Shrub-induced microclimate buffering on European alpine summits

High in the mountains, the air is often thin and cold, but the ground beneath a hiker's boot tells a different story. The temperature right at the soil surface can differ wildly from the air just a few feet above. This difference is called the microclimate, and it is the immediate environment where plants grow, insects crawl, and soil microbes work. For decades, scientists have known that the landscape itself shapes these tiny worlds. Snow acts as a thick blanket, keeping the ground warm even when the air freezes. Rocks and bare soil, however, heat up quickly in the sun and cool rapidly at night. Now, a new layer of complexity is being added to this picture: the plants themselves. As the climate warms, shrubs are spreading upward into areas that were once open grasslands. These woody plants are not just changing the view; they are actively rewriting the thermal rules of the mountain summit, creating pockets of warmth or coolness that can determine which species survive and which disappear.

A team of researchers set out to measure exactly how much these spreading shrubs change the temperature of the soil. They looked at 34 mountain summits across Europe, ranging from the lower edges of the tree line up to the middle of the alpine zone. These mountains are part of a long-term monitoring network that has been tracking vegetation and climate for over two decades. The scientists gathered a massive amount of data, combining 21 years of hourly soil temperature records from 136 sensors buried ten centimeters underground with daily air temperature data from weather models. They also meticulously measured what was growing around each sensor, noting the percentage of the ground covered by deciduous shrubs, evergreen shrubs, rocks, bare soil, lichens, and mosses. By comparing the soil temperature to the air temperature over this vast period, they could calculate how much the ground was buffered, or protected, from the extremes of the weather above.

The results revealed a clear and powerful pattern: the ground is generally much more stable than the air. On average, the soil temperature was only about 30 to 35 percent as variable as the air temperature, meaning the ground is naturally buffered against the wild swings of the weather. However, this protection is not uniform; it changes dramatically depending on the season, the direction the slope faces, and what is growing on the ground. The buffering effect was strongest on north-facing slopes and during the winter and spring months. This is largely because snow accumulates deeper and lasts longer on these cooler, shaded slopes. A thick layer of snow is an excellent insulator, keeping the soil near freezing even when the air above is bitterly cold. In contrast, during the summer and autumn, the presence of vegetation becomes the primary factor in keeping the soil cool.

The study found that shrubs are the most effective agents of this cooling and buffering, but their impact depends heavily on their type. Deciduous shrubs, which lose their leaves in the autumn, provided a much stronger cooling effect during the summer than evergreen shrubs. When these tall, leafy plants are in full bloom, their canopies cast shade and release moisture into the air, creating a cooler environment for the soil below. This effect was so strong that as the cover of deciduous shrubs increased, the connection between the hot air and the soil weakened significantly. Evergreen shrubs also helped buffer the temperature, but their effect was less pronounced. Interestingly, the researchers found that this buffering is not a simple, straight-line relationship. As shrub cover increases, the buffering effect grows, but the rate of change varies, suggesting a complex interplay between plant height, leaf density, and the amount of sunlight reaching the ground.

The research also highlighted the surprising role of smaller, often overlooked plants. Lichens and mosses, which form low mats on the ground, also contribute to temperature regulation, but their behavior flips depending on the season. In the summer and autumn, when the ground is free of snow, these organisms act as a shield, reflecting sunlight and keeping the soil cooler. However, in the winter and spring, when they are buried under snow or dried out by the cold, they lose this ability and can sometimes even increase the connection between the cold air and the soil. The study also noted that as shrubs spread, they often crowd out these smaller plants, creating a trade-off where the cooling benefit of the shrubs might replace the cooling benefit of the lichens.

Despite the dramatic increase in shrub cover across these European mountains over the last 21 years, the overall ability of the ground to buffer temperature extremes has remained surprisingly stable. The researchers did not see a long-term trend where the soil became more or less buffered over time. This stability appears to be the result of a balancing act. As shrubs spread and provide more cooling shade in the summer, they simultaneously reduce the cover of lichens and mosses, which also provide cooling. At the same time, the expansion of shrubs often leads to deeper snow accumulation in the winter, which enhances insulation. These opposing forces seem to cancel each other out, maintaining a relatively consistent level of protection for the soil ecosystem.

The implications of these findings extend far beyond simple temperature readings. The microclimate under a shrub is a different world than the open tundra, and it dictates which plants and animals can live there. By cooling the soil in the summer and warming it in the winter, shrubs act as mediators that can either help native species survive extreme heat or, conversely, create a welcoming environment for invasive species from lower elevations that are better adapted to warmer conditions. The study suggests that as the climate continues to change, the spread of shrubs will not just be a visual change in the landscape but a fundamental shift in the thermal architecture of the mountains. Understanding these subtle, ground-level changes is crucial for predicting how alpine ecosystems will respond to a warming world, as the survival of many species depends on the tiny, buffered spaces beneath the branches.