Limited stomatal regulation of the largest-size class of Dryobalanops aromatica in a Bornean tropical rainforest in response to artificial soil moisture reduction

The physiological response of trees to drought is crucial for understanding the risk of mortality and its feedbacks to climate under the increase in droughts due to climate change, especially for the largest trees in tropical rainforests because of their large contribution to total carbon storage and water use. We determined the response of the mean canopy stomatal conductance per unit leaf area (g_s) and whole-tree hydraulic conductance (G_p) of the largest individuals (38–53 m in height) of a typical canopy tree species in a Bornean tropical rainforest, Dryobalanops aromatica C.F.Gaertn., to soil moisture reduction by a 4-month rainfall exclusion experiment (REE) based on the measurements of sap flux and leaf water potentials at midday and dawn. In the mesic condition, the g_s at vapor pressure deficit (D) = 1 kPa (g_sref) was small compared with the reported values in various biomes. The sensitivity of g_s to D (m) at a given g_sref (m/g_sref) was ≥ 0.6 irrespective of soil moisture conditions, indicating intrinsically sensitive stomatal control with increasing D. The REE caused greater soil drought and decreased the mean leaf water potentials at midday and dawn to the more negative values than the control under the relatively dry conditions due to natural reduction in rainfall. However, the REE did not cause a greater decrease in g_s nor any clear alteration in the sensitivity of g_s to D compared with the control, and induced greater decreases in G_p during REE than the control. Thus, though the small g_s and the sensitive stomatal response to D indicate the water saving characteristics of the studied trees under usual mesic conditions, their limited stomatal regulation in response to soil drought by REE and the resulting decline in G_p might suggest a poor resistance to the unusually severe drought expected in the future.