The enduring question of why some species are rare while others are common has important implications for ecological theory, rare species conservation, and overall biodiversity. The southeastern U.S. has been long recognized for its disproportionately high numbers of rare endemic plant species, and the rapid pace of contemporary environmental change due to anthropogenic activities and influences has been implicated as a particular threat to rare species in the region and beyond. Adaptation to new conditions is possible for such species, but could be impaired by the time required relative to that of rapid environmental change and the low genetic diversity characteristic of many rare species. In contrast, acclimation could facilitate species expansion across varied environments and/or persistence in locations experiencing rapid environmental change. We use a combination of field- and growth chamber-based empirical investigations of ecophysiological plasticity and genetic diversity within and across a suite of rare endemic and common congeneric plant species of the southeastern U.S. to elucidate how the ability to adapt and/or acclimate could constrain rare species responses to contemporary environmental change. As broader objectives, we aim to provide a deeper conceptual understanding of species rarity and guidance to rare species conservation efforts in the region.
Results/Conclusions
Our preliminary investigations focused on responses of the rare terrestrial orchid Platanthera integrilabia (white fringeless orchid) to light and soil moisture availability across and within four natural occurrences toward evaluating its potential for adaptation and acclimation. Across sites, photosynthetic light-response and water-use efficiency suggest that P. integrilabia could adapt to maximize energy gain in contrasting light and soil moisture environments. However, minimal associations of these measures with abiotic variations within occurrences suggest that photosynthetic trait plasticity in this species may be limited. Recent and comparative growth chamber-based work focused on the rare Pityopsis ruthii (Ruth’s golden aster) similarly suggests that this rare species is characterized by significant local adaptations but limited plasticity in response to resource availability relative to the common congener P. graminifolia (narrowleaf silkgrass). Companion investigations aim to quantify the genetic diversity of these species toward further investigating their potential to adapt to environmental change. Current NSF-sponsored research is focused on comparing rare Boechera perstellata (Braun’s rockcress) to common B. laevigata, with further comparative congeneric research to focus on rare Echinacea tennesseensis (Tennessee coneflower), Helianthus longifolius (longleaf sunflower), Scutellaria montana (large-flowered skullcap), and Xyris tennesseensis (Tennessee yellow-eyed grass).