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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, delta C-13(leaf), and discrimination, Delta C-13(leaf), oxygen isotope composition, delta O-18(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 CO2 concentrations inside the leaf (C-i), stomatal conductance (g), and photosynthesis (A) were tested in order to understand leaf physiological response to increasing air CO2 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 delta C-13(leaf) since 1950, without significant change in Delta C-13(leaf) and leaf morphological traits. Furthermore, there was no clear change in delta O-18(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.