The C₄ pathway is most abundant in the Poaceae and is associated with warm temperatures.  In Hawai‘i, the grass family is represented by 47 native species and 100 nonnative grasses having both C₄ and C₃ photosynthetic pathways.  Most nonnative C₄ grasses have altered the grass fire cycle, making them a threat to native plant communities.  In Hawai‘i, extreme elevation gradients occur with C₄ and C₃ grasses distributed at different elevations.  Currently, C₄ grass invasions are found at low to mid elevations, sparing intact high elevation sites.  However, an increase of temperature of 0.164°C/decade has occurred in Hawai‘i with temperature increases 3x greater at high elevations, paving the way for C₄ grass expansion.  Previous studies have investigated plant distribution shifts upward in elevation, but these studies have been done in places of high latitude and not in the tropics.  The aim of this study was to compare Hawai‘i Volcanoes National Park (HAVO) grass distributions from 1968 to 2008 to determine if C₄ and C₃ grass distributions are shifting upward in elevation.  We hypothesized that C₄ and C₃ grasses are now establishing at higher elevations within HAVO, and this pattern is associated with climate warming over the past 40 years.  Forty-one 50m x 2m field plots were surveyed in 2008 for grass species and cover at 150m elevation intervals along the roadside and ~30m away from the road.  The transition elevation between C₄ and C₃ grasses in terms of cover and richness was determined from the inflection point of a best fit logistic regression model and compared to the 1968 inflection point.  Individual species distributions were also compared to 1968 values. 

Results/Conclusions

We found that the current transition point of C₄ and C₃ grasses of 1527m (% C_4, all data) is significantly different than the 1968 value of 1183m.  However, off-road transition points did not follow this trend.  We also found the 2008 maximum elevation for C₄ grasses (along road and off road) to be significantly higher than 1968 elevations (P=0.01) while maximum elevations for C₃ grass did not change during this period (P=0.70).  Our study, revealed that grass distributions have changed in association with temperature changes since 1968, exposing higher elevations within HAVO to the threat of invasion by nonnative C₄ grasses, however, changing disturbance regimes within the park may also be a factor that has affected species distributions.