Why are some ski slopes much worse for the ecosystem than others?

Gunbarell ski run at Heavenly Mountain Resort in South Lake Tahoe, California. "Cleared" or "graded?" Image credit, Constantine Kulikovsky.A new study finds that "not all ski slopes are created equal" when it comes to impacts to fragile, high-elevation ecosystems.

The article in Ecological Applications seems particularly timely as Thanksgiving approaches and most ski resorts in California are set to open for the season (if they have not already).

Jennifer Burt and Kevin Rice from the University of California, Davis compared ski runs created by two different methods - "clearing" and "grading." They found that grading was far more destructive to the sub-alpine ecosystem.

The clearing method involves cutting and removing tall, woody vegetation to open up skiing pathways while leaving the soil and seedbank largely intact. Grading, on the other hand, uses machines to level the slope, removing most of the vegetation and much of the topsoil.

The researchers compared the vegetation and soil characteristics of cleared versus graded ski runs on seven, large ski resorts in the Sierra-Nevada range near Lake Tahoe. They compared the results with adjacent undisturbed reference forests.

They found that graded ski runs had more visible erosion, compacted soils, bare ground and degraded soil chemistry. Cleared ski runs, in contrast, had very similar soil characteristics to undisturbed reference forest.

This indicates that grading ski causes long-term damage to important ecosystem functions such as the ability to retain water and sediment and prevent erosion. This is particularly important in the Tahoe Basin where degraded upland areas have caused reduced clarity in the lake.

The researchers also found that graded ski slopes had lower plant diversity, greater number of exotic species (likely from hyrdoseeding to prevent erosion) and were characterized by early successional plants such as annual grasses. Cleared ski runs had mid-succesional communities with shrubs and perennial herbs and tended to have greater species diversity than even forest reference sites. The authors state,

Cleared ski runs may even offer some ecological benefits in a surrounding matrix of dense forests. Because they represent diverse mid-successional communities, cleared ski runs could potentially provide important wildlife habitat and contribute to greater regional diversity through increased habitat heterogeneity.

So in an ideal world, all ski slopes would be cleared instead of graded. However, the level slope in graded ski runs allows them to build snow pack more quickly - the authors estimate that ski resorts with graded slopes open a week earlier. So unfortunately, business decisions trump a rational approach that takes the whole ecosystem into consideration - which is hard for me to understand given that most ski resorts in the region are located at least partially on US Forest Service Land.

--Reviewed by Rob Goldstein

Burt, J., & Rice, K. (2009). Not all ski slopes are created equal: Disturbance intensity affects ecosystem properties Ecological Applications, 19 (8), 2242-2253 DOI: 10.1890/08-0719.1