Decaying mountain spruce forests have a heightened risk of catching fire
In recent years, the frequency and intensity of forest fires have increased significantly. This year, fires have especially affected France, Spain, Greece, Germany, and Belgium. Additionally, fires are becoming more common in the northern regions of Europe.
Photo: There is a higher risk of fire in the decaying Tatra forests affected by the bark beetle outbreak; source: Pixabay-free
In both the Czech Republic and Slovakia, hundreds of incidents occur annually, leading to considerable economic damage and environmental harm. The majority of these fires are caused by human factors, including carelessness, negligence, a lack of knowledge, or even malicious intent. During hot weather, it becomes increasingly easy to ignite dry forests.
Slovak scientists from the Technical University in Zvolen studied the natural conditions that increase the risk of wildfires while working on project APVV-21-0224, alongside projects VEGA 1/0443/23, VEGA 1/0617/26, and VEGA 2/0115/25. Their findings were published in an article titled Kalamitní rozpad horských smrčin jako faktor formování požárního mikroklimatu lesů (Kalamitný rozpad horských smrečín ako faktor formovania požiarnej mikroklímy lesov / Disturbance-induced dieback of mountain spruce forests as a driver of forest fire microclimate), which appeared in the journal „Zprávy lesnického výzkumu“ (Reports of Forestry Research). These conclusions are also relevant to the Czech forestry sector. The researchers recommend that, in addition to the abiotic risks typically assessed for forest stands, the values of the fire danger indicator should be included among the relevant abiotic risks – a category that currently lacks this specific parameter in forestry practices.
The scientists conducted their research at the “Červenec” experimental site located in the Jalovecká Valley, near the Pod Náružím mountain cottage, which has coordinates of 49.183617°N and 19.641944°E. This site is situated at an elevation of 1,420 meters above sea level in the Šedý Vrch area of the Western Tatras. It is characterised by a cold mountain climate, with an average temperature of 3°C and total annual precipitation of 1,450 mm. The predominant vegetation in the area consists of spruce forests that are over 130 years old.
Photo: Červenec experimental trial in the Western Tatras (1,420 m a.s.l.; www.sorger.sk – modified, ZBGIS)
Experimental trials were conducted at the Červenec site to represent different stages of the catastrophic decline of a mountain spruce forest. The study area included three distinct zones: an open, unforested area; a dying spruce forest; and a healthy, living spruce forest. The open, unforested area does not have any tree cover and is characterised by herbaceous vegetation, grassy patches, or bare soil. During spring, when there is no snow cover, dry grass in this area presents a high risk of fire ignition and spread.
The dying forest contains spruce trees in an advanced stage of decline, with both standing and fallen dead trees present. Its stand density is significantly lower than that of the living forest, mainly due to the gradual loss of trees. In contrast, the living forest is a vigorous stand of trees that are exhibiting active physiological processes, indicating their health and vitality.
Measurements of specific meteorological parameters, including temperature, air humidity, and global solar radiation, were conducted during the fire season from May 1 to October 23, 2024. Fine fuel moisture content serves as a sensitive indicator of the immediate flammability of the fuel complex, responding to micrometeorological changes with minimal time lag.
The stand microclimate plays a crucial role in regulating fire danger by influencing fuel drying, evapotranspiration, and the site’s overall energy balance. The amount of incident solar radiation indirectly yet significantly impacts forest fire risk, particularly through its effects on air temperature, fuel moisture, and evapotranspiration. During the season, the total amount of global radiation reached 720.7 kWh/m² in open areas, while it was only 255.7 kWh/m² (35% of the value in open areas) in dying forests and just 133 kWh/m² (18% of the value in open areas) in living forests.
The ground-level layer in the dying stands received approximately twice as much radiation as that in the living stands (+92%), indicating a significant reduction in the shading function of the canopy and a considerable loss of shading even during the initial stages of stand decline.
