A University of Cambridge-led team has used aerial surveys across a huge swathe of the Cairngorms National Park in Scotland to create detailed 3D maps that show the beginnings of large-scale recovery of native woodland.
The maps document a success-story for the Cairngorms Connect nature restoration partnership: young trees are spreading naturally into areas where woodlands have been missing for centuries, following a landscape-scale approach to deer management across the partnership's 60,000 hectares.
From a light aircraft flown 600m above the ground, a laser scanning technology known as LiDAR collected the data used by the Cambridge team. Their new, AI-supported method of analysing the data revealed over six million individual trees across the 569km2 survey area, as well as the exact height of every tree.
The results show that 2.65 million trees - just over 40% of those surveyed - were under five metres tall. These small, regenerating trees grow mostly near the edges of existing forests, where seeds are most likely to fall - but some new trees were found growing over half a kilometre from the forest edge.
The maps also show that habitat type matters for tree regeneration: fewer new trees had established in boggy areas than in dry heathlands.
"The ability to map regenerating trees so precisely, at such a huge scale, is a breakthrough for monitoring nature restoration at a landscape scale. It means we can effectively map young trees, assess whether restoration targets are being met, and inform future restoration work," said Dr Aland Chan in the University of Cambridge's Centre for Landscape Regeneration, who co-led the work.
"The new LiDAR-based method is excellent at tracking where new woodlands are establishing across the landscape, and also for tracking the impacts of fires - which are increasing in severity with climate change," said Dr Pip Gullett from the Cairngorms Connect partnership, co-author of the study.
She added: "This is going to be a great way to monitor large-scale woodland expansion going forward, as more and more landowners across Scotland are looking to re-establish native woodlands."
The study is published today in the Journal of Applied Ecology.
Airborne LiDAR shoots laser beams at high frequency from a light aircraft towards the ground, which bounce back when they hit trees or any other surface. By recording the time taken for each laser pulse to bounce back to the plane, the team could create a three-dimensional image of the landscape with vastly more detail than is possible to collect through ground-based surveys.
Point measurements on the ground validated the accuracy of the results, and comparison with field surveys conducted by Cairngorms Connect and RSPB Scotland proved that the team's method can accurately assess forest expansion at a landscape scale.
Airborne LiDAR has previously only been piloted for young-tree mapping, but this new study shows it can be used to survey millions of trees in 3D, and can accurately map natural woodland expansion.
The team says its approach has potential to complement, or partially replace, hundreds of hours of work by staff and volunteers to physically survey sites on the ground - and could be used around the world to monitor forest management and restoration.
A reversal of fortunes
Scotland's only native coniferous forests have suffered centuries of decline through tree clearance for agriculture and hunting. Only 4% of the original native forests now remain and the landscape is extensively heath, bogs and grassland.
The study assessed land within the Cairngorms Connect partnership, the largest contiguous nature restoration partnership in the UK, which aims to double the size of the existing forest - largely through natural regeneration. This aligns with the Scottish Government's long-term strategy to capture more carbon by creating 3,000-5,000 hectares of native woodland across Scotland each year.
Until recently, deer browsing and the high cost of planting new trees within fenced areas has held back woodland expansion in Scotland. Now, a decade since the RSPB Scotland, Forestry and Land Scotland, NatureScot, and WildLand Ltd came together to form the Cairngorms Connect partnership, this study shows that over 5,235 hectares of previously open land supports young woodland - defined as land with over 100 trees per hectare. This has largely established naturally, with only small areas of planting where natural seed sources are not available.
The ability to track the success of forest regeneration is vital for informing future land-management approaches. The Scottish Government is now funding a nationwide LiDAR survey, and the team is hopeful that its method will be used for monitoring nature recovery in Scotland and the rest of the UK.
In addition to removing carbon dioxide from the atmosphere, more forested land is a boost for threatened species including Capercaillie and red squirrels, which are currently restricted to Scotland's remaining forest fragments.
