Increasing fire and the decline of fire adapted black spruce in the boreal forest

Abstract

Intensifying wildfire activity and climate change can drive rapid forest compositional shifts. In boreal North America, black spruce shapes forest flammability and depends on fire for regeneration. This relationship has helped black spruce maintain its dominance through much of the Holocene. However, with climate change and more frequent and severe fires, shifts away from black spruce dominance to broadleaf or pine species are emerging, with implications for ecosystem functions including carbon sequestration, water and energy fluxes, and wildlife habitat. Here, we predict that such reductions in black spruce after fire may already be widespread given current trends in climate and fire. To test this, we synthesize data from 1,538 field sites across boreal North America to evaluate compositional changes in tree species following 58 recent fires (1989 to 2014). While black spruce was resilient following most fires (62%), loss of resilience was common, and spruce regeneration failed completely in 18% of 1,140 black spruce sites. In contrast, postfire regeneration never failed in forests dominated by jack pine, which also possesses an aerial seed bank, or broad-leaved trees. More complete combustion of the soil organic layer, which often occurs in better-drained landscape positions and in dryer duff, promoted compositional changes throughout boreal North America. Forests in western North America, however, were more vulnerable to change due to greater long-term climate moisture deficits. While we find considerable remaining resilience in black spruce forests, predicted increases in climate moisture deficits and fire activity will erode this resilience, pushing the system toward a tipping point that has not been crossed in several thousand years.

Keywords: climate change; ecological state change; resilience; tree regeneration; wildfire.

Fig. 1.

A framework to evaluate the potential for state changes following fire. A represents possible trajectories for Species A, a prefire dominant taxon (Species A comprises ≥50% of prefire stem density). Resilience is the most likely outcome in the self-replacement panel, in which the prefire dominant taxon continues to dominate after fire and sufficient seedlings have established to at least maintain, or possibly increase, species-specific prefire stem densities; density reduction was also considered to be a more stable or resilient outcome than the other outcomes. We considered that the competition, poor establishment, and regeneration failure categories would all likely lead to state change. B represents possible trajectories for Species B, a prefire subordinate taxon (Species B comprises <50% stem density), and we were interested in understanding the potential for expansion (i.e., postfire increase in dominance and maintenance or increase in stem density) of Species B. In this case, postfire state change (i.e., loss of resilience of the prefire dominant) is most likely in the expansion category. However, in our study, given the greater competitive abilities of the prefire subordinates in question (i.e., broadleaf taxa and jack pine) compared to black spruce (the most common prefire dominant in our dataset), the competition category will also like lead to loss of resilience when the prefire dominant is black spruce. Poor establishment and density reduction categories in B were never observed in our dataset. In boreal North America, fires typically occur at intervals shorter than the lifespan of the postfire cohort of trees (<100 y); therefore, the pulse of seedling establishment immediately postfire typically determines the trajectory of future forest composition until the next fire.

Fig. 2.

Characterizing black spruce resilience and state changes represented by alternative trajectories of postfire recovery across boreal North America for fires that burned from 1989 to 2014. Sample sizes of fires and individual sites with postfire seedling counts are overlain on ecozones. For all analyses, ecozones were pooled as depicted by the colored regions on the map, based on biophysical similarities. Across ecozones, black spruce was the most common prefire stand dominant (1,140 of 1,538 sites). The fire-free interval for these stands ranged from 8 to 322 y; the range of stand ages was largely comparable across ecozones (SI Appendix, Table S1). Categories of change in the “Black Spruce Resilience” panel are defined in Fig. 1A. Where loss of resilience is expected (Regeneration Failure [n = 187], Poor Establishment [n = 106], and Competition [n = 144]), we quantified the likely outcome of state changes in the “Trajectory of Change” panel. The main alternative trajectories are depicted above the map for each pooled ecozone. Images showing landscape perspectives of these alternative trajectories are provided in SI Appendix, Fig. S2. SI Appendix, Table S1 provides details of individual studies included in this synthesis.

Fig. 3.

Relative importance of predictors of conifer and black spruce resilience and broadleaf and jack pine expansion from random forest analyses. Hargreaves’ CMD (millimeters) predictors included 30-y (1981 to 2010) normals (CMD_normal) and the difference between CMD_normal and CMD in years 1 and 2 after fire (CMD_postfire1 and CMD_postfire2, respectively). Other predictors include residual soil organic layer thickness (centimeters, Residual organic), site drainage classes (dry, moist, wet; Site drainage), fire-free interval (years, Time after fire), canopy combustion severity (ordinal scale from 0 to 3; Canopy combustion), prefire basal area for the species or taxa under consideration (m² ⋅ ha⁻¹; Basal area), and pooled ecozone (Ecozone). Text in the bottom right of each panel indicates the accuracy of the overall random forest model as well as resilience (self-replacement and density reduction) versus state change (regeneration failure, poor establishment, and competition) categories for black spruce and conifer models or expansion categories (expansion and competition) for broadleaf and jack pine. Model accuracy is based on out-of-bag error estimates. Sample sizes for each dataset are provided in the panels; details of sample size by pooled ecozone can be found in Materials and Methods and in SI Appendix, Table S1. The direction of the relationship between each variable and resilience (conifer and black spruce) or expansion (broadleaf and jack pine) is given for the three most important variables for each analysis. Relationships between common important variables and regeneration outcomes are also shown in SI Appendix, Figs. S5–S8, and partial dependence plots are provided in SI Appendix, Figs. S11–S14.