What drought is, and what it is not
A drought is a period of unusually low water availability compared with what is normal for a place. A desert is not in drought, because dryness is its normal state; a rainforest with a few weeks of unusual dryness can be. For trees, what matters is not only rainfall but how much water is in the soil and how strongly the air pulls water out of the leaves. Heat makes this pull stronger, so a warm dry spell is typically harder on trees than a cool one with the same rainfall.
How water stress affects a tree
Trees draw water up from their roots to their leaves through narrow tubes in the wood, a process described in the article on how trees move water from roots to leaves. The column of water is under tension, a little like a drinking straw under strong suction. When soil gets very dry, that tension rises.
Trees have a few ways to cope.
- Closing their pores. Leaves have tiny openings, called stomata, that let in carbon dioxide and let out water vapor. Closing them saves water, but also reduces the tree’s intake of carbon dioxide, so photosynthesis slows.
- Dropping leaves. Some trees shed leaves early in a dry period to cut water loss.
- Growing less. Growth often slows before any visible damage, which shows up as narrow rings in a tree ring record.
- Adjusting roots. Deeper or wider roots can reach more water, but they take time to build.
If water stress becomes severe, air bubbles can form in the water-carrying tubes and block them. This is often called hydraulic failure, and it can be hard for a tree to reverse. Another idea, usually called carbon starvation, suggests that a tree with its pores closed for a long time may run low on stored sugars. Researchers still debate how much each process contributes, and in many cases both are likely involved.
From stress to die-off
Not every dry spell kills trees. Many trees survive moderate drought with only slower growth. But when drought is long, severe or repeated, die-off can occur, in which many trees of a species or a whole stand die over a short period.
Several patterns are commonly reported.
- Larger, taller trees can be more vulnerable in some forests, because they must lift water higher and lose more through large crowns.
- Trees on shallow or sandy soils, on south- or west-facing slopes, or at the dry edge of their range tend to be hit first.
- Dense stands compete for the same limited water. In some cases, reducing density through careful stand thinning is discussed as a way to ease the competition, though results vary with site and timing.
- Trees may die one or more years after the drought has ended, because damage to their water system or reserves persists. This lag makes cause and effect harder to untangle.
Drought as a setup for pests
A tree under water stress usually has fewer resources to spend on its defenses. Many conifers, for example, defend against bark beetles by flooding tunnel entrances with resin. A drought-stressed tree may not make enough resin, which gives beetles a better chance of establishing. If beetle numbers build, they can overwhelm even healthier neighbors. This is one way forest insect pests and drought interact, and why outbreaks are often linked to dry and warm periods. For more on how trees protect themselves, see tree defenses.
Disease can follow a similar pattern. Some fungi that live quietly in healthy trees become damaging when the host is weakened.
Drought and fire
Drought also dries out the fuels on the forest floor and in the canopy. Dead trees, curled leaves, fallen branches and dry litter all burn more readily. A wildfire that starts in a dry year may spread faster and burn more intensely than usual. Forest fires are a natural process in many ecosystems, but the combination of drought, die-off and heavy fuels can shift fires toward conditions that forests are not adapted to. Smoke from such events can then travel far, which is why wildfire smoke is a public health topic in addition to an ecological one.
This chain, from drought to weakened trees to pests to fire, is not inevitable. It describes a set of linked risks, and each link depends on the forest type, the local weather and past management.
Other effects on the ecosystem
Drought changes more than the trees.
- Soil life. Dry soil slows decomposition and can reduce activity of fungi and microbes that support trees, as discussed with mycorrhizal fungi.
- Streams. Lower flows and warmer water affect fish, insects and amphibians in forest streams.
- Seedlings. Young trees have shallow roots and are often the first to die, which can alter which species replace the older trees.
- Wildlife. Animals can lose water sources, food crops and shelter. Seed and fruit production may drop in dry years.
Recovery and resilience
After drought ends, many forests recover, but not always fully or quickly. Survivors may regrow leaves, while heavily damaged stands may be replaced by other species or by shrubs and grasses. Several things generally help a forest withstand drought.
- Diverse species and ages, such as in mixed stands, spread the risk.
- Well-structured soils that hold water.
- Healthy, deep-rooted trees suited to the site.
- Avoiding additional stress, for example compaction, over-harvesting or heavy browsing.
Whether any given action helps depends on local conditions. Landowners worried about drought effects can ask a professional forester for guidance suited to their stand.