In late June 2021, air temperatures in Oregon, Washington, and British Columbia soared past 113°F in a heat wave unlike anything seen before. Hundreds of people lost their lives. In the weeks after, another impact appeared: evergreen forest canopies turned brown across an area as large as Rhode Island.
Scorching, described as a type of leaf death caused by thermal damage and hydraulic failure during extremely warm drought conditions in Nature Climate Change, is a sign of a problem now showing up in backyards and city parks. Two other papers in the same journal highlight a tough truth for tree lovers: extreme heat can directly harm leaves, and scorching damage often looks like early fall, but it is actually an injury the tree may carry for years. Learning to spot the difference and changing how we water, mulch, and plant is becoming a normal part of tree care.
Heat Can Damage a Leaf on Its Own
A tree cools itself much the way we sweat. It draws water up from its roots and releases it through pores in its leaves, a process called transpiration, and the evaporation carries heat away from leaf tissue. When soil water runs short, or the air simply gets too hot, transpiration can’t keep pace, leaf temperatures spike, and cells begin to die. Michigan State University Extension describes exactly this: when transpiration can no longer cool a leaf, high temperatures damage the tissue directly.
For decades, summer canopy browning was read mainly as a symptom of drought. Research on the 2021 heat dome complicates that picture. In their Nature Climate Change assessment, Michael Paul Nelson and Christopher Still report that the dominant driver of the die-off was extreme heat cooking leaf tissue: browning developed rapidly even in stands with adequate soil moisture, and it concentrated on the south- and west-facing slopes that absorbed the most sun.
The authors compare the event to coral bleaching, which is a quick, heat-driven breakdown that reveals a limit once thought to be far away. Trees that had survived for centuries proved out to be vulnerable to temperatures just a few degrees higher than they had ever experienced. The authors say extreme heat should be seen as its own type of disturbance, different from slow, ongoing stress.
Why Brown Leaves in July Behave Differently Than Fall Color
Autumn color is a controlled process. As the days shorten and cool, a tree deliberately breaks down chlorophyll and pulls nitrogen, phosphorus, and other nutrients back out of its leaves into its twigs and roots, banking them for spring before the leaves drop. Scientists call this senescence, and it is essentially an act of saving.
Heat scorch is a different process. In a 2026 Nature Climate Change letter, a group of researchers explained that severe heat and water stress can kill leaf tissue suddenly, sometimes in just a few hours, with little or no chance for the tree to recover nutrients. The leaf turns brown and dies before the tree can take back what is inside it.
The consequence reaches into the seasons that follow. A tree that senesces normally enters winter with its pantry stocked. A tree that scorches loses both the leaves and the nutrients they held, and can head into the next growing season depleted, with less capacity to leaf out, photosynthesize, and recover. Repeated scorching, the authors caution, can progressively drain those reserves and wear down a tree’s resilience.
You can often see the difference on the leaf itself. The Morton Arboretum notes that scorch usually starts at the tips and edges, which are the parts farthest from the veins and the last to get water. The damage then moves inward, leaving leaves dry, brittle, and sometimes curled. It is worst on the side of the tree facing the sun, wind, or heat reflected from pavement or a brick wall. On evergreens, needles turn brown from the tip downward.
During a heat wave, hold off on these tree care activities to preserve your trees:
- Nitrogen fertilizer pushes tender new growth that demands more water the tree can’t spare.
- Cuts stimulate growth and can expose shaded bark to sudden, direct sun.
- Moving a tree severs roots when it needs every one of them.
- Shallow daily sprinkling. It trains roots to stay near the hot surface instead of reaching for cooler, deeper moisture.
That advice to skip fertilizer, pruning, and transplanting until the heat passes comes from the Oregon State University Extension Service, which gets many questions like these whenever the temperature rises.
In the Amazon, the Forest Fights Back With New Chemistry
When a tree is stressed by heat and drought, it reacts not just through its leaves but also through its chemistry, releasing volatile organic compounds. Some of these act as defenses, while others serve as signals. A study published in Communications Earth & Environment observed this response across the entire Amazon.
During the record 2023–2024 El Niño, which was the most severe drought ever recorded in the Amazon basin, a team from the Max Planck Institute for Chemistry sampled the air above the canopy. Emissions of sesquiterpenes, a type of reactive compound trees produce as stress defenses, surged 122 percent over the course of the event. Then, as the rains returned, the forest started releasing three heavier “sesquiterpene alcohols,” primarily a compound called beta-eudesmol, that had not previously been recorded above the forest. The researchers interpreted the shift as a stress-defense response that outlasted the drought itself, perhaps in an attempt to make clouds form over the forest.
