Mountain summits, species losses and shrubs

The Bluff - a typical alpine summit in
the southern Australian alps
(Photo: John Morgan)

Alpine summits in Australia are thought to be one of the places most vulnerable to climate change. Afterall, the distance between treeline and summits is very small - from a few metres to perhaps only 100-150 m at the higher peaks. Hence, alpine species have nowhere to migrate when air temperatures rise, potentially lengthening the growing season and allowing subalpine species to grow at higher altitudes.

The scenario that is usually described is that trees will migrate up mountain slopes in response to climate warming, and alpine species are lost (presumably) because trees shade out the small herbs and grasses that dominate alpine peaks. The bioclimatic envelope of the alpine species will also shift, undoubtedly affecting the ability of some species to persist in their current location. While most ecologist probably might visualise the process, the mechanism of species loss is actually poorly understood. Indeed, it's perhaps not the trees that will drive local extinctions (nor shifts in climate envelopes in the short-term) but rather, changes in other woody plants (such as shrubs) that will strongly compete with low-statured plants.

Resurveying transects on Mt Sterling; note the
burnt snow gums in the background, a legacy of fire in 2006
(Photo: John Morgan)

Susanna Venn and I have just returned from several days in the field re-monitoring some long-term transects on alpine summits. These were last done about 8 yrs ago and it's clear that few trees have moved above the treeline in that time despite regional warming. Hence, there is a degree of resistance to change in treelines that has long fascinated Australian alpine ecologists.

What is more apparent is that there seems to be a (potentially strong) negative relationship between shrub cover and species diversity. This is really apparent on the different aspects of summits - southerly, colder aspects have far fewer shrubs and much more diversity. Shrubs, like Podolobium and Hovea, grow taller than the herbfield species, deposit lots of litter that accummulates because of slow decomposition, and can grow quickly after disturbance such as fire - in Phil Grime's CSR scheme, they might be considered "competitors". Perhaps the real driver of vegetation change at summits will be shrubs as they affect competitive interactions. Many of these species vegetatively spread so they have a clear "method of encroachment" into alpine areas from surrounding slopes that is not reliant on seed regeneration. And, because they resprout after fire, they are not so senstive to fire return intervals as are obligate-seeders such as Grevillea.

Susanna has just published a paper on shrub encroachment in tundra ecosystems with Shrub Hub Research Network collaborators - a popular press piece can be found here, while the scientific paper in Environmental Research Letters can be found here. It's worth checking out - shrub encroachment is a global phenomenon, from the prairies, to the semi-arid, to the temperate. Clearly, understanding the controls on shrub dynamics on alpine summits, and their potential (negative) feedbacks with plant diversity, is an exciting area of research that we will persue over the coming years. I'll update you when we've crunched our data.