Production,Respiration, and Overall Carbon Balance in an Old-growth <Emphasis Type="Italic">Pseudotsuga</Emphasis>-<Emphasis Type="Italic">Tsuga</Emphasis> Forest Ecosystem

Ground-based measurements of stores, growth, mortality, litterfall, respiration, and decomposition were conducted in an old-growth forest at Wind River Experimental Forest, Washington, USA. These measurements were used to estimate gross primary production (GPP) and net primary production (NPP); autotrophic respiration (R_a) and heterotrophic (R_h) respiration; and net ecosystem production (NEP). Monte Carlo methods were used to calculate uncertainty (expressed as ± 2 standard deviations of 200–400 calculations). Live carbon (C) stores were 39,800 g C m^–2 (34,800–44,800 g C m^–2). The store of C in detritus and mineral soil was 22,092 g C m^–2 (20,600–23,600 g C m^–2), and the total C stores were 61,899 g C m^–2 (56,600–67,700 g C m^–2). Total NPP was 597 g C m^–2 y^–1 (453 to 741 g C m^–2 y^–1). R_a was 1309 g C m^–2 y^–1 (845–1773 g C m^–2 y^–1), indicating a GPP of 1906 g C m^–2 y^–1 (1444–2368 g C m^–2 y^–1). R_h, including the respiration of heart rots in tree boles, was 577 g C m^–2 y^–1 (479–675 g C m^–2 y^–1). Long-term NEP was estimated to be +20 g C m^–2 y^–1 (–116 to +156 g C m^–2 y^–1), indicating this stand might be a small sink. These estimates contrast with the larger sink estimated at the same site using eddy-flux methods. Several hypotheses to explain this discrepancy were explored, including (a) undetected biomass increases, (b) underestimates of NPP, (c) unmeasured losses, and (d) a temporal mismatch between the two sets of measurements. The last hypothesis appears the most likely.     

Environmental controls on the photosynthesis and respiration of a boreal lichen woodland: a growing season of whole-ecosystem exchange measurements by eddy correlation

Measurements of net ecosystem CO₂ exchange by eddy correlation, incident photosynthetically active photon flux density (PPFD), soil temperature, air temperature, and air humidity were made in a black spruce (Picea mariana) boreal woodland near Schefferville, Quebec, Canada, from June through August 1990. Nighttime respiration was between 0.5 and 1.5 kg C ha^–1 h^–1, increasing with temperature. Net uptake of carbon during the day peaked at 3 kg C ha^–1 h^–1, and the daily net uptake over the experiment was 12 kg C ha^–1 day^–1. Photosynthesis dropped substantially at leaf-to-air vapor pressure deficit (VPD) greater than 7 mb, presumably as a result of stomatal closure. The response of ecosystem photosynthesis to incident PPFD was markedly non-linear, with an abrupt saturation at 600 mol m^–2 s^–1. This sharp saturation reflected the geometry of the spruce canopy (isolated conical crowns), the frequently overcast conditions, and an increase in VPD coincident with high radiation. The ecosystem light-use efficiency increased markedly during overcast periods as a result of a more even distribution of light across the forest surface. A mechanistic model of forest photosynthesis, parameterized with observations of leaf density and nitrogen content from a nearby stand, provided accurate predictions of forest photosynthesis. The observations and model results indicated that ecosystem carbon balance at the site is highly sensitive to temperature, and relatively insensitive to cloudiness.     

Fluxes of carbon dioxide and water vapor above paddy fields

Atmospheric fluxes of carbon dioxide and water vapor were measured by the eddy correlation technique over a paddy field in 1989. The carbon dioxide was transported downward during daylight hours due to photosynthesis of the paddy crop. The downward flux of carbon dioxide increased with increasing net radiation. Maximum values of downward flux varied with the growing stage of the paddy crop: ca. 0.3 mg m^–2 s^–1 at early vegetative growth stage and ca. 1.3 mg m^–2 s^–1 at ear formation stage. The daytime totals of downward flux of carbon dioxide also showed seasonal variation reflecting the photosynthetic activity of the paddy crop: ca. 6 g m^–2 at early vegetative growth stage in June and 40 g m^–2 at ear formation stage in September. The seasonal variation of daily totals of carbon dioxide flux shows that carbon dioxide of about 28 t ha^–1 is fixed by the paddy crop from transplanting to harvesting. Taking into account the water use efficiency, the paddy crop requires water in amounts at least 100 times that of carbon dioxide fixed by photosynthesis. It is noted that the correlation coefficients between carbon dioxide, water vapor and vertical wind velocity have constant values under near neutral and free convective regimes.     

