Boreal Forest Of Central Canada Research Articles

Foliar nutrient resorption dynamics of trembling aspen and white birch during secondary succession in the boreal forest of central Canada

Nutrient resorption is a key strategy in plant conservation, which minimizes nutrient loss and enhances productivity. However, the effects of stand development and the neighbourhood species diversity on the nutrient resorption of boreal tree species remain unclear. Using a detailed, replicated chronosequence spanning 7, 15, 33, 98, 146, and 209 years following fire, we examined the dynamics of leaf nitrogen (NRE), phosphorus (PRE), and potassium (KRE) resorption efficiencies of two common broadleaved boreal tree species (Populus tremuloides and Betula papyrifera) associated with stand development and overstory composition type (monoculture and mixedwood) in a central boreal forest of Canada. We found that the leaf NRE did not change; however, the PRE and KRE decreased with overstory succession. Further, we observed a higher leaf PRE in younger mixed stands, but a lower leaf PRE in older mixed stands, compared to monocultures for Populus tremuloides. Our results also revealed that the leaf NRE/PRE and NRE/KRE ratios increased with overstory succession, particularly in mixed stands, which demonstrated strong N limitations in old mixed boreal forests. Moreover, the overstory succession- and type-dependent dynamics of nutrient resorption efficiencies were driven by soil nutrient changes and relative tree growth rates associated with stand development. Nutrient resorption efficiency patterns may imply a transition from more P and K limitations in early succession, to more N limitations in late succession. Our findings highlight the need for modelling long-term primary productivity to account for overstory succession- and type-specific changes in nutrient resorption efficiencies.

Role of Forests of the Volga River Basin in the Mitigation of Climate Fluctuations and of Forthcoming global Warming (Predictive Empirical-Statistical Modeling)

In this article, the authors propose a predictive landscape-ecological analysis of the forest cover of the Volga basin, which highlights the problem of adsorption of greenhouse gases, which is included in the list of tasks set the Paris Climate Change Agreement (2015). As a result of the study, the authors established the adsorption potential of indigenous and derived boreal and nemoral forests, assessed their ability to mitigate climate change, including reducing anthropogenic warming. A quantitative assessment was also made of the loss of these ecological resources by the forests of the Volga basin since the beginning of intensive forest and land use in it. We have identified contrasting changes in the ecological resources of boreal and nemoral forests during their structural transformation in the course of global warming. We show that the process of thermal-arid transformation of forest ecosystems leads to a general decrease in the positive carbon balance in most groups of forest formations. The verification of the predicted calculations of the carbon balance, carried out by remote and ground measurements in the boreal forests of Central Canada in the first decade of modern warming, gave positive results.

Water availability regulates tree mixture effects on total and heterotrophic soil respiration: A three‐year field experiment

Soil respiration is of critical importance to the energy balance and element cycling of terrestrial ecosystems. Biodiversity loss and changes in rainfall regimes are predicted to increase, and in so doing have the potential to affect soil C cycling. However, uncertainties remain concerning the effects of tree diversity on soil respiration, and how these effects might be modified by altered water availability. Here, we measured the total soil respiration (RS) and heterotrophic respiration (RH) rates during the 2017 to 2019 growing seasons in stands dominated by Populus tremuloides, Pinus banksiana, and their relatively even mixtures under reduced (−25% throughfall), ambient, and added (+25% throughfall) precipitation in a boreal forest of Central Canada. We found that the RS and RH were 28% and 26% greater, respectively, in the mixtures than expected from those of monocultures that received ambient water availability, while the RH/RS were unaffected by tree mixtures. On average, the addition of water increased Rs and RH by 19%, and 10%, respectively, with more pronounced increases of Rs in tree mixtures, which resulted in 55% higher Rs in mixtures relative to monocultures. In contrast, water reduction did not affect the Rs but decreased RH in mixtures. Additionally, the effects of mixtures on RH increased over the three-year duration of the experiments. Both the Rs and RH decreased, while the RH/Rs increased with higher soil fungal to bacterial ratios. Our results revealed that plant species mixtures and water availability interactively influenced soil respiration under natural conditions. We provided novel experimental insights, which indicated that the establishment of diverse forests can increase soil C cycling over time, particularly under increased water availability.

