As Above so Below? Trampling Effects on Alpine and Subalpine Soil Seed Banks

Abstract

Subalpine and alpine meadows are a popular recreation destination, but heavy visitation results in off-trail vegetation trampling. Repairing trampling damage in the alpine and subalpine is a challenge because plant growth is slow, transporting materials to the restoration site is difficult, and restoration is resource-intensive. Climate-change-driven shifts towards hotter, drier growing seasons may make restoration even harder. Where nonnative species invasion risk is low, the soil seed bank could be a convenient, low-cost tool to buffer restored areas against climate change impacts by increasing diversity. I evaluated the size and composition of the germinable soil seed bank and compared it to aboveground vegetation communities in trampled and intact areas across an elevation gradient at Paradise, Mount Rainier National Park. I also investigated differences in soil characteristics between trampled and intact areas to evaluate potential barriers to seedling establishment and growth. Soils from the top 5 cm and from 5-10 cm below the surface of intact areas had higher seed density and species richness than the top 5 cm of trampled areas, with reductions in seed density in trampled areas increasing with elevation. Invasive and lowland species comprised 0.2% of the germinable soil seed bank. The aboveground vegetation communities of trampled areas at lower elevations resembled intact communities at high elevations. The soil seed bank community differed from the standing vegetation for both intact and trampled areas, and was dominated by Juncus and Carex species. Soil analysis showed that trampling increased compaction and pH while reducing percent organic matter and micronutrients. Abundant seeds in the soil seed bank but low overlap with the standing vegetation shows that the soil seed bank has potential to supplement, but not replace, restoration planting or seeding. Restoration practices to restore soil structure may be necessary to facilitate soil seed bank germination and seedling establishment.