Controlling landscape-scale bark beetle dynamics: Can we hit the right spot?

Abstract

Outbreaks of spruce bark beetle Ips typographus have devastating effects on European forest landscapes. Salvage removal of windfelled trees is deemed effective to prevent the outbreaks, although adverse effects on ecosystem recovery, ecologically valuable legacies, and unclear economic performance raise concerns about the widespread use of this practice. Here, we formulated a novel management framework aiming for the salvaging of a limited number of windfelled stands with the highest leverage for outbreak risk, potentially replacing the current large-scale uniform treatments. We demonstrated our concept in a Central European forest landscape using the forest disturbance model iLand and a newly developed optimization tool based on graph theory. We simulated the removal of 33% and 66% of windfelled trees in (i) stands randomly distributed on the landscape and (ii) stands identified by the optimization algorithm. Moreover, we tested the effect of (iii) a no-treatment strategy and (iv) complete removal of windfelled trees.We found that the spatially optimized salvaging that aimed to remove 66% of the windfelled trees reduced the level of damage from bark beetles by 55%, whereas the non-optimized treatments, which removed the same volume of windfelled trees, reduced the damage by only 10%. In contrast to the reference treatments, the optimized approaches also retained their efficiency under elevated air temperature, which amplified the outbreaks. Although effects on deadwood retention were variable, the optimized treatments mitigated the economic impacts of the outbreak more effectively. The total loss of 7487 EUR ha−1, incurred without any treatment, was reduced by 36.7%–68.9% (range for 33 % and 66 % salvaging intensity), whereas the loss reduction under the reference treatments was only 27.6%–57.7%.We conclude that the spatially optimized salvaging proposed here can better balance multiple management objectives than the broadly applied uniform treatment, including the reduction of secondary disturbance from bark beetles, deadwood retention and economic goals.