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This study presents a comparison and integration of three methods commonly used to estimate the amount of forest ecosystem carbon (C) available for storage. In particular, we examine the representation of living above- and below-ground biomass change (net accumulation) using plot-level biometry and repeat airborne laser scanning (ALS) of three dimensional forest plot structure. These are compared with cumulative net CO 2 fluxes (net ecosystem production, NEP) from eddy covariance (EC) over a six-year period within a jack pine chronosequence of four stands (~ 94, 30, 14 and 3 years since establishment from 2005) located in central Saskatchewan, Canada. Combining the results of the two methods yield valuable observations on the partitioning of C within ecosystems. Subtracting total living biomass C accumulation from NEP results in a residual that represents change in soil and litter C storage. When plotted against time for the stands investigated, the curve produced is analogous to the soil C dynamics described in Covington (1981). Here, ALS biomass accumulation exceeds EC-based NEP measured in young stands, with the residual declining with age as stands regenerate and litter decomposition stabilizes. During the 50–70 year age-period, NEP and live biomass accumulation come into balance, with the soil and litter pools of stands 70–100 years post-disturbance becoming a net store of C. Biomass accumulation was greater in 2008–2011 compared to 2005–2008, with the smallest increase in the 94-year-old “old jack pine” stand and greatest in the 14-year-old “harvested jack pine 1994” stand, with values of 1.4 (± 3.2) tC ha − 1 and 12.0 (± 1.6) tC ha − 1 , respectively. The efficiency with which CO 2 was stored in accumulated biomass was lowest in the youngest and oldest stands, but peaked during rapid regeneration following harvest (14-year-old stand). The analysis highlights that the primary source of uncertainty in the data integration workflow is in the calculation of biomass expansion factors, and this aspect of the workflow needs to be implemented with caution to avoid large error propagations. We suggest that the adoption of integrated ALS, in situ and atmospheric flux monitoring frameworks is needed to improve spatio-temporal partitioning of C balance components at sub-decadal scale within rapidly changing forest ecosystems and for use in national carbon accounting programs. [ABSTRACT FROM AUTHOR]

Abstract: The eddy-covariance method often underestimates fluxes under stable, low-wind conditions at night when turbulence is not well developed. The most common approach to resolve the problem of nighttime flux underestimation is to identify and remove the deficit periods using friction–velocity (u * ) threshold filters (u * Th ). This study modifies an accepted method for u * Th evaluation by incorporating change-point-detection techniques. The original and modified methods are evaluated at 38 sites as part of the North American Carbon Program (NACP) site-level synthesis. At most sites, the modified method produced u * Th estimates that were higher and less variable than the original method. It also provided an objective method to identify sites that lacked a u * Th response. The modified u * Th estimates were robust and comparable among years. Inter-annual u * Th differences were small, so that a single u * Th value was warranted at most sites. No variation in the u * Th was observed by time of day (dusk versus mid or late night), however, a few sites showed significant u * Th variation with time of year. Among-site variation in the u * Th was strongly related to canopy height and the mean annual nighttime u * . The modified u * Th estimates excluded a high fraction of nighttime data – 61% on average. However, the negative impact of the high exclusion rate on annual net ecosystem production (NEP) was small compared to the larger impact of underestimating the u * Th . Compared to the original method, the higher u * Th estimates from the modified method caused a mean 8% reduction in annual NEP across all site-years, and a mean 7% increase in total ecosystem respiration (R e ). The modified method also reduced the u * Th -related uncertainties in annual NEP and R e by more than 50%. These results support the use of u * Th filters as a pragmatic solution to a complex problem. [Copyright &y& Elsevier]

Abstract: The eddy-covariance method has become a cornerstone in the study of the surface energy and water balances, yet the oft-reported lack of energy-balance closure adds uncertainty to its measurements of evapotranspiration. This study explores the issue of closure based on 10 years of energy and water balance measurements at seven flux-tower sites in the southern boreal forests of western Canada. The sites include mature aspen, black spruce and jack pine forests, three young jack pine forests following harvesting, and one fen. Careful attention was paid to the measurement of evapotranspiration E, surface available energy (net radiation minus storage), precipitation P, and soil water storage S including root-zone soil water and water table elevation. Stand-level outflow O at the flux-tower sites was estimated annually as a residual term in the vertical water balance (O = P − E −ΔS), with and without energy-closure adjustments to E. Six of the flux towers were located within or near the gauged White Gull Creek watershed (603km2). The comparison of measured streamflow from the White Gull Creek with estimated outflow, scaled from the flux towers to the watershed, provides an independent test of the need for energy-closure adjustments to flux-tower E. Measured streamflow from the White Gull Creek watershed had a long-term mean (S.D.) of 101 (54)mmy−1 but was highly variable among years, ranging from 30mmy−1 during an extreme dry year to 204mmy−1 during an extreme wet year. Stand-level outflow, estimated with an energy-closure adjustment to E, varied by land-cover type, from a mean (S.D.) of 30 (35)mmy−1 at the mature aspen forest and 52 (125)mmy−1 at the fen to 108 (85)mmy−1 at the mature black spruce forest and 187 (55) and 250 (64)mmy−1 at the mature and young (harvested) jack pine forests, respectively. The dominant land-cover type, mature black spruce forest, typified the hydrology of the boreal forest landscape, with outflow similar to the watershed mean. The warm-season, eddy-covariance sensible and latent heat fluxes were 15±4% (annual mean±S.D.) smaller than surface available energy, with reasonable similarity among sites and years. When scaled from stand to watershed, the tower-based estimates of outflow agreed with measured streamflow only when outflow was calculated with an energy-closure adjustment to E. The annual means for the 2002–2009 hydrologic years were: 131mmy−1 (measured streamflow); 182mmy−1 (tower-based outflow calculated without energy-closure adjustments to E); and 112 and 127mmy−1 (tower-based outflow calculated with energy-closure adjustments to E from two different schemes). This result corroborates the need for energy-closure adjustments to eddy-covariance measurements of E, but it does not provide guidance about how the adjustments should be implemented. [Copyright &y& Elsevier]