Fig. 5: Forest cover as a function of precipitation at Tenaghi Philippon for the past 500 kyrs.

a Cross-plot of tree-pollen percentages and log(Ca/Fe) values illustrating the persistence of two distinct and stable vegetation regimes for a wide range of precipitation levels. Rather than a gradual increase in tree cover with increasing precipitation at Tenaghi Philippon, non-linear regime shifts between steppe (low tree-pollen %) and forest (high tree-pollen %) biomes occur above a precipitation threshold (as suggested by the step-like blue line from a generalized additive model [GAM] fit). Purple circles show values at periods of atmospheric CO₂ concentrations above 260 ppmv (ref. ¹³) representing interglacial conditions¹². The gray shaded areas show the density of the data points (darker color for higher density). b Histograms of tree-pollen percentages at different log(Ca/Fe) values demonstrating a bimodal distribution with steppe biomes (low tree-pollen %) prevailing below a value of ~0.2 in log(Ca/Fe) and forest biomes (high tree-pollen %) dominating above a value of ~0.8. Regime shifts between the two biomes occur at a threshold value of ~0.5 in log(Ca/Fe).