Stem girth changes in response to soil water potential in lowland dipterocarp forest in Borneo: a time-series analysis

Time series data offer a way of investigating the causes driving ecological processes. To test for possible differences in water relations between species of different forest structural guilds, daily stem girth increments (gthi), of 18 trees across six species were regressed individually on soil moisture potential (SMP) and temperature (TEMP), accounting for temporal autocorrelation (in GLS-arima models), and compared between a wet and a dry period. Coefficients were estimates of response in gthi to increasing SMP or TEMP. The best-fitting significant variables were SMP the day before and TEMP the same day. The first resulted in a mix of positive and negative coefficients, the second largely positive ones. Negative relationships for large canopy trees can be interpreted in a reversed causal sense: fast transporting stems depleted soil water and lowered SMP. Positive relationships for understorey trees meant they took up most water at high SMP. The unexpected negative relationships for these understorey trees may have been due to their roots accessing deeper water supplies (SMP being inversely related to that of the surface layer), this influenced by competition with larger neighbour trees. A tree-soil flux dynamics manifold may have been operating. Patterns of mean diurnal girth variation were more consistent among species than, but weakly related to, time-series coefficients, suggesting no simple trait-based differentiation of responses. Expected differences in response to SMP in the wet and dry periods did not support a previous hypothesis for drought and non-drought tolerant understorey guilds. Trees within species showed highly individual responses. Time-series gthi-SMP regressions might be applied as indicators of relative depth of access to water for small trees. Obtaining detailed information on individual tree9s root systems and recording SMP at many depths and locations are needed to get closer to the mechanisms that underlie complex tree-soil water relations in tropical forests.