Quaking aspen (Populus tremuloides) regenerates via asexual suckers and sexual seedlings. Unlike suckers, seedlings enable adaptation to climate change through increased genetic diversity and by migration facilitated by long-distance seed dispersal. However, aspen seedling regeneration has largely been ignored in the western United States until recently, because seedlings were thought unable to establish in hotter, drier climates. Though aspen seedlings are increasingly seen as a conceivable occurrence, untested assumptions still exist, notably that establishment is restricted to favorable climatic “windows of opportunity” (wetter, cooler periods than average). Utilizing every literature account we could find of aspen seedling establishment in the western United States (n = 20), we explored whether aspen seedling establishment is indeed confined to years of favorable climate. We quantified establishment year climate using a suite of variables from gridded TerraClimate datasets (annual and summer: mean temperature, precipitation, and climatic water deficit) calculated as z-scores. For each variable, we compared the probability distribution of establishment z-scores to the expected distribution of z-scores (standard normal distribution) using one-sample Kolmogorov AbstractSmirnov tests. Additionally, we compared the distribution of establishment site climate normals to aspen’s western climate envelope using two-sample Kolmogorov-Smirnov tests, to determine if establishment is restricted to a limited climate space.
Results/Conclusions
Documented seedling establishment occurred across much of aspen’s western range, and distributions of establishment site climate variables were not different from those of aspen’s western climate envelope. Contrary to the prevailing narrative that aspen seedling establishment is confined to wetter and cooler years, our analysis showed no evidence that climate during years of seedling establishment is more favorable than average. In fact, annual climatic water deficit z-scores in the year of establishment were higher than expected (i.e. greater drought stress; p < 0.05), while all other annual and summer climate variables had z-score distributions no different than expected by chance. Aspen seedlings established across a wide range of annual and summer climatic conditions, from >2 standard deviations cooler and wetter than average to >2 standard deviations hotter and drier. This work adds to the growing consensus that aspen seedling establishment is much more common than currently thought, and that the paucity of observed establishment is likely due to factors such as sampling bias and assumptions of seedling rarity. We encourage further research into aspen seedling ecology, exploring the disproportionately large role seedlings may play in the species’ response to climate change through increased genetic diversity and dispersal potential.