Understanding nutrient dynamics in an African savanna: local biotic interactions outweigh a major regional rainfall gradient

Abstract

1. Nutrient availability in terrestrial ecosystems has been found to vary along regional climatic and soil gradients and drive variation in plant community composition and vegetation structure. However, more local biotic feedbacks also affect nutrient availability, but their importance in determining vegetation structure relative to regional drivers is yet unclear. 2. Mesic African savannas form a transition zone between the dry grasslands with relatively low nitrogen availability (indicated by low plant N:P ratios) and the wet woodlands of the continent characterized by relatively low phosphorous availability (high plant N:P ratios). They host strong feedback mechanisms of both vegetation and consumers, where large grazers can create short grazing lawns that alternate over short distances with tall fire-dominated bunch grasslands, and patches dominated by woody species with abundant macrodetritivores. Here, we test if such local biotic interactions can overrule regional, landscape-level drivers of plant nutrient availability. 3. In a South African savanna, we find that plant N:P ratios, N and P resorption efficiencies and proficiencies, were indeed much stronger affected by small-scale biotic-created heterogeneity than by a regional rainfall gradient (530-830 mm yr À1 ). Furthermore, we observe that differences in N:P ratios between vegetation structural types are not caused by simple differences in inherent plant traits. 4. This suggests that availability of N and P to plants is strongly contingent on the local biotic interplay between different vegetation structural types and within-system feedbacks linked to each vegetation type. The regional rainfall gradient did affect nutrient availability, but mainly through its effect on the relative distribution of the different vegetation structural types thus setting the range of possible outcomes of biotic interactions. 5. Synthesis. Regional and global studies that model savanna carbon and nutrient cycling only on the basis of regional gradients in soil and climatic conditions may insufficiently capture the dominant ecosystem processes involved.

Patterns in woody vegetation structure across African savannas

“…8). These are factors that influence ecohydrological processes such as the propensity to form overland flows during rainfall events (Valentin et al, 1999). The results are in agreement with a global study on the biogeography of PVPs by Deblauwe et al (2008), who found similar effects in regions with strong seasonal variation in temperature and more constant rainfall (Australia and Mexico) and in regions with distinct rainfall seasonality but more constant temperatures (Africa).…”

“…Woody cover is fundamentally a function of crown sizes and crown density and by studying these components individually, it is possible to attain important insight into the function of ecosystems and what ecosystem services they provide. Two landscapes with similar woody cover but different sizes of individual trees will sequester different amounts of carbon (Shackleton and Scholes, 2011), harbor different fauna (Riginos and Grace, 2008), and differ in biogeochemical dynamics (Veldhuis et al, 2016a). The level of spatial aggregation among woody plants can help us understand facilitative and competitive processes determining survival of seedlings and saplings.…”

“…These short-range facilitative effects usually operate at spatial scales of a few meters but may increase the degree of aggregation among woody plants at larger scales (Scanlon et al, 2007;Xu et al, 2015). Overland flows of water can be especially effective at redistributing resources over longer distances, in some conditions leading to the emergence of periodic vegetation patterns (PVPs; Rietkerk and van de Koppel, 2008;Valentin et al, 1999). Contrasting infiltration rates between bare and vegetated patches lead to redistribution of water and soil resources, which reinforces an organized pattern.…”

“…On flat ground, PVPs take the form of spotted, labyrinthine, or gapped patterns depending on soil water availability. On a gentle slope, they develop into vegetated bands that run parallel to contour lines (Valentin et al, 1999). While PVPs have been studied extensively, their formative processes are seldom linked to ecohydrological processes in other types of savanna landscapes.…”

Nitrogen leaching from natural ecosystems under global change: a modelling study

“…Although uncertainty regarding N losses by fire is large (Gruber and Galloway, 2008), the general agreement with observations strengthens our finding of the important role of fire in the terrestrial N budget. Specifically for savannah ecosystems, the influence of fire on N dynamics is further supported by field and remote sensing studies (Veldhuis et al, 2016;Chen et al, 2010).…”