Seasonal net carbon dioxide exchange of a beech forest with the atmosphere

The seasonal carbon dioxide exchange of a beech forest of Central Italy was studied by means of the eddy covariance technique. Additional measurements of biomass respiration with cuvettes and relationship of carbon dioxide exchanges with temperature and light were used to interpolate missing data during the dormant and part of the growing season. The net ecosystem production of the forest equals 472 g C m^?2 y^?1 while the gross ecosystem production 1016 g C m^?2 y^?1 and respiration 544 g C m^?2 y^?1. These estimates are compared with the net primary production determined by direct biomass sampling which amounts to 802 g C m^?2 y^?1.     

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Summary Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day^–1 tree^–1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m² of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s^–1 that is equivalent to 112 mmol m^–2 s^–1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m^–2 s^–1 at the top and 150 mmol m^–2 s^–1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was –27.76±0.27 (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.This paper is dedicated to Prof. Dr. O.L. Lange on the occasion of his 65th birthday     

An initial intercomparison of micrometeorological and ecological inventory estimates of carbon exchange in a mid-latitude deciduous forest

The role of mid‐latitude forests in the sequestration of carbon (C) is of interest to an increasing number of scientists and policy‐makers alike. Net CO₂ exchange can be estimated on an annual basis, using eddy‐covariance techniques or from ecological inventories of C fluxes to and from a forest. Here we present an intercomparison of annual estimates of C exchange in a mixed hardwood forest in the Morgan‐Monroe State Forest, Indiana, USA for two years, 1998 and 1999. Based on eddy‐covariance measurements made at 1.8 times canopy height from a tower, C uptake by the forest was 237 and 287 g C m^?2 y^?1 for 1998 and 1999, respectively. For the same time period, biometric and ecophysiological measures and modelled estimates of all significant carbon fluxes within deciduous forests were made, including: change in living biomass, aboveground and belowground detritus production, foliage consumption, and forest floor and soil respiration. Using this ecological inventory method for these same two time periods, C uptake was estimated to be 271 and 377 g C m^?2 y^?1, which are 14.3% and 31.4% larger, respectively, than the tower‐based values. The relative change between this method's annual estimates is consistent with that of the eddy‐covariance based values. Our results indicate that the difference in annual C exchange rates was due to reduced heterotrophic soil respiration in 1999.     

Longitudinal Spatial Patterns of Bacterial Production and Respiration in a Large River–Estuary: Implications for Ecosystem Carbon Consumption

Rivers and estuaries transport organic carbon (C) from terrestrial and freshwater ecosystems to the marine environment. During this transit, bacteria actively utilize and transform organic C, but few studies have measured detailed spatial variation in rates of bacterial respiration (BR) and production (BP). We measured BP at 39 stations and BR at 12 stations at monthly intervals along a 200-km reach of the tidal Hudson River. We observed strong repeatable spatial patterns for both BP and BR, with rates declining in the downstream direction. Bacterial Production had much greater dynamic range of spatial variation than BR. We used the detailed seasonal and spatial data on BP and BR to measure the total C demand of bacteria at several scales. We calculated volumetric and areal rates for 12 sections of the Hudson, as well as the total C utilization. Volumetric BR averaged 20 g-C-m^–3 y^–1, but it was highest in the most upstream section at 30 g C m^–3 y^–1. Areal rates averaged over the entire river were 174 g C m^–2 y^–1, but they were 318 g C m^–2 y^–1 in the deepest section of the river, indicating the importance of morphometric variation. Total bacterial C demand increased downriver with increasing total volume. Overall, bacteria in the freshwater section of the river consumed approximately 18–25.5 × 10⁹ g C y^–1, about 20% of the total organic C load.     

