Action must be taken across all facets of society to avert the interconnected climate and biodiversity crises. But most governments, private sector investors, landowners and other organisations don’t seem that interested. Even many conservation organisations are constrained by limited resources or a lack of political backbone. One area that looks to be an exception is tree planting.
Whether it’s conservation organisations, governments or businesses looking to greenwash their reputations by trading carbon offsets, everybody appears to be on board with tree planting. There are understandable reasons for this: planting trees is a relatively straightforward, easily measurable and communicable intervention – “X number of trees planted, look how green we are!”
Indeed, the government has recently taken a short break from touting airport expansions and denigrating environmentalists in order to announce a new national forest to be created in the west of England, with 20m trees planted across 2,500ha of Gloucestershire, Wiltshire and Bristol.
However, like much in ecology and conservation, this penchant for sticking spades in the ground is not necessarily a silver bullet to be used as a shorthand for environmental sustainability.
Tree planting or natural recolonisation?
There’s plenty of debate around whether tree planting is really effective at restoring naturalistic and functioning ecosystems, or whether by contrast woodland restoration should happen mainly via natural recolonisation and rewilding. After all, trees have been perfectly capable of reproducing and dispersing for hundreds of millions of years without the help of our plastic tubes.
There are many advantages to natural recolonisation by trees rather than planting. Natural recolonisation usually involves native tree species, as opposed to many planting initiatives which create monocultures of more commercially valuable exotic conifers. It typically means that, depending on the condition of the land, the habitat must go through a process of ecological succession, with scrub gradually colonising open ground and then being overgrown by larger trees.
This often creates a far more dynamic and structurally complex habitat than planting – especially in the presence of large herbivores – with a multitude of biotic and abiotic factors influencing where trees will successfully grow and the diversity of herbaceous plants that grow around them.
Unlike planting, natural recolonisation doesn’t release carbon through the disturbance of topsoil. It is partly for this reason that tree planting on carbon-rich peat is seen as particularly environmentally detrimental. Moreover, natural recolonisation is likely to create forests with greater genetic diversity than trees grown in nurseries, and unlike nursery saplings, doesn’t risk introducing novel plant pathogens or invasive pests. Where feasible, it’s also, obviously, much cheaper than planting.
Neither natural recolonising or tree planting alone will be enough
However, in landscapes which, like much of Britain, have been deforested and overgrazed for centuries over large areas, natural colonisation of trees is unlikely to be effective any time soon. One of the key determinants of successful natural recolonisation is proximity to existing woodland. Close to these seed sources, trees can begin spreading in earnest as soon as sheep are removed from the land or burning stops, but the colonisation rate decreases steadily with distance, becoming almost non-existent beyond a few hundred metres, or in some cases even a few tens of metres.
Even for those trees whose seeds are commonly dispersed by birds, such as those producing nuts or berries, dispersal distances are usually limited to a few hundred metres and crucially, the foraging preferences of the birds mean that they tend to disperse seeds between existing wooded habitats rather than into open areas.
Therefore, when it comes to reforesting landscapes that have been grazed bare over wide areas, like much of the British uplands, natural recolonisation alone will be unable to restore forest ecosystems at the scale or speed required in our climate and biodiversity emergency. Planting will therefore be required over large areas to provide seed sources where none currently exist.
So, when it comes to restoring deforested landscapes, neither planting nor natural recolonisation alone is sufficient. Where natural recolonisation is feasible, it should certainly be prioritised. There are too many instances where, because of funding eligibility requirements or the desire for predictability or profit, organisations are too quick to send out volunteers with spades and plastic tubes to areas where trees and shrubs could naturally disperse if sheep and deer were kept off the land. This is an anthropocentric rather than an ecocentric approach, little more than gardening.
Making forests more sustainable
Often, planting is undertaken in areas still under grazing by sheep, which is rather like bailing water out of a boat before you’ve fixed the hole in the hull. In many cases, removing the sheep would remove the need for planting and the resulting forests would be more resilient, biodiverse and able to regenerate over time.
But those who look at all tree planting as superfluous, a step towards substandard human management of natural processes, arguably underestimate the extent of ecological damage already done, and the urgency of rectifying it. Often, there’s no alternative.
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