Observed changes in phenology across the US: A regional review for the National Climate Assessment

Background/Question/Methods

The 2013 National Climate Assessment (NCA) process calls for the identification of national indicators of change within geographic regions and sectors as a mechanism for establishing consistent methods of evaluation for future assessments. Phenology, the study of seasonal life cycle events in plants and animals (e.g., timing of leaf-out, blooming, hibernation, migration), is a well-recognized indicator of climate change impacts on people and nature. In an effort to synthesize climate-linked regional variation in phenological events across the United States, we conducted a review of the recent phenology literature as a Technical Input Report for the 2013 NCA. Specifically, we compiled studies that met the following criteria: (1) appears in the peer-reviewed literature between 2002 and the present; (2) provides documented analyses of long-term (at least 10 years) phenology data sets;  (3) includes concurrent analyses or other consideration of temperature and/or precipitation; and (4) focuses on non-marine, field-based, community-level observations of individual organisms or of landscapes captured by remote instrumentation. We organized this information into eight geographic regions specifically delineated by the NCA: Alaska and the Arctic, Great Plains, Hawaii and the Pacific Islands, Midwest, Northeast, Northwest, Southeast, and Southwest.   

Results/Conclusions

We reviewed over 175 studies and found that most published reports of recent long-term phenological patterns occurred in the Southwest region (ca 45 publications), whereas the Hawaii and the Pacific Islands region had the fewest published studies (ca 2 publications).  Overall, the majority of studies documented an advance in springtime phenological events across species. In contrast, organisms in the Southeast typically showed great variability in onset of spring events. This may be the result of a recently described ‘warming hole’ in the southeastern U.S. (Meehl et al., 2012). Bird and plant taxa were well-represented in the reviewed studies, whereas there was a paucity of phenological information for mammals and, in particular, insects and reptiles. We also found a growing body of research focused on examining the phenomenon of trophic mismatch, which can result from differential phenological responses to climate or other cues. Though few studies documented the specific consequences of asynchronous species interactions, most predicted population declines, local extirpations, and the emergence of novel biotic interactions. Our review not only highlights the existence of broad directional shifts in phenology, but also indicates a regional variability in response to climatic changes that ultimately has implications for ecosystem processes and services.