FW-I-3: Incremental growth in timber

The picture shows a wood store along a forest path.Click to enlarge
Depending on weather conditions, timber yields can vary considerably from year to year.
Source: Photograph: © Andreas Bolte / Thünen-Institut

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

Table of Contents

 

FW-I-3: Incremental growth in timber

The incremental growth in the period of 2008-2012 was lower for all main tree species than in the preceding period of 2002-2008. This suggests that individual drought years have so far not been crucial for incremental growth figures. In fact, the particularly warm and dry years of 2003 and 2006 occurred in the first period which still produced higher incremental growth figures.

The graph shows points for the annual basal area increment (increment of derb wood with bark) in square metres per hectare and year for spruce, beech, oak and pine for the period 2002-2008 and 2008-2012 respectively. The values for 2002-2008 are 0.81 for spruce, 0.65 for pine, 0.64 for beech and 0.58 for oak; for 2008-2012 0.75 for spruce, 0.53 for pine, 0.57 for beech and 0.53 for oak.
FW-I-3: Incremental growth in timber

The graph shows points for the annual basal area increment (increment of derb wood with bark) in square metres per hectare and year for spruce, beech, oak and pine for the period 2002-2008 and 2008-2012 respectively. The values for 2002-2008 are 0.81 for spruce, 0.65 for pine, 0.64 for beech and 0.58 for oak; for 2008-2012 0.75 for spruce, 0.53 for pine, 0.57 for beech and 0.53 for oak.

Source: Thünen-Institut für Waldökosysteme (analysis based on the National Forest Inventory)
 

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 on the prevailing temperatures. In mountainous topography or in cold hollows which have so far had limited warmth, temperature increases can indeed have positive impacts on incremental growth in localised stands. 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, however, have adverse 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 carbon dioxide concentrations in the atmosphere. This increase can, in principle, benefit productivity provided there is no restriction on other important growth factors. Another major influence is the age structure of forest stands as the growth performance of trees depends on their age. In young trees less than 20 years old volume growth is low. In subsequent years 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 interaction of all influencing factors combined is complex and it is hard to predict in terms of impacts on future growth. It is abundantly clear, however, even now that there will be winners and losers from the effects of climate change depending on the specific site conditions of 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 – e.g. owing to unfavourable weather conditions – targeted forestry management actions will be required in order to maintain the productivity function of the forest or forests concerned. Besides, timber increment 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. It as 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 a long-term time series pertaining to timber 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 not possible to find evidence, especially for spruce forests, in the former Länder for the period from 2002 until 2012 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 2008–2012 from which drought years were absent, the nationwide mean of timber increments for the four main tree species declined further, with pine trees most badly affected, followed by beech trees.

The outcomes of an interim inventory carried out for the purpose of reporting on greenhouse gas emissions (CI 2017) were not yet available at the point of preparing the 2019 Monitoring Report. It was therefore not yet possible at that time to describe the timber increments for the period 2012-2017. A longer-term time series will make it possible in future to illustrate the long-term impacts of climate change on timber increments.

 

Interfaces

LW-I-2: Yield fluctuations

 

Objectives

Safeguarding the production of timber from sustainable forestry; meeting the increasing domestic demand for timber even after 2020 from predominantly domestic production (Waldstrategie 2020, p. 7)