Table 2.

Some major climatic events since 23 Myr BP that have moulded the floristic diversity of modern forests in Uganda.

Time perioda	Climate	Rainforest
Last few centuriesb	Many climatic fluctuations, some geographically widespread, others apparently more local.	The influence of climatic events on rainforest in Uganda is difficult to discern against strong human influence on the vegetation.
From 4000 to 3500 BPc to the present	Drier than previously (but still wet compared with ice age aridity).	There was a transition to drier forest types at ∼4000–3500 BP in Uganda, experienced at all altitudes.
From 12,500 to 10,000 BPd to the present	Warmer and much wetter than previously.	The extent of forest expanded greatly in Uganda at ∼12,500–10,000 BP.
From 2.6 Myr BPe to the present	Marked climatic fluctuations in Africa, especially after 800,000 BP.	There were major contractions and expansions of forest in tropical Africa driven by the fluctuating climate. Differentiation and extinction of populations of forest species.
23–2.6 Myr BPf	Climate initially much wetter than now across tropical Africa, becoming progressively drier.	Forest was initially more extensive than now in tropical Africa, then retreating with species being lost.

^aDates are in years before present (BP), those based largely on radiocarbon dating (younger than ∼40,000 BP) being in ¹⁴C years before 1950 CE. Dates given in calendar years (BCE or CE) elsewhere in this paper are in calendar years, transposed from ¹⁴C years where necessary (Reimer et al., 2009).

^bThe most detailed climatic records for the last ∼1000 years reveal fluctuations in climate of short to medium term duration (decades to centuries) (Ryves et al., 2011, Ssemmanda et al., 2005). A dry phase at ∼1750–1850 CE has been detected widely across East Africa. Similar short-term climatic fluctuations are likely to have occurred at all times.

^cA mid-Holocene shift to a drier climate has been widely recorded across equatorial and northern Africa, with the abruptness of the transition debated (McGlynn et al., 2013, Tierney et al., 2011). The date of ∼4000–3500 BP given here (equivalent to ∼2050–1850 BCE in calendar years) is one quoted in regional reviews (Hamilton, 1982, Hamilton, 1992, Jolly et al., 1997, Kiage and Liu, 2006). A notable feature seen in many pollen diagrams from Uganda is a rise in the very well dispersed pollen type Podocarpus (produced by the gymnosperm genera Afrocarpus and Podocarpus).

^dThere is much evidence for a major transition from a relatively cool dry climate prevailing across equatorial Africa during the last global ice age (peaking at 18,000 BP) to warmer and much wetter conditions thereafter (the postglacial). The date of this transition given here is based on assessments of the pollen evidence for East Africa as a whole or parts thereof (Hamilton, 1982, Hamilton, 1992, Jolly et al., 1997, Kiage and Liu, 2006).

^eThis is the Quaternary Period, marked by a series of ice ages in temperate parts of the world.

^fSeveral publications discuss climatic change during this period and its effects on the flora (Hamilton and Taylor, 1991, Harris et al., 2000, Jacobs, 2004, Plana, 2004, Sosef, 1994).