Received: 12 October 2017 | Revised: 5 March 2018 | Accepted: 9 March 2018 DOI: 10.1111/gcb.14150 PRIMARY RESEARCH ARTICLE Waxing and waning of forests: Late Quaternary biogeography of southeast Africa Sarah J. Ivory1,2 | Anne-Marie Lezine 3 | Annie Vincens4 | Andrew S. Cohen5 1Department of Anthropology, Ohio State University, Columbus, OH, USA Abstract 2Department of Geosciences, Penn State African ecosystems are at great risk. Despite their ecological and economic impor- University, State College, PA, USA tance, long-standing ideas about African forest ecology and biogeography, such as 3LOCEAN, CNRS, Paris, France the timing of changes in forest extent and the importance of disturbance, have been 4CEREGE, CNRS, Aix-en-Provence, France 5Department of Geosciences, University of unable to be tested due to a lack of sufficiently long records. Here, we present the Arizona, Tucson, AZ, USA longest continuous terrestrial record of late Quaternary vegetation from southern Correspondence Africa collected to date from a drill core from Lake Malawi covering the last Sarah J. Ivory, Department of Geosciences, ~600,000 years. Pollen analysis permits us to investigate changes in vegetation Penn State University, State College, PA, USA. structure and composition over multiple climatic transitions. We observe nine Email: [email protected] phases of forest expansion and collapse related to regional hydroclimate change. Funding information The development of desert, steppe and grassland vegetation during arid periods is US National Science Foundation–Earth likely dynamically linked to thresholds in regional hydrology associated with lake System History Program, Grant/Award Number: EAR-0602350; International level and moisture recycling. Species composition of these dryland ecosystems Continental Scientific Drilling Program; varied greatly and is unlike the vegetation found at Malawi today, with assemblages National Science Foundation Graduate Research Fellowship, Grant/Award Number: suggesting strong Somali-Masai affinities. Furthermore, nearly all semiarid 2009078688 assemblages contain low forest taxa abundances, suggesting that moist lowland gal- lery forests formed refugia along waterways during arid times. When the region was wet, forests were species-rich and very high afromontane tree abundances suggest frequent widespread lowland colonization by modern high elevation trees. Further- more, species composition varied little amongst forest phases until ~80 ka when dis- turbance tolerant tree taxa characteristic of the modern vegetation increased in abundance. The waxing and waning of forests has important implications for under- standing the processes that control modern tropical vegetation biogeography as well as the environments of early humans across Africa. Finally, this work highlights the resilience of montane forests during previous warm intervals, which is relevant for future climate change; however, we point to a fundamental shift in disturbance regimes which are crucial for the structure and composition of modern East African landscapes. KEYWORDS Africa, biogeography, global change, hydrology, Lake Malawi, palaeoclimate, palaeoenvironments, tropical forests | Glob Change Biol. 2018;1–13. wileyonlinelibrary.com/journal/gcb © 2018 John Wiley & Sons Ltd 1 2 | IVORY ET AL. 1 | INTRODUCTION emergence of disturbance-maintained Zambezian miombo woodlands, have yet to be resolved due to the availability of sufficiently long Tropical African ecosystems are immensely important for their ecolog- terrestrial records which capture previous warm intervals. Marine ical value, as bastions of biodiversity and endemism, as well as for cores covering this period show a large expansion of afromontane essential resources for millions of people for daily subsistence (e.g. forest, suggesting that ancient forests may have been substantially Doherty, Sitch, Smith, Lewis, & Thornton, 2010; Mittermeier, Turner, different than lowland communities observed today, however, no Larsen, Brooks, & Gascon, 2011; White, 1981). However, African for- terrestrial record has ever documented their presence or extent ests and semiarid lands are already changing as a result of pressure (Dupont et al., 2011; Ning & Dupont, 1997). In addition, palaeocli- from climate and land-use change (Fugere, Kasangaki, & Chapman, mate records from across the continent show strongly antiphased 2016; Newmark & McNeally, 2018; Niang, Ruppel, & Abdrabo, 2014). signals implying that these earlier high amplitude wet–dry cycles of In turn, feedbacks associated with altered disturbance regimes, such as the late Pleistocene occurred asynchronously across the continent. wildfires, on landscapes processes could result in detrimental changes For example at Lake Malawi from ~600 to 100 