Macro-Climatic Distribution Limits Show Both Niche Expansion and Niche Specialization among C4 Panicoids

Abstract

Grasses are ancestrally tropical understory species whose current dominance in warm open habitats is linked to the evolution of C4 photosynthesis. C4 grasses maintain high rates of photosynthesis in warm and water stressed environments, and the syndrome is considered to induce niche shifts into these habitats while adaptation to cold ones may be compromised. Global biogeographic analyses of C4 grasses have, however, concentrated on diversity patterns, while paying little attention to distributional limits. Using phylogenetic contrast analyses, we compared macro-climatic distribution limits among ~1300 grasses from the subfamily Panicoideae, which includes 4/5 of the known photosynthetic transitions in grasses. We explored whether evolution of C4 photosynthesis correlates with niche expansions, niche changes, or stasis at subfamily level and within the two tribes Paniceae and Paspaleae. We compared the climatic extremes of growing season temperatures, aridity, and mean temperatures of the coldest months. We found support for all the known biogeographic distribution patterns of C4 species, these patterns were, however, formed both by niche expansion and niche changes. The only ubiquitous response to a change in the photosynthetic pathway within Panicoideae was a niche expansion of the C4 species into regions with higher growing season temperatures, but without a withdrawal from the inherited climate niche. Other patterns varied among the tribes, as macro-climatic niche evolution in the American tribe Paspaleae differed from the pattern supported in the globally distributed tribe Paniceae and at family level.

C4 photosynthesis evolved in warm climates but promoted migration to cooler ones

“…C 4 photosynthesis is specifically associated with increases in the upper bound of the species range towards higher temperatures during the warmest month (Table ), mirroring previous conclusions with a smaller species sampling (Aagesen et al . ). By contrast, the lower bound of the temperature ranges within species did not differ significantly between C 4 and C 3 groups, which indicates that C 4 evolution does not affect the lower range of the temperatures that are occupied (Table ).…”

“…; Aagesen et al . ). Temperature was the independent variable, with photosynthetic type as the categorical predictor.…”

“…; Aagesen et al . ). However, systematic tests for an effect of photosynthetic types on transition rates among different climatic zones are lacking.…”

Molecular phylogeny of Panicum s. str. (Poaceae, Panicoideae, Paniceae) and insights into its biogeography and evolution

“…str., added to the presence of C 4 photosyntesis, could have facilitated repeated shifts between habitats and the colonization of new areas. Evidence reported by [ 99 ] suggests that changes from C 3 to C 4 photosynthesis among panicoid grasses promoted niche expansion into hotter climates, and also into more arid climates for tribe Paniceae. These traits, together with the numerous dispersal events since the Late Miocene, could have generated the widespread distribution of the group.…”

Global thermal niche models of two European grasses show high invasion risks in Antarctica

“…Following the observed latitudinal pattern of decaying biodiversity on the Antarctic Peninsula north to south (Green et al ., ), we hypothesize a decreasing suitability gradient for the two non‐native species in relation to diminishing temperatures. Furthermore, we could also expect a niche expansion in Antarctica after facing their cold distribution limits, following similar trends to those recently observed on other grass tribes at warm extremes (Aagesen et al ., ). In this regard, we compare the bioclimatic thermal envelopes of the two Poa populations at each continent, including Antarctica.…”