Climatic Consequences of Leaf Presence in the Eastern United States

At the time of leaf emergence in deciduous forests, markedly enhanced evapotranspiration leads to a rapid drop in the Bowen ratio. A small fraction of this surface flux alteration converges into the boundary layer, and this can be detected in the mean temperature and humidity daily increments at the surface. A simple technique is presented for identifying this response in surface climate data and extracting time series for the date of spring onset and for the ‘‘spring intensity,’’ a measure of surface energy budget partition change in spring. A tendency Bowen ratio B9 is found from changes in the daily increment of temperature and humidity in multidecadal averages. The spring date determined using this criterion for stations along the U.S. east coast corresponds to the date at which the normalized difference vegetation index (NDVI) reaches 80% of its seasonal maximum. Northward movement of the vernal front is similar to that obtained using Hopkins’ empirical rule; it is linearly related to leaf emergence and flowering dates from the North American lilac phenology network. Spring intensity increases northward; the states from Virginia north exhibit distinctly higher values. There has been a trend in the most recent decades toward earlier spring dates, except for regions in Virginia and North Carolina. The same analyses performed using the small subset of stations with longer-term records indicate that a trend toward an earlier spring date is confined to recent decades. An inverse relationship between the spring date and spring average temperature was found for the Midwest but is inadequate for the northeast. Spring intensity has generally increased in northeastern North America throughout the twentieth century. However, large oscillations with an approximate 20-yr period distinguish the northeastern United States from the Midwest, indicating that the intensity of spring is not a simple function of spring temperature or of forest cover fraction.