Micro-site effects of trees and shrubs in dry savannas

Abstract

Abstract. The physiognomy of dry savannas is described as a combination of discontinuous woody perennials and a continuous grassland matrix. Interactions between these two components are of vital importance for the persistence of a savanna landscape. Earlier savanna models have emphasized competitive interactions for water between the two components. Recent studies have argued that small‐scale facilitating interactions between woody perennials and the herbaceous understorey are also important. This phenomenon has been given little theoretical consideration in the savanna literature, but it has been an important topic in agroforestry and arid‐grassland ecology. This paper reviews some of the evidence for micro‐site effects of trees and shrubs, and attempts to integrate their interactions with the surrounding open grassland. Woody perennials modify the microclimate by interception of solar radiation and rainfall. Their root systems extract nutrients horizontally and vertically, which are concentrated in the sub‐canopy soil from litter decomposition and root turnover. Legumes are abundant in dry savannas, and may have symbiotic relationships with Rhizobium bacteria. This symbosis increases the availability of nitrogen in the soil. Isolated trees and shrubs initiate feedback mechanisms in their interactions with other organisms, and contribute to an uneven distribution of water and nutrients in dry savanna. This influences the species composition, and community diversity. Small‐scale facilitating interaction between the woody and herbaceous components and competitive interaction on larger scales, are complementary processes which together explain a dynamic coexistence.

FACILITATION BYPINUS FLEXILISDURING SUCCESSION: A HIERARCHY OF MECHANISMS BENEFITS OTHER PLANT SPECIES

“…Facilitative mechanisms may also be direct, in which neighbors ameliorate the physical environment for less hardy species (Vetaas 1992, Callaway 1995, Hacker and Bertness 1995, Baumeister 1999, Choler et al 2001. Particular conditions that are ameliorated directly by stress-tolerant benefactors include high temperature and drought (Muller 1953, Muller and Muller 1956, Steenberg and Lowe 1969, Nobel 1984, Everett et al 1986), low temperature (Carlsson and Callaghan 1991, Blundon et al 1993, Choler 2001, high salinity (Bertness 1991 Fong et al 1996), low soil oxygen (Schat and Beckhoven 1991, Bertness and Shumway 1993, Callaway and King 1996, and low nutrients (Connell and Slatyer 1977, Walker and Chapin 1986, Wood and del Moral 1987, Blundon et al 1993.…”

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

“…Savannahs are described as a continuous grass layer interspersed with trees (Sankaran et al 2004), and their spatial heterogeneity strongly depends on the scale of observation (Adler et al 2001). At a landscape scale, the patchiness in spatial nutrient distribution, vegetation structure, and tree species composition has been the focus of many studies (Vetaas 1992;Scholes and Archer 1997;Sankaran et al 2004). This patchiness might be one important explanation for the high diversity of animals that inhabit these ecosystems.…”

Resource Use and Crop Productivity in a Colophospermum Mopane Tree Based Agro- Ecosystem in Indian Desert

“…However, population of these are very less that requires further integration of trees to improve vegetation status and would be beneficial for the poor by providing top feed for livestock and fuel for energy generation. Trees have been found to facilitate regeneration of native and existing vegetation in forest plantation (Bone et al, 1997), sub-alpine and alpine plant communities (Callaway et al, 2002), desert communities (Vetaas, 1992) and the Middle East (Tielborger and Kadmon, 1995). However, there are also reports of neighbour interference through allelopathic suppression (Friedman et al, 1977) and competition for resources (Singh et al, 2001;Tielborger and Kadmon, 1997).…”