Grass competition suppresses savanna tree growth across multiple demographic stages

Abstract

Abstract. Savanna ecosystems, defined by the codominance of trees and grasses, cover one-fifth of the world's land surface and are of great socioeconomic and biological importance. Yet, the fundamental question of how trees and grasses coexist to maintain the savanna state remains poorly understood. Many models have been put forward to explain tree-grass coexistence, but nearly all have assumed that grasses do not limit tree growth and demography beyond the sapling stage. This assumption, however, has rarely been tested. Here I show that grass can strongly suppress the growth of trees. I removed grass around trees of three size classes in an Acacia drepanolobium savanna in Laikipia, Kenya. For even the largest trees, grass removal led to a doubling in growth and a doubling in the probability of transitioning to the next size class over two years. These results suggest that grass competition in productive (nutrient-rich) savannas may limit tree growth as much as herbivory and fire (the main factors thought to determine tree demography within a rainfall region) and should be incorporated into savanna models if tree-grass coexistence and savanna dynamics are to be understood.

A dominance shift in arid savanna: An herbaceous legume outcompetes local C4 grasses

“…Thereby, our interaction experiments support the findings of Maestre, Bautista, and Cortina (2003) who found seedlings of legumes to grow largely unaffected in living grass tussocks under stressful conditions similar to those of our study region. In the case of Crotalaria, the C 4 grass Stipagrostis is not, or probably no longer able to exert its supposed dominance (February et al., 2013; Riginos, 2009; Sankaran et al., 2004; Scholes & Archer, 1997) over the legume and suppress its recruitment. This clearly contradicts the widespread opinion that grasses are able to outcompete legumes and encroachers due to their competitive advantage in water‐limited environments (O’Connor et al., 2014; Riginos, 2009).…”

Conserving Wildlife in African Landscapes Kenya’s Ewaso Ecosystem

“…For example, although tree density is higher on average in ungulate exclusion plots than in control plots, ungulates actually facilitate tree seedling establishment and survival by suppressing herbivorous rodents and competing grasses (Goheen et al, 2004Riginos, 2009). In the special case of the whistling thorn tree, aggressive ant defenders also limit the impacts of mammalian herbivory.…”

“…Likewise, arboreal geckos are suppressed by the ungulate community at large but facilitated by structural impacts of elephant on trees (Pringle et al, 2007;Pringle, 2008). Indeed, most of the experimental effects documented to date are likely to have multiple causes, although only for trees have these been systematically and experimentally untangled (Goheen et al, 2004Riginos, 2009). …”

“…First, high densities of shrubs and trees can substantially suppress grass production (Smit and Rethman, 2000;Smit, 2004) and hence reduce forage for cattle. In the black cotton habitats, sites with a low density of trees were found to have double the grass production as sites with a high density of trees (Riginos et al, 2009). Further research is needed to quantify the effects of tree density on grass productivity in the red sands habitat.…”

Bush encroachment control and risk management in semi-arid rangelands

“…bushes and grass, under the influence of stochastic precipitation and bushfire, and that are managed for the purpose of livestock grazing (Knoop and Walker 1985, Perrings and Walker 1997, Wiegand and Jeltsch 2000, Beukes et al 2002, Sullivan and Rohde 2002, Janssen et al 2004, Riginos 2009). The crucial ecosystem service that limits livestock production and shapes farming strategies, is production of green grass biomass, which serves as a forage for livestock and thus generates farm income.…”