By Randi Jandt

This research brief is available as a standalone PDF

Forestry research in Alaska indicates that coniferous-dominated interior boreal forests are being replaced by deciduous trees due to recent climate warming and changes in the wildfire regime. Mann et al. 2012¹ suggested a dramatic shift in forest species dominance was already happening, since the 1990’s. Interior Alaska forests have averaged about ⅔ coniferous to ⅓ deciduous over the last several hundred years. Using landscape disturbance models under various global climate scenarios, Mann’s team predicted the ratio would soon be 1:1 and even reverse by mid-century, with hardwood stands comprising ⅔ of the forests. Now, 30 years of satellite imagery and improved vegetation mapping methods using new technology offer a chance to test their predictions.

Post-fire succession: When Alaska spruce forests burn the trees are killed, but black spruce stands tend to “self-replace” about ⅔ of the time—by seeding into the remnant moss layers². On the other hand, it has been well-documented that “severe” (deep-burning) forest fires favor regrowth of hardwoods (birch, aspen and poplar). These species can grow fast in deeper active layers that result after severe fires in which most of the moss duff is removed_³. This can result in a “relay succession” recovery where hardwoods dominate the canopy for a time before slower growing spruce can regain dominance of the canopy. In either case, if the stand reburns within about 50 years, small black spruce in the understory are destroyed before developing a robust seed bank, and the stands tend to become increasingly hardwood dominated. Ongoing studies to refine these parameters in eastern interior Alaska reveal that pure black spruce stands transitioned to deciduous forest about half the time after a severe fire⁴. Coniferous forest with even a small percentage of deciduous stems (> 7%) succeeded into a mixed-wood composition (50:50) and stands which had >30% pre-fire deciduous stems became exclusively deciduous.

Why is a shift in forest species important? Spruce forests have vastly different properties than broadleaf forests. These differences include: higher flammability, lower water uptake, slower litter decomposition rates, different atmospheric heating properties (surface albedo), and carbon sequestration capacity. Of course, they also differ with respect to supplying wood, game, and berries to humans. Black spruce trees thrive in cold, moist soil conditions, and actually enhance these conditions by accumulating a deep mossy soil organic layer. The moss layer is often too moist–with snowmelt water, rainfall, and seasonally thawing ice below–to support combustion beyond a few centimeters deep. Most of the carbon (C) stored in these slow-growing forests is in the moss and soil organic layers. Warmer summers and extended burning seasons in late summer tend to allow the moss duff layers to dry faster and deeper. In western Canada, 90% of C combusted in extensive 2014 fires in black spruce forest came from the forest floor ⁵. Deciduous trees, on the other hand, thrive in nutrient rich, dry and (relatively) warm soils, and reinforce these conditions with high decomposition rates, so the soil organic layer tends to be shallow. Their faster growth rate (productivity 5-7 times greater than black spruce forest) means that they store most of their C aboveground in the trees themselves⁶. Deciduous trees also require more water to sustain that rapid growth—a lot more! It was recently discovered that Alaska broadleaf trees take up 25% of available spring snowmelt water (compared to about 1% for spruce)⁷. But mature spruce forests (aged 70-120 years) win out when it comes to long-term C storage — they had 4-10 times the C stored in their organic soils (2.0-5.7 kg C m-2) compared to 50-year-old stands⁸.

Was the forest shift prediction accurate? Back to the changing forest composition. What does the latest research tell us about the predictions of a decade ago? As part of a multi-year NASA study, Wang, et al. 2020⁹ used an enormous data set of satellite-derived vegetation cover type estimates covering all Alaska and northwestern Canada from 1984 to 20149. They documented a net loss of 14.7% (+/- 3%) coniferous forest along with an equivalent net gain of deciduous forest. These results seem consistent with the predicted forest composition shift caused by a more active fire regime in boreal forests relative to the last several millennia. In an ongoing separate study, preliminary data showed that in burned areas of the boreal biome the ratio of deciduous to evergreen forest cover increased 14.4% (from 23.4% to 37.8%) from 2001-2016, which was 3.3 times higher than in unburned areas¹⁰. However, these same authors also documented that mixed and deciduous forest types may be burning more now compared to previous decades, perhaps even burning at a faster rate than they were being replaced during the study time window. In addition, by coupling two types of remotely sensed data for vegetation, the researchers could estimate forest biomass and tree-canopy structure. This analysis showed that recovering burned forest, whether it succeeded to deciduous forest or remained evergreen, still had overall lower biomass and canopy density compared to areas that did not burn. This finding has implications for management scenarios involving fire use to reduce future fuel loadings, and also for carbon sequestration studies. Separate investigations confirm the loss of forest biomass across eastern interior Alaska and predict this will become the modern trend as the climate warms⁴. Impacts on wildfire and permafrost dynamics will result in overall decreases in biomass (particularly for spruce within the interior Tanana Valley, despite increases in quaking aspen biomass) and result in a continued shift towards a higher deciduous fraction. To a lesser extent, increased biomass is seen at certain locations, such as cold or wet locations, and at high elevations, such as along the north slopes of the Alaska range¹¹. In summary, recent studies seem to support the predictions made a decade ago1 with respect to spruce forest declining and broadleaf and mixed forest increasing across interior Alaska.

