The interrupted lives of urban trees

I walked a long avenue of mature nettle trees this morning. Their leaves were dry and many had turned yellow. A recent heavy shower could not undo four months of hot, dry weather.

During drought conditions, small openings on leaves - the stomata - close up. This prevents water loss but also stops carbon dioxide from entering the leaf by diffusion. The production of glucose by photosynthesis depends on both carbon dioxide and water.

Nevertheless, fruits are emerging due to hormonal changes and phenological influences such as temperature and photoperiod. Growth is possible because sugars stored earlier in the year are released.

The berries, or drupes, start out green, turn yellow, and ripen to a dark red or purple. They contain sugars and some fats and proteins, making them a good source of food for animals. Birds and mammals disperse the seeds of the nettle tree.

But in almost 150 trees, I saw no birds. The busy road and the urban setting precludes most other animals.

Reproduction is dependent on organisms being in sync. In the case of nettle trees, the fruit will persist through the mild winter. There is still plenty of light and temperatures remain high. If there is rain and the leaves remain functional rather than senescent, photosynthesis will continue.

In the absence of predation and the random dispersal of seeds by animals, the opportunities for reproduction are reduced. The natural cycle is replaced by an artificial one. Trees are selected for the benefits they bring to people but due to urban stresses, they die young without reproducing, and are replaced by saplings grown in nurseries.

These elegant trees, unique yet of a kind, rise to the height of the surrounding buildings, their canopies stretched towards one another until the road is covered by an arch several hundred metres long. The role given them is not a tree's life.