Why forest soil deserves attention
Forest soil is more than dirt. It is a living system made of mineral particles, decaying organic matter, water, air, roots, fungi, bacteria and animals from earthworms to tiny mites. It supplies trees with water and nutrients, stores large amounts of carbon, filters water and anchors roots. Because the soil forms slowly, over centuries or longer, it responds slowly to change and is also slow to recover once harmed.
This article concerns how temperature and moisture changes affect soil health in general. The specific question of how much carbon the soil holds is a separate topic, covered in the discussion of soil carbon storage.
Warmer soils, faster activity
Microbes that break down organic matter tend to work faster in warmer conditions, as long as there is enough moisture and oxygen. That has several possible effects.
- Faster nutrient release. Decomposition of leaf litter and dead wood releases nitrogen, phosphorus and other nutrients that trees can use. In cold forests, where decomposition is slow and nutrients are tied up in organic layers, warming may increase nutrient supply for a time.
- Faster loss of organic matter. Quicker decomposition can also mean that organic matter, the dark, crumbly material that helps soil hold water and nutrients, is used up faster than it is replaced.
- Uncertain balance. Whether a given soil gains or loses nutrients overall depends on the supply of new litter, the soil type and how wet it is. Researchers do not expect one outcome everywhere.
Dry and wet extremes
Moisture matters as much as temperature. Soil microbes and fungi are most active in damp, not saturated, conditions.
- Drying. During drought, soil life slows down. Fungal networks may shrink, and decomposition stalls. When rain returns after a long dry spell, a pulse of activity can release a burst of nutrients, some of which may wash away rather than be taken up. Dry soils can also become water-repellent, which makes later rain run off instead of soaking in.
- Waterlogging. In wet periods or in poorly drained spots, soil can become short of oxygen. Roots may suffer, and some microbes switch to processes that behave differently. Heavy rain also increases the risk of soil erosion, especially on slopes or where the ground is bare.
Because climate change often means more variable moisture, not only more or less, soils may swing between extremes more often. That variability can be hard on organisms adapted to steadier conditions.
Soil life under pressure
The community of organisms in the soil is closely tied to the trees above. Mycorrhizal fungi trade nutrients and water with tree roots in exchange for sugars. When trees are stressed by heat or drought, they may send less carbon to these partners, and the fungal network can weaken. In turn, a weakened network may leave trees less able to find water and nutrients.
Different species of fungi and microbes tolerate different conditions, so the makeup of the community can change, even if total activity stays similar. The consequences of such shifts are active research topics. It is generally unclear which changes will be harmful and which will be neutral.
Soil animals matter too. Earthworms, beetles, springtails and others mix and fragment litter and open channels for air and water. Some of these are sensitive to dryness and cold. In some northern forests, non-native earthworms moving into areas where they were previously absent have been associated with changes to the leaf litter layer, a reminder that biological change can interact with climate.
Structure, freezing and thawing
Healthy soil has a crumb-like structure, with spaces for air and water. Organic matter and fungal threads help bind particles together. When organic matter declines or soil is compacted, this structure weakens, and the soil holds less water and resists erosion less well.
In cold regions, winter conditions also shape soil. Deep, insulating snow keeps the soil from freezing hard. With less snow cover or more frequent winter thaws, soils may freeze more deeply, which can injure fine roots and affect microbes. In some northern forests, permafrost thaw can reshape the ground, creating wet hollows or collapsed areas.
Disturbance and the soil
Climate-related disturbance affects soil directly. Intense fire can burn off the organic layer, alter nutrients and sometimes make the surface water-repellent, raising the risk of erosion and landslides on steep ground afterward. Storm damage and windthrow can uproot trees and churn the soil. Insect outbreaks and drought die-off can change the amount and type of litter reaching the ground.
Management also matters. Heavy machinery on wet soil can compact it, and forest roads can concentrate runoff. Climate change can magnify these effects by making wet and dry extremes more intense.
Nutrients and tree growth
Healthy trees need a steady supply of nutrients. Warmer conditions might speed up release in some soils, but if trees are limited by water, they may not be able to use the extra nutrients. In soils that are already poor, a faster loss of organic matter could leave less fertility over time. As a rule, expect effects to be site-specific.
Supporting soil health
Soil health can’t be fixed quickly, but several practices are commonly recommended to avoid making it worse.
- Keep the forest floor covered with litter and plants, especially on slopes.
- Avoid working with heavy equipment when soil is wet.
- Maintain a mix of species and ages, which supplies varied litter and root systems.
- Leave some dead wood, which stores moisture and feeds soil organisms.
- Reduce other stresses, such as heavy grazing.
A landowner with specific concerns can get advice from a local forester or soil scientist, since soils and climates differ widely.