Eco-Physiological Responses of Nigerian Forest Tree Species to Elevated CO2 and Thermal Stress: Implications for Future Forest Composition

The eco-physiological responses of tropical forest tree species to the combination of elevated atmospheric CO2 and thermal stress determine whether future forest composition shifts will be gradual or exhibit threshold non-linearities, yet experimental eco-physiological data for West African forest tree species are virtually absent from the global literature, creating significant uncertainty in Earth System Model projections for the region. This study investigates the eco-physiological responses of twelve Nigerian forest tree species, spanning a range of functional types and climatic niches, to elevated CO2 (700 ppm) and two thermal stress treatments (+2 degrees Celsius and +4 degrees Celsius above ambient) in a controlled growth chamber experiment. Seedlings were grown from seeds collected from wild populations across a latitudinal gradient spanning humid rainforest to derived savanna. Photosynthetic gas exchange, water use efficiency, stomatal conductance, dark respiration, leaf nitrogen content, and growth rates were measured at six-weekly intervals over twenty-four weeks. Thermal tolerance limits were assessed through chlorophyll fluorescence-based high-temperature response curves. Results demonstrate significant species-specific divergence in responses to elevated CO2 and thermal stress combinations. Guinea-Congolian species showed CO2 fertilisation effects that were partially offset by thermal stress above two degrees Celsius. Sudan-Sahel adapted species showed greater thermal tolerance but less CO2 fertilisation response, suggesting differential competitive outcomes under future climate scenarios. Three species exhibited photosynthetic downregulation under elevated CO2, inconsistent with current Earth System Model assumptions. The study provides species-specific eco-physiological parameters for improving regional climate-vegetation model representations of Nigerian forest dynamics. Keywords: eco-physiology, elevated CO2, thermal stress, Nigerian forest trees, climate change