Effects of rainfall exclusion on leaf gas exchange traits and osmotic adjustment in mature canopy trees of Dryobalanops aromatica (Dipterocarpaceae) in a Malaysian tropical rain forest

Climate change exposes vegetation to unusual levels of drought, risking a decline in productivity and an increase in mortality. It still remains unclear how trees and forests respond to such unusual drought, particularly Southeast Asian tropical rain forests. To understand leaf ecophysiological responses of tropical rain forest trees to soil drying, a rainfall exclusion experiment was conducted on mature canopy trees of Dryobalanops aromatica Gaertn.f. (Dipterocarpaceae) for 4 months in an aseasonal tropical rain forest in Sarawak, Malaysia. The rainfall was intercepted by using a soft vinyl chloride sheet. We compared the three control and three treatment trees with respect to leaf water use at the top of the crown, including stomatal conductance (gs(max)), photosynthesis (A(max)), leaf water potential (predawn: Psi(pre); midday: Psi(mid)), leaf water potential at turgor loss point (pi(tlp)), osmotic potential at full turgor (pi(100)) and a bulk modulus of elasticity (epsilon). Measurements were taken using tree-tower and canopy-crane systems. During the experiment, the treatment trees suffered drought stress without evidence of canopy dieback in comparison with the control trees; e.g., Psi(pre) and Psi(mid) decreased with soil drying. Minimum values of Psi(mid) in the treatment trees decreased during the experiment, and were lower than pi(tlp) in the control trees. However, the treatment trees also decreased their pi(tlp) by osmotic adjustment, and the values were lower than the minimum values of their Psi(mid). In addition, the treatment trees maintained gs and A(max) especially in the morning, though at midday, values decreased to half those of the control trees. Decreasing leaf water potential by osmotic adjustment to maintain gs and A(max) under soil drying in treatment trees was considered to represent anisohydric behavior. These results suggest that D. aromatica may have high leaf adaptability to drought by regulating leaf water consumption and maintaining turgor pressure to improve its leaf water relations.