Because sesquiterpenes and their heavier alcohols are highly reactive, they contribute to the atmospheric chemistry that creates aerosols and helps form clouds. This means a drought that changes a forest’s emissions could, in theory, affect the weather above it. The study shows this chemical shift, but the effects on climate are still unknown.
The trees in your yard do something similar on a smaller scale. Heat and drought change how a forest behaves, even affecting the chemistry of the air above it, often in ways we cannot see.
Greening Globally, Cooking Locally
Zoom out, and the planet looks, improbably, greener. A separate 2026 Nature Climate Change review notes that vegetation has been expanding across much of the globe for decades, driven largely by rising carbon dioxide acting as a fertilizer for plant growth.
That overall trend exists alongside the problems we see in our own yards. Global greening happens over decades and over continents, while a deadly afternoon can happen in just a few hours to a single group of trees. A global average temperature does not protect an individual maple when its leaves reach a local breaking point. A warmer climate can bring more greenery overall and more sudden, local die-offs in the same year.
The baseline behavior of trees is changing as well. When the USDA updated its Plant Hardiness Zone Map in 2023, about half the country moved into a warmer half-zone, and the map was about 2.5°F warmer than the 2012 version of an analysis led by the PRISM Climate Group at Oregon State University. These zones track winter cold, not summer heat, so they do not measure the risk discussed in this article directly. Still, they clearly show that the climate our trees were planted in is not the same as the one they face now.
What You Can Do
You can’t lower the temperature, but you can change the odds for the trees you tend. The work involved must be focused on the trees you already have, the ones you plan to plant next, and the canopy your neighborhood shares.
For the trees already in your yard
- Water deeply and infrequently. Colorado State University Extension suggests roughly 10 gallons per week for every inch of trunk diameter for trees that don’t get lawn irrigation, or about 20 gallons a week for a 2-inch trunk. Deliver it slowly with a soaker hose or a hose set to a trickle, spread out under the canopy where the feeder roots live, not piled against the trunk.
- Check before you water. Push a screwdriver or soil probe 6 to 8 inches down; if it comes out damp, wait before watering. Deep, occasional soaking builds deep roots. Frequent surface watering does the opposite, ultimately starving trees of nutrition because their root systems are atrophied.
- Mulch, but keep it away from the trunk. A 2- to 4-inch layer of wood chips or bark over the root zone helps keep soil temperature steady and slows evaporation. Leave a few inches of bare ground around the trunk so the bark does not rot, and avoid piling mulch up like a volcano.
- Water in the morning. Early watering gets moisture into the soil before the day’s heat pulls it back out.
- Focus on the most vulnerable trees. Young and newly planted trees, as well as any tree surrounded by pavement, a driveway, or a south-facing wall, are the first and hardest hit by heat. Give these trees extra attention when water is limited.
- Shade the small ones. For a young or newly planted tree, temporary shade cloth rigged on a frame can cut the heat load during a severe spell without stopping airflow.
For what you plant next
- Plant in fall or early spring, not mid-summer. New roots need mild weather to establish before they’re asked to survive a heat wave.
- Choose trees that fit the site and will thrive in the coming decades. Pick species that have done well in hotter, drier summers in your area, and think about where a small tree’s shade will fall as it grows. Species that are likely to scorch, like many maples and birches, commonly struggle in paved, heat-reflecting areas. Save those spots for tougher, heat-adapted trees.
- Diversify. A mix of species means no single pest, disease, or heat event can take out the whole block at once.
For your parks and neighborhood
- Water the public trees, too. Young street and park trees rarely get watered during heat waves. A few buckets from a neighbor can keep a new tree alive; many cities run “adopt-a-tree” watering programs worth joining.
- Support more tree canopy and fair access to it. Tree cover acts as cooling infrastructure. A World Resources Institute review of 806 cities found that more canopy lowers midday land-surface temperatures by about 2.7°F, and the shade under a mature tree feels much cooler than open pavement. However, this cooling is not shared equally, as lower-income neighborhoods usually have much less tree cover.
- Close the loop on the debris. Fallen leaves and chipped branches make excellent free mulch; put them back under your trees. For yard waste you can’t reuse, find local composting and green-waste drop-off through Earth911’s recycling search.
The Trees, and Humans, Are Moving Into a New Climate
For many people, a familiar tree is more than just part of the landscape. It can mark a season, a street corner, or a childhood memory. Seeing a tree turn brown at the wrong time brings a real sense of loss, a feeling that researchers writing in Nature Climate Change are starting to recognize as part of the human impact of climate change.
We can’t hand our trees back the climate they were planted into. We can give them water at the right depth, shade when the heat spikes, and smarter choices about what goes into the ground next. For a lot of trees, that is enough to change whether they make it through.