Carbon capture, photosynthesis, and leaf gas exchange of shade tree species and Arabica coffee varieties in coffee agroforestry systems in Veracruz state, Mexico.

Agroforestry systems with native vegetation enhance climate adaptation and mitigation by improving coffee farm resilience, carbon storage, and income diversification. Seven native tree species were pre-selected as shade providers for Veracruz coffee agroforestry systems based on ecological, cultural, and economic criteria. The present study evaluated their physiological performance through above-ground biomass, carbon stocks, and in-situ chlorophyll fluorescence and gas exchange measurements under controlled light and temperature conditions. Five Coffea arabica varieties were also assessed under these shade canopies using the same leaf-level parameters, and leaf nitrogen and moisture content. Erythrina americana and Persea schiedeana had the highest carbon sequestration per tree. E. americana showed the highest water-use efficiency, whereas P. schiedeana showed the lowest transpiration and stomatal conductance, indicating a water-saving strategy via stomatal restriction. These traits reflect their ecological adaptations to shade and microclimate conditions in agroforestry systems. Inga inicuil achieved the highest carbon capture per hectare due to high tree density, despite lower individual performance. Species-specific strategies were identified: Psidium guajava and P. schiedeana exhibited high transpiration but limited carbon gain. E. americana and Inga punctata formed a drought-resilient group, having a high carbon assimilation and low water loss. Intermediate species (Heliocarpus appendiculatus, Inga vera, I. inicuil) balanced moderate CO2 assimilation rates with adaptable stomatal response. Photochemical efficiency remained stable across species. Shaded Coffea arabica var. Oro Azteca had significantly higher leaf nitrogen, moisture, and water-use efficiency than unshaded ones. These differences coincided with lower PAR under shade, aligning with known variations in shaded versus unshaded coffee plants. Principal component analysis showed that PC1 correlated strongly with stomatal conductance and transpiration, driven by P. guajava and P. schiedeana. PC2 showed a carbon economy trade-off between CO2 assimilation and internal concentration, dominated by E. americana. Collectively, these components highlight stomatal regulation and carbon management as adaptive strategies. Coffee PCA revealed contrasting water-use strategies: PC1 showed inverse stomatal regulation (especially in shaded varieties), and PC2 an energy allocation trade-off between photochemical efficiency and carbon assimilation, with shaded plants maintaining stable CO2 assimilation regarding unshaded ones. These results demonstrate notable interspecific variation in carbon storage, water-use efficiency, and light conditions among shade trees, offering empirical support for species selection in Veracruz coffee agroforestry.