Large herbivores may alter vegetation structure of semi-arid savannas through soil nutrient mediation

Abstract

In savannas, the tree–grass balance is governed by water, nutrients, fire and herbivory, and their interactions. We studied the hypothesis that herbivores indirectly affect vegetation structure by changing the availability of soil nutrients, which, in turn, alters the competition between trees and grasses. Nine abandoned livestock holding-pen areas (kraals), enriched by dung and urine, were contrasted with nearby control sites in a semi-arid savanna. About 40 years after abandonment, kraal sites still showed high soil concentrations of inorganic N, extractable P, K, Ca and Mg compared to controls. Kraals also had a high plant production potential and offered high quality forage. The intense grazing and high herbivore dung and urine deposition rates in kraals fit the accelerated nutrient cycling model described for fertile systems elsewhere. Data of a concurrent experiment also showed that bush-cleared patches resulted in an increase in impala dung deposition, probably because impala preferred open sites to avoid predation. Kraal sites had very low tree densities compared to control sites, thus the high impala dung deposition rates here may be in part driven by the open structure of kraal sites, which may explain the persistence of nutrients in kraals. Experiments indicated that tree seedlings were increasingly constrained when competing with grasses under fertile conditions, which might explain the low tree recruitment observed in kraals. In conclusion, large herbivores may indirectly keep existing nutrient hotspots such as abandoned kraals structurally open by maintaining a high local soil fertility, which, in turn, constrains woody recruitment in a negative feedback loop. The maintenance of nutrient hotspots such as abandoned kraals by herbivores contributes to the structural heterogeneity of nutrient-poor savanna vegetation.

Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna

“…Additionally, Acacia sapling growth rates were higher in nutrient-rich soils when compared to growth in nutrient-poor grassland soils (Wakeling et al 2010). In contrast, certain studies have found that abundant soil nutrients improve herbaceous productivity which in turn suppresses tree sapling establishment (Cohn et al 1989, Kraaij and Ward 2006, van der Waal et al 2011). Although we did not find any significant nutrient-grass interactive effect on sapling growth, the negative effects exerted by grasses were greater than the positive effects of nutrients (see also Kambatuku et al 2013 for Acacia mellifera).…”

The Stoichiometry of Nutrient Release by Terrestrial Herbivores and Its Ecosystem Consequences

“…Terrestrial herbivores typically do not graze and excrete randomly, but are attracted to landscape features, such as nutrient-rich areas with high food quality, resulting in a net import of nutrients and creating nutrient hotspots in the landscape. At the same time, large parts of the landscape with poorer quality vegetation experience a net removal of nutrients (McNaughton et al, 1997;Augustine et al, 2003;van der Waal et al, 2011). Water bodies may also induce spatial patterns; semi-aquatic herbivores such as hippopotamus, beaver or water birds can transport nutrients across ecosystem boundaries and thus strongly impact nutrient redistribution Subalusky et al, 2015;Bakker et al, 2016).…”

Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots

“…Grasses often compete intensely with young trees (e.g., van der Waal et al 2009;Cramer et al 2010). Glades in our study site typically have very high grass cover (Veblen 2008;Porensky 2011), creating an environment in which grass competition may be severe enough to kill young trees (e.g., van der Waal et al 2011). Areas between nearby glades have unusually low cover of dense glade grasses, which may release young trees from competition and help explain increased tree densities between nearby glades (Porensky 2011).…”

“…Herbivory has major impacts on woody cover in savanna ecosystems (Pellew 1983;Augustine and McNaughton 2004;Goheen et al 2010) and could play a particularly important role in determining woody cover patterns associated with glades, where use by mid-sized, mixed-feeder herbivores is elevated (Young et al 1995;van der Waal et al 2011). Herbivore preference may lead to elevated browsing pressure and reduced tree survival or growth within glades, particularly for young trees (seedlings and saplings) (Goheen et al 2004;Midgley et al 2010).…”

“…Intermediate in scale are nutrient-rich sites derived from traditional livestock corrals (bomas) (Western and Dunne 1979;Blackmore et al 1990;Lamprey and Reid 2004). Throughout African savannas, former corral sites develop into long-term, nutrient-rich patches with unique vegetation (Blackmore et al 1990; Reid and Ellis 1995;Young et al 1995;Augustine 2003;Muchiru et al 2009;van der Waal et al 2011), distinctive plant community dynamics (Veblen 2008(Veblen , 2011Veblen and Young 2010) and broad-ranging ecological impacts (Augustine 2004;Gregory et al 2010).…”