Montane springs provide regeneration refugia after high-severity wildfire

Abstract

In the mountainous regions of the Western United States, increasing wildfire activity and climate change are putting forests at risk of regeneration failure and conversion to non-forests. During periods with unfavorable climatic conditions, locations that are suitable for post-fire tree regeneration (regeneration refugia) may be essential for forest recovery. These refugia could provide scattered islands of recovering forest from which broader forest recovery may be facilitated. Spring ecosystems provide cool and wet microsites relative to the surrounding landscape and may act as regeneration refugia, though few studies have investigated their influence on post-fire regeneration. To address this knowledge gap, we quantified coniferous tree regeneration adjacent to and away from springs in mixed-conifer forests in a mountainous region of central Idaho, USA. Our research objectives were to (1) quantify post-fire conifer density near and away from springs, (2) assess the relative importance of distance to a spring compared with other biophysical factors important to post-fire regeneration, and (3) examine the temporal trends of post-fire seedling establishment near and away from springs. In areas burned at high severity from fires in 1988, 2000, and 2006, we sampled transects at 27 springs for the count, age, and height of extant conifer seedlings, as well as topographic factors and distance to surviving seed source. We modeled the relative effects of distance to a spring, topographic variables (slope, heat load index, elevation), post-fire climate, and distance to surviving seed source for the two dominant species, Douglas fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta), using a generalized linear mixed-effects model. Our study revealed that proximity to springs resulted in higher conifer density and earlier establishment after high-severity wildfire when conditions for available seeds and topography were also met. Our results demonstrate that springs are important and previously undescribed regeneration refugia with landscape-scale implications for post-fire forest recovery in increasingly water-limited environments. Springs are relatively abundant features of montane landscapes and may offer continued regeneration refugia for post-fire recovery into the future, but additional springs mapping and hydroclimatic considerations are needed.