Photosynthetic traits scale linearly with relative height within the canopy in an African tropical forest

Abstract

• Understanding leaf photosynthetic traits and their variation in tropical forests is crucial for improving model predictions of forest productivity, and accurately representing the high functional diversity in these forests remains a challenge. Moreover, leaf photosynthesis data are lacking for the tropical forest of the Congo basin.
• We observed photosynthetic, chemical and structural leaf traits of 24 woody species in a Congolese tropical forest and studied their variance across functional guilds, within-tree crown positions and overall canopy positions defined by their relative height within the canopy.
• Guild and crown position jointly influenced leaf traits, with a significant effect observed (marginal R² > 0.43). The traditional guild classification explained a significant portion of the observed interspecies variation, revealing a clear gradient from shade-tolerant to light-demanding species. Crown position significantly affected intraindividual leaf trait variability, with bottom crown leaves exhibiting trait values at least 19.3% lower than top leaves. Importantly, the linear relationship between relative canopy height and leaf traits emerged as a robust and continuous metric, effectively integrating both inter- and intraspecific variability.
• We conclude that while guild-based classifications provide a useful framework for identifying plant functional groups, relative canopy height offers a robust and quantitative approach for capturing overall canopy trait variation, valuable for modeling canopy processes.