FW-I-7: Forest condition

The picture shows a pine crown with severe drought damage.Click to enlarge
Even drought-tolerant pines suffer from excessive drought like the black pines near Würzburg.
Source: Photograph: © Tanja Sanders / Thünen-Institut

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

FW-I-7: Forest condition

Up until 2017 there had been no indications that the condition of forests was deteriorating continuously owing to climate-related changes. However, the impacts of particularly hot and dry years such as 2003 find expression – with the exception of pine trees – in distinct needle and leaf losses observed in all main tree species. As far as deciduous trees are concerned, greater fluctuations in crown defoliation have been observed.

A line graph shows the mean crown defoliation in percent for the total forest and the tree species spruce, pine, beech and oak for the period 1991 to 2017. For beech there is a significant increasing trend, for oak it is quadratically decreasing. There is no trend for the other tree species and the total forest.
FW-I-7: Forest condition

A line graph shows the mean crown defoliation in percent for the total forest and the tree species spruce, pine, beech and oak for the period 1991 to 2017. For beech there is a significant increasing trend, for oak it is quadratically decreasing. There is no trend for the other tree species and the total forest.

Source: BMEL (forest condition survey)
 

Climate-related crown defoliation?

For many years, the condition of crowns was considered a suitable indicator for illustrating the impacts of pollutants on the vitality of forest trees. Nowadays it is known that the causes are more numerous, interacting in complex ways. Developments on the weather front have meanwhile come into much sharper focus to the same degree as the links between the temporal progress of needle and leaf loss and summer weather patterns have become obvious.

In all statistics on the development of forest condition, the hot year 2003 figures prominently owing to records of increased losses of leaves and needles affecting the main tree species. The drought-tolerant pine tree, in comparison with previous years, was the only tree species that showed no significant changes in the subsequent year of 2004. Leaf and needle losses can be the direct consequence of drought and heat but they can also be caused indirectly by increased pest infestation. In view of the relationships described above, it seems apt to discuss the influence of climate change on the condition of forest tree crowns.

However, trees losing their leaves or needles does not necessarily mean that they are damaged. In the case of deciduous trees, the spontaneous shedding of leaf mass under unfavourable conditions is frequently an appropriate adaptation response. This response helps trees to take precautions against losing too much water. The situation becomes only critical when an accumulation of droughtstress years causes crown defoliation to become a permanent feature in the affected trees. This kind of development is bound to lead to productivity losses or even to trees dying. As far as coniferous trees are concerned, the situation is slightly different, because they respond less spontaneously with needle loss owing to their greater investment into making permanent leaves. In these cases it seems more reasonable to assume that crown defoliation suggests some kind of damage affecting the trees.

The interpretation of needle and leaf defoliation becomes even more complicated when in addition potential interactions with fruiting processes are taken into account. These processes can also have considerable influence on crown condition. In so-called mast years with particularly heavy fruiting, trees reduce their investment into leaf and needle mass. That is when a tree’s crown appears more transparent.

So far, there is only scant information available on fruiting regularities to inform discussions about links with climate change. For example, in former times beech and oak trees would have mast years approximately every six or seven years, whereas these days the frequency is every two or three years. However, any tangible links with climate change remain to be proven.

Up until 2017 there had been no indications that the condition of forests was deteriorating continuously owing to climate-related changes. There is evidence, however, that unfavourable weather patterns, especially hot and dry summers, can lead to needle and leaf losses. In 2018 it was noted in some locations that even pine trees suffered heavy drought-related damage. If the frequency of such extreme conditions increases in future, and restructuring the forest to create drought and heat-tolerant stands does not progress fast enough, this might indeed lead to increased forest damage.

 

Interfaces

FW-I-5: Extent of timber infested by spruce bark beetle - case study

 

Objectives

To increase the stability and diversity of forests (Waldstrategie 2020, p. 23)

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 adaptation to climate change  KomPass  monitoring report  crown defoliation  forest condition  leaf loss  needle loss