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Tree-ring chronologies for tamarack (Larix laricina (Du Roi) K. Koch) growing in four stand types covering a dry-to-wet gradient were developed to investigate the association between radial growth and climate as well as evidence of larch sawfly (Pristiphora erichsonii Hartig) herbivory near Churchill, subarctic Manitoba. The chronologies, produced using both living and subfossil material, were well replicated for the period 1800 to 2000. Our results indicated that climate explained more than 60% of the growth variation in tamarack with temperature from May to July of the growing season being most important. This was unusual considering that other studies showed a weak radial growth-climate association in tamarack because of signal contamination due to severe larch sawfly defoliation. Potential outbreak episodes were investigated by looking at pointer years and by contrasting the variance within the tamarack tree-ring series with that observed in climate and in nonhost species. A weak correspondence was observed among pale latewood rings, growth suppression period, and incomplete rings. Comparison of host and nonhost chronologies revealed synchronized growth suppression periods in tamarack during 1818-1824, 1857-1864, 1891-1892, 1903-1912, and 1959-1964, with seldom more than 25% of the trees being affected. The 20th-century suppressions corresponded to known outbreaks identified in Manitoba. However, they also corresponded to periods of little residues in the climate model suggesting that they may be due to short-term changes in site hydrology. This study stresses the difficulties to use dendrochronology to identify what may be low severity or "subepidemic" defoliation events. It also stresses the potential differences in the larch sawfly dynamics between the boreal forest and the forest-tundra. Further studies using an extended network of chronologies will be needed to decipher the short-term impacts of climate from those of low severity defoliation episodes. [ABSTRACT FROM AUTHOR]

Investigates the influence of climate change, forest fires and insects outbreaks on eastern larch stand dynamics in Quebec. Use of larch and black spruce in the stem analysis conducted; Occurrence of larch establishment from surviving stems during a severe outbreak; Ability of eastern larch to tolerate high soil moisture and acidity.

With the objective of understanding how vegetation was structured in four Larix laricina (Du Roi) K. Koch dominated wetlands in north-western Quebec, 186 point-centred quarters were sampled in four stands. For each point, both biotic and abiotic variables were collected and species cover was recorded. Divisive hierarchical classification analysis (Twinspan) identified nine vegetation clusters: i) Larix laricina & Spiraea alba, ii) Larix laricina & Kalmia angustifolia, iii) Larix laricina, Picea mariana & Alnus rugosa, iv) Larix laricina & Betula pumila, v) Thuja occidentalis & Trientalis borealis, vi) Abies balsamea & Betula papyrifera, vii) Fraxinus nigra & Onoclea sensibilis, viii) Alnus rugosa, and ix) Eleocharis smallii. Results of the canonical correspondence analyses indicated that the distribution of these clusters was mainly related to (i) distance from shore, (ii) shade (canopy cover), (iii) substrate nitrate concentration (in relation to the abundance of Kalmia angustifolia and Alnus rugosa), (iv) substrate pH (in relation to the abundance of Sphagnum spp.), and (v) substrate conductivity. Several characteristics of the water table also affected species distribution, including pH, depth, and carbon concentration. Further studies should address the effect of the presence of Kalmia angustifolia and Alnus rugosa on larch growth.Key words: larch, wetland, vegetation analysis, flooding, boreal forest.Avec pour objectif de comprendre la distribution de la végétation dans quatre tourbières dominées par Larix laricina (Du Roi) K. Koch au Nord-Ouest de la forêt boréale québécoise, 186 quadrants centrés sur le point ont été échantillonnés à l'intérieur de quatre sites. Pour chacun des points, des variables abiotiques et biotiques ont été récoltées et le recouvrement en espèces a été noté. L'analyse par classification divisive hierarchique (Twinspan) a désigné neuf groupements végétaux: (i) Larix laricina & Spiraea alba, (ii) Larix laricina & Kalmia angustifolia, (iii) Larix laricina, Picea mariana & Alnus rugosa, (iv) Larix laricina & Betula pumila, (v) Thuja occidentalis & Trientalis borealis, (vi) Abies balsamea & Betula papyrifera, (vii) Fraxinus nigra & Onoclea sensibilis, (viii) Alnus rugosa et (ix) Eleocharis smallii. Les analyses canoniques des correspondances ont indiqué que la distribution de ces groupements était principalement reliée à (i) la distance de la rive, (ii) l'interférence lumineuse et certaines caractéristiques du substrat, entre autre (iii) la concentration en nitrate du substrat (notamment en relation avec l'abondance de Kalmia angustifolia et d'Alnus rugosa), (iv) le pH (en relation avec l'abondance de Sphagnum spp.) et (v) la conductivité. Un lien a été démontré entre les caractéristiques de la nappe phréatique et la distribution des espèces, notamment en relation avec le pH, la profondeur et la concentration en carbone. Des études plus approfondies devraient être conduites afin de déterminer l'impact de la présence des espèces Kalmia angustifolia et Alnus rugosa sur la croissance du mélèze.Mots clés : mélèze, tourbière, analyse de végétation, inondation, forêt boréale. [ABSTRACT FROM AUTHOR]