Climate and geochemistry as drivers of eucalypt diversification in Australia

Abstract

Eucalypts cover most of Australia. Here, we investigate the relative contribution of climate and geochemistry to the distribution and diversity of eucalypts. Using geostatistics, we estimate major element concentrations, pH, and electrical conductivity at sites where eucalypts have been recorded. We compare the median predicted geochemistry and reported substrate for individual species that appear associated with extreme conditions; this provides a partial evaluation of the predictions. We generate a site-by-species matrix by aggregating observations to the centroids of 100-km-wide grid cells, calculate diversity indices, and use numerical ecology methods (ordination, variation partitioning) to investigate the ecology of eucalypts and their response to climatic and geochemical gradients. We find that β-diversity coincides with variations in climatic and geochemical patterns. Climate and geochemistry together account for less than half of the variation in eucalypt species assemblages across Australia but for greater than 80% in areas of high species richness. Climate is more important than geochemistry in explaining eucalypts species distribution and change in assemblages across Australia as a whole but there are correlations between the two sets of environmental variables. Many individual eucalypt species and entire taxonomic sections (Aromatica, Longistylus of subgenus Eucalyptus, Dumaria, and Liberivalvae of subgenus Symphyomyrtus) have distributions affected strongly by geochemistry. We conclude that eucalypt diversity is driven by steep geochemical gradients that have arisen as climate patterns have fluctuated over Australia over the Cenozoic, generally aridifying since the Miocene. The diversification of eucalypts across Australia is thus an excellent example of co-evolution of landscapes and biota in space and time and challenges accepted notions of macroecology.

Increased diversification rates are coupled with higher rates of climate space exploration in Australian Acacia (Caesalpinioideae)

“…Tests for phylogenetic localization of salinity and alkalinity tolerance would be worthwhile. Within ES 1, rhizosphere adaptations may be a key trait to cope with the annual extremes of cyclically waterlogged and parched tropical monsoon soils (Bui et al , ). Soil pH tends to decrease with rainfall: tropical monsoon soils tend to be acidic, whereas those to the south are often more alkaline (Ahren et al , ; Henderson et al , ).…”

“…Macroevolutionary patterns have been linked to historical trends in global and regional climate in many Australian lineages, including elevated speciation in arid‐zone pygopodoid geckos (Brennan & Oliver, ), macropods (Couzens & Prideaux, ), cockroaches (Beasley‐Hall et al , ), and cicadas (Owen et al , ). In plants, the eucalypts provide evidence that climate and historical events are important in explaining species distribution and turnover at the continental scale (Ladiges et al , ; Bui et al , ), and Hakea has expanded coincident with a transition to more open, drought and fire‐prone habitats promoted by aridification (Lamont et al , ; Cardillo et al , ). Aridity also drove the evolution of embolism resistance in Australian Callitris (Larter et al , ) and xeromorphy in other lineages (Hill & Brodribb, ).…”

Climate, soil or both? Which variables are better predictors of the distributions of Australian shrub species?

“…Bui et al (2017) found that incorporating soil variables with climate was efficient for defining the distribution of Eucalyptus species and strongly influenced some specific species in taxonomic sections (e.g., Aromatica and Dumaria), although that climate was more important factor. These results are similar to Martinson et al (2011), who used Maxent to model the distributions of 30 species, including shrubs, across arid areas of North America using climate and soil variables.…”

A dated molecular perspective of eucalypt taxonomy, evolution and diversification

“…After the vicariance, the eudesmids radiated into the north-western arc of Australia, and the monocalypts radiated around the south-eastern arc. Bui et al (2017) noted that soil geochemistry was influential in the turnover of monocalypt species around the south-east. Further, the eudesmids and 10 moncalypts did not adapt to grow on the soil created by the inland sea, and only some symphyomyrts adapted and radiated into that part of Australia.…”

“…Ladiges et al (2003) suggested that eucalypt species are breaking into fragmented ranges, which could easily be a driver of diversifying large 45 widespread species into isolated populations that become unique entities. Bui et al (2017) suggested that climate and geochemistry are major factors in controlling the current distribution of eucalypt species. By combining different climate with a mosaic of soil types, it is possible to create a 50 great number of niches into which the ancestral taxa could spread and then diversify.…”