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Aims Serpentine barrens of the Mid-Atlantic United States are globally rare, grass-dominated ecosystems thought to exclude C3 species and characterized by unique soil composition. However, like many grassland ecosystems globally, these sites are presently facing encroachment by surrounding forest. In this ecosystem, the tendril climbing vine, Smilax rotundifolia, forms dense thickets around barrens that typically precede forest encroachment. While numerous factors speculatively initiate and promote encroachment in these systems, few studies have examined photosynthetic responses of serpentine grasses to changes in resource availability initiated by encroachers or physiological ability of encroaching S. rotundifolia to colonize high-light barren environments. We sought to understand both environmental and physiological dynamics of encroachment along an irradiance gradient in these systems. Methods At three serpentine barren sites in southeastern Pennsylvania, physiological responses of the native grasses Sorghastrum nutans and Schizachyrium scoparium to increased shading by S. rotundifolia were examined. Additionally, the physiological performance of S. rotundifolia in the forest understory was compared to that in sunlit barrens environments. Light-saturated photosynthesis (Asat), stomatal conductance (gs), instantaneous transpiration efficiency (ITE), maximum efficiency of photosystem II (Fv/Fm), midday leaf water potential (ψleaf) and specific leaf area (SLA) were measured for all species over the course of the 2014 growing season. Two-way analyses of variance were used to assess changes in these parameters across growing environments, as well as seasonally for each species. Multiple regression analyses were also performed to investigate the influence of vapor pressure deficit (D) and leaf temperature (Tleaf) on Asat and ITE. Important Findings Light-saturated photosynthesis was significantly greater in S. rotundifolia growing in sun than in the shade (P < 0.001) and early in the season (P = 0.012). Sun S. rotundifolia had up to 27% greater Asat than shaded plants. Even with reduced ψleaf in the barrens, S. rotundifolia maintained high gs though Asat was limited by high D later in the growing season. These data are in agreement with recent research suggesting that plants are not subject to water limitation in Mid-Atlantic barrens. Unexpectedly, shaded grasses at the encroachment interface did not exhibit any significant reduction in Asat. Declines in grass photosynthesis do not likely occur until S. rotundifolia rhizomes transition into monotypic thickets, completely excluding grasses. Encroachment by S. rotundifolia appears to be primarily facilitated by its ability to capitalize on light resources early on in the summer growing season, when environmental conditions are less stressful. [ABSTRACT FROM AUTHOR]

Premise of the Study: Microsatellite markers were developed for Smilax rotundifolia (Smilacaceae), an understory vine widely distributed in eastern North America, to investigate genetic diversity and structure. Cross‐amplification was tested in three congeneric species: S. china, S. riparia, and S. walteri. Methods and Results: A total of 6153 simple sequence repeat primer pairs were detected from the de novo–assembled transcriptome data (88,3962 contigs) of S. rotundifolia. Thirty‐three polymorphic microsatellite loci were selected for further analysis among 96 individuals representing four natural populations of the species. The number of alleles ranged from two to 15, and 87.9% of the developed primer pairs could be cross‐amplified in at least one of three congeneric Smilax species. Conclusions: The simple sequence repeat markers developed in this study will facilitate further studies on genetic diversity and phylogeographic patterns of S. rotundifolia and provide additional potential microsatellite resources for other Smilax species. [ABSTRACT FROM AUTHOR]