Search

Abstract: A long-term flux measurement station has been established in a 74-year-old mixedwood forest ecosystem, located approximately 80km west of Timmins in northern Ontario, as part of the Fluxnet-Canada Research Network (FCRN). Measurements of energy, water vapour, and carbon dioxide fluxes have been made continuously since August 2003 using the eddy covariance technique, along with ancillary meteorological variables. The spatial structure of the site was evaluated using a variety of sources and techniques, including remote sensing, showing that this forest is mixed but relatively homogeneous. The canopy top height is remarkably constant at between 30 and 32m. The basal area varies from 18 to 27m2 ha−1, and the aboveground biomass ranges from 82 to 122Mgha−1. In this paper, we summarize the diurnal and seasonal patters of carbon dioxide exchange and water loss from September 1, 2003 to August 31, 2004. Net ecosystem productivity (NEP) is strongly related to temperature. Atmospheric vapour pressure deficit (VPD) in this ecosystem exerted strong biophysical control on the daily gross ecosystem productivity (GEP) and evapotranspiration. Seasonal change in shortwave albedo, as a result of the presence of mixed deciduous and coniferous species, was clearly evident. Albedo changes were comparable to the seasonal pattern of NEP. The dormant season lasts more than 6 months of the year at this station. This forest was a moderate sink of carbon over the measurement period. Annual values of GEP, ecosystem respiration (R), and NEP were 1075, 919, and 156±35gCm−2, respectively. [Copyright &y& Elsevier]

Copyright of Canadian Journal of Forest Research is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: The spatial variability of the fraction of photosynthetically active radiation absorbed by the canopy (fPAR) was characterized for a heterogeneous boreal mixedwood forest site located in northern Ontario, Canada, based on relationships found between fPAR and light detection and ranging (lidar) data over different canopy architectures. Estimates of fPAR were derived from radiation measurements made above the canopy at a flux tower and below-canopy radiation was measured across a range of species compositions and canopy architectures. Airborne lidar data were used to characterize spatial variability of canopy structure around the flux tower and a map of mean canopy chlorophyll concentration was derived from airborne hyperspectral imagery. Different volumes of lidar points for the locations directly above each photosynthetically active radiation (PAR) sensor were examined to determine if there is an optimal method of relating lidar returns to estimated fPAR values. The strongest correlations between mean lidar height and fPAR occurred when using points that fell within a theoretical cone which originated at the PAR sensor having a solid angle α =55°. For diffuse conditions, the correlation (r) between mean lidar height versus fPAR×chlorophyll was stronger than between mean lidar height versus fPAR by 8% for mean daily fPAR and from 10 to 20% for diurnal fPAR, depending on solar zenith angle. For direct light conditions, the relationship was improved by 12% for mean daily fPAR and 12–41% for diurnal relationships. Linear regression models of mean daily fPAR×chlorophyll versus mean lidar height were used in conjunction with gridded lidar data and the canopy chlorophyll map to generate maps of mean daily fPAR for direct and diffuse sunlight conditions. Site average fPAR calculated from these maps was 0.79 for direct light conditions and 0.78 for diffuse conditions. When compared to point estimates of mean daily fPAR calculated on the tower, the average fPAR was significantly lower than the point estimate. Subtracting the direct sunlight fPAR map from the diffuse sunlight fPAR map revealed a distinct spatial pattern showing that areas with open canopies and relatively low chlorophyll (e.g., black spruce patches) have a higher fPAR under direct sunlight conditions, while closed canopies with higher chlorophyll (e.g., deciduous species) absorb more PAR under diffuse conditions. These findings have implications for scaling from point measurements at flux towers to larger resolution satellite imagery and addressing local scale heterogeneity in flux tower footprints. [Copyright &y& Elsevier]

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]

