Maximal stomatal conductance to water and plasticity in stomatal traits differ between native and invasive introduced lineages of Phragmites australis in North America.

The fitness costs of reproduction by clonal growth can include a limited ability to adapt to environmental and temporal heterogeneity. Paradoxically, some facultatively clonal species are not only able to survive, but colonize, thrive and expand in heterogeneous environments. This is likely due to the capacity for acclimation (sensu stricto) that compensates for the fitness costs and complements the ecological advantages of clonality. Introduced Phragmites australis demonstrates great phenotypic plasticity in response to temperature, nutrient availability, geographic gradient, water depths, habitat fertility, atmospheric CO2, interspecific competition and intraspecific competition for light. However, no in situ comparative subspecies studies have explored the difference in plasticity between the non-invasive native lineage and the highly invasive introduced lineage. Clonality of the native and introduced lineages makes it possible to control for genetic variation, making P. australis a unique system for the comparative study of plasticity. Using previously identified clonal genotypes, we investigated differences in their phenotypic plasticity through measurements of the lengths and densities of stomata on both the abaxial (lower) and adaxial (upper) surfaces of leaves, and synthesized these measurements to estimate impacts on maximum stomatal conductance to water (gwmax). Results demonstrated that at three marsh sites, invasive lineages have consistently greater gwmax than their native congeners, as a result of greater stomatal densities and smaller stomata. Our analysis also suggests that phenotypic plasticity, determined as within-genotype variation in gwmax, of the invasive lineage is similar to, or exceeds, that shown by the native lineage.

Intra-clonal variation and a similarity threshold for identification of clones: application to Salix exigua using AFLP molecular markers.

Although molecular methods are a major advance over phenological or root connectivity studies in the identification of clonal plants, there is still a level of ambiguity associated with two types of error: misidentification of genetically similar seedlings as clones and misidentification of dissimilar fingerprints from clones as genetically distinct individuals. We have addressed the second of these error types by determining the level of variation for AFLP fingerprints in Salix exigua, and then by developing a threshold value of Jaccard's similarity index for assigning individuals to clones or to siblings. Variation in AFLP banding patterns among clones was partitioned into three potential sources; clones, stems within-clones and foliage within-stems. Most of the variation was attributable to clones and then to stems within-clones. To provide an objective means of identifying clones, we developed a method for establishing a threshold similarity index to assign individuals to the same clone. Our method yielded a Jaccard similarity threshold of 0.983 that resulted in a potential pairwise error rate of 8.1% putative clone assigned to siblings and 1.5% sibling assigned to clones. The method was tested on independent clonal and sibling individuals resulting in the same threshold value and similar error rates. We applied our method to assign individuals to clones in a population of S. exigua along the Cosumnes River, California. A total of 11 clones were identified, with one clone including 43% of the individuals sampled. Our results show that this approach can be useful in the accurate identification of clones.