© 2007 Heron Publishing—Victoria, Canada
Changes in net ecosystem productivity with forest age following clearcutting of a coastal Douglas-fir forest: testing a mathematical
model with eddy covariance measurements along a forest chronosequence
R. F. Grant (1, 4), T. A. Black (2), E. R. Humphreys (3) and K. Morgenstern (2)
1. Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada T6G 2E3 / 2. Faculty of Agricultural Sciences, University of British Columbia, Vancouver, BC, Canada / 3. Department of Geography, Trent University, Peterborough, ON, Canada / 4. Corresponding author (Robert.Grant@afhe.ualberta.ca) / Received June 8, 2005; accepted December 19, 2005; published online October 2, 2006
Summary
We hypothesized that changes in net ecosystem productivity (NEP) during aging of coastal Douglas-fir (Pseudotsuga menziesii Mirb. Franco) stands could be explained by (1) changing nutrient uptake caused by different time scales for decomposition
of fine, non-woody and coarse woody litter left after harvesting, (2) declines in canopy water status with lengthening of
the water uptake pathway during bole and branch growth, and (3) increases in the ratio of autotrophic respiration (Ra) to gross primary productivity (GPP) with phytomass accumulation. These hypotheses were implemented and tested in the mathematical
model ecosys against eddy covariance (EC) measurements of forest CO2 and energy exchange in a post-clearcut Douglas-fir chronosequence. Hypothesis 1 explained how (a) an initial rise in GPP
observed during the first 3 years after clearcutting could be caused by nutrient mineralization from rapid decomposition of
fine, non-woody litter with lower C:N ratios (assart effect), (b) a slower rise in GPP during the next 20 years could be caused
by immobilization during later decomposition of coarse woody litter, and (c) a rapid rise in GPP between 20 and 40 years after
clearcutting could be caused by nutrient mineralization with further decomposition of coarse woody litter and of its decomposition
products. During periods (a) and (b), heterotrophic respiration (Rh) from decomposition of fine and coarse litter greatly exceeded net primary productivity (NPP = GPP – Ra) so that Douglas-fir stands were large sources of CO2. During period (c), NPP exceeded Rh so that these stands became large sinks for CO2. Hypothesis 2 explained how declines in NPP during later growth in period (c) could be caused by lower hydraulic conductances
in taller trees that would force lower canopy water potentials and hence greater sensitivity of stomatal conductances and
CO2 uptake to vapor pressure deficits. Enhanced sensitivity to vapor pressure deficits was also apparent in the EC measurements
over the post-clearcut chronosequence. Hypothesis 3 did not contribute to the explanation of forest age effects on NEP.
Keywords:
autotrophic respiration, ecosys, GPP, heterotrophic respiration, hydraulic conductance, litter decomposition, NEP, nutrient
mineralization, Pseudotsuga menziesii
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