Choosing the right trees matters more than planting a greater variety, study finds

An oak forest | JHI

Planting a greater variety of trees may not be enough to protect Scotland’s woodlands from climate change and disease, with new research suggesting that the species chosen can matter far more than the number planted.

A study led by the James Hutton Institute (JHI) found that the right combination of trees could support up to 94 per cent of the wildlife associated with woodland, while a poorly chosen mixture could sustain as little as one per cent.

The findings challenge the assumption that greater tree diversity will automatically make forests more resilient, suggesting that the benefits depend on which species are included rather than simply how many.

Researchers worked with the Royal Society for the Protection of Birds (RSPB), the Royal Botanic Garden Edinburgh and an independent consultant to examine four common British woodland trees: ash, oak, Scots pine and Sitka spruce.

Using data from previous studies, the team identified the species associated with each tree and assessed whether they could also find food or shelter in other tree species. They then modelled two contrasting scenarios. This included one in which additional trees supported the greatest possible number of other species, and another in which they supported very few.

In the best-case mixtures, between 77 and 94 per cent of species associated with the original trees that did not depend exclusively on a single tree species could still be supported. In the worst-case scenarios, that figure fell to between one and 11 per cent, the report found.

Researchers concluded the difference suggests that planting more species is not, in itself, a guarantee of a healthier woodland.

Dr Ruth Mitchell, a woodland ecologist at the JHI and lead author of the study, said: “We generally assume a diversity of tree species will provide resilience for biodiversity. By this I mean that if one tree species declines due to a pest, pathogen or climate change, the tree-associated biodiversity will use the other tree species.

Dr Ruth Mitchell, of the James Hutton Institute | JHI

“However, this study shows that this isn’t necessarily true. To provide resilience for tree-associated biodiversity we really need to think about which tree species we are planting in the mix.”

Researchers said the results have implications for woodland managers deciding how to replace trees lost to disease, drought and storms, as well as for forestry policy as the UK seeks to make its forests more resilient to a changing climate.

Paul Bellamy, senior conservation scientist at the RSPB, said: “Woodland managers face the decision of how to replace trees lost to disease, drought or storms and keep the woodland resilient to future threats.

“This study highlights the importance of understanding how the choice of tree species affects biodiversity when managing woodland to withstand changing environmental pressures and maintain their ecological importance.”

The study, titled Not all trees are equal: Which tree species is more important than tree species diversity when considering provision of resilience for tree-associated biodiversity?, was published in the Quarterly Journal of Forestry as part of DiversiTree, a collaborative project seeking to improve woodland resilience through greater tree species diversity.