FW-I-2: Incremental growth in timber | Umweltbundesamt

FW-I-2: Incremental growth in timber

The incremental growth rates for beech, oak and pine trees have declined distinctly since 2002. It is assumed that dry years and their aftereffects on forest ecosystems have a detrimental influence on growth rates. The higher growth rate in respect of spruce stands in the period of 2012 to 2017 can probably be attributed to the fact that there were no supraregional damage events during that period and that precipitation was of average extent.

Changes in incremental growth

How quickly trees grow and how much timber volume per time unit develops, is essentially dependent on the nutrient and water supply at their location and also on the prevailing temperatures. In montane topography or in cold hollows which have so far had limited warmth, temperature increases can indeed have positive impacts on incremental growth in stands with stagnating growth. However, in areas such as the Upper Rhine plateau where growth is already limited now in many places owing to heat or drought, further increases in temperature and increasing drought caused by climate change will have detrimental impacts on timber growth. In general it is expected that changes in the weather associated with climate change will have different impacts on timber growth in respect of specific locations and stand compositions.

Apart from weather-related effects, there is also a lot of discussion regarding the fertilising effect of increased CO2 concentrations in the atmosphere. This increase can, in principle, benefit productivity unless there are other factors restricting carbon absorption. Another important influential factor is the age structure of forest stands. In young trees less than 20 years old, volume growth is low. In subsequent years of the life of these trees, volume growth will increase considerably while in old trees it will decline according to the species concerned. Any analysis of growth data will therefore have to take the age factor into account. The values of the indicator illustrated have therefore been adjusted according to relevant age categories.

The interaction of all influencing factors combined is complex, which is why the resulting impacts on future growth are hard to predict. It is abundantly clear, however, even now that there will be winners and losers regarding the effects of climate change depending on the specific site conditions in a forest. In principle, productive timber growth, apart from the quality of the timber, is a relevant variable in forestry, as it ultimately determines the level of achievable timber yields. If growth increments in commercial forests decline continuously to a considerable extent – for instance owing to unfavourable weather conditions – it may be possible to use targeted silvicultural management measures to actively reduce stand density and competition within stands, thus counteracting a decrease in the incremental growth of trees in a forest. Besides, incremental timber growth is also important insofar as it is a prerequisite for a forest’s ability to function as a carbon sink. The more timber accrues in a forest, the more carbon dioxide can be extracted from the atmosphere for storage as carbon in the timber (cf. Indicator FW-R-4). It is assumed that every cubic metre of timber stores approximately 250 kg of carbon. This means that forests with a positive carbon regime make a valuable contribution to protection from climate change.

The outcomes of National Forest Inventories available so far are starting points for the establishment of a long-term time series pertaining to timber growth increments. From 2002 onwards, there are nationwide inventory data available. The data provide opportunities to draw inferences regarding the effects of extreme weather situations in a specific observation period. For example, it was no longer possible to find evidence, especially for spruce forests, in the old-established Länder for the period from 2002 until 2008 in the same volumes as the high average timber increments which were found in those areas until the end of the 20th century. It is assumed that, especially during the hot and dry years of 2003 and 2006, productivity losses will have occurred. However, it must be said that also in the subsequent period of 2008 to 2012 in which there were no drought years, the nationwide mean of timber increments for the four main tree species declined further, with pine trees most badly affected, followed by beech trees. One reason for this may be the increasing average age of all main tree species which would lead to decreasing timber increments.

During the observation period of 2012 to 2017 the incremental growth rates for beech, oak and pine trees declined further. By contrast, timber increments for spruce forests have shown a surprising increase – higher in that period than in the two preceding periods. More than the other main tree species, the spruce tree benefitted from the absence of supraregional damage events (such as storm, drought and infestation with insects), while also benefitting from average precipitation during the period in question. However, it must be taken into account that the surveys were completed prior to the heat and drought years of 2018 to 2020. The current situation would present a distinctly different picture.

A longer time series will make it possible in future to illustrate the long-term impacts of climate change on timber growth increments.