What about negative feedbacks to boreal burning? Deciduous forests are more resistant to fires than spruce forests—a guiding principle of much fire suppression and fuel treatments strategy¹². Under average summer conditions, coniferous forest is more conducive to large fire spread and accounts for at least half of the average yearly burned extent. Since hardwood forests are less flammable, why doesn’t a negative feedback kick in to reduce flammability as deciduous forests become more common on the landscape? The short answer is that most mega-fires burn in unusually hot/dry summers, and once the fire weather gets extreme enough, deciduous forests and regenerating forests become equally receptive to burning¹³. The “unusual” heat is becoming more “usual” in recent decades, and future climate scenarios predict dramatic increases in high fire danger days. As for the future of forests in Alaska, new models at the stand level are being developed which include the latest understanding of forest environmental drivers and physiological constraints^4,11. A research study funded by the Department of Defense in eastern interior will be sharing findings regarding fire-prone areas of Alaska from Fairbanks to the Canadian border, complete with web visualization tools for managers (beta version:
https://masseyr44.users.earthengine.app/view/decidfractreecoverserdpv1).

The takeaway: Alaska’s boreal forest is undergoing major changes driven by the domino effects of changing climate. Boreal forests are experiencing changes in the fire regime, which through interactions with the ecological and physiological attributes of trees are causing widespread shifts in forest composition. Those shifts in turn are causing a shift towards net C release to the atmosphere which could accelerate global warming. Overall, these shifts in forest species dominance are a fascinating example of how even a relatively simple biome like boreal forest can have complex responses to changes in climate.

Acknowledgements:
Many thanks to Dr. Dan Mann and Dr. Eric Deutsch for helpful discussion and
interpretation and to Zav Grabinski for editing help.

References: 1 Mann, D. H., T. S. Rupp, M. A. Olson, and P. A. Duffy. 2012. Is Alaska’s boreal forest now crossing a major ecological threshold? Arctic Antarctic and Alpine Research, v. 44, no. 3, p. 319-331. 2 Baltzer, Jennifer et al. 2019. Widespread ecological reorganization of boreal forests following severe wildfires. AGU Poster B33B-07, San Francisco, CA Dec. 9-12, 2019. 3 Hollingsworth, T. N., J. F. Johnstone, E.L. Bernhardt, S. F. Chapin. 2013. Fire severity filters regeneration traits to shape community assembly in Alaska’s boreal forest. Plos One 8(2):e56033. 4 Goetz, Scott, et al. (ongoing) SERDP Project RC18-C2-1183: Resiliency and Vulnerability of Boreal Forest Habitat across DoD Lands of Interior Alaska. 5 Walker, X. et al. 2018. Cross-scale controls on carbon emissions from boreal forest megafires. Global Change Biology 24 (9): 4251-4265. 6 Alexander, H. and M. Mack. 2016. A Canopy Shift in Interior Alaskan Boreal Forests: Consequences for Above and Belowground Carbon and Nitrogen Pools during Post-fire Succession. Ecosystems 19: 98-114. 7 Young-Robertson, J. M., W.R. Bolton, U.S. Bhatt, J. Cristóbal, R. Thoman. 2016. Deciduous trees are a large and overlooked sink for snowmelt water in the boreal forest. Scientific Reports 6:29504. 8 Hoy, E.E., M.R. Turetsky and E.S. Kasischke. 2016. More frequent burning increases vulnerability of Alaskan boreal black spruce forests. Environ. Res. Lett.11 095001 9 Wang, J. A., D. Sulla-Menashe, C.E. Woodcock, O. Sonnentag, R.F. Keeling and M.A. Friedl. 2020. Extensive Land Cover Change Across Arctic-Boreal Northwestern North America from Disturbance and Climate Forcing. Global Change Biology 26, 807–822. 10 Deutsch, E.J. and M.L. Chipman. 2020. Observations of Post-Wildfire Land Cover Trends in Boreal Alaska Using Geospatial Analyses. AGU Poster, Virtual, Dec. 1-17, 2020. 11 Foster, A.C. et al. 2019. Importance of tree- and species-level interactions with wildfire, climate, and soils in interior Alaska: Implications for forest change under a warming climate. Ecol. Modeling 409: 108765. 12 Beverly, J. L. 2017. Time since prior wildfire affects subsequent fire containment in black spruce. International Journal of Wildland Fire 26:919–929. 13 Barrett, K, T. Loboda, A.D. McGuire, H. Genet, E. Hoy, and E. Kasischke. 2016. Static and dynamic controls on fire activity at moderate spatial and temporal scales in the Alaskan boreal forest. Ecosphere7(11): e01572. 10.1002/ecs2.1572. 14 Dash, C.B., J.M. Fraterrigo, and F.S. Hu. 2016. Land cover influences boreal forest fire responses to climate change: geospatial analysis of historical records from Alaska. Landscape Ecology 31:1781–1793.