Soil Temperature Rise May Hinder Forest Expansion

Forests play a vital role in the ecosystem by offering habitats for indigenous species, recreational spaces for humans, and facilitating carbon cycling to help stabilize the global climate. Research indicates that the loss of forested areas disrupts local water cycles and threatens biodiversity. However, what environmental factors hinder the expansion of forests? Ecologists suggest that soil temperature might be a significant factor.

Scientists propose that soil temperatures may influence the periphery of forests. Seedlings are particularly sensitive—if conditions aren’t conducive to their early growth, forest regeneration and expansion become challenging. Rising soil temperatures bring difficulties such as increased heat, drought, wildfires, and other stressors, complicating recovery efforts for these ecosystems.

Previous studies have indicated that soil temperatures impact the development of humid, highland forests found in regions like Maine and New York. However, scientists are keen to understand how these temperatures interact in semi-arid areas such as New Mexico, Nevada, and Utah. They theorize that lower elevations, with drier soils and frequent droughts, could experience severe consequences as temperatures rise.

Zachary Holden and his team aimed to explore the effect of soil surface temperatures on forest boundaries. They hypothesized that the temperature at the soil surface significantly restricts forest recovery and expansion, particularly since these temperatures are often elevated at the forest edges due to increased sunlight exposure.

To validate their hypothesis, they conducted experiments with two tree species from western North America: Ponderosa pine and Douglas fir. Their studies identified the range of soil surface temperatures conducive to seedling germination and healthy growth. They discovered that both seedling groups could only withstand temperatures between 50°-60°C (122°-140°F) before suffering damage. They concluded that extreme soil temperatures may create natural barriers that prevent seedlings from establishing beyond the tree line.

Next, the researchers identified regions in the western United States most susceptible to rising soil temperatures. They combined existing climate data, historical soil temperature records, and satellite imagery to model the temperatures likely to be recorded in areas with minimal soil coverage. This value was termed Potential Soil Surface Temperature (PSST).

Using the calculated PSST values, the researchers assessed the likelihood of forest expansion and seedling survival in various forested regions in the western United States. They categorized areas with PSSTs of 50°C (122°F) as having low probabilities for forest growth and seedling survival, while those below 50°C (122°F) were deemed more favorable for forest expansion and seedling health.

Based on current soil surface temperature data, the team found that numerous forest edges in the western United States have already reached maximum surface temperatures around 50°C (122°F), complicating seedling establishment. They predict that the number of treelines that cannot support reforestation efforts will double by 2050, underscoring the urgent need for forest protection.

The researchers concluded that calculating PSST can aid scientists in pinpointing areas where seedlings are most at risk from dehydration or heat damage. They recommended strategies such as planting more heat-resistant species at the forest margins to help conservationists cool the soil and enhance forest growth.

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