Trees: the solution to all our problems or not that simple?

Trees are a great way to soak up carbon emissions, improve nature and biodiversity. Right? Well, not always. We take a dive into why where you plant trees can have hidden, perhaps surprising, implications.

Tree planting is big. Governments across the UK and beyond have ambitious targets for planting large areas of trees. The UK’s Climate Change Committee has recommended planting 30,000 ha of new woodland in the UK every year until 2050. The Scottish Government’s vision is to expand woodland cover to 21% of Scotland by 2032.

It’s hard not to like the idea. As well as soaking up carbon, trees also provide shade, a home for biodiversity and green space for us to enjoy – and timber as a resource.

But it’s not quite that simple. Depending what you plant and where, planting trees could actually unleash more carbon than they store and even lower biodiversity.

A carbon sink or source?

At the Hutton, we look at both what happens above and below the ground, as both areas can be carbon sinks and there’s more interaction between the two than you might think.

What’s important to note is that soil is a major carbon store, with some soils holding more than others. Soils with lots of organic material, decomposed plant matter, in them, such as peatland and moorland, are known as carbon rich, as that plant matter is rich in carbon. While planting on peatlands is no-longer allowed, planting on soils that are not classified as deep peat, but are still  carbon rich soils, does occur.

This can be an important factor, if, say, we decide to grow faster growing trees on it, says Dr Alessandro Gimona, a spatial ecologist at the Hutton.

“Our research showed that more carbon would be released than is stored (both in the tree and in the soil) by planting slower growing trees on organic (carbon) richsoils, with an overall gain in storage of carbon only happening decades after the trees are planted,” he says. “That’s because just disturbing these carbon rich soils, through ploughing or tree planting, for example, can release carbon.”

“We’ve also found that planting trees with minimal interference with the (carbon rich) soil (such as in upland areas) could also result in carbon being released from the soil,” adds Dr Ruth Mitchell, who leads the biodiversity and ecosystems group at the institute. “We’ve seen some evidence of this at our long-term MOORCO(moorland colonisation) experiment plots. They were planted with very little disturbance to the soil, but we still found carbon lossesin those plots after 12 years.” 

“It is well known that disturbing the soil does release carbon (the more intensive ground preparation, the more carbon is released),” says Dr Gimona. “What was less expected is that, on carbon-rich soils, carbon seems to be released, even if you do not disturb the soil, by the very act of growing trees.”

“This phenomenon is something that the science community is only really starting to understand,” says Dr Mitchell. “It’s thought that when trees are establishing, even through natural regeneration, when trees grow from seeds that fall to the ground, the additional carbon from the roots and leaves drives changes in the soil microbial community, which in turn drives the breakdown of the stored soil carbon.”

There has been initial evidence of this happening in the Arctic tundra, where trees and shrubs are starting to grow where they hadn’t before, due to climate change. The Hutton Institute is involved in experiments in Sweden to further test these ideas.

Faster growing trees, faster results, but…

Another option is to grow faster growing trees like Sitka spruce, which would soak up carbon faster. But, even then, intense ground preparation works could undermine how much carbon, overall, is stored in a timeframe that could contribute to 2045/50 net zero targets. And if the trees are then felled and used for wood pulp or paper or even biofuel, that carbon isn’t being stored for very long. So we need to think about the full value chain, says Dr Mitchell.

We could also lock up more carbon faster by planting trees on more mineral rich soils, which are made more from minerals or rocks and have less organic matter and won’t release as much carbon when they’re disturbed by planting.

We’ve done work to model how much carbon is stored according to the soil they’re grown on, the tree species and also the climate  But there are also other factors to take into consideration.

“There’s a lot of mineral rich soil in Scotland, particularly on the east coast,” says Dr Mitchell. “But it is also highly fertile soil; it puts tree planting into conflict with arable crops on prime agricultural land. This is why tree planting tends to be targeted on the more organic, less agriculturally valued, uplands.”

