Street trees in Lisbon

A tree pit (caldeira de árvore in Portuguese) is the excavated space around a tree designed to protect it.

According to Lisbon municipal tree regulations, the caldeira "must be of a size compatible with the healthy and full growth of the tree species planted there."

For a square, the internal width should be no less than 1.2 metres, and for a circular, or non-rectangular shape, no less than 1.2 metres in diameter. Traditional caldeira, formed from several curved pieces of limestone, are attractive but too small for a mature tree.

City trees provide shade, mitigate the urban heat island effect, and retain water. Their leaves absorb traffic pollutants.

But the street is an alien and unkind environment for trees. They die young. Compact soil damages roots and harms fungal hyphae and microbial communities. Vehicles hit them and people break limbs and their natural form is cut to accommodate both.

Deprived of water and nutrients, trees will send out lateral roots that can disrupt and puncture surrounding surfaces including tarmac and, in Portugal, the small, limestone blocks of calçada.

The sparsity of street trees, made worse when one dies, lessens the opportunity for interactions with birds and mammals. Insects are affected by pesticides. There are fewer worms and beetles due to poor and polluted soils and the absence of dead branches and leaf litter which are removed for aesthetic and safety reasons. Mulching is rare. Where grids are used the protected soil allows plants to grow and, in rare cases, trees are connected by subterranean troughs.

The European nettle tree, Celtis australis, a native to the Iberian Peninsula, is commonly planted in Lisbon's streets and parks. Tolerant of air pollution, drought, and poor soil, it withstands hot, dry summers, with little or no watering. The leaves, which grow fresh and bright green in early spring, fade with the year and take on dust and dirt. In the heat of the day the dry, rugose leaves droop.

They provide deep shade in summer but, once their leaves have fallen, let in the low winter sun.