Species diversity: reducing risk without losing landscape coherence

A tree planting scheme that depends on a single species is vulnerable to pests, diseases and weather events that affect every tree in much the same way.

Diversity reduces that exposure, but it should not be applied as a random mix. Streets, parks and neighbourhoods need to stay legible and continuous.

The analysis can be done by tree, street, neighbourhood, genus and botanical family. A city with many species can still be vulnerable if much of its canopy depends on a few families.

Age matters too. A group planted in the same period ages, and needs replacing, all at once.

Emerging pests and diseases can spread quickly through a uniform population. EPPO maintains alerts, lists and distribution data that help to keep track of new risks.

The design should avoid species with known problems locally, and should not put the whole investment into a solution that is still little tested.

Diversity also spreads the responses to drought, heat, frost, wind and waterlogging. A species that performs well for decades may show its limits when the climate changes.

Selecting alternatives does not mean abandoning proven species. It means reducing dependence and trialling new material in a controlled way.

A street can use one dominant species and another at specific points. A neighbourhood can be organised in different sequences, keeping heights, rhythms or forms compatible.

In parks, diversity can follow soils and microclimates. In formal avenue planting, the changes can come section by section, so that the avenue does not read as fragmented.

The nursery needs time to produce uniform trees. A diversified plan should take account of actual availability, quality and the capacity to supply replacements.

Swapping a species on site simply because another is not available can break up the scheme. Selecting and reserving in advance reduces this risk.

A long list does not make up for insufficient soil, poor watering or poor maintenance. Trees of several species can fail from the same planting fault.

There are ecological conflicts too. An exotic species may be suitable in an urban setting and a problem next to sensitive habitats. Origin, behaviour and site all need to be assessed.

Inventories make it possible to compare mortality, growth and symptoms between species. This information helps to adjust future proportions.

Diversity becomes a management strategy when it is planned, monitored and reviewed. Without that cycle, it is reduced to an initial count.

The distribution should be analysed by species, genus, family, age and location. A city can have many species in its inventory and still depend on a few dominant trees.

The strategy should also avoid every specimen ageing at the same time. Biological diversity and age diversity reduce different risks.

1. EPPO Global Database — distribution, hosts and plant health alerts.
2. Plan Arbre de Paris.
3. London Urban Forest Plan 2025 Actions Update.
4. Trees for Life: Master Plan for Barcelona’s Trees 2017–2037.