Abstract
AbstractA 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.
Highlights
Current analyses of the global carbon budget indicate that the terrestrial biosphere is presently a significant net sink for carbon (e.g. Siegenthaler & Sarmiento 1993); several recent studies locate this terrestrial sink in temperate and/or boreal forests (e.g. Tans et al 1990; Enting & Mansbridge 1991; Dai & Fung 1993; Ciais et al 1995; Denning et al 1995), but at this point there are not enough field data to demonstrate this
In anticipation of the field data from the BOREAS behaviours has several implications: (i) it can illuminate campaign, we developed a daily time-step, ecosystem model deficiencies, such as neglected ecosystem componcarbon balance model of spruce / moss boreal ecosystems. ents or processes, and/or poorly functioning algorithms; Table 1 Model parameters—site description
Soil respiration rates through the summer season (415 g C m"~) compared closely to those measured in 1980 and 1981 in an Alaskan black spruce/feathermoss forest (367-370 g C m"-) by Schlentner and Van Cleve (1985), Soil dark respiration was the sum of fine root respiration, moss metabolic respiration, and heterotrophic respiration associated with litter decomposition
Summary
Sink for carbon (e.g. Siegenthaler & Sarmiento 1993); several recent studies locate this terrestrial sink in temperate and/or boreal forests (e.g. Tans et al 1990; Enting & Mansbridge 1991; Dai & Fung 1993; Ciais et al 1995; Denning et al 1995), but at this point there are not enough field data to demonstrate this. Moss photosynthesis was calculated daily as a maximum rate (1.0 mg CO2 g'' h"'), reduced by light availability and model-generated temperature and moisture conditions (see Fig. 2a,b,e and Table 3), and multiplied by daylength. DRQ was determined by using long term mean daily weather data for the site, running the soil climate submodel to determine the temperature and water content of the top of the litter layer (first year cohort), finding the annual total of R(T,W) by summing (20) over the year, and normalizing it to the annual mass loss value. All root litter decomposition was modelled, like humus, as a single exponentially decaying pool (i.e, a = 0.0 in Eq, 15), with an annual rate equal to that of fresh needle and moss litter {RQ), modified by the temperature and moisture conditions at each level in the soil profile. 40 80 120 160 200 240 280 320 day of year field studies is to try to partition daily NEE into separate components
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