An upward elevation shift of native and non-native vascular plants over 40 years on the island of Hawai'i

Questions: Have vascular plants shifted in elevation along a tropical mountain gradient on Mauna Loa, Hawai'i over the past 40 yr? If so, do native and non-native species differ in their shifts of mean elevation range, upper elevation limit or lower elevation limit? Location: Hawai'i Volcanoes National Park, southeast slope of Mauna Loa, Hawai'i. Methods: We resurveyed an elevation belt transect of vegetation plots that was first surveyed in the early 1970s. Differences between 1970 and 2010 were calculated for 69 plant species' mean elevation, upper elevation limit, lower elevation limit and elevation range. Statistical significance of differences was assessed by Monte Carlo permutation tests. Results: Averaging across all 69 species, a statistically significant upward shift of 65.2 m was detected in mean elevation. Both upper and lower elevation limits shifted significantly upward by 31.6 and 90.5 m, respectively. Elevation range did not change significantly. For the 49 native species, the upper elevation limit did not change, but the lower elevation limit shifted significantly upward by 94.1 m. In contrast, the 20 non-native species displayed a different pattern of a significant upward shift in both their upper and lower elevation limits, by 126.4 and 81.6 m, respectively. Native species seemed to display a contracting elevation range of 101.2 m, but this was not significant (P = 0.053), while non-native species maintained a stable elevation range (P = 0.365). Conclusions: Vascular plants have shifted upward in elevation on the tropical island of Hawai'i, but the shifting trend differs between native and non-native species. Non-native species, which were often found at lower elevations, are in an upward niche expansion phase. Native species show trends toward range contraction due to lack of expansion in upper elevation limit, probably due to drought stress correlating with a trade-wind inversion (TWI) that severely limits rainfall at higher elevations. As more non-native species reach the TWI elevation, their upward expansion may also become more limited.