Photo: Layout of research trials; A – open, non-forested area, B – dying forest, C – living forest; author L. Malovcová
Daily radiation totals showed a significant decrease in values across different environments: 4 kWh/m² in the open area, 1.4 kWh/m² in the dying forest, and 0.8 kWh/m² in the living forest. This striking difference highlights a key energy mechanism that accelerates the drying of fine fuels.
Understanding daily temperature extremes is crucial, as they ultimately influence whether a fire ignites and how it behaves. Most forest fires start between 12:00 PM and 4:00 PM, coinciding with peak temperatures regardless of the daily average. This period, along with increased human activity, creates ideal conditions for the ignition and rapid spread of fires. There is a strong correlation between maximum air temperature and minimum relative humidity; it is during the daily maximum temperatures that the evaporation of water from the fuel occurs most rapidly.
At the Červenec site, the highest daily temperature amplitude was recorded in the open area, measuring 19.2°C, with an average daily maximum (Tmax) of 24.0°C and a minimum (Tmin) of 4.8°C. In the dying forest, the amplitude was approximately half that at 10.5°C (Tmax 17.9°C; Tmin 7.4°C), while in the living forest it was even lower at 9.5°C (Tmax 17.0°C; Tmin 7.5°C). Stands of withered grasses and herbaceous plants exacerbate extreme air temperatures, particularly in the layer directly above the ground and the adjacent soil layer. Dry vegetation hinders heat dissipation into the surface soil layers, leading to elevated near-ground air temperatures during hot days.
Photo: There is a higher risk of fires in the decaying Tatra forests damaged by the bark beetle disaster, source: Pixabay-free
A living spruce forest is characterized by lower daytime temperatures because the canopy limits the amount of solar radiation that reaches the forest floor. At night, however, the temperatures are higher, as the heat retained by the soil surface is radiated back into the air. This results in a more stable temperature regime, with microclimatic conditions showing less seasonal and daily variability.
When it comes to fire-related microclimate conditions, the most critical parameter is minimum relative humidity. This humidity was lowest in open areas at 49.4% and highest in the living forest at 67.7%, with the dying stand recording a value of 60.3%. The significant 18.3% difference in minimum humidity between the open area and the living forest greatly contributes to the drying of fine fuels during the peak afternoon temperatures.
In their assessment of meteorological fire risk, scientists reached several conclusions. The highest fire danger level (Level 5) was observed in open areas during the study period, particularly in May, June, July, and August. By October, the risk decreased, with Level 1 becoming the predominant rating. The dying forest, which has a sparse canopy, had a lower fire danger level, registering at Level 2. In contrast, low to very low fire danger levels were most common in the living spruce forest.
The large-scale collapse of mountain spruce forests significantly changes the fire-related microclimate within these stands. This creates a range of conditions that vary from the interior of a living forest (where the risk of fire ignition and spread is lower) to open areas that are highly exposed to fire risk. Scientific research shows that the loss of the microclimate-buffering function provided by a healthy canopy is a crucial factor that increases fire susceptibility in mountain spruce forests affected by large-scale disturbances.
Fire exposure is highest in open, non-forested areas. Here, a combination of increased solar radiation, greater temperature extremes, lower relative humidity, and higher potential evapotranspiration creates conditions that promote the drying of fine fuels.
On the other hand, living spruce stands maintain a stabilizing effect throughout the fire season. This results in a more favourable temperature and moisture regime, reduced fire danger, and higher moisture content in fine fuels compared to dying stands.
Authors: Lenka Malovcová, Veronika Babničová, Milan Ostrihoň, Paulína Nalevanková, Jaroslav Škvarenina, Technical University in Zvolen, Faculty of Forestry, e-mail: xnalevankova@tuzvo.sk
The paper Kalamitní rozpad horských smrčin jako faktor formování požárního mikroklimatu lesů (Disturbance-induced dieback of mountain spruce forests as a driver of forest fire microclimate) is available for download here.
Prepared from the original by Jan Řezáč, VÚLHM, v. v. i., e-mail: rezac@vulhm.cz