Environmental factors determining the phylogenetic structure of C4 grass communities


Colin P. Osborne, Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
E-mail: c.p.osborne@sheffield.ac.uk


Aim  To determine how the distribution of species richness is associated with environmental factors for the four major C4 grass lineages in South Africa, as a means to explore the mechanisms responsible.

Location  South Africa, Lesotho and Swaziland.

Methods  The geographical distributions of species richness for four major C4 grass lineages (Aristidoideae, Chloridoideae, Andropogoneae and Paniceae) were sourced from a recently published flora that divided the study region into different vegetation types. Mean values of potential environmental correlates were calculated for each vegetation type, and the relative importances of these were determined using single- and multiple-predictor generalized linear models, with and without control for spatial autocorrelation. Model selection of the multiple-predictor generalized linear models was conducted using an Akaike’s information criterion–information theoretic approach. Association with wet, intermediate or dry, shady or open, and disturbed or undisturbed habitats was also determined for each C4 grass clade using habitat data for all the grass species, and analysed using chi-square tests of independence.

Results  Andropogoneae and Paniceae are most species-rich in areas of high precipitation and in mesic habitats. Andropogoneae are associated with high fire frequencies. Species richness in Andropogoneae decreases and in Paniceae increases in relation to livestock density. Chloridoideae species richness is relatively constant across South Africa, but is highest where there are infrequent fires, high temperatures and basic soils, and in mesic and disturbed habitats. Aristidoideae are most species-rich in arid regions and in habitats with high temperatures, and are associated with disturbed habitats.

Main conclusions  Environmental variables other than precipitation, including temperature, fire frequency and grazing pressure, are strongly associated with the contrasting distributions of species richness for the various C4 grass clades in South Africa. Our results suggest that ecological sorting is an important determinant of phylogenetic patterns in the species richness of these C4 grass lineages.