The leaves of Lisbon
The leaves are still holding on in Lisbon in mid September but they are beginning to turn.
The leaves of the horse chestnuts appear to be dead or dying perhaps from the leaf miner moth (Cameraria ohridella).
The leaves of the plane trees are ready to drop. Their fruiting heads (achenes) will soon release their wind borne seeds. Fewer leaves helps dispersal but leaf drop and seed dispersal are both determined by changes in day length and temperature.
The sun is still warm and the hours of sunlight long but there is no rain. Without it the stomata on the leaves remain closed so as to prevent water loss through evaporation.
For the nettle tree, a rare native broadleaf on the Iberian peninsula, the timing of leaf loss is determined by a number of factors. Soon, nutrients will be withdrawn from the leaves, they will wither, die and fall, leaving a scar where the petiole joined the branch. Without the means to produce the energy it requires to live and grow, the tree will remain dormant until there is new growth in spring.
The leaves of the nettle tree are dry and rough to the touch. The specialised leaves of the evergreen conifers - needles - do not fall with the season but are replaced continually. They are modified to withstand hot, dry summers. In species such as the native stone (Pinus pinea) and maritime (Pinus pinaster) pines they photosynthesise throughout the year and take advantage of the warm winter days that come after rain.
Their smaller surface area reduces water loss thereby limiting the amount of carbon dioxide they can absorb which, in turn, slows the rate at which they can photosynthesise.
In an adaptation common among Mediterranean evergreens such as the olive tree and some conifers, the stomata are sunken in tiny inlets in the epidermis called stomatal crypts. Together with tiny hairs, which slow the passage of air, these reduce evaporation.
In many broad leafed trees, the stomata are on the abaxial (the underside) of leaves. The waxy cuticle of the adaxial (the upper surface) may also be thicker. The reverse is true in aquatic plants, and some trees have stomata on both sides (amphistomatic leaves).
Although it may seem awkward, the absorption of carbon dioxide and the output of oxygen and water occur only at the stomata. This structural constraint creates a natural balance. When water evaporates, it draws water up through the plant from the roots. When there is insufficient water, the cells around the stomata deflate, closing the opening. Carbon dioxide cannot enter but without sufficient water, photosynthesis could not proceed.
There are variations on how photosynthesis takes place (desert plants take in carbon dioxide at night and store it for photosynthesis during the day). These are known as CAM plants. Grasses and maize have evolved a process (the C4 pathway) which is more efficient at high light, high temperature, and low CO₂, than most other plants (C3 pathway).
Leaf morphology, cuticle thickness, location of the stomata, shape and surface area provide adaptations which affect the efficiency and regulation of photosynthesis and reflect local conditions.
Visual differences provide clues as to the strategy for reproduction and survival any particular tree is adapted to. The annual shedding of leaves comes at a cost because they must be replaced. The nettle tree minimises loss by withdrawing resources before a leaf is severed and benefits from the leaf litter at its base.
Where there is water, either naturally in proximity to rivers and lakes, or in cities where they are watered, broadleaf deciduous trees prosper. But where there is no relief from the sun, and roots must extend to find what water there is, attrition is required. Growth is slow but persistent, occurring whenever conditions are favourable.
September reveals most clearly the two strategies available to trees in the Mediterranean. Enter another cycle of death and renewal or maintain a quiet, enduring resistance.