Upward expansion of fire-adapted grasses along a warming tropical elevation gradient

High mountain regions in the tropics have thus far been impacted relatively little by anthropogenic activity or plant invasions, however, they are unlikely to be immune to impacts of global change, including climate change and other anthropogenic disturbances. Changes in fire regimes are known to accelerate the spread of invasive C4 grasses and interactions between changes in fire and climate can alter species distributions. The aim of this study was to compare grass distributions along an elevational gradient in Hawaiʿi between 1966 – 1967 and 2008 to determine whether C4 and C3 grass distributions are shifting upward in response to alterations in fire and climate patterns. Field plots at Hawaiʿi Volcanoes National Park were surveyed for grass species and cover at ∼150 m elevation intervals and compared to previous surveys done in 1966 – 1967. We found that the transition elevation, marking a shift in dominance between C4 and C3 grasses based on relative cover, shifted upward over 40 yr (95% confidence interval = 1476 m ± 130 m in 2008 versus 1200 m ± 106 m in 1966 – 1967). On the other hand, maximum elevations of all C4 or C3 grasses as a group were not significantly greater than 1966 – 1967 elevations; however, a subset of C4 (and fewer C3 ) grasses moved to substantially higher elevations, and these were the species adapted to fire. 100% of fire-adapted grasses moved up in elevation compared to 29% of non-fire adapted species, and the change in elevation of those species (x̄ = + 454 m) was significantly greater than the change in elevation of non-fire adapted species (p = 0.003). Our study documents an upward expansion of fire-adapted grasses at high elevations in the tropics as an important threat that seems to be compounded by warming trends.