Soil Drought Impairs Forest Microclimate and Increases Temperature, Study Reports

Climate change is expected to increase the frequency and intensity of extreme weather events, such as heat waves, droughts, and storms.

These events can have negative impacts on forest ecosystems, such as reducing tree growth, increasing mortality, and altering species composition.

However, some forests may be more resilient than others, depending on their microclimate and soil moisture conditions.

Microclimate is the local variation of climate within a small area, influenced by factors such as vegetation, topography, and soil.

Soil moisture is the amount of water in the soil, which affects the availability of water for plants and the exchange of energy and water between the land and the atmosphere.

A recent study by scientists from the University of Michigan and the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL) investigated how soil moisture affects the microclimate of temperate broadleaf forests in the eastern United States and Switzerland.

They found that higher soil moisture increased the buffering capacity of the forest canopy, meaning that it reduced the daily and seasonal fluctuations of air temperature and vapor pressure deficit (VPD) below the canopy.

VPD is a measure of the dryness of the air, which affects the rate of water loss from plants through transpiration.

The buffering effect of soil moisture was stronger in summer than in winter, and stronger in forests with higher leaf area index (LAI), which is the amount of leaf area per unit of ground area.

Why Soil Moisture Matters for Forest Microclimate

(Photo : GUILLAUME SOUVANT/AFP via Getty Images)

The researchers explained that soil moisture influences the microclimate of forests through two main mechanisms: evapotranspiration and thermal properties.

Evapotranspiration is the combined process of evaporation from the soil and transpiration from the plants, which cools the air and increases the humidity.

Thermal properties refer to the ability of the soil to store and release heat, which affects the temperature of the soil and the air above it.

Higher soil moisture enhances both mechanisms, resulting in a cooler and more humid microclimate below the forest canopy.

The researchers used data from 11 forest sites in the eastern United States and Switzerland, where they measured air temperature, VPD, soil moisture, and LAI for several years.

They calculated the moderating capacity of the canopy, which is the difference between the below-canopy and the open-area microclimate, as an indicator of the buffering effect of the forest.

Then, they analyzed how the moderating capacity varied with soil moisture and LAI, and how it changed over time and space.

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Implications for Forest Resilience and Management

The study showed that higher soil moisture can help forests maintain a more stable and favorable microclimate, which can enhance the growth and survival of trees and understory plants.

This can have important implications for forest resilience and management, especially in the face of climate change.

The researchers suggested that maintaining or increasing soil moisture in forests could be a potential adaptation strategy to mitigate the negative effects of climate change, such as heat stress, drought, and fire risk.

They also pointed out that soil moisture is influenced by both natural factors, such as precipitation and soil type, and human factors, such as land use and management practices.

Therefore, they recommended that forest managers should consider the effects of their actions on soil moisture and microclimate, and monitor these variables to assess the health and functioning of forest ecosystems.

The study also highlighted the importance of considering the spatial and temporal variability of soil moisture and microclimate in forests, and how they interact with other factors, such as vegetation structure and composition.

The researchers noted that their findings were consistent with another study that they conducted in the same forest sites, where they found that higher soil moisture increased the diversity and productivity of understory plants.

To conclude, soil moisture is a key driver of forest microclimate and ecosystem processes and that more research is needed to understand its role and dynamics in different forest types and regions.

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