Search

Stand-replacing wildfires are a keystone disturbance in the boreal forest, and they are becoming more common as the climate warms. Paleo-fire archives from the wildland–urban interface can quantify the prehistoric fire regime and assess how both human land-use and climate change impact ecosystem dynamics. Here, we use a combination of a sedimentary charcoal record preserved in varved lake sediments (annually layered) and fire scars in living trees to document changes in local fire return intervals (FRIs) and regional fire activity over the last 500 years. Ace Lake is within the boreal forest, located near the town of Fairbanks in interior Alaska, which was settled by gold miners in AD 1902. In the 400 years before settlement, fires occurred near the lake on average every 58 years. After settlement, fires became much more frequent (average every 18 years), and background charcoal flux rates rose to four times their preindustrial levels, indicating a region-wide increase in burning. Despite this surge in burning, the preindustrial boreal forest ecosystem and permafrost in the watershed have remained intact. Although fire suppression has reduced charcoal influx since the 1950s, an aging fuel load experiencing increasingly warm summers may pose management problems for this and other boreal sites that have similar land-use and fire histories. The large human-caused fire events that we identify can be used to test how increasingly common megafires may alter ecosystem dynamics in the future. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Forest Research is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Many boreal forests grow in regions where climate is now warming rapidly. Changes in these vast, cold forests have the potential to affect global climate because they store huge amounts of carbon and because the relative abundances of their different tree species influence how much solar radiation reflects back to space. Both the carbon cycling and albedo of boreal forests are strongly affected by wildland fires, which in turn are closely controlled by summer climate. Here we use a forest disturbance model in both a retrospec-tive and predictive manner to explore how the forests of Interior Alaska respond to chang-ing climate. Results suggest that a widespread shift from coniferous to deciduous vegetation began around A.D. 1990 and will continue over the next several decades. This ecological regime shift is being driven by old, highly flammable spruce stands encountering a warmer climate conducive to larger and more frequent fires. Increased burning promotes the spread of early successional, deciduous species at the expense of spruce. These striking changes in the vegetation composition and fire regime are predicted to alter the biophysics of Alaska's forests. The ground will warm, and a surge of carbon emission is likely. Our modeling results support previous inferences that Alaska's boreal forest is now shifting to a new ecological state and that positive feedbacks to global warming will accompany this change. [ABSTRACT FROM AUTHOR]

Postfire succession in the Alaskan boreal forest follows several different pathways, the most common being self-replacement and species-dominance relay. In self-replacement, canopy-dominant tree species replace themselves as the postfire dominants. It implies a relatively unchanging forest composition through time maintained by trees segregated within their respective, ecophysiological niches on an environmentally complex landscape. In contrast, species-dominance relay involves the simultaneous, postfire establishment of multiple tree species, followed by later shifts in canopy dominance. It implies that stand compositions vary with time since last fire. The relative frequencies of these and other successional pathways are poorly understood, despite their importance in determining the species mosaic of the present forest and their varying, potential responses to climate changes. Here we assess the relative frequencies of different successional pathways by modeling the relationship between stand type, solar insolation, and altitude; by describing how stand age relates to species composition; and by inferring successional trajectories from stand understories. Results suggest that >70% of the study forest is the product of self-replacement, and tree distributions are controlled mainly by the spatial distribution of solar insolation and altitude, not by time since last fire. As climate warms over the coming decades, deciduous trees will invade cold sites formerly dominated by black spruce, and increased fire frequency will make species-dominance relay even rarer. La succession après feu dans la forêt boréale de l’Alaska adopte différents modes, les plus communs étant le retour des mêmes espèces et la dominance successive de différentes espèces. Dans le cas du retour des mêmes espèces, les espèces d’arbres qui dominent la canopée sont remplacées par les mêmes espèces qui deviennent dominantes après un feu. Cela implique que la composition de la forêt qui demeure relativement stable dans le temps soit maintenue par des arbres qui sont restreints à leur niche écologique respective dans un paysage complexe du point de vue environnemental. Au contraire, la dominance successive de différentes espèces suppose l’établissement après feu de plusieurs espèces d’arbres suivi par des changements ultérieurs de dominance dans la canopée. Cela signifie que la composition du peuplement change avec le temps après un feu. Les fréquences relatives de ces modes de succession et d’autres sont peu connues malgré leur importance dans la détermination de la mosaïque d’espèces de la forêt actuelle et de leurs différentes réactions potentielles aux changements climatiques. Dans cet article, nous évaluons la fréquence relative de différents modes de succession en modélisant la relation entre le type de peuplement, l’ensoleillement et l’altitude; en décrivant comment la composition en espèces est reliée à l’âge du peuplement et en déduisant le mode de succession à partir du sous-étage d’un peuplement. Les résultats indiquent que plus de 70 % de la forêt étudiée est le résultat du retour des mêmes espèces et que la distribution des arbres est régie principalement par la distribution spatiale de l’ensoleillement et l’altitude, non par le temps écoulé depuis le dernier feu. À mesure que le climat se réchauffera au cours des prochaines décennies, les espèces décidues vont envahir les stations froides autrefois dominées par l’épinette noire et l’augmentation de la fréquence des feux va faire en sorte que la dominance successive de différentes espèces sera encore plus rare. [ABSTRACT FROM AUTHOR]

The article explores the relationships between weather variables and the annual area burned in Alaska. The boreal forest contains roughly 40% of the world's reactive soil carbon, an amount similar to that held in the atmosphere. The biophysical phenomena affecting carbon storage and high-latitude albedo make the boreal forest an integral component of the global climate system. Fire-initiated succession underlies the biophysical factors, and there is a pressing need to characterize sensitivities and potential responses of the boreal forest disturbance regime to climatic change. The article addresses variation in the seasonality of the atmospheric circulation-fire linkage through an evaluation of both the East Pacific teleconnection field and a Pacific Decadal Oscillation index keyed to an annual fire index. It uses multiple linear regression (MLR) method. In the MLR, seven explanatory variables and an interaction term collectively explain 79% of the variability in the natural logarithm of the number of hectares burned annually by lightning-caused fires in Alaska from 1950 to 2003. Average June temperature alone explains one-third of the variability in the logarithm of annual area burned.