The influence of emergent trees on rainfall distribution in a cacao agroforest (Sulawesi, Indonesia)

Emergent trees may have an influence on the volume and the spatial distribution of water input into agroforestry stands and may thus affect water availability for the main crops. Our goal was to analyze the influence of such trees on rainfall distribution in a cacao agroforest area in the rainforest margin zone of Central Sulawesi, Indonesia. The emergent trees studied belong to the species Bischofia javanica (Phyllanthaceae) and were 15 m high remnants from the natural forest. A set of 96 throughfall gauges was systematically distributed underneath canopies of cacao only, and underneath canopies of cacao plus emergent trees (cacao plus trees). From an earlier study we knew that stemflow can safely be estimated with less than 1% of gross precipitation (Pg).Median throughfall tended to be lower in gauges underneath cacao plus trees than under cacao only (p⩽0.1), and the estimated rainfall interception loss was 4% and 16% of Pg in cacao only and cacao plus trees plots, respectively. This difference was most likely caused by a tree-induced enhancement of the canopy water storage capacity and an increase in canopy roughness. Underneath the canopy of emergent trees (cacao plus trees), throughfall exceeded gross precipitation east of the tree stems (113%) and was significantly lower (p⩽0.05) west and north of the tree stems (67–77% of Pg). Significant effects of trees on throughfall did not extend beyond their canopy area. We assume that wind-driven rain was stripped out east of the stems causing an increase in throughfall, while rain-shadow effects led to a decrease in throughfall west and north of the tree stems. Thus, at our study site emergent trees tended to reduce rain water input, and produced clear spatial patterns in throughfall distribution. A reduced water availability may lead to reduced cacao bean yields in times of water scarcity but a more complete assessment of the hydrological function of shade trees in agroforestry systems may also reveal positive influences of shade trees on cacao trees. Such an advanced analysis of hydrological functions still remains to be done.