Browsing by Subject "savanna"

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    Ecosystem-scale spatial heterogeneity of stable isotopes of soil nitrogen in African savannas
    (2013-04) Wang, Lixin; Okin, Gregory S.; D'Odorico, Paolo; Caylor, Kelly K.; Macko, Stephen A.
    Soil 15N is a natural tracer of nitrogen (N) cycling. Its spatial distribution is a good indicator of processes that are critical to N cycling and of their controlling factors integrated both in time and space. The spatial distribution of soil δ15N and its underlying drivers at sub-kilometer scales are rarely investigated. This study utilizes two sites (dry vs. wet) from a megatransect in southern Africa encompassing locations with similar soil substrate but different rainfall and vegetation, to explore the effects of soil moisture and vegetation distribution on ecosystem-scale patterns of soil δ15N. A 300-m long transect was set up at each site and surface soil samples were randomly collected for analyses of δ15N, %N and nitrate content. At each soil sampling location the presence of grasses, woody plants, Acacia species (potential N fixer) as well as soil moisture levels were recorded. A spatial pattern of soil δ15N existed at the dry site, but not at the wet site. Woody cover distribution determined the soil δ15N spatial pattern at ecosystem-scale; however, the two Acacia species did not contribute to the spatial pattern of soil δ15N. Grass cover was negatively correlated with soil δ15N at both sites owing to the lower foliar δ15N values of grasses. Soil moisture did not play a role in the spatial pattern of soil δ15N at either site. These results suggest that vegetation distribution, directly, and water availability, indirectly, affect the spatial patterns of soil δ15N through their effects on woody plant and grass distributions.
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    The interactive nutrient and water effects on vegetation biomass at two African savanna sites with different mean annual precipitation
    (2012-12) Wang, Lixin; Katjiua, Mutjinde; D'Odorico, Paolo; Okin, Gregory S.
    Savannahs cover more than 40% of Africa and provide a variety of important ecosystem services. Their productivity is constrained by disturbance and limiting resources. In southern Africa, fine-leaf savannahs typical of arid environments are known for being richer in nutrients than broad-leaf mesic savannahs. However, despite numerous recent studies on the dynamics of southern African savannahs, the interplay between water and nutrient limitations remains poorly investigated in these systems. To better understand the interactions between water, nutrients (N and P in this manuscript) and grazing on the productivity of grasses and trees in fine-leaf savannah ecosystems, a fertilization experiment with controlled grazing was conducted at two sites with different mean annual rainfall in Namibia. The experiment demonstrated that the vegetation at the drier site may not be nutrient-limited (N, P or N + P). At the wetter site, however, vegetation showed significant response to nutrient addition. Grasses exhibited N limitation and trees exhibited P limitation. This experiment also showed that grazing reduces the overall grass biomass, but may not modify the response to nutrient treatments. The results indicated a switch from water to nutrient limitation between dry and wet sites and demonstrated different tree and grass responses to nutrient additions.
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    Responses and feedbacks of African dryland ecosystems to environmental changes
    (Elsevier, 2021-02) Wei, Fangli; Wang, Shuai; Brandt, Martin; Fu, Bojie; Meadows, Michael E.; Wang, Lixin; Wang, Lanhui; Tong, Xiaowei; Fensholt, Rasmus; Earth Sciences, School of Science
    Drylands occupy 43% of the African continent and play an important role in the global carbon cycle and in supporting local livelihoods. Understanding how dryland ecosystems respond to environmental changes, both structurally and functionally, is of great significance for sustainable dryland management. In this article, we review the current remote sensing-based knowledge on African dryland ecosystem dynamics and the main drivers of changes. Global CO2 enrichment, changes in rainfall regimes, and a decline in fire activity have collectively driven vegetation greening, woody plant increase and carbon dynamics in African drylands over recent decades, challenging the long-held desertification narrative. Here we also highlight the importance of rainfall–vegetation–fire feedbacks in enhancing dryland ecosystem resilience and predicting future ecosystem responses.