Each option has its trade-offs.

Does it have to be either or? Not necessarily, says Professor Alison Hester, a senior ecological scientist who leads the Hutton’s Climate-Positive Farming Initiative.

“There are ways we can integrate more trees into agricultural areas, through agroforestry, riparian (riverside) planting trees on farms and within fields, and windbreaks, for example,” she says. “We are actively integrating trees into the farm-scape at our research farm at Glensaugh as part of our Climate-Positive Farming Initiative. We planted an extra 100 ha of trees through 2021-22, taking tree coverage at the farm to more than 17%, and we will continue expanding our woodland resource.”

“We’re also working with the University of Aberdeen on the FarmTree project, which is looking at ways to help farmers do just this: integrate tree planting onto their agricultural land,” adds Prof Hester. This project will include practical tools and ways for farmers and land managers to plan, visualize and assess the potential environmental benefits of tree planting on their land, but also recommendations for government policymakers on strategies to support these initiatives.

It’s a complex and ever evolving landscape. “While trees give us many benefits, tree planting shouldn’t be seen as a silver bullet to removing carbon emissions,” Dr Gimona adds. “Our research says that, even if we’re successful in planting all the trees that we’re targeted to, it will only account for around 10% of Scotland’s emissions and only 1% of the UK’s emissions. So we still need to reduce carbon emissions as well.”

Trees are good for biodiversity, surely?

We can also inadvertently damage biodiversity if we plant trees in areas such as moorlands and grasslands, adds Dr Mitchell. “These are important habitats that support protected species like curlews, which nest inland. It’s not just the direct effect of loss of breeding habitat. It’s been shown that woodlands planted up to 1km from curlew breeding grounds can harbor predators like foxes and crows, that raid curlew nests, pitching woodland creation against biodiversity targets,” she says.

Hutton research has also shown that how woodlands are planted and then managed can also impact biodiversity. Dense, uniform plantations tend to support less biodiversity. “But with more variety, like a native woodland with different species trees at different ages, sizes and shapes, as well as scrub and shrubs, more biodiversity is supported,” says Dr Mitchell, who is leading DiversiTree, a UK Treescapesfunded project looking into these themes led by the Hutton. “It’s also a more resilient woodland too. A plant disease might kill one type of tree and leave the others unscathed. But it takes longer to grow and deliver the carbon storage levels targeted.”

For commercial forestry, species diversity can be a challenge to manage. “But you might think about having a block of a single species then a small block of different species, to maintain support for biodiversity,” says Dr Mitchell. “Another way to support biodiversity is through corridors between areas, to help species disperse.

“The decisions we make depends on our goals. For example, if we had more long-term carbon storage targets, we might plant oak trees, which can store carbon for hundreds of years and are great for biodiversity, supporting support a far wider variety of species then many other trees.”

Our research has showed that oak trees support 2,300 species, 326 of which are entirely dependent on oak. However, oak trees take longer to grow than other species such as Sitka spruce. But if we decide to convert more commercial forestry to native woodland, we also risk outsourcing the biodiversity losses, simply pushing the problem elsewhere, as noted by our recent Understanding the Indirect Drivers of Biodiversity Loss in Scotlandreport for NatureScot.

Trees in climate adaptation

Trees are not only beneficial for locking away carbon and enhancing biodiversity, they can also help environments adapt to climate change, particularly rivers.

“Our research has shown that Scottish rivers are warming. River temperature is a critical control on Atlantic salmon populations which are adapted to live in relatively cool water. If river temperatures exceed 23°C this can cause them thermal stress and behavioural change and beyond 27°C can severely impact the success rate when salmon spawn,” says Dr Rachel Helliwell, a senior research scientist and Director of CREW, Scotland’s Centre of Expertise for Waters, based at the Hutton.

