Invasion of Norway Spruce Diversifies the Fire Regime in Boreal European Forests

Since the Holocene ¾a geological epoch that began around 10,000 BC, global wildfire activity and biomass burning has varied greatly. At a regional to continental scale, it is generally accepted that macroclimate is the primary control that regulates wildfire. Forest and vegetation composition is usually treated as a secondary factor in studies assessing temporal variation in regional wildfire activity. Ohlson and team (2011) gather a spatially comprehensive data set of macroscopic charcoal records that illustrate forest landscapes and local burning spread over a large area of the European boreal forest. They demonstrate that the invasion of the Norway spruce Picea abies has greatly reduced wildfire activity, thereby changing forest disturbance dynamics at a subcontinental scale (Ohlson et al. 2011). Their findings reveal that independent of regional climate change, a biotic change in the forest ecosystem altered the region’s fire regime, demonstrating that forest composition is a significant factor that must be considered when modeling carbon dynamics and fire risk in boreal forests. –Loren Stutts
Ohlson, M., K. Brown, H.J.B. Birks, J. Grytnes, G. Hörnberg, M. Niklasson, H. Seppä, and R.H.W. Bradshaw 2011. Invasion of Norway spruce diversifies the fire regime in boreal European forests, Journal of Ecology, 99, 395-403, doi: 10.1111/j.1365-2745.2010.01780.

Changes in the amount of a single species can bring about significant alterations in the properties and behaviors of an ecosystem. Forest structure and biodiversity were profoundly affected with the invasion of Norway spruce Picea abies in northern Europe as the spruce emerged as a new boreal forest keystone species. Fire regime is a significant ecosystem process that was also affected by this ecosystem transformation. Fire regimes vary as a result of a complex interplay between human activities, climatic variability, sources of ignition, and vegetation and fuel characteristics. Yet at regional and continental scales, climatic factors are considered to be the primary controls that regulate fire regime. Forest tree composition is usually viewed as subsidiary factor in studies evaluating variation in wildfire activity. However Ohlson and team claim that new research on the interactions between forest tree species composition and fire in the boreal forests of Alaska have shown that vegetation composition can be an important driver of wildfire activity.
Ohlson and team assemble a network of peat, tree-ring, and humus records from forests landscapes down the longitudinal axis of Scandinavia to analyze fire disturbance and forest composition in the boreal forest of northern Europe. The records gained from their sampling reveal the history of local forest composition and a comparison of fire history before and after local spruce invasion at both local and regional spatial scales. Ohlson and team assert they can evaluate the influence of both climate and vegetation composition on fire regime through the comparison of charcoal and spruce pollen records. They sampled 75 spruce forest sites located in 24 forest landscapes in the boreal forest of northern Europe. The difference between pre and post-spruce charcoal concentrations were tested statistically while partial cross-sections were cut out of living trees to record fire scars and dated by counting tree rings.
The differences in charcoal values between pre- and post-spruce sections of the 75 soil profiles lead the team to make several conclusions. In terms of the general pattern of charcoal deposition across the soil sites, there was a significant decrease in abundance and deposition rate of charcoal following the local invasion of spruce. Concurrence between local spruce invasion and local cessation of wildfire is revealed by the permanent and immediate absence of historically substantial charcoal deposition following the emergence of spruce forest. Tree-ring analysis revealed that in spruce forests, no wood recorded past fires while charcoal records illustrated a complete lack of fire activity at various sites following the local invasion of spruce.
Ohlson and team draw several conclusions from their results.  First they assert that fire disturbance is not ubiquitous. Of the 75 forest sites analyzed in their study, 13 produced no macroscopic charcoal at all while seven produced records that showed a sporadic single peak occurrence of charcoal particles. They claim that such a large proportion of sites yielding little or no charcoal challenges the common notion that wildfire is a ubiquitous, important and frequent disturbance factor in the boreal European forest. Secondly, Ohlson and team claim that regional macroscale climate exerts a broad influence on fire regime. The marked variation of their charcoal records revealed a significant variability in the fire regime across forest sites.
 Next, Ohlson and team conclude that spruce invasion had a critical influence on fire regime. Specifically they claim their results reveal a close correspondence between the invasion of spruce and the decline in charcoal concentration, thereby suggesting that a change in the dominant tree species had a critical effect on the fire regime that exceeded the broad influence of climate change. Lastly they assert that the fire regime of northern Europe was diversified by the spruce invasion, which induced a more variable spatial occurrence of fire at the landscape scale. Specifically their records showed that the spread of spruce led to a marked reduction in local fire frequency and severity. The spruce invasion further diversified the fire regime by reducing wildfire activity significantly in mesic/moist forest types that typically occur in concave landscape forms. Ohlson and team also discuss how the reduction in burning as a result of the spread of spruce in northern Europe has most likely resulted in increased sequestration of carbon in forest ecosystems and thus initiated a major biological feedback to the climate system.

Ohlson and team do not claim their specific findings on the linkages between boreal forest composition and wildfire to be universal, but instead wish to raise awareness to the fact that tree species composition is an important factor capable of regulating the fire regime. They suggest replacing the concept of fire disturbance as a major determinant of boreal forest composition in favor of maintaining biological continuity

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