The relative importance of biotic and abiotic factors influencing aspen recruitment in Arizona

Poor recruitment in some quaking aspen (Populus tremuloides) stands has led to debate over which factors play the largest role in aspen forest persistence. Understanding the relative importance of the many relevant factors over a large landscape could inform management strategies regarding aspen recruitment by focusing efforts on the most important factors. Therefore, between 2011 and 2016 we collected data on 29 biotic and abiotic factors thought to affect aspen recruitment from 92 randomly-selected aspen stands growing along the southwestern limit of its distribution in Arizona, USA. We assessed the condition of selected aspen stands by quantifying the number of recently recruited aspen stems (saplings > 2 m tall and < 5 cm dbh) in each sampling plot. We used negative binomial regression to estimate the relationship between aspen recruitment and the measured coyariates. We fit a balanced set of models, calculated AIC weights for those models, and summed the weights of the models containing each covariate as a measure of covariate relative importance. Six covariates had relative importance values that were significantly greater than random: fire severity in 2011, conifer encroachment, rust presence, fire severity in 2015, blight presence, and the standardized precipitation-evapotranspiration index. There were no significant differences in the ranking of these six covariates by relative importance. Although we estimated that cattle and elk had a significant negative impact on aspen recruitment, these factors were relatively unimportant. This seemingly counter-intuitive result arose because many sites lacked ungulates, but still failed to recruit aspen, indicating that other factors were more important for aspen recruitment. Our results indicate that conifer removal and increased fire activity could be among the most effective management tools to help promote aspen recruitment.