Processes linked to woody plant encroachment in grassland are still not well understood, especially the interactions between trees and soil water availability. Our aim was to study the depth of water uptake by trees in grassland and its controlling factors. We studied water uptake in Vachellia sieberiana trees (hereafter called ‘acacias’) in a humid grassland of South Africa. We especially studied the effect of size classes of acacias and their position in the catena on water potential (ψ) of acacia leaves and water uptake depth of acacias across seasons, as inferred from isotope δ¹⁸O values of soil and stem water samples. Acacias were more water stressed in February (wet season) with the most negative water potentials. Taller acacias (>3 m height) were more water-stressed (ψ ± SD = −1.62 ± 0.81 MPA) than smaller acacias (<1 m height) (ψ ± SD = −1.32 ± 0.80 MPA). In September (end of dry season), all tree size classes uptake water equally in A and B soil horizons. In February (middle of the wet season), tall, medium and even small trees favoured the B horizon. Small trees had similar plasticity in water uptake to taller trees and could modify depth of water uptake between seasons. This process could increase their probability of becoming adults, and may favour tree encroachment in grasslands.

与木本植物侵占草地相关的过程仍不清楚，尤其是树木与土壤水分可用性之间的相互作用。我们的目的是研究草地树木吸水深度及其控制因素。我们研究了南非潮湿草原中的Vachellia sieberiana树（以下称为“金合欢”）的吸水量。我们特别研究了金合欢的大小类别及其在链中的位置对金合欢叶片水势 (ψ) 和金合欢跨季节吸水深度的影响，这是从同位素 δ ¹⁸推断的土壤和茎水样品的 O 值。金合欢树在 2 月（雨季）的水压力更大，水势最负。较高的金合欢（> 3 m 高度）比较小的金合欢（<1 m 高度）（ψ ± SD = -1.32 ± 0.81 MPA）更受水胁迫（ψ ± SD = -1.62 ± 0.81 MPA）。在 9 月（旱季结束），所有大小等级的树木在 A 和 B 土壤层中均等地吸收水分。在二月（雨季中期），高大、中小甚至小树都青睐 B 地平线。小树在吸水方面与较高的树具有相似的可塑性，并且可以改变季节之间的吸水深度。这个过程可能会增加它们成年的可能性，并可能有利于草原上的树木侵占。