Airborne lasers count 2.65 million trees under five metres across the Cairngorms

The lasers measure height, so age has to be inferred from it. A stem under five metres in the Cairngorms is probably recent, because Scotland's native forests were cleared over centuries for agriculture and hunting and only 4% of the original woodland survives ^[14]. But establishment depended on the ground: the team found fewer new trees in boggy areas than in dry heathlands ^[5], and the same height on different soils need not mean the same age. Dating the cohort would take a second flight.

The survey area is large enough that the density is worth working out. 569 square kilometres is 56,900 hectares ^[1], so six million trees averages about 105 per hectare across everything, open ground included ^[2]. That is barely above the 100-trees-per-hectare line the study uses to call previously open land "young woodland" ^[19]. Trees are not spread evenly, of course. The 5,235 hectares that clear the threshold are about 9% of the area flown ^[3]. And 2.65 million divided by six million is 44%, where the source describes it as just over 40%, so the true count sits nearer 6.5 million ^[4].

Cairngorms Connect took the landscape-scale deer-management route across its 60,000 hectares and aims to double existing forest largely through natural regeneration ^[20][15]. The cost case for that is explicit: deer browsing and the price of planting inside fences are what had held Scottish woodland expansion back ^[17]. The survey is consistent with the approach working, though it cannot test it: there is no comparison area where deer were left unmanaged and no earlier LiDAR flight to difference against.

"The ability to map regenerating trees so precisely, at such a huge scale, is a breakthrough for monitoring nature restoration at a landscape scale," said Dr Aland Chan of the University of Cambridge's Center for Landscape Regeneration, who co-led the work ^[6]. Dr Pip Gullett of Cairngorms Connect, a co-author, said the method "is excellent at tracking where new woodlands are establishing across the landscape, and also for tracking the impacts of fires" ^[7]. Ground point measurements and comparison with field surveys run by Cairngorms Connect and RSPB Scotland confirmed the approach can assess forest expansion at landscape scale ^[11], though no detection rate or height error is given.

The Scottish Government's strategy calls for creating 3,000 to 5,000 hectares of native woodland a year ^[16]. The 5,235 hectares inside the UK's largest contiguous restoration partnership is roughly one year of that target at its upper end ^[6]. Spread across the ten years since RSPB Scotland, Forestry and Land Scotland, NatureScot and WildLand Ltd formed the partnership ^[18], that is about 520 hectares a year ^[5], though the survey does not date when individual trees established. Airborne LiDAR had previously only been piloted for young-tree mapping ^[12]. The team says the method could complement or partly replace hundreds of hours of ground survey by staff and volunteers ^[13].