Leaf functional response to increasing atmospheric CO 2 concentrations over the last century in two northern Amazonian tree species: A historical δ 13C and δ 18O approach using herbarium samples

Abstract

We assessed the extent of recent environmental changes on leaf morphological (stomatal density, stomatal surface, leaf mass per unit area) and physiological traits (carbon isotope composition, δ 13C leaf, and discrimination, Δ 13C leaf, oxygen isotope composition, δ 18O leaf) of two tropical rainforest species (Dicorynia guianensis; Humiria balsamifera) that are abundant in the Guiana shield (Northern Amazonia). Leaf samples were collected in different international herbariums to cover a 200 year time-period (1790-2004) and the whole Guiana shield. Using models describing carbon and oxygen isotope fractionations during photosynthesis, different scenarios of change in intercellular CO 2 concentrations inside the leaf (C i), stomatal conductance (g), and photosynthesis (A) were tested in order to understand leaf physiological response to increasing air CO 2 concentrations (C a). Our results confirmed that both species displayed physiological response to changing C a. For both species, we observed a decrease of about 1.7‰ in δ 13C leaf since 1950, without significant change in Δ 13C leaf and leaf morphological traits. Furthermore, there was no clear change in δ 18O leaf for Humiria over this period. Our simulation approach revealed that an increase in A, rather than a decrease in g, explained the observed trends for these tropical rainforest species, allowing them to maintain a constant ratio of C i/C a. © 2011 Blackwell Publishing Ltd.