“As well as helping to cool rivers through shading, we’ve shown that planting riverbanks with native, especially broadleaf woods can stabilize eroding banks, interrupt sediment and nutrient passage to watercourses and restore vital ecological connections between river habitats and bordering land,” says senior scientist Professor Marc Stutter. “We are supporting and providing evidence for national initiatives such as Riverwoods, which will  drive a step-change in implementing trees for rivers, as well as  Scottish Forestry’s new targets for river wood planting. Huge environmental and societal benefits can be achieved with strategic planting bordering streams and rivers, we are not talking about substantial land change, or loss of productive land.”

Looking to the future; tree resilience in a changing environment

With all the benefits that trees have the potential to provide, we also need to look at their health. How resilient are our forests and woodlands to climate change and things like diseases and pests?

We’re looking at these issues through a UK Research and Innovation funded Future of UK Treescapes project. Specifically, we’re looking at the rates at which some of our most important tree species are able to adapt to the changing environment and if intervening to help them could impact the species that they also rely on.

Working with the UK Centre for Ecology and Hydrology(CEH), we’re  looking at the genetic diversity within species and if they can  adapt to new challenges, such as warmer or wetter weather, new diseases and pests. The newLEAFproject, a UK Treescapes-funded project led by CEH, is also looking at how quickly trees can adapt. “This will help us to see how they might fare in a rapidly changing climate and what we might be able to do to help them and increase resilience against pests and disease,” says research scientist Dr Jenni Stockan, who co-leads the Hutton work in the newLEAF project.

“Our long-term pine provenance experiment, set up in 2012 in collaboration with Forest Research, the CEH and the National Trust for Scotland, is a great resource for this type of work,” says Dr Stockan, a co-leader of this long-term project. “We took Scots pines grown from seeds collected from 21 of the remaining Caledonian pinewoods across Scotland and are following how they fare when grown in different locations around Scotland.”

In these different locations, they’re exposed to different climates, pests and diseases. One of those locations is our research farm at Glensaugh. It’s an upland farm with a cooler drier environment, while other sites are warmer and wetter or warmer and drier.

“We are also using young pine trees grown from these different Caledonian pinewoods to experimentally test responses to predicted changes in climate such as drought and late frost,” adds Professor Hester. “This work will help us understand the potential for resilience and adaptation by Scotland’s native pine trees to predicted changes in climate into the future.”

The DiversiTreeprojectis also exploring what potential there might also be for starting to plant selected non-native trees that are predicted to be more resilient to a future warmer climate and drought. But a key issue is also how well these species would support biodiversity, which is currently poorly understood. “This project is assessing the biodiversity supported by non-native Sitka spruce forests and Scotland’s native pine forests and how diversification of these with other tree species may increase biodiversity and forest resilience,” explains Mitchell.

Tools for real world impact

All of this work by the Hutton helps those in forestry or looking at landscape-scale change to make decisions that will help them achieve their goals, now and into the future.

Tools developed by Hutton researchers allow land managers to test what they’re prioritizing, such as flood risk management, conifer productivity or avoiding curlew breeding grounds, taking into account the impact of variables like climate, soil, and multiple benefits afforded by planting schemes and management. “In the future, processes like changes in the soil microbial community that we’re still learning about, could also be incorporated into our models and also these tools,” says Dr Gimona, whose team has led this work.

But we mustn’t forget people

“Engaging with communities is also important” adds Dr Alice Hague, an environmental social scientist at the Hutton. “Our research has shown that managing land for multiple benefits – carbon storage, biodiversity, recreation and wellbeing, is important, rather than only focusing on biodiversity, or only on carbon storage, for example, which  can be taken negatively by the public.  We’ve shown that information and ensuring ease of access also helps people make greater use of woodlands – and benefits their sense of wellbeing.”

“It’s all a balancing act, between tree species, how fast the tree grows, where you plant them and over what timescale you’re looking for a benefit, alongside other targets, such as biodiversity and human wellbeing,” says Dr Mitchell. “There’s not a simple single answer.”

Disclaimer: The views expressed in this blog post are the views of the author(s), and not an official position of the institute or funder.