2001-2006 Mission Kearney Foundation of Soil Science: Soil Carbon and California's Terrestrial Ecosystems Final Report: 2003319, 1/1/2004-12/31/2006 1 University of California, Berkeley 2 University of California, Santa Cruz Controls of Canopy Photosynthetic Activity on Roots and Soil Carbon Dynamics in Ponderosa Pine and Oak/Savanna Ecosystems Allen Goldstein 1 , Weixin Cheng 2 , Dennis Baldocchi 1 , Laurent Misson 1 , Alex Gershenson 2 , Jorge Curiel Yuste 1 Summary The first objective of this research is to make the link between the seasonality of fine root dynamics and the carbon balance of the ecosystem. The second objective of this research is to improve our understanding of how canopy photosynthesis influences fine root initiation, growth and mortality. In order to reach these objectives, we combined CO 2 flux measurements (at ecosystem, soil, and leaf level) with aboveground (shoot, needle, stem) and belowground (fine root) growth dynamics. The first part of this research was conducted in a young Ponderosa pine plantation at the Blodgett Forest Ameriflux site, located at 1,315 m above sea level in the Sierra Nevada Mountains of California. One of our hypotheses was that fine root development at our site is a high priority and is tightly coupled to canopy photosynthesis and available soil water. This hypothesis is partially confirmed and mainly holds for the first part of the vegetation period when a tight coupling between photosynthesis and root growth was observed. This relationship appears to change during dry years, when belowground growth is extremely limited, while aboveground growth does not appear to be similarly affected. We found that the seasonal window for optimal root growth is extremely reduced at our site, partially due to low soil temperature during the winter and partially due to soil water stress during summer. However, when large rates of photosynthesis were observed following summer rains during the second part of the vegetation period in 2003, while temperature was optimal, root growth did not resume and mortality rates did not decrease. The second hypothesis was that fine roots exert a major control over the seasonal patterns of soil respiration; and that such control is most apparent when roots are actively growing. This hypothesis is also partially confirmed because increases in soil and ecosystem respiration corrected for temperature variations were observed during the active growing period. However, the increase in respiration during root growth was limited in magnitude. The largest variation in soil respiration at our site occurred due to an increase in heterotrophic respiration during unusual rain pulse events in the second part of the vegetation period (summer). However, the activity of these heterotrophs was highly dependent on the earlier input of fresh soil labile carbon by the roots. Objectives 1. Our first objective was to examine the effects of the overstory canopy density on the understory micrometeorology, and on the coupling between overstory and understory conditions, as well as the effects of the overstory canopy density on CO 2 flux partitioning between understory and overstory, and the environmental and biological factors that control