In this post, Associate Editor Nathalie Butt discusses a recent paper by Jennifer Burt and Jeffrey Clary Initial disturbance intensity affects recovery rates and successional divergence on abandoned ski slopes

Fragile habitats

Montane or alpine ecosystems are among the most fragile we have, and they are therefore places where human impact can be very damaging. Of course we are attracted to these beautiful snow-covered mountains and forests, often as slopes down which to hurtle ourselves (not me though, I’ll be up there measuring trees and recording species). During the second half of the last century, cheaply available air travel and increasing leisure time hugely increased the numbers of ski holidays, skiers, and thus ski slopes, resulting in forest clearing on mountains in many areas. As conservationists, we are interested in how we can best protect or restore fragile or damaged ecosystems, and as ecologists, we want to know how habitats are affected by and respond to disturbance, and how natural processes operate.

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Skier at Alaska’s Alyeska Resort. Photo credit: Climb7moniz, Wikimedia Commons.

Ski slope impacts

Burt and Clary’s recent paper elegantly makes use of abandoned ski slopes in the northern Sierra Nevada Mountains in California and Nevada to investigate what happens when a ski slope is no longer a ski slope. They were interested in establishing the post-use effects of pre-use preparation on forest succession and regrowth, and compared graded and cleared ski runs. Grading is done with heavy machinery; tree stumps and rocks are removed along with topsoil and seed bank, to create an even surface. Clearing, on the other hand, is where trees are cut and removed, with the stumps left flush with the ground, leading to minimal soil and seed bank disruption. So, clearing is a much smaller disruption than grading, but what exactly does that mean in ecological terms, and what (and how much) difference does it make to post-use forest recovery/restoration?

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Ski slope. Photo credit: MOs810, Wikimedia Commons.

Disturbance and forest succession

Although the classical Clementsian succession paradigm tells us that a return to a climax forest state is inevitable and happens through fixed seral stages, we know now that there are many factors other than climate determining this process. The type of disturbance itself is in fact a key driver of what happens afterwards, and seed availability and order of species arrival, recruitment and establishment are also important. The greater the initial perturbation, the less predictable the successional trajectory, and Burt and Clary had accordingly predicted that graded ski runs would show a slower recovery to forest conditions than cleared ski runs. However, this was not the case: while tree cover and composition on cleared ski runs did become more similar over time-since-abandonment to reference forests, graded runs did not, even after several decades. They established that the initial disruption to the soil and the seed bank is the principal driver of differences in recovery.

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Forest succession. Photo credit: LucasMartinFrey, Wikimedia Commons.

Planning ahead for conservation and restoration

So, using grading to construct ski slopes can mean that post-use recovery, of both soils and vegetation, is unreliable and variable. Reduction in availability and predictability of sufficient snow during ski seasons has already become a problem in many places, and is set to continue under climate change: we can probably expect many more ski slopes to be abandoned. Given that assumption, and considering the long term impacts, clearing should be the preferred or default method for ski slope preparation. Perhaps most worryingly, the lack of recovery shown by the graded runs means that not only may they not return to reference forest condition, they may not be able to achieve any ecologically functional state, and therefore be unable to provide ecosystem services such as soil conservation, rainfall infiltration and storage, and plant regeneration. This is where active restoration could play a role, and the costs and practicalities of that should also be considered. Effective planning at the outset of such large-scale land cover changes as ski run construction can minimise the long-term ecological impacts, and enhance post-use conservation outcomes.

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An alpine meadow. Photo credit: Katja Schulz, Wikimedia Commons.