Koide, D., Yoshida, K., Daehler, C.C., & Mueller-Dombois, D. (2017). An upward elevation shift of native and non-native vascular plants over 40 years on the island of Hawai’i. Journal of Vegetation Science, 28: 939-950. https://www.doi.org/10.1111/jvs.12549

Summary

Studies have demonstrated that climate change is likely to impact mountain ecosystems across the globe, most typically through changes in temperature and shifts in water and nutrient availability, which could lead to upward shifts in species ranges over time. It is unclear, however, the extent to which plant species are shifting their ranges upslope in Hawaiʻi, and whether the rate of movement is different between native and non-native plant species. Mauna Loa, a 13,100 ft. (3,992 m) active volcano on Hawaiʻi Island, experienced a mean annual temperature increase of 1.07 degrees C (1.93℉) between 1970 and 2010, with no detected change in annual precipitation across the gradient, and the authors found it an excellent place to test for species range shifts.

The researchers analyzed long-term vegetation monitoring data from 1970 and 2010 from transects on the southeastern slope of Mauna Loa, Hawaiʻi in order to explore potential shifts in the elevation of both native and non-native plants. To do this, the team re-surveyed 46 vegetation plots in 2010 using the same methodology as the original 1970 surveys, which included collecting presence/absence information and the percentage cover in each vegetation layer (i.e., tree, shrub, and herb layers). To compare shifts in native and non-native species they calculated mean elevation, elevation range, and the upper and lower elevation limits of 69 species that occurred in both the 1970 and 2010 data sets.

The authors found that over a 40-year period, the direction and magnitude of shifts and whether they were driven by changes in the lower, mean, or upper limits depended on the individual species. Strawberry guava shifted its mean elevation ~233 ft. (71 m) upslope due to changes in both its upper and lower limits; weeping-grass had a large increase in mean elevation driven by an increase in its upper limit but not lower limit; molasses grass shifted downward in mean elevation by ~244 ft. (74 m), and Asian sword fern increased in mean elevation with a change in its upper, but not lower elevational limits

Take Home Points

• Individual non-native plant species showed wide variability in their elevation shifts with some species raising their upper elevational limit significantly but not their lower limit (e.g., weeping-grass, Microlaena stipoides), and others increasing both their lower and upper limits (e.g., bamboo orchid, Arundina graminifolia). Some non-native species actually shifted downward over the 40-year period (e.g., red-top grass, Melinis repens).
• The mechanisms driving shifting distributions of non-native plants are largely unknown and could be due to a variety of factors, including: 1) non-native species are still spreading to fill their climate niche, 2) increased disturbances/greater spread (e.g., changing fire regime), 3) enhanced anthropogenic dispersal (building of roads and their use), or 4) changes in habitat suitability due to a changing climate.
• The authors found that native species were generally experiencing shrinking ranges, because the lower limits of their range had increased in elevation and the upper range stayed the same. The authors believe that this stationary upper limit is likely due to the trade-wind inversion (TWI) which abruptly limits rainfall at that higher elevation. In contrast, non-native species experienced a rise in lower and higher elevation limits – perhaps because many non-native species had not yet reached the TWI elevation.

Management Considerations

• Consider increased monitoring efforts that may help detect shifts in non-native plant distributions, especially at the lower and upper elevation limits of species.
• Consider assisting native plant species dispersal across their ranges as conditions change, especially if natural dispersal corridors are absent or degraded. Non-native plants may be able to respond more rapidly to changes in environmental conditions, putting native species at a disadvantage.
• Consider increasing protection efforts for native plants or collecting seeds for future propagation and reintroduction efforts at low elevations, as non-native tree cover is increasing more rapidly than native species cover in these areas.