Seasonal Water Relations and Leaf Temperature in a Deciduous Dipterocarp Forest in Northeastern Thailand

Deciduous dipterocarp forests across mainland Southeast Asia are dominated by two families: the Dipterocarpaceae and Fabaceae. Monsoon conditions produce strong seasonal climates with a hot dry season of 5-7 months extending from late November or early December through April or early May. Seasonal measurements of stomatal conductance and plant water potential found important differences between members of the two families. Despite their long dry season, Shorea siamensis and S. obtusa (Dipterocarpaceae) showed little significant patterns of seasonal change in xylem water potentials, with midday potentials never dropping below -1.3 MPa. These species present a classic example of isohydric strategies of adaptation where stomatal regulation maintains a relatively stable minimum water potential over the course of the year. However, maximum rates of stomatal conductance dropped sharply in the late dry season as the leaves heated in full sun without significant transpirational cooling, reaching as high as 44-45 degrees C, making them potentially sensitive to global increases in extreme temperature. The woody legumes Xylia kerrii and Dalbergia oliveri present different patterns of seasonal water relations and leaf response to high temperatures. The legumes exhibit anisohydric behavior where water potential decreases over the dry season as evaporative demand increases. Dry season midday water potentials dropped from high wet season levels to -2.4 to -3.2 MPa, moderately lowering maximum stomatal conductance. The relatively small leaflets of these legumes responded to the high temperatures of the late dry season by temporarily wilting, reducing their exposure to solar radiation and taking advantage of convective cooling. Large leaf size of dipterocarps in this community may not be an adaptive trait but rather an ancestral condition compensated for with ecophysiological adaptations.