Wildfire combustion and carbon stocks in the southern Canadian boreal forest: Implications for a warming world.

Boreal wildfires are increasing in intensity, extent, and frequency, potentially intensifying carbon emissions and transitioning the region from a globally significant carbon sink to a source. The productive southern boreal forests of central Canada already experience relatively high frequencies of fire, and as such may serve as an analog of future carbon dynamics for more northern forests. Fire-carbon dynamics in southern boreal systems are relatively understudied, with limited investigation into the drivers of pre-fire carbon stocks or subsequent combustion. As part of NASA's Arctic-Boreal Vulnerability Experiment, we sampled 79 stands (47 burned, 32 unburned) throughout central Saskatchewan to characterize above- and belowground carbon stocks and combustion rates in relation to historical land use, vegetation characteristics, and geophysical attributes. We found southern boreal forests emitted an average of 3.3±1.1kgC/m2 from field sites. The emissions from southern boreal stands varied as a function of stand age, fire weather conditions, ecozone, and soil moisture class. Sites affected by historical timber harvesting had greater combustion rates due to faster carbon stock recovery rates than sites recovering from wildfire events, indicating that different boreal forest land use practices can generate divergent carbon legacy effects. We estimate the 2015 fire season in Saskatchewan emitted a total of 36.3±15.0TgC, emphasizing the importance of southern boreal fires for regional carbon budgets. Using the southern boreal as an analog, the northern boreal may undergo fundamental shifts in forest structure and carbon dynamics, becoming dominated by stands <70years old that hold 2-7kgC/m2 less than current mature northern boreal stands. Our latitudinal approach reinforces previous studies showing that northern boreal stands are at a high risk of holding less carbon under changing disturbance conditions.

Using machine learning to synthesize spatiotemporal data for modelling DBH-height and DBH-height-age relationships in boreal forests

Sustainable forest management requires the ability to accurately model forest dynamics under a changing environment, which is difficult using conventional statistical methods as many factors that interactively affect forest growth must be considered. As well, statistical model development is often limited by the lack of broad-scale repeated forest measurements needed to capture changes in 1 or more variables and the corresponding changes in forest dynamics (e.g., growth in diameter and height), while assuming other variables do not change, or their changes do not significantly affect the forest dynamics of interest. In many forested countries, comprehensive monitoring programs have amassed large amounts of diverse forest measurement data. Here we propose a new approach for using artificial neural network-based machine learning to synthesize spatiotemporal tree measurement data collected over a vast area of boreal forest in central Canada to model diameter at breast height (DBH)-height and DBH-height-age relationships for 6 dominant tree species. More than 30 potentially important stand structure, site, and climate variables were considered. We used an individual-based modelling approach by considering each individual tree measurement as an instance of the complex relationships modelled; together, broad-scale long-term monitoring data contain many such instances, representing considerable spatial and temporal scale variation in forest growth and growing conditions. Using this approach, we significantly improved DBH-height and DBH-height-age models. And the models developed allowed us to analyze the effects of environmental conditions or changes in these conditions on forest growth. This may be the first attempt at applying this type of approach, which can be used to more accurately model, for example, forest growth, mortality, and how they are affected by changing climate in a variety of forest types.

The effect of species diversity on tree growth varies during forest succession in the boreal forest of central Canada

Although major advances have demonstrated that species diversity has a general positive effect on forest ecosystem productivity, some studies report negligible or even negative effects, highlighting remaining uncertainty in our knowledge of the ecological mechanisms that influence diversity–productivity relationships. In particular, ecological succession is postulated to drive temporal shifts in the strength and direction of diversity–productivity relationships, but few studies have explicitly tested this hypothesis because long-term succession data (from forest initiation to eventual climax) are rare.Using a detailed, replicated chronosequence (space-for-time substitution) study design of 53 natural forest stands (ages 8 to 210 years) in the boreal forests of central Canada, we investigated the relationship between neighbourhood species diversity and tree growth of five dominant boreal tree species, covering entire, long-term secondary successional sequences following stand-replacing wildfire.We found compelling evidence that the strength of the relationship between species diversity and tree growth changes over the course of secondary succession, following a general “hump-shaped” pattern, with mid-succession stages of higher functional diversity exhibiting the strongest growth–diversity relationships. However, tree species exhibited individualistic responses to succession-driven changes in species diversity, with broadleaf species (e.g., Populus tremuloides) generally showing negative responses, whereas conifers (e.g., Pinus banksiana) responded more favorably to higher neighbourhood diversity. Furthermore, our results show the effect of individual tree size on the relationship between species diversity and tree growth to be highly variable, contradicting the hypothesis that larger trees benefit more from complementarity due to size-asymmetric competitive ability. These results contribute to disentangling the mechanisms that link species diversity to forest growth and function, which is important to sustainable forest management planning and for predicting the consequences of global biodiversity loss, especially for the boreal forest, which plays a critical role in controlling global carbon flux and climate.

