Biology subjects

Raman, T. R. S.

Publications and source records attributed to Raman, T. R. S..

6 recordsLinked to original sources

bioRxiv2026

Phenology of 50 tree species across 9 years in a South Asian tropical rainforest indicates complex influence of climate, traits, and phylogeny

In relatively aseasonal tropical rainforests, few studies have explored long-term phenological patterns of a high diversity of tree species in relation to climate, phylogeny, and functional traits. In these systems, short-duration seasonal pulses of irradiance and water deficit are expected to provide narrower windows for leafing and flowering and wider windows for fruiting across prolonged wet seasons, potentially mediated by functional traits and phylogenetic relatedness. Here, we document leafing, flowering, and fruiting phenology of 50 tree species (920 - 1077 trees, 10 - 42 trees/species) monitored monthly over an 9-y period (2017 - 26) in a relatively aseasonal south Asian tropical rainforest in the Anamalai Hills, Western Ghats, India. We examined correlations between climatic variables (irradiance, daylength, temperature, precipitation) and tree phenology and Mantel correlations among similarity in monthly phenology, functional traits (wood density, seed size, and maximum height), and phylogenetic relatedness. We then investigated the phylogenetic signal of phenological traits (frequency, amplitude, duration, and peak month) using Pagels {lambda}. Leaf flushing and flowering showed distinct seasonality and negative associations with daylength and precipitation, whereas fruiting showed greater temporal spread and weaker associations with climate. Functional traits or phylogeny did not significantly influence leaf flushing and flowering, whereas dissimilarity in fruiting was correlated with phylogenetic distance and peak fruiting month showed a significant phylogenetic signal ({lambda} = 0.96). The results indicate that in relatively aseasonal tropical rainforests, proximate climatic cues more strongly influence leaf flushing and flowering, whereas phylogenetic constraints affect timing of fruiting and may cause lineage-specific vulnerabilities to climate change.

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↗

bioRxiv2024

Distribution models predict climate-related range alteration or extinction of eleven threatened tropical rainforest trees in the Western Ghats

Evidence for climate-change related alteration in distributions and ranges of forest trees is accumulating, but information from Asian tropical forests, particularly for threatened and endemic species, remains limited. Here, we examine landscape-level distribution-abundance patterns of 11 endemic and threatened tropical rainforest tree species in the Anamalai Hills and model their distribution and responses to future climate scenarios in the Southern Western Ghats (SWG), India. Six of the primarily low- and mid-elevation species were more abundant in protected reserves than forest fragments. Occurrence data from the Anamalais and SWG (N = 3004, range: 41 - 706 per species) were used to model distributions using maximum entropy (maxent) species distribution modelling in R. Maxent model performances indicated excellent fits for 9 of 11 species (AUC>0.90) with precipitation and temperature variables showing higher permutation importance. There was high interspecies variability in range size (197 - 12,221 km2) and niche width (0.04 - 0.50). Models of distribution under future climate in 2061 - 2080 predict range reductions in six species (including near-extinction for two species), increases for three species, and no substantial change for two species. Predicted southward and westward shifts in ranges and persistence in parts of the SWG indicate the importance of identifying and conserving micro-climatic zones or refugia to ensure the persistence of tree species under anticipated climate change.

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

Forest bird decline and community change over 19 years in long-isolated South Asian tropical rainforest fragments

Recent evidence of forest bird declines worldwide is attributed to climate change and its interactive effects with recent land-use changes such as forest loss and fragmentation, and avian life-history traits. In Asian tropical forests, such effects are poorly understood as long-term data are lacking from fragments that are long-isolated rather than recently fragmented. Here, we use data from ~2000 point-counts from bird surveys carried out between 2000 - 2005 and 2019 in 19 long-isolated (~80 y) South Asian tropical rainforest fragments to examine changes in bird species richness, density, and composition in relation to fragment area (0.7 - 4310 ha), habitat structure, and time. Over the 19 y timespan, despite stable fragment areas, we uncovered a 29% decline in rainforest bird density and 7% decline in individual-rarefied species richness of rainforest birds, while density and richness of open-country birds remained stable. With increasing fragment area, rainforest bird species richness (jackknife estimate) increased, while open country bird richness (individual-rarefied) and density decreased. Larger fragments housed more compositionally stable bird communities, while poorer habitat was associated with lower diversity of rainforest birds but higher diversity, density, and compositional variation of open-country birds. Threshold analysis however indicated relatively small area thresholds (~20 ha) for rainforest bird species abundance. Besides identifying alarming declines in rainforest birds, the study confirms some but not all predictions for bird diversity in long-isolated forest fragments with stable forest-matrix boundaries, indicating that small fragments and habitat quality also matter.

ecology↗