Examining the onset of spring in Wisconsin

Vegetation phenological events, such as bud break, flowering, and leaf coloring, are closely associated with lower atmospheric conditions as seasons change. Plant phenology during springtime is particularly sensitive to climatic factors, especially temperature variations. Therefore, the occurrence of specific plant events can be used to identify the onset of spring. Advance or delay in these timings can serve as potential climate change detection measures over long periods. In this paper, changes to spring's onset in Wisconsin were examined using the first-bloom event of several introduced and native species in t965-1998. Due to the incompleteness of these observations, satellite data were applied to derive 3 phenological regions across the state. Next, average first-bloom time-series were formed at this regional scale. Several multi-species indices were then created based on regional first-bloom variations. Trends toward earlier first-bloom dates over the study period, especially for early-spring species, were revealed in the southwestern and central/eastern regions of Wisconsin. Two of the most important aspects of our study are: (1) phenology is regarded as a multi-species problem that can be more easily manipulated by reducing species variations to several indices; and (2) satellite data, weather data, and phenological observations are integrated to create and validate phenological regions, a process that can be used in other areas to facilitate long-term phenological data reconstruction.