Light rings are characterized by one or a very few latewood-cell layers, an indication of shortened growing seasons, and are particularly frequent in black spruce (Picea mariana (Mill.) BSP) at the treeline in Québec. The construction of a light-ring chronology spanning the period AD 1398–1982 showed that the highest frequency (>25%) of light rings among 160 trees and krummholz occurred in 1593, 1620, 1634, 1784, 1816, 1817, 1853, 1969, and 1972. These diagnostic rings be a useful cross-dating tool for dendroecologists working with living and dead krummholz with a low-growth variability. About two-thirds of the 65 light-ring years coincide with years (or triads) of major volcanic eruptions. The climatic conditions (low temperature) occurring at the end of the growing season, in part induced by the climatic effect of volcanism, seem to initiate light rings.

White spruce (Piceaglauca (Moench) Voss) populations at their northern limit of distribution, the tree line along the east coast of Hudson Bay, expanded significantly during the last 100 years in response to recent climatic warming. Expansion began around 1880 but was more important during the 20th century, especially between 1920 and 1965. Although no major change in the position of the forest limit and the latitudinal tree line occurred, local altitudinal tree lines rose a few tens of meters. The seed regeneration line increased to about 100 meters above the present altitudinal tree line during the same period. Tree density increased below the local tree lines at many sites. There, spruce expansion was stepwise, as indicated by widely distributed clustered cohorts belonging to the 10–29, 50–59, and 70–89 age-class groups. Although the expansion process was stimulated by nearby old seed bearers, long-distance seed dispersal did occur. The development of contagious dispersion during the formation of white spruce groves or forests caused important local ecological changes, particularly in snow patterns and tree growth. Fire selectively influenced tree regeneration during the past centuries. Black spruce krummholz were progressively decimated during the last 400 years before present, although the species was characterized by low postfire regeneration. Vigorous postfire white spruce regeneration was stimulated by favorable climatic conditions 100 years ago in the northernmost forest site. Conclusions from this study support the hypothesis that Holocene tree line displacements were of low magnitude in northern Québec, and that the primary influence of climatic change was on tree density.

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The recent history and disturbance regime of an old-growth sugar maple – yellow birch forest located in the Tantaré Ecological Reserve, Québec, were determined using tree-ring growth patterns of individual trees that had undergone suppression and release. Within a sampling quadrat (0.25 ha) where all living and dead trees were mapped, the age, size, and spatial pattern of gaps formed since the mid-19th century were inferred from tree-ring signatures of standing trees. From 1860 onwards, more than 30 gaps of various form and size occurred, most gaps being < 200 m2 and covering a total area of 3775 m2. During the 1930–1985 period, the tree-fall frequency was 0.45 per year, the tree fall free interval was 3.2 years, and the tree-fall rotation period (turnover rate) was estimated to be 45 years. The rather short life-span of most trees (<125 years) may be attributed to the cool and humid conditions prevailing at the site, which stimulated self-pruning of mature trees. The spatiotemporal development of the sugar maple forest has been active during the past 2 centuries through small-scale disturbances associated with successve, often overlapping, single and multiple tree falls. This situation explains the uneven age distribution of sugar maple and yellow birch populations subjected to differential survival conditions caused by gap dynamics. It is concluded that tree-ring patterns corresponding to periods of suppression and release will likely give a finer resolution record of gap dynamics than other methods of determining natural disturbance regime and reconstruction of recent stand history.

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Macrofossil evidence indicates that the mid-Holocene hemlock [Tsuga canadensis L. (Carr.)] decline that occurred over a wide area in eastern North America was associated with phytophagous insect activity. In situ hemlock macrofossils and insect remains found in a paludified dunefield at the northern limit of hemlock testify that two defoliation events occurred at 4910 ± 90 and 4200 ± 100 yr B.P., respectively. The sharp coincidence of remains from hemlock needles with chewing damage typical of hemlock looper feeding, head capsules from the hemlock looper (Lambdina fiscellaria) and the spruce budworm (Choristoneura fumiferana), absence of hemlock fruiting remains, and tree-ring anomalies in fossil hemlocks that died prematurely (<165 yr) suggest that defoliation affected hemlock reproductive capacity and pollen productivity, or more likely caused mass mortality. Our findings indicate that defoliation can affect ecosystems for centuries, especially when long-lived tree species are involved.

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