Forest Of Central Canada Research Articles

Predictive modelling of boreal forest resources in regulation of the carbon cycle and mitigation of the global warming

It is described that the ecological resources of forests is their ability to absorb the greenhouse gases and accordingly to mitigate the climate fluctuations. A forecast empirical-statistical analysis of carbon cycle regulation by the forest cover of the Volga River basin is presented. The adsorption potential of primary and derivatives of boreal and nemoral forests has been established, their ability to mitigate climate changes, including reducing anthropogenic warming, has been evaluated. The losses of ecological resources in forests since the beginning of intensive forest and land use in them were quantitatively assessed. Thermo-arid transformation of forest ecosystems leads to the general decrease in positive carbon balance in most groups of forest formations. Verification of the forecast calculations of the carbon balance was carried out using measurements in the forests of Central Canada. A numerical experiment was executed to assess the effect of the elastic stability of forest formations on their carbon balance.

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.

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.

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 <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.

Vegetation of the Continental Northwest Territories at 6 ka BP

Pollen records are used to reconstruct vegetation in the continental Northwest Territories at 6 ka (6000 14C yr BP). Picea glauca, P mariana, Larix laricina, Populus tremuloides, P. balsamifera, Alnus crispa and A. incana were present throughout their modern ranges in the Boreal and Subarctic Forest Zones by 6000 BP. Pinus banksiana, however, had not yet reached its present northern limits. Population densities of the dominant trees, Picea glauca and Picea mariana, were close to, or as high as, present. In the Mackenzie Delta region the range limit of Picea glauca was approximately 25 km north of its modern location just prior to 6000 BP. In contrast, the northern limits of the forest in central Canada were similar to present. The tundra vegetation close to the edge of the forest was similar to modern Low Arctic Tundra. Development of extensive Sphagnum peatlands had begun in the forested areas and the adjacent Low Arctic Tundra. Palaeoecological information regarding vegetation at 6000 BP remains lacking for the northeastern half of the study area. Therefore, the nature of the vegetation in much of the area now occupied by Low Arctic and Middle Arctic Tundra remains unknown. Important vegetation changes that occurred following 6 ka include : (1) the advance of Pinus banksiana to its present northern range limits, (2) the retreat of the northern range limits of Picea glauca in the Mackenzie Delta region between 6000 and 3500 BP and (3) the rapid and marked increase in the population density of Picea mariana in the treeline zone of the central Northwest Territories at 5000 BP followed by a decline at 4000 BP.

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.

Boreal forest CO2 exchange and evapotranspiration predicted by nine ecosystem process models: Intermodel comparisons and relationships to field measurements

Nine ecosystem process models were used to predict CO2 and water vapor exchanges by a 150‐year‐old black spruce forest in central Canada during 1994–1996 to evaluate and improve the models. Three models had hourly time steps, five had daily time steps, and one had monthly time steps. Model input included site ecosystem characteristics and meteorology. Model predictions were compared to eddy covariance (EC) measurements of whole‐ecosystem CO2 exchange and evapotranspiration, to chamber measurements of nighttime moss‐surface CO2 release, and to ground‐based estimates of annual gross primary production, net primary production, net ecosystem production (NEP), plant respiration, and decomposition. Model‐model differences were apparent for all variables. Model‐measurement agreement was good in some cases but poor in others. Modeled annual NEP ranged from −11 g C m−2 (weak CO2 source) to 85 g C m−2 (moderate CO2 sink). The models generally predicted greater annual CO2 sink activity than measured by EC, a discrepancy consistent with the fact that model parameterizations represented the more productive fraction of the EC tower “footprint.” At hourly to monthly timescales, predictions bracketed EC measurements so median predictions were similar to measurements, but there were quantitatively important model‐measurement discrepancies found for all models at subannual timescales. For these models and input data, hourly time steps (and greater complexity) compared to daily time steps tended to improve model‐measurement agreement for daily scale CO2 exchange and evapotranspiration (as judged by root‐mean‐squared error). Model time step and complexity played only small roles in monthly to annual predictions.

