The chronology of eastern spruce budworm (Choristoneurafumiferana (Clem.)) outbreaks in the vicinity of Lake Duparquet in the Abitibi region, Quebec, was determined for a period of 200 years using dendroecological methods. Growth patterns of two budworm host species, balsam fir (Abiesbalsamea (L.) Mill.) and white spruce (Piceaglauca (Moench) Voss), were compared with white cedar (Thujaoccidentalis L.), a nonhost species, and with maps of defoliation surveys. The method permitted precise dating of past outbreaks prior to the last two decades, where cartographic information is either imprecise or nonexistent. Growth reductions associated with insect defoliation situate three outbreak periods in this century between 1970 and 1987, between 1930 and 1950, and between 1919 and 1929. A fourth outbreak period may have occurred in the 19th century between 1810 and 1832. The outbreaks in this region were less severe than those that have occurred further south of the insect's northern limit of dispersion. Nevertheless, they did have a major impact on the dynamics of balsam fir populations in the region, as revealed by a massive invasion of fir between 1915 and 1940, and a marked growth release between 1940 and 1950. Dry periods, reflected in the growth curve of cedar, were associated with recorded outbreak periods.

The chronology of spruce budworm (Choristoneurafumiferana (Clem.)) oubreaks for the past 200 years has been reconstructed by dendrochronological analysis of balsam fir (Abiesbalsamea (L.) Mill.) and white spruce (Piceaglauca (Moench) Voss) samples from virgin forests, unaffected by insecticide spraying programs and recently protected against fires in the north and northwest of Lac Saint-Jean. Outbreaks began around 1974, 1944, 1909, and possibly, 1832. They are closely linked with those described in the Laurentide wildlife reserve. The frequency, duration and severity of growth rings suppression associated with outbreaks are similar to those registered during outbreaks in the southern forests, except for the 1974 outbreak, which was shorter and less severe. Other outbreaks registered in southern regions, where the proportion of balsam fir is higher, have reached this region where balsam fir populations are marginal. These results underline the importance of these ecological disturbances in the population dynamics of the host trees of the region.

Mast seeding is considered a reproductive trait resulting from several potential evolutionary forces. Although the mechanisms driving reproduction have been thoroughly investigated and discussed, their relative importance and possible coexistence remain an open question. Seed rain abundance and viability of balsam fir Abies balsamea, white spruce Picea glauca, and white birch Betula papyrifera were monitored during 1994-2007 along a chronosequence of developmental stages at different densities represented by four stands in the boreal forest of Quebec, Canada. The aim of the study was to verify whether seeding temporal dynamics and abundance were affected by stand density, and to test the causal relationships between the hypotheses of pollination efficiency and predator satiation. Seed rain abundance varied substantially among years and species, with the highest values being observed in 1994 and 1996, and attaining up to 28.2  10 3 seeds m 2 year 1 . However, the annual dynamics of seed production was similar in the three sites with the higher densities. The greater proportions of germinating and dead seeds were observed in white birch, while balsam fir showed an average of 11.2% of damaged seeds, which were infected by larvae. The conifer species had the higher proportions of empty seeds. The causal models tested on balsam fir demonstrated that seed rain abundance influenced the amount of both viable and larval-infected seeds, although no causal relationship existed between these last two variables. The comparable and synchronous seed rain abundances observed among sites demonstrated that stand density can significantly affect the reproduction of trees, probably by limiting the access to resources, but not the annual dynamics of seeding. Pollination efficiency and predator satiation are important forces in the reproductive effort of trees and could be not mutually exclusive because both these mechanisms of natural selection synergistically coexist in driving the seeding dynamics of balsam fir.

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During the past 100 years, palsa bogs located on the eastern coast of Hudson Bay have undergone major changes associated with global climatic warming of the northern hemisphere. The recent main developmental stages of palsas, collapse scars, and thermokarstic pools were reconstructed within a representative permafrost peatland located in the southern part of the forest–tundra, using detailed mapping and tree-ring analyses. Maps of the peatland in 1957, 1973, and 1983 indicate a 49% decrease of the total cover of palsas and collapse scars between 1957 and 1983 and a 44% increase of the thermokarstic pool surface. Degradation of the palsa bog was more pronounced between 1957 and 1973 than between 1973 and 1983. Tree-ring analysis of reaction wood on black spruce (Picea mariana (Mill.) BSP.) exposed to unstable peat substratum indicates that permafrost degradation, which began as early as 1880, increased markedly between 1930 and 1965. During the 19th century, the permafrost peatland was mainly composed of large peat plateaus, which subsequently disintegrated into residual palsas, collapse scars, and thermokarstic pools. In addition the increase in temperature during the 20th century, it seems that milder winters with heavier snowfalls promoted conditions conducive to permafrost degradation. The constantly increasing contrast in the microtopographic pattern of the peatland, resulting from the transformation of peat plateaus into smaller palsa units, created more snow cover on east and southeast palsa slopes, thus accelerating permafrost degradation. This autocatalytic process seems to have also played a role in some sections of the peatland with abundant thermokarstic pools, where major changes in drainage conditions have stimulated thermokarstic erosion.