Vegetation Composition, Structure, and Ecophysiology of Maritime Ledge Ecosystems

Ledge and rocky outcrops are common geomorphologic features along the eastern coastline of the Gulf of Maine. Plant communities consisting of pines, spruce and ericaceous shrubs typically dominate these localities. However, coastal currents that regulate local climates create a sharp distinction between communities lying to the east of Frenchman’s Bay and those from Mount Desert Island westward. These maritime climates provide for contrasting vegetation assemblages dominated by two pine species. Pitch pine (Pinus rigida) is the dominant tree in ecosystems at more southwestern sites and jack pine (Pinus banksiana) at more eastern localities. Within each ecosystem two co-occurring vegetation communities are present, one developing under the direct influence of pines and the other lacking pines. Pines appear to have keystone roles, meaning they are necessary for the survival of their associated vegetation. This study focuses on species composition and diversity differences between ecosystems and community-types (pine-dominated and non-pine), how the role of temperature and atmospheric humidity (vapor pressure deficit) affect species distributional patterns, and the ecophysiological relationships of plants and their environment in these four ecosystem-community combinations. The ability of plants to be successful in these high-stress coastal environments is largely related to strategies to avoid or compensate for moisture stress. Species differed in their ability to respond to vapor pressure deficit, which enabled the categorization of species into response functional groups which were associated with species distributional patterns. The Schoodic jack pine ecosystem experiences lower temperatures and increased atmospheric humidity than the Wonderland pitch pine ecosystem. Lower transpirational demand for plant species in Schoodic makes it a more suitable environment for species that lack the ability to respond to high moisture-stress environments, resulting in greater species richness and diversity. Higher temperatures and moisture-stress in Wonderland favor more stress-adapted plants, which contribute to lower richness and diversity. Pines appear to play powerful roles in these ecosystems, as both ecosystem engineers and keystone species. Pines provide a more suitable microclimate for plant communities by decreasing high-light and temperature extremes and increasing atmospheric humidity beneath the canopy. In addition, pines qualify as keystone species as their ecophysiological characteristics make them adapted to establish plant communities in environments prohibitive to other tree species. Plants growing in pine- dominated community-types experience less seasonal water stress and exhibit greater productivity compared to their non-pine counterparts. Pine-dominated communities also have greater soil depths, contributing to a higher availability of water and nutrients for understory plants. Significantly higher concentrations of nutrients associated with marine aerosols were found in plants associated with pines, suggesting that pines may efficiently capture fog and marine aerosols, providing additional water and nutrients to surrounding plant communities. These ecosystems have been identified as having global conservation priority. Understanding the physiological differences between species and how this relates to species distributional patterns can be valuable for resource managers to protect the pines and other species within these unique communities in a warming climate