24 April 2025 – In light of the growing threats from climate change, experts are calling for the implementation of mixed species forests as a vital ‘insurance policy’ for UK forestry. This recommendation is based on a new literature review supported by DEFRA and Teagasc.
Published in the Quarterly Journal of Forestry, the paper highlights the numerous benefits of incorporating a diverse range of tree species in productive forests. Not only can this approach help mitigate climate change and biodiversity loss, but it can also address issues of housing and domestic timber shortages.
The review, authored by Andrew Leslie of Forest Research and Ian Short of Teagasc, suggests that mixed species forests, which consist of broadleaves or a mix of broadleaves and conifers, can increase resilience to extreme weather events, benefit wildlife, reduce damage from pests and diseases, increase productivity, and improve financial returns from broadleaved trees.
This research is especially significant for the UK, as broadleaved trees, also known as hardwoods, make up the majority of British woodlands. These findings not only address the pressing issues of climate change and biodiversity decline but also align with the government’s new Timber in Construction Roadmap, which aims to meet sustainable housing targets by promoting the use of sustainable building materials and strengthening the UK’s domestic timber market.
A comparison of hardwood sawlog production volume in the UK with that of France and Germany reveals a significant gap. Based on data presented in the paper, the UK produces six times less hardwood sawlogs per hectare annually than France and fifteen times less than Germany.
Dr. Ian Short, co-author and Silviculture Senior Research Officer with Teagasc, stated, “Considering the mounting pressures on our forests from escalating pest and disease outbreaks and the accelerating impacts of climate change, the timing of this review is critical. We recognized that while mixed species stands, especially those with broadleaves, constitute a significant portion of the forest area in Great Britain, there is a noticeable scarcity of formal research specifically investigating their potential for timber production.”
Co-author Andrew Leslie, Head of Silviculture and Wood Properties at Forest Research, added, “The potential for increased timber productivity in these diverse forest stands can contribute to a greater supply of domestically grown wood, potentially supporting the construction sector’s growing demand for timber. The increased resilience of mixed stands to climate impacts, coupled with potentially higher overall biomass production, can also play a role in climate change mitigation through carbon sequestration. Additionally, the species diversity inherent in these stands supports a more complex ecosystem, which is vital for enhancing biodiversity.”
The review also highlights the benefits of mixing tree species in terms of building resilience. Different tree species respond differently to physical stresses such as storms, drought, and extreme temperatures. Research compiled in the paper shows that species such as sycamore and lime (diffuse porous species) respond differently to drought than species like oak and ash (ring porous). This diversity in response means that when one species struggles, others may continue to thrive.
In upland areas with challenging growing conditions, mixed stands have demonstrated particular benefits. Evidence from trials in the North York Moors showed improved growth of broadleaves when mixed with conifers like Scots pine and Japanese larch, while studies at Gisburn forest showed that sessile oak performed better with Scots pine and alder than in monoculture.
The literature review also points to improvements in soil health through strategic mixing. Incorporating nitrogen-fixing trees like alder, for example, can enhance overall growth, particularly in nitrogen-poor soils.
Moreover, mixed stands offer economic advantages as well. They have the potential to increase yield if managed well, addressing the gap in UK hardwood production. Growing oak with species that create a dense understorey canopy can inhibit epicormic growth and improve stem form. This practice is already common in Germany, where beech, lime, or hornbeam are planted with sessile oak to achieve quality stems managed on long rotations.
The paper also discusses “overyielding,” where mixed stands produce more volume than monocultures of the same species. Analysis across multiple experiments in Europe showed that mixing beech and oak can increase productivity by approximately 30% compared to monocultures, particularly on poorer quality sites. Furthermore, financial returns can be further enhanced by combining species with different rotation lengths, allowing for earlier revenue from fast-growing species like Scots pine, while maintaining the longer-term value of slow-growing species like oak.
Threats from pests and diseases are intensifying with a changing climate. Statistics released by Forest Research show that between April 2023 and March 2024, there were 2350 tree pests and diseases reported via the TreeAlert service. The catastrophic impact of ash dieback on monoculture ash stands across Britain highlights the vulnerability of single-species forests.
In light of this, the authors describe mixed species stands as an ‘insurance policy’ against potentially devastating pest or pathogen incidents. They explain that mixing tree species can
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