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]

Abstract Plants with crassulacean acid metabolism ( CAM) are increasing their abundance in drylands worldwide. The drivers and mechanisms underlying the increased dominance of CAM plants and CAM expression (i.e., nocturnal carboxylation) in facultative CAM plants, however, remain poorly understood. We investigated how nutrient and water availability affected competition between Mesembryanthemum crystallinum (a model facultative CAM species) and the invasive C3 grass Bromus mollis that co-occur in California's coastal grasslands. Specifically we investigated the extent to which water stress, nutrients, and competition affect nocturnal carboxylation in M. crystallinum. High nutrient and low water conditions favored M. crystallinum over B. mollis, in contrast to high water conditions. While low water conditions induced nocturnal carboxylation in 9-week-old individuals of M. crystallinum, in these low water treatments, a 66% reduction in nutrient applied over the entire experiment did not further enhance nocturnal carboxylation. In high water conditions M. crystallinum both alone and in association with B. mollis did not perform nocturnal carboxylation, regardless of the nutrient levels. Thus, nocturnal carboxylation in M. crystallinum was restricted by strong competition with B. mollis in high water conditions. This study provides empirical evidence of the competitive advantage of facultative CAM plants over grasses in drought conditions and of the restricted ability of M. crystallinum to use their photosynthetic plasticity (i.e., ability to switch to CAM behavior) to compete with grasses in well-watered conditions. We suggest that a high drought tolerance could explain the increased dominance of facultative CAM plants in a future environment with increased drought and nitrogen deposition, while the potential of facultative CAM plants such as M. crystallinum to expand to wet environments is expected to be limited. [ABSTRACT FROM AUTHOR]

The coexistence of woody plants and grasses in savannas is determined by a complex set of interacting factors that determine access to resources and demographic dynamics, under the control of external drivers and vegetation feedbacks with the physical environment. Existing theories explain coexistence mainly as an effect of competitive relations and/or disturbances. However, theoretical studies on the way facilitative interactions resulting from hydraulic lift affect tree-grass coexistence and the range of environmental conditions in which savannas are stable are still lacking., We investigated the role of hydraulic lift in the stability of tree-grass coexistence in savannas. To that end, we developed a new mechanistic model that accounts for both competition for soil water in the shallow soil and fire-induced disturbance., We found that hydraulic lift favors grasses, which scavenge the water lifted by woody plants. Thus, hydraulic lift expands (at the expenses of woodlands) the range of environmental conditions in which savannas are stable., These results indicate that hydraulic lift can be an important mechanism responsible for the coexistence of woody plants and grasses in savannas. Grass facilitation by trees through the process of hydraulic lift could allow savannas to persist stably in mesic regions that would otherwise exhibit a forest cover. [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]

Highlights: [•] We investigate mechanism of hydraulic redistribution (HR) in relatively shallow tree roots. [•] We model the climate, vegetation, and soil controls on HR in shallow tree roots. [•] We investigate the effect of HR on the water stress of trees and grasses. [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]