E-mail : corinne.vezeau.1@ulaval.ca

Spatiotemporal analysis of black spruce mortality in northern old-growth forests
Direction :
Serge Payette

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Mortality in forest ecosystems has become such a familiar concept that its understanding is often taken for granted. Nevertheless, tree mortality is a complex ecological process, given its large variability in time and space as well as its numerous causes. Knowledge about tree death in boreal forests is limited, since few studies have focused on this phenomenon. We thus propose to examine the causes and patterns of black spruce (Picea mariana (Mill.) B.S.P.) mortality in the forest tundra of subarctic Québec. To do so, spruce-lichen woodlands located mid-slope on two pre-determined hills in the Boniface River region (57°45' N; 76°20' W) will be investigated. These forests were selected because they enclose zones in which there is a high concentration of dead black spruce trees and an absence of regeneration. Landscape openings in spruce-lichen woodlands at these latitudes are usually attributed to a failure of post-fire regeneration under cool climatic conditions. However, the absence of charcoal in these sites suggests that there has been an absence of fire. As a result, these old-growth spruce-lichen woodland stands appear to have maintained themselves since the initial establishment of black spruce in the area. The old-growth tree formations within these stands thus provide an ideal environment to study the spatiotemporal dynamics of tree mortality. The absence of disturbance permits the assessment of the direct influence of the ecologic and climatic factors in the regression or the maintenance of these ecosystems. Moreover, the oldness and the scarcity of these northern forests bestow onto them a particular nature. It is important to understand the past dynamics of these marginal ecosystems in order to predict the impact of future climate change on their structure. The main objectives of this study are to i. characterize the spatio-temporal pattern of black spruce mortality in these spruce-lichen woodlands not influenced by fire, ii. determine the causal factors of this mortality, and iii. describe the dead individuals in order to establish a decay class of dead wood in function of time. Thus, two transects (oriented north-south and east-west) will be laid along the mountainsides of the old-growth spruce-lichen woodlands to ensure sampling in both the mortality zones and the adjacent wooded zones. A detailed description of all individuals (dead and alive) encountered will be performed. Age and diameter-size structure will also be analyzed for the dead and living individuals, respectively. This will allow us to reconstruct the history of these spruce-lichen woodlands and will enable us to predict the future of these unique stands.