Tree cover for a warming world
(appeared in May 2015)

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Trees need to adapt if they should be there to mitigate climate change, says S.Ananthanarayanan.

The earth’s tree cover is its largest asset, other than the flora of the sea, for fixing atmospheric CO2, and hence holding down the pace of global warming. But tree cover is itself at peril in a warming world, and temperature rise may set off a spiral of less trees and more warming. This observation is another challenge to the primary need of saving the earth’s tree cover from competing demands of land for bio-fuel or food crops.

The Potsdam Institute for Climate Impact Research, PIK, has published a study of what it would take to hold global warming down to 1.5°C, in place of 2°C, which is what the world is talking about. There is a widespread belief that 2°C may be too warm for effective adaptation, apart from the view that speaking of temperature rise in numbers of °C may take the focus off the action needed to limit the quantity of greenhouse gases being added to the atmosphere. PIK notes that 1.5°C is technologically feasible and what we need to do is similar to what we need for 2°C, but with no time to lose and some of the action to be taken sooner.

“In 1.5°C scenarios, the remaining carbon budget for the 21st century is reduced to almost half compared to 2°C scenarios,” says PIK researcher Gunnar Luderer, who co-led the study. A key action to help this happen is better energy efficiency, but the study says at some point in the century, there would have to be negative emissions, which is to say we would need to remove more CO2 from the atmosphere than we are adding. One way of doing this would be large scale use of bio-energy, combined with carbon capture and storage. Some of this technology is untested, and increased use of bio-fuels would encroach on land used for growing the greater demand for food. The imperative then, is to grow more forests and sequester carbon in the leaves and branches of trees. While here again, land use would have to be balanced against food needs, the emphasis is that containing climate change depends sensitively on the role of forests in capturing CO2.

Can the forest do it?

This is where the paper of Nathan G. McDowell and Craig D. Allen of the National Laboratory and the US Geological Survey, at Los Almos, New Mexico, in the journal, Nature Climate Change raises an alarm and also shows a way forward. The paper starts by noting that the world over, the rates of tree mortality have been on the increase in recent decades. Scientific literature is packed with studies that document rapid and severe reduction of tree species – and the reason is seen consistently as regional warming, which, with periods of low rainfall, leads to increase in ‘water deficit’. “Given forecasts of continued rising temperatures and more extreme droughts globally, there are increasing risks of massive disruption of today's forests during this century”, says the paper.

The paper then analyses the factors that help plants and trees grow and arrives at conclusions of what trees may survive and what trees may not. And the paper goes on to outline a plan of replacing the trees at risk with species that can survive, to help forests to adapt and continue to sequester carbon, before climate change wipes them out.

The analytical tool the Los Almos researchers used is based on a formula devised by Henry Darcy, a French engineer who made important contributions to the way liquids diffused through granular media, like beds of sand. The original formula says the rate of flow depends directly on how easily the medium allows it, the cross section area and the pressure difference, but falls if the liquid is viscous, the medium resisting and according to how long the passage through the medium is. This formula has been adapted for use with the flow of sap through the barks of trees, and takes into account the surface area of leaves, and also the capacity of the air around the tree to take in more water vapour, as opposing upward flow into the tree.

But the formula, known as Darcy’s Law, also shows which characteristics of plants and trees can mitigate the effects of rising temperature and increase survival. The first is height – the shorter the tree, the less the resistive path the sap has to traverse. Next is the behavior of the pores, the stomatae, of leaves when the air becomes dry. Plants that close the pores, to conserve water, would draw less from the ground, while plants that let the pores stay open, would keep drawing water, and continue photosynthesis, and do better in drought conditions. And finally, plants with wider stems and which allow easier flow would do better too.

Darcy’s formula also suggests which plant varieties would be the first to die, under rising temperatures. A first reaction of a forest system is to reduce the overall leaf area, by mortality of tall and older trees, to be replaced by shrubs, grasses, flowering plants and shorter and more vigorous trees. Following the method of forest systems, a first strategy to minimize mortality would then be selective thinning, or reduction of the number of trees, or the stand density, of forests. The next strategy would to replace varieties most at risk with shorter trees and which are adapted to keep leaf pores open even in drought conditions.

And along with strategies that arise from Darcy’s law, would be real world course correction, based on species and location specific peculiarities, or the behavior of competing species or even pests, which may also decline with rising temperatures. But overall, the law provides a framework for managing adaptation and the new global vegetation patterns that must emerge, under the plant water stress that rising temperature would bring.

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