Significant Increase in Deciduous Forest Cover from Disturbance in North American Boreal Forest
Recent field and remote sensing-based studies have suggested that the balance between deciduous and evergreen tree cover is shifting in the North American boreal forest, raising important questions regarding the long-term forest structure and composition of North American boreal forests. In particular, models project that deciduous forests are likely to expand at high latitudes due to more favorable climate conditions and more frequent disturbance events. However, current understanding regarding if and how such changes are occurring is incomplete. To address this, we refined and extended the NASA ABoVE (Arctic-Boreal Vulnerability Experiment) land cover data set (Wang et al. 2019), which covered the ABoVE core study domain (4.6 x 106 km2 across western Canada and Alaska) for the period 1984-2014, to encompass all of boreal Canada and Alaska (1.7 × 107 km2) at 30 m from 1984 to 2022. To create annual land cover maps and quantify land cover changes at moderate spatial resolution (30 m) across 39 years we used Landsat imagery, supplemented with training data from Wang et al. (2019) and Canada’s National Forest Inventory. We found that approximately two-thirds of the study area had stable land cover. These stable areas were composed of evergreen forests (32.87 ± 1.15% of the study area), deciduous forests (9.70 ± 0.82% of the study area), shrublands (12.98 ± 1.12% of the study area) and herbaceous land cover (9.99 ± 0.83% of the study area). Our results identify a net loss of areas dominated by evergreen forests of 2.17% (3.82 × 105 km2 ), while deciduous forests and shrublands expanded by 2.51% (4.42 × 105 km2) and 0.83% (1.46 × 105 km2), respectively, during the same period. Disturbance was the main driver of land cover change; 76% of evergreen forest loss and 45% of deciduous forest changes occurred in locations that experienced either fire or harvest. These shifts in the distribution of plant functional types, observed through remote sensing at continental scale, are consistent with field observations and model projections, and suggest that the intensified disturbance regimes are driving systematic increases in the proportion of deciduous forest at the expense of evergreen species in North American boreal forests.