Carbon Storage Declines in Old Boreal Forests Irrespective of Succession Pathway

The boreal forest plays a critical role in regulating global atmospheric carbon dioxide and is highly influenced by wildfire. However, the long-term recovery of forest carbon (C) storage following wildfire remains unclear, especially during late succession. Uncertainty surrounding C storage in old forests largely stems from both a lack of repeated measurements in forest stands older than the longevity of the pioneer cohort and a lack of consideration of multiple succession pathways. In this study, we constructed a replicated chronosequence, which covered a wide range of forest stand age classes (up to 210 years old) following fire in the boreal forest of central Canada. We selected stands of different overstorey types (that is, broadleaf, conifer, or mixedwood) and age classes to account for multiple succession pathways known to occur in our study area. Our results show a strong relationship between total ecosystem C storage and stand age following fire. Broadleaf stands had on average higher total ecosystem C; however, the inferred temporal dynamics of total ecosystem C were similar among all three overstorey types. Importantly, we found that total ecosystem C storage declined from canopy transition to late-succession stages, irrespective of succession pathway, contradicting views that old forests continually accumulate C as they age. Our findings emphasize that further study of stands older than the longevity of the pioneer cohort is critical to better understand the contribution of old forests to the global C cycle.

Response of saproxylic insect communities to logging history, tree species, stage of decay, and wood posture in the central Nearctic boreal forest

Saproxylic insect assemblages are essential functional components of forest ecosystems that can be affected by forest management. We used a split-plot ANOVA design to analyze differences in selected saproxylic insects (all arthropod orders and dipteran and parasitic hymenopteran families) emerging from dead wood of sites with different logging histories (horse-logged, mechanically-logged and unlogged), tree species (Populus and Picea), stage of decay (early- and late-decay stages) and posture (standing and downed logs) in the boreal forest of central Canada. No clear effects of logging history were seen for the studied taxa; however, interaction between logging history and other dead wood features was apparent. Cecidomyiidae consistently emerged more from Populus than from Picea dead wood. Most of the studied saproxylic families were more abundant in late-decay than in early-decay wood. Dipterans of the Cecidomyiidae, Ceratopogonidae, Empididae, Mycetophilidae and Sciaridae families, and hymenopterans of the Diapriidae and Ichneumonidae families were significantly more abundant in downed than in standing dead wood. In contrast, Mymaridae was most abundant in standing dead wood. Our study provides evidence that some insects at high taxonomic levels respond differently to dead wood quality, and this could inform future management strategies in the boreal forest for the conservation of saproxylic fauna and their ecological functions.

Soil C:N:P dynamics during secondary succession following fire in the boreal forest of central Canada