Abstract: Large-scale weather events such as the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and droughts are known to cause substantial interannual variation in the net ecosystem productivity (NEP) of tropical, temperate and boreal forests. Hypotheses for the impacts on NEP of changes in air temperature (T a) and precipitation associated with these events were tested at diurnal, seasonal and annual time scales using the terrestrial ecosystem model ecosys with measurements of CO2 and energy exchange from 1998 to 2006 at eddy covariance (EC) flux towers along a transcontinental transect of forest stands in the Fluxnet-Canada Research Network (FCRN). 1 [1] Now the Canada Carbon Project. These tests were supported at seasonal time scales by remotely-sensed vegetation indices, and at decadal time scales by wood growth increments from tree-ring and inventory studies. Collectively, results from this testing indicate that large-scale weather events during the study period caused spatially coherent changes in NEP, although these changes may vary with climate zone, species and topography. High T a episodes, such as occurred with greater frequency during ENSO/PDO events, adversely affected diurnal CO2 exchange of temperate and boreal conifers, but had little effect on that of a boreal deciduous forest. These contrasting responses of CO2 exchange to T a were attributed in the model to greater xylem resistance to water uptake in coniferous vs. deciduous trees. Sustained warming such as occurred during ENSO/PDO events extended the period of net C uptake and thus raised annual NEP at boreal coniferous and deciduous sites, but did not do so at a temperate coniferous site where annual NEP was reduced. However the rise in NEP of boreal conifers with warming was partially offset by the adverse effects of high T a on diurnal CO2 exchange, so that the rise in NEP with warming remained smaller than that at a boreal deciduous site. A 3-year drought during the study period adversely affected annual NEP of well-drained boreal deciduous forests but did not affect that of poorly-drained boreal conifers. This lack of effect was attributed in the model to low coniferous evapotranspiration rates and to subsurface water recharge. Drought effects on NEP were therefore largely determined by topography. These contrasting responses of different forest stands to warming and drought indicate divergent changes in forest growth with interannual changes in weather. Such divergent changes are consistent with the complex changes in forest NDVI and net C uptake observed over time in several large-scale remote-sensing studies. [Copyright &y& Elsevier]

Abstract: Fire and harvesting are major forest renewal processes in the Canadian boreal forest. The eddy covariance method was used to compare ecosystem fluxes of carbon dioxide between harvested and burned boreal forest sites in Saskatchewan, Canada. The harvest chronosequence had sites harvested in 2002 (HJP02), 1994 (HJP94) and 1975 (HJP75), whereas the fire chronosequence sites were burned in 1998 (F98), 1989 (F89), 1977 (F77) and 1929 (OJP). All sites were dominated by jack pine prior to the disturbance. During 2004 and 2005, net ecosystem production showed an average carbon gain (gCm−2 year−1) at F89=84, HJP75=80, HJP94=14 and OJP=20. The other sites lost carbon (gCm−2 year−1) at HJP02=−139, F98=−20, and F77=−58. Gross ecosystem production (GEP), ecosystem respiration (R e) and evapotranspiration tended to be greater at the burned sites than the harvested sites. The F89 and F77 sites had the strongest response of GEP to photosynthetically active radiation, and the strongest response of R e to soil temperature at the 2-cm depth. HJP02 had the weakest responses, followed by HJP94. This apparent greater ecosystem activity at the burned sites is likely caused by local differences in soil moisture and nutrients, differences in vegetation development, and differences in the decomposition of coarse woody debris. [Copyright &y& Elsevier]

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]

Abstract: Fire and harvesting are important disturbances in the boreal forest, driving net biome production. Measurements of net ecosystem production (NEP) over mature forest stands have been made from flux towers for about a decade at the Boreal Ecosystem Research and Monitoring Sites (BERMS) in central Saskatchewan, Canada. Over the last few years, the network of towers has been expanded to include stands that were recently disturbed by fire or harvesting. These new towers are part of Fluxnet-Canada, a national network that ties into the international Fluxnet network. Data from 2001 and 2002 show that forests burned in 1998 (F98) or harvested in 1994 (HJP94) were net carbon sources to the atmosphere with annual NEP values in the range of −50 to −130gCm−2 y−1. A site burned in 1989 (F89) was a carbon sink of about 68gCm−2 y−1 in 2002. NEP at a mature jack pine site (SOJP) ranged from 41 to −23gCm−2 y−1 in this period, whereas, an old black spruce site (SOBS) ranged from 68 to 21gCm−2 y−1. A mature aspen site (SOA) had the greatest NEP with 361gCm−2 y−1 in 2001 and 139gCm−2 y−1 in 2002, the contrast between years attributed to differences in spring temperature and leaf emergence, as well as a long-term drought at the site. Evapotranspiration (ET) followed similar patterns although the F89 site had the highest ET. The deciduous tree components of SOA and F89 are likely responsible for lower summer Bowen ratios with greater energy going into ET, compared to coniferous sites. These data indicate that there is still a broad gap in our knowledge of carbon fluxes to forest stands between about 10 and 50 years of age. This is especially true in understanding the dynamics of the growth of successional vegetation along with heterotrophic respiration, highlighted by predictions of more boreal fire disturbance in the future. [Copyright &y& Elsevier]