Functional diversification enabled grassy biomes to fill global climate space

Abstract

Global change impacts on the Earth System are typically evaluated using biome classifications based on trees and forests. However, during the Cenozoic, many terrestrial biomes were transformed through the displacement of trees and shrubs by grasses. While grasses comprise 3% of vascular plant species, they are responsible for more than 25% of terrestrial photosynthesis. Critically, grass dominance alters ecosystem dynamics and function by introducing new ecological processes, especially surface fires and grazing.However, the large grassy component of many global biomes is often neglected in their descriptions, thereby ignoring these important ecosystem processes. Furthermore, the functional diversity of grasses in vegetation models is usually reduced to C3 and C4 photosynthetic plant functional types, omitting other relevant traits. Here, we compile available data to determine the global distribution of grassy vegetation and key traits related to grass dominance. Grassy biomes (where > 50% of the ground layer is covered by grasses) occupy almost every part of Earth's vegetated climate space, characterising over 40% of the land surface. Major evolutionary lineages of grasses have specialised in different environments, but species from only three grass lineages occupy 88% of the land area of grassy vegetation, segregating along gradients of temperature, rainfall and fire. The environment occupied by each lineage is associated with unique plant trait combinations, including C3 and C4 photosynthesis, maximum plant height, and adaptations to fire and aridity.

The global distribution of grass functional traits within grassy biomes

“…Traits are: (LNC) leaf nitrogen content (%), (SLA) specific leaf area (cm 2 /g), (LTS) leaf tensile strength (MPa), (Height) maximum culm height (cm), (Leaf width) maximum leaf width (cm) and (Leaf area) maximum leaf area (cm 2 ). Mapping is based on the mean trait values for dominant species in each of the grassy vegetation types determined by Lehmann et al () [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Variance partitioning was used to compare how much of the trait variation occurred within and between each of the grassy vegetation types defined by Lehmann et al (). This showed that 60% of variation in LTS occurred within rather than between vegetation types, and a large amount of variation within the vegetation types was also evident for SLA (95%), LNC (64%), maximum culm height (55%), maximum leaf area (83%) and maximum leaf width (81%), suggesting that global and regional‐scale changes in environment are not key drivers of variation in grass traits.…”

Biomes as evolutionary arenas: Convergence and conservatism in the trans‐continental succulent biome

“…As pointed out by Dexter et al (2018) and setting aside true deserts, most research on tropical biodiversity divides the vegetation of the lowland tropics into two broad categories: rain forests and savannas, or forest versus ‘open’ vegetation (Antonelli et al, 2018; Oliveras & Malhi, 2016; Staver, Archibald, & Levin, 2011). Savannas are found in areas with a pronounced dry season and abundant C 4 grasses, and are prone to regular fires (Lehmann et al, 2019; Lehmann & Parr, 2016). Rain forests, in contrast, receive year‐round precipitation, have few grasses, and do not experience fire (Eiserhardt et al, 2017).…”

Widespread lateral gene transfer among grasses

“…Paniceae grasses generally grow in biodiverse savannas which are dominated by Andropogoneae species in wetter areas, and Chloridoideae grasses in drier habitats (Lehmann et al 2019). The effects of phylogenetic distance are therefore confounded with those of biogeography, but specific examples indicate that biogeography can take precedence.…”

“…Andropogoneae) for a majority of the range of each of the two species, and the area from which the individual for the genome assembly was sampled (Lovell et al 2018;Lehmann et al 2019).…”