North-facing aspects, shade objects, and microtopographic depressions promote the survival and growth of tree seedlings planted after wildfire

Abstract

Background: Planting tree seedlings may help promote forest recovery after extensive high-severity wildfire. We evaluated the influence of growing environment characteristics on the performance of seedlings planted in the 2016 Cold Springs Fire, Colorado, USA. In 2021, four growing seasons after planting, we measured survival, height, and 2021 height growth for 300 ponderosa pine, limber pine, and Douglas-fir seedlings permanently marked along “stake rows.” For each seedling, we also recorded one site-level growing environment characteristic, aspect, and two microsite-level characteristics, the presence of coarse wood or other shade object and the presence of water-capturing microtopographic depressions. To examine a potential mechanism through which these growing environment characteristics could influence seedling responses, we also measured summer soil moisture at each ponderosa pine seedling. We used generalized linear mixed models to examine the influence of aspect, shade object presence, and depression presence on seedling survival, height, and height growth, and on soil moisture. Results: The growing environment had a clear influence on tree seedlings. We found greater seedling survival on more northerly aspects, in shade, and in depressions. Across all species, seedlings on north aspects had 37% greater survival than those on south aspects (76% vs. 39%, respectively). Seedlings planted in shaded microsites had 20% greater survival, and seedlings planted in depression microsites had 14% greater survival relative to microsites without shading or depressions, respectively. Seedling height was greater on more northerly aspects and in shade. Likewise, seedling height growth was generally greater on north aspects and in shade, although the influence of aspect and shade depended on species. Soil moisture was greater in depressions. Conclusions: The findings of this opportunistic study demonstrate how positioning seedlings to take advantage of cooler, wetter growing environments can increase their performance in what are often climatically stressful post-fire landscapes. Overall, planting seedlings on north-facing aspects, in shaded microsites, and in depression microsites, practices commonly employed by land managers, were effective at promoting survival and growth, thereby facilitating planting success. These practices will likely become ever more relevant as the pace and scale of post-fire planting increases and as planting conditions become more stressful due to ongoing climate change.