Why drop leaves at all
A deciduous tree sheds its leaves once a year, usually in fall. In temperate regions, this is a strategy for surviving winter. Broad, thin leaves are efficient at photosynthesis in warm, bright, moist conditions, but they are vulnerable to freezing and lose water quickly. When the ground freezes and roots can barely take up water, keeping a full set of broad leaves would risk drying out or frost damage and could load branches with snow.
By dropping leaves and going dormant, the tree avoids these risks. The cost is that it must grow a new set of leaves every spring. In places with a long, mild growing season, the high output of those leaves repays that cost. Where the season is too short or the soil too poor, evergreen strategies tend to win, as described in the article on conifer forests.
Deciduous habits are not limited to cold regions. Some tropical and subtropical trees drop leaves in the dry season, which shows that the underlying trigger can be drought as well as cold. The focus here is on temperate broadleaf forests, with trees such as oak, maple, beech, birch, ash and hickory.
Late summer and fall
As days shorten and nights cool, trees receive signals that winter is coming. Day length is a dependable cue, since it does not vary from year to year the way temperature does. Before leaves fall, the tree breaks down much of the green pigment chlorophyll and pulls valuable nutrients, such as nitrogen, back into its branches and trunk to be stored.
As the green fades, other pigments become visible. Yellows and oranges come from pigments that were present all along, while reds and purples in many species are made fresh in fall. The brightness of the display varies from year to year with weather and differs among species and regions, and it is hard to predict precisely.
At the base of each leaf stalk, the tree forms a layer of cells that seals the point where the leaf attaches. When this layer is complete, the leaf separates and falls, leaving a clean scar. The tree has effectively closed the wound before the leaf is gone.
The forest floor in fall
Leaf fall delivers a large pulse of material to the ground. Fallen leaves form a litter layer that protects the soil, insulates small animals and plants, and feeds the soil food web. Fungi, bacteria, earthworms, millipedes and many other small animals break the litter down, releasing nutrients that trees take up again through their roots, often with the help of mycorrhizal fungi. The process is described in detail in the article on leaf litter.
Leaves of different species decompose at different rates. Some, such as those of ash or linden, break down relatively quickly, while oak and beech leaves are tougher and last longer. The mix of species in a stand therefore influences how fast nutrients cycle.
Winter dormancy
Over winter, a deciduous tree is not dead, but its activity is greatly reduced. Buds formed in the previous summer sit sealed in protective scales. Water inside the living cells is adjusted so that ice crystals are less likely to damage them, and the tree can survive temperatures far below freezing. Many species also need a certain amount of cold exposure before the buds will open, which prevents them from reacting to a brief warm spell in midwinter.
The bare canopy lets light reach the forest floor, which makes winter a good time to see forest structure, bird nests and animal tracks. Wildlife has its own strategies. Some animals migrate, some hibernate, and others stay active, feeding on stored seeds, buds and bark.
Spring regrowth
In spring, rising temperatures and lengthening days trigger bud break. Sap movement increases, buds swell, and new leaves expand. The first leaves are small and often tinged with color before they mature. Different species leaf out at different times, and in many forests the canopy closes over a few weeks. The mechanisms by which water gets to the new leaves are explained in the article on water transport.
Timing matters a great deal. Leaves that emerge too early risk frost damage, while leaves that emerge too late lose part of the growing season. In recent decades, many temperate species have been leafing out earlier on average, a topic covered under spring phenology. Because insects, birds and plants all keep their own schedules, shifts in timing can in principle cause mismatches, for instance if caterpillars that feed on young leaves hatch at a different time from the birds that feed on caterpillars. The size of these effects varies and is still being studied.
The spring window below the canopy
Before the canopy closes, a short period of strong light reaches the ground. Many woodland wildflowers, sometimes called spring ephemerals, take advantage of it. They sprout, flower and store energy in underground structures within a few weeks, then die back above ground by early summer. Trillium, bloodroot and wood anemone are typical examples in parts of eastern North America and Europe. Other understory plants are adapted to the shade of summer and remain green longer, as covered under understory plants.
Summer growth
Once the canopy has closed, most of the tree’s growth takes place. Leaves capture sunlight and produce sugars that are used to build wood, roots, flowers and seeds, or stored for the winter. Broadleaf trees put on an annual growth ring, wider in favorable years, and a record of conditions builds up in the trunk, as described under tree rings.
Some species, notably oaks and beeches, produce large seed crops only in certain years. This is discussed in the article on mast seeding. The seeds feed squirrels, jays, deer, mice and many other animals.
Variation and change
Not all broadleaf forests behave identically. Some trees, like certain oaks, hold dead leaves through winter (a trait called marcescence), especially when young. Others, in warmer regions, mix evergreen broadleaf species with deciduous ones. Water availability, soil and elevation change the picture further.
A warmer climate may lengthen the growing season, but it can also bring drought stress, late frosts after early leaf-out, and shifts in species ranges. The forests will probably continue to be seasonal, but their timing and mix of species may change.