a systematic review of the impacts of woody encroachment
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ContentsA systematic review of the impacts of woody encroachment on ecosystem services in African and North American
savannas and grasslands.
Felix V. Skhosana
PhD Candidate
2
Woody encroachment (WE)
Photo By Vision2030 - Own work, CC BY-SA 4.0,
• WE – The directional increase of indigenous woody plants in savannas and the
‘invasion’ of the formerly grassland biomes by indigenous woody plants (O’Connor et al.
2014).
• Eg. WE at the Waterberg Plateau Park in Namibia
3
Woody Encroachment (WE) across the world
Stevens et al. 2018
• WE occurs mostly in savannas and grasslands worldwide.
4
Drivers of Woody encroachment (WE)
Luvuno et al. 2018
• Substantial knowledge on drivers and encroachers of WE.
• A combination of local and global drivers causing WE.
5
Encroaching species
Photo By Vision2030 - Own work, CC BY-SA 4.0,
• Mostly N-fixing legumes to search and review.
S. melifera P. glandulosa
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Gap in literature
• Impacts of WE on ecosystem services (ES) and ecosystem
disservices (EDS) warrants a clear synthesis.
– AIM: Systematically reviewing the impacts of WE on ES & EDS.
– Objective: Use online databases to search and review literature.
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Ecosystem services (ES) and disservices (EDS)
• ES: Benefits and services humans obtain from ecosystems (MA, 2005).
– Provisioning, Regulating and Cultural
• EDS: goods and services that harm human well-being (Lyytimäki, 2014).
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Methods
• Searching electronic databases for literature
• Using MA, TEEB & CICES categories as guidelines.
Provisioning ES & EDS Regulating ES and EDS Cultural ES & EDS
Grass forage availability Carbon sequestration Tourism & view obstruction
Water availability Drought regulation Herding obstruction
Woody fuels Erosion regulation Cultural heritage
Woody materials Flood regulation Cultural standing
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Preliminary Results
• 313 published cases from 186 publications documented the impacts of WE on ES & EDS.
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Impacts on provisioning ESN
. Am
eric
aA
fric
a
Canada: Decline in grass cover from 50% to 0% from open to 80% shrub cover (Bork et al. 2009).
E.g. Decrease provisioning E.g. Increase provisioning
Namibia: 60% reduction in cattle carrying capacity (de
Klerk 2004); decline in cattle from 2,5milllion in 1958 to 800 000 in 2001 resulting to a 64% decline in meat production (Jones et al. 2009; Demas et al., 2012)
Namibia: Use 0.5mil to 1mil tons of firewood per yr. and exports 60 000 – 158 000 tons/yr. of charcoal (Rothauge
2014; Trede and Patt, 2015).
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Impacts on regulating ES
Eg. Increase regulation Eg. Decrease regulating Eg. EDS
RSA: Increase of 12100 kg/ha in AGC (Coetsee et al., 2013).
WE facilitates weed invasion (e.g. Chromolaena), causing bloating or even death (Solomon et al. 2007; Wigley
et al. 2009).
WE causing diseases (e.g. Trypansomiasis) in animals spread by Glossima fly (Bollig and Osterle
2008).
Afr
ica
Eg. Increase regulating Eg. Decrease regulating Eg. EDS
USA : Increase in AGC ranged from 1200 to 80958kg/ha (Collins et al. 1998; Asner et al.
2003; Campbell et al. 2012).
USA: Increase in SOC ranging from 300 to 61200 kg/ha (Hibbard et al., 2003; Liao et
al., 2006; Neff et al., 2009).
USA: Decrease in SOC ranging from -50 000 to -30kg/ha from grass-dominated to tree-dominated areas(McCulley and Jackson 2012; Shifa 2017).
N. A
me
rica
16
Conclusion
• Review presents both the benefits and negative impacts of WE on ES.
• At local scale, whether benefits outweigh negative impacts depends on the type of
landuse and the ES of interest.
• At wider scale WE is largely a problem than a benefit.
– WE beneficial in C sequestration but huge negative impact on livestock production.
– 50% to 80% of savannas and grasslands are used as rangelands & > 70% of these rangelands are degraded due to WE (Lukomska et al., 2014; Belayneh a.nd Tessema, 2017; Stevens et
al. 2017).
– > 1bil. people earn their livelihood from livestock farming in these impacted areas (Lukomska et al., 2014; MA, 2005).
• Control measures are essential to reduce WE.
Acknowledgements
Graham von Maltitza, Malebajoa Anicia Maoelaa, Gregor T. Feigac and Guy Midgleyb
• aCouncil for Scientific and Industrial Research, Pretoria, South Africa
• bDepartment of Botany and Zoology, Stellenbosch University, Matieland, South Africa
• cDepartment of Geography, Geoinformatics and Meteorology, University of Pretoria, South Africa
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