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.

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Shrub encroachment in semi‐arid savannas is induced by interacting effects of climate, fire suppression, and unsustainable livestock farming; it carries a severe risk of land degradation and strongly influences natural communities that provide key ecosystem functions. However, species‐specific effects of shrub cover on many animal groups that act as indicators of degradation remain largely unknown. We analysed the consequences of shrub encroachment for ground‐dwelling beetles in a semi‐arid Namibian savanna rangeland, where beetles and vegetation were recorded along a shrub cover gradient (<1% to >30%). Focusing on species niche breadths and optima, we identified two crucial shrub cover thresholds (2.9% and 10.0%), corresponding to major changes in the beetle communities with implications for savanna ecosystem functioning. Niche optima of most species were between the first and second thresholds; beyond the second threshold, saprophagous, coprophagous, and rare predatory beetles declined in numbers and diversity. This is problematic because beetles provide important ecosystem functions, such as decomposition and nutrient cycling. However, we also found that certain species were adapted to high shrub cover, thus providing examples of niche differentiation. Despite the predominantly negative effects of heavy shrub encroachment on beetle communities, shrubs in their early life stages apparently provide essential structures, which enhance habitat quality for ground‐dwelling beetles. Our results demonstrate that shrub encroachment can have mixed effects on ground‐dwelling beetle communities and hence on savanna ecosystem functioning. We, therefore, conclude that rangeland management and restoration should consider the complex trade‐offs between species‐specific effects and the level of encroachment for sustainable land use.

Fertilization causes species loss and species dominance changes in plant communities worldwide. However, it still remains unclear how fertilization acts upon species functional traits, e.g. seed mass. Seed mass is a key trait of the regeneration strategy of plants, which influences a range of processes during the seedling establishment phase. Fertilization may select upon seed mass, either directly by increased nutrient availability or indirectly by increased competition. Since previous research has mainly analyzed the indirect effects of fertilization, we disentangled the direct and indirect effects to examine how nutrient availability and competition influence the seed mass relationships on four key components during seedling establishment: seedling emergence, time of seedling emergence, seedling survival and seedling growth. We conducted a common garden experiment with 22 dry grassland species with a two‐way full factorial design that simulated additional nutrient supply and increased competition. While we found no evidence that fertilization either directly by additional nutrient supply or indirectly by increased competition alters the relationship between seed mass and (time of) seedling emergence, we revealed that large seed mass is beneficial under nutrient‐poor conditions (seedlings have greater chances of survival, particularly in nutrient‐poor soils) as well as under competition (large‐seeded species produced larger seedlings, which suffered less from competition than small‐seeded species). Based on these findings, we argue that both factors, i.e. nutrient availability and competition intensity, ought to be considered to understand how fertilization influences seedling establishment and species composition with respect to seed mass in natural communities. We propose a simple conceptual model, in which seed mass in natural communities is determined by competition intensity and nutrient availability. Here, we hypothesize that seed mass shows a U‐shaped pattern along gradients of soil fertility, which may explain the contrasting soil fertility‐seed mass relationships found in the recent literature.

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