Disturbances of the Alaskan Boreal Forest
The major disturbances of the Alaskan boreal forest include wildfire, flooding, insect/pathogen outbreaks, permafrost thaw, and drying of lakes and streams. These disturbances impact ecosystem function directly through the changes they impose on the physical environment, and indirectly through their effects on vegetation. The nature of disturbance strongly influences the pattern of plant succession, and the properties of the vegetation, in turn, influence the disturbance regime. These disturbances, which are closely related to climate, have all become more extensive now than at any point in recorded history, and will likely result in substantial changes in the structure and functioning of the Alaskan boreal forest and its role in the global climate system (Chapin et al., in press).
Thermokarst photo by Ted Schuur
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Wildfire photo by La'ona DeWilde
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Rosie Creek fire photo by Glenn Juday
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The areal extent of wildfire has increased throughout the boreal forest (Soja et al. 2007), with the annual area burned in interior Alaska twice as high over the last decade than any decade over the previous 40 years (Duffy et al. 2007) (Fig. 1). Warm, dry weather allows fires to continue burning in late summer, when soils are deeply thawed, have lower soil moisture, and therefore burn more deeply (Kasischke et al. 2007), creating a radically different environment for seedling establishment of mineral soil seedbeds (Johnstone and Kasischke 2005). The increase in mineral soil seedbeds as well as a shortened fire interval (Johnstone and Chapin 2006), promotes the establishment and dominance of deciduous trees, rather than evergreen trees, except in extremely dry sites where no tree recruitment may occur (Johnstone et al., in press) (Fig. 2). The increase in fire frequency, area, and severity, therefore suggests potential shifts in the relative abundance of forest types that currently dominate the Alaskan boreal forest: a decline in abundance of black spruce, which has dominated the lowland landscape and north-facing slopes for the last 6,000 years, a potential increase of deciduous forests in former black spruce habitat, and a conversion to grass or shrublands on dry sites (Johnstone et al. 2008).
Figure 1. Characteristics of the Alaska fire regime over the past 5 decades (drawn from Kasischke et al. in press).
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Figure 2. Results of a boosted regression tree model predicting the relative dominance of black spruce vs. deciduous trees in early post-fire communities following the 2004 fires (n=78). The bar chart shows the relative influence of different variables in the model. Line charts are partial dependency plots representing the estimated marginal effect of the three most important variables on y when all other variables are held at their average. Results are presented for the simplest model that minimized prediction error (final prediction error of 42%) (redrawn from Johnstone et al., in press).
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Deciduous forests, until now, have been largely restricted to south-facing uplands and floodplain corridors and have acted as a stabilizing feedback to fire probability and spread because of their high leaf moisture content and low flammability. As climate warms, however, vegetation effects on flammability decline, weakening this stabilizing feedback, so the areal extent of fire is projected to continue increasing with climate warming despite the shift to deciduous vegetation (Kasischke et al. in press). Hardwoods accumulate less soil organic soil than spruce ecosystems (Mack et al. 2008), so increased hardwood dominance might reduce carbon sequestration at landscape scales.
Spruce budworm photo by Robert Ott
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Insect and pathogen outbreaks are characteristic of the North American boreal forest and in recent decades have caused widespread mortality and sub-lethal damage in interior Alaska (Figs. 3, 5). Many of the dominant tree and shrub species the Alaskan boreal forest are susceptible to drought stress, which is expected to intensify with climate warming and further increase host vulnerability to insect and disease damage. In this region, insect and pathogen species acting as biotic disturbance agents are themselves responsive to environmental cues, causing a complex pattern of environmental signals, pathogen and host responses, disturbance effects, and forest adjustments (Juday et al, in review). In addition to the effects of long-established insects and pathogens, biological invasions of non-native insect species, possibly including larch sawfly and amber-marked birch leaf miner, have affected large areas of the Alaskan boreal forest.
