Drought induces opposite changes in the concentration of non-structural carbohydrates of two evergreen Nothofagus species of differential drought resistance

One current explanation for worldwide drought-induced tree mortality states that reduced photosynthesis and continued respiration lead to carbon depletion and eventually to carbon starvation. To determine if variations in gas exchange are consistent with variations in carbon storage, and if such consistency may depend on the drought resistance of a species, I examined the non-structural carbohydrates (NSC) concentration and gas exchange in seedlings of two Nothofagus species of differential drought resistance under severe drought-just before death-and under well-watered conditions. Drought provoked decreased photosynthesis and had no effect on leaf respiration in both species, whereas NSC concentrations varied oppositely: it decreased in the relatively more drought-susceptible species (Nothofagus nitida) whilst it increased in the relatively more drought-resistant species (Nothofagus dombeyi). Thus, if carbon balance would have been inferred from gas exchange alone, I would have wrongly concluded that carbon depletion occurred in both species. In stressed seedlings of N. nitida, photosynthesis and NSC concentrations were negatively correlated in roots (r (2) = -0.57, p = 0.03) and not correlated in stems (r (2) = -0.05, p = 0.58), indicating that carbon depletion due to reduced photosynthesis was not occurring at harvest time, but it took place earlier when water stress was milder. Results demonstrate that carbon depletion cannot be predicted from measurements of gas exchange. Drought-induced mobilization of carbon storage appeared influenced by the drought resistance of the species and by drought intensity.