Tree plantations and carbon capture

在过去的十年中,已经成为一种植树option, backed by governments, to capture carbon dioxide in the fight against climate change. But a new study shows that this can leave the planet worse off than it was before. Researchers have analysed the effects of a long-running campaign in Chile to subsidise tree planting. And as Phil Sansom heard from Robert Heilmayr, the resulting ‘plantations’ weren’t good news...

Robert - In many cases, those new plantations were encroaching upon native forests and shrublands. And so that conversion of native forest to plantations really led to a reduction in biodiversity across the landscape and an undermining of the carbon benefits that we would hope we would get.

Phil - Really? So in this case, planting trees ended up bad for the environment.

Robert - Yeah. What it ended up highlighting is that it really depends how you go about planting those trees, right? If you're replacing some of the existing standing forest that's been growing for centuries and replacing them with kind of young plantation forest that you plan to clear within 20 years, obviously you're not going to get as much carbon in that above ground biomass that you would, if you just left those forests standing.

Phil - Can you explain that? I'm not quite sure I understand the whole biomass concept.

Robert - Yeah. So, you know, all vegetation is made out of carbon. And so as plants go through photosynthesis, they're storing carbon in their structure. One of the ways that people think a potential contribution to addressing climate change would be to increase the amount of vegetation out there. We're kind of increasing the storage of carbon.

Phil - Basically more tree, more tree material, equals good.

Robert - Yes. That's usually the case. Yeah. And so what ends up happening with this kind of plantation setting is that you have less tree material, less plant in a forest than you would within kind of a natural dense native forest.

Phil - That's kind of perverse though. Why were people chopping down beautiful old forest just to plant a new single type of tree?

Robert - I mean, the initial intent of the policy was largely to build kind of a domestic timber sector. This was implemented under the Pinochet regime in the 1970s. And they were really looking for ways to kind of drive economic growth. But at the same time there were environmental concerns in this kind of central and Southern parts of Chile. There were large areas that had been degraded through, um, really aggressive agricultural practices. And so the government felt that if they encouraged people to reforest those landscapes, they would reduce the rates of erosion and help kind of reclaim some of these lands that were really marginal and not being put to use. The problem with it was that it, although they initially had a policy in place that said, "you can't convert native forest to plantations and get the subsidy", they were really poorly enforced. And so you had a lot of cases where you were actually getting the kind of conversion of a native forest to agriculture for a year, maybe a pasture, and then people would apply for these subsidies and convert it to plantation forest.

Phil - And then if the point was for timber, were people just getting paid to plant trees and then chop them down again?

Robert - Exactly. So they do provide some soil erosion protection. They do provide some carbon benefits, but the main goal of those plantations is really as a feedstock for large pulp mills and timber mills that exist in Chile.

Phil - Can you then compare this to any of the big tree planting campaigns that are going on today?

Robert - We don't necessarily think that planting trees is a bad idea. It can be, and I think it will be an important part of the solution to climate change. However, our study really cautions that you want to design the policies in a way that doesn't lose a native forest at the same time. We want to be reforesting landscapes with kind of the native species, not an unending kind of row by row monoculture of one species of trees.

Phil - It reminds me a little bit of the whole biofuels thing, where people thought "here's an environmentally friendly fuel. It just grows out of the ground", but in reality, it takes precious land to grow that biofuel. And then you're just burning it anyway.

Robert - Yeah. I think that's true. And even, you know, in the timber sector, a growing trend right now is to use wood pellets for home heating. And I think that's especially common in Europe. And what you're finding is that in some cases, those wood pellets are associated with clearing of natural forests at a pretty large scale. I think policy makers today are making some of the same mistakes that we saw happen 40 years ago. When you look at the commitments that people are making in things like the Bonn Challenge, which is one of these kind of global reforestation campaigns, almost half, I think, of the commitments under that are to replant with plantations.