Carbon exchange of a mature, naturally regenerated pine forest in north Florida.

Global Change Biology (Impact Factor: 8.22). 01/2008; DOI: 10.1111/j.1365-2486.2008.01675.x
Source: OAI

ABSTRACT We used eddy covariance and biomass measurements to quantify the carbon (C) dynamics of a naturally regenerated longleaf pine/slash pine flatwoods ecosystem in north Florida for 4 years, July 2000 to June 2002 and 2004 to 2005, to quantify how forest type, silvicultural intensity and environment influence stand-level C balance. Precipitation over the study periods ranged from extreme drought (July 2000-June 2002) to above-average precipitation (2004 and 2005). After photosynthetic photon flux density (PPFD), vapor pressure deficit (VPD) >1.5 kPa and air temperature <10 °C were important constraints on daytime half-hourly net CO₂ exchange (NEEday) and reduced the magnitude of midday CO₂ exchange by >5 μmol CO₂ m⁻² s⁻¹. Analysis of water use efficiency indicated that stomatal closure at VPD>1.5 kPa moderated transpiration similarly in both drought and wet years. Night-time exchange (NEEnight) was an exponential function of air temperature, with rates further modulated by soil moisture. Estimated annual net ecosystem production (NEP) was remarkably consistent among the four measurement years (range: 158-192 g C m⁻² yr⁻¹). In comparison, annual ecosystem C assimilation estimates from biomass measurements between 2000 and 2002 ranged from 77 to 136 g C m⁻² yr⁻¹. Understory fluxes accounted for approximately 25-35% of above-canopy NEE over 24-h periods, and 85% and 27% of whole-ecosystem fluxes during night and midday (11:00-15:00 hours) periods, respectively. Concurrent measurements of a nearby intensively managed slash pine plantation showed that annual NEP was three to four times greater than that of the Austin Cary Memorial Forest, highlighting the importance of silviculture and management in regulating stand-level C budgets.

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    ABSTRACT: Old growth forests have traditionally been viewed as an insignificant sink or source in the global carbon cycle and therefore, flux tower studies of net ecosystem exchange (NEE) and evapotranspiration (LE) using flux measurements in these ecosystems are limited. Here we report eddy covariance (EC) fluxes of carbon dioxide and water above and below the canopy of an old growth Mountain Ash (Eucalyptus regnans) forest over an 18 month period. Mountain Ash species are the world’s tallest angiosperm and recognized as the most carbon-dense forests, which potentially makes them an important component of the terrestrial carbon and water budgets in Australia. Results showed that for 2006, the ecosystem was a large net sink of carbon of 377 ± 49 g C m−2 year−1. Throughout the study period, daytime Gross Primary Productivity (GPP) was limited mainly by radiation, but there were important secondary drivers regulating carbon uptake, especially in summer, when atmospheric and soil water deficits were high. The highest rates of NEE occurred during spring, when the ecosystem was not limited by radiation or moisture, and the lowest rates were observed during autumn and winter. In 2006, GPP for the ecosystem was 2615 g C m−2 year−1, and ecosystem respiration (Re) was 2238 g C m−2 year−1. During the summer and autumn of 2006, the understorey fluxes accounted for 29% of ecosystem GPP, 33% of evapotranspiration, and 53% of night time Re, a significant proportion of carbon dioxide and water exchange given that the understorey biomass is only one tenth of the ecosystem biomass. Results from this study highlighted the importance of the understorey vegetation in regulating old growth forest carbon and water balances, which has important implications for forest management practices.
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    ABSTRACT: Global terrestrial atmosphere–ecosystem carbon dioxide fluxes are well constrained by the concentration and isotopic composition of atmospheric carbon dioxide. In con-trast, considerable uncertainty persists surrounding regional contributions to the net global flux as well as the impacts of atmospheric and biological processes that drive the net flux. These uncertainties severely limit our ability to make confi-dent predictions of future terrestrial biological carbon fluxes. Here we use a simple light-use efficiency land surface model (the Vegetation Photosynthesis Respiration Model, VPRM) driven by remotely sensed temperature, moisture, and phe-nology to diagnose North American gross ecosystem ex-change (GEE), ecosystem respiration, and net ecosystem ex-change (NEE) for the period 2001 to 2006. We optimize VPRM parameters to eddy covariance (EC) NEE observa-tions from 65 North American FluxNet sites. We use a sep-arate set of 27 cross-validation FluxNet sites to evaluate a range of spatial and temporal resolutions for parameter esti-mation. With these results we demonstrate that different spa-tial and temporal groupings of EC sites for parameter estima-tion achieve similar sum of squared residuals values through radically different spatial patterns of NEE. We also derive a regression model to estimate observed VPRM errors as a function of VPRM NEE, temperature, and precipitation. Be-cause this estimate is based on model-observation residuals it is comprehensive of all the error sources present in modeled fluxes. We find that 1 km interannual variability in VPRM NEE is of similar magnitude to estimated 1 km VPRM NEE errors.
    Biogeosciences 01/2014; 11:217-235. · 3.75 Impact Factor

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