Contemporary high resolution European forest structure assessed using tree-level National Forest Inventory data

Area of Science:

• Forestry Science
• Ecology
• Environmental Science

Background:

• Assessing forest structure across diverse European National Forest Inventories (NFIs) presents harmonization challenges due to varying plot designs and measurement protocols.
• Understanding forest structure, including tree species and diameter at breast height (DBH) distributions, is crucial for effective forest management and ecological studies.

Purpose of the Study:

• To develop and apply an objective, consistent methodology for assessing European forest structure using harmonized NFI data.
• To classify forest plots into six structural types based on species composition and DBH distribution.
• To provide a spatially explicit overview of forest structure across multiple European countries.

Main Methods:

• Harmonized tree-level data (basal area, species, diameter) from 18 European countries were compiled, using a common DBH threshold and scaling plot radii to one hectare.
• Six forest structural classes were defined, ranging from single-species, narrow DBH distributions (regular) to multi-species, wide DBH distributions (irregular).
• Data from 255,418 inventory plots were analyzed to determine the prevalence of each structural class across Europe at a 0.2-degree resolution.

Main Results:

• Approximately 56% of the analyzed forest area was dominated by a single species with a narrow DBH distribution (single-species regular).
• The single-species regular class was the most prevalent structural type, ranging from 35.8% (Switzerland) to 79.7% (Spain).
• Overall, 94% of European forests exhibited a regular structure (either single-species or multi-species regular), indicating a widespread pattern of organized forest development.

Conclusions:

• The study successfully established a consistent approach for European forest structure assessment, revealing a predominantly regular structure across the continent.
• The findings provide a valuable baseline for monitoring temporal and spatial changes in forest structure, informing forest management strategies and policy, such as the EU Nature Restoration Law.
• Despite harmonization challenges inherent in NFI data, the results offer significant advantages over aggregated statistics, providing detailed, spatially explicit insights into forest structure.