How long does tropical forest take to recover from agricultural clearance?

Intermediate secondary forest in Paragominas, Para, Brazil – Photo credit to the fantastic Ricardo Solar, you can see more of his pics here

Today our work on the recovery of secondary tropical forests got published in Royal Society Proceedings B. I’m really chuffed with this piece of work and in this blog I’m going to summarise what we found out and why I think it’s important. If you want to read the paper you can get it here.

Large areas of tropical forest have been cleared for agriculture over the last 100 years.

Why does this matter? Well it matters because these forests are vital for the unique biodiversity in the tropics but also because humans can benefit from them remaining intact.

Their loss causes extinction, release of carbon into the atmosphere – worsening climate change, and changes the ecosystem services we get from these forests.

Because of the importance of these forests their restoration is seen as a priority by some. There are valiant attempts to restore tropical forests in Brazil and various Central American countries. In addition there are also international initiatives that aim to encourage the restoration of carbon and biodiversity (E.g. CBD & REDD+). These are great and ambitious aims but, until now, we didn’t really know how long these recoveries took, or whether recovery was different for different disturbance and forest types.

To solve all this we collected the biggest dataset yet compiled on recovery of aboveground, belowground and soil carbon as well as plant species richness and community composition following agricultural clearance. All this data came from previous studies.


We found that after about 80 years aboveground carbon storage was around 85% of that found in undisturbed forests, while belowground carbon storage seemed to recover more slowly. Soil carbon showed no relationship with time since clearance.


In terms of biodiversity both tree and epiphyte species richness seemed to increase over time, with tree richness recovering after around 50 years since disturbance but epiphytes took around 100 years.


However, when we looked at species that are found in the undisturbed forests, relatively few of them are found in the recovering forests. They didn’t seem to accumulate over time either. Given that these species are likely to be more prone to extinction it is worrying that they don’t seem to be doing very well in secondary forests.

We think that carbon recovers relatively well following abandonment of land since there tends to be a rapid influx of woody species. However, we also think that complete recovery of carbon is likely to take more than a century since this is likely to be dependent upon large, slow-growing trees.

Differences between tree species richness recovery and that of epiphytes is likely to be because tree seeds are more easily transported between forests than those of epiphytes. Also epiphytes seem to be found more on big trees, and there don’t tend to be many of these in secondary forests.

The lack of recovery of species found in undisturbed forests is perhaps the most disturbing thing that we found. We think that to improve this situation there may be a need for management of these forests by planting trees and helping to increase dispersal of seeds throughout the non-forest areas.

Disturbed forests like this are not worthless.
Regrowing forests like this are vital if we wish to conserve biodiversity in our human dominated world. Photo credit again to Ricardo Solar

There’s been lots of great work recently on the value of disturbed forests. We hope our work goes into a bit more detail where the soon-to-be-classic work of Luke Gibson etl al  left off which showed that primary forest has greater conservation value than any types of disturbed forest in the tropics. We agree with this, particularly for specialist species. However, most tropical forests are not primary forests and have been logged, cut down or burnt at some point in recent history. Because of this we think that older secondary forests need to be recognised as important for conservation and carbon storage and their clearance should be avoided. These forests are not worthless.

The role of ecologists in influencing policy

Policy not based on evidence is dangerous.

Or at this is what Ben Goldacre would have you believe. Goldacre, a doctor, writer and journalist claims that much policy making is ad-hoc and that we should be carrying out more randomised control trials of policy relevant questions.

I agree with him.

However, there is much to be positive about as well.

The  UK government recognises the importance of science in forming policy. It acknowledges that evidence-based environmental policy is important and there are various independent bodies which help politicians access this evidence. There have also been positive moves  in following expert advice with the recent approval of Nature Improvement Areas, as suggested in the Making Space for Nature report. In addition at the recent British Ecological Society Annual Conference I heard about the government funding for work into Ash dieback in an attempt to combat the disease.

So obviously the government does value science in policy making. We are actually reasonably lucky in this country compared to some less enlightened places.

Now the bad.

The real problem for a lot of environmental issues is not that there is not enough evidence, but rather that they are fringe issues. While reducing crime will always be a priority, many issues related to conservation and environmental policy making are lower down the pecking order.

