Abstract
Key messageWhereas cold temperatures and artificial smoke pollution (during World War II) cause negative pointer years in northern Fennoscandian downy birches, mass outbreaks of Epirrita autumnata L. and Operophtera brumata Bkh. are the strongest growth-influencing and -synchronizing factor.Variations in radial tree growth of downy birch can be affected by short-term environmental changes. Here we examine the tree-ring width of downy birch trees for signals caused by three different disturbance factors: climatic extremes, insect outbreaks, and artificial smoke employed to hide the German battleship Tirpitz during World War II at the Kåfjord in northern Norway. Besides growth/climate response analysis using gridded climate data and a tree ring width-chronology of the studied birch trees, we analyze missing rings, pointer years, and the ability of recovery after severe disturbances on an individual tree basis using the percent change of the trees’ basal area increment with respect to a reference period. The downy birches reveal (1) a significant positive correlation with May and June temperatures, (2) a high growth sensitivity to moth epidemics (Epirrita autumnata L. and Operophtera brumata Bkh.), and (3) a distinct growth deviation in the year of intense but short-term artificial smoke pollution. We conclude that downy birches are not exclusively sensitive to unusual cold temperatures, but short-term artificial pollution and insect outbreaks cause similar growth declines in the year of occurrence. Whereas for temperature and pollution, the growth response is less coherent among trees, and fades within 2 years, the insect outbreaks synchronize growth among all downy birches. Growth declines during years of mass insect outbreaks can further be amplified by cold May and June temperatures. Our findings thereby improve the understanding of competing disturbance factors on single birch trees but also on the growth of a whole site in Fennoscandia and facilitates the detection of disturbances in birch chronologies.
Highlights
Disturbances are short-term, temporal variations of environmental conditions and force impulses in changes of the nature of forest ecosystems (Seidl et al 2017; Dale et al 2001)
We focused on single tree growth responses of B. pubescens in years of different disturbance factors in the Kåfjord in northern Norway
We find a close association between the chemical impact of the Tirpitz concealing tasks in 1944 and a growth decline for three quarter of the birch trees with a strong effect on the cambial activity of the following year
Summary
Disturbances are short-term, temporal variations of environmental conditions and force impulses in changes of the nature of forest ecosystems (Seidl et al 2017; Dale et al 2001). Its ability of growing rapidly on well-drained soils as well as on poorly developed sites predestinate downy birch trees to be a pioneer tree species, even in boreal regions (Seppä et al 2002; Beck et al 2016). Downy birches are well adapted to the environmental conditions at the latitudinal tree-line, but, there are growth-limiting disturbances, especially for rapid changes in climate conditions Downy birches are well adapted to the environmental conditions at the latitudinal tree-line, but, there are growth-limiting disturbances, especially for rapid changes in climate conditions (e.g. Opala et al 2015; Levanic and Eggertsson 2008), moth outbreaks (e.g. Karlsen et al 2013; Jepsen et al 2011) and, around the Kåfjord, short-term artificial pollution (Hartl et al 2019)
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