Be. Law et al., Carbon dioxide and water vapor exchange by young and old ponderosa pine ecosystems during a dry summer, TREE PHYSL, 21(5), 2001, pp. 299-308
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 Siena Nevada Mountains and an old-growth ponderosa pine forest
(mix of 45- and 250-year-old trees) on the east side of the Cascade Mounta
ins, from June through September 1997. At both sites, we operated eddy cova
riance systems above the canopy to measure net ecosystem exchange of carbon
dioxide and water vapor, and made concurrent meteorological and ecophysiol
ogical measurements, Our objective was to understand and compare the contro
ls 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 de
creasing 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 ecosys
tem 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, pos
sibly because of xylem cavitation, Net CO2 exchange generally decreased wit
h 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 c
onditions between fall and spring offset the more severe limitations impose
d by summer drought.