We’re currently seeing record levels of human migration which is a direct result of a worsening climate crisis that disproportionately affects countries within the “Global South”1. This Global North – Global South divide is based on GDP per capita, and is a division that is shaped by colonialism and its lasting impact. In extreme cases, entire communities are being forced to leave their homes as they suffer natural disasters, war and persecution. The place they were born no longer having the safety and conditions needed for them to thrive – or in some cases – survive. When we think about migration in a human context, it is defined by political borders. Lines drawn on a map.
Nature, however, is not bound by political borders even though we do use them to define what is ‘native’. Trees have what’s called a ‘native range’ which is the areas of the world in which they occur naturally, without human intervention. They are boundaried only by abiotic factors (nonliving things) such as climate, rainfall, availability of sunlight and topography. And increasingly by biotic factors (living things) that affect a tree’s habitat potential. We’re learning more and more about their deep relationship with (and often dependence on) other living things.
There are many species that here in the UK we classify to be native and some that are held up as symbols of British nationalism, the Common oak (Quercus robur) being a prime example. In fact, Common oak is also known (in Britain at least) as English oak for this patriotic reason. However, even these native species, throughout history, have moved and migrated and are migrating still. Oaks, like most species vanished from the UK during the last ice age and then took around 6,000 years to slowly recolonise the whole of the British Isles. 2
In the UK we’re seeing hotter summers, milder winters and heavier, sudden rainfall known as cloudburst events. All of these changes can add stress to trees and fluctuate their habitat potential. In cities, where Urban Heat Island Effect can play a big part in how severe a drought is, we’re seeing a decline in some native species such as birch, rowan, willow and alder and they have started a northward migration toward cooler climates. At the same time we are seeing an increase in species that would otherwise have struggled here, now able to survive as winters that were historically too cold are now becoming milder. For orchard fans this means an opportunity for figs, almonds, peaches and many more.
A paper recently produced in partnership with Kew Gardens 3 in London showed how through plotting future climate scenarios for the UK against known existing native ranges for commonly planted tree species we can now see which trees are going to struggle first and which species will be beneficial migrants that can support our future urban forests. It’s predicted that the UK climate by 2090 will resemble the existing climate of southeastern Europe (think Romania, Bulgaria, Georgia etc.) and so by looking at species that are currently thriving there, or that are most common there, we can prioritise these species when planting.
Red markers indicate past, present and future UK conditions. Black markers show existing observations of species. The closer the red marker labelled “2090” is to the centre of the black markers, the better suited that species will be to the projected UK climate in years to come.
We still have a deep relationship and emotional connection to our historically native trees and will continue to do so even as their numbers lessen. To support our native and naturalised trees we should celebrate where they are adapting to climate change and we should explore new opportunities for habitat changes that could help secure their future. We already know that if we want silver birch or rowan in our cities or alder and willow along our river and canal edges we need to protect their habitat potential. If we can engage those involved in city planning and management we can increase rainwater harvesting, install better sustainable urban drainage systems and even underplant large climate resilient species with our native trees.
However, we should be careful to avoid a native-only or native-first approach that risks our communities. We must also open our arms to the migrants – the future trees of our cities – to whom we will become more and more indebted for the health and well being benefits they bring. If we’re unable to accept help from new migrants to support the existing tree stock, we will lose huge numbers of urban trees, the benefits of which will take years to recover.
At Birmingham TreePeople we’re planting a huge range of species each year that have proven abilities to thrive in the warmest urban settings, withstand drought and flood events and high levels of air pollution. This year we’re planting nine new Golden Rain Trees (Koelreuteria paniculata) which shows great potential to thrive in future UK climate scenarios.
Golden Rain tree (Koelreuteria paniculata) in flower. By Didier Descouens – Own work, CC BY-SA 4.0,
References
- Independent Commission on International Development Issues, North–South: A Programme for Survival (London: Pan, 1980)
- Lowe, et al. (2005). Route, speed and mode of Oak Postglacial colonisation across the British Isles: Integrating molecular ecology, palaeoecology and modelling approaches. (Botanical Journal of Scotland. 57. 59-81.)
- Kevin W E Martin, Henrik Sjöman – Evaluating urban tree population fitness for a changing climate: Using climatic moisture index (Trees, Forests and People, Volume 22, 2025)