Measures of soil nutrient availability such as concentrations of nitrogen and phosphorus ([N] and [P]) are important indicators of terrestrial productivity. Optimal plant growth and ecosystem functioning are also strongly correlated with nutrient ratios in soils. Long-term trends in soil [C], [N], [P], and their stoichiometric ratios during secondary succession in the fire-driven boreal forest remain unclear. We used replicated 7- to 209-year chronosequences to examine the influence of stand age and overstory composition on [C], [N], and [P] and their ratios in the forest floor, surface (0–15cm), and subsurface (15–30cm) mineral soil in the boreal forest of central Canada. In the forest floor, [C] and [N] increased rapidly during the first three decades following fire, after which they fluctuated, but remained larger than in the youngest stands. Surface soil [C] and [N] increased from young to intermediate-aged stands, but decreased in the oldest stands. Subsurface [C] and [N] followed a similar trend, but was higher in the 7-year-old stands. Forest floor [P] followed a gradual, linear increase throughout stand development but of smaller magnitude than [C] and [N]. The temporal pattern of [P] in both mineral soil layers was lower in the 33-year-old stands of all overstory types, suggesting that P resources may be outpaced by their demand during this highly competitive stage of advanced stem exclusion during forest succession. In the forest floor and surface soil, C:N differed not only with stand age but also with overstory type, due to higher C:N in intermediate-aged conifer stands. Forest floor C:P and N:P were higher in the 33- to 146-year-old stands in all overstory types and soil layers, but particularly so in the conifer stands. In the mineral layers, C:N, C:P, and N:P all followed a similar trend. Our results demonstrate the important influence of stand age, overstory composition (as influenced by succession), and soil depth on forest soil nutrient dynamics in boreal forest ecosystems.

Recovery of Ecosystem Carbon Stocks in Young Boreal Forests: A Comparison of Harvesting and Wildfire Disturbance

Corresponding with the increasing global resource demand, harvesting now affects millions of hectares of boreal forest each year, and yet our understanding of harvesting impacts on boreal carbon (C) dynamics relative to wildfire remains unclear. We provide a direct comparison of C stocks following clearcut harvesting and fire over a 27-year chronosequence in the boreal forest of central Canada. Whereas many past studies have lacked measurement of all major C pools, we attempt to provide complete C pool coverage, including live biomass, deadwood, forest floor, and mineral soil C pools. The relative contribution of each C pool to total ecosystem C varied considerably between disturbance types. Live biomass C was significantly higher following harvesting compared with fire because of residual live trees and advanced regeneration. Conversely, most live biomass was killed following fire, and thus post-fire stands contained higher stocks of deadwood C. Snag and stump C mass peaked immediately following fire, but dramatically decreased 8 years after fire as dead trees began to fall over, contributing to the downed woody debris C pool. Forest floor C mass was substantially lower shortly after fire than harvesting, but this pool converged 8 years after fire and harvesting. When total ecosystem C stocks were examined, we found no significant difference during early stand development between harvesting and fire. Maximum total ecosystem C occurred at age 27 years, 185.1 ± 18.2 and 163.6 ± 8.0 Mg C ha−1 for harvesting and fire, respectively. Our results indicate strong differences in individual C pools, but similar total ecosystem C after fire and clearcutting in boreal forests, and shall help improve modeling terrestrial C flux after stand-replacing disturbances.

Decline in Net Ecosystem Productivity Following Canopy Transition to Late-Succession Forests

Boreal forests are critical to the global carbon (C) cycle. Despite recent advances in our understanding of boreal C budgets, C dynamics during compositional transition to late-succession forests remain unclear. Using a carefully replicated 203-year chronosequence, we examined long-term patterns of forest C stocks and net ecosystem productivity (NEP) following stand-replacing fire in the boreal forest of central Canada. We measured all C pools, including understorey vegetation, belowground biomass, and soil C, which are often missing from C budgets. We found a slight decrease in total ecosystem C stocks during early stand initiation, between 1 and 8 years after fire, at -0.90 Mg C ha -1 y -1 .A s stands regenerated, live vegetation biomass increased rapidly, with total ecosystem C stocks reaching a maximum of 287.72 Mg C ha -1 92 years after fire. Total ecosystem C mass then decreased in the 140- and 203year-old stands, losing between -0.50 and -0.74 Mg Ch a -1 y -1 , contrasting with views that old-growth forests continue to maintain a positive C balance. The C decline corresponded with canopy transition from dominance of Populus tremuloides, Pinus banksiana ,a nd Picea mariana in the 92-year-old stands to Betula papyrifera, Picea glauca ,a ndAbies balsamea in the 203-yearold stands. Results from this study highlight the role of succession in long-term forest C dynamics and its importance when modeling terrestrial C flux.