Total Belowground Carbon Allocation in a Fast-growing Eucalyptus Plantation Estimated Using a Carbon Balance Approach

Trees allocate a large portion of gross primary production belowground for the production and maintenance of roots and mycorrhizae. The difficulty of directly measuring total belowground carbon allocation (TBCA) has limited our understanding of belowground carbon (C) cycling and the factors that control this important flux. We measured TBCA over 4 years using a conservation of mass, C balance approach in replicate stands of fast growing Eucalyptus saligna Smith with different nutrition management and tree density treatments. We measured TBCA as surface carbon dioxide (CO₂) efflux (“soil” respiration) minus C inputs from aboveground litter plus the change in C stored in roots, litter, and soil. We evaluated this C balance approach to measuring TBCA by examining (a) the variance in TBCA across replicate plots; (b) cumulative error associated with summing components to arrive at our estimates of TBCA; (c) potential sources of error in the techniques and assumptions; (d) the magnitude of changes in C stored in soil, litter, and roots compared to TBCA; and (e) the sensitivity of our measures of TBCA to differences in nutrient availability, tree density, and forest age. The C balance method gave precise estimates of TBCA and reflected differences in belowground allocation expected with manipulations of fertility and tree density. Across treatments, TBCA averaged 1.88 kg C m⁻² y⁻¹ and was 18% higher in plots planted with 10⁴ trees/ha compared to plots planted with 1111 trees/ha. TBCA was 12% lower (but not significantly so) in fertilized plots. For all treatments, TBCA declined linearly with stand age. The coefficient of variation (CV) for TBCA for replicate plots averaged 17%. Averaged across treatments and years, annual changes in C stored in soil, the litter layer, and coarse roots (−0.01, 0.06, and 0.21 kg C m⁻² y⁻¹, respectively) were small compared with surface CO₂ efflux (2.03 kg C m⁻² y⁻¹), aboveground litterfall (0.42 kg C m⁻² y⁻¹), and our estimated TBCA (1.88 kg C m⁻² y⁻¹). Based on studies from similar sites, estimates of losses of C through leaching, erosion, or storage of C in deep soil were less than 1% of annual TBCA. Received 6 March 2001; accepted 7 January 2002.     

Effects of climatic variability on the annual carbon sequestration by a boreal aspen forest

To evaluate the carbon budget of a boreal deciduous forest, we measured CO₂ fluxes using the eddy covariance technique above an old aspen (OA) forest in Prince Albert National Park, Saskatchewan, Canada, in 1994 and 1996 as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). We found that the OA forest is a strong carbon sink sequestering 200 ± 30 and 130 ± 30 g C m^–2 y^–1 in 1994 and 1996, respectively. These measurements were 16–45% lower than an inventory result that the mean carbon increment was about 240 g C m^–2 y^–1 between 1919 and 1994, mainly due to the advanced age of the stand at the time of eddy covariance measurements. Assuming these rates to be representative of Canadian boreal deciduous forests (area ≈ 3 × 10⁵ km²), it is likely they can sequester 40–60 Tg C y^–1, which is 2–3% of the missing global carbon sink. The difference in carbon sequestration by the OA forest between 1994 and 1996 was mainly caused by the difference in leaf emergence date. The monthly mean air temperature during March–May 1994, was 4.8 °C higher than in 1996, resulting in leaf emergence being 18–24 days earlier in 1994 than 1996. The warm spring and early leaf emergence in 1994 enabled the aspen forest to exploit the long days and high solar irradiance of mid-to-late spring. In contrast, the 1996 OA growing season included only 32 days before the summer solstice. The earlier leaf emergence in 1994 resulted 16% more absorbed photosynthetically active radiation and a 90 g C m^–2 y^–1 increase in photosynthesis than 1996. The concomitant increase in respiration in the warmer year (1994) was only 20 g C m^–2 y^–1. These results show that an important control on carbon sequestration by boreal deciduous forests is spring temperature, via the influence of air temperature on the timing of leaf emergence.     

Carbon balance of a tropical savanna of northern Australia

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO₂ efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204±53 ton C ha^–1, with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0–1 m) was 151±33 ton C ha^–1, accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53±20 ton C ha^–1, 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha^–1, of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha^–1 year^–1, of which 8.0 ton C ha^–1 (73%) was contributed by below-ground net primary production and 3.0 ton C ha^–1 (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha^–1 year^–1. Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5–6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha^–1 year^–1.     

