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Emily Warner is a NERC-funded PhD student on the University of Oxford’s Doctoral Training Partnership in Environmental Research.
The mission of Trees for Life to rewild the Scottish Highlands has also created an opportunity to research the impact of native tree planting, or reforestation, in upland areas. There has been a recent increase in forest expansion targets in the UK, as a way to tackle the climate and biodiversity crises. Understanding more about how different reforestation methods can positively contribute to these goals is therefore important to guide future action.
I spent an inspiring two months volunteering with Trees for Life in 2015. When I began my PhD in ecology two years later, I was keen to revisit the upland replanting areas at Glen Affric and Dundreggan, which provide a unique opportunity to study native reforestation over a 30-year period. This June, the Journal of Applied Ecology will publish our research on the response of plants, carabid beetles and birds to reforestation, based on fieldwork carried out at the two sites.
The aim of the research was to characterise the effect of native tree planting on different aspects of biodiversity, and compare this to unforested areas, as well as mature Caledonian pinewood - the target habitat. Trees for Life team members Alan McDonnell and Doug Gilbert helped to define the focus of the work and supported the fieldwork. Alongside my supervisory team, we developed an experimental design that aimed to capture the response of biodiversity to the reforestation.
Our research showed that there were more bird species in reforested areas than in unforested areas, but this was still lower than in mature forest. On average, we found one bird species in our study plots in the treeless areas, four species in the reforested areas, and seven species in the mature forest. The study also looked at plants and carabid beetles, for these two groups we found similar numbers of species across the mature forest, reforested and unforested areas.
We were also interested in how the communities of each group responded to the reforestation. For plants and birds, we found that the reforested areas had an assemblage of species that was intermediate to the unforested and mature forest areas. Suggesting that the bird and plant species present at these sites are starting to look more like the mix of species found in mature forest. The carabid beetle communities were quite similar in the mature forest and reforested areas, suggesting that the reforested areas provide habitat for the same beetle species found in the mature forest.
We were particularly interested to understand how woodland specialist species respond to reforestation. We assessed changes in ancient woodland plants, such as wood anemone and common cow-wheat, and specialist woodland birds, such as tree pipit and willow warbler. For both plants and birds, we found that some woodland specialist species were present in the reforested sites. The more mobile birds have likely colonised the new forest, while the plant species present may have regenerated from the soil seedbank.
The understorey of Caledonian pinewood has a thick layer of mosses, abundant heathers and berry-producing shrubs such as blaeberry, cowberry and crowberry. Further analysis found that the number of these characteristic plant species is similar in the unforested and reforested areas, both of which are lower than in the mature forest. As the forest matures, we hope that these characteristic species will increase in the reforested sites, to generate an understorey with the composition found in mature pinewood.
Although the development of the new forest habitat is ongoing at Glen Affric and Dundreggan, it is already supporting woodland specialists. The newly created habitat has not yet reached the composition and structure of the mature forest that it aims to recreate, but the value of the new forest will increase over time. Areas where trees have been planted could support a similar richness of bird species to the mature forest within several decades, but the timescale over which the plant community will develop is less clear. Future interventions in the young forest, including the introduction of rarer specialist plants, could speed up the increase in value of the new forests.
Our research shows the long timescales over which restored habitats develop, emphasising the need for a reduction in threats to existing mature forest, the creation of new forest with connectivity to mature remnants, and the timely creation of new forests that will take decades, even centuries, to reach their full potential.
A second journal article based on our research, which assessed the response of ecosystem functions to reforestation, is also scheduled for publication later this year. You can read more about my experience carrying out this research in The Clearing, an online journal published by Little Toller Books.