Tree nurseries and climate change: the impacts

LAST December I spent a week attending and speaking at the Landscape Below Ground Conference at the Morton Arboretum in Illinois, USA. The conference was a great success, but a frequently repeated joke amongst the locals was, ‘We are OK, we have not got climate change.’ This was obviously all ‘tongue in cheek’, referring to their current President’s denial that such a thing even exists.

Who could have forecast then where we would be today, with the emergence of COVID-19 affecting all of our lives so significantly? The US President remains in the news, with his suggestions of ingested and injected disinfectant coupled with regular use of anti-malaria drugs grabbing the headlines rather than climate change denial. Irrespective of that singular individual’s thoughts and prognostications, the challenges of climate change remain.

One of the side effects of the pandemic has been renewed attention on the value of nature and how important it is that people have accessible green space for leisure, health, and general well-being. Pre-pandemic there was, in the UK, an increased focus on the planting of trees, with central and local government making policy statements around increasing tree canopy cover and large-scale planting. While such a positive push for trees is to be welcomed (and there is little to say this is diminishing even while the pandemic is with us), there has been little discussion about where the trees are going to come from or which tree species should be planted and where to effectively mitigate against some aspects of climate change.

The real implications of climate change are unknown, with modelled forecasts fuelling debate. However, some of the emerging certainties include sea levels rising in coastal zones, increased winter precipitation in north-western Europe, temperatures rising much higher than the global average for northern Europe and an increase in warm temperature extremes in central and eastern Europe.

Rainfall across the British Isles and Ireland is predicted to fall by between five and 15 per cent. On top of this, there is the unpredictability of extreme weather events, which it is suggested will increase in frequency.

Climate change and the problems it poses for the future are taken seriously across the world (except for in the White House, perhaps), with many international reports having been published in recent years laying out the situation. The Impact of Climate Change on American Forests, Why Forests? Why Now?, The Science, Economics and Politics of Tropical Forests and Climate Change, and Boreal Forest and Climate Change are just a few of the titles to hit the shelves.

Koelreuteria paniculata growing in urban Dresden.

So what are the implications – surrounded as they are with a veil of uncertainty – for the tree nursery industry?

One of the most likely outcomes is the demand for a more resilient urban forest, one which can cope with the challenges of unpredictable weather and the increase in the spread of imported pests and diseases. The UK has already been badly affected by ash dieback and oak processionary moth with plane wilt, emerald ash borer and Asian long-horned beetle on the horizon. The method suggested by many as the most effective is increasing tree diversity within the urban tree population.

This begs the question: how resilient are current urban tree populations? At the conference referred to above, a speaker from Shanghai reported that there are over 1.2 million street trees in the city represented by 30 species, yet just two species accounted for 75 per cent of the total. In Barcelona, three species account for 79 per cent of the population, compared to three species accounting for 45 per cent in Pittsburgh, USA, and in the London Borough of Camden, six species accounting for 61 per cent. These figures are taken from a few of the many i-tree studies carried out both in the USA and mainland Europe. If tree population resilience is to be achieved through species diversity, then it would seem none of these populations are going to be resilient into the future. The work of Treeconomics (www.treeconomics.co.uk) and others continues to provide detailed information about urban tree populations, their value, the services they deliver and their resilience in terms of species composition.

It is likely then that the nursery industry is going to be asked to produce a wider range of species, some of which it is going to be unfamiliar with. Already in the UK there is increased demand for Celtis australis and C. orientalis, Styphnolobium japonicum, Ostrya carpinifolia, Koelreuteria paniculata and Carya illinoinensis, to name but a few. The production of these species presents challenges to a nursery industry unused to either propagating them (where this is possible), and growing them to a standard where they will survive in the urban environments of the future.

Then there is the question of provenance. At this moment in time, most tree nurseries would be unable to state with any confidence where the selection of a species they produce originated from. The natural range of a species can vary enormously. To provide one example, the natural range of Liriodendron tulipifera extends from New England to Florida in eastern USA. Climatic conditions between the northerly and southerly extremes of the range are obvious, but this has implications with regard to the genotype being offered by the nursery and the ability of that genotype to thrive as the impacts of climate change begin to emerge into reality.

The question of imported pests and diseases has already been mentioned, but the demand for an increasing number of species to be available from the nursery can actually fuel the demand for imported tree stock. This is particularly true where a species cannot be propagated or produced economically on the home nursery, with importing the only way of supplying the tree in question. Inevitably, this increases the risk.

Mature Celtis australis in Venice.

The demand for high-quality tree stock is also likely to increase. In recent years, I have travelled extensively in the UK and have visited Spain, Italy, Portugal, Holland, Canada, USA, Australia and Russia. There appears to be a consensus among practitioners that the quality of tree stock offered by the nursery industry is often not good enough. I cannot verify how wide-scale this issue is, though I have seen some real horrors not only supplied by the nursery industry but planted into the landscape by unquestioning contractors.

One of the impacts of climate change is likely to be that our cities will warm and the provision of shade and cooling will become increasingly important. It is already apparent that many municipalities are looking at trees to at least mitigate some of the extreme effects. Many cities are now beginning to set targets for increased tree canopy cover and have ambitious tree-planting programmes in place. If this trend continues (and the evidence suggests it will), then the demand for trees from the nursery is likely to increase proportionately. However, these trees will have to be fit for purpose if continued investment is to be made in tree planting. Investment is fuelled by success and rows of dead trees caused by poor-quality tree stock from the nursery, poor planting and poor maintenance are unlikely to stimulate repeated investment.

Has the nursery industry the skills it needs to capitalise on what would appear to be a golden opportunity? Again, the indications are not good. It would appear to be increasingly difficult to get young people to work on the land and many of the old craft skills associated with high-class nursery practice appear to be slowly disappearing. In the UK, many of the growers currently producing trees are the same folk who were in place 20 or more years ago.

I think it safe to say climate change is real. Some of the realities of that change have been outlined above, as have some of the implications of those changes for the nursery industry. There are many which have not been discussed here with the limited space available and many which I am sure I have not even thought of.

However, the one thing I am certain of is the tree nursery has an important role to play in the development and resilience of the urban forest in face of climate change. It has the opportunity to become central to the debate rather than peripheral. It will be interesting to see how many are capable of rising to the challenge.

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