As climate change and other human-induced environmental changes progress, accurately predicting the responses of forests becomes increasingly urgent. While environmental changes will impact forests across broad spatial scales, responses will rely on underlying fine-scale population processes including growth, survival and establishment. Seedling establishment in particular determines the environments in which species may persist in and/or move to, and directly determines the ability of species to respond to stressful environments. The factors influencing seedling establishment may differ substantially from those determining the persistence and success of adult trees, but are not well understood for many species and systems. Here, we quantify the relative influences of light availability, biotic interactions (i.e. competition and facilitation), and edaphic factors on seedling establishment and distributions of seven dominant Rocky Mountain tree species to clarify the relative effects of factors underlying broad-scale forest persistence and migration. Specifically, we collected data on seedling abundance and spatial distribution across a variety of biotic and abiotic conditions and used a Bayesian regression approach to directly quantify the influences of edaphic factors, light availability, and biotic interactions on seedling establishment patterns. We then performed a Bayesian sensitivity analysis to estimate the relative importance of each factor to overall establishment trends.
Bayesian regression models for each species revealed that a substantial proportion of the variance in seedling abundance was explained by proximity and density of neighbors, microtopography, soil nutrient status and light availability, yet a notable proportion of variance remained unexplained by our models. Of the modeled covariates, biotic interactions had the highest relative influence on the establishment of shade-tolerant species Picea engelmannii and Abies lasiocarpa, while edaphic factors (net mineralization, microtopography) had the greatest effect on montane species Pseudotsuga menziesii and Pinus flexilis. The shade-intolerant species Pinus contorta, Populus tremuloides and Pinus ponderosa were most impacted by light availability. These findings reveal that seedlings of individual species vary in their response to abiotic and biotic factors depending on life history traits, and that seedling responses to broad-scale environmental change may depend on very local environmental conditions. We suspect that unexplained variance is due to disturbance and climate, two important drivers of seedling establishment that operate on broader scales than those considered in our analysis.