alternative stable states; tropical biomes; tree species composition; precipitation and seasonality; fire
Abstract :
[en] The idea that tropical forest and savanna are alternative states is crucial to how we manage these biomes and predict their future under global change. Large-scale empirical evidence for alternative stable states is limited, however, and comes mostly from the multimodal distribution of structural aspects of vegetation.
These approaches have been criticized, as structure alone cannot separate out wetter savannas from drier forests for example, and there are also technical challenges to mapping vegetation structure in unbiased ways. Here, we develop an alternative approach to delimit the climatic envelope of the two biomes in Africa using tree species lists gathered for a large number of forest and savanna sites distributed across the continent. Our analyses confirm extensive climatic overlap of forest and savanna, supporting the alternative stable states hypothesis for Africa, and this result is corroborated by paleoecological evidence. Further, we find the two biomes to have highly divergent tree species compositions and to represent alternative compositional states. This allowed us to classify tree species as forest vs. savanna specialists, with some generalist species that span both biomes. In conjunction with georeferenced herbarium records, we mapped the forest and savanna distributions across Africa and quantified their environmental limits, which are primarily related to precipitation and seasonality, with a secondary contribution of fire.
These results are important for the ongoing efforts to restore African ecosystems, which depend on accurate biome maps to set appropriate targets for the restored states but also provide empirical evidence for broad-scale bistability.
J. Ratnam et al., When is a ‘forest’ a savanna, and why does it matter? Glob. Ecol. Biogeogr. 20, 653–660 (2011).
C. E. Lehmann, S. A. Archibald, W. A. Hoffmann, W. J. Bond, Deciphering the distribution of the savanna biome. New Phytol. 191, 197–209 (2011).
A. C. Staver, S. Archibald, S. A. Levin, The global extent and determinants of savanna and forest as alternative biome states. Science 334, 230–232 (2011).
P. L. Silva de Miranda et al., Using tree species inventories to map biomes and assess their climatic overlaps in lowland tropical South America. Glob. Ecol. Biogeogr. 27, 899–912 (2018).
M. Hirota, M. Holmgren, E. H. Van Nes, M. Scheffer, Global resilience of tropical forest and savanna to critical transitions. Science 334, 232–235 (2011).
A. C. Staver, S. Archibald, S. Levin, Tree cover in sub-Saharan Africa: Rainfall and fire constrain forest and savanna as alternative stable states. Ecology 92, 1063–1072 (2011).
C. Favier et al., Abrupt shifts in African savanna tree cover along a climatic gradient. Glob. Ecol. Biogeogr. 21, 787–797 (2012).
G. P. Hempson, S. Archibald, J. E. Donaldson, C. E. R. Lehmann, Alternate grassy ecosystem states are determined by palatability-flammability trade-offs. Trends Ecol. Evol. 34, 286–290 (2019).
W. A. Hoffmann et al., Ecological thresholds at the savanna-forest boundary: How plant traits, resources and fire govern the distribution of tropical biomes. Ecol. Lett. 15, 759–768 (2012).
T. Charles-Dominique, A. C. Staver, G. F. Midgley, W. J. Bond, Functional differentiation of biomes in an African savanna/forest mosaic. S. Afr. J. Bot. 101, 82–90 (2015).
M. D. Swaine, J. B. Hall, J. M. Lock, The forest-savanna boundary in west-central Ghana. Ghana J. Sci. 16, 35–52 (1976).
J. C. Aleman, A. C. Staver, Spatial patterns in the global distributions of savanna and forest. Glob. Ecol. Biogeogr. 27, 792–803 (2018).
C. Xu et al., Remotely sensed canopy height reveals three pantropical ecosystem states. Ecology 97, 2518–2521 (2016).
V. de L. Dantas, M. Hirota, R. S. Oliveira, J. G. Pausas, Disturbance maintains alternative biome states. Ecol. Lett. 19, 12–19 (2016).
M. C. Hansen et al., Towards an operational MODIS continuous field of percent tree cover algorithm: Examples using AVHRR and MODIS data. Remote Sens. Environ. 83, 303–319 (2002).
M. C. Hansen et al., High-resolution global maps of 21st-century forest cover change. Science 342, 850–853 (2013).
J. C. Aleman, O. Blarquez, C. A. Staver, Land-use change outweighs projected effects of changing rainfall on tree cover in sub-Saharan Africa. Glob. Change Biol. 22, 3013–3025 (2016).
M. Torello-Raventos et al., On the delineation of tropical vegetation types with an emphasis on forest/savanna transitions. Plant Ecol. Divers. 6, 101–137 (2013).
J. W. Veldman, Clarifying the confusion: Old-growth savannahs and tropical ecosystem degradation. Philos. Trans. R Soc. Lond. B Biol. Sci. 371, 20150306 (2016).
C. P. Osborne et al., Human impacts in African savannas are mediated by plant functional traits. New Phytol. 220, 10–24 (2018).
H. P. Linder et al., The partitioning of Africa: Statistically defined biogeographical regions in sub‐Saharan Africa. J. Biogeogr. 39, 1189–1205 (2012).
H. P. Linder, The evolution of African plant diversity. Front. Ecol. Evol. 2, 38 (2014).
G. P. Hempson, S. Archibald, W. J. Bond, A continent-wide assessment of the form and intensity of large mammal herbivory in Africa. Science 350, 1056–1061 (2015).
V. Droissart et al., Beyond trees: Biogeographical regionalization of tropical Africa. J. Biogeogr. 45, 1153–1167 (2018).
M. S. Sosef et al., Exploring the floristic diversity of tropical Africa. BMC Biol. 15, 15 (2017).
A. Fayolle et al., Patterns of tree species composition across tropical African forests. J. Biogeogr. 41, 2320–2331 (2014).
