Fire has been an important driver of forest dynamics in the Carpathian Mountains during the Holocene

Abstract

Fire frequency and severity are key parameters in evaluating fire-mediated changes in ecosystems, but these metrics are rarely reconstructed at extensive temporal scales. Notably our understanding of the role of fire regime dynamics in the functioning and biodiversity of Central Eastern European temperate forests is limited because investigation of the effect of fire has been neglected. To fill this gap in knowledge, we applied a multi-proxy approach (macrocharcoal, charred remains, pollen, plant macrofossils) to two sedimentary sequences spanning stands of closed canopy Picea abies to the P. abies treeline located in the northern Carpathians, Romania. We found that climate exerts a broad-scale influence, whereas vegetation feedbacks strongly modulate this fire-climate relationship. Fire has been almost continuously present throughout the Holocene with a remarkably stable mean fire rotation (∼250yr) with fires of mostly low to mid severity and/or small to medium size. Humans have shifted the fire regime during the last 2800years to slightly longer fire return intervals (mean 300yr) and more biomass consumption per fire. We found that P. abies was favoured by low to moderate fire severity/area burned. The establishment of late-successional, shade tolerant Fagus sylvatica was facilitated by fire disturbances, but its expansion coincided with major gaps in fire events. This highlights the key role of fire in the expansion of F. sylvatica that seems only possible in a low/small to mixed severity/size fire regime with a sufficiently long fire return interval. High magnitude charcoal peaks negatively affected F. sylvatica. We found more diverse pollen assemblages, especially taxa linked to anthropogenic impact, at times of moderate fire disturbance corroborating the intermediate disturbance hypothesis. In terms of forest management, our results show that, contrary to current understanding, fire is a natural and important driver of vegetation change in this region. The anticipated increase in fire activity with the climate warming and/or augmented fuel accumulation may threaten the dominant forest ecosystems, given that these are adapted to low frequency and severity fires. We advise forestry to consider the effects of fire as part of climate-change conservation strategies. Diversifying the forest tree mixture with more fire-resistant native species is desirable in order to mitigate the effects of increased fire occurrence and severity.