These kind of things need work to make them an issue even before we try to work out what to do about them. Even policies on climate change, which is widely seen as a disaster waiting to happen, have shown little meaningful progress over the last 20 years.

This is simply because any changes would require a change in more or less everyone’s lifestyle. The public don’t want this and governments around the world want to stay in power, so they don’t do anything.

Even when issues don’t conflict with people’s day to day lives getting good environmental policy can take a long time. Ecologists often want action quickly on their pet topic but this rarely happens.

Just how long things can take was emphasised when I saw Hugh Possingham speak in Glasgow this year. He has been working on designing  protected areas so they are as representative of the marine environment as possible for 15 years. In the last year or two the Australian government has started taking this seriously and is now planning the largest MPA network in the world (though Possingham doesn’t think it’s perfect). Although he wasn’t the only one involved in bringing this to fruition it has been a long road for Possingham and the MPA network. This is just one example of how long things can take to develop into policies.

So how can we put environmental concerns higher up the political pecking order? Firstly, we need to talk to people more about why we care about the environment.

We need to work in schools and do more outreach projects. The public need to care before most politicians will do much about things.

Secondly, we should be willing to communicate policy relevant work to politicians directly. We should be happy to suggest policies to politicians – though it is down to them as to what they do with this advice.

There have been many column inches filled about the lack of scientific education amongst politicians. This is true but there is also a lack of political education amongst scientists. We should be looking to create the biggest impact possible with our work. We won’t do this by sitting in rooms talking to each other.

Testing environmental policies with randomised control trials

How do we know that what we do to manage biodiversity is doing what we want?

The sad truth is that a lot of the time we don’t. A lot conservation management is done in particular ways because of a mate once told the ranger a certain method was good or because they wish to maintain the status quo, but very little is actually based on evidence.

The best way we have of understanding the effects of management on biodiversity is through properly controlled studies. Despite the increasing complexity in ecology, which is increasingly resembling a branch of applied mathematics, many conservation questions are about whether treatments a or b are better than doing nothing at all.

This is one of the few ways we can attribute causation in ecology and is held as the gold standard for other disciplines. It bears repeating that correlation does not equal causation and most ecology is observational in nature, rather than manipulative.

Although there is a push by some  to answer these questions and compile evidence for particular management techniques (notably the excellent, much remains unevaluated. There is a feeling by some publishers that such experiments are boring and this stops researchers pursuing this type of research as well as reducing replication even when they are published.

Though we sometimes have little idea about how to manage biodiversity at the site scale, environment policy can be equally difficulat to implement  Randomised controlled trials (RCTs) have the potential to be powerful tools to aid this decision making. Many policy questions relate to how people interact with their environment. For example, is it best to pay up-front for agri-environment schemes or to pay based on the results they deliver?

Would it be better to pay farmers up front for planting wildflower margins or should we pay them based on delivery?

Questions like this could feasibly be addressed by a large RCT. However, these would need government backing and funding. Some people might see the use of RCTs, when government is cutting such large parts of it’s budget, as an unnecessary indulgence. However, without these trials we are fumbling around in the dark trying to find a sticking plaster when we might be in need of a visit to the hospital. It’s a clumsy analogy but without RCTs of such policy options we do not know whether what we are doing is the best thing to do, or even if it is working at all.

This type of thing is being championed by Ben Goldacre and Tim Harford, who argue we should have RCTs for almost everything. Goldacre has also written a report for Cabinet Office in the UK, so it looks like this idea is gaining a bit of traction. RCTs have been identified as having potential for social policies such as the use of different teaching techniques in aiding reading. However, there is no reason why similar trials couldn’t be designed to test environmental policy.

This big idea would put an end to uninformed policy making on the environment. It would mean that we wouldn’t have to guess at which policy delivers the best results. Politicians would still get it wrong some of the time but at least they’d be presented with good evidence to help them make these decisions.

Politics would still however come first. Making decisions is a messy process and science doesn’t have the last say on such issues. As seen with the recent badger cull debate even the best evidence can have little value when faced with the might of industry lobbyists. But without this evidence, policy making will continue to fumble around in the dark.