Summer is the season when trees are doing the most, even though they look the most static. Leaves are running photosynthesis at or near peak capacity, roots are pumping water upward at rates that can exceed hundreds of liters per day in a large tree, and the trunk is actively laying down new wood. But summer also presents threats: scorching heat, drought, insects, and intense sunlight that can overwhelm the very photosynthetic machinery the tree depends on. What looks like a tree quietly standing in the sun is actually an organism juggling enormous productivity with constant damage control.
Peak Photosynthesis and Its Limits
Summer is when most trees reach their highest rates of photosynthesis. In deciduous species, leaves that unfurled in spring need weeks to mature internally before they can photosynthesize at full speed. Studies on canopy oaks found that leaves took roughly 50 to 70 days after budbreak to develop full photosynthetic capacity, meaning the leaves that emerged in April or May only hit their stride in June or July.1PubMed. Seasonal changes in the photosynthetic capacity of canopy oak (Quercus robur) leaves: the impact of slow development on annual carbon uptake Evergreen species, which keep their leaves year-round, also peak in summer because the combination of long days and strong sunlight maximizes the energy available for carbon fixation.
Yet summer photosynthesis is not a flat plateau through the day. Many tree species experience what researchers call a “midday depression,” a noticeable dip in photosynthesis during the hottest, brightest hours. When leaf temperatures climb too high and the air gets very dry, leaves partially close their stomata (the tiny pores that let carbon dioxide in and water vapor out). This protects the tree from losing too much water, but it also chokes off the supply of CO₂ the leaf needs to keep photosynthesizing.2Tree Physiology. Midday depression of net photosynthesis in the tropical rainforest tree Eperua grandiflora: contributions of stomatal and internal conductances, respiration and Rubisco functioning The result is a morning peak, a midday slump, and sometimes a partial recovery in the afternoon as conditions cool. Satellite-based monitoring of a subtropical fir forest showed midday depression occurring far more often in summer than in other seasons, with drought and heat waves making it worse by roughly 50 to 75 percent.3Forests. Heat and Drought Have Exacerbated the Midday Depression Observed in a Subtropical Fir Forest by a Geostationary Satellite
This midday slowdown is not a malfunction. It is a calculated trade-off: the tree sacrifices some carbon gain to avoid drying out. In species that face intense Mediterranean-type summers, leaves even adjust their internal chemistry over the season. Cork oaks, for instance, ramp up production of light-absorbing pigments in the xanthophyll family that can safely dissipate excess sunlight as heat rather than letting it damage photosynthetic proteins.4Tree Physiology. Diurnal changes in photoprotective mechanisms in leaves of cork oak (Quercus suber) during summer The pool of these protective pigments grows larger as summer wears on and water stress intensifies, peaking by late summer.5Journal of Experimental Botany. Seasonal changes in xanthophyll composition and photosynthesis of cork oak (Quercus suber L.) leaves under mediterranean climate Think of it like a leaf putting on sunglasses that get progressively darker as summer heat builds.
How Trees Move Water in the Heat
A large tree can pull several hundred liters of water from the soil to its canopy on a hot summer day. This upward flow, called sap flow, is driven mostly by evaporation from the leaves: as water vapor exits through open stomata, it creates a tension that pulls liquid water up through the xylem, the network of tiny tubes running from roots to leaf tips. The biggest driver of sap flow rate is vapor pressure deficit, essentially how dry the air is relative to the moisture inside the leaf. In boreal forests, vapor pressure deficit and solar radiation together explained most of the variation in daily sap flow for both balsam fir and black spruce, while soil moisture and wind speed had almost no measurable effect.6Agricultural and Forest Meteorology. Vapour pressure deficit and solar radiation are the major drivers of transpiration of balsam fir and black spruce tree species in humid boreal regions, even during a short-term drought
That relationship sounds simple, but there is a catch. When vapor pressure deficit gets too high, as it does during a heat wave, the tree faces a dilemma. Very dry air pulls water out of leaves faster than roots can replace it, and if the tension in the xylem gets too extreme, air bubbles can form inside the water columns, a process called cavitation. Those bubbles block flow the way an air lock blocks a pipe. Some Mediterranean evergreens have been shown to actively repair these blockages by refilling embolized vessels, coordinating the process with changes in ion concentrations in the sap.7PubMed. Coping with drought-induced xylem cavitation: coordination of embolism repair and ionic effects in three Mediterranean evergreens Not every species can do this efficiently, however, and chronic water stress pushes trees closer to the point where their plumbing fails.
