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

Philip Rundel,Mark Patterson,Kansri Boonpragob

doi:10.3390/f8100368

Philip Rundel, Mark Patterson + Show 1 more

Open Access

https://doi.org/10.3390/f8100368

Copy DOI

Abstract

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 °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.

Highlights

Tropical dry forests which exhibit diverse patterns of leaf lifespan among dominant species provide interesting systems to investigate how interactions of seasonal environmental conditions influence leaf structural traits, gas exchange, and water relations
We focused attention on two species in each family: Shorea siamensis Miq. and S. obtusa Wall. in the Dipterocarpaceae and Xylia xylocarpa Craib and Hutch. and Dalbergia oliveri Prain in the Fabaceae
There were significant differences in individual leaf area of four dipterocarp species compared to the range of individual leaflet area, the functional photosynthetic unit, in legumes, but specific leaf weight was not significantly different

Summary

Introduction

Tropical dry forests which exhibit diverse patterns of leaf lifespan among dominant species provide interesting systems to investigate how interactions of seasonal environmental conditions influence leaf structural traits, gas exchange, and water relations. Unlike temperate forests where leaf phenology and longevity are strongly synchronized by predictable seasonal progressions of temperature and major changes in photoperiod, tropical dry forests with 4–6 months or more of drought commonly lack such cues [1]. Both deciduous and evergreen species often co-occur as dominants. There has been an expanding research interest in comparing and contrasting the ecophysiology of evergreen and deciduous leaf phenologies in dry tropical forests [2] Much of this focus has centered on divergent strategies of hydraulics and seasonal gas exchange. These latter studies have shown differences in ecophysiological adaptations of evergreen compared to drought deciduous trees in Thai forests

Methods

Results

Discussion

Conclusion