The use of eddy covariance to infer the net carbon dioxide uptake of Brazilian rain forest

• 1 Eddy covariance measurements of CO₂ flux, based on four and six week campaigns in Rondôdnia, Brazil, have been used in conjunction with a model to scale up data to a whole year, and thus estimate the carbon balance of the tropical forest ecosystem, and the changes in carbon balance expected from small interannual variations in climatological conditions.
• 2 One possible source of error in this estimation arises from the difficulty in measuring fluxes under stably stratified meteorological conditions, such as occur frequently at night. Flux may be ‘lost’ because of low velocity advection, caused by nocturnal radiative cooling at sites on raised ground. Such effects may be detected by plotting the net ecosystem flux of CO₂, F_eco is a function of wind speed. If flux is ‘lost’ then F_eco is expected to decline with wind speed. In the present data set, this did not occur, and F_eco was similar to the nocturnal flux estimated independently from chamber measurements.
• 3 The model suggests that in 1992/3, the Gross Primary Productivity (GPP) was 203.3 mol C m^?2 y^?1 and ecosystem respiration was 194.8 mol C m^?2 y^?1, giving an ecosystem carbon balance of 8.5 mol C m^?2 y^?1, equivalent to a sink of 1.0 ton C ha^?1 y^?1. However, the sign and magnitude of this figure is very sensitive to temperature, because of the strong influence of temperature on respiration.
• 4 The model also suggests that the effect of temperature on the net carbon balance is strongly dependent on the partial pressure of CO₂.

     

Impacts of the C4 sedge Cyperus papyrus L. on carbon and water fluxes in an African wetland

Fluxes of CO₂ and H₂O vapour were measured by eddy covariance from a stand of the C₄ emergent sedge Cyperus papyrus (papyrus), which formed a fringing swamp on the north-west shore of Lake Naivasha, Kenya. The fluxes of CO₂ and H₂O vapour between the papyrus swamp and the atmosphere were large but variable, depending on the hydrology of the wetland system and the condition of the vegetation. These measurements, combined with simulation modelling of annual fluxes of CO₂, show that papyrus swamps have the potential to sequester large amounts of the carbon (1.6 kg C m^–2 y^–1) when detritus accumulates under water in anaerobic conditions, but they are a net source of carbon release to the atmosphere (1.0 kg C m^–2 y^–1) when water levels fall to expose detritus and rhizomes to aerobic conditions. Evapotranspiration from papyrus swamps (E) was frequently lower than evaporation from open water surfaces (E _o) and plant factors have a strong influence on the flux of water to the atmosphere. For the period of measurement E/E_o was 0.36.     

Methane consumption in two temperate forest soils

Forest soils are thought to be an important sink for atmospheric methane. To evaluate methane consumption,¹⁴C-labeled methane was added to the headspace of intact soil cores collected from a mixed mesophytic forest and from a red spruce forest located in the central Appalachian Mountains. Both soils consumed the added methane at initially high rates that decreased as the methane mixing ratio of the air decreased. The mixed mesophytic forest soil consumed an average of 2 mg CH₄ m^–2 d^–1 versus 1 mg CH, m^–2 d^–1 for the spruce forest soil. The addition of acetylene to the headspace completely suppressed methane consumption by the soils, suggesting that an aerobic methane-consuming microorganism mediated the process. At both forest sites, methane mixing ratios in soil air spaces were greater than that in the air overlying the soil surface, indicating that these soils had the ability to produce methane. Models of methane emission from forest soils to the atmosphere must represent methane flux as the balance between production and consumption of methane, which are controlled by very different factors     

Fine Root Production and Turnover in a Norway Spruce Stand in Northern Sweden: Effects of Nitrogen and Water Manipulation

