Tree-ring evidence of larch sawfly outbreaks in western Labrador, Canada

As many insect outbreak reconstructions are typically based on targeted single-site sampling, researchers have often been limited in their ability to draw conclusions about regional trends as opposed to local trends in the data. The results of this paper demonstrate the value of a systematic sampling design when studying spatio-temporal processes that can vary greatly within large continuous areas of forest. Many single-site research programs have been conducted to reconstruct the history of larch sawfly ( Pristiphora erichsonii Htg.) outbreaks in the eastern boreal region of North America. However, no such research has yet been conducted in the region of Labrador. In an attempt to illustrate the strength of a systematic gridded sampling protocol over a single-site study, we sampled a 12-site grid in western Labrador. Dominant and codominant species were sampled at each grid point, resulting in 24 master chronologies. Six eastern larch ( Larix laricina (Du Roi) K. Koch) chronologies (host) and a regional black spruce ( Picea mariana (Mill.) Britton, Sterns, Poggenb.) chronology (nonhost) were used to establish a host–nonhost analysis of past sawfly outbreaks on a regional scale. Both regional and localized larch sawfly outbreaks were identified, but in general, larch sawfly outbreaks in western Labrador appeared to be spatially synchronous and regional in scale.

Dynamics of eastern larch stands and its relationships with larch sawfly outbreaks in the northern Clay Belt of Quebec

Climate change, fires, and insects outbreaks can affect eastern larch (Larix laricina (Du Roi) K. Koch) stand dynamics. To determine which of these factors had the greatest influence on stand dynamics, we sampled four wetlands dominated by larch on the margin of Lake Duparquet in the northern Clay Belt of Quebec. The ages of seedlings, saplings, and trees were determined in twelve 400-m2 plots. Increment cores were taken at breast height to determine past disturbance episodes related, among others, to larch sawfly (Pristiphora erichsonii (Hartig)) activity. Stem analysis was conducted using larch and black spruce (Picea mariana (Mill.) BSP) for identification of post-disturbance releases in height growth. Analyses at the stand and cluster levels showed that larch age structures were characterized by many unsynchronized establishment periods. In addition, a seedlings bank not much older than 10 years characterized many plots. Two larch sawfly outbreak episodes (1895–1912 and 1955–1962) identified by tree-ring and stem analysis were associated with larch establishment. A smaller outbreak in the late 1970s could also have contributed to the initiation of establishment in one stand. Our results suggest that the length (severity) of an outbreak may be a critical factor in explaining the different patterns of establishment observed in these stands. During a severe outbreak, larch establishment may occur mainly from surviving stems (increased seed production), whereas during a mild outbreak, larch establishment may occur from increased survival of pre-established seedlings and saplings. Further studies on the distribution of gaps within larch stands may provide better information on the pattern of mortality (gap dynamics) during an outbreak and may help to better understand larch establishment in these stands.