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Accelerating sea-level rise will overwhelm the beneficial effects of elevated CO 2 on coastal wetland plant growth.

Tidal marsh plant species commonly zonate along environmental gradients such as elevation, but it is not always clear to what extent plant distribution is driven by abiotic factors vs. biotic interactions. Yet, the distinction has importance for how plant communities will respond to future change such as higher sea level, particularly given the distinct flooding tolerances and contributions to elevation gain of different species. We used observations from a 33-year experiment to determine co-occurrence patterns for the sedge, Schoenoplectus americanus, and two C4 grasses, Spartina patens and Distichlis spicata, to infer functional group interactions. Then, we conducted a functional group removal experiment to directly assess the interaction between sedge and grasses throughout the range in which they cooccur. The observational record suggested negative interactions between sedge and grasses across sedge- and grass-dominated plots, though the relationship weakened in years with greater flooding stress. The removal experiment revealed mutual release effects, indicating competition was the predominant interaction, and here, too, competition tended to weaken, though nonsignificantly, in more flooded, lower elevation zones. Whereas zonation patterns in undisturbed portions of marsh suggest that the sedge will dominate this marsh as flooding stress increases with sea level rise, we propose that grasses maymore »exhibit a competition release effect and contribute to biomass and elevation gain even in sedge-dominated communities as sea level continues to rise. Even as abiotic stresses drive changes in the relative contributions of sedges and grasses, competition among them moderates fluctuations in total plant biomass production through time.« less

Plants are subject to tradeoffs among growth strategies such that adaptations for optimal growth in one condition can preclude optimal growth in another. Thus, we predicted that a plant species that responds positively to one global change treatment would be less likely than average to respond positively to another treatment, particularly for pairs of treatments that favor distinct traits. We examined plant species abundances in 39 global change experiments manipulating two or more of the following: CO2, nitrogen, phosphorus, water, temperature, or disturbance. Overall, the directional response of a species to one treatment was 13% more likely than expected to oppose its response to a another single-factor treatment. This tendency was detectable across the global dataset but held little predictive power for individual treatment combinations or within individual experiments. While tradeoffs in the ability to respond to different global change treatments exert discernible global effects, other forces obscure their influence in local communities.