Moister climate reduces spruce's ability to withstand weather extremes
Research from the University of Tartu shows that increased humidity slows Norway spruce growth and makes the trees more vulnerable to drought, storms, and pests, threatening this key species in northern Europe.
Norway spruce (Picea abies) is one of the most economically and ecologically important tree species in northern Europe, but it is highly sensitive to climate change due to its shallow root system. In southern and Central Europe, increasingly frequent droughts are already slowing growth, causing physiological stress and widespread tree mortality. Studies indicate that high temperatures, rising atmospheric vapor pressure deficit, and soil dryness will limit the species' distribution in the southern part of its range in the future.
Species distribution models based on climate scenarios show that Norway spruce is gradually shifting northward. By the end of the century, it could lose much of its current range in Central and Eastern Europe, with the overall range shrinking by as much as half.
Trees weakened by unfavorable weather are more susceptible to pests, particularly the European spruce bark beetle (Ips typographus). Warmer, longer summers allow beetles to produce more generations each year, increasing damage. In Estonia, spruce stands cover about one-fifth of the forest area. In the past decade, bark beetle damage reached unprecedented levels, necessitating extensive sanitary logging. The geographic focus of damage has shifted from southern to northern, central, and western Estonia.
Additionally, root rot (Heterobasidion) threatens spruce stands. The fungus causes stem and root decay, reducing growth and making trees more vulnerable to storms. Climate warming, including milder winters, promotes the spread of root rot.
Unlike southern Europe, northern Europe is expected to become wetter. Under a moderate global warming scenario, summer precipitation in Estonia could increase by an average of 15 percent by the end of the century. More frequent rainfall raises relative humidity, especially within forest canopies.
Research by the University of Tartu's Chair of Ecophysiology indicates that a 15 percent increase in precipitation does not significantly affect Norway spruce's physiological processes or stand productivity on moderately moist sites. However, increased atmospheric humidity can slow foliage development and reduce growth rates. Trees growing under humid conditions produce smaller needles and shoots, leading to lower foliage biomass. Resource allocation shifts toward fine roots, increasing respiratory costs and reducing resources for growth.
Humidity also affects stomatal regulation – foliage becomes less sensitive to changes in vapor pressure deficit, reducing water-use efficiency. This makes trees more vulnerable to drought stress and embolism. Additionally, foliage developed under humid conditions has a higher light compensation point for photosynthesis and higher dark respiration rates, indicating greater physiological stress.
The study concludes that climate change impacts on forests are more complex than previously thought. While global warming is expected to increase boreal forest productivity, increased precipitation and humidity in northern Europe counteract this effect. The future of Norway spruce in Estonia and across Europe poses a major challenge for forestry and forest protection.
