Seasonal variation in Norway spruce response to inoculation with bark beetle-associated bluestain fungi one year after a severe drought

In 2018, up to 4 million m3 Norway spruce was killed by the spruce bark beetle Ips typographus in Sweden. The event was unique for Sweden, in terms of both affected volume and the fact that it was triggered by severe drought stress, not by ample availability of relatively defenseless storm-felled trees. The outbreak continued in 2019 and 2020, each year with twice as many trees killed as in 2018. The aim of this study was to quantify seasonal variation and potential lag-effects in tree defense capacity the year after a severe drought stress. Inoculation with a bark beetle-associated bluestain fungus, repeated four times with one-month-intervals between May and August 2019, were carried out at three field sites with spruce provenances of Swedish and East European origin representing early and late bud burst, respectively. All sites had experienced moderate to severe drought stress in 2018, and site-specific defense capacity correlated positively with the cumulative precipitation two months before inoculation. Sites with two-month precipitation levels