Abstract. 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 CAD 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 data sets. 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. However, the
simulation led to the underestimation and overestimation of fire frequency at
both the northern and southern limits of the boreal forest in Québec. 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 PFT Picea. In these northern areas,
the predictive ability of LPJ-LMfire is likely being affected by the low
density of weather stations, which leads to underestimation of the strength
of fire–weather interactions and, therefore, vegetation consumption during
extreme fire years. Agreement between the spatiotemporal patterns of fire
frequency and the observed data across a vast portion of the study area
confirmed that fire therein is strongly ignition limited. A drier climate
coupled with an increase in lightning frequency during the second half of the
20th century notably led to an increase in fire activity. Finally, our
simulations highlighted the importance of both climate and fire in
vegetation: despite an overarching CO2-induced enhancement of NPP in
LPJ-LMfire, forest biomass was relatively stable because of the compensatory
effects of increasing fire activity.
Resilience of the boreal forest in response to Holocene fire-frequency changes assessed by pollen diversity and population dynamics
