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Measurements of Gross and Net Ecosystem Productivity and Water Vapour Exchange of a Pinus Ponderosa Ecosystem, and an Evaluation of Two Generalized Models

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Measurements of Gross and Net Ecosystem Productivity and Water Vapour Exchange of a Pinus Ponderosa Ecosystem, and an Evaluation of Two Generalized Models Global Change Biology (2000) 6, 155±168 Measurements of gross and net ecosystem productivity and water vapour exchange of a Pinus ponderosa ecosystem, and an evaluation of two generalized models B. E. LAW,*. R. H. WARING,² P. M. ANTHONI³ andJ. D. ABER§ *Department of Forest Science, Peavy Hall 154, Oregon State University, Corvallis, OR 97331, USA, ²Department of Forest Science, Peavy Hall 154, Oregon State University, Corvallis, OR 97331, USA, ³College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA, §Complex Systems Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824, USA Abstract Net ecosystem productivity (NEP), net primary productivity (NPP), and water vapour exchange of a mature Pinus ponderosa forest (44°30¢ N, 121°37¢ W) growing in a region subject to summer drought were investigated along with canopy assimilation and respiratory ¯uxes. This paper describes seasonal and annual variation in these factors, and the evaluation of two generalized models of carbon and water balance (PnET-II and 3-PG) with a combination of traditional measurements of NPP, respiration and water stress, and eddy covariance measurements of above-and below-canopy CO2 and water vapour exchange. The objective was to evaluate the models using two years of traditional and eddy covariance measurements, and to use the models to help interpret the relative importance of processes controlling carbon and water vapour exchange in a water-limited pine ecosystem throughout the year. PnET-II is a monthly time-step model that is driven by nitrogen availability through foliar N concentration, and 3-PG is a monthly time-step quantum-ef®ciency model constrained by extreme tempera- tures, drought, and vapour pressure de®cits. Both models require few parameters and have the potential to be applied at the watershed to regional scale. There was 2/3 less rainfall in 1997 than in 1996, providing a challenge to modelling the water balance, and consequently the carbon balance, when driving the models with the two years of climate data, sequentially. Soil fertility was not a key factor in modelling processes at this site because other environmental factors limited photosynthesis and restricted projected leaf area index to ~1.6. Seasonally, GEP and LE were overestimated in early summer and underestimated through the rest of the year. The model predictions of annual GEP, NEP and water vapour exchange were within 1±39% of ¯ux measure- ments, with greater disparity in 1997 because soil water never fully recharged. The results suggest that generalized models can provide insights to constraints on produc- tivity on an annual basis, using a minimum of site data. Keywords: carbon allocation, eddy covariance, micrometeorology, net ecosystem production, photosynthesis, respiration Received 5 January 1999; resubmitted 8 April and accepted 18 May 1999 Introduction Climatic variation and management practices in¯uence et al. 1993). Models of gross ecosystem production (gross carbon uptake, storage and release by forests (Cohen et al. photosynthesis, GEP), net primary production (NPP) and 1996), and water vapour exchange with the atmosphere, net ecosystem production (NEP) are being driven with which can affectmicro- and meso-scale climate(Pielke daily meteorological data and remotely sensed vegeta- tion characteristics to provide regional to global esti- Correspondence: Beverly E. Law, tel +1/541-737-2996, fax +1/ mates of vegetation±atmosphere interaction (Prince & 541-737-2540, e-mail [email protected] Goward 1995; Hunt et al. 1996). Until advanced sensors # 2000 Blackwell Science Ltd. 155 156 B.E. LAW et al. and techniques are developed and made available, there imate stand age, maximum quantum ef®ciency) than is no direct way of evaluating model predictions at 0.25 for PnET-II. km2 scale and larger. Alternatively, eddy covariance Our research site offers a challenging test for these measurements of CO2 and water vapour ¯uxes (LE) and models because the dominant species are evergreen complementary ®eld measurements of ecosystem pro- conifers that have the potential to continue photosynth- cess rates can provide a means of evaluating model esis throughout the year. However, carbon uptake is estimates of LE, GEP, NEP, NPP and respiration by constrained in winter by subfreezing conditions and ecosystem components. reduced solar radiation, and in summer by soil water We conducted our study at a ponderosa pine (Pinus de®cits and extreme vapour pressure de®cits when ponderosa Laws.) site in Oregon that is a member of the incidentsolar radiationlevels are high. The soils are AmeriFlux and Fluxnet networks of ¯ux sites. At these relatively infertile, thus nutrient de®ciencies play an sites, suites of measurements are being made at various important role in carbon allocation only for a few months levels of