Late-stage canopy tree species with extremely low δ13C and high stomatal sensitivity to seasonal soil drought in the tropical rainforest of French Guiana

We assessed the daily time-courses of CO2 assimilation rate (A), leaf transpiration rate (E), stomatal conductance for water vapour (g(s)), leaf water potential (Psi(w)) and tree transpiration in a wet and a dry season for three late-stage canopy rainforest tree species in French Guiana differing in leaf carbon isotope composition (delta(13)C). Th, lower sunlit leaf delta(13)C values found in Virola surinamensis (-29.9 parts per thousand) and in Diplotropis purpurea (-30.9 parts per thousand), two light-demanding species, as compared to Eperua falcata (-28.6 parts per thousand), a shade-semi-tolerant species, were clearly associated with higher maximum g(s) values of sunlit leaves in the two former species. These two species were also characterized by a high sensitivity of g(s), sap flow density (Ju) and canopy conductance (g(c)) to seasonal soil drought, allowing maintenance of high midday Psi(w) values in the dry season. The data for Diplotropis provided an original picture of increasing midday Psi(w) with increasing soil drought. In Virola, stomata were extremely sensitive to seasonal soil drought, leading to a dramatic decrease in leaf and tree transpiration in the dry season, whereas midday Psi(w) remained close to -0.3 MPa. The mechanisms underlying such an extremely high sensitivity of stomata to soil drought remain unknown, In Eperua, g(s) of sunlit leaves was nonresponsive to seasonal drought, whereas Ju and g(c) were lower in the dry season. This suggests a higher stomatal sensitivity to seasonal drought in shaded leaves than in sunlit ones in this species.