The effect of nitrogen availability and water conditions on competition between a facultative CAM plant and an invasive grass.

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 (a model facultative CAM species) and the invasive C grass that co-occur in California's coastal grasslands. Specifically we investigated the extent to which water stress, nutrients, and competition affect nocturnal carboxylation in . High nutrient and low water conditions favored over , in contrast to high water conditions. While low water conditions induced nocturnal carboxylation in 9-week-old individuals of , 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 both alone and in association with did not perform nocturnal carboxylation, regardless of the nutrient levels. Thus, nocturnal carboxylation in was restricted by strong competition with 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 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 to expand to wet environments is expected to be limited.