Fig. 3: Results of the modern stable carbon isotope analysis.

a, Samples of P. spiralis collected from different locations along the Stuart Highway, between Darwin and Katherine. MAP is calculated from 24 meteorological sites (see Supplementary Section 2) and displayed as a 10-km buffer surrounding the sampling transect, with blue and yellow indicating higher and lower precipitation, respectively. Additionally, we indicate the mean daily evaporation for June by the fill colour of each dot, which reflects the first month of fruiting of the P. spiralis drupe²³. Individual P. spiralis trees are indicated by their specimen number (white numbers; Supplementary Table 3). b, δ¹³C values of P. spiralis collected from different growth environments (floodplain fringe, seasonal floodways, and open forest and woodland vegetation communities) with the same MAP (~1,510 mm yr⁻¹), near Madjedbebe in the Alligator Rivers region. The grey horizontal band demarcates the interquartile range of the δ¹³C values of modern P. spiralis from the Alligator Rivers region. Boxes show the interquartile range, the midline shows the median value, and the whiskers extend vertically 1.5 times the interquartile range from the end of the box to the furthest datum within that distance. Data beyond that distance are represented individually as points (‘outliers’). c, A linear relationship is observed between precipitation and δ¹³C values of P. spiralis with 95% confidence intervals (grey area) and 95% prediction intervals (dashed lines) when removing all sites affected by microclimate effects (blue dots with mean evaporation <5 mm per day). Error bars on data points indicate one standard deviation. Note that the modern δ¹³C value for P. spiralis collected near Madjedbebe (white dot) falls directly on our best-fit line (sample not used to generate this regression). RMSE, root-mean-square error. The data and R code for this figure are available from ref. ⁵⁴. Panel a adapted with permission from ref. ⁶³, Geoscience Australia.