Nettle trees in summer
A common tree throughout Lisbon is the European hackberry or Nettle tree. It is a native, well adapted to the heat and drought of long summers in the city.
Its sharply toothed leaves are asymmetric as though someone has held them in their right hand and pushed up the left side a little with their thumb. They are medium-sized, ovate, four or five inches long and two inches wide.
Leaves exposed to the sun are dry and pale and coarse to the touch. The shade leaves are green and smooth with bright yellow-green veins.
In late July, with daytime temperatures in the high twenties and thirties, the trees are mostly dormant, their stomata closed against water loss. This reduces their intake of carbon dioxide through diffusion and limits the production of sugars.
By late August, there has been no serious rain for months. The nettle trees are not alone in prioritising reproduction; water and nutrients are withdrawn from less efficient, or sun-damaged leaves in a process called senescence. Their fruits begin to ripen, drawing on carbohydrates, nutrients and water stored earlier in the year.
