FW-I-6 + 7: Endangered spruce stands and spruce bark beetle
2023 Monitoring Report on the German Strategy for Adaptation to Climate Change
Table of Contents
FW-I-6: Endangered spruce stands
The risk from bark beetles breeding in the bark of spruce trees – thus endangering the trees – is very serious, especially during and after hot and dry years; it can bring about the collapse of entire spruce stands. Bark beetles thrive in warmer weather, and provided there are adequate amounts of wood to breed in, they can produce up to six generations annually in those conditions. Extremely high pressure from pest infestation can kill even healthy, vigorous trees.
The stacked columns chart FW-I-6 ’Endangered spruce stands’ shows – for the period of 1960 to 2021 – the proportion in per cent of areas in Germany planted with spruce trees, in which the spruce bark beetle would be able to achieve a specific number of potential reproduction cycles. In this process, a differentiation was made between the number of potential reproduction cycles between 1 and 6. Until roughly the end of the 1980s, between three and four potential reproduction cycles were feasible in the majority of spruce stands. As of the late 1980s, an increased number of stands were assessed as being able to support 5 potential breeding cycles. In the years of 2014 and 2018, the proportion of spruce stands with 5 potential reproduction cycles of the spruce bark beetle amounted to 60 per cent and to just under 70 per cent respectively. In those years, there were stands on roughly 5 per cent of the areas where 6 reproduction cycles were feasible. Whilst in the period of 1960 to 2021 the proportion of spruce stands with 1 or 3 potential reproduction cycles decreased significantly, the proportion in stands with 4 and more reproduction cycles increased significantly.
FW-I-7: Volume of damaged timber due to spruce bark beetle – case study
The spruce bark beetle benefits from dry hot weather and prefers to infest trees that are already damaged or weakened in some way. Infestation data from eight Federal Länder demonstrate that hot and dry years and also storm events lead to increased volumes of damaged timber. After the drought years of 2018 to 2020, the volume of beetle-damaged timber was 189 times as high as the multi-annual average recorded for the period of 1998 to 2017.
The XY scatter diagram FW-I-7 ’Volume of damaged timber due to spruce bark beetle infestation – case study’ indicates – for the period of 1998 to 2021 – the deviation of the annual volume of damaged timber infested by spruce bark beetle as a multiple of the multi-annual average for the period of 1998 to 2017. Eight individual dots represent the infestation data for eight Länder for each year, based on a logarithmic scale. As of 2018, the volumes of timber damaged owing to spruce bark beetle infestation had distinctly increased, and in 2019 this value was 189 times as high as the multi-annual average.
Prolific bark beetle reproduction
Many trees suffer from diminished vitality caused – as projected – by climate-related changes, especially during the increasing summer droughts. Thermophilic insects and pathogens can benefit from such conditions. As far as spruce trees are concerned, bark beetles – such as the European spruce bark beetle and the six-dentated bark beetle which breed in their bark – cause particular problems to this tree species. However, there are other pest organisms whose development also benefits from climate change: Coniferous trees are particularly affected by silver fir adelges and by fungi. As far as deciduous trees are concerned, pests include the common European cockchafer, the oak processionary moth, gipsy moth, the oak-boring beetle, the leaf-mining moth on horse chestnuts, the small beech bark beetle and the beech jewel beetle on beech trees. The increasing incidence of these species is thought to be associated with warm and dry summer weather.
In the case of spruce trees, damage from bark beetles has reached devastating proportions as this tree species is now extensively planted in locations which – owing to climatic changes – no longer provide the cool and moist conditions preferred by it. These issues originated some 200 years ago – the beginning of a deliberate expansion of spruce cultivation in German forests. In those days many forests were badly fragmented and thinned due to forest pasture and intensive use of timber. Owing to their undemanding nature, sturdiness, and easy propagation, spruce trees were considered the ideal tree species for rapid reforestation of large areas. The useful and versatile timber was regarded as suitable for overcoming an impending timber shortage. It is true to say, however, that spruce trees – owing to their mostly shallow root system – are vulnerable to storms and droughts. Hence, there were early indications that pure stands of spruce trees were fraught with high cultivation risks. From the late 19th century onwards, there were already repeated incidents of pest infestation or storm events which destroyed localised stands. Eventually, the extensive storm events of recent decades and latterly, the severe impacts of heat and drought became unmistakably obvious (cf. Indicator FW-I-4).
Even if climate change is not the only cause of increased pest infestation, it is assumed – for instance with regard to the spruce bark beetle – that higher temperatures cause this species to swarm earlier in the year thus enabling the development of additional generations. It is possible to calculate the potential number of reproduction cycles of the spruce bark beetle on the basis of data on temperature developments within the course of a year. Normally, the spruce bark beetle raises two generations per summer, with one female able to lay up to 80 eggs. Given optimum conditions, however, several broods are feasible, producing several generations, as well as sibling broods. Every new brood increases the number of individuals – and thus the damage potential – exponentially. In years such as 2018, a female beetle can raise a six-figure number of offspring117. The area of spruce forests available to bark beetles for four and more reproduction cycles annually has – owing to changed weather patterns – increased significantly. As a result, the risks faced by forestry organisations in terms of pest infestations have risen noticeably. This increased risk is reflected also in the data collected by eight Länder in respect of damaged timber caused by infestation with bark beetles.
These data clearly show that beetle infestations caused as a result of the heat and drought year 2003 increased rapidly in nearly all the Federal Länder examined. The after-effects continued for some subsequent years, reinforced by another very warm and dry summer in 2006. In 2007 – owing to the rather rainy month of May and a cold September – the bark beetle population was not able to reproduce quite as successfully. This species was not able to regain that opportunity until 2010 when the volume of damaged timber again reached roughly the level experienced in the hot summer of 2003. After the warm summer of 2015, the volume of damaged timber first increased moderately but ‘exploded’ eventually owing to the drought years of 2018 to 2020. Compared to the average for the period of 1998 to 2017, a 189-fold deviation from the volume of damaged wood occurred. In all eight of the Länder which filed reports, the volume of damaged timber increased by several multiples.
It is also possible to infer – from the time series – the impacts of extreme storm years (such as Lothar in 1999 and Cyril in 2007) incurred as a result of the typically increased beetle infestations of damaged or thrown trees. However, recent developments have eclipsed these events.
66 - Renner K., Fritsch U., Zebisch M., Wolf M., Schmuck A., Ölmez C., Schönthaler K., Porst L., Voß M., Wolff A. Jay M. 2021: Klimawirkungs- und Risikoanalyse 2021 für Deutschland. Teilbericht 2: Risiken und Anpassung im Cluster Land. Climate Change 21/2021, Dessau-Roßlau, 339 pp. https://www.umweltbundesamt.de/publikationen/KWRA-Teil-2-Cluster-Land
