Functional diversity or number of species: what makes a planting more resilient?

A planting does not become resilient simply by having many species. If they all share the same sensitivity to heat, drought, compaction or disease, the risk stays concentrated. Functional diversity spreads the responses and creates some redundancy: several species fulfil similar functions through different strategies.

A palette of ten deciduous species with large leaves and a high water requirement may have great botanical richness but low response diversity. Another with fewer species, spread across different strategies, may cope better with climatic variation.

The comparison should include water, root, crown, phenology, tolerance and susceptibility.

Redundancy does not mean repetition without criteria. It means that an essential function, such as shade or summer flowering, does not depend on a single species. If one fails, another maintains part of the service.

This logic is especially important in urban networks and large projects.

Adding a rare species in small numbers can raise the count without changing performance. Spatial distribution and the proportion of each function carry more weight.

Trait tools and matrices help to visualise gaps and overlaps.

A matrix can show species in the rows and attributes in the columns, using simple classes and an indication of confidence. Grouping the columns for water, heat, root and phenology reveals sets that are too similar and functions with no alternative.

The tool also helps to communicate trade-offs. The most drought-tolerant species may give lighter shade; the densest crown may need more water. Resilience comes from accepting these differences and distributing them in space.

The most common mistakes are counting species without comparing attributes; using token percentages just to meet targets; concentrating all the functions in one dominant species; and ignoring distribution in space.

At Diaplant, diversity is assessed by the functions it adds and by the existence of alternatives able to respond to the same problem. The species count, on its own, does not measure resilience.

Resilience comes from diversity of responses. The number of species matters, but it gains meaning when it is tied to functions and risks.

• Functional traits are better than climate niche approach at predicting the vital growth and mortality rates of urban tree species. (2026).
• How do nature-based solutions contribute to biodiversity in cities? (2025).
• Reviewing designed plant communities’ potential for optimizing the performance of urban nature-based solutions. (2025).