ka, lake levels to important ecosystems services (e.g. Bond, Woodward, & Midgley, decreased multiple times by as much as À550 m, whereas north of 2005; Midgley & Bond, 2015; Tilman et al., 1997; Van Langevelde the equator, lakes expanded in the Kenyan rift valley (Ivory et al., et al., 2003). Therefore, it is increasingly important to be able to pre- 2016; Salzburger, Van Bocxlaer, & Cohen, 2014; Trauth et al., 2007). dict long-term species responses to climate. However, particularly in These large hydroclimatic shifts and patchiness of continental cli- Africa, modern ecological observations are sparse, and the range of mate likely resulted in massive reorganization of species as they observations is short (e.g. Feeley & Silman, 2011; Willis & Birks, 2006). tracked optimal climate. Perhaps more importantly, although we Furthermore, as tropical climate is likely moving to a state that does know that species respond individualistically to climate (e.g. Palmer not exist today, projecting species’ responses to future change relies et al., 2015; Stewart, 2009), it is yet unclear how ecosystem compo- on extrapolation (Niang et al., 2014; Williams & Jackson, 2007; Wil- sition and structure changed or how differential dispersal may have liams, Jackson, & Kutzbach, 2007). Thus, long-term (>105 year) records influenced community composition and forest succession. which observe the climatic processes that control vegetation structure Lake Malawi, southeast Africa, sits currently at the southern and composition are required (Dietl et al., 2015; Dietl & Flessa, 2017; extent of the Intertropical Convergence Zone (ITCZ) and is sensitive Lindbladh, Fraver, Edvardsson, & Felton, 2013; ). to fluctuations in regional climate (Figure 1; Cohen et al., 2007; Ivory Palaeoecological records have provided insights about long-term et al., 2016; Scholz et al., 2007). Long drill core records from this ecological processes and allowed us to look at vegetation change in lake preserve a regional climate signal over the late Quaternary with response to conditions warmer than today and over periods of rapid rapid transitions from arid to humid climates (Ivory et al., 2016). Fos- change which can be used as future analogues (e.g. Blois, Zarnetske, sil pollen analysis of sediments from this record provides the ideal Fitzpatrick, & Finnegan, 2013; Hughen, Eglinton, Xu, & Makou, opportunity to better understand ecological processes that operate 2004; Salzmann & Hoelzmann, 2005). For example in Africa, the over many climatic transitions. The unique position of the lake with prevalence of forests throughout the region when global tempera- respect to tropical circulation as well as ecologically important tures are higher, such as during interglacials, has been thought of as ecosystems within the African tropics allows us to look at changing canonical, despite some evidence to the contrary (Beuning, Zimmer- forest composition and vegetation structure through time as well man, Ivory, & Cohen, 2011; Ivory, Lezine, Vincens, & Cohen, 2012; alterations to disturbance regimes. Furthermore, this unique record Vincens, Garcin, & Buchet, 2007). Warming and wetting over the last is the first long, terrestrial record of vegetation change from East deglacial period associated with the reinforcement of regional mon- Africa covering the entirety of the late Quaternary (>500 ka) and soons resulted in an expansion of woodlands and forests as well as provides insights into large-scale reorganization of biomes over mul- the retreat of dryland ecosystems, which dominated during the Last tiple wet-dry cycles in African climate. Glacial Maximum (LGM; e.g. Izumi & Lezine, 2016; Maley, 1992). However, hydrological records reconstructed from ancient lake 1.1 | Modern setting deposits and marine cores suggest that LGM to modern climate changes in Africa were small in amplitude and occurred over thou- Lake Malawi, the southernmost lake in the East African Rift System, sands of years with respect to those which occurred earlier in the is composed of a series of alternating N-S oriented half-graben late Pleistocene (Dupont & Kuhlmann, 2017; Dupont et al., 2011; basins (Figure 1). The lake is hydrologically open and is drained by Scholz et al., 2007; Trauth et al., 2007). Thus, as most existing the Shire River to the south (sill depth = 6 m), although most water records do not extend beyond the last glacial period, it is unclear loss occurs via evaporation (Eccles, 1974). Mountain ranges in the whether the LGM-modern vegetation response is indicative of larger northern (Livingstone Mountains and Rungwe Highlands) and the alterations in climate over multiple cycles. However, the lack of long western (Nyika Plateau) watershed rise steeply from the lake shore records has made it challenging