On the spatio-temporal radial growth response of four alpine treeline species to climate across central Labrador, Canada.

摘要

This thesis applies standard dendrochronological techniques to contrast and compare between the radial growth responses of alpine treeline species across a climate gradient in central Labrador, Canada. Four species were examined: black spruce (Picea mariana (Mill.) B.S.P.), white spruce (Picea glauca (Moench) Voss), balsam fir (Abies balsamea (L.) Mill.) and eastern larch (Larix laricina (DuRoi) K. Koch). Eighteen tree ring width (TRW) chronologies were constructed at five alpine treeline sites across a 600 km long transect from the Labrador Sea to the Quebec border. The effects of climate and forest disturbance events on radial tree growth were examined at various spatial and temporal scales.Key words. Tree ring width, climate, central Labrador, dendroclimatology, disturbance, divergence.This study is the first to conduct dendrochronological studies on black spruce, fir and larch in Labrador and to use samples from across central Labrador. The chronology statistics are in accordance with regional publications, with the exception of the larch chronologies, which have exceptionally high sensitivity values (MS > 0.3). These results suggest that the radial growth response of each one of the tree species sampled is uniquely adapted to monthly and/or seasonal temperature and precipitation values during the growing season and, in some cases, to winter snowfall amounts. These climate sensitivities are also site-specific and indicative of a strong reliance on the proximity of the Labrador Sea. This study simultaneously compares the radial growth patterns of the four alpine treeline species in a region that, to date, has been largely understudied. As a result, this study has identified possible periods of outbreaks of spruce budworm (Choristoneura fumiferana (Clem.)) and larch sawfly (Pristiphora erichsonii Hartig), which are synchronous with outbreaks reported from eastern Quebec. The intensity and frequency of these insect outbreaks is also climate-dependent, and can be so severe as to mask the climate sensitivity in the larch trees. A palaeoclimatic reconstruction (1847-2004) using merged spruce chronologies at one of the sites, the Mealy Mountains, is in accordance with other published records but has weak significance values due to a high degree of variability in climate sensitivity throughout the instrumental record (1942-present). The high degree of variability in the climate sensitivity of spruce trees across central Labrador is partly due to the effect of spruce budworm outbreaks on the radial growth of trees, but other site-specific factors such as moisture effects must also be contributing to the variability. Further studies are needed to determine the cause of this episodic divergence in the relationship between radial tree growth and climate in central Labrador in order to comprehend more fully which environmental factors influence radial tree growth, and how this changes with time.