Relationships between the weather and phenology of Populus tremuloides in interior Alaska in recent decades
Roland CA, Schaible L, Nicklen E. 2021. Relationships between the weather and phenology of Populus tremuloides in interior Alaska in recent decades. National Park Service
Plant phenology can be a sensitive indicator of changing climatic conditions, especially temperature. High latitude regions of the northern hemisphere such as interior Alaska have been warming at twice the rate of temperate areas. Phenological changes stimulated by this warming may have major impacts on how ecosystems respond to earth’s increasingly altered climates, potentially resulting in important and cascading effects in these ecosystems. We recorded multi-year records of phenological events in quaking aspen (Populus tremuloides) in three different areas of interior Alaska: Fairbanks, Denali National Park and Copper Center (near Wrangell-St. Elias National Park and Preserve) over the period 2005-2020, including the timing of flowering, leaf-out (green-up) and leaf senescence in fall. In addition, we recorded other relevant data in our plot network including the progress of snowmelt in spring, and for the latter part of the observation period, installed temperature dataloggers in our plots. In this report we summarize various elements of the dataset we have acquired over this period and present analyses linking annual variation in phenological events in aspen with variation in yearly weather patterns. Specifically, we evaluated a set of indices based on spring temperature (and other relevant factors) for predicting annual timing of green-up by comparing the relationships of index values to observed bud burst dates in our annual observations. We found that a weighted index based on thawing degree days accumulated above a 0° C threshold using values of maximum daily temperature was the ‘best” index for predicting aspen bud burst in interior Alaska for our dataset. We then used this index to make a set of predictions of the historical dates of bud burst in the past. We analyzed the variation in estimated budburst timing over recent decades to determine if there was a significant linear trend over time. We estimated that budburst occurred 6-10 days earlier per century in Fairbanks, Denali National Park, and Gulkana and at a median rate of 16 days earlier per century throughout mainland Alaska based on extrapolations using a spring green-up Index. Our model suggested that it is possible that warming and the advancement of budburst earlier in spring may have been particularly evident in maritime-influenced regions, while the trends are less pronounced in the interior. However, this conclusion is strictly based on extrapolations using a spring weather index, without the benefit of phenological observations. As such, our findings regarding this larger geographic domain are preliminary and speculative, and require investigation using longer term records of observed plant phenology patterns from maritime sites.