Deadwood Density of Five Boreal Tree Species in Relation to Field-Assigned Decay Class

Aboveground deadwood, consisting of downed woody debris (DWD), snags, and stumps, is an important component of boreal forest ecosystem structure. Accurate deadwood density estimates are essential for evaluating ecosystem biomass and carbon stocks. The objective of this study was to examine the relationships between deadwood density, tree species, and decay status, identified in the field by morphological characteristics. We sampled DWD, snags, and stumps of trembling aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera Marsh.), jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana [Mill.] B.S.P.), and balsam fir (Abies balsamea [L.] Mill.) in the boreal forest of central Canada. A total of 240 samples (99 DWD, 94 snags, and 47 stumps) were collected. Decay class and tree species explained >80% of the variation in wood density of all deadwood types. Wood density decreased consistently from the lowest to the highest decay class. Tree species identity was also important in determining the relationships between wood density and field-assigned decay class for snags and stumps, but not for DWD, probably because of the five-class system used for DWD, rather than the three-class system used for snags and stumps. These results indicate that decay class and tree species are adequate predictors of deadwood density.

Stand age structural dynamics of conifer, mixedwood, and hardwood stands in the boreal forest of central Canada

To study the effects of stand development and overstory composition on stand age structure, we sampled 32 stands representing conifer, mixedwood, and hardwood stand types, ranging in ages from 72 to 201 years on upland mesic sites in northwestern Ontario. We defined the stages of stand development as: stem exclusion/canopy transition, canopy transition, canopy transition/gap dynamics, and gap dynamics. Stand age structure of conifer stands changed from bimodal, bimodal, reverse-J, and bimodal, respectively, through the stages of stand development. Mixedwood and hardwood stands revealed similar trends, with the exception of missing the canopy transition/gap dynamic stage in mixedwoods. Canopy transition/gap dynamic stage in hardwoods showed a weaker reverse-J distribution than their conifer counterparts. The results suggest that forest management activities such as partial and selection harvesting and seed-tree systems may diversify standard landscape-level age structures and benefit wildlife, hasten the onset of old-growth, and create desired stand age structures. We also recommend that the determination of old-growth using the following criteria in the boreal forest: 1) canopy breakdown of pioneering cohort is complete and stand is dominated by later successional tree species, and 2) stand age structure is bimodal, with dominating canopy trees that fall within a relatively narrow range of age and height classes and a significant amount of understory regeneration.

Attributing uncertainties in simulated biospheric carbon fluxes to different error sources

[1] Estimating the current sources and sinks of carbon and projecting future levels of CO2 and climate require biospheric carbon models that cover the landscape. Such models inevitably suffer from deficiencies and uncertainties. This paper addresses how to quantify errors in modeled carbon fluxes and then trace them to specific input variables. To date, few studies have examined uncertainties in biospheric models in a quantitative fashion that are relevant to landscape-scale simulations. In this paper, we introduce a general framework to quantify errors in biospheric carbon models that “unmix” the contributions to the total uncertainty in simulated carbon fluxes and attribute the error to different variables. To illustrate this framework we apply and use a simple biospheric model, the Vegetation Photosynthesis and Respiration Model (VPRM), in boreal forests of central Canada, using eddy covariance flux measurement data from two main sites of the Canadian Carbon Program (CCP). We explicitly distinguish between systematic errors (“biases”) and random errors and focus on the impact of errors present in biospheric parameters as well as driver data sets (satellite indices, temperature, solar radiation, and land cover). Biases in downward shortwave radiation accumulated to the most significant amount out of the driver data sets and accounted for a significant percentage of the annually summed carbon uptake. However, the largest cumulative errors were shown to stem from biospheric parameters controlling the light-use efficiency and respiration-temperature relationships. This work represents a step toward a carbon model-data fusion system because in such systems the outcome is determined as much by uncertainties as by the measurements themselves.

Debating the greening vs. browning of the North American boreal forest: differences between satellite datasets

