Abstract
Introduction: Many semiarid regions around the world are presently experiencing significant changes in both climatic conditions and vegetation. This includes a disturbed coexistence between grasses and bushes also known as bush encroachment, and altered precipitation patterns with larger rain events. Fewer, more intense precipitation events might promote groundwater recharge, but depending on the structure of the vegetation also encourage further woody encroachment. Materials and Methods: In this study, we investigated how patterns and sources of water uptake of Acacia mellifera (blackthorn), an important encroaching woody plant in southern African savannas, are associated with the intensity of rain events and the size of individual shrubs. The study was conducted at a commercial cattle farm in the semiarid Kalahari in Namibia (MAP 250 mm/a). We used soil moisture dynamics in different depths and natural stable isotopes as markers of water sources. Xylem water of fifteen differently sized individuals during eight rain events was extracted using a Scholander pressure bomb. Results and Discussion: Results suggest the main rooting activity zone of A. mellifera in 50 and 75 cm soil depth but a reasonable water uptake from 10 and 25 cm. Any apparent uptake pattern seems to be driven by water availability, not time in the season. Bushes prefer the deeper soil layers after heavier rain events, indicating some evidence for the classical Walter’s two-layer hypothesis. However, rain events up to a threshold of 6 mm/day cause shallower depths of use and suggest several phases of intense competition with perennial grasses. The temporal uptake pattern does not depend on shrub size, suggesting a fast upwards water flow inside. δ2H and δ18O values in xylem water indicate that larger shrubs rely less on upper and very deep soil water than smaller shrubs. It supports the hypothesis that in environments where soil moisture is highly variable in the upper soil layers, the early investment in a deep tap-root to exploit deeper, more reliable water sources could reduce the probability of mortality during the establishment phase. Nevertheless, independent of size and time in the season, bushes do not compete with potential groundwater recharge. In a savanna encroached by A. mellifera, groundwater will most likely be affected indirectly.
Highlights
Many semiarid regions around the world are presently experiencing significant changes in both climatic conditions and vegetation
According to the linear stable isotope mixing model outputs soil water was in general the main source of water taken up by A. mellifera
This is consistent with a study by Kulmatiski and Beard (2013) in the Lowveld savannas of southern Krüger National Park in South Africa, where the dominating shrubs and trees seem not to rely on soil water of potential groundwater recharge to any great extent
Summary
Many semiarid regions around the world are presently experiencing significant changes in both climatic conditions and vegetation. This includes a disturbed coexistence between grasses and bushes known as bush encroachment, and altered precipitation patterns with larger rain events. In arid and semiarid regions, the problem is exacerbated by extremely small recharge fluxes that are highly variable in space and time (Phillips, 1994; Stephens, 1994; Izbicki et al, 2000). Mainly large but rare rain events with deep percolation result in groundwater recharge (Le Maitre et al, 1999; Scanlon et al, 2006; Moore et al, 2012). The deep-rooted soil-plant system of drylands is known to be highly rain-use efficient (Schenk and Jackson, 2002; Seyfried et al, 2005)
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