Norway spruce (Picea abies) is one of the most economically and ecologically important tree species in northern Europe. Its range extends from the mountain regions of Central Europe to the (hemi)boreal forests of the Baltic states and Scandinavia. At the same time, it is among the tree species most sensitive to climate change, as its shallow root system makes it vulnerable to drought, storms and extreme temperatures, writes University of Tartu Professor Emeritus Arne Sellin.

Across southern and Central Europe, increasingly frequent droughts are already slowing the growth of Norway spruce, causing physiological stress and leading to widespread tree mortality. Studies conducted in Central Europe indicate that high temperatures, increasing atmospheric vapor pressure deficit and the resulting soil dryness will limit the species' distribution in the southern part of its current 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 is likely to lose much of its current range in Central and Eastern Europe. According to the models, the species' overall range could shrink by as much as half compared with its present extent.

Trees weakened by unfavorable weather conditions are also more susceptible to pests, particularly the European spruce bark beetle (Ips typographus) whose populations have grown explosively in many parts of Europe as the climate has warmed. Warmer, longer summers allow the beetles to produce more generations each year, increasing the extent of the damage they cause.

Norway spruce is also one of the most important tree species in Estonia's commercial forests. Pure spruce stands and mixed stands dominated by spruce account for about one-fifth of the country's forest area. Unfortunately, spruce is also especially vulnerable to climate change in Estonia. Dry, hot periods reduce the species' resilience and promote mass outbreaks of the European spruce bark beetle.

In many parts of Estonia, bark beetle damage reached unprecedented levels during the previous decade. As a result, extensive sanitary logging and clear-cutting became necessary, causing significant economic losses for forest owners. In recent years, the geographic focus of European spruce bark beetle damage has also shifted considerably. While southern Estonia was hit hardest toward the end of the previous decade, more recent monitoring data show a rapid increase in bark beetle activity and the spread of damage across northern, central and western Estonia.

In addition to bark beetles, spruce stands are threatened by root rot (Heterobasidion spp.), one of northern Europe's most troublesome forest pathogens. The fungus causes stem and root decay, reducing tree growth, making trees more vulnerable to storms and lowering timber quality.

Climate warming, including milder winters and a longer growing season, also promotes the spread of root rot and increases the extent of the damage it causes. Infection with root rot significantly increases mortality among Norway spruce trees. Trees either die directly as a result of stem decay or are more likely to be brought down by storms and strong winds.

Northern European peculiarities

One of the defining characteristics of global climate change is that the climate is becoming more variable and increasingly uneven, with differences between geographic regions expected to grow rather than diminish. Unlike Central and southern Europe, northern Europe is projected to become wetter. Under a scenario of moderate global warming, summer precipitation in Estonia is expected to increase by an average of 15 percent by the end of the century.

The weather over the past several summers has already provided a glimpse of the direction regional climate trends are taking. Along with increasing precipitation, atmospheric specific humidity is rising globally, while relative humidity (RH) is increasing locally as both the amount and frequency of rainfall grow, particularly in forested landscapes.

Forest ecology research cannot ignore the fact that humidity dynamics within a forest stand differ substantially from those recorded at meteorological stations located in open areas. Within and beneath the forest canopy, relative humidity is consistently much higher because the foliage has a large evaporative surface area and airflow through the canopy is restricted. In temperate forests, the daily minimum RH is typically 10 to 20 percent higher than in adjacent open areas.

More frequent rainfall means the foliage becomes wet more often, allowing humidity to remain high for extended periods as rainwater retained by the canopy gradually evaporates. This effect is especially pronounced in spruce forests where the canopy of a mature spruce stand intercepts 44 to 47 percent of total precipitation.

In addition, sunlight penetrates only weakly into the forest canopy, resulting in lower air temperatures than in open areas. The combination of higher relative humidity and lower air temperatures significantly reduces vapor pressure deficit (VPD), the atmospheric condition that drives water loss from leaves and initiates the flow of water through plants during transpiration. The combination of increasing precipitation and the continued risk of drought in northern Europe illustrates the complex nature of climate change and its nonlinear effects on the water cycle.

At higher latitudes, forest growth is generally expected to accelerate because of earlier springs and warmer, longer growing seasons. However, a longer growing season does not necessarily lead to higher forest productivity.

Spruce growth limitations

Recent studies show that different processes limit the length of the growing season and the photosynthetic activity of trees. Earlier and more vigorous physiological activity before the summer solstice leads to an earlier decline in photosynthesis, an earlier end to primary shoot growth and earlier autumn senescence. Since the turn of the millennium, Nordic researchers have observed signs of slowing forest growth, although the underlying causes have yet to be fully explained.

