Long-term vegetation change in Scotland's native forests

Forests play a key role in climate change mitigation, adaptation and delivery of a range of ecosystem services. There is increasing evidence for impacts of climate and other drivers on plant community change, and fragmented habitats are predicted to be much less resilient to negative impacts on biodiversity and other services. Within Europe, Scotland's native forests are highly fragmented and now cover 4% of the land after many centuries of degradation and loss, but little is known about how their species composition has changed. We recorded long-term vegetation change (from resurvey data) and examined the relationships with climate, pollutant deposition and grazing as key drivers of change, focusing on four forest types: pine, ash, acid- and base-rich oak-birch. All four forest types showed dynamic compositional change during 30–50 years between surveys, with increased species richness and decreased diversity. There was no evidence for homogenisation - the opposite was the case for all except pine (no change). Analyses indicate significant and varied climate, pollution and grazing impacts; NHy deposition showed the most frequent association with species compositional changes. Notable species changes include increases in pteridophytes and declines in forb cover, and a doubling in frequency and cover of Fagus sylvatica between surveys. Our findings suggest a possible extinction debt, with many more species declining than increasing between surveys. This trajectory of change and our other findings indicate a pressing need for mitigation management to reduce the risks of future species losses, with forest expansion planning explicitly considering spatial location in relation to existing native forest and those plant species identified as most at risk.