Pathogen induced disturbance and succession in temperate forests: Evidence from a 100-year data set in southern Sweden

Major tree species are declining in many temperate forests due to changing disturbance regimes, including invasive pests and pathogens. We examined the interaction of secondary succession and Dutch elm disease in the Swedish temperate forest reserve Dalby Soderskog, based on five tree surveys made between 1909 and 2011. The forest is characterized by the coexistence of four major European tree species: wych elm (Ulmus glabra), European ash (Fraxinus excelsior), European beech (Fagus sylvatica) and pedunculate oak (Quercus robur). After protection of the forest in 1918, lack of disturbance mainly favoured elm, while the oak population declined due to mortality of old oaks and lack of regeneration. Dutch elm disease has caused high and continuous elm mortality since 1988. As a result, increased light availability at the forest floor favoured abundant regeneration of ash, beech, and lately also oak. The recent arrival of an invasive fungal pathogen causing ash dieback may once again change the course of succession. Open space emerging from loss of elm and ash in forest reserves may be used by reserve managers to favour oak regeneration and biodiversity of semi-open woodlands once lost during succession to closed forest. We conclude that winners and losers change places as an effect of invasive pathogens, resulting in unexpected successions and both losses and gains in valuable ecological niches and habitat structures in temperate broadleaf forests.

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