Boreal forest vegetation and fuel conditions 12 years after the 2004 Taylor Complex fires in Alaska, USA

Authors: HAMMOND, DARCY - University Of Idaho; STRAND, EVA - University Of Idaho; HUDAK, ANDREW - Us Forest Service (FS); Newingham, Beth

Submitted to: Fire Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2019
Publication Date: 8/26/2019
Citation: Hammond, D.H., Strand, E.K., Hudak, A.T., Newingham, B.A. 2019. Boreal forest vegetation and fuel conditions 12 years after the 2004 Taylor Complex fires in Alaska, USA. Fire Ecology. 15(32):1-19. https://doi.org/10.1186/s42408-019-0049-5.
DOI: https://doi.org/10.1186/s42408-019-0049-5

Interpretive Summary: Wildfires in the North American boreal forest are experiencing increased fire frequency and size. In 2004, 2.7 million hectares burned in the largest single fire complex, the Taylor Complex. Little is know about long-term fire effects in these regions. We sampled 32 sites along a burn severity gradient (unburned to high severity) using a remotely-sensed index (differenced Normalized Burn Ratio; dNBR) 12 years after the Taylor Complex. Our objectives were to 1) examine changes in understory species over time, 2) understand effects of burn severity on overstory trees, tree regeneration, understory vegetation, and woody fuels, and 3) characterize the influence of elevation, burn severity, woody fuel loading, forest floor depth on understory species assemblages. Density of standing live and dead trees did not change over time; however, the total number of understory species doubled over time. Burn severity decreased live tree density and hardwood saplings, while increasing dead tree density twelve years after fire. Hardwood saplings and 1000-hour fuel loads increased with elevation, while 1- and 100-hour fuel loads were influenced by aspect. Understory species composition was also influenced by burn severity. We conclude that using long-term data across burn severity gradients helps to understand long-term ecosystem responses to wildfire.

Technical Abstract: Background: Fire has historically been a primary control on succession and vegetation dynamics in boreal systems, although modern changing climate is potentially increasing fire size and frequency. Large, often remote fires necessitate large-scale estimates of fire effects and consequences, often using Landsat satellite-derived dNBR (differenced Normalized Burn Ratio) to estimate burn severity. However, few studies have examined long-term field measures of ecosystem condition in relation to dNBR severity classes in boreal Alaska, USA. The goals of this study were: 1) assess changes in dominant vegetation at plots resampled one and 12 years post fire; 2) use dNBR classes to characterize vegetation and downed woody fuels 12 years post fire; and 3) characterize the relationship between biophysical, topographic, and remotely sensed characteristics (e.g., moss and duff depth, canopy cover, elevation, aspect, dNBR) and understory species assemblages 12 years post fire. Results: Understory species richness doubled (from 39 to 73) between 2005 and 2016; some common species increased in cover over time (e.g., Ledum groenlandicum Oeder) while others decreased (e.g., Hylocomium splendens [Hedw.] Schimp.). In 2016, live and dead tree densities, tall shrub cover, and 1- and 100-h woody fuels were significantly different among dNBR classes; moss and duff depth, canopy cover, and spruce seedling density were not. Elevation and aspect significantly influenced tall shrub cover, hardwood sapling density, and downed woody fuel loads. Understory plant communities differed between unburned and all burn classes, as well as between low and high dNBR severity. Ordination analysis showed that overstory (e.g., live tree density), understory (e.g., moss depth, woody fuel loading), and site (elevation, aspect, dNBR) significantly influences understory species assemblages. Conclusion: Remeasured sites (sampled one and 12 years post fire) showed recruitment of new understory species and differing, diverse responses to burning by several common plant species. In 2016, low-severity burned sites had generally the highest woody fuel loading, which may increase risk of repeated surface burning, although the reduction in live tree density would still result in decreased fire risk and behavior. Understory community composition correlated with multiple biotic and abiotic factors, including moss depth, canopy cover, elevation, aspect, and dNBR. Overall, our findings can improve landscape-level predictions of ecosystem condition following fire based on dNBR.