Positive, Negative, and Net Effects in Grass–shrub Interactions in Mediterranean Semiarid Grasslands

Abstract

Current theoretical models and field evidence suggest that facilitation and interference act simultaneously in the field, but there is little information on their joint dynamics under varying abiotic conditions. We evaluated spatial and temporal variations in the positive, negative, and net effects of the tussock grass Stipa tenacissima on the shrub Pistacia lentiscus in Mediterranean semiarid grasslands. We performed a field experiment in which positive (microclimatic amelioration and water inputs from runoff) and negative (belowground competition) effects were experimentally manipulated under contrasting levels of stress. The environment provided by S. tenacissima (tussock treatment) improved the survival and physiological status of planted P. lentiscus seedlings. The elimination of shade and competition from S. tenacissima significantly decreased and increased, respectively, seedling performance as compared to the tussock treatment. We found evidence that microclimatic amelioration is the main positive factor involved in the interaction under study, and that it can balance the negative effects of belowground competition. The latter was also important, especially after major rainfall events. In contrast with previous studies, both the water inputs from runoff and the soil chemical fertility were found to have a relatively weak effect in the interaction under study. The net effect of S. tenacissima on P. lentiscus was always facilitative, but the magnitude of this effect increased with the harshness of the environmental conditions in both space and time. Despite this, the mechanisms underlying plant–plant interaction responded unequally to small‐scale environmental variation. Our results agree with models suggesting that the magnitude of facilitation may increase under stressful conditions, and provide evidence of the dynamic nature of positive and negative interactions in the field.

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

“…This negative effect of Crotalaria on Stipagrostis extends over the later growth stages of Crotalaria, increases with higher legume density, and is, other than the recruitment of Crotalaria , not compensated for by higher rainfalls and water availability. 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.…”

Modeling Dryland Landscapes

“…in woody patches), when the precipitation rate is low. This prediction is in line with recent field observations along rainfall gradients and along other gradients of environmental stress [45,41,46,47,42]. …”

Facilitation of tree saplings by nurse plants: Microhabitat amelioration or protection against herbivores?

“…2). This finding suggests that the nurse effect of shrubs, reported mainly for the first year of life of tree seedlings (Maestre et al 2003;Gómez-Aparicio et al 2004), is consistent throughout the sapling stage (see also Castro et al 2004Castro et al , 2006. At the intra-annual scale, facilitation by shrubs due to microclimatic amelioration in mediterranean environments has been reported mainly during the summer season (Callaway 1992;Rousset & Lepart 1999;Castro et al 2004a).…”