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

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.

The competitive advantage of a constitutive CAM species over a C4 grass species under drought and CO2 enrichment

“…Previous studies showed that high drought resistance accounted for the competitive advantage of a model facultative CAM species ( M. crystallinum ) over its co‐occurring grass species ( B. mollis ) found in California grasslands under global change scenarios (Yu et al. , b). This study investigated whether water stress and concurrent CO 2 enrichment—which are known to affect the interactions between C 3 and C 4 plants (Smith et al.…”

“…Yu et al. (, b) experimentally found that a model facultative CAM species ( Mesembryanthemum crystallinum ) had a competitive advantage over a co‐occurring grass ( Bromus mollis ) in California under global change scenarios of increased drought, soil salinity, and nitrogen deposition. An empirical evaluation of the response of constitutive CAM plants and their competitive relationships with other functional groups (i.e., C 4 plants) to CO 2 enrichment and changing climate, however, is still missing (Poorter and Navas ).…”

Response of a facultative CAM plant and its competitive relationship with a grass to changes in rainfall regime

“…2d). In fact, B. mollis is not drought tolerant (Yu et al 2017b(Yu et al , 2017c) and appeared to be severely damaged by early extreme droughts, thus restricting the ability of B. mollis to recover and benefit from later rainfall events (Walter et al 2011). This Bmemory^of early droughts was not significant for M. crystallinum which as facultative CAM plants have higher plant water contents and are more drought (Lüttge 2004;Borland et al 2009;Yu et al 2017bYu et al , 2017c.…”

“…Studies of CAM plants are engendering scientific interest, particularly in recent years, because of their ability to adapt to drought through photosynthetic plasticity which may allow them to increase their dominance in a future (drier and warmer) climate (Drennan and Nobel 2000;Lüttge 2004;Borland et al 2009;Yu et al 2017bYu et al , 2017c. This study investigates the response of a model facultative CAM species (Mesembryanthemum crystallinum, occasional crop species) and its competitive relationship with a C 3 grass (Bromus mollis) to changes in rainfall regime (i.e., intra-seasonal rainfall variability and rainfall seasonality).…”

“…This photosynthetic plasticity performed by M. crystallinum was found to increase its fitness and reproduction in dry conditions (Winter and Ziegler 1992;Cushman et al 2008). A recent study indicates that M. crystallinum could also adapt to biotic stress (i.e., moderate competition from grasses) because of its photosynthetic plasticity (Yu et al 2017b). It remains unclear, however, how changes in rainfall regime (i.e., increased intra-seasonal variability and rainfall seasonality) may affect nocturnal carboxylation in M. crystallinum.…”