PhD Oral Exam - Sachin Medigeshi Harish, Biology

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Biodiversity-rich tropical forests act as critical carbon sinks, regulating the global carbon cycle. However, deforestation and degradation threaten these ecosystems, causing biodiversity loss and shifting them into carbon sources. Understanding how woody plant communities assemble and how their functional and phylogenetic diversity influence aboveground carbon (AGC) accumulation is crucial for gaining insights into their evolutionary ecology and role in global carbon dynamics. The niche complementarity hypothesis, which suggests species occupying distinct ecological niches more efficiently use resources and enhance AGC. In addition, the mass ratio hypothesis posits that a few dominant species with traits promoting high growth and survival drive AGC accumulation. To evaluate these perspectives, I analysed functional trait and phylogenetic diversity using presence, abundance and size weighted metrics in relation to spatial and temporal patterns of AGC storage and accumulation in the tropical forests of the Western Ghats biodiversity hotspot in India. First, I examined climatic drivers of woody plant community assembly across four vertical strata, including lianas. Stronger phylogenetic clustering in understory and lianas under high climatic water deficit (CWD) suggested environmental filtering under water-limited conditions. Conversely, greater phylogenetic divergence with elevation in emergent and canopy layers indicated niche differentiation in montane habitats. Second, I assessed the independent and additive effects of functional and phylogenetic diversity on AGC stocks while accounting for environmental stress. AGC declined with increasing stress but increased with elevation. Abundance and size weighted phylogenetic divergence increased AGC and supported the mass ratio hypothesis. Combined functional and phylogenetic diversity metrics and environmental variables significantly improved AGC prediction. Third, I analysed temporal AGC accumulation across forest types. Evergreen forests had the highest accumulation rates, while El Nino-induced CWD significantly reduced AGC accumulation. A few dominant species disproportionately contributed to carbon dynamics. Dense-wooded species enhanced carbon storage in drier forests, and large-seeded species boosted accumulation in evergreen forests. Abundance-weighted phylogenetic basal divergence and size-weighted terminal divergence enhanced AGC accumulation, further supporting the mass ratio hypothesis. By highlighting the mechanisms shaping spatial and temporal patterns of AGC storage and accumulation, this study provides insights to maximize conservation and restoration efforts in tropical forests of Western Ghats.