Leaf shape is not random. Each outline is a compromise between catching light and losing water.
Why leaves are the shape they are
The leaf's job
A leaf catches sunlight and uses it, with carbon dioxide from the air and water from the roots, to make sugar. The wide, flat blade of a broadleaf tree is a good light trap. The pores on the underside, called stomata, let gas in and water vapour out.
Big or small
Leaves in shady forests are often large, thin and wide to collect weak light. Leaves in dry or windy places are often small, thick or waxy to cut water loss. A tree's own shade leaves are usually larger and thinner than its sun leaves, which is one reason to sample leaves from the sunny outer crown.
Lobes and teeth
Deeply lobed or compound leaves let wind and light through the crown and help the leaf shed heat. Many botanists see lobed leaves as a way to cool the leaf, although there is still discussion about how much each shape matters.
Needles and scales
Needles have a small surface area and a thick waxy coat, which suits cold, dry or poor conditions and lets the tree keep leaves through winter. Evergreens save the cost of growing new leaves each year but pay for it in a lower rate of photosynthesis.
Why trees drop leaves
Deciduous trees drop their leaves to avoid frost damage and water loss when the ground is cold. Before the leaves fall, the tree takes back useful nutrients, and the green chlorophyll breaks down, unmasking the yellows and oranges beneath. Red pigment is made fresh in autumn in some trees, notably red maple and sumac.
Why this helps you identify
Leaf shape is under strong genetic control, which is why it works for identification. But the same species varies with sun and shade, so the leaf shapes page and the leaves page both suggest sampling several leaves from the sunny part of the crown.