Climate sets a tree’s possible home
Every tree species has a climate envelope: a range of temperature and moisture conditions in which it can survive, grow and, most importantly, reproduce. The mapped area of a species, its range, roughly follows that envelope, though soils, competition and history also play a part.
When the climate warms, the envelope does not stay put. Areas that were too cold for a species may become suitable, while the warm or dry edge of its range may become hard to live in. In broad terms, the expectation is that suitable climate for many species moves toward the poles and up mountainsides. How far and how fast depends on the species and the region.
Uphill and poleward
On a mountain, a few hundred feet of elevation can mimic a large north-south distance in climate. That is why mountains offer a short route for species to track cooler conditions: moving uphill is much shorter than moving far north. Over time, species at lower elevations may expand upslope, while those near the summit have less and less space. The alpine treeline is one of the places where such shifts are watched closely, although local conditions such as wind, snow and soil can slow the advance.
At high latitudes, the northern edge of the boreal forest may gain areas that were once too cold, while the southern edge may face drying and heat. The picture is complicated where permafrost thaw changes the ground itself, sometimes creating waterlogged ground that does not favor trees.
The trailing and leading edges
Ecologists distinguish the leading edge, where a species is colonizing new ground, from the trailing edge, where it is declining. The two often move at different speeds.
- At the leading edge, a species must get seeds there, find a place to germinate and survive the early years, which are usually the most vulnerable.
- At the trailing edge, adult trees may persist for a long time in less suitable conditions, producing fewer or weaker seedlings. The range may seem stable for decades even though no replacement is coming.
This lag is one reason forests can look unchanged while their future composition is already shifting. Young trees are generally more sensitive to heat and drought than established adults, so changes often show first in the regeneration layer.
The speed problem
Trees are rooted for life, and they move only through seeds. Seed dispersal is therefore the limit. Wind-borne seeds, such as those of many conifers, maples and birches, can travel some distance. Heavy seeds like acorns and nuts often depend on animals that carry and bury them, such as jays and squirrels. Many of these animals move seeds only short distances, and in a tropical setting the same dependence on animals applies.
Climate envelopes can shift faster than trees can follow. Some analyses suggest that for certain species the suitable climate may move more quickly than natural dispersal, though estimates vary widely and depend on the climate scenario. A generation of a long-lived tree can also take decades to mature and produce seed, which slows the advance further.
Obstacles along the way
A seed arriving in a new area still needs somewhere to land. Several things can block range shifts.
- Fragmented landscapes. Where forests are broken up by farms, roads and towns, seeds and the animals that carry them may not cross. Forest fragmentation and the loss of wildlife corridors can make shifting harder.
- Unsuitable soil. A climate that is right may sit over soil that is wrong for the species, for example too shallow, too acidic or lacking the right mycorrhizal fungi.
- Competition. Established vegetation can crowd out newcomers.
- Pests and browsing. Seedlings may face deer browsing or new insect pests, and non-native invasive species can occupy gaps first.
- Mountain tops. Species already near the top have no higher ground to move to.
What it means for forest composition
Range shifts rarely move whole forests together. Each species responds individually, so existing communities may break apart and recombine into new mixes. A forest of the future might include some familiar species at lower numbers, newcomers from warmer areas, and more weedy or disturbance-tolerant species.
In practice this can mean:
- a gradual change in dominant trees, for instance from one oak or conifer to another that tolerates heat
- more open or shrubby stands where tree regeneration fails
- different timber, wildlife habitat and shade conditions than a landowner is used to
- possible loss of some local populations that carry unique genetic traits
Forests may also change type altogether after major disturbance. A severe fire or die-off in a spot where the old species no longer regenerates can lead to grassland or shrubland rather than the same woodland.
How foresters and researchers respond
Researchers study range shifts with long-term plots, tree ring records, seedling surveys and models of climate suitability. These models are useful, but they simplify. They tend to describe where a climate might be suitable, not where a species will actually appear.
Management ideas under discussion include favoring natural regeneration of species that appear well matched to future conditions, maintaining mixed stands and genetic variety, and in some cases planting seed from warmer or drier sources. The last idea, sometimes called assisted migration, is debated because moving species or seed sources carries risks as well as potential benefits. Which approach makes sense depends on local goals and conditions, and anyone considering it should seek advice from a qualified forester.