Is the growth of our forests getting worse?

SLU statistics have shown how Swedish forests have seen a dip in increment in recent years. Meanwhile, total losses have increased due to logging and natural losses caused by storms and pests such as bark beetle. When combined, these factors could result in a negative net.

Logging not a factor

The volume of all living forest is commonly referred to as the growing stock. Over the past 100 years, this has increased, simply because annual losses have been less than the annual increment. In terms of sheer volume, the current growing stock is more than twice what it was at the start of the 20^th century.

The large growing stock has had a positive effect on the annual increment – larger stock means greater forest growth each year. This in turn has enabled an increase in annual logging. However, this is not the reason for the growth decline we are seeing, as there was no drastic change in annual logging during the 2000s and 2010s.

Falling curve

The past ten years have seen Swedish forest growth slow down. Between 2012 and 2018, total increment in Sweden decreased by 15 per cent, with the greatest drop being in the Götaland region.

Sweden faces several consequences if this decrease continues – especially in relation to its climate work. As long as the growing stock increases, forests will act as carbon sinks. This has been highlighted recently, for example in an article from the Swedish Environmental Protection Agency. Fortunately, it appears as though we will be seeing growth increase going forward.

Cornelia Roberge, environmental assessment specialist at the department of forest resource management, division of forest resource data at Swedish National Forestry Inventory (NFI) explains,

“There are clear signs that the decline in southern Sweden is reversing, and that forest growth is picking up again. This is indicated by our analyses from 2019 and 2020. However, this change isn’t fully visible yet, as the NFI works with 5-year averages”.

Drought a likely culprit

It is difficult to pin down the causes of declining growth; several factors may play a part, including management methods, the forest’s age distribution, tree species and nitrogen deposits. Although none of these are thought to be the main cause, Cornelia Roberge continues,

“As far as we can tell, the decline is mainly due to individual trees not growing as fast as they used to. The reason for that is most likely worsening growing conditions, caused by drought, for example”.

Task force appointed

SLU researchers and experts from the National Forestry Inventory recently formed a task force to look more closely at potential explanations behind the decline in Swedish forests. Göran Ståhl, professor of forest inventory at SLU and one of the task force experts explains,

“Historically we have seen dips in growth from time to time. The duration of the current dip is what stands out here, which is obviously a problem. Nevertheless, our analyses indicate there will be an improvement in growth in the coming years, which may mean the trend is already broken”.

Göran Ståhl continues, “Generally, the increment has grown steadily since the early 1960s, when we took a more active approach to forestry that focused on high yields. Our prognosis models indicate that climate change will lead to increased growth – at least here in Sweden. Countries that suffer more severely from droughts may find things more difficult”.