Tree effects on grass growth in savannas: competition, facilitation and the stress‐gradient hypothesis

Abstract

Summary1. The stress-gradient hypothesis (SGH) predicts an increasing importance of facilitative mechanisms relative to competition along gradients of increasing environmental stress. Although developed across a variety of ecosystems, the SGH's relevance to the dynamic tree-grass systems of global savannas remains unclear. Here, we present a meta-analysis of empirical studies to explore emergent patterns of tree-grass relationships in global savannas in the context of the SGH. 2. We quantified the net effect of trees on understorey grass production relative to production away from tree canopies along a rainfall gradient in tropical and temperate savannas and compared these findings to the predictions of the SGH. We also analysed soil and plant nutrient concentrations in subcanopy and open-grassland areas to investigate the potential role of nutrients in determining grass production in the presence and absence of trees. 3. Our meta-analysis revealed a shift from net competitive to net facilitative effects of trees on subcanopy grass production with decreasing annual precipitation, consistent with the SGH. We also found a significant difference between sites from Africa and North America, suggesting differences in tree-grass interactions in the savannas of tropical and temperate regions. 4. Nutrient analyses indicate no change in nutrient ratios along the rainfall gradient, but consistent nutrient enrichment under tree canopies. 5. Synthesis. Our results help to resolve questions about the SGH in semi-arid systems, demonstrating that in mixed tree-grass systems, trees facilitate grass growth in drier regions and suppress grass growth in wetter regions. Relationships differ, however, between African and North American sites representing tropical and temperate bioclimates, respectively. The results of this meta-analysis advance our understanding of tree-grass interactions in savannas and contribute a valuable data set to the developing theory behind the SGH.

Facilitation, competition and parasitic facilitation amongst invasive and native liana seedlings and a native tree seedling

“…Montgomery et al 2010, Dohn et al 2013, we have measured both sides of the interaction in this experiment, and the positive effect was only seen for one of the participating species (the liana) with a strong negative effect for the native tree seedling. The interaction is, therefore, more associated with the directions of advantage associated with a parasitic interaction (+,-), a term not usually applied to plant-plant interactions where both species are physiologically independent.…”

Evaluación de prácticas de desmonte selectivo y clausuras temporales en sistemas degradados del Chaco árido (Argentina)

“…Son ambientalmente sustentables cuando se respetan umbrales ecológicos propios de cada sistema y ecorregión, referidos a biodiversidad, cobertura del suelo, frecuencia de especies clave, calidad de mantillo depositado en el suelo, tasas de extracción de nutrientes, entre otros indicadores ecológicos (Dohn et al, 2013;Karlin et al, 2009;Montagnini & Nair, 2004;Sánchez, 1995).…”

“…Estos sistemas han sido estudiados a nivel mundial (Crespo, 2008;Dohn et al, 2013;Moreno & Pulido, 2009; Revista de Ciencias Ambientales (Trop J Environ Sci). (Julio-Diciembre, 2017 (Ayerza et al, 1988;Carranza & Ledezma, 2005;Carranza et al, 2012;Cora, 2009;Díaz, 2003;Hanh et al, 1995;Karlin et al, 2009;Nai Bregaglio et al, 2001;Mazzarino et al, 1991;Oliva et al, 1993;Patt & Ayan, 2005;Vega, 1988) y se ha podido encontrar, en estos últimos indicadores de sustentabilidad que podrían ser utilizados para definir umbrales críticos de las variables del sistema estudiados, entendiéndose por umbral crítico a aquel valor de un indicador que define un punto de no retorno del sistema en relación a con la capacidad de ofrecer algún servicio ecosistémico.…”

Spatial vegetation patterns and neighborhood competition among woody plants in an East African savanna

“…Established woody plant canopies may also improve microsite conditions by reducing evaporative losses due to light interceptance (O'Connor 1995, Salazar et al 2012), and by increasing subcanopy nutrient concentrations through litter deposition (Belsky et al 1989, Ludwig et al 2004, Hagos and Smit 2005. Furthermore, the effects of woody plants on understory herbaceous production vary predictably with mean annual precipitation such that woody plants in dry regions generally promote subcanopy grass growth (Dohn et al 2013). Indeed, clusters at MRC typically have greater grass biomass than inter-plant spaces (Augustine 2003).…”