Changes in northern hardwood growing season length and productivity during an 18-year warming trend

Temperate forest ecosystem productivity can be affected by growing season length and moisture stress, both of which will be impacted by predicted levels of global and regional climatic change.  We have measured temperature, precipitation, soil moisture availability, length of foliar display and aboveground biomass production in four northern hardwood forests in Michigan since 1988.  During that time period, mean annual temperature increased by 1.0 degree C, with the most pronounced change occurring at the end of the growing season.  The average length of the growing season also increased, from 142 days to 154 days.  Both earlier leaf out and later leaf drop contributed to this change.   In conjunction with the longer growing season, average annual aboveground woody biomass production increased by 27 percent.  A clear trend for increased productivity over time existed at three of the four sites, with a neutral trend at the fourth site.   Greater frequency and duration of moisture deficits appear to be at least partly responsible for the absence of increased productivity at the fourth location.   Together, these results suggest that longer growing seasons associated with climatic warming may initially result in greater productivity in northern hardwood forests for which moisture availability is adequate.