Clonal diversity and resistance to invasion in remnant salt marsh patches

Abstract: The importance of using local ecotypes of species as building blocks in restored communities and the need of including information on genotypic differentiation has been stressed in restoration ecology. Large portions of brackish East Coast marshlands have been invaded by a non-native, European genotype of the common reed, Phragmites australis. As a result, only a small fraction of the New Jersey Hackensack Meadowlands is now dominated by native marsh species and only isolated patches of Spartina patens remain. As these patches vary in size and seem to resist encroachment by Phragmites differentially, we investigated (a) whether larger patches are able to resist invasion more than smaller patches and (b) whether large-patch clones are better suited for restoration efforts. We aim at understanding whether the extant mosaic of small and large patches is the result of a random disintegration of one former large Spartina stand or rather it is the result of a network of patches that resisted invasion differently. In a combined approach that includes surveys of permanent transects, common garden transplant experiments, competition experiments, genetic and chemical analysis, we monitored border dynamics and assessed genetic identity and performance of clones of different patch sizes. Results of the project indicate (1) clear genetic differentiation between large and small-patch clones even when they are in close proximity to each other and (2) that clonal diversity in large patches is greater than in small-patches. There are differential competitive capacities of these distinct clones: (a) border zones between the invader and Spartina tend to be more defined in large Spartina remnant patches than in small patches; (b) Spartina increases in dominance at large-patch borders but decrease in small-patch borders; and (c) large-patch Spartina is able to reduce the growth of invading Phragmites fragments and is less reduced by it. We conclude that current small patches are not the remnants of the formerly extensive, homogenous stands. Rather, the current mosaic of small and large patches is the result of a large number of clones that resisted invasion differentially. Little is known about the underlying mechanisms, and clone-specific root exudates were explored but no clear differences between ramets from successful and less successful patches were found so far. Our results strongly suggest that some genetically defined Spartina clones are more suitable for restoration efforts than others, a notion that has strong implication for future restoration projects.