Stomatal anatomy, leaf structure and nutrients of tropical rainforest tree species respond to altitude in a coordinated manner in accordance with the leaf economics spectrum

Understanding taxon level responses of key plant traits to environmental variation across tropical rainforests (TRFs) is important to determine their response to climate change. We used an altitudinal gradient (from 150 to 2100 m above sea level, asl) across TRFs in Sri Lanka to answer the following questions: (a) Does the response to altitude by stomatal traits differ among plant taxa in TRFs?; (b) Are the altitudinal responses of key leaf structural traits (e.g. leaf mass per area, LMA) and major leaf nutrient (nitrogen, N, and phosphorus, P) concentrations linked to the corresponding variation in stomatal traits in a coordinated response across taxa?; (c) How strong is the influence of climatic variation on responses of leaf traits to altitude?. Leaf samples were collected in permanent sampling plots within rainforest reserves at Kanneliya (150 m asl), Sinharaja-Enasalwatta (1050 m), Hakgala (1800 m) and Piduruthalagala (2100 m) from 19 species in three plant genera (Calophyllum, Semecarpus and Syzygium). Stomatal density, guard cell length and epidermal density showed variation among taxa, but did not respond to altitude. Potential conductance index (PCI), a proxy for photosynthetic capacity, decreased with increasing altitude, in a common response across taxa. We found evidence that altitudinal responses of LMA, leaf N and P were linked to stomatal responses in a coordinated manner, where key features were the negative correlations between PCI and LMA and between proxy photosynthetic N- and P-use efficiencies ( PNUE and PPUE) and LMA. We found strong responses to climatic variation across taxa and altitudes, where PCI, PNUE and PPUE increased and LMA decreased with increasing temperature, precipitation and solar irradiance. We conclude that stomatal traits of tree species in TRFs form part of a coordinated leaf trait response to environmental change which is in accordance with the leaf economics spectrum.