Heteropogon‐Themeda grasses evolve to occupy either tropical grassland or wetland biomes

Abstract

Species of the Heteropogon‐Themeda clade are ecologically important grasses distributed across the tropics, including widespread species, such as the pantropical Heteropogon contortus and Themeda triandra, and range‐restricted species such as Heteropogon ritchiei and Themeda anathera. Here, we examine habitat preferences of the grassland/savanna and wetland species by describing bioclimatic niche characteristics, characterizing functional traits, and investigating the evolution of functional traits of 31 species in the Heteropogon‐Themeda clade in relation to precipitation and temperature. The climatic limits of the clade are linked to mean annual precipitation and seasonality that also distinguish seven wetland species from 24 grassland/savanna species. Tests of niche equivalency highlighted the unique bioclimatic niche of the wetland species. However, climatic factors do not fully explain species geographic range, and other factors are likely to contribute to their distribution ranges. Trait analyses demonstrated that the wetland and grassland/savanna species were separated by culm height, leaf length, leaf area, awn length, and awn types. Phylogenetic analyses showed that the wetland species had tall stature with long and large leaves and lack of hygroscopic awns, which suggest selective pressures in the shift between savanna/grassland and wetland. The two most widespread species, H. contortus and T. triandra, have significantly different bioclimatic niches, but we also found that climatic niche alone does not explain the current geographic distributions of H. contortus and T. triandra. Our study provides a new understanding of the biogeography and evolutionary history of an ecologically important clade of C4 tropical grasses.

The biogeography of grasses (Poaceae)

“…Four of the manuscripts in this issue include new dated biogeographical analyses (Gallaher et al, 2022;Peterson et al, 2022b;Soreng et al, 2022a;Zhou et al, 2022); three comprise new phylogenetic analyses with subsequent taxonomic novelties (Da Silva et al, 2022;Peterson et al, 2022aPeterson et al, , 2022c, one is an ecological study investigating functional traits in a clade of savanna/wetland grasses (Arthan et al, 2022), one looked at the current distribution of the grasses over the Australian continent to determine migration directionality (Bryceson & Morgan, 2022), and one is an updated classification of the family primarily based on molecular phylogenetic studies (Soreng et al, 2022).…”

“…Arthan et al (2022) investigated the evolution of functional traits of 31 species in the Heteropogon ‐ Themeda clade and found culm height, leaf length, leaf area, awn length, and awn types separated species occupying wetland and grassland/savannas; and that two widespread species, Heteropogon contortus (L.) P. Beauv. ex Roem.…”