For a typical encroacher tree of the thornbush savanna, we studied the responses of water consumption to changes in soil water availability within 80 cm depth, vapor pressure deficit (VPD), and global radiation. Therefore, we monitored the sap velocities of Senegalia mellifera trees over 2 years in a maximum of 20 stems (6–15 plant individuals) per measurement period on a site in central Namibia. For this water-restricted ecosystem we aimed to understand the role of an encroacher tree on soil water dynamics and potential groundwater recharge.

At the day-to-day scale, soil water was the primary driver of cumulative daily sap velocities (Q_n-day). In the dry season, Q_n-day decreased with increasing soil drought. Rainy seasons triggered multiple sapflow phases, initiated by soil-wetting rain events and associated with increase of soil water. Maximum attainable Q_n-day increased with VPD and radiation. We hypothesize a 3-stage response of tree water use to soil water tension (SWT): (1) SWT < pF 2.6–2.7: soil water is optimal for transpiration; (2) pF 2.6–2.7 < SWT < pF 3.2–3.7: transpiration decreases with soil drought; (3) SWT > pF 3.2–3.7: trees minimize transpiration. Our findings contribute to the understanding of tree–grass coexistence in savannas. The responses of tree water use to soil water within 80 cm depth indicate that S. mellifera exploits the same depths as grasses, in contrast to the classic hypothesis of vertical niche differentiation.

对于荆棘大草原的典型入侵者树，我们研究了水消耗对80厘米深度内的土壤水分，蒸气压亏缺（VPD）和全球辐射的变化的响应。因此，我们在纳米比亚中部的一个站点上，在每个测量期内最多监测20个茎（6–15个植物个体）的2年内塞纳利亚（Senegalia mellifera）树的树液速度。对于这个缺水的生态系统，我们旨在了解入侵者树对土壤水动力学和潜在的地下水补给的作用。

在日常规模上，土壤水是累积每日树液速度（Q _n-day）的主要驱动力。在干旱季节，Q _n-day随着土壤干旱的增加而减少。雨季引发了多个树液流阶段，这是由土壤湿润的降雨事件引起的，并与土壤水的增加有关。最大可达到的Q_天随着VPD和辐射的增加而增加。我们假设树木用水对土壤水分张力（SWT）有3个阶段的响应：（1）SWT <pF 2.6–2.7：土壤水分最适合蒸腾作用；（2）pF 2.6–2.7 <SWT <pF 3.2–3.7：土壤干旱导致蒸腾作用降低；（3）SWT> pF 3.2–3.7：树木将蒸腾作用降到最低。我们的发现有助于了解稀树草原中树木与草的共存。与垂直生态位分化的经典假设相反，树木水分对80 cm深度土壤水的响应表明，S。mellifera利用与草相同的深度。