Structural and physiological changes with forest stand age: A modeling approach.
Licata, Julian*,1, George, Kate2, Pypker, Tom1, Bible, Ken3, Kerrigan, Julia1, Williams, Mathew4, Bond, Barbara1, 1 Oregon State University, Corvallis, OR2 USDA-ARS, Beltsville, MD3 Wind River Canopy Crane Research Facility, Carson, WA4 University of Edinburgh, Edinburgh, Scotland
ABSTRACT- Many studies have shown that net primary production in old-growth forests is lower than in younger forests in similar sites, although the cause is still not clear. One possibility is that overall carbon assimilation, or GPP, is lower in older forests. Although it is relatively easy to measure water fluxes in forest stands, direct measurement of GPP is more difficult. One approach is to use process-based models linking carbon and water fluxes at the stand level. We used a detailed soil-plant-atmosphere continuum (SPA) model, which explicitly links CO2 and H2O fluxes through stomatal conductance. We calibrated the model to accurately represent transpiration in a young (ca 25 years old) and an old-growth (ca 450 years old) Douglas-fir/western hemlock stand at Wind River, WA, based on detailed measurements of soil water depletion and sap flow. We then used the model to examine variation in GPP among the sites. The model accounts for many structural and physiological differences in forest stands. This allowed us to perform sensitivity tests to quantify the effect on GPP and transpiration of the variables found to differ among stands of different age. Among the variables tested, the most influential of these was height. Moreover, height increase was the only change from the young to the old-growth stand that had a considerable negative impact on transpiration and GPP. The change in the other variables from the young to the old-growth stand appeared to be compensating for the potential negative impact of the increase in height. Among them, above ground leaf specific conductivity and minimum mid-day water potential had the highest compensatory effect. SPA suggested that the old-growth stand achieved a total compensation of GPP and a partial compensation of transpiration when all variables were accounted for. This modeling analysis did not agree completely with hypotheses suggesting that the age-related decline in productivity coincides with a decline in GPP.
Key words: stand age, GPP, physiology, modeling
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