The effect of forest establishment on biodiversity and ecosystem function
Keywords: forest expansion
forest creation
plantation
biodiversity
tree diversity
carbon sequestration
forest restoration
reforestation

Forest expansion is increasingly proposed as a mechanism to tackle the biodiversity and climate crises. It is therefore crucial we understand the impacts of forest establishment on biodiversity and ecosystem functions, so that we can optimise future forest creation.
In this thesis I use a long-term, large-scale native forest creation project in the Scottish Highlands to assess the impact of forest expansion on biodiversity and ecosystem functions in an upland context in the UK. Within the UK, the uplands are an area where forest expansion is likely to conflict less with other land-uses, so understanding the impact of forest expansion in these locations is important. I show that the newly created forest provides habitat for some plant and bird woodland specialists, but specialists are more abundant in mature forest. Plant, carabid beetle and bird communities are transitioning away from those found in unforested areas, towards the composition in mature forest. Ecosystem functions (including aboveground carbon, topsoil carbon, topsoil nitrogen, decomposition rates, soil invertebrate feeding, vegetation structure, tree regeneration) respond variably to the forest creation. Notably, topsoil carbon is lower in the forest creation sites relative to unforested and mature forest areas. Together these results highlight the biodiversity conservation value of forest creation in the Scottish Highlands, but the impact on carbon sequestration is less clear. This emphasises that forest creation targets must consider trade-offs with unforested habitats that may contain high soil carbon stocks and support different aspects of biodiversity.
I also consider the need for the integration of ecological research into rewilding projects from their outset. I provide a framework for understanding the links between ecological concepts and goals of rewilding projects, and highlight the need for baseline data collection, long-term monitoring and integration of experimental manipulation in future rewilding projects. A coherent framework for monitoring rewilding and addressing uncertainties experimentally could inform future rewilding projects and allow their biodiversity conservation and carbon sequestration potential to be assessed.
Finally, I assess the effect of tree diversity on carbon storage in planted forests. I use a meta-analysis to compare carbon stocks in mixed forests to monocultures, using data from the literature and Tree Diversity Network. I show that carbon stocks in mixed forests outperform the average of associated monoculture plantations and monocultures of commercial species, with no clear difference between carbon stocks in mixed plantations and their best associated monoculture. These results provide further support for the diversification of future planted forests.
Overall, my thesis provides data that will inform future forest expansion at a range of scales. Where forest expansion is deployed due to its perceived benefits for biodiversity and carbon sequestration, it is important we understand the extent to which it will achieve this. My research highlights that carbon and biodiversity responses are not always congruent, and that approaches such as the diversification of planted forests could optimise carbon storage. Taking opportunities to incorporate data collection into future forest expansion projects from their outset will play an important role in filling research gaps and monitoring progress towards targets.