Drought intensification increases extinction risk in the recalcitrant-seeded oak Quercus thorelii in the Indo-Burma biodiversity hotspot

Escalating anthropogenic disturbance and climate change are increasing extinction risks in the Indo-Burman region, yet the mechanisms driving rarity in many threatened trees remain poorly understood. Quercus thorelii, a low-elevation evergreen oak with only 11 known occurrences, exemplifies this pattern of rarity and vulnerability. This study combined seed-to-seedling trait assays with species distribution modeling (SDM) to identify the physiological and environmental mechanisms underlying its rarity. We assessed seed desiccation sensitivity, germination dynamics, and early seedling tolerance to drought and predation. We then identified key environmental factors influencing current habitat suitability and projected its future distribution under climate change scenarios.

Q. thorelii seeds are dispersed during the dry season but are highly desiccation-sensitive, with a broad germination temperature range and high tolerance to predation, suggesting that desiccation may be a primary constraint on seedling recruitment. Although its seeds germinate rapidly, thereby reducing exposure to drought, their large size restricts dispersal into moist microhabitats essential for successful germination. Despite a sequential seedling establishment strategy characterized by elongated cotyledonary petioles that position the embryo deeper in the soil, rapid allocation of cotyledon reserves to promote early taproot development, and temperature-regulated epicotyl physiological dormancy synchronized with the rainy season, successful recruitment remains constrained by seed desiccation. Habitat suitability for Q. thorelii was influenced more strongly by precipitation than by other environmental variables, including anthropogenic disturbance, highlighting precipitation as the dominant environmental predictor of its distribution. Under all future scenarios, total suitable habitat is projected to decline by > 80%, with only ~ 1% of future refugia located within protected areas. Based on these findings, Q. thorelii warrants reassessment as Critically Endangered under IUCN criteria. Assisted germination and reintroduction into climatically suitable refugia represent practical conservation strategies.

Our findings highlight the exceptional vulnerability of recalcitrant-seeded trees to increasing drought and demonstrate how seed biology shapes extinction risk under climate change.