Climate change: Alpine shrubs as ecosystem engineers

Worldwide, shrub cover is increasing across alpine tundra. In Australia, alpine shrub increases match a trend spanning four decades of rising temperatures and declining snowpack. Repeat photography, long-term monitoring, field warming experiments and dendrochronology have revealed that alpine shrubs are responding by encroaching into otherwise non-shrubby communities, such as alpine herbfields and grasslands. Alpine shrubs readily restrict the growth of other plants via shading and smothering with leaf litter, and they can alter wildlife habitats. Warmer conditions may also exacerbate a feedback between shrubs and fire, whereby increased fire activity due to highly flammable foliage and leaf litter, stimulate vigorous re-sprouting and seeding, resulting in further increases in shrub cover. Some shrub growth forms interact with winter processes; they can accumulate snow in their lee, thereby insulating soils from extreme winter temperatures. These effects may also promote a second feedback whereby deeper snowpack, warmer soils and higher soil moisture, coupled with leaf litter under shrub canopies, increases microbial activity. These effects in turn, can enhance soil nutrient cycling and ultimately promote shrub growth. Deeper snowpack around shrubs also contributes to winter and spring water yields in mountain catchments. Given that alpine shrub range-expansion has the potential to significantly modify existing landscape flammability, winter processes and ecosystem function, alpine shrubs effectively act as ecosystem engineers. There is an urgent need for land managers to monitor changes in shrub abundance, and for stakeholders to understand these processes in order to determine whether increases in shrub cover and shrub encroachment will result in alternate stable states in alpine vegetation, local plant and/or animal extinctions, and whether an overall declining snowpack will mitigate or exacerbate these processes.