Forests support the environment by providing habitats for native species, offering recreational spaces for humans, and cycling carbon to stabilize the global climate. Scientists have shown that the loss of forest land disrupts a region’s water cycle and biodiversity. But what environmental conditions prevent forests from expanding beyond their current range? Ecologists think one answer might lie in soil temperatures.
Researchers suspect soil temperatures could play a part in determining the edges of forests because they directly influence the survival and root development of young plants, known as seedlings. Without suitable conditions for the early growth of seedlings, forests struggle to regenerate and expand. As soil temperatures rise, forests face challenges like excessive heat, droughts, and wildfires, making it more difficult for these habitats to recover from periods of stress.
Researchers in the past have shown that soil temperature impacts forest growth in humid, high-elevation forests like those in Maine and New York. But scientists want to determine how soil temperature affects forests in semi-arid lowland regions like New Mexico, Nevada, and Utah. They reasoned that lowland areas could be more severely impacted by rising temperatures because they have drier soils and more frequent droughts.
Zachary Holden and colleagues set out to investigate how soil surface temperatures affect forest boundaries. They hypothesized that the soil surface temperature at the treeline was the primary factor limiting forest recovery and expansion. This is because soil surface temperatures are significantly higher at the edges of forests since the soil is exposed to more direct sunlight.
To test their hypothesis, they experimented on 2 tree species in western North America – the Ponderosa pine and the Douglas fir. In their experiments, they determined the range of soil surface temperatures that allowed seedlings to successfully germinate and grow. They discovered that both sets of seedlings can only handle temperatures of 50° to 60°C, or 122° to 140°F, before being damaged. They interpreted these findings to indicate that extreme soil temperatures could create a natural barrier that prohibits seedlings from establishing themselves outside the tree line.
Next, the researchers determined which regions of the western United States are most vulnerable to rising soil temperatures. To do so, they combined preexisting climate data, historical soil temperature records, and satellite imagery to simulate the highest possible temperature the soil surfaces could reach in areas with minimal tree cover. They referred to this value as the potential soil surface temperature or PSST.
The researchers used their calculated PSST values to determine the probability of forest expansion and seedling survival in forest regions in the western United States. They interpreted regions with PSSTs of 50°C or 122°F and higher as having a lower chance of forest expansion and seedling survival. They interpreted regions with PSSTs lower than 50°C or 122°F as having a higher chance of forest expansion and seedling survival.
From current soil surface temperature data, the team found that many forest edges in the western United States already reach maximum soil surface temperatures of around 50°C or 122°F, making it more difficult for seedlings to establish themselves. They predicted that the number of tree lines that can’t support forest regeneration will double by 2050, meaning that forests will need more protection than ever before.
The researchers concluded that PSST calculations can help scientists identify regions where seedlings are most at risk of drying out or being damaged by heat. They suggested conservationists develop strategies like planting more heat-resistant seeds around the edges of forests to help cool the soil and promote forest growth.
