Strategies of a Bornean tropical rainforest water use as a function of rainfall regime: Isohydric or anisohydric?

Although Bornean tropical rainforests are among the moistest biomes in the world, they sporadically experience periods of water stress. The observations indicate that these ecosystems tend to have little regulation of water use, despite episodes of relatively severe drought. This water-use behaviour is often referred to as anisohydric behaviour, as opposed to isohydric plants that regulate stomatal movement to prevent hydraulic failure. Although it is generally thought that anisohydric behaviour is an adaptation to more drought-prone habitats, we show that anisohydric plants may also be more favoured than isohydric plants under very moist environments where there is little risk of hydraulic failure. To explore this subject, we examined the advantages of isohydric and anisohydric species as a function of the hydroclimatic environment using a stochastic model of soil moisture and carbon assimilation dynamics parameterized by field observations. The results showed that under very moist conditions, anisohydric species tend to have higher productivity than isohydric plants, despite the fact that the two plant types show almost the same drought-induced mortality. As precipitation decreases, the mortality of anisohydric plants drastically increases whereas that of isohydric plants remains relatively constant and low; in these conditions, isohydric plants surpass anisohydric plants in their productivity. The observations indicate that a Bornean tropical rainforest ecosystem tends to have little regulation of water use referred to as anisohydric behavior, as opposed to isohydric plants that have a wide range of stomatal regulation to prevent hydraulic failure. Although it is generally thought that such an anisohydric behavior is an adaptation to more drought-prone habitats, we show that anisohydric plants may also be more favored than isohydric plants under very moist environments, e.g., a tropical rainforest. In this study, we examined the advantages of isohydric and anisohydric species as a function of the hydroclimatic environment using stochastic model of soil moisture and carbon assimilation dynamics parameterized by field observations.