Does rain forest grow back? Archaeology might have the answer

Most international archaeological work in South America has concentrated on the Andes for various reasons. It’s more accessible, the ruins are more visible, there’s a better ethnohistorical record from the conquistadors, there’s variety over short distances because change in height makes vertical economies possible where different foods grow at different heights and they’re just the reasons that come off the top of my head.

It means the Amazon has been overlooked.

Slash and Burn in Brazil
Slash and Burn Agriculture in Brazil. Photo Pat Heslop-Harrison

Now discoveries in the Amazon reveal human occupation, with large areas dominated by geometrical earthworks. One of interesting things with this find what it means about rain forest regeneration. It’s thought that primary rainforest is irreplaceable. What you get back after cutting down a large area of rainforest is secondary rainforest. On the ground secondary rainforest is much more like the popular image of a jungle where you hack your way through the undergrowth. In primary forest the canopy keeps a lot of light reaching the ground. Though it looks empty primary forest creates a rich habitat in the canopy making it among the most biodiverse habitats in the world. Secondary forest, without the canopy doesn’t do that.

This is why regrowth of cut-back forest isn’t as good as not cutting it down in the first place. There’s also been a puzzle of how long it would take the scars of secondary forest to heal and primary forest to return. Study of the area, it’s size and its abandonment date could help. On the downside because someone’s already cut down the trees you don’t know if this area would have stood out as a biodiversity coldspot in the forest.

Sadly I doubt you could neatly extrapolate the data to say when the current cuts will heal. I’m willing to bet however big these clearances were, they weren’t as big as the modern clearances. Despite this, an archaeological/ecological investigation could still provide useful data on the relationship between area cleared and the time taken to grow back forest.

Forest at the edge of clearance
Forest at the edge of clearance, Photo Pat Heslop-Harrison

This kind of environmental approach to human settlement can be seen elsewhere. There’s a Botanical Briefing: Fire, Forest Regeneration and Links with Early Human Habitation: Evidence from New Zealand by Ogden, Basher and McGlone that you can pick up as a free PDF from Annals of Botany. The background here is that prior to humans New Zealand was covered in temperate rain-forest. Crucially this hadn’t evolved fire adaptations. The periods between major fires in any place were more often thousands, not hundreds, of years. Polynesians discovered New Zealand late, around AD 1200. That seems strange, because New Zealand has the largest Polynesian islands, but it’s also a long way south. Polynesians preferred to travel east-west. When they did arrive they brought agriculture with them and a great way to clear areas for crops is with fire. There is an increase in the number of fires post-colonisation, but sifting the natural fires from the man-made fires at any given site is difficult. If you find regenerating forest in New Zealand, it doesn’t automatically follow that it’s evidence of prehistoric human settlement.

However, if you can find human artefacts at a regenerating site, you can date the initial damage. A number of sites with different dates or areas gives you a series of snapshots instead of having to run a series of thousand year experiments. I’m not disparaging the work of The Long Now Foundation, but sometimes it’s nice to have an answer quickly.

Amazonian hat-tip @alexbellos on Twitter and +Tom Elliot on Google+

  • An unasked question is why you want climax forest to regrow? Valuable timber species tend to be slow-growing and shade-intolerant. They regenerate best in clearing, like those caused by hurricanes and lightning, for which one might presume they are adapted. The mahogany rich forests of the Yucatan may well be so because they were felled by the Maya, and abandoned 1000 years ago.

    Does climax primary forest supply any ecosystem services in significantly greater measure than climax secondary forest?

    The problem is the delicate balance between population pressure and available land. That is what determines the length of fallows. With a long enough fallow, what is the ecological problem of clearing some forest?

  • Dorian Fuller’s archaeobotanical blog at has some valuable and relevants highlights of 2011

    And the comment Dorian makes at is pertinent too: “Rice presents a fascinating opportunity to study parallel evolution in crop diversity, crop ecology and cultural traditions with the comparison of African O. glaberrima and and Asian O. sativa. Sadly, at present the archaeobotany of most West African countries is a complete blank, and archaeobotany in Africa needs more researchers, but some exciting finds are coming out (see my blog for some recent examples: Asian rice has also attracted more historical linguistic research and discussion. Unravelling the cultural histories of African rice remains a good challenge to get to work on!”

    … and of course the brilliant and seminal paper “Contrasting Patterns in Crop Domestication and Domestication Rates: Recent Archaeobotanical Insights from the Old World”