Summer sap flow does not stop at night, either. Multiple woody species from a range of ecosystems continue transpiring after dark, sometimes at meaningful rates. Researchers have proposed several reasons for this: it may allow stomata to be open and ready for photosynthesis at first light, it may help deliver nutrients dissolved in soil water to distant parts of the crown, or it may simply reflect that some older leaves have leaky cuticles and cannot fully seal their stomata.8Tree Physiology. Nighttime transpiration in woody plants from contrasting ecosystems In drought-stressed trees, nighttime sap flow can actually increase, possibly as a mechanism to rehydrate tissues that lost water during the day.9PubMed Central. Sap Flow Velocity in Fraxinus pennsylvanica in Response to Water Stress and Microclimatic Variables
Growing Wood and Storing Fuel
Summer is when most of a tree’s trunk growth happens, and the wood being laid down tells a story about conditions outside. In temperate climates, the large, thin-walled cells formed in spring (earlywood) gradually give way to smaller, thick-walled cells in late summer (latewood). This shift happens because sugar availability is high later in the season while the hormonal signals that promote rapid cell expansion have faded, so the tree uses those sugars to build denser, stronger wood.10PubMed Central. The Physiological Mechanisms Behind the Earlywood-To-Latewood Transition: A Process-Based Modeling Approach That contrast between earlywood and latewood is what creates visible growth rings. A summer drought can make latewood even narrower and denser than usual, which is one reason dendrochronologists can read past climate from tree rings.
Not all the carbon a tree fixes in summer goes to building wood. A substantial share is shuttled into storage as starch and sugars, particularly in the trunk and roots. Research on Mediterranean beeches found that carbon assimilated early in the growing season fuels growth, while carbon fixed later in the season is preferentially converted to storage reserves before the leaves drop.11Tree Physiology. Seasonal and inter-annual dynamics of growth, non-structural carbohydrates and C stable isotopes in a Mediterranean beech forest These reserves are critical: they fund the next spring’s leaf flush, power root respiration through winter, and provide emergency energy if the tree is damaged. In deciduous species, starch pools in branches and stems tend to peak at the start of the dormant season, right after the tree has spent the summer banking surplus carbon.12PubMed Central. Whole‐tree nonstructural carbohydrate storage and seasonal dynamics in five temperate species
Keeping Cool Under Extreme Heat
Trees have a surprisingly effective air-conditioning system: transpiration. As water evaporates from leaf surfaces, it absorbs heat, cooling the leaf in the same way sweat cools skin. When water supply is adequate, trees can keep their leaves two to five degrees Celsius below the surrounding air temperature, even during a record heat wave.13PubMed Central. Intensive leaf cooling promotes tree survival during a record heatwave During an extreme Australian heat event, canopy photosynthesis dropped to nearly zero, but transpiration kept going, maintaining cooling even as carbon fixation shut down.14PubMed. Trees tolerate an extreme heatwave via sustained transpirational cooling and increased leaf thermal tolerance That decoupling between photosynthesis and water loss is a survival strategy, not a sign of collapse. The tree gives up productivity to stay alive.
Drought changes this equation dramatically. When soil moisture drops, trees close their stomata to conserve water, which also shuts off the transpirational cooling system. Without evaporative cooling, leaf temperatures can exceed air temperatures, and the risk of permanent heat damage rises sharply.15PubMed Central. Water Availability and Hydraulic Strategies Control Leaf Thermoregulation and Damage During Heat Stress and Recovery in Temperate Tree Species This is one reason combined heat and drought events are far more dangerous for trees than either stress alone.
