COS 55-4 - Hawaiian subalpine: Implications for plant community conservation under climate change

Abstract: Globally, subalpine and alpine plant communities are receiving increasing attention due to disproportionate rapid warming at high altitudes and the resultant habitat shrinkage leaving high-altitude specialists with nowhere to migrate. The Hawaiian subalpine zone (1530 – 2500m asl), above the trade wind inversion, is an interesting example of this potential phenomenon because of the high endemism in the vegetation. We analyzed vascular plant species richness, cover, and density from 89 plots (1000m²) sampled between 2010 and 2018 across two volcanic mountains on two islands, Haleakalā (3055m) on Maui, and Mauna Loa (4169m) on Hawaiʻi. Most of the 138 plant species recorded were non-native (56%) with the remainder being endemic (30%) and indigenous (14%). Plot-level species richness differed from gamma diversity, with endemic species more abundant than non-native species, indicating that while many non-natives are present, their distribution is patchy. Non-native species richness was higher on Haleakalā than Mauna Loa. These subalpine communities are characterized by low-lying (< 1 m tall) vegetation and substantial open space, with lower cover on Mauna Loa (36%), which is younger, higher, and drier than Haleakalā (56%). Cluster analysis based on species cover using the Sorensen (Bray-Curtis) distance measure identified two habitat types, a shrubland and a woodland (Mauna Loa – only). When comparing volcanoes, multiple species were important indicators of the Haleakalā subalpine, with two of the strongest being non-native, Holcus lanatus grass and Hypochoeris radicata herbs. Conversely, only two species were identified as indicators of Mauna Loa (including shrublands and woodlands), the endemic tree, Metrosideros polymorpha, and the endemic sedge, Morelotia gahniiformis. Density was largely consistent with the understory cover data, with endemic Vaccinium reticulatum ( >3500/ha) and indigenous Leptecophyllya tameiameiae ( >2430/ha) shrubs dominant on both volcanoes. Woodland communities, encountered only on Mauna Loa, had endemic trees Metrosideros polymorpha dominant on the wetter, south aspect, and Sophora chrysophylla dominant on the drier, leeward side. The Hawaiian subalpine varies among islands, volcanoes, and aspects, yet remains largely native-dominated, but with increasing threats from climate change, non-native species, and wildfire. Critically, this sparsely vegetated, environmentally extreme zone may represent the last habitat for many of Hawaiʻi’s disappearing endemic plants, insects, and avi-fauna. We recommend continued monitoring of biotic communities and climate in this sensitive zone, physiological in situ studies for the native matrix subalpine species, stricter non-native species biosecurity and sanitation protocols, wildfire prevention, and improved documentation of the effects of feral ungulates.