Can planting trees too close together make soil absorb more rain even as the trees compete for water?

Details

Plant evapotranspiration affects soil suction on slopes and landfill covers. Previous work has mostly examined suction induced by a single plant, paying little attention to planting density. This study aimed to quantify how planting density changes tree growth and tree-induced soil suction during evapotranspiration and rainfall in single-species plantations.

In a controlled planting experiment, researchers planted Schefflera heptaphylla, a tree species commonly found in Asia, in silty sand at spacings of 60, 120 and 180 mm, representing three planting densities. Each spacing was tested with three replicates to account for variability among trees. In total, soil suction responses were measured for 297 seedlings subjected to drying and a rainfall event with a 10-year return period.

Reducing tree spacing from 180 to 60 mm increased competition between trees for water, as indicated by a 364% increase in peak suction during evapotranspiration. These tree–tree interactions led to a 19–35% reduction in leaf area index, a 17–36% decrease in root length and obvious root decay.

During the rainfall event, the infiltration rate in soil with trees spaced 60 mm apart was up to 247% higher than in soil with wider tree spacing, where mainly fresh roots were found. At 60 mm spacing, increased infiltration caused most suction within the root zone—the top 100 mm—to be lost. However, suction at deeper depths below the root zone was largely preserved.