Abstract
Sea level rise (SLR) imposes increasing salinity and inundation stresses in salt marshes which simultaneously face invasions by exotic plant species. We aimed to improve and apply knowledge on the ecophysiological responses of halophytes to SLR to conservation management of salt marshes. In a mesocosm experiment, we measured and compared phosphoenolpyruvate carboxylase (PEPC) activity and related functional traits of the California-native Spartina foliosa, invasive S. densiflora and their hybrid S. densiflora × foliosa in response to increasing levels of salinity and inundation. S. foliosa was moderately sensitive to salinity, showing a 57% reduction in PEPC specific activity from freshwater to hypersalinity. This native species compensated for the reduction of PEPC activity with increased salinity through 80% higher enzyme activation by phosphorylation. PEPC functional trait responses of S. foliosa were mostly independent of inundation depth. In view of these results, managers should conserve undeveloped lands for accommodation space above current high tide lines to facilitate colonization of stress-tolerant S. foliosa. Our results on functional responses of PEPC traits recorded high sensitivity to salinity for S. densiflora. This was reflected by 65% lower PEPC specific activity together with increasing accumulation of free proline (+96%) and total proteins (+23%) with elevated salinity. These results suggest prioritized eradication of S. densiflora populations in brackish habitats. Measured PEPC responses support the high stress tolerance of the S. densiflora × foliosa hybrid. PEPC traits for the hybrid were mostly independent of salinity and inundation. The hybrid showed higher PEPC-specific activity than S. foliosa (+70%) and S. densiflora (+15%) in freshwater under intermediate inundation. Results suggest that eradication of the hybrid should be the highest management priority. Our study shows that the responses of key functional physiological traits to environmental stresses serve as biological indicators that can guide ecosystem management practices in a scenario of climate change.
Highlights
Sea level rise (SLR) due to global warming is increasing salinity, and inundation depth and duration in the world’s salt marshes (IPCC, 2015)
S. foliosa was moderately sensitive to salinity, showing a 57% reduction in phosphoenolpyruvate carboxylase (PEPC) specific activity from freshwater to hypersalinity
We reviewed 28 studies related to the response of S. foliosa, S. densiflora and their hybrids to salinity, 20 works related to inundation, and only one publication analyzing the combined effect of salinity and inundation on these taxa (Supplementary information: Literature review)
Summary
Sea level rise (SLR) due to global warming is increasing salinity, and inundation depth and duration in the world’s salt marshes (IPCC, 2015) These environmental changes are highly significant since salinity and flooding are among the main abiotic stress factors determining the performance and distribution of halophytes in salt marshes (Engels and Jensen, 2010). Alien species hybridize with native species producing hybrids with high stress tolerance and competitive ability that displace native biodiversity in salt marshes (Wong et al, 2018; Williams et al, 2019) This improved performance of hybrids may be related to transgressive traits due to non-additive gene expression (Favre and Karrenberg, 2011). Improved knowledge of functional and evolutionary traits supporting the invasiveness and relative impacts of exotic plant species is a priority (Drenovsky et al, 2012)
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