Forests and Soils Are Essential to Mitigate Climate Change
A recent focus collection of papers in Environmental Research Letters summarises the importance of forests, and forest soils, in meeting climate change mitigation goals. The authors highlight that the global emphasis has been placed on reducing fossil fuel emissions, while there has been a lack of emphasis on the removal and storage of existing atmospheric carbon. It should not be a question of one or the other, but rather of both reducing emissions and increasing sequestration simultaneously. The long term storage of carbon in plant biomass and soils presents itself as an obvious choice for this sequestration.
Three of the most important aspects of forests and their soils are:
“(i) Carbon accounting of forest sinks and reservoirs, process emissions and carbon storage in forest products,
(ii) the carbon dioxide dynamics of using forest bioenergy, and
(iii) the carbon cycle of tropical forests.”
Carbon Accounting
Although wood is a renewable resource and a popular building material, the benefits of its use are contentious. One must consider the carbon displacement, the lifespan of the buildings, as well as how a forest’s carbon budget has been altered by the harvesting of said materials. In fact, the authors state that the “substitution of wood for more fossil carbon intensive building materials […] results in major climate mitigation benefits”. Harvest on public lands should be decreased, and rotation times on private forests increased to reduce emissions from the forestry sector. This is because increasing forest productivity and harvest rates will always lead to an increase in net emissions when compared to business-as-usual. Accordingly, forest recovery and afforestation can have very positive effects and even be more effective than using carbon accounting alone because of increased evaporative cooling witnessed in forested areas.
Forest Bioenergy
Bioenergy is electricity and gas generated from organic materials. However, burning wood for energy contributes significantly to atmospheric carbon dioxide emissions. In fact, “the combustion of biomass generates gross GHG emissions roughly comparable to the combustion of fossil fuels.” The commonly used argument by forest bioenergy proponents is that the carbon dioxide released will be re-sequestered by the growth of a replacement forest. While this may seem true on the surface, the issue is much more complex. The atmospheric CO2 concentration level is actually higher during the growth period of a new forest than it would have been if the original forest had been left untouched. This then contributes to planetary warming which in turn has irreversible effects (i.e. the melting of glaciers and ice sheets, and thawing of permafrost) that cannot be fixed by simply replacing the harvested trees. Allowing forests to grow beyond the first rotation, results in the forest carbon reservoir storing significantly more carbon as carbon sequestration rate increases with size. Additionally, forest soil’s capacity to sequester carbon increases with the age of the forest. Moreover, this does not take into account the nitrogen-based fertiliser or the other inputs used on forest plantations. Despite this, in the EU, the Renewable Energy Directive states that forest bioenergy is zero carbon by definition — regardless of scientific analysis determining this is not necessarily the case. The US government has also made a similar statement with regard to the ‘carbon neutrality’ of forest bioenergy.
Tropical Forests
By now, most, if not all, of us know how important tropical forests are for biodiversity, ecosystem services, and climate change mitigation. Deforestation in the Amazon leads to increased air and land surface temperatures as far as 50km (!!!!) from the disturbed site. The change in temperature can then lead to changes in air circulation and precipitation. Large portions of tropical forests in southeast Asia have been converted to plantations for economic purposes. This conversion can be very detrimental to the environment, especially when the plantations are placed on peatlands as drying peat soils lead to significant carbon emissions. Should things continue on their current trajectory it is estimated that approximately one-seventh of “the total of tropical forests [existing] in the year 2000 (some 289 million hectares, the size of India) would be deforested between 2016 and 2050, with annual deforestation emissions rising by 42% and cumulatively releasing some 169 Gt CO2.” To avoid this outcome becoming reality, carbon pricing policies can be put into effect.
In light of this focus collection of papers, it is clear that forest carbon must be accurately accounted for by governments when managing atmospheric carbon dioxide levels and climate change. Natural forests are able to achieve their ecological potential for carbon storage, and as such are much better at sequestering carbon than managed forests. Forest soils are also important carbon sinks and reservoirs. The responsible management of tropical, boreal, and temperate forests is essential, not only for reaching climate change mitigation goals but also for providing ecosystem services and overall resiliency. For this reason, accurate and transparent carbon accounting for forest products and forest biomass used for bioenergy must be publicly available.
