Wood microsites at timberline-alpine meadow borders: implications for conifer seedling regeneration and alpine meadow conifer invasion

Abstract

The importance of climate warming on forests is recognized worldwide and has increased attention on the significance of both timberline advance and alpine meadow invasion by forests. Successful seedling regeneration in alpine meadows depends on availability of suitable substrates, or microsites, for seedling establishment. We sought to determine whether wood microsites (i.e., nurse logs), which are regeneration sites in Pacific Northwest subalpine forests, promoted regeneration at timberline-alpine meadow borders. To determine the ecological role of wood microsites, we examined mechanisms forming wood microsites; compared density, survival, and percent nitrogen content of seedlings growing on wood microsites to adjacent soil substrates; and compared substrate moisture, temperature, and percent nitrogen content. Wood microsites, at 13 of 14 randomly selected sites, were characterized by highly decayed downed wood (> 75% decay class five) originating from tree fall (66%), snow avalanches (17%), forest fires (15%), and by human cutting (2%). Although no differences in percent nitrogen content were detected, greater seedling densities, greater seedling survival, higher temperatures, and higher moisture contents were found on wood microsites compared to adjacent soil. We suggest that greater seedling density and seedling survival on wood microsites was associated with factors including heightened moisture and increased temperature. Assuming sustained downed wood input from timberline trees and continued viable seed input, we expect wood microsites will facilitate accelerated alpine meadow conifer invasion via wood microsites associated with climate warming.