Opportunities for forest sector emissions reductions: a state-level analysis.

The forest sector can play a significant role in climate change mitigation. We evaluated forest sector carbon trends and potential mitigation scenarios in Vermont using a systems-based modeling framework that accounts for net emissions from all forest sector components. These components comprise (1) the forest ecosystem, including land-use change, (2) harvested wood products (HWP), and (3) substitution effects associated with using renewable wood-based products and fuels in place of more emission-intensive materials and fossil fuel-based energy. We assessed baseline carbon trends from 1995 through 2050 using a business as usual (BAU) scenario. Emission reductions associated with different forest management and HWP scenarios were evaluated relative to the BAU scenario from 2020 to 2050. We estimated uncertainty for each forest sector component and used a Monte Carlo approach to estimate the distribution of cumulative total mitigation for each scenario relative to baseline. Our analysis indicates that the strength of the forest sector carbon sink in Vermont has been declining and will continue to decline over coming decades under the BAU scenario. However, several scenarios evaluated here could be effective in reducing emissions and enhancing carbon uptake. Shifting HWP to longer-lived commodities resulted in a 14% reduction in net cumulative emissions by 2050, the largest reduction of all scenarios. A scenario that combined extending harvest rotations, utilizing additional harvest residues for bioenergy, and increasing forest productivity resulted in a 12% reduction in net cumulative emissions. Shifting commodities from pulp and paper to bioenergy showed a 7.3% reduction in emissions. In contrast, shortening rotations to increase harvests for bioenergy use resulted in a 5.5% increase in emissions. In summary, model simulations suggest that net emissions could be reduced by up to 14% relative to BAU, depending on the management and HWP-use scenario. Combining multiple scenarios could further enhance reductions. However, realizing the full climate mitigation potential of these forests may be challenging due to socioeconomic barriers to implementation, as well as alternative management objectives that must be considered along with carbon sequestration.

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

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.

A synthesis of radial growth patterns preceding tree mortality.

Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-continental tree-ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1-100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long-lasting declines were found for gymnosperms, shade- and drought-tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark-beetles) typically showed relatively small and short-term growth reductions. Our analysis did not highlight any universal trade-off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark-beetle attack, while long-term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth-based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark-beetle outbreaks.

Patterns of inter-annual variation in the size asymmetry of growth in Pinus banksiana.

A large body of literature suggests that asymmetric competition, where large individuals suppress the growth of smaller individuals by intercepting a disproportionate share of incoming light, is a dominant process in tree population development. This has not been examined extensively for long-lived tree species that accumulate growth over many years under varying growing conditions. Using dendrochronological techniques, we reconstructed annual growth and mortality rates at ten stands of jack pine (Pinus banksiana Lamb.) in Western Canada. We used these data to calculate an annual index of the size asymmetry of growth for each stand for the last 50 years. Jack pine is a shade-intolerant species found in even-aged monoculture stands, so the simple hypothesis is that large trees should consistently perform relatively better than small trees. Inter-annual variation in the index of size-asymmetric growth was positively associated with interannual variation in stand productivity at eight of ten sites. The size asymmetry of growth also showed a positive trend with age at eight of ten sites, even though all sites were in a period of declining leaf area. This should have reduced the intensity of asymmetric competition for light and reduced the size asymmetry of growth over time. Alternate hypotheses for this trend are (1) that physical collisions between crowns result in asymmetric competition for growing space because they are more damaging to small trees, or (2) that a differential build up of diseases in susceptible trees suppresses their growth, even in the absence of competition.

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.

Early-Warning Signals of Individual Tree Mortality Based on Annual Radial Growth.

Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter-annual growth variability and a decrease in growth synchrony in the last ∼20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.