Fire and Climate

Large fires in the boreal forests of Alaska may stimulate transitions from black spruce (see the patch of unburned trees in the foreground) to a landscape matrix dominated by deciduous tree species. Photo taken near the Caribou Poker Creek Research Watershed of the Bonanza Creek LTER site.
J. Johnstone

Plant ecologists working with the Bonanza Creek LTER program in Alaska have found that fire effects on soil organic layer depths is a key factor in the disruption of stable patterns of conifer dominance in the boreal forest. Plant-soil-microbial (PSM) feedbacks between vascular plants, mosses, and microbial decomposition maintain deep organic soils in black spruce forests and wetlands of Interior Alaska (Johnstone et al. 2010a). This internal feedback has been a key source of ecosystem resilience under the historical fire regime; moist, cold soils, poorly drained due to permafrost, burn at low severity and leave the surface organic layer largely intact. Thick organic layers burned at the surface create a seedbed that favors the re-establishment of black spruce and mosses. The system quickly returns to a structure similar to that of the conifer-dominated, pre-fire forest.

However, the stabilizing feedbacks between plants and soil in the boreal forest can be disrupted by unusual fire events. In extreme fire years, severe fires can consume much of the soil organic layer. When less than about 5 cm depth of organic soil remains after fire, deciduous tree species such as aspen and birch can establish at high densities (Johnstone et al. 2010b). From long-term studies of forest succession at the Bonanza Creek LTER sites, scientists have found that this period of initial post-fire succession sets the stage for decades to centuries of plant succession. Thus, the change in seedbed conditions caused by a severe fire can catalyze a switch from conifer dominance to alternate plant successional trajectories dominated by deciduous trees.

Once deciduous forests are established, a new domain of PSM feedbacks emerges where shallow organic soils are maintained by rapidly decomposing litter from highly productive deciduous species. Deciduous broadleaf trees increase rates of evapotranspiration and export moisture from the soil to the atmosphere.

Once thick organic layers are consumed by fire, permafrost degradation is likely, leading to a state change that permanently alters ecosystem structure and function. Shifts between domains of spruce vs. deciduous dominance and the resulting effects on permafrost have large implications for ecosystem productivity and carbon storage, feedbacks to regional climate--the goods and services that boreal ecosystems provide to humans. Indeed, BNZ scientists have shown that shifts from conifer to deciduous forest cover can largely compensate for the carbon emissions caused by increasing wildfire (Randerson et al. 2006). This research demonstrates that stabilizing feedbacks within the dominant forest type can be disrupted by changes to climate and disturbance regimes, initiating rapid transformations of the forest landscape, with regional to global consequences.

Output from a boosted regression tree model showing the variables most important in predicting the proportional dominance of black spruce in post-fire regenerating stands. Fire severity, measured by the Composite Burn Index (CBI), shows the strongest effects in determining whether burned black spruce stands will recover to be dominated by spruce (low severity) or deciduous species (high severity). Stand age and site moisture also have important effects on the dominance of spruce in the post-fire regenerating forests.
Johnstone, J. F., F. S. I. Chapin, T. N. Hollingsworth, M. Mack, V. E. Romanovsky, and M. Turetsky. 2010a. Fire, climate change, and forest resilience in interior Alaska. Canadian Journal of Forest Research 40:1302-1312.
For further reading: 
Johnstone, J. F., F. S. I. Chapin, T. N. Hollingsworth, M. Mack, V. E. Romanovsky, and M. Turetsky. 2010a. Fire, climate change, and forest resilience in interior Alaska. Canadian Journal of Forest Research 40:1302-1312.
Johnstone, J. F., T. N. Hollingsworth, F. S. Chapin III, and M. C. Mack. 2010b. Changes in fire regime break the legacy lock on successional trajectories in Alaskan boreal forest. Global Change Biology 16:1281-1295.
Randerson, J. T., H. Liu, M. G. Flanner, S. D. Chambers, Y. Jin, P. G. Hess, G. Pfister, M. C. Mack, K. K. Treseder, L. R. Welp, F. S. I. Chapin, J. W. Harden, M. L. Goulden, E. Lyons, J. C. Neff, E. A. G. Schuur, and C. S. Zender. 2006. The impact of boreal forest fire on climate warming. Science 314:1130-1132.
For further information: 
Dr. Jill Johnstone
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