AbstractA number of remote sensing studies have evaluated the temporal trends of the normalized difference vegetation index (NDVI or vegetation greenness) in the North American boreal forest during the last two decades, often getting quite different results. To examine the effect that the use of different datasets might be having on the estimated trends, we compared the temporal trends of recently burned and unburned sites of boreal forest in central Canada calculated from two datasets: the Global Inventory, Monitoring, and Modeling Studies (GIMMS), which is the most commonly used 8 km dataset, and a new 1 km dataset developed by the Canadian Centre for Remote Sensing (CCRS). We compared the NDVI trends of both datasets along a fire severity gradient in order to evaluate the variance in regeneration rates. Temporal trends were calculated using the seasonal Mann–Kendall trend test, a rank‐based, nonparametric test, which is robust against seasonality, nonnormality, heteroscedasticity, missing values, and serial dependence. The results showed contrasting NDVI trends between the CCRS and the GIMMS datasets. The CCRS dataset showed NDVI increases in all recently burned sites and in 50% of the unburned sites. Surprisingly, the GIMMS dataset did not capture the NDVI recovery in most burned sites and even showed NDVI declines in some burned sites one decade after fire. Between 50% and 75% of GIMMS pixels showed NDVI decreases in the unburned forest compared with &lt;1% of CCRS pixels. Being the most broadly used dataset for monitoring ecosystem and carbon balance changes, the bias towards negative trends in the GIMMS dataset in the North American boreal forest has broad implications for the evaluation of vegetation and carbon dynamics in this region and globally.

Effects of time since stand-replacing fire and overstory composition on live-tree structural diversity in the boreal forest of central Canada

Stand structure diversity is hypothesized (i) to increase with stand development and (ii) to be greater in mixedwood stands than in conifer and broadleaf stands. We examined the effects of time since stand-replacing fire (TSF) and overstory type on stand volume, stand density, and tree-size variability, which is measured using Shannon’s diversity index (H′) and coefficient of variation, in fire-origin boreal forest stands. We sampled 36 stands representing conifer, mixedwood, and broadleaf overstory types, ranging in ages from 72 to 201 years TSF on upland mesic sites in northwestern Ontario, Canada. Stand volume decreased in older mixedwood and broadleaf stands, but followed a U-shaped pattern in conifer stands with TSF. Diameter-at-breast-height-based H′ followed an inverse U-shaped pattern with TSF for all overstory types. Height-based H′ decreased with TSF in conifer and mixedwood stands but peaked at the intermediate age class in broadleaf stands. Diameter-at-breast-height- and height-based coefficient of variation indices followed an inverse U-shaped distribution with TSF. Our results partially supported the two hypotheses, as (i) the 124- to 139-year-old stands were most diverse and (ii) mixedwood stands were more than or as equally diverse as conifer and broadleaf stands, depending on stand development stage and the diversity indices used.

Asymmetrical Reinforcement and Wolbachia Infection in Drosophila

Reinforcement refers to the evolution of increased mating discrimination against heterospecific individuals in zones of geographic overlap and can be considered a final stage in the speciation process. One the factors that may affect reinforcement is the degree to which hybrid matings result in the permanent loss of genes from a species' gene pool. Matings between females of Drosophila subquinaria and males of D. recens result in high levels of offspring mortality, due to interspecific cytoplasmic incompatibility caused by Wolbachia infection of D. recens. Such hybrid inviability is not manifested in matings between D. recens females and D. subquinaria males. Here we ask whether the asymmetrical hybrid inviability is associated with a corresponding asymmetry in the level of reinforcement. The geographic ranges of D. recens and D. subquinaria were found to overlap across a broad belt of boreal forest in central Canada. Females of D. subquinaria from the zone of sympatry exhibit much stronger levels of discrimination against males of D. recens than do females from allopatric populations. In contrast, such reproductive character displacement is not evident in D. recens, consistent with the expected effects of unidirectional cytoplasmic incompatibility. Furthermore, there is substantial behavioral isolation within D. subquinaria, because females from populations sympatric with D. recens discriminate against allopatric conspecific males, whereas females from populations allopatric with D. recens show no discrimination against any conspecific males. Patterns of general genetic differentiation among populations are not consistent with patterns of behavioral discrimination, which suggests that the behavioral isolation within D. subquinaria results from selection against mating with Wolbachia-infected D. recens. Interspecific cytoplasmic incompatibility may contribute not only to post-mating isolation, an effect already widely recognized, but also to reinforcement, particularly in the uninfected species. The resulting reproductive character displacement not only increases behavioral isolation from the Wolbachia-infected species, but may also lead to behavioral isolation between populations of the uninfected species. Given the widespread occurrence of Wolbachia among insects, it thus appears that there are multiple ways by which these endosymbionts may directly and indirectly contribute to reproductive isolation and speciation.