Previous articles on Novaator have used findings from the Free Air Humidity Manipulation (FAHM) experiment to explain how broadleaf trees respond to increasingly humid conditions. But how does Norway spruce, a species highly susceptible to drought, fare against the backdrop of these climate trends? In general, conifers are considered physiologically less flexible than broadleaf trees in regulating water use, making them potentially more vulnerable to the effects of climate change.

Research conducted by the University of Tartu's Chair of Ecophysiology seeks to determine how climate change alters tree function and how those changes affect growth. So far, the results indicate that a 15 percent increase in precipitation does not significantly affect the physiological processes or stand productivity of Norway spruce growing on moderately moist sites. At the same time, we have observed that increased atmospheric humidity can slow foliage development and reduce tree growth rates.

One of the central findings of our research is that tree responses depend more on the source of moisture — atmospheric humidity or soil moisture — than on the overall level of environmental moisture. In Norway spruce, slower foliage development is reflected in smaller needles and shoots, resulting in lower overall foliage biomass.

The observed reductions in tree growth are therefore driven by a smaller assimilating surface area — the total green leaf area available for photosynthesis — while our latest studies also show a decline in the photosynthetic capacity of the foliage itself. Smaller needles exhibit lower net photosynthetic rates per unit of leaf area, likely because of a less developed photosynthetic apparatus, greater mesophyll resistance or changes in the balance between photosynthetic and structural tissues.

A second group of mechanisms that helps explain slower growth involves the allocation of resources within the tree. In trees growing under more humid conditions, biomass allocation shifts toward the root system, particularly fine roots. This provides clear evidence of more difficult nutrient uptake, which also increases respiratory costs.

Because transpiration is weaker, trees absorb fewer mineral nutrients from the soil. This particularly affects nutrients transported through the mass flow of water, including nitrate (NO₃⁻). Lower vapor pressure deficit (VPD) and reduced water loss, together with higher soil moisture, increase the proportion of hydrophilic taxa in mycorrhizal communities, which also influences phosphate uptake.

This shift in biomass allocation reduces the ratio of photosynthetic to nonphotosynthetic tissues, altering the internal balance between carbon-producing and carbon-consuming tissues. Under conditions of higher atmospheric humidity, this balance is also affected by a redistribution of resources away from primary shoot growth and toward secondary growth. Because primary growth is closely linked to foliage development, this changes the proportion of photosynthetic and nonphotosynthetic tissues in the aboveground parts of the tree.

Greater investment in fine roots increases respiratory losses, leaving fewer resources available for growth. First, maintenance respiration increases because fine roots consist largely of living cells. Second, growth respiration rises because fine roots have a short lifespan and high turnover rate. Third, Norway spruce is an ectomycorrhizal species whose fine roots are covered by a thick fungal sheath, resulting in higher levels of mycorrhizal respiration.

Atmospheric humidity also has a significant effect on stomatal regulation and the functioning of the photosynthetic apparatus in Norway spruce. In foliage that develops under more humid conditions, stomata become less sensitive to changes in VPD, while the water-use efficiency of photosynthesis declines.

During drought or periods of high atmospheric demand for water, these trees are less able to adjust their water-use efficiency to changing conditions, increasing the risk of water stress and embolism that disrupts the tree's water transport system. This makes the trees more vulnerable to extreme weather events, which climate projections indicate will become more frequent in the future.

Trees that develop under humid conditions also exhibit a higher light compensation point for photosynthesis and higher rates of dark respiration. As a result, the balance of leaf gas exchange becomes positive later in the morning and turns negative earlier in the evening. Elevated dark respiration indicates greater physiological stress, leading to higher respiratory losses and leaving fewer resources available for growth.

In the context of climate change, both responses reduce the capacity of trees to sequester carbon: respiratory losses increase, tree growth potential declines and the productivity of northern forests decreases.

Diverse effects

Our research shows that the mechanisms through which climate change affects forests are far more complex than previously thought. Although global warming is expected to increase the productivity of boreal forests, at higher latitudes this effect is counteracted by increased precipitation and the accompanying rise in atmospheric humidity.

The future of Norway spruce in Estonia and across Europe presents a major challenge for forestry and forest protection. Addressing it will require accounting for both the direct and indirect effects of climate change. Understanding how trees adapt to a changing climate requires knowledge of their responses at the level of physiological processes. That knowledge makes it possible to manage forests on a scientific basis and better mitigate the impacts of climate change.

Readers interested in our findings can learn more through a series of scientific papers published in the journals Frontiers in Forests and Global Change, Science of the Total Environment and Tree Physiology.

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