Plants within tropical treeline ecotones are theoretically very sensitive to climate variability because this zone represents tree species’ altitudinal limits. Mountains on tropical islands may be especially vulnerable to climate change, because tropical high elevation ecosystems are expected to be among the first to experience future “novel” climates. Hawaiian volcanic mountains in Hawaiʻi Volcanoes and Haleakalā National Parks support extremely young, isolated, trade wind inversion (TWI) driven treeline ecotones. Hotter and drier conditions associated with more frequent TWI events have occurred at high elevations (>800m) in Hawaii during the past fifty years. Little is known about how these climatic changes have influenced treeline vegetation across treeline ecotones. Previous studies highlighted an abrupt treeline transition between grassland and wet forest on windward Haleakalā, but this unique site does not represent the diversity of treeline ecotones among volcanoes, lava flow ages and types, and regional climatic conditions in Hawaii. We used National Park Service inventory and monitoring vegetation data from 225 plots surrounding treelines (1500-2500 m) on Haleakalā and Mauna Loa volcanoes to test the hypotheses that (1) species composition across treeline ecotones differs by moisture and temperature and (2) treelines are higher in elevation and more abrupt at wetter sites.
Results of nonmetric multidimensional scaling analysis indicate that mean annual temperature and rainfall are important in explaining differences in plant community composition across Hawaiian treeline ecotones. Sampling plots separated into three distinct communities using cluster analysis, wet forest and subalpine shrubland on Haleakalā and Mauna Loa and subalpine woodland on Mauna Loa. Community indicator species ranged from single community type indicators (e.g., Cheirodendron trigynum trees, many pteridophytes in wet forest) to widespread species with abundance and/or growth form defining the indicator (e.g., high cover of Leptecophylla tameiameiae shrubs in subalpine shrublands or high cover and upright growth of Metrosideros polymorpha trees in wet forest). Treelines at wetter sites (>2200 mm), where cloud forests meet dry subalpine shrublands, are higher (~2000 m) and more visually distinct on aerial imagery than drier sites (Haleakalā and the wetter regions of Mauna Loa). Alternatively, on drier, leeward Mauna Loa, where sparser subalpine woodlands meet shrublands, elevation varies and temperature is better than moisture as a treeline indicator. Interestingly, these results indicate lower elevation treelines for the Hawaiian Islands than previously documented in the literature. We expect Hawaiian treelines to continue migrating down based on future climatic drying scenarios, contrary to temperate treeline predictions.