<S1-11> Effect of through-fall exclusion on leaf water use in the tropical canopy tree species Dryobalanops aromatica

Abstract

Climate change exposes vegetation to unusual drought, causing risk of a decline in productivity and increased mortality. It still remains unclear how trees and forests respond to such unusual drought, particularly in tropical rain forests. For a detailed study of tropical tree responses to soil drying, a through-fall exclusion experiment was conducted on three target trees of Dryobalanops aromatica, which is one of the dominant canopy tree species, for three months at Lambir Hills National Park. We compared the control and treatment trees regarding leaf water use at the top of the crown, including transpiration, stomatal conductance, photosynthesis, leaf water potential (predawn: Ψpre, midday: Ψmid and at turgor loss point: πtlp), osmotic potential at full turgor (π100), and a bulk modulus of elasticity (ε), by use of tree-tower and canopy-crane systems. During the experiment, there was no evidence of canopy dieback. However, the treatment trees showed higher water stress than the control trees, because predawn and midday leaf water potential decreased with soil drying. Additionally, the drought treatment led to a reduction in stomatal conductance (gs), and the treatment trees showed about half the value of gs compared with the control trees. Consequently, transpiration and photosynthetic rates clearly decreased compared with those in the control trees. Minimum values of midday leaf water potential in the treatment trees decreased during the drought, and were lower than πtlp in the control trees. However, the treatment trees also decreased their πtlp by osmotic adjustment, and their πtlp values were lower than the minimum values of their midday leaf water potential. These results suggest that this species may have plasticity in drought tolerance by maintaining turgor pressure to improve its leaf water relations and regulating water consumption during severe drought.