Direct and indirect facilitation of plants with crassulacean acid metabolism (CAM)
Plants with Crassulacean Acid Metabolism (CAM) are increasing their cover in many dryland regions around the world. This effect is typically related to photosynthetic plasticity and efficient water-use strategies of CAM plants which could adapt changes in climate or increasing atmospheric CO2 concentrations. However, the effect of interspecies interactions and the role of CAM plant facilitation by trees and grasses remain poorly understood. Trees 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. To this end, 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. A comparison of transpiration of CAM plants in tree-CAM-grass (T-C-G) and CAM-grass (C-G) associations can explain whether trees directly or indirectly facilitate CAM plants.
Grasses and CAM plants both have shallow roots and strongly compete for soil water in the shallow soil layer. Trees having a high root overlap with CAM plants compete with CAM plants for soil water in the shallow soil layer. This competitive effect of woody plants on CAM plants is outweigh by the significant reduction of grass transpiration in shaded conditions in tree-CAM-grass associations. Overall, transpiration of CAM plants in tree-CAM-grass associations is higher than that in tree-CAM associations, which indicates that trees having a high root overlap with CAM plants indirectly facilitate CAM plants by suppressing grass transpiration in shade conditions. In the cases of a low to moderate root overlap, the indirect effect is confounded by the simultaneous occurrence of the direct effect because transpiration of CAM plants in tree-CAM asscoiations is higher than that in CAM alone. Conversely, the competitive effects of grasses on CAM plants outweigh the facilitation induced by hydraulic lift and thus grasses are unable to indirectly facilitate CAM plants through the mechanism of hydraulic lift.