle Andaman. In 2017-18, we placed seventy-six 0.49ha plots across evergreen and deciduous patches and compared large-tree ([≥]180cm girth) density and diversity across forests that were logged after 2005 focussing on sustainability, logged in 1990s focussing on timber, logged twice in 1990s and after 2005, and unlogged forests. We assessed forest regeneration in thirty 0.01ha plots along a coupe road within forests logged after 2005. Forests logged after 2005 had similar density of large trees as forests logged in 1990s (despite having 1/3rd the recovery time), indicating reduced offtake or better recruitment. Along the unlogged--once-logged--twice-logged gradient, the dominance of Pterocarpus dalbergioides in deciduous patches decreased while the dominance of Diptercarpus sp. in evergreen patches increased. Compared to natural regeneration, proportionately more deciduous saplings were planted in both evergreen and deciduous patches. The new working plan maintains timber stock but not diversity. We make six simple recommendations to better align practice with the Working Plan mandate. SynthesisPost-2005 timber extraction policy in the Andaman Islands is partially successful but long-term forest health, in line with the working plan mandate, requires (1) lower timber offtake from deciduous patches and (2) targeted assisted regeneration of non-timber tree species.

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↗

bioRxiv2019

Functional diversification enabled grassy biomes to fill global climate space

Global change impacts on the Earth System are typically evaluated using biome classifications based on trees and forests. However, during the Cenozoic, many terrestrial biomes were transformed through the displacement of trees and shrubs by grasses. While grasses comprise 3% of vascular plant species, they are responsible for more than 25% of terrestrial photosynthesis. Critically, grass dominance alters ecosystem dynamics and function by introducing new ecological processes, especially surface fires and grazing. However, the large grassy component of many global biomes is often neglected in their descriptions, thereby ignoring these important ecosystem processes. Furthermore, the functional diversity of grasses in vegetation models is usually reduced to C3 and C4 photosynthetic plant functional types, omitting other relevant traits. Here, we compile available data to determine the global distribution of grassy vegetation and key traits related to grass dominance. Grassy biomes (where > 50% of the ground layer is covered by grasses) occupy almost every part of Earths vegetated climate space, characterising over 40% of the land surface. Major evolutionary lineages of grasses have specialised in different environments, but species from only three grass lineages occupy 88% of the land area of grassy vegetation, segregating along gradients of temperature, rainfall and fire. The environment occupied by each lineage is associated with unique plant trait combinations, including C3 and C4 photosynthesis, maximum plant height, and adaptations to fire and aridity. There is no single global climatic limit where C4 grasses replace C3 grasses. Instead this ecological transition varies biogeographically, with continental disjunctions arising through contrasting evolutionary histories.\n\nSignificance statementWorldviews of vegetation generally focus on trees and forests but grasses characterize the ground layer over 40% of the Earths vegetated land surface. This omission is important because grasses transform surface-atmosphere exchanges, biodiversity and disturbance regimes. We looked beneath the trees to produce the first global map of grass-dominated biomes. Grassy biomes occur in virtually every climate on Earth. However, three lineages of grasses are much more successful than others, characterizing 88% of the land area of grassy biomes. Each of these grass lineages evolved ecological specializations related to aridity, freezing and fire. Recognizing the extent and causes of grass dominance beneath trees is important because grassy vegetation plays vital roles in the dynamics of our biosphere and human wellbeing.

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↗