Some species have a chemical backup plan. Many broadleaf trees, especially poplars, oaks, and willows, emit isoprene, a volatile organic compound, at high rates during hot summer days. Isoprene production ramps up with rising leaf temperature. Experiments with genetically modified plants that were engineered to emit isoprene showed clear heat tolerance compared to plants that did not emit it, along with lower leaf surface temperatures under heat stress.16Plant and Cell Physiology. Plants Utilize Isoprene Emission as a Thermotolerance Mechanism The exact mechanism is still debated, but isoprene may stabilize cell membranes or quench reactive oxygen molecules that form when photosynthetic machinery overheats. You may have noticed the hazy blue look that hangs over forested mountains on hot summer days; a good portion of that haze is isoprene and other volatile compounds released by stressed trees reacting with sunlight in the atmosphere.
Bark as a Shield
Leaves get most of the attention, but bark plays its own summer role. On a sunny July afternoon, the south-facing side of an exposed trunk can get hot enough to damage the living tissue underneath. Trees have evolved several strategies to deal with this. Species with white or light-colored bark reflect sunlight, keeping the surface cooler. Species with deeply fissured bark shade their inner tissues with ridges and crevices. Some thick-barked species provide genuine insulation, maintaining a temperature gradient from the hot outer surface to the cooler cambium beneath. Smooth, thin-barked species like birch and beech have the fewest defenses against overheating and are among the most susceptible to a condition called sunscald, where intense solar heating kills a patch of cambial tissue on the trunk.
Fighting Off Insects
Summer is prime season for insect herbivory. Caterpillars, beetles, aphids, and leaf miners are all active when foliage is lush, and trees do not just sit there and take it. Many species mount chemical counterattacks. When insect chewing crosses a certain intensity threshold, some deciduous and evergreen trees ramp up production of tannins in their leaves, bitter compounds that make the foliage less nutritious or outright toxic to herbivores.17PubMed Central. Leaf defenses of subtropical deciduous and evergreen trees to varying intensities of herbivory Mediterranean oaks that suffered insect damage increased lignin and flavonoid concentrations in their leaves, toughening the tissue and making it harder for insects to digest.18Tree Physiology. Photosynthetic and defensive responses of two Mediterranean oaks to insect leaf herbivory
These defenses are not free. Producing tannins, lignin, and flavonoids costs carbon and nitrogen that could otherwise go to growth or storage. A tree that spends a summer fighting off heavy insect attack may enter fall with depleted reserves and narrower growth rings. Some species hedge their bets by maintaining a baseline level of defensive chemicals year-round and only boosting production when actual damage occurs. Others, particularly long-lived oaks, invest heavily in constitutive defenses and barely change their chemistry in response to attack. The strategy depends on how predictable insect pressure is in a given environment.
Feeding the Underground Network
Not all of a tree’s summer carbon stays above ground. A substantial portion is shipped downward to the roots and, from there, to the symbiotic fungi that colonize the root tips. These ectomycorrhizal fungi extend the tree’s effective root system and help it absorb water and nutrients, especially phosphorus. In return, the fungi get sugars. Research in a boreal pine forest found that below-ground allocation of recently fixed carbon was five times higher in August than in June, and the production of fungal fruiting bodies, mushrooms, basically, depended entirely on the allocation of fresh photosynthetic carbon in late summer.19PubMed. Quantification of effects of season and nitrogen supply on tree below-ground carbon transfer to ectomycorrhizal fungi and other soil organisms in a boreal pine forest The autumn mushroom flush you see on the forest floor is, in a real sense, funded by the tree’s summer photosynthesis.