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.

An analytical hybrid GORT model for bidirectional reflectance over discontinuous plant canopies

The geometric optical (GO) bidirectional reflectance model, combined with a new component spectral signature submodel, can be used to estimate the bidirectional reflectance distribution function (BRDF) of discontinuous canopies. This approach retains the GO approach of incorporating the effect of shadows cast by crowns on the background. The newly developed submodel uses an analytical approximation of the radiative transfer (RT) within the plant canopies to model the spectral properties of each scene component. A multiple scale-hotspot function that incorporates effects for smaller canopy objects like branches, stems and leaves was also well modeled. Comparison of model results with field measurements (ASAS, POLDER and PARABOLA) over an old black spruce forest in central Canada demonstrated that the model ran predict the basic features of the BRDF, i.e., bowl shape and the hotspot. The benefits of the model presented are simplicity, improved treatment of multiple scattering and a new method of estimating the component signatures.

Temperature and humidity effects on branchlet gas-exchange in white spruce: an explanation for the increase in transpiration with branchlet temperature

In situ gas-exchange data, for branchlets of white spruce [Picea glauca (Moench) Voss.] in a mature mixed-wood boreal forest in central Canada (53°44´N 105°14´W), were subjected to a multiple regression analysis. Vapor pressure deficit (VPD) and branchlet temperature (tleaf) were both significant predictors (P<0.0001) of stomatal conductance to water vapor (gsw) and net photosynthesis (An), together explaining 67 and 64% of the variation in gsw and An, respectively. Since VPD and tleaf were autocorrelated in these field data, but also to further explore the nature of independent effects of temperature and humidity on water and CO2 exchange in white spruce, steady-state gas-exchange was performed on well-watered greenhouse-grown seedlings of white spruce. Results from laboratory experiments supported the following conclusions: (1) Transpiration (E) increases with VPD to an inflection point that increases linearly with tleaf. This tleaf effect on E could not be explained by trends in VPD, RH, An or PFD. Rather, our data support a model in which E and gsw are influenced by the balance between ’supply’ and ’loss’ of water to and from leaf tissue, respectively. The supply of water appears to be in accordance with Darcy’s law, where supply of water is proportional to the driving gradient in pressure/ tension, specific permeability (k), and inverse of water viscosity (n–1). Approximately half of the increase in E could be explained by the linear increase in n–1 with increasing tleaf. We propose that increases in k explain the remainder of the increase in E with tleaf. (2) VPD and tleaf appear to have independent effects on gsw. In contrast, RH effects on gsw or E were subtle and could be explained by a combination of effects of tleaf and VPD. (3) An was affected primarily by tleaf, being reduced at low (10°C) and high (40°C) temperatures, and only indirectly by humidity parameters via stomatal conductance, viz. intercellular CO2 concentrations. Our results have implications for the prediction of water fluxes from plants and canopies in areas where plant temperatures vary diurnally or seasonally.

Photosynthesis and carbon isotope discrimination in boreal forest ecosystems: A comparison of functional characteristics in plants from three mature forest types