Fine root length production, biomass production, and turnover in forest floor and mineral soil (0–30 cm) layers were studied in relation to irrigated (I) and irrigated-fertilized (IL) treatments in a Norway spruce stand in northern Sweden over a 2-year period. Fine roots (<1 mm) of both spruce and understory vegetation were studied. Minirhizotrons were used to estimate fine root length production and turnover, and soil cores were used to estimate standing biomass. Turnover was estimated as both the inverse of root longevity (RTL) and the ratio of annual root length production to observed root length (RTR). RTR values of spruce roots in the forest floor in I and IL plots were 0.6 and 0.5 y⁻¹, respectively, whereas the corresponding values for RTL were 0.8 and 0.9 y⁻¹. In mineral soil, corresponding values for I, IL, and control (C) plots were 1.2, 1.2, and 0.9 y⁻¹ (RTR) and 0.9, 1.1, and 1 y⁻¹ (RTL). RTR and RTL values of understory vegetation roots were 1 and 1.1 y⁻¹, respectively. Spruce root length production in both the forest floor and the mineral soil in I plots was higher than in IL plots. The IL-treated plots gave the highest estimates of spruce fine root biomass production in the forest floor, but, for the mineral soil, the estimates obtained for the I plots were the highest. The understory vegetation fine root production in the I and IL plots was similar for both the forest floor and the mineral soil and higher (for both layers) than in C plots. Nitrogen (N) turnover in the forest floor and mineral soil layers (summed) via spruce roots in IL, I, and C plots amounted to 2.4, 2.1, and 1.3 g N m⁻² y⁻¹, and the corresponding values for field vegetation roots were 0.6, 0.5, and 0.3 g N m⁻² y⁻¹. It was concluded that fertilization increases standing root biomass, root production, and N turnover of spruce roots in both the forest floor and mineral soil. Data on understory vegetation roots are required for estimating carbon budgets in model studies.     

Seasonal variations of CO2 and water vapour exchange rates over a temperate deciduous forest

Long-term and direct measurements of CO₂ and water vapour exchange are needed over forested ecosystems to determine their net annual fluxes of carbon dioxide and water. Such measurements are also needed to parameterize and test biogeochemical, ecological and hydrological assessment models. Responding to this need, eddy covariance measurements of CO₂ and water vapour were made ever a deciduous forest growing near Oak Ridge, TN, between April 1993 and April 1994. Periodic measurements were made of leaf area index, stomatal resistance, soil moisture and pre-dawn leaf water potential to characterize the gas exchange capacity of the canopy. Four factors had a disproportionate influence on the seasonal variation of CO₂ flux densities. These factors were photon flux densities (during the growing season), temperature (during the dormant season), leaf area index and the occurrence of drought The drought period occurred during the peak of the growing season and caused a significant decline in daily and hourly CO₂ flux densities, relative to observations over the stand when soil moisture was plentiful. The annual net uptake of carbon was calculated by integrating flux measurements and filling missing and spurious data with the relations obtained between measured CO₂ fluxes and environmental forcing variables. The net flux of carbon for the period between April 1993 and April 1994 was -525 g C m^?2 y^?1. This value represents a net flux of carbon from the atmosphere and into the forest. The net annual carbon exchange of this southern temperate broadleaved forest exceeded values measured over a northern temperate forest (which experiences a shorter growing season and has less leaf area) by 200 g C m^?2 y^?1 (cf. Wofsy et al 1993). The seasonal variation of canopy evaporation (latent heat flux) was controlled mostly by changes in leaf area and net radiation. A strong depression in evaporation rates was not observed during the drought Over a broadleaved forest large vapour pressure deficits promote evaporation and trees in a mixed stand are able to tap a variety of deep and shallow water sources.     

Fluxes of gases in an ice related ecosystem in the north-western Weddell Sea

Summary During the European Polarstern Study (EPOS leg 1 and leg 2) measurements of temperature, salinity, inorganic nutrients, chlorophyll-a, oxygen and total inorganic carbon dioxide were performed from October to January 1988–1989 in north-south sections at 47–49 °E in the NW Weddell Sea from approximately 58 °S to 63 °S (Hempel 1989; Hempel et al. 1989). In order to explain parts of the obtained data, a time-dependent ecological model was constructed by Svansson (1991). He found that a moderate mixing with a constant diffusion coefficient from sea surface downwards resulted in good agreement between computed and measured chlorophyll. In this paper we introduce the gas fluxes, mainly oxygen but also carbon dioxide, into the model work. It turns out that air-sea fluxes are necessary to explain the vertical oxygen distribution. The annual development of chlorophyll, phosphate, oxygen and total inorganic carbon dioxide are computed. Hours of day-light, losses and the eddy diffusion coefficient are allowed to vary during the year with the condition that the mean total chlorophyll at 14 selected leg 1 stations was nearly double the magnitude of that of 18 selected leg 2 stations. This yields variations consistent with the observations. Different steady-state solutions after 91 days are also tested to show effects of one selected variation at a time, for example the eddy diffusion coefficient or the loss rate. The oxygen air-sea flux, of about 90 mmol m^–2 day^–1 in the time variable model computation, is compared to estimated fluxes by a gas transfer formula. The formula used gives a flux which is about 5 times smaller than the model flux. Some of the 91 days solutions give results of fluxes which are less than 90 mmol m^–2 day^–1 but still higher than the transfer formula result. Fluxes of total inorganic carbon dioxide in the model computation are always directed from air to sea.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Canopy Carbon Gain and Water Use: Analysis of Old-growth Conifers in the Pacific Northwest

