Facilitative or competitive effects of woody plants on understorey vegetation depend on N‐fixation, canopy shape and rainfall

Abstract

Summary 1.A recent meta-analysis suggested that differences in rainfall are a cause of variation in tree-grass interactions in savannas, with trees facilitating growth of understorey grasses in low-rainfall areas, but competing with them under higher rainfall. We hypothesized that this effect of rainfall upon understorey productivity is modified by differences in the growth form of the woody plants (i.e. the height of the lower canopy) or by their capacity to fix nitrogen. 2. We performed a meta-analysis of the effects of woody plants on understorey productivity, incorporating canopy height and N-fixation, and their interaction with rainfall. 3. N-fixing woody plants enhanced understorey productivity, whereas non-fixers had a neutral or negative effect, depending on high or low canopy, respectively. We found a strong negative correlation between rainfall and the degree to which trees enhanced understorey productivity, but only for trees with a high canopy. 4. Synthesis. The effect of woody plants on understorey productivity depends not only on rainfall, but also on their growth form and their capacity to fix N. Facilitation occurs mostly when woody plants ameliorate both water and nitrogen conditions. However, a low canopy suppresses understorey vegetation by competing for light, regardless of water and nutrient relations.

Woody Plant Encroachment: Causes and Consequences

“…We now turn to mechanisms that govern the ecological interactions between grasses and trees or shrubs. There is a large body of ecological and rangemanagement literature on the effects of woody plants on grasses (Scholes and Archer 1997;Blaser et al 2013;Dohn et al 2013). Here, we focus on factors that influence recruitment and abundance of woody plants into grass-dominated communities.…”

Wood-pastures of Europe: Geographic coverage, social–ecological values, conservation management, and policy implications

“…Positive effects are mainly due to shelter and improved soil fertility, negative impacts due to competition for light, water and nutrients . Further studies are needed to understand under which ecological conditions and plants traits the tree effects change from net competitive to net facilitative (Blaser et al, 2013). The number of young trees to be encouraged in open ground or left after thinning in-filled wood-pasture must allow for mortality (Lonsdale, 2013): not all the young trees will survive more than the approximately 150 years needed before they start to develop hollows and other veteran tree characteristics.…”

Contrasting effects of two Acacia species on understorey growth in a drylands environment: interplay of canopy shading and litter interference

“…They also showed, along with others, that trees with low canopies inhibit understorey growth through the competition for light, regardless of the water and nutrient relations (Blaser et al. ; Metz & Tielbörger ; Noumi et al. ).…”

“…), and some have shown that the SGH alone cannot explain the tree–grass co‐existence in savanna ecosystems (Blaser et al. ; He et al. ).…”

“…Only recently, a meta‐analysis by Blaser et al. () showed that trees facilitate understorey productivity mostly when the trees ameliorate both water and N conditions. They also showed, along with others, that trees with low canopies inhibit understorey growth through the competition for light, regardless of the water and nutrient relations (Blaser et al.…”