In this paper we compare measurements of photosynthesis and carbon isotope discrimination characteristics among plants from three mature boreal forest types (Black spruce, Jack pine, and aspen) in order to help explain variation in ecosystem‐level gas exchange processes. Measurements were made at the southern study area (SSA) and northern study area (NSA) of the boreal forest in central Canada as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). In both the NSA and the SSA there were significant differences in photosynthesis among the major tree species, with aspen having the highest CO2 assimilation rates and spruce the lowest. Within a species, photosynthetic rates in the SSA were approximately twice those measured in the NSA, and this was correlated with similar variations in stomatal conductance. Calculations of the ratio of leaf intercellular to ambient CO2 concentration (ci/ca) from leaf carbon isotope discrimination (Δ) values indicated a relatively low degree of stomatal limitation of photosynthesis, despite the low absolute values of stomatal conductance in these boreal tree species. Within each ecosystem, leaf Δ values were strongly correlated with life‐form groups (trees, shrubs, forbs, and mosses), and these differences are maintained between years. Although we observed significant variation in the 13C content of tree rings at the old Jack pine site in the NSA during the past decade (indicating interannual variation in the degree of stomatal limitation), changes in summer precipitation and temperature accounted for only 44% of the isotopic variance. We scaled leaf‐level processes to the ecosystem level through analyses of well‐mixed canopy air. On average, all three forest types had similar ecosystem‐level Δ values (average value ± standard deviation, 19.1‰±0.5‰), calculated from measurements of change in the concentration and carbon isotope ratio of atmospheric CO2 during a diurnal cycle within a forest canopy. However, there were seasonal changes in ecosystem discrimination for aspen forests, while the evergreen conifer forests exhibited relatively constant discrimination values throughout the active growing season.

Seasonal variation of energy and water vapor exchange rates above and below a boreal jack pine forest canopy

Fluxes of energy and water vapor over boreal forest stands are expected to vary during the growing season due to temporal variations in solar energy, soil and air temperature, soil moisture, photosynthetic capacity, and leaf area. To investigate this hypothesis, we measured fluxes of energy balance components (solar, latent and sensible heat, and soil and canopy heat storage) over and under a boreal jack pine forest in central Canada during the 1994 growing season. Temporal trends of daily‐integrated energy fluxes were significant during a 117 day period between spring and autumn. Mean fluxes of net radiation and latent heat peaked near the summer solstice. By the autumnal equinox their magnitudes were half of their peak values. On a day‐to‐day basis the presence or absence of clouds modulated solar energy fluxes, while evaporation rates were dependent on whether the canopy was dry or wet. When the canopy was dry, daily evaporation was generally less than 2.5 mm d−1. This amount was less than one‐half the rate of equilibrium evaporation and was low compared to evaporation from vegetation in temperate zones. When the canopy was wet, daily evaporation approached 3 mm d−1 and exceeded predicted rates of equilibrium evaporation. Evaporation from the dry forest was weakly coupled to available energy and was restricted by the canopy AEs low‐surface conductance. Biotic factors limiting the forest AEs surface conductance include the forest AEs low‐leaf area index and partial stomatal closure. Abiotic and physiological factors restricting stomatal opening included a scarce supply of soil moisture, limiting vapor pressure deficits and the low photosynthetic capacity of the needles. The fluxes of solar energy and latent and sensible heat at the floor of the forest were a significant portion of energy exchange between the forest and the atmosphere. Typically, 20 to 40% of the total energy exchange by the jack pine stand originated at the understory. Since a substantial amount of energy occurs under the forest, two‐layer, not a big‐leaf AE, evaporation models are recommended as a tool for estimating water vapor fluxes from this open forest stand.

Growth and maintenance respiration rates of aspen, black spruce and jack pine stems at northern and southern BOREAS sites.

We measured stem respiration rates during and after the 1994 growing season of three common boreal tree species at sites near the northern and southern boundaries of the closed-canopy boreal forest in central Canada. The growth respiration coefficient (r(g); carbon efflux per micro mole of carbon incorporated in structural matter) varied between 0.25 and 0.76, and was greatest for black spruce (Picea mariana (Mill.) B.S.P.), least for jack pine (Pinus banksiana Lamb.) and intermediate for trembling aspen (Populus tremuloides Michx.). There was a consistent trend for higher r(g) at northern sites than at southern sites. Maintenance respiration rates at 15 degrees C (r(m)) varied from 0.5 to 2.7 nmol C mol(-1) C(sapwood) s(-1). Values of r(m) were high at sapling-stage jack pine sites and mature black spruce sites, and low at mature trembling aspen and mature jack pine sites. We found significant relationships between annual maintenance respiration and sapwood relative growth rate and sapwood volume per unit of stem surface area that explained much of the within-stand and between-stand variability. Because of the large differences in parameter values among sites, we conclude that the use of stand-specific respiratory parameters may improve model predictions of ecosystem process models over the use of generic parameter values.

