27 February 2019

Study: Mature trees weakened and saplings lost due to drought


Up to 100 percent of newly planted trees in Danish forests were lost as a result of last year’s drought. A significant portion of adult trees were weakened. Trees face serious consequences in the event of another hot dry summer in 2019. These are some of the conclusions in a study on the effects of drought upon Danish trees conducted by researchers at the University of Copenhagen and the Technical University of Denmark (DTU).

Beech and Norway spruce are particularly vulnerable to low levels of precipitation. Combined, these two species account for more than a quarter of all forested area in Denmark. Foto: Bruno Bilde Jørgensen.

The outlook for trees in urban areas and forests could worsen if yet another extremely hot, dry summer strikes Denmark this year. A recently published memo from the University of Copenhagen’s Department of Geosciences and Natural Resource Management reviews the extent of the 2018 drought and its impact on Danish forests.

"The central theme is that trees with small root systems were significantly affected by last year's drought. These included urban trees, with constricted space for root systems, and newly planted saplings. In forests beech and Norway spruce are vulnerable and will be impacted by a repeat drought," says Senior Adviser Iben M. Thomsen of the Department of Geosciences and Natural Resource Management.

Trees planted in spring 2018 wilted and died all over the country. In private forests, which comprise 74 percent of all forested land in the country, the mortality rate of newly planted trees reached up to 100 percent, while public forests reported up to 30 percent mortality — with higher outliers in a few locations. For example, in the Danish Lake District, near Silkeborg, 50 percent of saplings died off as a result of the drought. Among Danish Christmas Tree Association members, 36 percent of newly planted Christmas trees were lost in average from spring 2018 onwards.

25 percent of forests face severe odds in a new drought
Another dry spring and hot, parched summer will worsen the odds for trees. Beech and Norway spruce are particularly vulnerable to low levels of precipitation. Combined, these two species account for more than a quarter of all forested area in Denmark, covering 625,600 hectares — or 14.5 percent of the Danish landscape.

"The experience from dry summers in the mid-1970s and early 1990s demonstrates that repeated years of drought are a source of stress, especially for beech and spruce forests," according to Iben M. Thomsen.

Water is of unquestionable importance for trees, but it’s only now water levels are beginning to approach normal in the wake of last year's drought. This could be problematic if we experience another hot and dry spring/summer period.

"Last year, we were partly rescued by a wet autumn in 2017 that lead to large ground water reserves in the forest. This year’s starting point is not as favourable as last year’s," notes Thomsen, adding:

"On the other hand, some trees died when their roots drowned in 2017. Sudden and extreme shifts in yearly weather conditions are harder to deal with for trees and forest managers. If such weather extremes are a secondary effect of climate change, they could be more common in the future."

Lowest carbon sequestration in 23 years
In research projects, measurements of tree growth in 2018 indicate little to no growth. For example, DTU measured the amount of carbon dioxide sequestered by trees in Lille Bøgeskov forest near Sorø, Denmark. During photosynthesis, trees absorb the CO2 needed for them to grow and store carbon.

Initial analyses in Lille Bøgeskov found significantly less carbon sequestration during July and August of 2018 compared against data gathered in the same forest over the past 23 years.

"This fits with our forest health monitoring, where we saw signs of beech trees being drought-stressed in late summer, with thin crowns due to loss of leaves. Drought-stricken Norway spruce also began to shed needles, a process that typically continues throughout autumn and winter. Therefore, the full effects of the drought will become apparent in 2019. The same applies to other consequences, such as insect-related damage," said Iben M. Thomsen.


  • Beech and Norway spruce will be particularly impacted by repeat drought events.
  • Twenty-seven percent of Denmark's forested area is composed of beech and Norway spruce.
  • The drought impacted newly planted stands and urban trees most.
  • Oaks fared better and are more vulnerable to water logging of soils in wet years.
  • Private forest owners (who manage 74% of Denmark’s total forested area) have lost between 25-100% of their newly planted trees.
  • In Danish Nature Agency managed forests, up to 30 percent of newly planted trees were reported lost.
  • Among Christmas tree growers, an average of 36 percent of newly planted Christmas trees were lost due to the drought.
  • Many Christmas trees and new forests must now be partially or completely replanted, at a high cost to growers.
  • The drought also means a loss of volume growth, and an increased risk of collateral damage caused by insects.