Climatic niche differentiation in a widely distributed species, trembling aspen (Populus tremuloides)

Background/Question/Methods

For broadly distributed species like quaking aspen (Populus tremuloides ), wide climate tolerances potentially complicate species distribution models (SDMs) that predict species range. This is because most models create predictions using a single climate characterization as opposed to leveraging regional differences in climate that may be important to local phenotypes. In this study, the aim was to identify differences of within range climate to infer climate niche differentiation in trembling aspen (Populus tremuloides). SDMs and statistical analyses were used to quantify relationships of aspen occurrence to climate. Aspen presence points across the US were divided into climatically similar groups (identified from Hierarchical Cluster Analysis) and distributions and inferred climatic niches were compared. Time series data also provide context to climate norms and coarse analysis suggests that future climate that may not be hospitable to aspen in their current range within the conterminous United States of America if warming trends continue.

Results/Conclusions This study found two spatially distinct groupings of aspen in the eastern and western United States that are strongly distinguished by climatic characteristics. Potential evapotranspiration, Growing Degree Days, and Mean Annual Precipitation were the most important climate layers in predicting aspen presence, and their relative rankings were different between the eastern and western groups. Predictive models of aspen range derived from presence data spanning the entire range, from only eastern aspen points, or from only western points, produced distinct aspen maps and climatic responses. Additionally, time series analysis demonstrated that available climatic niche space within aspen’s current range could be shrinking. Niche differences align with biological differences such as clone size in the eastern and western United States. Ultimately, the results indicate that the eastern and western groupings of aspen inhabit different climates, and thus it can be inferred that they inhabit different climatic niches. This suggestst that, for at least this cosmopolitan species, niche differentiation within the range was important for species distribution modelling and in predicting range shifts under climate change.