Juha M. Metsaranta

An inventory-based analysis of Canada's managed forest carbon dynamics, 1990 to 2008.

Canadas forests play an important role in the global carbon (C) cycle because of their large and dynamic C stocks. Detailed monitoring of C exchange between forests and the atmosphere and improved understanding of the processes that affect the net ecosystem exchange of C are needed to improve our understanding of the terrestrial C budget. We estimated the C budget of Canadas 2.3 × 106 km2 managed forests from 1990 to 2008 using an empirical modelling approach driven by detailed forestry datasets. We estimated that average net primary production (NPP) during this period was 809 ± 5 Tg C yr−1 (352 g C m−2 yr−1) and net ecosystem production (NEP) was 71 ± 9 Tg C yr−1 (31 g C m−2 yr−1). Harvesting transferred 45 ± 4 Tg C yr−1 out of the ecosystem and 45 ± 4 Tg C yr−1 within the ecosystem (from living biomass to dead organic matter pools). Fires released 23 ± 16 Tg C yr−1 directly to the atmosphere, and fires, insects and other natural disturbances transferred 52 ± 41 Tg C yr−1 from biomass to dead organic matter pools, from where C will gradually be released through decomposition. Net biome production (NBP) was only 2 ± 20 Tg C yr−1 (1 g C m−2 yr−1); the low C sequestration ratio (NBP/NPP=0.3%) is attributed to the high average age of Canadas managed forests and the impact of natural disturbances. Although net losses of ecosystem C occurred during several years due to large fires and widespread bark beetle outbreak, Canadas managed forests were a sink for atmospheric CO2 in all years, with an uptake of 50 ± 18 Tg C yr−1 [net ecosystem exchange (NEE) of CO2=−22 g C m−2 yr−1].

Implications of future disturbance regimes on the carbon balance of Canada's managed forest (2010-2100).

Recent increases in fire and insect disturbances have contributed to a transition of Canada’s managed forest carbon balance from sink to source. Further increases in area burned could contribute positive feedback to climate change. We made probabilistic forecasts of the recovery of C sinks in Canada’s managed forest between 2010 and 2100 under two assumptions about future area burned by wildfire: (1) no increase relative to levels observed in the last half of the 20th century and (2) linear increases by a factor of two or four (depending on region) from 2010 to 2100. Recovery of strong C sinks in Canada’s managed forest will be delayed until at least the 2030s because of insect outbreaks, even if predicted increases in area annually burned do not occur. After 2050, our simulations project an annual probability of a sink near 70% with no increase in area burned and 35% with increasing area burned. All simulations project a cumulative C source from 2010–2100, even if annual area burned does not increase. If the sink strength of terrestrial ecosystems is reduced because of increasing natural disturbances, then it will become more difficult to achieve global atmospheric CO2 stabilization targets.

No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO2 fertilization

Significance Limited knowledge about the mechanistic drivers of forest growth and responses to environmental changes creates uncertainties about the future role of circumpolar boreal forests in the global carbon cycle. Here, we use newly acquired tree-ring data from Canada’s National Forest Inventory to determine the growth response of the boreal forest to environmental changes. We find no consistent boreal-wide growth response over the past 60 y across Canada. However, some southwestern and southeastern forests experienced a growth enhancement, and some regions such as the northwestern and maritime areas experienced a growth depression. Growth–climate relationships bring evidence of an intensification of the impacts of hydroclimatic variability on growth late in the 20th century, in parallel with the rapid rise of summer temperature. Considerable evidence exists that current global temperatures are higher than at any time during the past millennium. However, the long-term impacts of rising temperatures and associated shifts in the hydrological cycle on the productivity of ecosystems remain poorly understood for mid to high northern latitudes. Here, we quantify species-specific spatiotemporal variability in terrestrial aboveground biomass stem growth across Canada’s boreal forests from 1950 to the present. We use 873 newly developed tree-ring chronologies from Canada’s National Forest Inventory, representing an unprecedented degree of sampling standardization for a large-scale dendrochronological study. We find significant regional- and species-related trends in growth, but the positive and negative trends compensate each other to yield no strong overall trend in forest growth when averaged across the Canadian boreal forest. The spatial patterns of growth trends identified in our analysis were to some extent coherent with trends estimated by remote sensing, but there are wide areas where remote-sensing information did not match the forest growth trends. Quantifications of tree growth variability as a function of climate factors and atmospheric CO2 concentration reveal strong negative temperature and positive moisture controls on spatial patterns of tree growth rates, emphasizing the ecological sensitivity to regime shifts in the hydrological cycle. An enhanced dependence of forest growth on soil moisture during the late-20th century coincides with a rapid rise in summer temperatures and occurs despite potential compensating effects from increased atmospheric CO2 concentration.

