High elevation Hawaiian plant communities: implications for conservation under climate change
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2021
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University of Hawaii at Manoa
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Globally, subalpine, and alpine plant communities are receiving increasing attention due to disproportionate rapid warming at high altitudes and the resultant shrinking habitat leaving high-altitude specialists with less habitat and nowhere to migrate. Plant communities on tropical high islands, such as the Hawaiian Islands, are predicted to experience rapid climate change, and if increased temperature and/or drought exceed plant species’ current tolerances, species that are unable to adapt or shift ranges, risk extinction. While extensive climate modeling is underway in Hawaiʻi, few studies have assessed the potential impacts of climate change on high elevation vegetation. To address this knowledge gap, I first quantified habitat specialization for 170 plant species using species co-occurrence data from over one thousand plots to rank species’ realized habitat niche breadth using the Jaccard index. This indirect method of estimating species’ potential climatic flexibility uses increasingly available large plant community data sets with output rankings which represent species’ realized habitat niches. The distribution of species along this continuum differed by species’ biogeographic origin, with endemic plant species ranked on the specialist end and non-native plant species ranked on the generalist end. Habitat specialization rankings also differed by number of habitat moisture types, minimum elevation, number of Hawaiian Islands, and life form. Volcanic mountains in Hawaiʻi have distinct treeline ecotones driven by trade wind inversion. During the past fifty years, periods of increased temperature and drought associated with increasingly frequent cloud inversion events have occurred, but little is known about how these climatic changes have influenced treeline vegetation. Vegetation data from 225 plots spanning treelines (1500-2500 m) on Haleakalā and Mauna Loa were used to categorize ecotonal plant communities. Treeline indicator species differ by moisture and temperature variables with common native species important for all types: wet forest (Cheirodendron and Metrosideros trees, ferns), subalpine woodland (Myoporum and Sophora trees), and subalpine shrubland (Vaccinium and Leptecophylla shrubs, native graminoids). The subalpine woodland contains the most habitat generalist indicator species, likely due to high non-native species richness. Moisture best explains the described patterns in plant community composition, with wet canopy evaporation, mean annual precipitation, and aridity index values differentiating between wet forest and subalpine communities. A wider extent of the subalpine zone (i.e., beyond treeline ecotone) was analyzed to further identify commonalities and differences between plant communities on Haleakalā and Mauna Loa volcanoes. I compared plant species richness, cover, and density data from 89 plots. A total of 138 plant species were recorded and over half of these species were non-native (56%) with the remainder being endemic (30%) and indigenous (14%). Gamma diversity differed between volcanoes with one-third found only on Haleakalā, one-third found only on Mauna Loa, and one-third shared. Species richness per plot differed from gamma diversity in that endemic species were more abundant than non-native species for both volcanoes indicating that while many non-natives are present, their populations remain patchy and not yet widespread. Non-native species richness was higher on Haleakalā than Mauna Loa. In general, the subalpine communities are characterized by patchy low-lying (<1 m) vegetation with lower cover on the younger drier site – Mauna Loa (36%) than on Haleakalā (56%). Community structure was largely consistent with the understory cover data, with endemic Vaccinium (>3500/ha) and indigenous Leptecophyllya (>2430/ha) shrubs dominant. I recommend continued monitoring of biotic communities and climate in this sensitive high elevation zone, additional physiological in situ studies for the few native matrix subalpine plant species, stricter non-native species biosecurity and sanitation protocols, wildfire prevention, and improved documentation of the effects of feral ungulates including their ongoing removal.
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