Editing IPCC:AR6/SRCCL/Chapter-3 (section)

=== 3.3.3 Changes in vegetation and greenhouse gas fluxes ===

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Terrestrial ecosystems have the ability to alter atmospheric GHGs through a number of processes (Schlesinger et al. 1990 <sup>[[#fn:r519|519]]</sup> ). This may be through a change in plant and soil carbon stocks, either sequestering atmospheric CO <sub>2</sub> during growth or releasing carbon during combustion and respiration, or through processes such as enteric fermentation of domestic and wild ruminants that leads to the release of methane and nitrous oxide (Sivakumar 2007 <sup>[[#fn:r520|520]]</sup> ). It is estimated that 241–470 GtC is stored in dryland soils (top 1 m) (Lal 2004 <sup>[[#fn:r521|521]]</sup> ; Plaza et al. 2018 <sup>[[#fn:r522|522]]</sup> ). When evaluating the effect of desertification, the net balance of all the processes and associated GHG fluxes needs to be considered.

Desertification usually leads to a loss in productivity and a decline in above – and below-ground carbon stocks (Abril et al. 2005 <sup>[[#fn:r523|523]]</sup> ; Asner et al. 2003 <sup>[[#fn:r524|524]]</sup> ). Drivers such as overgrazing lead to a decrease in both plant and SOC pools (Abdalla et al. 2018 <sup>[[#fn:r525|525]]</sup> ). While dryland ecosystems are often characterised by open vegetation, not all drylands have low biomass and carbon stocks in an intact state (Lechmere-Oertel et al. 2005 <sup>[[#fn:r526|526]]</sup> ; Maestre et al. 2012 <sup>[[#fn:r527|527]]</sup> ). Vegetation types such as the subtropical thicket of South Africa have over 70 tC ha– <sup>1</sup> in an intact state, greater than 60% of which is released into the atmosphere during degradation through overgrazing (Lechmere-Oertel et al. 2005 <sup>[[#fn:r528|528]]</sup> ; Powell 2009 <sup>[[#fn:r529|529]]</sup> ). In comparison, semi-arid grasslands and savannahs with similar rainfall, may have only 5–35 tC ha– <sup>1</sup> (Scholes and Walker 1993 <sup>[[#fn:r530|530]]</sup> ; Woomer et al. 2004 <sup>[[#fn:r531|531]]</sup> ).

At the same time, it is expected that a decline in plant productivity may lead to a decrease in fuel loads and a reduction in CO <sub>2</sub> , nitrous oxide and methane emissions from fire. In a similar manner, decreasing productivity may lead to a reduction in numbers of ruminant animals that in turn would decrease methane emissions. Few studies have focussed on changes in these sources of emissions due to desertification and it remains a field that requires further research.

In comparison to desertification through the suppression of primary production, the process of woody plant encroachment can result in significantly different climatic feedbacks. Increasing woody plant cover in open rangeland ecosystems leads to an increase in woody carbon stocks both above – and below-ground (Asner et al. 2003 <sup>[[#fn:r532|532]]</sup> ; Hughes et al. 2006 <sup>[[#fn:r533|533]]</sup> ; Petrie et al. 2015 <sup>[[#fn:r534|534]]</sup> ; Li et al. 2016 <sup>[[#fn:r535|535]]</sup> ). Within the drylands of Texas, USA, shrub encroachment led to a 32% increase in aboveground carbon stocks over a period of 69 years (3.8 tC ha– <sup>1</sup> to 5.0 tC ha– <sup>1</sup> ) (Asner et al. 2003 <sup>[[#fn:r536|536]]</sup> ). Encroachment by taller woody species can lead to significantly higher observed biomass and carbon stocks. For example, encroachment by ''Dichrostachys cinerea'' and several Vachellia species in the sub-humid savannahs of north-west South Africa led to an increase of 31–46 tC ha– <sup>1</sup> over a 50–65 year period (1936–2001) (Hudak et al. 2003 <sup>[[#fn:r537|537]]</sup> ). In terms of potential changes in SOC stocks, the effect may be dependent on annual rainfall and soil type. Woody cover generally leads to an increase in SOC stocks in drylands that have less than 800 mm of annual rainfall, while encroachment can lead to a loss of soil carbon in more humid ecosystems (Barger et al. 2011 <sup>[[#fn:r538|538]]</sup> ; Jackson et al. 2002 <sup>[[#fn:r539|539]]</sup> ).

The suppression of the grass layer through the process of woody encroachment may lead to a decrease in carbon stocks within this relatively small carbon pool (Magandana 2016 <sup>[[#fn:r540|540]]</sup> ). Conversely, increasing woody cover may lead to a decrease and even halt in surface fires and associated GHG emissions. In their analysis of drivers of fire in southern Africa, Archibald et al. (2009) <sup>[[#fn:r541|541]]</sup> note that there is a potential threshold around 40% canopy cover, above which surface grass fires are rare. Whilst there have been a number of studies on changes in carbon stocks due to desertification in North America, southern Africa and Australia, a global assessment of the net change in carbon stocks – as well as fire and ruminant GHG emissions due to woody plant encroachment – has not been done yet.

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