Alpine summit vegetation cover change over 18 years: divergent trajectories driven by climate warming and fire
By Iris T. Hickman, Susanna E. Venn, James S. Camac, John W. Morgan
Alpine summits, the highest point of species geographical distributions, are potentially vulnerable to climate change (thermal niche contraction), and there is evidence of change in northern hemisphere summits. In Australia, summits are experiencing multi-faceted change due to warming and increasing fire frequency. Little is known about how these factors are affecting Australian alpine summit vegetation.
We used a revisitation approach to capture the long-term dynamic changes in Australian alpine summit plant community patterns and to understand the mechanisms of change. To quantify shifts in the dominant vegetation over time, we utilised state and transition models (STMs) and developed hierarchical models to estimate environmental factors (time since fire, growing degrees, elevation) influencing transitions in vegetation on summits.
We observed vegetation change influenced by climate and moderated by site-specific factors. The STMs showed that over time, summit vegetation was largely stable unless disturbed; however, there was evidence of increasing shrub cover. Fire-disturbed summits experienced higher instability in their vegetation cover over time. Linear mixed-effect models indicated that the effect of cumulative growing degree days depended on fire occurrence. 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, high-elevation summits.
These findings indicate the multifaceted nature of change that must be accounted for in alpine vegetation studies. We show distinct trajectories of change for dominant vegetation types on summits in response to different, co-occurring drivers. These results indicate that alpine summit vegetation will respond multi-directionally to challenges posed by a warming climate and changing fire regimes, with outcomes likely contingent on life history characteristics.