On paper, but perhaps only on paper, the prospects for restoring the environmental health and diversity of our planet can look quite bright. Consider the 2011 Bonn Challenge: based on commitments from earlier international environmental summits, it urged governments, UN institutions and conservation organisations to restore 150 million hectares of the world's deforested and degraded land by 2020. Major world players, including the US, China, and South Africa signed up.

The New York Declaration on Forests followed in 2014, increasing the target to restoring 350 million hectares by 2030. The 2015 UN Sustainable Development Goals are even more ambitious: oceans as well as land-based ecosystems are designated for restoration on a grand scale.

To put these abstract figures in some perspective, the New York target represents about half the current forest cover of north and central America, while the entire estate of our semi-State forestry company Coillte is less than half a million hectares. So the international aspirations for restoration are pretty ambitious.

On the other hand, the world was still losing five million hectares of forest a year in the first decade of this century, so a monumental turnaround is required to come close to meeting these commitments.

Let's imagine for the moment, difficult though it is in the current political atmosphere, that the governments and institutions involved have made these commitments in good faith, and are dedicating sufficient resources to honour them. A host of challenging questions still arises, many of which came up for animated discussion at a symposium I moderated entitled Ecological Restoration in a Changing Biosphere at Missouri Botanical Garden last month.

The talk explored questions such as what do we mean by “restoring” on so vast a scale, in so many very different parts of the globe? Do we really know how to succeed in such endeavours? And to what extent does the climate crisis make restoration an even trickier enterprise than it was in the past?

Diverse aspects

The symposium speakers were all specialists in diverse aspects of restoration and conservation, with experience largely in the Americas. They offered a range of evidence of historical and contemporary restoration successes, tempered by keen awareness that rapid global change will severely test their survival and replication.

They recognised that the kind of landscape-scale restoration that the Bonn and New York agreements demand will test current restoration theory and technology, and must engage local communities to have a chance of succeeding. They also accepted that there is no universal restoration template: every project is conditioned by site-specific ecological and social factors.

This was often ignored in classic ecological restoration prescriptions from the 1970s and 1980s, which were largely focused on temperate northern latitudes, especially in the US. Temperate ecosystem recovery from degradation is generally much slower, and much more ordered and predictable, than the very speedy and – to European eyes – chaotic recovery of tropical systems. In north America especially, early restoration advocates tended to target their work towards an idealised and “pristine” past, prior to European settlement.

Yet we now know that some Mayan cultures were very skilled at simultaneously exploiting and restoring the rainforests in which they farmed, long before white settlers arrived.

And at the symposium, Leighton Reid, who works as a restoration ecologist for Missouri Botanical Garden’s conservation and sustainable development centre, reminded us that one of the oldest and largest enduring restoration projects outside Europe is in Brazil’s Atlantic rainforest.

The Tijuca forest north of Rio de Janeiro had been chronically degraded by unsustainable agriculture by the mid-19th century. Its eroding slopes threatened the city’s water supply. Fifty years later, a combination of extensive tree-planting and rapid natural regeneration have produced a flourishing national park, home to a number of endangered species. In the process, Rio’s fresh water source was rescued.

This combination of biodiversity gains and the provision of natural capital and ecosystem services to local economies characterises many of the best restorations. But the balance between the two will always be up for debate.

Tropical forests

Robin Chazdon, who has written extensively about the restoration of tropical forests, took the view that the kind of landscape-scale restoration envisaged in the international declarations will not focus on the recovery of the biological communities found on remaining "old-growth" sites. It will rather aim at recovering ecosystem services such as timber, water, and carbon sequestration from naturally regenerating second-growth forests, which she said are more biodiverse than traditional conservationists suppose.

However, one member of the audience pointed out that, in many other ecosystems, natural regeneration without active management would often produce monocultures of alien invasive plants. Once again, it was clear that one size does not fit all in restoration.

James Aronson, whose life-long study of restoration theory and practice has been exceptionally wide-ranging globally, laid great stress on the importance of using a site's ecological history as a reference to set restoration goals. The broad community of species that evolved together over many millennia to the recent past is likely to be the most appropriate target for the future.

But he also recognised that this community will itself have changed many times during its evolution; the restoration process is not so much about “reconstructing” past ecosystems as about recovering their “historical continuity”, enabling them to recover and flourish in as diverse and dynamic a manner as possible.

Unprecedented rate

Just how difficult that task will be in some regions, with climate changing at a rate unprecedented in recent human history, was demonstrated in the keynote address by Don Falk of the University of Arizona, in a survey of the forests of the southwestern US.

The mutually amplifying effects of increasing drought, wildfire and the spread of new pests led him to question whether these forests can persist into the near future with anything resembling their current species make-up.

Falk’s tone was gentle, but his content was compelling and disturbing. The idea that a vast and regional forest complex might vanish within a generation suggests that human-generated environmental change is leading not just towards the rapid extinction of species, but of entire ecosystems.

Unlike some fashionable science writers on this theme, Falk did not counsel abandonment of biodiversity conservation and the embrace of so-called novel ecosystems. But he did make it very clear that the challenge to the science and practice of restoration will in many places be very exacting indeed, unless we rapidly adjust our way of living on the earth.

Paddy Woodworth is a research associate at Missouri Botanical Garden, and author of Our Once and Future Planet: Restoring the World in the Climate Change Strategy (Chicago, 2015)