Comparisons of predictions of net primary production and seasonal patternsin water use derived with two forest growth models in Southwestern Oregon

In this paper, we compare predictions made with two forest growth models of maximum annual net primary production and seasonal trends in the constrain ts imposed by different climatic variables at IS sites in the Siskiyou Moun tains of southwestern Oregon. One model, 3-PGS, is a production model drive n by remote sensing data, running at monthly time steps, while the other, B IOME-BGC, is a complex eco-physiological model run at daily time steps. Bot h models include subroutines for predicting the interception of radiation a nd its dissipation as energy for evaporating water and the absorbed fractio n that is photosynthetically active (400-700 nm). The models differ in a nu mber of ways, including the estimation of canopy dynamics, calculation of r espiration, use of growth modifiers and below ground mechanisms. In 3-PGS, canopy dynamics are derived from remote sensing inputs, and autotrophic res piration is assumed a constant fraction of gross photosynthesis=0.53; in BI OME-BGC, the canopy biomass is accumulated through allocation, with respira tion a function of live biomass, temperature, and nitrogen content. BIOME-B GC includes decomposition and nitrogen mineralization subroutines, while 3- PGS incorporates these processes through an index of soil fertility. Plot-b ased information was available at each site on species composition, site pr oductivity, phenology, and seasonal trends in plant water relations. Long-t erm averages of minimum/maximum temperature and precipitation were extrapol ated from local meteorological stations and converted into estimates of sol ar radiation, daytime vapor pressure deficits, and frequency of subfreezing temperatures for the sites, which ranged in elevation from 550 to 2135 m a nd had varying slopes and aspects. State-wide soil survey data were interpr eted to estimate soil water holding capacity and fertility. Satellite-deriv ed data were used to drive 3-PGS and to validate predictions of leaf area b y BIOME-BCG. The two models gave similar annual estimates of total net prim ary production (r(2) = 0.85, slope = 0.64, intercept: 2.26 Mg ha(-1) year(- 1)) but differed in their presentation of photosynthetic activity seasonall y. 3-PGS has a suboptimal temperature function that provides more realistic ally limits on photosynthesis during the dormant season than assumed by BIO ME-BGC. BIOME-BGC predicted seasonal variation in the ratio of autotrophic respiration to gross photosynthesis from 0.4 to 0.7, but over the year, the average was similar to that assumed by 3-PGS (0.58 +/-0.05). We discovered that Landsat imagery with 30 in spatial resolution was reasonably correlat ed with leaf area indices as predicted by the BIOME-BGC model, but a variat ion still occurred associated with small areas where outcrops of serpentine restricted canopy development. (C) 2001 Elsevier Science B.V. All rights r eserved.