Abstract
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 <100 mm had larger necrotic lesions in the phloem following inoculation, an indication of lower tree defense capacity. Lesion size did not differ between provenances, and all trees were able to confine fungal infection successfully.There were some seasonal differences in necrotic lesion size, with the sites Skärsnäs and Norberg having significantly larger lesions in June than in May, and site Lugnet having large lesions also in May. Lesions were generally smaller in July and August than in June.The cross-sectional area and number of traumatic resin ducts was measured in sapwood samples from one site, Lugnet, to quantify an additional aspect of tree defenses. The area of resin ducts produced in May and June were larger than that in July and August. This is in line with a positive correlation between lesion area and resin duct area, indicating that a stronger fungal infection following inoculation in spring triggered a stronger induced defense response. The East European provenances had more resin ducts than Swedish provenances, but the area of resin ducts did not differ significantly between provenances.
Highlights
The spruce bark beetle (Ips typographus) is a major forest pest in Europe and the risk of beetle outbreaks is strongly influenced by climatic factors
Larger trees tended to have larger necrotic lesions, and the same differences in response were detected between sites, time of inoculation and provenance classes when tree defense capacity was analyzed in terms of absolute lesion area or normalized lesion area per stem diameter (Table 2 and 3)
The relationship between tree resistance and drought stress is complex and non-linear, since there is evidence that mild drought may enhance tree resistance (Christiansen and Glosli, 1996; Zhao et al, 2011a; 2011b). In this field study we found evidence for a threshold in the sensitivity of Norway spruce to drought stress, where trees receiving less than about 100 mm precipitation in the two months preceding fungal infection had lower defense capacity than other trees
Summary
The spruce bark beetle (Ips typographus) is a major forest pest in Europe and the risk of beetle outbreaks is strongly influenced by climatic factors. In 2018 the beetles killed 3–4 million m3 spruce, and 7 million m3 in both 2019 and 2020 (Wulff and Roberge, 2021). This extreme mortality was triggered by the warm and dry summer in 2018, leading to the completion of two beetle generations and rapid population build-up. Bivoltine development of the spruce bark beetle has rarely been observed in Sweden (Ohrn et al 2014), but is expected to occur much more frequently in a warmer climate (Jonsson et al, 2012)
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