Inequality of Size and Size Increment in Pinus banksiana in Relation to Stand Dynamics and Annual Growth Rate

Background and Aims Changes in size inequality in tree populations are often attributed to changes in the mode of competition over time. The mode of competition may also fluctuate annually in response to variation in growing conditions. Factors causing growth rate to vary can also influence competition processes, and thus influence how size hierarchies develop. Methods Detailed data obtained by tree-ring reconstruction were used to study annual changes in size and size increment inequality in several even-aged, fire-origin jack pine (Pinus banksiana) stands in the boreal shield and boreal plains ecozones in Saskatchewan and Manitoba, Canada, by using the Gini and Lorenz asymmetry coefficients. Key Results The inequality of size was related to variables reflecting long-term stand dynamics (e.g. stand density, mean tree size and average competition, as quantified using a distance-weighted absolute size index). The inequality of size increment was greater and more variable than the inequality of size. Inequality of size increment was significantly related to annual growth rate at the stand level, and was higher when growth rate was low. Inequality of size increment was usually due primarily to large numbers of trees with low growth rates, except during years with low growth rate when it was often due to small numbers of trees with high growth rates. The amount of competition to which individual trees were subject was not strongly related to the inequality of size increment. Conclusions Differences in growth rate among trees during years of poor growth may form the basis for development of size hierarchies on which asymmetric competition can act. A complete understanding of the dynamics of these forests requires further evaluation of the way in which factors that influence variation in annual growth rate also affect the mode of competition and the development of size hierarchies.

Estimation of snag carbon transfer rates by ecozone and lead species for forests in Canada

Standing dead trees (snags) and downed woody debris contribute substantially to the carbon (C) budget of Canadas forest. Accurate parameterization of the C transfer rates (CTRs) from snags to downed woody debris is important for forest C dynamics models such as the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3), but CTRs are rarely measured or reported in the literature. Therefore, forest C models generally use snag fall rates (FRs) available in the literature, as a proxy for CTRs. However, FRs are based on stem counts while CTRs refer to mass transfers. Stem mass and stem number are not linearly related, with small diameter trees representing disproportionately lower C mass transfers. Therefore this proxy, while convenient, may bias C transfer from standing dead to downed woody material. Here, we combined tree data from 10802 sample plots and previously published species-specific individual-tree relationships between tree diameter (diameter at breast height, dbh) and fall rate to derive stand-level estimates of CTRs for the CBM-CFS3. We estimated CTRs and FRs and used the FR values to validate this approach by comparing them with standardized FR values compiled from the literature. FRs generally differed from CTRs. The overall CTR (4.78% +/- 0.02% per year, mean +/- SE) was significantly smaller than the overall FR (5.40% +/- 0.02% per year; mean +/- SE). Both the difference between FR and CTR (FR - CTR) and the CTR itself varied by ecozone, with ecozone means for CTR ranging from 3.94% per year to 10.02% per year. This variation was explained, in part, by heterogeneity in species composition, size (dbh distribution), structure, and age of the stands. The overall mean CTR estimated for the Snag_Stemwood (4.78% per year) and the Snag_Branches (11.95% per year) pools of the CBM-CFS3 were approximately 50% and 20% higher than the current default rates used in the CBM-CFS3 of 3.2% and 10.0%, respectively. Our results demonstrate that using CTRs to estimate the annual C transfer from standing dead trees to downed woody biomass will yield more accurate estimates of C fluxes than using a FR proxy, and this accuracy could be further improved by accounting for differences in ecozone, stand component (hardwood or softwood), or lead species.

Potentially limited detectability of short-term changes in boreal fire regimes: a simulation study.

Climate change is expected to increase area burned in the boreal plains ecozone of Canada in the early 21st century (2001–50). I examined the influence of inter-annual variability in area burned and short observed time series on the probability of detecting if an increase has occurred, using a null model of present and future fire regimes. A wide range of fire cycles are consistent with annual area burned in the late 20th century (1959–99). Fire cycles estimated from the reciprocal of the average annual burn fraction over a 50-year period are not very precise, and overestimate the fire cycle if years with large annual area burned have not recently occurred. Under the default assumptions, the probability of detecting a doubling of annual area burned during 2001–50 is 73% if it occurred instantaneously, but only 31% if it occurred gradually. Imprecise estimates and uncertainty in the ability to detect changes in fire cycles poses challenges for implementing aspects of sustainable forest management. Alternate empirical or model-based statistics, such as return periods for annual areas burned of a given magnitude, may be useful for inferring frequencies and magnitudes of large fire years that have not yet been observed.

