Monocultures or mixed species? Surprising research shows how forests cope with drought

Monocultures of some of the UK’s most economically important conifers may be more resilient to spring drought than mixed species forests, new research has shown.

Although mixed-species forests can be more productive and provide a wider range of social, environmental and economic benefits than those containing a single species, they may not be as resilient to drought, the University of Stirling researchers found.

Using a long-term experimental forest in Ardross, near Inverness in Scotland, they measured the impact of a spring drought in 2012 on monocultures of two species - Sitka spruce and Scots pine - compared to mixtures of the same two species growing together in different proportions.

Scots pine and Sitka spruce are two of the most economically important timber species in the UK, collectively making up 68% of all the UK’s coniferous forest area, with Sitka spruce alone comprising 51%.

Tom Ovenden

PhD researcher Tom Ovenden, of the Department of Biological and Environmental Sciences, who led the study, said: “As expected, we found evidence that Scots pine was more resistant to drought than Sitka spruce. However, to our surprise, monocultures of both species appeared to be more resilient to spring drought than any of the mixtures of the two species that we considered.

Increasing resilience

“As we rapidly try to adapt our forests to deal with the challenges of a changing climate, it’s important that decisions on how best to achieve this are based on robust scientific evidence. This work is important because it demonstrates that simply adding more tree species to a forest does not automatically increase its resilience. Instead, the existence of any beneficial effects of species mixture likely depends on which species are mixed, their characteristics and how they interact.

“Understanding how to effectively increase forest resilience is important, as the ability of forests to sequester carbon, provide habitat for a range of species and to continue to deliver a range of ecosystems services is dependent on them being robust to climate change.”

Measuring growth in tree rings

Mr Ovenden and his team collected tree cores – samples extracted from the trunks – to examine the tree rings that document the annual variation in tree growth. He said: “As tree growth is partly driven by how favourable a given year’s climate is, by collecting and measuring tree rings, we can quantify how much of an impact an extreme event such as this 2012 drought had on tree growth.

“Unexpectedly, we also found no evidence that competition from surrounding trees had a role in regulating either tree resistance - their ability to withstand the impact of drought - or their resilience - their ability to get back to normal after to drought. This may be because the forest was still relatively young, at 24 years, when the drought occurred.”

More research needed

Professor Alistair Jump, Dean of Stirling’s Faculty of Natural Sciences and a co-author of the study, highlighted the need for more research to understand how resilient native and non-native trees are under a range of UK soils and climate conditions.

Professor Jump said: “We currently know very little about how resilient tree species growing in the UK are to drought and other extreme events, and this lack of evidence makes it difficult for forest managers to effectively adapt to climate change.

“As a result, there is an urgent need for us to understand the historic impact of drought and other extreme weather events, as well as predict the impact of future events under a range of possible climate change scenarios for different species and forest types.”

Mr Ovenden’s research is funded by the University of Stirling, Forest Research, and the Scottish Forestry Trust. The paper ‘Intimate mixtures of Scots pine and Sitka spruce do not increase resilience to spring drought’ is published in the journal Forest Ecology and Management.