Abstract
In northern Europe, a long history of human exploitation effectively eliminated legacies of natural disturbances in mixed oak forests and we currently lack understanding of the role of natural disturbance factors in affecting oak regeneration into the forest canopies. We compiled dendrochronological, observational and paleochronological data from Southern Sweden to discuss the role of forest fires in oak (Quercus spp.) dynamics. We analyzed oak age structure and its growth dynamics in six southern Swedish forests, which experienced fires between 42 and 158 years prior to our sampling. Extending our analysis over longer time frames, we studied the relationship between sediment charcoal and oak pollen in an area of south-eastern Sweden, where oak has been a common canopy species. In three of the study sites, forest fires resulted in increased oak regeneration. Although fires were generally not associated with a wave of growth releases in surviving trees, the mean basal area growth rate of oaks increased by a range of 108% to 176%, following the fires. The overall pattern indicated that historical fires in oak-dominated forests were of low severity, did not kill canopy oaks, and yet provided a window of regeneration opportunities for that species. Post-fire sprouting of oak and an increase in oak seedling densities following modern prescribed fires are consistent with this explanation. Consistent with this conclusion were significant positive correlations between charcoal concentration and the oak pollen percentage in a site in southeastern Sweden. We discuss the co-occurrence of oak and pine in the historical southern Swedish landscape, as a possible analogy to eastern North American oak-pine forests. Modern conservation policies aimed at the preservation of oak in the southern Swedish landscape should consider the use of low severity fires to maintain natural oak regeneration.
Highlights
Forests which experienced little human impact are rare in Europe and it applies to mixed oak ecosystems
The overall pattern suggested that historical fires in oak-dominated forests were of low severity as they tended to not kill a considerable proportion of canopy oaks (Fig. 2), occasionally resulted in a wave of growth releases in surviving trees (Figs. 3 and 4), and provided a win dow of regeneration opportunities for that species (Fig. 2)
The occur rence of these fires during generally wetter fire seasons (Fig. 5) points to the human-related nature of fire ignitions, consistent with a history of fire used for agricultural purposes in this part of Scandinavia (Niklasson et al 2002)
Summary
Forests which experienced little human impact are rare in Europe and it applies to mixed oak ecosystems. Oaks have been generally associated with the late and intermediate serial stages of forest development, both in Europe and North America (McCune & Cottam 1985; Abrams et al 1997; Gotmark and Kiffer, 2014; Selochnik 2014), they generally exhibit only a moderate tolerance to shade (Larsen & Johnson 1998; Niinemets & Valladares 2006; Kharchenko 2012) It is, not surprising that oaks reveal a generally positive response to natural and human-mediated disturbances that result in the complete or partial removal of the shade-tolerant forest understory, opening up forest canopies, reducing competition on the ground floor and improving access to mineral seedbeds (Lof et al 2006; Harmer & Mor gan 2007; Gotmark and Kiffer, 2014). In southern Sweden, tree-ring re constructions in combination with pollen and charcoal analysis at the scale of single stands have revealed the presence of oak under frequent fires (Niklasson et al 2002; Hannon et al 2010)
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