Invasion of Norway spruce diversifies the fire regime in boreal European forests

Abstract

1. Global wildfire activity and biomass burning have varied substantially during the Holocene in both time and space. At the regional to continental scale, macroclimate is considered to be the predominant control regulating wildfire activity. By contrast, the role of forest tree composition is often considered as a subsidiary factor in studies addressing temporal variation in regional wildfire activity.2. Here, we assemble a spatially comprehensive data set of 75 macroscopic charcoal records that reflect local burning and forest landscapes that are spread over a substantial part of the European boreal forest, spanning both oceanic and continental climates. 3. We show that the late-Holocene invasion of Norway spruce Picea abies, a new forest dominant in northern Europe, significantly reduced wildfire activity, thus altering forest disturbance dynamics at a subcontinental scale. 4. Synthesis. Our findings show that a biotic change in the local forest ecosystem altered the fire regime largely independent of regional climate change, illustrating that forest composition is an important parameter that must be considered when modelling future fire risk and carbon dynamics in boreal forests.

Holocene fire in Fennoscandia and Denmark

“…Fire appears to decrease in sites as P. abies becomes regionally established in Finland (7800-5500 cal years BP) and in Sweden (5500-3000 cal years BP). This negative correlation between P. abies and fire has been well documented for Fennoscandia, with both the spread of P. abies being held responsible for a reduction in fire (Ohlson et al 2011), and a decrease in fire being blamed for the subsequent spread of P. abies (Bjune et al 2009). Here, our compilation indicates that the change in fire regime occurs once P. abies is regionally established.…”

Biodiversity, carbon storage and dynamics of old northern forests

“…Yet smaller scale disturbances, caused by heart-rot, insect attacks, wind throws, snow breaks, may result in patch or gap dynamics where single trees (gaps) or small groups of trees (patches) die and the forest develops a more varied age structure (Kuuluvainen & Aakala 2011). The variable role of fire as a disturbance agent is also emphasized by Ohlson et al (2009Ohlson et al ( , 2011 who showed that fires have varied considerably across Fennoscandia, having often been less extensive and frequent than previously considered (e.g., Zackrisson 1977). The frequency and effects of fires depended on both climate (few fires in oceanic climates) and the establishment of spruce as a dominant forest tree (resulting in far fewer fires than before) (although Hellberg et al (2009) found that fire was an important disturbance agent in spruce forests in their study area).…”

Management-driven evolution in a domesticated ecosystem

“…Natural fire frequencies in these boreal habitats vary considerably in space and time but are much lower than those of the managed coastal heathlands [22][23][24][25]. To evaluate evolutionary impacts of the anthropogenic fire regimes, we assessed germination responses to smoke of Calluna seeds [9,11,13 -15]).…”