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−1year−1 at ages of 35–55years except for the afforested white pine where it was 6.9MgCha−1year−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.
Highlights
Anthropogenic greenhouse gas emissions have been increasing since the beginning of the industrial age and the resulting increase in atmospheric CO2 concentrations are widely believed to be changing the planet’s climate
Estimates of net ecosystem productivity (NEP) obtained from exponential curves fit to the data from subsets of this full dataset indicate that the number of years needed to reach the C compensation point, the amount of carbon lost during this period and the time it takes to offset initial C losses, all depend on the stand type (Table 3)
Negative correlations between growing season length and NEP have been reported for sub-alpine forests and linked to decreased water availability due to an earlier snow-melt (Hu et al, 2010; Sacks et al, 2007), we believe the negative correlations we found between SLGEP and gross ecosystem productivity (GEP) and ecosystem respiration (ER) were caused by the presence of different plant functional types in the analysis of mature stands
Summary
Anthropogenic greenhouse gas emissions have been increasing since the beginning of the industrial age and the resulting increase in atmospheric CO2 concentrations are widely believed to be changing the planet’s climate. Using forest inventory data and long-term ecosystem carbon studies, Pan et al (2011) concluded that boreal forests accounted for 21% (0.50 ± 0.08 Pg C year−1) of the overall global carbon sink from established forests (2.41 ± 0.42 Pg C year−1) between 1990 and 2007. It is well recognized that northern forests contain large amounts of carbon (C) in both biomass and soils (Kurz and Apps, 1995, 1999; Tarnocai et al, 2009) and these reservoirs may be even more vulnerable to future changes in climate. Since Canada contains 10% of the world’s forests, proper accounting and management of these large C stocks requires a solid scientific understanding of how disturbance and climate variability impact the emission and sequestration of carbon by these forests and how we might separate the effects of these two factors on regional C budgets
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