Search

Background and aims: The intensity of extreme rainfall events is increasing in many regions of the world, with important impacts on community dynamics and ecosystem functioning especially in water-limited ecosystems. The impact of the intensification of extreme precipitation on mixed communities of CAM plants and dryland C4 grasses remains poorly understood. Methods: Through a set of greenhouse experiments we investigated the effect of increasing intraseasonal rainfall variability on two separate pairs of CAM and C4 grass species (C. imbricata and B. eriopoda; O. phaeacantha and B. curtipendula) that coexist in arid grasslands across the southwest USA. Results: The increased rainfall variability did not significantly change the biomass of C. imbricata while increasing the biomass of B. eriopoda when these two species coexisted. More extreme rainfall regimes caused a 24.8% decrease in B. curtipendula (grass) biomass, compared with a 71.3% decline in the O. phaeacantha CAM plant. Significantly lower nocturnal carbon assimilation rates and higher stem volume-to-surface area ratios were found in CAM plants in mixture than in monoculture. Conclusions: Our study suggests that C4 grasses may outcompete CAM plants through greater access to deep soil water under increased rainfall variability and highlights the important role of hydrologic conditions as determinants of the competitive relations between CAM plants and grasses, their community composition, and ecological resilience in dryland ecosystems. [ABSTRACT FROM AUTHOR]

Savannas are characterized by the coexistence of trees and flammable grasses. Yet, tree-grass coexistence has been labeled as paradoxical-how do these two functional groups coexist over such an extensive area, despite being generally predisposed to excluding each other? For instance, many trees develop dense canopies that limit grass growth, and many grasses facilitate frequent/intense fires, increasing tree mortality. This study revisits tree-grass coexistence with a model of hierarchical competition between pyrogenic grasses, 'forest trees' adapted to closed-canopy competition, and 'savanna trees' that are inferior competitors in closed-canopy communities, but more resistant to fire. The assumptions of this model are supported by empirical observations, including a systematic review of savanna and forest tree community composition reported here. In general, the model simulations show that when savanna trees exert weaker competitive effects on grasses, a self-reinforcing grass community is maintained, which limits forest tree expansion while still allowing savanna trees to persist (albeit as a subdominant to grasses). When savanna trees exert strong competitive effects on grasses, savanna trees cover increases initially, but as grasses decline their inhibitory effect on forest trees weakens, allowing forest trees to expand and exclude grasses and savanna trees. Rather than paradoxical, these results suggest that having weaker competitive effects on grasses may be advantageous for savanna trees, leading to greater long-term abundance and stability. We label this the 'enemy of my enemy hypothesis,' which might apply to species coexistence in communities defined by hierarchical competition or with species capable of generating strong ecological feedbacks. [ABSTRACT FROM AUTHOR]

Plants with crassulacean acid metabolism (CAM) are increasing their cover in many dryland regions around the world. Their increased dominance has been related to climate warming and atmospheric CO fertilization, while the effects of interspecies interactions and the role of CAM plant facilitation by trees and grasses remain poorly understood. Woody plants are known for their ability to directly facilitate CAM plants through amelioration of the abiotic environment. Mechanisms of indirect facilitation of trees on CAM plants in tree-grass-CAM associations, however, have received less attention. It is also unclear whether grasses might facilitate CAM plants in mixed tree-grass-CAM communities. For instance, the inclusion of grasses in tree-CAM associations could enhance hydraulic lift and facilitate CAM plants in their access to shallow soil moisture at the expenses of deep-rooted trees. If this effect outweighs the competitive effects of grasses on CAM plants, grasses could overall facilitate CAM plants through hydraulic lift. Here we develop a process-based ecohydrological model to investigate the direct and indirect facilitation in tree-CAM-grass associations; the model quantifies transpiration of CAM plants when isolated as well as in associations with trees and/or grasses. It is found that woody plants having a high root overlap with CAM plants indirectly facilitate CAM plants by significantly reducing grass transpiration in shaded conditions. For situations of a low-to-moderate root overlap, facilitation may occur both directly and indirectly. Conversely, grasses are unable to indirectly facilitate CAM plants through the mechanism of hydraulic lift because the competitive effects of grasses on CAM plants outweigh the facilitation induced by hydraulic lift. [ABSTRACT FROM AUTHOR]