Discrimination against C18O16O during photosynthesis and the oxygen isotope ratio of respired CO2 in boreal forest ecosystems

Our objective was to analyze factors that influence changes in the oxygen isotope ratio (δ18O) of atmospheric CO2 within boreal forest ecosystems. We made measurements in the three major forest types (black spruce, jack pine, and aspen) at the southern and northern ends of the boreal forest in central Canada. This research was part of a larger study, the Boreal Ecosystem‐Atmosphere Study (BOREAS). In terrestrial ecosystems the δ18O value of atmospheric CO2 is strongly influenced by isotope effects that occur during photosynthesis and respiration. Of primary importance is an equilibrium isotope effect that occurs between oxygen in CO2 and oxygen in soil water and plant chloroplast water. During the equilibrium reaction the oxygen isotope ratio of CO2 becomes enriched in 18O relative to that of water. We measured seasonal changes in the oxygen isotope ratio of (1) water input to the ecosystems (precipitation), (2) water taken up by the major plant species from the soil (plant stem water), and (3) water in plant leaves. We used this information in calculations of isotope discrimination during photosynthesis and soil respiration. Discrimination against C18O16O during photosynthetic gas exchange (ΔA) (influenced by equilibration with chloroplast water) averaged approximately 21‰ at midday and was similar for all forest types. In contrast, CO2 released during plant and soil respiration had an average δ18O value of −14.4‰ but was less depleted in 18O than would be expected for respired CO2 in isotopic equilibrium with soil water. This effect was most pronounced in black spruce sites because of the extensive coverage of moss on the ground surface and the observation that water in the upper moss layers can have an oxygen isotope ratio substantially different from water in deeper soil layers.

Carbon isotope discrimination during photosynthesis and the isotope ratio of respired CO2in boreal forest ecosystems

Our objective was to measure the carbon isotope ratio of CO2released by respiration (δr) within forest canopies at different times during the growing season and to use this information to estimate forest ecosystem carbon isotope discrimination. We made measurements in the three major forest types (black spruce, jack pine, and aspen) at the southern and northern ends of the boreal forest in central Canada. This research was part of a larger study, the Boreal Ecosystem‐Atmosphere Study (BOREAS). The δrvalues, calculated from measurements of change in the concentration and carbon isotope ratio of atmospheric CO2in air samples collected at night, ranged from −28.1‰ to −25.9‰ with an average (± s.d.) of −26.‰ ± 0.5‰. There was good correlation between calculated δr values and measurements of (1) the carbon isotope ratio of CO2released directly from the soil and (2) the δ13C values of foliage collected from the dominant tree species at each site. Carbon isotope discrimination during photosynthetic gas exchange (ΔA) by each forest ecosystem was estimated as the difference between the carbon isotope ratio of atmospheric CO2at the top of the canopy (δa) and the isotopic composition of respired CO2: ΔA= δa− δr. All three of the major forest types had similar values of ΔA, with an average (± s.d.) of 19.1‰ ± 0.5‰. However, a seasonal change in forest discrimination was observed for aspen forests in both the northern and southern study areas, with an increase in ΔAoccurring between the middle and end of the growing season. In contrast, the evergreen conifer canopies exhibited relatively constant discrimination values throughout the active growing season.

A THEORETICAL TREE LINE IN CENTRAL CANADA∗

ABSTRACT An index of tree growth based on the mean growth curve of black spruce shows that tree growth in the boreal forest of central Canada decreases linearly as the forest-tundra boundary is approached. A theoretical tree line is defined by black spruce growth data. The distribution of tree growth and the theoretical tree line indicate that the forest-tundra boundary is the mean summer position of the arctic front. The growth of white spruce is briefly considered, but because of a small sample size, no conclusions are drawn.