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.

在2018年，达到400万米³的挪威云杉被云杉小蠹虫杀死Ips的typographus在瑞典。这一事件对瑞典来说是独一无二的，无论是受影响的数量还是由严重的干旱压力引发的事实，而不是由于相对无防御的风暴砍伐树木的充足供应。疫情在 2019 年和 2020 年继续爆发，每年被杀死的树木数量是 2018 年的两倍。本研究的目的是量化季节性变化和严重干旱胁迫后一年树木防御能力的潜在滞后效应。2019 年 5 月至 8 月期间，在三个田间地点进行了与树皮甲虫相关的蓝染真菌的接种，重复四次，间隔一个月，分别代表早发和晚发芽的瑞典和东欧云杉原产地。

2018 年所有地点都经历了中度至重度干旱胁迫，特定地点的防御能力与接种前两个月的累积降水量呈正相关。两个月降水量 <100 毫米的地点在接种后韧皮部有较大的坏死病变，表明树木防御能力较低。病灶大小在来源之间没有差异，并且所有树木都能够成功地限制真菌感染。

坏死病灶大小存在季节性差异，Skärsnäs 和 Norberg 6 月份病灶明显大于 5 月份，Lugnet 5 月份病灶也较大。7 月和 8 月的病灶一般小于 6 月。

在来自 Lugnet 的一个地点的边材样品中测量了外伤性树脂导管的横截面积和数量，以量化树木防御的另一个方面。5、6月份树脂风管生产面积大于7、8月份。这与病变面积与树脂管面积呈正相关，表明春季接种后较强的真菌感染引发了较强的诱导防御反应。东欧出处比瑞典出处有更多的树脂导管，但树脂导管的面积在不同出处之间没有显着差异。