VULNERABILITY TO DROUGHT-INDUCED EMBOLISM OF BORNEAN HEATH AND DIPTEROCARP FOREST TREES

Occasional droughts may be important in controlling the distribution a nd structure of forest types in relatively aseasonal north Borneo. The low water retention capacity of the coarse, sandy soils on which trop ical heath forest occurs may cause drought to develop more quickly and severely than on the finer textured soils of nearby dipterocarp fores t. Resistance to drought-induced embolism is considered an important c omponent of drought tolerance. We constructed embolism vulnerability c urves relating loss in hydraulic conductivity to xylem tension by the air-injection method for understory trees of 14 species from both trop ical heath and mixed dipterocarp forests in Brunei Darussalam. There w as no significant difference (Mann-Whitney U-test, P = 0.11) between f orest types in the xylem tension at which 50% loss of hydraulic conduc tivity occurred. Most species from both forest types were highly vulne rable to embolism compared with species from seasonal tropical forests . We speculate that other mechanisms, such as stomatal control to prev ent development of embolism-inducing xylem tensions: are more cost-eff ective adaptations against occasional drought, but that the attendant reduction in productivity and competitive ability places a greater pre mium on resistance to embolism when drought is annual and predictable.