A. Fayolle et al., A sharp floristic discontinuity revealed by the biogeographic regionalization of African savannas. J. Biogeogr. 46, 454–465 (2019).
F. White, The Vegetation of Africa: A Descriptive Memoir to Accompany the UNESCO/ AETFAT/UNSO Vegetation Map of Africa (ORSTOM–UNESCO, 1983).
A. Vincens et al., Forest response to climate changes in Atlantic Equatorial Africa during the last 4000 years BP and inheritance on the modern landscapes. J. Biogeogr. 26, 879–885 (1999).
M. Sankaran et al., Determinants of woody cover in African savannas. Nature 438, 846–849 (2005).
U. Salzmann, P. Hoelzmann, The Dahomey gap: An abrupt climatically induced rain forest fragmentation in West Africa during the late Holocene. Holocene 15, 190–199 (2005).
J. C. Aleman et al., Palaeo-trajectories of forest savannization in the southern Congo. Biol. Lett. 15, 20190284 (2019).
U. Salzmann, Are modern savannas degraded forests?-A Holocene pollen record from the Sudanian vagetation zone of NE Nigeria. Veg. Hist. Archaeobot. 9, 1–15 (2000).
R. R. Klopper, L. Gautier, C. Chatelain, G. F. Smith, R. Spichiger, Floristics of the angiosperm flora of sub-Saharan Africa: An analysis of the African plant checklist and Database. Taxon 56, 201–208 (2007).
J. C. Aleman, M. A. Jarzyna, A. C. Staver, Forest extent and deforestation in tropical Africa since 1900. Nat. Ecol. Evol. 2, 26–33 (2018).
A. Vincens, Y. Garcin, G. Buchet, Influence of rainfall seasonality on African lowland vegetation during the late quaternary: Pollen evidence from Lake Masoko, Tanzania. J. Biogeogr. 34, 1274–1288 (2007).
L. Laestadius et al., Opportunities for forest landscape restoration. Unasylva 62, 238 (2011).
M. Greve, B. Reyers, A. Mette Lykke, J.-C. Svenning, Spatial optimization of carbon-stocking projects across Africa integrating stocking potential with co-benefits and feasibility. Nat. Commun. 4, 2975 (2013).
R. C. R. Abreu et al., The biodiversity cost of carbon sequestration in tropical savanna. Sci. Adv. 3, e1701284 (2017).
B. P. Murphy, A. N. Andersen, C. L. Parr, The underestimated biodiversity of tropical grassy biomes. Philos. Trans. R Soc. Lond. B Biol. Sci. 371, 20150319 (2016).
C. M. Ryan et al., Ecosystem services from southern African woodlands and their future under global change. Philos. Trans. R Soc. Lond. B Biol. Sci. 371, 20150312 (2016).
N. Stevens, C. E. Lehmann, B. P. Murphy, G. Durigan, Savanna woody encroachment is widespread across three continents. Glob. Change Biol. 23, 235–244 (2017).
E. Buisson et al., Resilience and restoration of tropical and subtropical grasslands, savannas, and grassy woodlands. Biol. Rev. Camb. Philos. Soc. 94, 590–609 (2019).
A. T. Kaptué, L. Prihodko, N. P. Hanan, On regreening and degradation in Sahelian watersheds. Proc. Natl. Acad. Sci. U.S.A. 112, 12133–12138 (2015).
L. Gillson, R. Marchant, From myopia to clarity: Sharpening the focus of ecosystem management through the lens of palaeoecology. Trends Ecol. Evol. 29, 317–325 (2014).
S. E. Fick, R. J. Hijmans, WorldClim 2: New 1-km spatial resolution climate surfaces for global land areas. Int. J. Climatol. 37, 4302–4315 (2017).
L. Giglio, J. Descloitres, C. O. Justice, Y. J. Kaufman, An enhanced contextual fire detection algorithm for MODIS. Remote Sens. Environ. 87, 273–282 (2003).
G. P. Hempson, S. Archibald, W. J. Bond, The consequences of replacing wildlife with livestock in Africa. Sci. Rep. 7, 17196 (2017).
T. Hengl et al., Mapping soil properties of Africa at 250 m resolution: Random forests significantly improve current predictions. PLoS One 10, e0125814 (2015).
M. Dufrêne, P. Legendre, Species assemblages and indicator species: The need for a flexible asymmetrical approach. Ecol. Monogr. 67, 345–366 (1997).
R Development Core Team, R: A Language and Environment for Statistical Computing, Version 3.5.0. https://www.R-project.org/. Accessed 23 April 2018.
R. J. Hijmans, raster: Geographic Data Analysis and Modeling, R package Version 2.6-7. https://CRAN.R-project.org/package=raster. Accessed 14 June 2018.
S. Dray, A. B. Dufour, The ade4 package: Implementing the duality diagram for ecologists. J. Stat. Softw. 22, 1–20 (2007).
R. J. Hijmans, S. Phillips, J. Leathwick, J. Elith, dismo: Species Distribution Modeling, R package Version 1.1-4. https://CRAN.R-project.org/package=dismo. Accessed 14 June 2018.
J. Oksanen et al, vegan: Community Ecology Package, R package Version 2.5-2. https://CRAN.R-project.org/package=vegan. Accessed 14 June 2018.
D. W. Roberts, labdsv: Ordination and Multivariate Analysis for Ecology, R package Version 1.8-0. https://CRAN.R-project.org/package=labdsv. Accessed 14 June 2018.
E. J. Pebesma, Multivariable geostatistics in {S}: The gstat package. Comput. Geosci. 30, 683–691 (2004).
A. Liaw, M. Wiener, Classification and regression by randomForest. R News 2, 18–22 (2002).
