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Simultaneous biometric measurements of aboveground net primary production (ANPP) and eddy-covariance measurements of gross primary production (GPP) were made at 18 forest stands with 80 site-years of data across Canada – to assess the fraction of photosynthesis that is used to produce plant tissues and the consistency of carbon allocation patterns across forest ecosystems. The stands included boreal and temperate forests and spanned very young to mature stand ages. Across all sites, ANPP averaged 298±138gCm−2 yr−1 (mean±1 s.d.), with the highest values for temperate white pine plantations (307–630gCm−2 yr−1) and harvested Douglas-fir stands (219–459gCm−2 yr−1), and the lowest values for boreal harvested jack pine stands (97–185gCm−2 yr−1). ANPP more than doubled from newly established (≤12 years) to young (13–25 years) stands, then stabilized in young to mature (≥51 years) stands but with diverging trends among species. Inter-site variations in ANPP and GPP were closely related to site characteristics, in particular, to leaf area index, which explained 66% of the variation in ANPP and 80% of the variation in GPP, and absorbed photosynthetically active radiation, which explained 80% of the variation in ANPP and 82% of the variation in GPP. Both ANPP and GPP were also positively correlated with mean annual air temperature, mean annual precipitation, and total soil nitrogen in the upper 10cm of the mineral soil. ANPP was strongly, positively correlated with GPP and the ANPP/GPP ratio was relatively constant (0.29±0.06), with no consistent differences among species or age classes. The results support the use of a constant ANPP/GPP ratio as a reasonable assumption in models of forest productivity for boreal and northern temperate forests. A similar conclusion is reached for the NPP/GPP ratio when published values of belowground NPP are considered. [ABSTRACT FROM AUTHOR]

Abstract: Proper management and accounting of forest carbon requires good knowledge of how disturbances and climate affect the carbon dynamics of different stand types. We have investigated such relationships by measuring, over a 5-year period (2003–2007), the net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (ER) of 26 forest sites in Canada using the eddy covariance technique. The study included black spruce, jack pine, Douglas-fir, aspen, boreal mixedwood and white pine forest ecosystems ranging in age from 1- to 153-years. The dataset included six chronosequences (one afforested plantation, three harvested and two burned). Following planting, the afforested white pine stands quickly became carbon sinks and offset initial carbon losses after 4years. Depending on forest type, the other forest stands were carbon sources for 10–18years following a disturbance, offset initial carbon losses after 19–47years, and showed net total gains ranging from 38 to 86MgCha−1 at 80years. Peak NEP ranged from 0.9 to 2.9MgCha−1 year−1 at ages of 35–55years except for the afforested white pine where it was 6.9MgCha−1 year−1 at 15–20years. Stepwise regression and Pearson correlation analyses indicated that the GEP and ER of mature stands (>70years old) were driven mainly by climate, while fluxes of young stands (<19years old) were driven by both leaf area index and climate. Although stand age of the afforested white pine plantations did not affect the GEP growing season lengths, the growing season length of the other forests increased with age until about 20years and this coincided with the switch from carbon source to sink. With the exception of the afforested white pine, peak GEP/ER ratios of the youngest sites occurred later in the growing season compared to older sites. The strong influence of stand age on the seasonal dynamics of GEP fluxes needs to be considered to avoid confounding the impacts of climate change with those of disturbance. These age-related seasonality effects are continental in scope and should be important in interpreting the time series of atmospheric CO2 concentration measurements at regional and global scales. [Copyright &y& Elsevier]