Leaf functional response to increasing atmospheric CO 2 concentrations over the last century in two northern Amazonian tree species: a historical d 13 C and d 18 O

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, d 13 Cleaf, and discrimination, D 13 Cleaf, oxygen isotope composition, d 18 Oleaf) of two tropical rain- forest species (Dicorynia guianensis; Humiria balsam- ifera) that are abundant in the Guiana shield (Northern Amazonia). Leaf samples were collected in different inter- national 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 CO2 concentrations inside the leaf (Ci), sto- matal conductance (g), and photosynthesis (A) were tested in order to understand leaf physiological response to increasing air CO2 concentrations (Ca). Our results con- firmed that both species displayed physiological response to changing Ca. For both species, we observed a decrease of about 1.7‰ in d 13 Cleaf since 1950, without significant change in D 13 Cleaf and leaf morphological traits. Further- more, there was no clear change in d 18 Oleaf 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, allow- ing them to maintain a constant ratio of Ci/Ca.