© 2001 Heron Publishing—Victoria, Canada
Carbon dioxide and water vapor exchange by young and old ponderosa pine ecosystems during a dry summer
B. E. Law (1), A. H. Goldstein (2), P. M. Anthoni (3), M. H. Unsworth (3), J. A. Panek (2), M. R. Bauer (2), J. M. Fracheboud (2) and N. Hultman (4)
1. Department of Forest Science, 328 Richardson Hall, Oregon State University, Corvallis, OR 97331, USA / 2. 151 Hilgard Hall, Division of Ecosystem Sciences, University of California, Berkeley, CA 94720, USA / 3. College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA / 4. Energy and Resources Group, University of California, Berkeley, CA 94704, USA / Received August 30, 1999
Summary
We investigated key factors controlling mass and energy exchange by a young (6-year-old) ponderosa pine (Pinus ponderosa Laws.) plantation on the west side of the Sierra Nevada Mountains and an old-growth ponderosa pine forest (mix of 45- and
250-year-old trees) on the east side of the Cascade Mountains, from June through September 1997. At both sites, we operated
eddy covariance systems above the canopy to measure net ecosystem exchange of carbon dioxide and water vapor, and made concurrent
meteorological and ecophysiological measurements. Our objective was to understand and compare the controls on ecosystem processes
in these two forests.
Precipitation is much higher in the young plantation than in the old-growth forest (1660 versus 550 mm year–1), although both forests experienced decreasing soil water availability and increasing vapor pressure deficits (D) as the summer of 1997 progressed. As a result, drought stress increased at both sites during this period, and changes in
D strongly influenced ecosystem conductance and net carbon uptake. Ecosystem conductance for a given D was higher in the young pine plantation than in the old-growth forest, but decreased dramatically following several days
of high D in late summer, possibly because of xylem cavitation. Net CO2 exchange generally decreased with conductance at both sites, although values were roughly twice as high at the young site.
Simulations with the 3-PG model, which included the effect of tree age on fluxes, suggest that, during the fall through spring
period, milder temperatures and ample water availability at the young site provide better conditions for photosynthesis than
at the old pine site. Thus, over the long-term, the young site can carry more leaf area, and the climatic conditions between
fall and spring offset the more severe limitations imposed by summer drought.
Keywords:
canopy architecture, carbon exchange, energy exchange, leaf area, Pinus ponderosa, radiative transfer.