organization for estimating carbon and water when other factors are not limiting. vapour exchange and for parameterizing and validating Our objectives are to evaluate the models using ecosystem models to improve estimates across larger traditional and eddy covariance measurements and spatial scales. determine sensitivity of key model parameters, and to In this study, we used two years of eddy covariance use the models to help interpret the relative importance measurements of CO2 and water vapour ¯uxes and of processes controlling carbon and water vapour traditional measurements of respiration, leaf area index exchange in a water-limited pine ecosystem throughout (LAI) and NPP to check assumptions and predictions the year. made by two generalized models of ecosystem processes, PnET-II and 3-PG. We chose these models because they Materials and methods require few parameters and have the potential for watershed to regional applications through parameter- Site description ization with GIS coverage of vegetation type, climate and topography (Aber et al. 1995; Coops et al. 1998). When The site is a ponderosa pine forest in central Oregon, driven by local meteorological data, the models were located in the Metolius Research Natural Area (44°30¢ N, able to match seasonal measurements of GEP acquired 121°37¢W, elevation 940 m). The pine forest extends at through eddy covariance measurements in a temperate least 12 km in all directions. The site consists of about deciduous forest(Aber et al. 1995; Waring et al. 1995). 27% old trees (~250 years), 25% younger trees (~45 years) Once the models have been evaluated with several and 48% mixed-age trees. The understorey is sparse with measures of ecosystem function in a range of forest patches of bitterbrush (Purshia tridentata), bracken fern types, we will have con®dence in broader applications of (Pteridium aquilinum) and strawberry (Fragaria vesca). the models. Most of the precipitation in the region occurs between The models predict carbon uptake and respiration at October and June, with the summer months lacking monthly or longer time steps, applying principles effective precipitation (normally ~20 mm July through developed from numerous comparative studies includ- August). Winters on the eastside of the Cascade ing whole ecosystem daily and annual carbon Mountains are cool, and there is a large diurnal variation balances. Both models use monthly summaries of in temperature in summer, typical of the semiarid region. weather data. PnET-II assesses canopy photosynthetic Snow cover in the winter is intermittent, reaching a capacity based on nitrogen status while 3-PG is maximum depth of about 50 cm. Freezing temperatures primarily driven by quantum ef®ciency and environ- occur mostly at night and early morning. mental constraints on gross photosynthesis. PnET-II Soil at the site is a sandy loam (73% sand, 21% silt, and and 3-PG both limit photosynthesis and alter carbon 6% clay; Law et al. 1999a) and is classi®ed as a light- allocation based on constraints from environmental coloured andic inceptisol that is low in nutrients. Soil factors, in particular soil water availability and soil water-holding capacity to 1 m depth is 163 mm (S. fertility, and climatic variables: subfreezing tempera- Remillard, pers. comm.). Surface litter was scant in the tures, vapour pressure de®cit (D), and limited pre- old stands (194 g m±2), and averaged 38% less than litter cipitation during some months. Both models are able mass in the young stands (315 g m±2). to provide some estimate of GEP under equilibrium conditions where soil carbon content and litterfall Environmental measurements accumulation are in apparent balance. More detailed parameterization is needed for 3-PG (e.g. local Using the eddy covariance method, we made continuous allometric equations for biomass, soil fertility, approx- ¯ux measurements on a tower 14 m above the canopy # 2000 Blackwell Science Ltd., Global Change Biology, 6, 155±168 GROSS AND NET ECOSYSTEM PRODUCTIVITY 157 and 2 m above the soil surface to determine half-hourly equivalent to chamber measurements of net assimila- sensible and latent heat ¯uxes (LE, evaporation from tion (Goulden et al. 1998). surfaces and transpiration), and CO2 ¯uxes in 1996 and The methods used to estimate foliage, wood and root 1997. Details on the instrumentation, ¯ux corrections and production in 1996 are reported in Law et al. (1999a). We calculations were reported in Law et al. (1999a,b) and repeated these measurements to obtain NPP estimates Anthoni et al. (1999). We summarize methods here to for 1997 in this study. The above-ground net primary brie¯y explain processing of the ¯ux data. The data were production (ANPP) was determined from mensuration screened to remove possible eddy covariance instrumen- measurements of stem diameter, growth increment and tation and sampling problems (Law et al. 1999a). After height, and from foliage biomass per age class and leaf screening, about75% of theabove-canopy carbon ¯ux area index measurements. Allometric equations that and 85% of the water vapour ¯uxes were available for were developed on-site were used to estimate current further analysis.
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