Résumé - CAFD

The pyrogeography of eastern boreal Canada from 1901 to 2012 simulated with the LPJ-LMfire model.

Emeline Chaste, Martin-Philippe Girardin, Jed O. Kaplan, Jeanne Portier, Yves Bergeron, Christelle Hely-Alleaume.

Wildland fires are the main natural disturbance shaping forest structure and composition in eastern boreal Canada. On average, more than 700?000?ha of forest burns annually, and causes as much as C$?2.9?million worth of damage. Although we know that occurrence of fires depends upon the coincidence of favourable conditions for fire ignition, propagation and fuel availability, the interplay between these three drivers in shaping spatiotemporal patterns of fires in eastern Canada remains to be evaluated. The goal of this study was to reconstruct the spatiotemporal patterns of fire activity during the last century in eastern Canada's boreal forest as a function of changes in lightning ignition, climate and vegetation. We addressed this objective using the dynamic global vegetation model LPJ-LMfire, which we parametrized for four Plant Functional Types (PFTs) that correspond to the prevalent tree genera in eastern boreal Canada (Picea, Abies, Pinus, Populus). LPJ-LMfire was run with a monthly time-step from 1901 to 2012 on a 10-km2 resolution grid covering the boreal forest from Manitoba to Newfoundland. Outputs of LPJ-LMfire were analyzed in terms of fire frequency, net primary productivity (NPP), and aboveground biomass. The predictive skills of LPJ-LMfire were examined by comparing our simulations of annual burn rates and biomass with independent datasets. The simulation adequately reproduced the latitudinal gradient in fire frequency in Manitoba and the longitudinal gradient from Manitoba towards southern Ontario, as well as the temporal patterns present in independent fire histories. Nevertheless, the simulation led to underestimation and overestimation of the fire frequency at both the northern and southern limits of the boreal forest in Quebec. The general pattern of simulated total tree biomass also agreed well with observations, with the notable exception of overestimated biomass at the northern treeline, mainly for Picea PFT. In these northern areas, the predictive ability of LPJ-LMfire is likely being affected by a low density of weather stations, which has led to underestimation of the strength of fire–weather interactions during extreme fire years and, therefore, vegetation consumption. Agreement of the spatiotemporal patterns of fire frequency with the observed data confirmed that fire in the study area is strongly ignition-limited. Overall, climate and lightning ignition variability at multi-decadal and -annual time-scales was the primary driver of fire activity since the beginning of the 20th C. However, our simulations highlighted the importance of both climate and vegetation on fire: despite an overarching CO2-induced enhancement of NPP in LPJ-LMfire, forest biomass was relatively stable because of compensatory effects of increasing fire activity.