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What drives future changes in the world’s forest carbon?

Climate change likely leads to a poleward migration of forests (photo: Province of British Columbia, www.flickr.com)
Climate change likely leads to a poleward migration of forests (photo: Province of British Columbia, www.flickr.com)

Climate mitigation can have both benefits (reduced wildfires) and costs (reduced forest carbon) for forests globally, according to a study by US researchers, who assessed climate change impacts and benefits of mitigation on forests under a range of scenarios.

The future of the world’s forests will be shaped by multiple driving forces that will have complex interactions among them, including climate change, economics and development, mitigation policies, natural resource management, land use and land use change, logging, wildfire, and insect and pathogen outbreaks.

Climate change will impact the major forest regions of the world in several ways. Forest productivity will change due to higher temperatures and changes in rainfall; in some parts there will be a loss of productivity, and in others an increase.

Forests will benefit from the higher carbon dioxide concentration in the atmosphere that has a fertilisation effect, stimulating photosythesis. Climate change will drive migration of forests: they will expand in some regions, and contract in others. Wildfires will increase in most of these major forest regions. Competition will change between different types of vegetation. As a result, carbon dynamics will change.

More wildfires versus higher productivity

Future potential changes were assessed with models that simulate potential future forest growth and decline under different scenarios of climate change and carbon dioxide emissions. The assessment focused on the combined effects of wildfire, climate-induced vegetation migration and productivity in relation to climate change scenarios on a global scale.

Main outcomes include how, at the global scale mitigating climate change can be beneficial in terms of reducing the impact of wildfires, and costly since it reduces primary production and thus forest carbon. Another outcome is the interplay between direct climate change impacts (changes in temperature and precipitation) and the fertilisation effect of carbon dioxide on the world’s forests. This interplay is complex.

Two scenarios of climate change were studied: a high-end scenario, and a scenario where climate change is mitigated such that a two degrees celsius global mean warming from pre-industrial by 2100 is not exceeded. Modelled changes were studied from 1980-2009 to 2070-2099.

Poleward migration of forests

Under both scenarios of climate change a poleward migration of forests was simulated: in the leading-edge of the migration, grassland and woodlands convert to forests while at the trailing edge, forests convert to shrubland, grassland, or woodland due to lower productivity or frequent fires.

Large expanses of boreal forests in Canada and Russia shifted northward, especially under the high-end scenario of climate change. In the southern hemisphere, forest expanded southward in southern Africa. Poleward migration of forests was not distinct in western South America, where simulations show forests to contract along elevation gradients. In Australia, the tropical forests in the north contracted northward as they lost productivity and became woodlands; simultaneously, increased growth of trees was simulated in the woodlands in western Australia, converting those areas to forests.

Dramatic increase total live forest carbon stock

In these simulations, total live forest carbon stock increased dramatically and consistently under both climate change scenarios, gaining 59 per cent and 54 per cent under the high- and low-end scenario, respectively.

The vast majority of the total live forest carbon gain was simulated to occur in the southern hemisphere: western South America, South America, and South Asia. For Europe only small increases were projected, while Russia was projected to see a significant decline, mainly due to forest contraction and more wildfires.

Both positive and negative effects are generally higher for the high-end scenario compared with the low-end scenario of climate change: a higher increase of both carbon stocks and productivity, and of the impact of wildfires.

Limitations

This study focused on evaluating the role of wildfire as a major disturbance regime. There is an array of disturbance regimes, however, including land cover change, logging, and insect and pathogen outbreaks. These disturbances were not included in the simulations. Also, current developed and agricultural areas, land use change, and forest management practices were left out. Besides, the study’s results are based on a single climate model; the effects of mitigation policies on the forest carbon stock may be sensitive to climate model selection.


This article first appeared on ClimateChangePost, and  is a digest of a study by Kim et al., 2017 in Environmental Research Letters 12.

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