The future trajectory of trembling aspen (Populus tremuloides Michx.) health and persistence in the Western United States is uncertain. Many theorize that predicted increasing temperatures and extended periods of drought will have a detrimental effect on the current distribution, composition, and structure of trembling aspen, especially in arid and semi-arid regions of the Intermountain West. Our project is exploring these relationships, and we are interested in whether riparian zones of the Intermountain West can potentially increase aspen stand stability, regeneration and resistance to drought.
We sampled nine riparian aspen stands and nine paired, adjacent non-riparian aspen stands of the Caribou National Forest of southeastern Idaho during the summer months of 2019. Plant community composition, structure, age distribution and regeneration dynamics were assessed within multiple 100-m2 plots established along 2-4 100-m transects oriented along the long axis of each aspen stand. Differences in plant community composition between the riparian and non-riparian stands were determined using multivariate analysis, including non-metric dimensional scaling (NMDS). Additionally, aspen stem size and age distributions were compared between riparian and non-riparian aspen stands with histogram analysis; statistical differences among size class and age group categories were compared with a chi square test for independence.
Results/Conclusions
While both the riparian and non-riparian plant communities are dominated by aspen, the riparian stands were dominated by a mixture of forbs, graminoids and woody shrubs, the non-riparian stands had a lower herbaceous species density, and different woody shrub species. Additionally, we observed higher conifer encroachment in the non-riparian aspen stands. Overall, the stand structure of riparian stands exhibit an even size distribution, while non-riparian aspen stands are dominated by larger, older stems. Additionally, riparian aspen stands have significantly higher seedling and sapling densities than adjacent non-riparian stands (P < 0.05).
Our results suggest that riparian aspen stands have a higher variation in aspen stem size and age, and higher aspen regeneration densities relative to adjacent non-riparian stands. The compositional and structural complexity of riparian aspen stands may prove more resistant to future environmental stressors caused by climate change. Furthermore, the higher densities of seedlings within riparian zones show evidence of a higher likelihood of stand stability (i.e. not dependent on disturbance for persistence). We suggest riparian zones could provide a refugia for aspen in areas where increasing temperature and drought are expected.