How Do Rubber (Hevea brasiliensis) Plantations Cope with Seasonal Drought in Northern Thailand and Central Cambodia?

Abstract:

Rubber (Hevea brasiliensis Müll. Arg.) plantaitons are rapidly expanding throughout mainland Southeast Asia, potentially changing the partitioning of water, energy, and carbon at multiple scales, compared with the traditional land covers they are replacing. We have conducted eddy flux measurements in two rubber plantation sites: Som Sanuk (SS), located northern Thailand; and Cambodian Rubber Research Institute (CRRI), central Cambodia. We used combination of actual evapotranspiration (E_T) flux measurements and an inversed version of a simple 2-layer E_T model for estimating the mean canopy stomatal conductances (g_s), which is among the most effective measures for describing the exchange characteristics. It is demonstrated how each studied rubber plantation copes with each strong seasonal drought via tree water use strategies. Potential tree water use deficit (precipitation (P) - potential evaporation (E_{T_POT})) for each season (i.e., December-February: DJF, March-May: MAM, June-August: JJA, and September-November: SON) revealed in which season and how the water use should be controlled. We found that in the season when actual tree water use deficit (P - E_T) was negative (DJF and MAM), the deficit was compensated with soil water from the previous season at a depth of 0-2 m at the Thailand site, and from a depth of 0-3 m at CRRI. Two ecophysiological parameters, the reference value of g_s (g_sref) and the sensitivity of g_s to atmospheric demand (m), as well as their proportionality (m/g_sref), were derived from the logarithmic response curve of g_s to vapor pressure deficit (D) for each season and each site. In both sites, g_sref and m appeared to be less in DJF and MAM than each in the other three month periods (seasons). On average in a whole year, m/g_sref was less than 0.6 at SS and almost 0.6 at the CRRI site, suggesting that there was less sufficient stomatal regulation at SS, where there might be little risk of water stress-induced hydraulic failure because of much annual rainfall amount. In comparison, at CRRI where annual P - E_{T_POT} was negative, there was stricter stomatal regulation, preventing excessive xylem cavitation. These tendencies imply that in the drier season, i.e., DJF and MAM, the rubber trees in SS and CRRI adopt the stomatal control strategy of changing g_sref with reluctance and positive to change m, respectively.