3: Tree species composition

What information does the survey provide and why does it matter?

The NWSS collects data on tree and shrub composition in every structural stage of woodland vegetation. The dominant position of trees and shrubs mean that they largely determine ecosystem processes and the character and other values of the woodland.

Native trees and shrubs in general support a greater diversity of associated species than non-native species, especially amongst groups of invertebrates which depend directly on trees for food and shelter.

There are many plants and animals which use or co-exist with non-native trees, but many rare and threatened woodland species are specialists adapted to one or a few native trees or shrub species (birches, willows and oaks, are examples of trees that host many specialist insect species).

Native woodland biodiversity value is usually greater when:

  • a wide range of native trees and shrubs characteristic of the site-type are present, as this will support a larger community of wildlife, more rare and specialist species and a wider range of ecosystem processes like decomposition and nutrient cycling
  • native species make up all or nearly all of the woodland.

Regional patterns of tree species diversity for each priority type or National Vegetation Community (NVC) type are valuable to set a context for site planning, e.g. to help managers assess where more diversity is desired.

The NVC type can provide an indication of the characteristic range of trees and shrubs that might be expected, to help guide management, although this needs to be tempered with local knowledge and should not be use as a ‘painting by numbers’ prescription.

Potential seed collection areas that might be registered as seed stands could be identified as well.

Policy relevance

Classifying woods as native requires 50% or more of canopy cover to be composed of native species. Targets for improving condition for native woods include removing or reducing the non-native tree and shrub species share.

How can the NWSS data be used?

Type of useNationalRegional/local authorityLandscape/site scale
Assess if a wood is native woodland or not Yes Yes Yes
Planning silvicultural management to increase native species share Yes Yes Yes
Percentage of native species in the established regeneration, pole, mature, veteran layers (for specific queries or to plan silvicultural management) n/a Yes Yes
Amounts of particular tree species of interest (for varied reasons) Yes Yes Yes
Distribution pattern of individual tree species Yes Yes n/a
A diversity measure: mean no. of native species in polygon for each woodland type Yes Yes Yes