Comparing an exotic shrub's impact with that of a native life form analogue: Baccharis halimifolia vs Tamarix gallica in Mediterranean salt marsh communities

• Published in: Journal of vegetation science 2016-07, Vol.27 (4), p.812-823
• Main Authors: Fried, Guillaume, F. Dane Panetta
• Format: Article
• Language: English
• Subjects: Baccharis; Baccharis halimifolia; Biotic resistance; canopy; coasts; community structure; Impact thresholds; indigenous species; ions; Juncus; juveniles; meadows; Nestedness; Risk assessment; salt marshes; Shrub invasion; shrubs; species diversity; Species richness; Species turnover; Tamarix gallica; Vegetation cover
• ISSN: 1100-9233; 1654-1103
• DOI: 10.1111/jvs.12407

QUESTIONS: To what extent may impact vary according to the invaded community, region or sites? Do impact thresholds exist, above which native species richness and/or cover declines rapidly? Does impact following invasion by the exotic shrub Baccharis halimifolia differ from that of a native shrub (Tamarix gallica) during natural successions? LOCATION: Mediterranean coast, SE France. METHODS: Vegetation was sampled on 120 4‐m² quadrats, half of which were in areas invaded by B. halimifolia and the other half in nearby non‐invaded areas, in two distinct community types nested in three sites in each of two regions. Mixed models were built to explain variations in impacts on native vegetation according to community types, sites and regions. Next, 85 additional plots variously invaded by B. halimifolia were used to assess the nature of the relationships between B. halimifolia cover and impact in Juncus communities. Linear regressions were compared to polynomial regressions, and when relevant, regression trees were built to identify threshold values. Finally, we compared the vegetation from 90 plots either dominated by B. halimifolia, by T. gallica (a native shrub) or without a shrub layer (control) using dissimilarity indices in wet meadows. RESULTS: The magnitude of B. halimifolia impacts depended mainly on community type, with no differences between regions. Community structure (species richness, Shannon's diversity) declined linearly with increasing cover of B. halimifolia, with the most rapid decline in annual species. Native species cover fitted better with a cubic regression, with an impact threshold of around 86% cover of B. halimifolia. Impact of B. halimifolia on community diversity was higher than that of the native shrub T. gallica. While the magnitude of species composition changes was comparable for both shrubs, the higher nestedness component of dissimilarity found for B. halimifolia may indicate a higher short‐term impact. CONCLUSION: Improved understanding of the impacts of exotic shrubs on community structure and composition can be gained via comparisons with the impacts caused by native shrubs. Higher impacts of the exotic B. halimifolia on Mediterranean salt marsh vegetation are likely explained by a denser canopy, shorter juvenile period and more frequent recruitment.