Long-term alpine summit vegetation cover change: Divergent trajectories driven by climate warming and fire

Alpine summit vegetation, the highest point of species geographical distributions, is vulnerable to climate change (thermal niche contraction), and there is evidence of change in Northern Hemisphere summits. However, summits are experiencing multifaceted change due to warming and increasing fire frequency. Little is known about how these factors are affecting alpine summit vegetation. We used a revisitation approach to capture the long-term (eighteen years) dynamic changes in Australian alpine plant summit community patterns and to understand the mechanisms of change. We found that vegetation change was influenced by climate and moderated by site-specific factors. There was increased shrub cover over time; however, summit vegetation was largely stable unless disturbed. Fire-disturbed summits experienced higher instability in their vegetation cover over time. Linear mixed-effect models indicated that as time since fire increased and the growing degrees accumulated, there was a strong positive effect on forb and graminoid cover and a negative effect on shrub cover. Forb cover was higher at cooler, wetter, higher-elevation summits. These findings indicate the multifaceted nature of change that must be accounted for in alpine vegetation studies. We show that alpine summit vegetation will respond multidirectionally to a warming climate and changing fire regimes, with outcomes likely contingent on life history characteristics.