Past studies have revealed the effects of climate change on the growth of vegetation in northern forests.  Evidence of increased growing season length within northern latitudes has resulted in a specific focus on these forests as potential CO₂ sinks.  Measurements of vegetation phenology can facilitate further analyses of the relationship between the onset of spring and the exchange of CO₂ because they are indicative of photosynthetic activity and lower atmospheric processes.  The purpose of this study was to examine the connection between surface-atmosphere CO₂ exchange and the onset of spring in a northern mixed forest.  This comparison used measurements of net ecosystem exchange (NEE) from a large tower, detailed phenological observations of multiple native tree species in northern Wisconsin, and microclimatic measurements of air temperature and relative humidity.  In addition to examining coincident fluctuations in the spring growth of these species and NEE, the phenological development of each species was weighted according to its abundance within the footprint of the flux tower.  Using the weighted phenological measurements, an analysis of the composite effect of the species’ development on NEE was conducted.  The results of this analysis were used to identify the species that had the largest effect on the exchange of CO₂ between the forest and the atmosphere.

Results/Conclusions

Preliminary results suggest that certain species within the footprint of the flux tower exert greater controls on tower-measured NEE.  Moreover, there are fluctuations in NEE at the onset of the growing season that appear to be linked to the assimilation of CO₂ by the observed species.   Overall, the results of this study convey the effects of native species phenology on the exchange of CO₂ and highlight the phenological stages that are most related to this exchange.  Consequently, the findings contribute to our understanding of how forest ecosystems directly interact with the lower atmosphere.