Phenological changes reflect climate change in Wisconsin

Abstract

A phenological study of springtime events was made over a 61-year period at one site in southern Wisconsin. The records over this long period show that several phenological events have been increasing in earliness; we discuss evidence indicating that these changes reflect climate change. The mean of regressions for the 55 phenophases studied was -0.12 day per year, an overall increase in phenological earliness at this site during the period. Some phenophases have not increased in earliness, as would be expected for phenophases that are regulated by photoperiod or by a physiological signal other than local temperature.

Figure 1

Regression analysis of the date of ice-melt from Lake Mendota over the years of phenological records. (A) The Julian calendar day of ice-melt is plotted against the year. The regression indicates a change in earliness of −0.124 day per year (R² = 0.046). (B) the Julian calendar day of ice-melt is plotted against the average temperature for the month of march. The regression in this case indicates a change in earliness of −2.719 days per degree of March temperature (R² = 0.453). Data on ice break-up are from State Climatology Office, 1999: Lake Mendota Ice Summary, 1853–1999, Electronic database appearing at http://www.uwex.edu/sco/icemend.html, Wisconsin Geological and Natural History Survey, Madison. Data for Madison temperatures are from National Climatic Data Center, Local Climatological Data for Madison, WI, at the Environmental Data and Information Service, National Oceanic and Atmospheric Administration, Asheville, NC.

Figure 2

On the left are regressions of four selected springtime phenophases that do show significant increases in earliness during the 61-year period of record: arrival dates of migrating eastern phoebe (Sayornis phoebe) and rose-breasted grosbeak (Pheucticus ludovicianus) and first-bloom date of forest phlox (Phlox divaricata) and baptisia (Baptisia leucantha). These are compared with four selected phenophases that do not show significant increases in earliness: arrival dates of fox sparrow (Passarella iliaca) and eastern towhee (Pipilio erythrophthalamus) and first-bloom dates of slender pentstemon (Pentstemon gracilis) and St. John’s wort (Hypericum perforatum). The regression values are recorded in Table 1.