Improved quality of beneath‐canopy grass in South African savannas: Local and seasonal variation

Abstract

Questions: Do large trees improve the nutrient content and the structure of the grass layer in savannas? Does the magnitude of this improvement differ with locality (soil nutrients) and season (water availability)? Are grass structure and species composition beneath tree canopies influenced by soil fertility and season? Location: South Africa. Methods: We compared grass leaf nutrient contents and grass sward structure beneath and outside tree canopy areas in three savannas of different soil fertility during the dry and the wet seasons. Results: Grass nitrogen contents were twice as high during the wet season as compared to the dry season, being more strongly elevated underneath tree canopies during the wet season. Grasses had significantly less stem material and provided less dead material underneath trees on the high soil fertility site. Grass species composition differed significantly beneath and outside tree canopies, with more nutritious grass species found sub-canopy. Grass species richness was significantly lower beneath than outside of trees at the site of high soil fertility. Conclusions: Trees improve overall quality of savanna grasses by enhancing grass growth and nutrient uptake during the wet season, and by delaying grass wilting in the dry season. The positive effect of trees on the grass layer might attract grazing herbivores in otherwise nutrient-poor savannas. Hence, single standing large trees should be maintained to sustain high grass quality and, consequently, grazer populations in savanna habitats.

Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

“…Grasses growing under the tree canopy contained more green leaf material and stayed green for longer into the dry season than grasses growing further away from trees (Treydte et al 2008). Overall, trees contribute substantially to the grass species layer and to vertical and horizontal structural diversity in African savannah systems.…”

“…Trees can improve the conditions for grass layer growth directly, for example, by providing shade and reducing evapo-transpiration, by accumulating soil nutrients close to their root systems (Belsky 1994;Dean et al 1999), and by changing the micro-climate (Jackson et al 1990;Belsky 1994;Ludwig et al 2001;Power et al 2003). Recent studies have shown that both soil and grass nitrogen (N) and phosphorus (P) contents were elevated beneath tree canopies compared to inter-canopy sites (Ludwig et al 2004;Treydte et al 2007Treydte et al , 2008 whereas some studies did not find any or only slight differences in soil properties between canopy and inter-canopy sites (Witkowski and Garner 2000).…”

“…Belsky et al (1993) found higher grass productivity and slightly different soil nutrients and organic matter beneath canopies of Acacia tortilis than beneath Adansonia digitata trees but found that overall the climatic condition, i.e., xeric versus mesic environment mainly determined differences in soil beneath and outside tree canopies rather than tree species. Treydte et al (2007Treydte et al ( , 2008 compared the effect of Nfixing trees versus non-N-fixing trees on the grass and soil N contents beneath their canopies and did not find significant differences for these two species. In contrast, Power (2003) found higher soil N and higher grass yield beneath Acacia trees, which they related to the Acacia tree's potential of fixing atmospheric N. However, as there are some constraints such as low P soil nutrients or very dry conditions (Hartwig 1998), under which N-fixation does not take place, actual measurements of N-fixing activities should be conducted before general conclusions can be drawn.…”

“…This was contrary to our expectation that more grass leaves should be eaten and grass height and cover would be reduced in areas that herbivores frequently used as feeding grounds. One of the main factors affecting these characteristics is seasonality influencing grass structure and nutrient contents (Treydte et al 2008), which in turn influences the use of different nutrient patches. Herbivores tend to use patches such as termite mounds and sodic sites early in the growing season (Grant and Scholes 2006), thereafter they use the most nutritious vegetation under trees more heavily later in the growing season (personal observation).…”

Analysis of Vegetation Phytosociological Characteristics and Soil Physico-Chemical Conditions in Harishin Rangelands of Eastern Ethiopia

“…They frequently clear most of the shrubs and trees inside the enclosures by leaving a few species, which are used to shade their animals from direct hot sun. A low density of woody species has been shown to increase soil nutrient status and thus encourage grass-growing conditions [55,56]. The overall density threshold of woody species in terms of bush encroachment is estimated to be more than 2500 trees equivalent ha -1 [57].…”