Plant community assembly and succession is a complex process with many variables potentially pushing a system into alternative stable states. Landscape scale disturbances, changes in climate, competition and/or facilitation from neighboring species are often the stimulus for such change. Quaking aspen (Populus tremuloides) is widespread and known for its ecological and economical value. Recent declines in aspen populations have been attributed to increased ungulate herbivory, changes in climate, fire suppression, and encroachment from conifers. While much research has been done on aspen regeneration in post-fire conditions less has been done on regeneration in intact stands in which condition the majority of aspen lands exist. This causes us to ask the question “which of these processes is contributing the most to aspen decline in intact stands?” In the spring of 2013 we set up 92 study blocks over 9 different mountain ranges throughout central and southern Utah. Each block consisted of a fenced and unfenced plot in a pure aspen stand, and a fenced and unfenced plot in a mixed aspen-conifer stand. Over 1100 individual aspen suckers were tagged and measured then revisited in the fall of 2016.
Results/Conclusions
After four growing seasons we found that fencing the aspen suckers increased their height nearly 2.5x in both pure aspen and mixed aspen-conifer stands. Suckers in pure aspen stands, whether fenced or not, were over 2x as tall as suckers in mixed-aspen conifer stands. Mixed effects models using tag nested within block as a random factor indicated that fence, winter lag precipitation, and annual temperature range were the best predictors of aspen sucker growth. No covariates could predict sucker density better than the intercept only model. Based on these data we suspect that ungulate herbivory, conifer encroachment, and climate all affect aspen sucker growth in intact stands varying in that order from greatest to least. While these data focus on aspen sucker growth and potential recruitment of younger age classes, we believe that these results may change when considered at different temporal or spatial scales.