Ecology and Habitat Selection of a Woodland Caribou Population in West-central Manitoba, Canada

Abstract This study examines the ecology of Rangifer tarandus caribou (woodland caribou) in the Naosap range in west-central Manitoba, Canada. This population is considered to be of high conservation concern because of potential resource-development impacts; therefore, baseline data are required to guide and evaluate the management of this species in this area. Radio-telemetry data were collected every two weeks from February 1998 to April 2001 and used in combination with forest-inventory data to evaluate habitat selection, site fidelity, movement, and grouping patterns. In both summer and winter, selected habitats were mature upland spruce and pine forests, as well as treed muskeg. Hardwood forests were least selected at all scales. Mature coniferous forest was preferred over immature coniferous forests in a pair-wise comparison in winter, but not in summer. Home-range sizes were within expected ranges of variation. Animals used distinct areas in summer and winter, showing broad fidelity to seasonal ranges. However, small shifts in the core areas were observed, particularly in winter. Movement rates and grouping behavior were typical of other caribou. Habitats used in winter were common in the study area, but the ability of the animals to disperse to alternate winter areas is not known. Management efforts could focus on protecting known calving and winter-use areas, and regenerating coniferous forests after logging, which is consistent with regional forest-management objectives.

If forest dynamics in Canada's west are driven mainly by competition, why did they change? Half-century evidence says: Climate change.

In a recent paper (1), Zhang et al. present analyses of “forest dynamics” inferred from measurements collected during 1958–2009 at permanent sample plots (PSP) distributed across Canada’s western forests. Their results are almost unanimous in showing widespread increases in mortality, and declines in relative growth and recruitment (figure 2 in ref. 1). Zhang et al. conclude these trends are explained primarily by changes in stand-scale competition, and that recent changes in climate are of secondary importance. Surprisingly, Zhang et al. do not explain the temporal changes in competition they detected. We accept that stand dynamics depend upon competition for light, nutrients, and water, but argue that climate affects the supply of these resources. We find some major problems with the report by Zhang et al., including misinterpretation of results and a critical lack of clarity on key model assumptions, which cast serious doubt on their conclusions.

Integrating forest fuels and land cover data for improved estimation of fuel consumption and carbon emissions from boreal fires

Estimating carbon emissions from wildland fires is complicated by the large variation in both forest fuels and burning conditions across Canada’s boreal forest. The potential for using spatial fuel maps to improve wildland fire carbon emission estimates in Canada’s National Forest Carbon Monitoring, Accounting and Reporting System (NFCMARS) was evaluated for select wildfires (representing a transect across western Canada) occurring in 2003 and 2004 at four study areas in western Canada. Area-normalised emission rates and total emissions differed by fuels data source, mainly as a function of the treatment of open fuels in the higher resolution spatial fuel models. The use of spatial data to refine the selection of stand types that probably burned and the use of fire weather conditions specific to the fire increased the precision of total carbon emission estimates, relative to computational procedures used by Canada’s NFCMARS. Estimates of total emissions from the NFCMARS were consistent with the regional and national data sources following the spatial approach, suggesting the two approaches had equivalent accuracies. Though it cannot be said with certainty that the inclusion of this detailed information improved accuracy, the spatial approach offers the promise or potential for more accurate results, pending more consistent fuel maps, especially at finer scales.

A fifty-year reconstruction of annual changes in the spatial distribution of Pinus banksiana stands: does pattern fit competition theory?

We used tree-ring reconstruction data to study changes in the spatial pattern of live and dead trees at an annual resolution over a 50-year period at four unmanaged, even-aged fire origin jack pine (Pinus banksiana Lamb.) stands in Saskatchewan and Manitoba, Canada. Previous studies of the spatial pattern in P. banksiana have either looked at only a snapshot from a survey done at a single point in time, or repeated measurements of permanent plots taken at 10-year intervals. With annual data, we could examine detailed changes in spatial patterns and relate these to events during stand development and external disturbances. Trees were initially clustered at all sites, but at different distances at each site, most likely because of variability in seedbed distribution at stand initiation. Clustering disappeared over time at all sites, and at a similar mean tree spacing at each site. However, significant regularity only appeared sporadically at one site, indicating that competition with neighbours was not the only factor influencing changes in spatial pattern. At two of the four sites, clustering disappeared suddenly at the same time that mortality rate reached a peak, in one case also coinciding with a jack pine budworm (Choristoneura pinus pinus Freeman) defoliation event. Dead trees were also initially more clustered than the distribution of all trees, but at different distances than the clustering of live trees. This also disappeared over time so that dead trees were eventually a random sample from the distribution of all trees. After the peak of mortality had passed, factors other than competition were determining the dynamics of these forests.