The scale of the fieldwork alone sets the study apart. Using a stratified random quadrat method, the team laid out a total of 1,050 quadrats of different sizes across the forest, a sampling intensity that allowed them to capture the fine-grained structure of the tree community rather than relying on a handful of broad transects. Within each quadrat they recorded tree species identity, counted individual stems, measured trunk diameters at breast height, the standard forestry metric abbreviated as DBH, and calculated total basal cover, a measure of the ground area occupied by tree trunks that serves as a proxy for the dominance and developmental stage of a stand. From these raw measurements they derived a battery of classical ecological indices, including the Shannon-Weiner diversity index, Margalef’s species richness index, Simpson’s index of dominance, and Pielou’s species evenness index, each of which captures a different facet of how biological variety is organized in a community.

The contrast between the two habitats was stark. The interior habitat harbored 38 tree species, compared with only 22 in the edge habitat, and its stem density reached 1,460 stems per hectare against 1,093.33 stems per hectare at the edge. Total basal cover followed the same pattern, standing at 84.41 square meters per hectare in the interior versus 50.05 square meters per hectare at the edge, indicating that interior trees were not merely more numerous but also larger and structurally more dominant. The Shannon-Weiner diversity index, which blends species richness with the relative abundance of each species, scored 3.49 in the interior against 2.90 at the edge, while Margalef’s richness index registered 5.29 versus 3.13. In every measure of diversity and structural complexity, the deep forest outperformed its own margin.

Yet the edge habitat was not simply an impoverished version of the interior. Two indices actually favored the boundary zone: Simpson’s index, which measures the probability that two randomly drawn individuals belong to the same species, was higher at the edge at 0.07 compared with 0.04 in the interior, and Pielou’s evenness index reached 1.07 at the edge against 1.04 inside. These values suggest that although fewer species live at the edge, the individuals that do survive there are distributed more evenly among those species, a pattern often observed in disturbed environments where hardy, generalist species proliferate without any single one dominating. When the researchers subjected the tree communities of the two habitats to statistical comparison, the difference in species diversity proved highly significant at the p less than or equal to 0.01 level, confirming that the edge-interior gradient represents a genuine ecological discontinuity rather than random variation.

Disturbance emerged as the most likely driver of the divide. By applying a disturbance index to each habitat, the team found that the edge habitat had suffered 38.69 percent disturbance, nearly four times the 9.25 percent recorded in the interior. Fatasil Reserve Forest sits within the Kamrup Metropolitan District, pressed against the expanding city of Guwahati, and its margins absorb the brunt of human activity, from fuelwood collection and grazing to encroachment and the microclimatic changes that follow when a closed canopy is opened to the sky. Edge effects of this kind are among the most extensively documented phenomena in conservation biology. Fragmented habitats expose interior species to altered light, temperature, wind, and humidity regimes, and the landmark long-term studies of Amazonian forest fragments cited in the paper have shown that such changes can trigger cascading losses of biodiversity that deepen over decades.

The carbon story proved equally dramatic, if statistically subtler. Total biomass in the interior habitat reached 1,023.62 megagrams per hectare, nearly double the 535.86 megagrams per hectare found at the edge. Converting biomass to carbon using standard allometric formulas and the widely applied assumption that roughly half of dry biomass is carbon, the researchers estimated a total carbon stock of 511.81 megagrams of carbon per hectare in the interior versus 267.93 megagrams of carbon per hectare at the edge. Expressed in terms of climate impact, the carbon dioxide equivalent came to 1,878.34 megagrams per hectare in the interior and 983.30 megagrams per hectare at the edge. In other words, the protected heart of this forest holds almost twice the climate-warming carbon of its exposed rim, and the difference between the two zones amounts to hundreds of megagrams of carbon dioxide equivalent per hectare.

Interestingly, when the researchers ran their statistical tests, the differences in total biomass, total carbon stock, and carbon dioxide equivalent between the two habitats did not reach significance at the p less than or equal to 0.01 threshold, even though the raw values differed substantially. The authors attribute the pattern to the enormous contribution of large-diameter trees, whose individual biomass can outweigh that of dozens of smaller stems, introducing high variability into the estimates. What did emerge as highly significant was the distribution of carbon across diameter classes. Carbon stock differed significantly among DBH classes in both habitats, peaking in the 20.1 to 30 centimeter class. This finding carries practical weight: mid-sized trees, large enough to accumulate serious woody biomass but still in their vigorous growth phase, are the workhorses of carbon sequestration in this forest, and any management action that removes or suppresses this size class would disproportionately undermine the forest’s climate function.

The biomass estimates themselves rest on allometric equations, the mathematical models that translate easily measured variables such as trunk diameter into harder-to-measure quantities like total aboveground mass. The study draws on the pantropical allometric frameworks refined by Chave and colleagues, which were calibrated through destructive sampling of trees across the tropics, alongside root biomass allocation factors developed for upland forests to account for the belowground component. This methodological lineage matters because carbon accounting for forests underpins everything from national greenhouse gas inventories to REDD+ programs that pay developing countries to keep their forests standing. When a study demonstrates that a single reserve forest can vary by nearly 250 megagrams of carbon per hectare between its edge and its interior, it underscores how coarse national averages can mask the fine-scale heterogeneity that determines where conservation dollars actually buy the most carbon.

The context of Fatasil Reserve Forest makes these findings especially urgent. Guwahati is one of the most rapidly urbanizing cities in northeastern India, and its green fragments serve double duty as biodiversity refuges and as buffers against urban heat, air pollution, and flooding. The reserve also features in documented human-wildlife interactions, including leopards moving through the urban matrix, a reminder that these fragments are living ecosystems rather than ornamental parks. Previous work by the same research group has examined tree diversity in Assam’s tropical moist deciduous forests, biomass in urban campus trees, and carbon stored by roadside plantings in Guwahati, building a picture of how vegetation across the urban-rural gradient of the region stores and cycles carbon. The new edge-interior analysis adds a critical dimension to that picture by showing that the value of a forest fragment is not uniform across its area.

The management implications are clear and the authors state them directly: the interior habitat’s ecological significance and carbon sequestration potential deserve protection, while the edge habitat requires targeted interventions to arrest its degradation. Buffer plantings, restrictions on biomass extraction, and restoration of degraded margins could shrink the effective edge zone and allow interior conditions to expand, a strategy consistent with global evidence that edge effects diminish as fragments grow or as their boundaries are softened. For a forest holding over 1,800 megagrams of carbon dioxide equivalent per hectare in its interior, the stakes of such interventions extend from the neighborhoods of Guwahati to the global climate ledger. As tropical forests worldwide continue to be carved into ever smaller pieces, studies like this one provide the quantitative grounding needed to argue that what happens at the edge of a forest is never confined to its edge.

Subject of Research: Edge-interior variation in tree diversity, community structure, and carbon stock in a tropical reserve forest in Assam, India

Article Title: Disparities in species diversity, community characteristics, and carbon stock of trees along the edge-interior gradient of Fatasil Reserve Forest, Assam, India

Article References: Kalita, P., & Yumnam, J. Y. (2026). Disparities in species diversity, community characteristics, and carbon stock of trees along the edge-interior gradient of Fatasil Reserve Forest, Assam, India. Environmental Monitoring and Assessment, 198(10), Article 1120. https://doi.org/10.1007/s10661-026-15966-y

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15966-y

Keywords: forest ecology, edge effects, carbon stock, biodiversity, tree biomass, Fatasil Reserve Forest, Assam, habitat fragmentation, disturbance, allometric equations, tropical forest, carbon sequestration