Forests respond to several changes at once
A forest is a long-lived community, and its trees often stand for decades or centuries. That is what makes climate change hard for forests to absorb. The conditions a tree was suited to when it sprouted may differ noticeably from the conditions it faces when it is old.
Climate change is not one pressure. It brings higher average temperatures, changed patterns of rain and snow, longer warm seasons and, in many regions, more frequent or more intense extreme events. These act together, and the combined effect is usually different from any single one. Responses also vary a great deal from place to place, so general statements here describe typical patterns, not guarantees for any particular woodland.
Warming and the growing season
Warmer temperatures often lengthen the period in which trees can grow. In cool, moisture-rich regions this can raise growth for a time. But the benefit is not automatic. Higher temperatures also make the air draw more water out of leaves and soil, and if water or nutrients are limiting, a longer season does not help much.
Warming also shifts the timing of seasonal events. Buds open earlier in many places, and that has consequences for the insects and birds that depend on the new foliage, a topic covered in more detail in the article on spring leaf-out and climate warming. Trees that depend on winter chilling to break dormancy, or on cold to kill off pests, can be affected in less obvious ways.
Changing water: too little, too much
Rainfall and snowpack matter as much as temperature. Where seasonal dry periods lengthen or become hotter, trees can run short of water, and prolonged shortage can weaken or kill them. The effects of drought on forest ecosystems are important enough to deserve their own treatment, including how stressed trees become more vulnerable to other problems.
In other places the change is toward heavier downpours. Intense rain can saturate soils, erode slopes and flood low-lying forests. Snow that melts earlier changes the timing of streamflow, which in turn affects forest streams and the life in them. How forests handle all of this is tied to the global water cycle, since trees move large amounts of water from soil to air.
Disturbance: fire, pests and storms
Many forests have always been shaped by disturbance. What changes with the climate is how often, how widely and how severely disturbance occurs.
- Fire. Hotter, drier conditions can lengthen the fire season and dry out fuels. Forest fires are natural in many ecosystems, but fires burning outside their usual pattern can be more damaging, and wildfire smoke can reach communities far from the flames.
- Insects and disease. Warm winters may allow some forest insect pests to survive in larger numbers or expand into new areas. Drought-stressed trees often defend themselves less effectively.
- Wind and storms. Where storms strengthen or where soils stay wet, trees can be toppled more easily. Windthrow is a distinct hazard with its own causes.
Disturbances also interact. A drought can weaken trees, pests can then kill many of them, and the dead wood can add to fire risk. Ecologists often describe this as a chain or compounding effect.
Shifts in species and communities
As conditions change, the places where a given tree species does well also change. Over long periods, ranges tend to move toward cooler areas, uphill or toward the poles, though trees move slowly and many cannot keep pace. This matters at the edges of cold-limited forests, such as the alpine treeline and the boreal margin, where permafrost thaw can alter the ground itself.
Tree species are only part of the story. Understory plants, insects, fungi, birds and mammals each respond in their own way, and a forest can lose or gain species even while it still looks green and intact. That is why change in the biodiversity of a forest can lag behind, or differ from, change in its trees.
Invasive species add another layer. Disturbed or stressed forests are often easier for non-native plants and pests to colonize, and a changing climate can open new areas to them.
Below ground and in the carbon balance
Soils store a great deal of carbon and support the fungi and microbes that feed trees. Warmer, wetter or drier soils change how fast organic matter breaks down and how nutrients are released. Because forests also take up carbon through growth, changes in tree health and disturbance can shift a forest from absorbing more carbon to releasing more, at least temporarily. The role of forests in the global carbon cycle is covered separately, along with what this means for climate mitigation.
Why some forests cope better
Not every forest is equally exposed. Several features tend to improve a forest’s chances, though none is a guarantee.
- Variety. Stands with several tree species and ages, sometimes called mixed stands, are less likely to be wiped out by a single pest or event.
- Connection. Forests linked by wildlife corridors give plants and animals more room to shift.
- Size and condition. Large, relatively intact forests with healthy soils generally have more capacity to recover than small, degraded or fragmented patches.
- Management. Thoughtful practices, including natural regeneration with suitable seed sources, may help forests adjust. Whether a specific action helps depends on the site and the species.
Time lags and thresholds
Forest change is rarely smooth. Long-lived trees can persist for decades under conditions that no longer suit their seedlings, so a stand may look stable while its future makeup is already shifting. Then a single severe event, such as a large fire or a multi-year drought, can push the system across a threshold in a short time. Because of these lags, observations made over a few years can understate what is happening, and long monitoring records, tree rings and repeat surveys are valuable for seeing slow trends.
What remains uncertain
Scientists are confident that climate is changing and that forests are responding, but projecting exactly how a particular forest will change is difficult. Local soils, topography, past management and chance events all matter. Some forests may adapt or recover well, others may shift into different types, such as shrubland or grassland, after severe disturbance. Treat any single prediction with caution, and favor information specific to your region.