This report summarizes our current knowledge of leaf-level physiological processes that regulate carbon gain and water loss of the dominant tree species in an old-growth forest at the Wind River Canopy Crane Research Facility. Analysis includes measurements of photosynthesis, respiration, stomatal conductance, water potential, stable carbon isotope values, and biogenic hydrocarbon emissions from Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western red cedar (Thuja plicata). Leaf-level information is used to scale fluxes up to the canopy to estimate gross primary production using a physiology-based process model. Both light-saturated and in situ photosynthesis exhibit pronounced vertical gradients through the canopy, but are consistently highest in Douglas-fir, intermediate in western hemlock, and lowest in western red cedar. Net photosynthesis and stomatal conductance are strongly dependent on vapor-pressure deficit in Douglas-fir, and decline through the course of a seasonal drought. Foliar respiration is similar for Douglas-fir and western hemlock, and lowest for western red cedar. Water-use efficiency varied with species and tree height, as indexed using stable carbon isotopes values for foliage. Leaf water potential is most negative for Douglas-fir and similar for western hemlock and western red cedar. Terpene fluxes from foliage equal approximately 1% of the net carbon loss from the forest. Modeled estimates based on physiological measurements show gross primary productivity (GPP) to be about 22 Mg C m^–2 y^–1. Physiological studies will be necessary to further refine estimates of stand-level carbon balance and to make long-term predictions of changes in carbon balance due to changes in forest structure, species composition, and climate.     

The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha^–1 y^–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha^–1 y^–1 below the litter layer to around 50 kg C kg C ha^–1 y^–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO₃⁻). The dominance of NO₃^– relative to the total amount nitrate of N leaching from the soil shows that NO₃^– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.     

Modelling temporal variability in the carbon balance of a spruce/moss boreal forest

A model of the daily carbon balance of a black spruce/feathermoss boreal forest ecosystem was developed and results compared to preliminary data from the 1994 BOREAS field campaign in northem Manitoba, Canada. The model, driven by daily weather conditions, simulated daily soil climate status (temperature and moisture profiles), spruce photosynthesis and respiration, moss photosynthesis and respiration, and litter decomposition. Model agreement with preliminary field data was good for net ecosystem exchange (NEE), capturing both the asymmetrical seasonality and short-term variability. During the growing season simulated daily NEE ranged from -4 g C m^-2 d^-1 (carbon uptake by ecosystem) to + 2 g C m^-2 d^-1 (carbon flux to atmosphere), with fluctuations from day to day. In the early winter simulated NEE values were + 0.5 g C m^-2 d^-1, dropping to + 0.2 g C m^-2 d^-1 in mid-winter. Simulated soil respiration during the growing season (+ 1 to + 5 g C m^-2 d^-1) was dominated by metabolic respiration of the live moss, with litter decomposition usually contributing less than 30% and live spruce root respiration less than 10% of the total. Both spruce and moss net primary productivity (NPP) rates were higher in early summer than late summer. Simulated annual NEE for 1994 was -51 g C m^-2 y^-1, with 83% going into tree growth and 17% into the soil carbon accumulation. Moss NPP (58 g C m^-2 y^-1) was considered to be litter (i.e. soil carbon input; no net increase in live moss biomass). Ecosystem respiration during the snow-covered season (84 g C m^-2) was 58% of the growing season net carbon uptake. A simulation of the same site for 1968–1989 showed = 10–20% year-to-year variability in heterotrophic respiration (mean of + 113 g C m^-2 y^-1). Moss NPP ranged from 19 to 114 g C m^-2 y^-1; spruce NPP from 81 to 150 g C m^-2 y^-1; spruce growth (NPP minus litterfall) from 34 to 103 g C m^-2 y^-1; NEE ranged from +37 to -142 g C m^-2 y^-1. Values for these carbon balance terms in 1994 were slightly smaller than the 1969–89 means. Higher ecosystem productivity years (more negative NEE) generally had early springs and relatively wet summers; lower productivity years had late springs and relatively dry summers.