Methods of estimating CO 2, latent heat and sensible heat fluxes from estimates of land cover fractions in the flux footprint

We present a description of the process of estimating surface fluxes of CO 2, latent heat and sensible heat from estimates of fractions of satellite-based land cover types in the flux footprint. The study is conducted at two heterogeneous sites in the boreal forest of Central Canada. Using a Twin Otter aircraft, fluxes were measured in a grid pattern during three Intensive Field Campaigns (IFCs) and Landsat thematic mapper data were used for land cover classification. Using a footprint function developed from tracer gas release experiments in the boreal forest, the fractions of cover types within the footprint were determined, and used in a regression analysis against observed fluxes. The results showed that the surface cover types within the flux footprint accounted for about 90% of the variations in the measured airborne fluxes of CO 2, sensible heat and latent heat, at two different study sites. The attempted validation of the regression models, by comparing flux estimates over regional transects outside the grid area for which the regression model had been developed or over site-specific runs within the grid area against observed fluxes, based on fractional distributions of surface cover types, were encouraging. They indicate the potential for extrapolating models developed for a given location to another location, based simply on the fractions of cover types, at least for similar land cover types.

Comparison of boreal ecosystem model sensitivity to variability in climate and forest site parameters

Ecosystem models are useful tools for evaluating environmental controls on carbon and water cycles under past or future conditions. In this paper we compare annual carbon and water fluxes from nine boreal spruce forest ecosystem models in a series of sensitivity simulations. For each comparison, a single climate driver or forest site parameter was altered in a separate sensitivity run. Driver and parameter changes were prescribed principally to be large enough to identify and isolate any major differences in model responses, while also remaining within the range of variability that the boreal forest biome may be exposed to over a time period of several decades. The models simulated plant production, autotrophic and heterotrophic respiration, and evapotranspiration (ET) for a black spruce site in the boreal forest of central Canada (56°N). Results revealed that there were common model responses in gross primary production, plant respiration, and ET fluxes to prescribed changes in air temperature or surface irradiance and to decreased precipitation amounts. The models were also similar in their responses to variations in canopy leaf area, leaf nitrogen content, and surface organic layer thickness. The models had different sensitivities to certain parameters, namely the net primary production response to increased CO2 levels, and the response of soil microbial respiration to precipitation inputs and soil wetness. These differences can be explained by the type (or absence) of photosynthesis‐CO2 response curves in the models and by response algorithms of litter and humus decomposition to drying effects in organic soils of the boreal spruce ecosystem. Differences in the couplings of photosynthesis and soil respiration to nitrogen availability may also explain divergent model responses. Sensitivity comparisons imply that past conditions of the ecosystem represented in the models' initial standing wood and soil carbon pools, including historical climate patterns and the time since the last major disturbance, can be as important as potential climatic changes to prediction of the annual ecosystem carbon balance in this boreal spruce forest.

Mapping net primary production and related biophysical variables with remote sensing: Application to the BOREAS region

Maps of net and gross primary production, autotrophic respiration, biomass, and other biophysical variables were generated for 106 km2 of boreal forest in central Canada (the Boreal Ecosystem‐Atmosphere (BOREAS) region) using a production efficiency model (PEM) driven with remotely sensed observations at 1 km2 spatial resolution. The PEM was based on carbon yields of absorbed photosynthetically active radiation for both gross and net primary production (GPP and NPP), accounting for environmental stress and autotrophic respiration (Ra). Physiological control was modeled using remotely sensed maps of air temperature, vapor pressure deficit, and soil moisture. The accuracy of the inferred variables was generally within 10–30% of point measurements at the surface and independent model results (both at the stand level). Biomass maps were derived from visible reflectance measurements and were also compared to independently derived maps. Area‐averaged GPP was 604 g C m−2 yr−1 compared with average canopy respiration of 428 g C m−2 yr−1 and NPP of 235 g C m−2 yr−1. Net annual carbon uptake in net primary production for the region totaled 175 teragrams. Canopy carbon exchange (GPP and Ra) differed widely between land cover types even though the model does not use land cover information. Extensive areas of the least productive cover types (e.g., lowland needleleaf species) accounted for the greatest amount of NPP.