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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 4 years. Depending on forest type, the other forest stands were carbon sources for 10–18 years following a disturbance, offset initial carbon losses after 19–47 years, and showed net total gains ranging from 38 to 86 Mg C ha−1 at 80 years. Peak NEP ranged from 0.9 to 2.9 Mg C ha−1 year−1 at ages of 35–55 years except for the afforested white pine where it was 6.9 Mg C ha−1 year−1 at 15–20 years. Stepwise regression and Pearson correlation analyses indicated that the GEP and ER of mature stands (>70 years old) were driven mainly by climate, while fluxes of young stands (

stage of the vegetation determines the radiative budget, energy balance partitioning, evapotranspiration and carbon dioxide flux. Here, we synthesize energy balance measurements from across the western boreal zone of North America as a function of stand age following fire. The data are from 22 sites in Alaska, Saskatchewan and Manitoba collected between 1998 and 2004 for a 150-year forest chronosequence. The summertime albedo immediately after a fire is about 0.05, increasing to about 0.12 for a period of about 30 years and then averaging about 0.08 for mature coniferous forests. A mature deciduous (aspen) forest has a higher summer albedo of about 0.16. Wintertime albedo decreases from a high of 0.7 for 5- to 30-year-old forests to about 0.2 for mature forests (deciduous and coniferous). Summer net radiation normalized to incoming solar radiation is lower in successional forests than in more mature forests by about 10%, except for the first 1–3 years after fire. This reduction in net radiative forcing is about 12–24 W m−2 as a daily average in summer (July). The summertime daily Bowen ratio exceeds 2 immediately after the fire, decreasing to about 0.5 for 15-year-old forests, with a wide range of 0.3–2 for mature forests depending on the forest type and soil water status. The magnitude of these changes is relatively large and may affect local, regional and perhaps global climates. Although fire has always determined stand renewal in these forests, increased future area burned could further alter the radiation balance and energy partitioning, causing a cooling feedback to counteract possible warming from carbon dioxide released by boreal fires.

Abstract: Light detection and ranging (lidar) is becoming an increasingly popular technology among scientists for the development of predictive models of forest biophysical variables. However, before this technology can be [...]

Abstract: Direct estimation of photosynthetic light-use efficiency (LUE) from space would be of significant benefit to LUE-based models which use inputs from remote sensing to estimate terrestrial productivity. The Photochemical Reflectance Index (PRI) has shown promise in tracking LUE at the leaf- to small canopy levels, but its use at regional to global scales still remains a challenge. In this study, we used different formulations of PRI calculated from the MODIS ocean band centered at 531 nm and a set of alternative reference bands at 488, 551, and 678 nm to explore the relationship between PRI and LUE where LUE was measured at eight eddy covariance flux towers located in the boreal forest of Saskatchewan, Canada. The magnitude and variability of LUE was significantly lower at the times when useful MODIS ocean band images were available (i.e. around midday under clear-sky conditions) relative to the rest of the growing season. PRI678 (reference band at 678 nm) showed the strongest relationship (r 2 =0.70) with LUE90a (i.e. 90-minute mean LUE calculated using Absorbed Photosynthetically Active Radiation, APAR), but only when all sites were combined. Overall, the relationships between the MODIS PRIs and LUE90a were always stronger for observations closer to the backscatter direction and there were no significant differences in the strength of the correlations whether LUE was calculated based on incident PAR or on APAR. Predictions of ecosystem photosynthesis at the time of the MODIS overpasses were significantly improved by multiplying either PAR or APAR by MODIS PRI (r 2 improved from 0.09 to 0.44 and 0.54 depending on the PRI formulation). We used our PRI-LUE model to create a regional LUE90a map for the three cover types covering 47,500 km2 around the flux sites. The MODIS PRI-derived LUE90a map appeared to capture more realistic spatial heterogeneity of LUE across the landscape compared to a daily LUE map derived using the look-up table in the MODIS GPP (MOD17) algorithm. While our LUE map is only a snapshot of minimum regional LUE90a values, with appropriate gap-filling methods it could be used to improve regional-scale monitoring of GPP. Moreover, the strong relationship between midday and daily LUE on clear days (r 2 =0.93) indicates that instantaneous MODIS observations of LUE90a could be used to estimate daily LUE. Finally, pixel shadow fraction from the 5-Scale geometric-optical model was closely related to both MODIS PRI and tower-derived LUE suggesting that differences in stand leaf area and in diffuse illumination among flux sites play a role in the relationship we observed between LUE and MODIS PRI. [Copyright &y& Elsevier]

Abstract: Closure of the surface energy balance provides an objective criterion for evaluating eddy-covariance (EC) flux measurements. This study analyses 5 years of EC carbon dioxide, water vapor, and sensible heat flux measurements from three mature boreal forest stands in central Saskatchewan, Canada. The EC sensible and latent heat fluxes, H and λE, underestimated the surface available energy by 11% (aspen), 15% (black spruce), and 14% (jack pine). At all sites, the energy-closure fraction CF responded similarly to the friction velocity u *, atmospheric stability, and time of day. At night, CF increased from ∼0.3 at very low-u * to an asymptotic maximum of ∼0.9 at u * above 0.35ms−1. During unstable-daytime periods, CF varied linearly from ∼0.7 at low-u * to ∼1.0 at high-u *. The energy imbalance pattern was similar among sites and may be characteristic of the continental, boreal forest. EC measurements of net ecosystem exchange F NEE have no objective, diagnostic parameter that is equivalent to CF. We therefore derived an analogous F NEE “closure fraction” CFNEE by normalizing measured F NEE against estimates from an empirical model that was tuned to the high-u * data. CF and CFNEE responded similarly to u *, atmospheric stability, and time of day. We discuss two implications for EC flux data post-processing. The results uphold the common practice of rejecting EC measurements during low-u * periods. They also lend support to the application of energy-closure adjustments to H, λE, and F NEE. [Copyright &y& Elsevier]