This belowground transfer also feeds a broader soil food web. Carbon that moves from roots into the soil through exudates and fungal turnover supports bacteria, springtails, and other tiny organisms that cycle nutrients the tree will need.20Biogeosciences. Seasonal variations of belowground carbon transfer assessed by in situ 13CO2 pulse labelling of trees Summer, in other words, is when the underground economy beneath a forest is at its most active.
When Summer Goes Wrong
Moderate summer heat and occasional dry spells are part of normal life for most trees. But extreme events can push them past their limits. During the Pacific Northwest heat dome of late June 2021, multiple days of record-breaking temperatures caused immediate and widespread death of canopy foliage. Leaves that had been green before the event turned red or orange shortly afterward, a change visible from space in satellite imagery and confirmed on the ground as heat scorch.21PubMed. Extreme Heatwave Causes Immediate, Widespread Mortality of Forest Canopy Foliage, Highlighting Modes of Forest Sensitivity to Extreme Heat This was not autumn color change. It was tissue death in the middle of summer.
Drought that persists through summer can trigger a different kind of damage: premature senescence. Instead of leaves staying green until autumn’s shorter days signal them to shut down, water-stressed trees begin breaking down chlorophyll and shedding leaves early, cutting their growing season short and reducing the carbon they can store for winter.22PubMed Central. Early Autumn: Drought Accelerates Leaf Senescence in Temperate Tree Species In cities, the problem compounds. Tracking of 23 common urban tree species in Sydney during the record-breaking 2019–2020 summer found that more than half were considered vulnerable due to crown dieback, poor health requiring tree removal, or outright mortality.23PubMed. Crown dieback and mortality of urban trees linked to heatwaves during extreme drought
Why Urban Trees Have It Harder
If you have noticed that street trees look more stressed in August than trees in a nearby park or forest, you are not imagining it. The urban environment amplifies nearly every summer challenge. Pavement absorbs and re-radiates heat, raising air temperatures. Impervious surfaces also prevent rain from reaching roots, creating artificial drought even in years with normal rainfall. Research comparing downtown trees with nearby campus trees found that downtown air temperatures and vapor pressure deficits were much higher, and the trees responded with lower water status and reduced gas exchange.24Arboriculture & Urban Forestry. Tree Moisure Stress and Insect Damage in Urban Areas in Relation to Heat Island Effects
The amount of pavement around a tree matters more than you might expect. Trees surrounded by more than about 75 percent impervious surface were found to be the most drought-stressed, particularly small-diameter trees that lack deep root systems.25Landscape and Urban Planning. Understanding urban tree heat and drought stress by tracking growth and recovery following an extreme year As pavement cover increased, urban trees also showed reduced safety margins against xylem cavitation, meaning they were operating closer to the point of hydraulic failure.26PubMed. Drought-induced xylem cavitation and hydraulic deterioration: risk factors for urban trees under climate change? This helps explain why newly planted street trees so often die in their first or second summer: they have small root systems, limited access to soil water, and are surrounded by surfaces that make the local climate harsher than conditions a few blocks away in a park.
Preparing for What Comes Next
Even at the height of summer, trees are already responding to signals about the approaching autumn. The two main environmental cues are photoperiod and temperature. As days begin to shorten after the summer solstice, the changing light triggers molecular pathways that gradually slow shoot elongation and begin the process of growth cessation and bud set in many temperate species.27PubMed Central. Photoperiod- and temperature-mediated control of growth cessation and dormancy in trees: a molecular perspective This is why height growth in most deciduous trees effectively stops by midsummer, even though leaves continue photosynthesizing for months. The tree has shifted priorities from getting taller to getting fatter (adding trunk girth) and stockpiling energy reserves for the dormant season ahead.
That transition from growth mode to storage mode is worth appreciating. A tree in July may look the same as it did in June, but its internal allocation has changed. Less carbon is being invested in new shoots and more is being packed into starch granules in the trunk and roots. By the time leaves begin changing color in October, the tree has already been preparing for winter for weeks, driven by those gradually shortening summer days that most of us barely notice.