This study focused on sensitivity of high-elevation ecosystems in Hawai‘i to climate change. These Hawaiian ecosystems are becoming warmer and drier, and are relevant because they house many rare species, represent the last remaining stretches of uninvaded landscapes, and include wao akua – the small-statured cloud forests of great cultural significance that are the ‘realm of the gods’. Rapid climate change here presents a disproportionately high climate change impact risk. We provided models that relate current, past, and future distribution of plant species from 6000 – 7500’ feet in elevation on Haleakalā, to mean climate, extreme drought events, and soil properties. We constructed 24 models of current vegetation and found that moisture – both overall mean moisture and moisture during an El Niño drought event – was a strong driver of vegetation patterns today; whereas temperature and soils were less important. We tested whether El Niño frequency was related to changes in vegetation over very long time scales with paleorecords of the forest’s upper limit and climate that extend over 3000 years. We found that indeed, the upper limit of forest shifted up or down the mountain depending on the frequency of El Niño drought events (downslope shift with greater drought frequency) and local moisture availability. We analyzed the sensitivity of vegetation to future changes in rainfall – from a 30% reduction in rainfall to a 30% gain. We found that vegetation is often more sensitive to reduced rainfall than to an equal amount of increased rainfall. With reduced rainfall, vegetation tends to move downslope. These data establish the importance of moisture and El Niño to high-elevation ecosystems and cast doubt on a common expectation of upslope movement of vegetation with warming in Hawai‘i. Downslope movement of vegetation with reduced moisture is an important scenario for planning and management. However, the future of El Niño, which is largely unknown, may also be important for predicting future vegetation changes.
