Metolius AmeriFlux Sites

Summary

We have been making eddy covariance and biological measurements of carbon and energy fluxes (soil respiration, photosynthesis, transpiration), and C storage at the Metolius sites since 2000 (Figure 1). We are currently focusing on the effects of disturbance on processes controlling carbon storage, fluxes, and energy exchange with the atmosphere in relation to climate and disturbance in chronosequences of ponderosa pine. We moved the flux system from an old-growth site (250yrs) in 2002 to a mature site (90yrs). We are currently making micrometeorological and meteorological measurements at our mature (90 yrs) and young (20yrs) pine flux sites (Table 1). We will continue biological measurements and model parameter measurements necessary to develop carbon budgets for the flux sites (e.g. soil surface CO₂ fluxes, foliage respiration, litterfall, annual productivity, LAI, foliar and soil chemistry), and begin identical measurements in a recently burned pine forest nearby so that we cover the range of disturbances (logging, fire) commonly observed in the region. The carbon budgets will be used to estimate annual net ecosystem production (NEP = -NEE), evaluate where and how much carbon is stored at different successional stages (e.g. soils, above- and belowground living biomass), and to evaluate controls on CO₂ exchange. We have built and installed an automated soil chamber system at each site, so that we have continuous soil flux measurements.

Results Highlights

• Soil respiration response to pulse rain events (Figure 2) show strong influence of moisture on soil fluxes in the semi-arid forest (Irvine and Law, 2002).
• Soil respiration accounts for ~70% of total ecosystem respiration in young and old pine forests (Law et al. 2001).
• The biological data show that aboveground net primary productivity (ANPP), NPP and net ecosystem production (NEP) were greater at the old pine site than the young (Y) site. Monte Carlo estimates of NEP show that the young site is a source of CO2 to the atmosphere, and is significantly lower than NEP of the old site by ~ 100 g C m-2 y-1. Eddy covariance measurements also show that the O site was a stronger sink for CO2 than the Y site (Law et al, 2001).
• Carbon, water, and nitrogen cycle simulations with the Biome-BGC model suggest that disturbance type and frequency, time since disturbance, age-dependent changes in below-ground allocation, and increasing atmospheric concentration of CO2 all exert significant control on the net ecosystem exchange of carbon at the two sites. Model estimates of major carbon flux components agree with budget-based observations to within +/- 20%, with larger differences for NEP and for several storage terms. Simulations (Figure 3)showed the period of regrowth required to replace carbon lost during and after a stand-replacing fire (O) or a clearcut (Y) to be between 50 and 100 years. In both cases, simulations showed a shift from net carbon source to net sink (on an annual basis) 10 to 20 years after disturbance. These results suggest that the net ecosystem production of young stands may be low because heterotrophic respiration, particularly from soils, is higher than the NPP of the regrowth. The amount of carbon stored in long-term pools (biomass, soils) in addition to short-term fluxes has important implications for management of forests in the Pacific Northwest for carbon sequestration(Law et al, 2001).