Photosynthetic responses of two dominant grasses and an encroaching vine in serpentine barrens of southeastern Pennsylvania
Serpentine barrens of northeastern North America are nationally and globally rare ecosystems which favor C4 grass assemblages due to unique low nutrient, ultramafic soil composition as well as high light and high temperatures during the growing season. Remaining barren grasslands are threatened by forest encroachment often preceded by invasions of dense waves of Smilax rotundifolia, a C3 forest understory species. Physiological studies detailing mechanisms by which S. rotundifolia invades the barrens are limited. This vine likely modifies the light and edaphic environment for the dominant grasses specializing in this unique ecosystem, possibly altering photosynthetic efficiency and performance of these species. We examined changes in photosynthetic gas exchange and chlorophyll fluorescence of S. rotundifolia growing in shaded understory and open grassland encroachment areas, as well as Schizachyrium scoparium and Sorghastrum nutans growing in open grassland and ecotone areas overtopped by S. rotundifolia throughout the 2014 growing season at three sites in southeastern Pennsylvania. Mid-day water potential was also measured daily on five leaves or blades per species per light environment. Specific leaf area (SLA) was determined for each species. Analysis of variance was used to assess measured parameters for differences between light environment and month of growth.
Net photosynthesis (Anet) was significantly greater in S. rotundifolia growing in the sun (p = 0.001) and early in the season (p = 0.012). Light environment did not affect Anet for either grass species, though Anet was lowest in July for both species. Significantly lower SLA was observed in sun plots for all species (p < 0.0001), suggesting these species can acclimate morphologically in response to light availability. Water potential, water use efficiency, and SLA also declined throughout the growing season in all species. Efficiency of photosystem II (Fv/Fm) was greater in shaded plots for all species. Smilax rotundifolia clearly benefits from increased light availability, especially during early summer, allowing for increased photosynthetic efficiency. Initially, Anet in the grasses may not be negatively affected by shading because these species can increase leaf surface area for light interception. Shaded grasses may also be subject to less water or light stress than their barrens counterparts, as indicated by higher Fv/Fm values. It is probable that barrens encroachment is chiefly driven by the growth potential of S. rotundifolia. Once this species has spread into the barrens and excluded grass species, it likely continues to alter the light and edaphic environment, initiating forest encroachment.