Protected areas (PAs) are critical to biodiversity and climate policies because they help conserve valuable forest ecosystems and store large amounts of carbon. Natural forests tend to be more diverse - in terms of vegetation structure - than intensely managed forests and dominated by older and taller trees. However, measuring how well PAs preserve such characteristics is challenging.
This is where satellite forest monitoring comes in. By comparing satellite-derived forest indicators inside and outside PAs, one can assess the effectiveness of conservation and protection measures. While such comparisons have been made for area-based metrics (e.g. the proportions of different ecosystem extents), assessments based on forest heights and structure are lacking.
The debate about the impact of protected areas on the conservation of forest diversity is quite relevant in Europe given the various policy instruments recently developed on biodiversity and forest management. As researchers of the Joint Research Centre (JRC) of the European Commission was immediately apparent that the question to ask ourselves was: How well do European PAs conserve forest structural diversity?
Luckily, the new developments in satellite technology helped us, particularly the data generated by the spaceborne Light Detection And Ranging (LiDAR) mission “Global Ecosystem Dynamics Investigation” (GEDI).
GEDI retrieves accurate information on forests’ vertical structure using LiDAR, i.e. a 3D laser scanning technology. Because it uses the International Space Station as a platform, GEDI collects data across the globe between latitudes of 52˚ N and 52˚S since 2019, providing a major advantage over field measurements and airborne laser scanning, which have limited and non-uniform spatial and temporal coverage.
GEDI enables the estimation of canopy height, a robust proxy for biomass, ecosystem structure, and biodiversity. Canopy height is known to influence maximum productivity and water use efficiency, by moderating air temperature extremes and water stress. Taller forests often provide a greater diversity of microhabitats, harbouring a bewildering array of evolutionary and functionally distinct species. Also, canopy height and its spatial variation are often indicative of the stand age and the intensity of disturbances or management. GEDI also enables the estimation of structural diversity metrics, such as the Foliage Height Diversity (FHD) describing the vertical heterogeneity of the foliage, which are strong predictors of species richness.
We then compared over 30 million records describing forest height and structural complexity indicators inside and outside almost 10,000 PAs. Then we assessed the effectiveness of PAs included in the Natura 2000 network, the de facto European reference network of protected areas.
Natura 2000 PAs can vary considerably in their protection levels, and in many cases, silvicultural activities are allowed within these areas. To have a baseline for forest habitats with minimal human impact, the researchers also used a subset of PAs which had particularly high levels of protection (i.e. Strict Nature Reserves, Wilderness Areas and National Parks) as recorded in the World Database on Protected Areas (WDPA) (UNEP-WCMC, IUCN), the most comprehensive global dataset on protected areas.
We found that forests inside European PAs are on average 2 m taller and more structurally complex than those outside, although there are some regional differences. Moreover, forests outside PAs have greater variations in canopy height, likely because production-oriented forest management creates a mix of older and younger stands.
The results were consistent for both Natura 2000 areas and the World Database on Protected Areas, thus demonstrating the effectiveness of PAs in Europe.
This assessment supports the post-2020 global biodiversity targets and the EU's nature conservation and restoration policies. Specifically, spatially exhaustive, consistent and robust information on forest height and structure in PAs is crucial to monitor the condition of forests providing baseline information for ecosystem restoration initiatives.
Results of the study highlight the utility of systematically collected LIDAR data to answer questions on forest biodiversity and habitat quality that were previously hardly addressable.
To learn more about our work, visit https://doi.org/10.1038/s43247-023-00758-w