Trees are on the move. The composition of the world's savanna landscapes is changing and carbon dioxide, more famously responsible for tweaking the Earth's climate, is playing a leading role. This is the message from a group of distinguished ecologists who argue that carbon emissions, by providing extra fuel for photosynthesis, give trees an advantage over grasses in savanna ecosystems. Bush encroachment is what results; among other effects it could place large portions of Africa's unique biodiversity at risk.
Bush encroachment has been a known problem since the early 20th century when the link between overgrazing and the spread of unpalatable woody vegetation was first made. Back then, as now, it was considered highly undesirable: bush encroachment makes farming more costly by necessitating expensive clearing operations. It also threatens thousands of grassland plant and animal species, which don't enjoy living among trees. What is new is the realisation that carbon emissions aid the process. An interacting set of ecological mechanisms is responsible. Climate change mitigation strategies – which encourage planting trees to offset emissions – could exacerbate the problem.
"By now most people are getting the message about climate change," said William Bond, professor of botany at the University of Cape Town. "But carbon dioxide is likely to have other, more direct effects on the environment. This fertilisation business is carbon dioxide's hidden hand."
In savannas, two rival vegetation types, grasses and forests, have been engaged in a battle over resources for millions of years. At the heart of the dispute is a difference in evolutionary strategy: grasses grow faster than trees and don't waste energy building branches, but trees capture sunlight more effectively and shade out grasses. Currently, neither competitor is completely able to dominate the other. The African plain – rolling open fields interspersed with trees – is the result.
Changing environmental conditions can shift the balance either way: more frequent fires, for example, favour grasses, which recover more quickly from disturbance. Fewer fires, on the other hand, allow trees to gain the upper hand. This is because trees that grow large enough – approximately three metres tall – can withstand the moderate flames of a grass burn. This can kick-start a positive feedback loop: trees suppress fire, begetting more trees.
Usually the savanna burns too frequently for this to happen. Razed to the ground repeatedly, saplings can remain trapped below flame-retardant size for decades. Ecologists term these individuals "Gullivers", held in check by armies of flammable "Lilliputian" grass. Each time they burn they sprout and try again.
Food from the sky
This is where carbon dioxide comes in. During photosynthesis, plants convert molecules of this gas into sugar, which they use to fuel growth. In theory, more carbon dioxide in the atmosphere – current levels are 40% greater than in pre-industrial times – should enable plants to grow more quickly and thus help Gullivers to escape the savanna fire trap.
"In most cases, feeding plants extra carbon does result in an initial boost in productivity," said Guy Midgley, chief director of the South African National Biodiversity Institute's climate change division. "But other factors, like nutrient limitation, often attenuate this effect."
Midgley grew fynbos species under artificially high carbon dioxide conditions to try to predict their physiological responses to climate change. He found that above a certain threshold the plants couldn't cope. "It was like they got constipation," he said. "They weren't able to process the extra sugar they produced and slowed their photosynthetic rates down to avoid getting clogged up."
The depletion of phosphorous, a nutrient required to utilise additional sugar, was responsible. Phosphorous occurs at naturally low levels in fynbos ecosystems, which means carbon fertilisation is unlikely to occur there on a meaningful scale. But in savannas something different happens.
"After a fire, nutrients are relatively abundant," said Midgley. Nitrogen and phosphorous from the tissues of living vegetation return to the ground as ash and there are fewer established plants remaining to compete over them. This means sprouting trees can respond to carbon fertilisation for longer than fynbos and stand a better chance of growing big enough to survive the next blaze.
Carbon curve balls
If carbon dioxide is causing trees to spread – and evidence is mounting that it does – then halting their progress will require new approaches. Of course, curbing emissions is the ultimate fix; in the meantime, we need creative approaches to deal with the excess wood.
The Cheetah Conservation Fund, a Namibian non-profit organisation, has a creative approach: it uses massive combine harvesters to clear congested land. The resulting debris gets chipped and compressed into briquettes and marketed as a smoke-free, ecofriendly alternative to braai wood. Midgley winced when we discussed this: the method is a violent one and leaves a visibly scarred landscape behind. But the bush is gone.
Cheetahs, which can't hunt in thickets, are not the only species threatened by bush encroachment. "Hundreds of different mammals and birds are specifically adapted to living in savannas," Midgley said. "We're talking the iconic African fauna here, which people from around the world flock to visit. It could be on the way out."
In other words, it might be time to make some tough decisions.
Bush encroachment has an economic twist that could make tackling it difficult. Woody vegetation can earn carbon credits, possibly giving landowners (and governments) an economic incentive to let trees invade. Carbon trading schemes were developed as a way for developed countries to offset some of their emissions by funding sequestration projects elsewhere. They have been criticised for inadvertently facilitating environmental degradation.
"Expanding forests in South Africa may take a little pressure off our government to cut emissions, because they technically count as captured carbon under the rules of the United Nations Framework Convention on Climate Change," said Midgley.
But these rules fail to take the full picture into account: according to Bond, they are biased in favour of trees because they ignore carbon stored below ground. They also ignore the heat-reflecting properties of grasses, which are cooler than trees in the same way that white road paint is cooler than tarmac. Scientists term this phenomenon "the albedo effect".
"Grasses hold most of their carbon in their roots," said Bond. "By increasing the Earth's albedo, they make a huge difference to its energy budget."
In other words, the convention on climate change's rules, which aim to combat climate change, could be at odds with reality. "The science is more complicated than the regulations let on," said Midgley, who has participated in the Intergovernmental Panel on Climate Change negotiations for almost 15 years. "But at some stage comes the inevitable compromise between what is accurate and what is practically achievable."
A new message
Despite the added complexity that bush encroachment brings to climate change, the key message remains simple: we need to reduce emissions, not just devise clever ways to trade them between ourselves. Current mitigation strategies require more nuance. Trees provide many benefits besides carbon storage in cities, where they intercept rainwater, soften harsh concrete surfaces and cast shade. In tropical regions devastated by deforestation, replanting them must be a priority. But in savanna ecosystems, our best approach may be pegging them back.
"Some environmentalists might not like it," said Midgley. "Saying that trees are sometimes bad for the environment clashes with conventional wisdom."