Investigation of plant growth and transpiration-induced matric suction under mixed grass-tree conditions

Although evapotranspiration-induced matric suction for single species has been widely studied, little is known about how mixed-species planting would affect plant growth and induced matric suction. This study aims to explore the effects of grass-tree interaction on their growth and induced matric suction during evapotranspiration (ET) and rainfalls. Field monitoring was carried out to measure matric suction responses in compacted soil that was vegetated with (i) single tree species, Schefflera heptaphylla, and (ii) mixed species of the trees and a grass species, Cynodon dactylon. In each condition, three tree spacings (120, 180, and 240 mm) were planted. When tree spacing increased from 120 to 240 mm, the peak tree root area index (RAI, for fine roots with diameter <2 mm) decreased by 16%, but the peak grass RAI increased by 29%. At mixed planting plots, the ET-induced peak matric suction for a tree spacing of 240mm was 20% higher than that for a spacing of 180mm because of increased contribution of grass-root water uptake as the trees were more widely spaced. Without grass, a reverse trend was observed as tree-tree interaction reduced at wide spacings. The peak ET-induced matric suction had a significant linear correlation with RAI. During rainfalls, the highest matric suction was preserved for the case of 240mm spacing due to the greatest reduction of soil hydraulic conductivity by the presence of roots.