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Figure 3. Alaska statewide area affected by outbreaks of the four principal tree-damaging insects affecting BNZ during the period of record (Juday et al., in review)
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Figure 4. Spruce budworm temperature predictive index at Fairbanks (F-SBW), for the period of weather record, 1907-2007 (black line) and BNZ spruce budworm monitoring data, 1975-2007 (red line). Years of extreme weather with known or potential significant relationship to spruce budworm population status are indicated; P = potentiating (1913, 1942, 1958, 1969, 1975), S = sterilizing (1910, 1932-33, 1949), E = exclusionary (1981-82, 1985, 1999-2000), O = outbreak (1978, 1989-90, 1993&95, 2005-06) (Juday et al. in review).
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Irruptions of spruce budworm, larch sawfly, aspen leaf miner, and Ips/ engraver beetles have had particularly strong or widespread impact on forests (Fig. 3). The spruce budworm has appeared at outbreak levels in central Alaska since 1989. Spruce budworm population sizes are tightly linked to air temperatures at key developmental stages of the budworm life cycle, which are projected to increase with continued climate warming (Fig. 4).
Figure 5. Relationship between the proportion of Alnus tenuifolia ramets either dead or colonized by stem canker at replicate in early-successional stands along the Eagle River (ER), Quartz Creek (QC) and Tanana River (TAN) (Ruess et al. 2009)
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Dana Nossov
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Recent outbreaks of a fungal stem canker (Valsa melanodiscus) are causing widespread dieback and mortality of thinleaf alder (Alnus tenuifolia) throughout central and southcentral Alaska (Fig. 5), causing significant declines in ecosystem nitrogen inputs derived from N-fixation (Ruess et al. 2009). Thinleaf alder plays a fundamental role in influencing ecosystem processes and community dynamics, and the long-term impacts of the canker on biogeochemistry and successional dynamics will depend on whether alder populations recover from the current epidemic.
References:
Chapin FS, III, et al. (in press) Resilience to climate change in Alaska's boreal forest. Canadian Journal of Forest Research.
Duffy PA, Epting J, Graham JM, Rupp TS, & McGuire AD (2007) Analysis of burn severity patterns using remotely sensed data. International Journal of Wildland Fire 16:277-284.
Johnstone JF & Chapin FS, III (2006) Effects of burn severity on patterns of post-fire tree recruitment in boreal forests. Ecosystems 9:14-31.
Johnstone JF & Kasischke ES (2005) Stand-level effects of soil burn severity on postfire
regeneration in a recently-burned black spruce forest. Canadian Journal of Forest
Research 35:2151-2163.
Johnstone JF, Hollingsworth TN, & Chapin FS, III (2008) A key for predicting postfire successional trajectories in black spruce stands of interior Alaska (USDA, US Forest Service, Pacific Northwest Research Station, General Technical Report PNW-GTR-
767).
Johnstone JF, Hollingsworth TN, Chapin FS, III, & Mack MC (in press) Changes in boreal fire regime break the legacy lock on successional trajectories in the Alaskan boreal forest. Global Change Biology doi: 10.1111/j.1365-2486.2009.02051.
Kasischke ES, Bourgeau-Chavez LL, & Johnstone JF (2007) Assessing spatial and temporal variations in surface soil moisture in fire-disturbed black spruce forests in Interior Alaska using spaceborne synthetic aperture radar imagery - Implications for post-fire tree recruitment. Remote Sensing of Environment 108(1):42-58.
Kasischke ES, et al. (in press) Alaska's changing fire regime - implications for the vulnerability of its boreal forests. Canadian Journal of Forest Research.
Mack MC, et al. (2008) Recovery of aboveground plant biomass and productivity after fire in mesic and dry black spruce forests of interior alaska. Ecosystems 11(2):209-225.
Ruess RW, McFarland JM, Trummer LM, & Rohrs-Richey JK (2009) Diseasemediated declines in N-fixation inputs by Alnus tenuifolia to early-successional floodplains in interior and south-central Alaska. Ecosystems 12:489-501.
Soja AJ, et al. (2007) Climate-induced boreal forest change: Predictions versus current observations. Global and Planetary Change 56(3-4):274-296.
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