THE ROLE OF CLOUD COVER IN NET ECOSYSTEM EXCHANGE OF CO_2 OVER TWO MID-WESTERN MIXED HARDWOOD FORESTS

摘要

The presence of clouds modifies surface environmental conditions both directly and indirect via atmospheric and hydrological processes. Since plant functioning is strongly linked with these processes, it is logical that the presence of clouds will also influence biophysical processes. One important relation that has emerged from field observations of net ecosystem exchange of CO_2 (NEE), links the presence of clouds with enhanced levels of canopy scale net CO_2 uptake (e.g. Price and Black 1990, Hollinger et al. 1994, Fitzjarrald et al. 1995). Several mechanisms have been suggested to explain this relation, including the enhancement of diffuse relative to direct beam photosynthetically active radiation (PAR), for which plant canopies have a higher use efficiency (Gu et al. 1999). Less direct influences include cooler leaf and soil temperatures under cloudy conditions which can lower respiration rates and decreases in vapor pressure deficit, enhancing stomatal openness. In this study, we examine the relation between clouds and NEE observed over two different mid-latitude forest systems and over multiple growing seasons. By quantifying this relationship over different ecosystems with different climatic controls as well as inter-annual differences, we hope to improve a regional understanding for this role and potential responses in carbon sequestration to changing levels of cloudiness.