Biodiversity is more than trees
When people picture a forest, they picture trees. But most of a forest’s species are not trees. They are insects, mosses and lichens, flowering plants on the floor, amphibians, birds, mammals and a huge variety of fungi and microbes in the soil and in dead wood. Climate change affects this whole community, and it can alter who lives in a forest even when the tree canopy appears unchanged.
This article focuses on which species a forest can keep supporting. The related question of where tree species themselves can grow is covered separately, in the discussion of species ranges.
Why species have limits
Each species has a range of conditions it tolerates: temperature, moisture, soil, light, and the presence of the other species it relies on. A salamander may need cool, damp leaf litter. A woodland orchid may need a particular fungus. A bird may need a certain insect to be abundant at nesting time.
When climate shifts, a species faces one of a few outcomes: it may tolerate the change, adjust its behavior or timing, move to a more suitable place, or decline locally. These outcomes are not equally available to everyone.
Who tends to be more vulnerable
Patterns seen across many studies suggest some groups tend to be at greater risk, though exceptions are common.
- Cold-adapted and mountaintop species. Species living near the cool upper limits of their habitat may have nowhere higher to go. Forests at high elevation or near the alpine treeline can be especially exposed.
- Moisture-dependent species. Amphibians, mosses, ferns and many fungi depend on damp conditions. Longer dry spells can shrink the microhabitats they use.
- Specialists. Species that depend on a single food, host plant or partner are harder hit when that partner shifts or declines.
- Small or isolated populations. Populations with few individuals have less genetic variety and less room to recover from a bad year. Forest fragmentation makes this worse by cutting off movement.
- Slow dispersers. Plants with heavy seeds and animals that rarely cross open ground may not be able to track suitable climate quickly.
Generalists, such as animals that eat many foods and plants tolerant of a wide range of conditions, often fare better. Some of them may expand, and in doing so can change the balance of the community.
Changes in timing and connection
Biodiversity depends on relationships. A change in the timing of leafing, flowering or insect emergence can separate species that evolved to meet at the same time. Pollinators, seed dispersers, predators and prey are all examples of links that can be strained. Seed dispersal by animals, for instance, depends on fruit ripening when the animals are present and willing to forage.
Even one mismatch rarely causes a collapse on its own, but many small mismatches can lower the success of individual species.
The role of disturbance and invasion
Climate change can also affect biodiversity indirectly through disturbance. Severe fires, drought die-off and storm damage can remove habitat quickly. Some species benefit from the open, sunny conditions that follow, and fire ecology describes plants and animals that depend on periodic fire. But when disturbance becomes more frequent or severe than the community is adapted to, the net result is often a loss of sensitive species.
Invasive species add pressure. Stressed forests may be easier for non-native plants, pests or diseases to establish in, and some invaders thrive in warmer conditions. This can crowd out native understory plants and alter the food available to wildlife.
What lies below the surface
Soil life is a major part of forest biodiversity. Mycorrhizal fungi form partnerships with tree roots, helping trees take up water and nutrients. Decomposers recycle dead wood and leaves. Changes in temperature and moisture can alter these communities, and because trees rely on them, effects can carry upward. Because these organisms are hard to observe, their responses are among the least well documented.
Lag, debt and surprise
One difficult feature of biodiversity change is delay. Long-lived trees may persist for decades in conditions no longer ideal for them, while the animals and plants that depend on certain habitat conditions fade sooner. Ecologists sometimes call this an extinction debt: species that are still present but whose long-term survival in that place is in doubt. How large this debt is for any given forest is hard to know.
There can also be surprises. Species sometimes persist in cool, moist pockets such as ravines, north-facing slopes and stream corridors that offer shelter from regional warming. These refuges can be disproportionately important for keeping species through difficult periods.
What helps
No single action reverses climate effects on biodiversity, but several approaches are commonly discussed for giving forest life more options.
- Protecting large, intact forests and protected areas that cover a range of elevations and conditions.
- Maintaining or restoring wildlife corridors so species can move.
- Keeping variety within stands, including mixed ages, dead wood and structural complexity, which supports many specialized species.
- Reducing other stresses, such as over-harvesting and habitat fragmentation, so species face fewer simultaneous pressures.
- Monitoring over time, so changes are noticed early.
Outcomes remain uncertain, and what works in one forest may not in another. The main idea is that the more variety and connection a forest retains, the more ways its living community has to respond.