Clonal Diversity and Resistance to Invasion in Remnant Salt Marsh Patches Dominated by Spartina patens

Participants: Claus Holzapfel, RUN Department of Biology; Francisco Artigas, MERI Director; Edward Kirby and Huimin Man, RUN Department of Biology

Background:

Large portions of brackish marsh have been invaded by non-native genotypes of the common reed (Phragmites australis). Within the Hackensack Meadowlands only fractions are still (or again) dominated by low marsh Spartina alterniflora and high marsh Spartina patens (both being native species). From the latter species, only isolated patches remain that nevertheless appear to be stable (Artigas pers. com.). These patches vary in size and seem to resist encroachment by other species through their peculiar growth form: the formation of dense “cowlick” mats consisting of prostrate grass shoots. This high marsh has been used in historic time for hay making (Smith 1942) or was burned to facilitate goose grazing and/or muskrat trapping. Current dense mats of S. patens apparently developed after these management practices ceased. Today, the presence of the Spartina high marsh is contributing to the landscape diversity and species diversity in the Meadowlands and is attracting a wide range of wildlife.

Goals:

We are interested in exploring the factors that allow these patches to persist in the presence of dominant and highly competitive Phragmites. In particular we would like to investigate (a) whether the peculiar prostrate growth form of S. patens allows it to compete with other species and (b) whether larger patches of S. patens consists of more competitive clones than small patches. Three goals drove the initial stage of this research:

Establishment of permanently marked transects along the border zone between Phragmites and S. patens that will allow documentation of medium and long-term development along these borders.
Establishment of molecular methods for the analysis of the genetic structure of clones of large and small patches with the long-term goal to establish whether these patches consist of many or few distinct clones differing in competitive potential.
Further trial-stage investigations to establish methodology to measure relative fitness and function of Phragmites along the ecotone zone at the edges of present S. patens patches.

Methods and Results:

1. Border Transects

We established 13 permanent transects in 5 different locations along the lower Hackensack River (see Fig. 1). The transects were placed perpendicular to the Pharagmites – S. patens border with the center of the transect approximately placed at the center of the contact zone. The data from this transect are used to calculate the area that is dominated by the interacting species (A and B in Fig. 2); special consideration is given for the overlap zone (O).

Exemplary results are shown in Fig. 3. For these transects it becomes evident that the overlap zone is more extensive for small patches than for large patches. An analysis of the relative extent of overlap (O / A+B) revealed that this difference is statistically significant (p>0.002 (ANOVA), Fig. 4).

2. Genetic description of clonal structure of patches

Plant material has been sampled along the transects from the edge to the centers in selected small and large patches of S. patens. All locations listed in Fig. 1 have been sampled, and DNA from Spartina patens isolated and amplified. In the initial phase we decided to use RAPDs (randomly amplified polymorphic DNA) in order to describe the genetic variability within and between Spartina patches.

The genetic variation of collected samples was examined by twelve primers but only one produced best result. According to RAPD banding patterns, samples from different sites had no significant genetic variation (see Fig. 5). This suggests molecular data show the genetic diversity in Spartina patens is low.

3. Descriptive measurements of the effects of S. patens mats on isolated stems of competitors such as Phragmites australis

A number of above-ground structural and ecophysiological measurements were conducted to screen for possible negative (=competitive) effects of S. patens on merging fronts of Phragmites in border zones. No clear indications were gained thus we conclude that likely negative interactions (if existent) are more likely acting below-ground and therefore require additional investigations (most likely of experimental nature).

Conclusions and outlook

We found evidence that border zones between Phragmites and Spartina patens stands are different between small or large remnant patches of Spartina high marsh. The fact that the stand overlaps in these border areas are smaller and the border therefore sharper, suggests that differences in competitive interaction exist.

The preliminary result, that the sampled Spartina patens clones have very similar genetic structure, has to be confirmed with additional studies. These are currently conducted by graduate student Tingling Wu in cooperation with the lab of Edward Kirby.

Meadowlands-Riverbend-Fish Creek site, 2006-08-31:
‘Cowlick’ mats of Spartina patens with bend culms of Phragmites australis