Comparing carbon flux and high-resolution spring phenological measurements in a northern mixed forest

Vegetative canopies play a crucial role in the energy balance and composition of the atmospheric boundary layer via biotic control over evapotranspiration and carbon sources/sinks. Accurately predicting the onset/increase of carbon uptake/transpiration during the spring leaf development period using coarse resolution tower-based and satellite-derived data alone is difficult. Thus, understanding stand-level spatial patterns of spring plant phenological development and the processes that drive them may be crucial for improving landscape level estimates of evapotranspiration and carbon accumulation. In this study, high-resolution spatial and temporal tree phenology data were recorded in field campaigns over approximately 5 weeks during spring 2006 and 2007 (within a 625 m × 275 m area), and over similar periods during spring 2008, 2009, and 2010 (within two 625 m × 625 m areas) near the WLEF eddy covariance flux tower site near Park Falls in northern Wisconsin. Our findings demonstrate that phenological variations between individual trees in a specific microclimate can be adequately represented with a sample of 30 or more individuals. Further, visual phenological observations can be generally related to under-canopy light levels, and for spring phenology measurements in similar microclimates, a sampling interval of every 4 days minimizes data uncertainty and field work expenses. An analysis of the relationships among phenology, climate, and gross primary productivity (GPP) during the spring indicate that the phenology of the dominant tree species is responsible for an overall positive trend in carbon assimilation, but climate is the cause of day-to-day variation.