<p class="first" id="d1691571e117">Poaceae (the grasses) is arguably the most successful
plant family, in terms of its
global occurrence in (almost) all ecosystems with angiosperms, its ecological dominance
in many ecosystems, and high species richness. We suggest that the success of grasses
is best understood in context of their capacity to colonize, persist, and transform
environments (the "Viking syndrome"). This results from combining effective long-distance
dispersal, efficacious establishment biology, ecological flexibility, resilience to
disturbance and the capacity to modify environments by changing the nature of fire
and mammalian herbivory. We identify a diverse set of functional traits linked to
dispersal, establishment and competitive abilities. Enhanced long-distance dispersal
is determined by anemochory, epizoochory and endozoochory and is facilitated via the
spikelet (and especially the awned lemma) which functions as the dispersal unit. Establishment
success could be a consequence of the precocious embryo and large starch reserves,
which may underpin the extremely short generation times in grasses. Post-establishment
genetic bottlenecks may be mitigated by wind pollination and the widespread occurrence
of polyploidy, in combination with gametic self-incompatibility. The ecological competitiveness
of grasses is corroborated by their dominance across the range of environmental extremes
tolerated by angiosperms, facilitated by both C3 and C4 photosynthesis, well-developed
frost tolerance in several clades, and a sympodial growth form that enabled the evolution
of both annual and long-lived life forms. Finally, absence of investment in wood (except
in bamboos), and the presence of persistent buds at or below ground level, provides
tolerance of repeated defoliation (whether by fire, frost, drought or herbivores).
Biotic modification of environments via feedbacks with herbivory or fire reinforce
grass dominance leading to open ecosystems. Grasses can be both palatable and productive,
fostering high biomass and diversity of mammalian herbivores. Many grasses have a
suite of architectural and functional traits that facilitate frequent fire, including
a tufted growth form, and tannin-like substances in leaves which slow decomposition.
We mapped these traits over the phylogeny of the Poales, spanning the grasses and
their relatives, and demonstrated the accumulation of traits since monocots originated
in the mid-Cretaceous. Although the sympodial growth form is a monocot trait, tillering
resulting in the tufted growth form most likely evolved within the grasses. Similarly,
although an ovary apparently constructed of a single carpel evolved in the most recent
grass ancestor, spikelets and the awned lemma dispersal units evolved within the grasses.
Frost tolerance and C4 photosynthesis evolved relatively late (late Palaeogene), and
the last significant trait to evolve was probably the production of tannins, associated
with pyrophytic savannas. This fits palaeobotanical data, suggesting several phases
in the grass success story: from a late Cretaceous origin, to occasional tropical
grassland patches in the later Palaeogene, to extensive C3 grassy woodlands in the
early-middle Miocene, to the dramatic expansion of the tropical C4 grass savannas
and grasslands in the Pliocene, and the C3 steppe grasslands during the Pleistocene
glacial periods. Modern grasslands depend heavily on strongly seasonal climates, making
them sensitive to climate change.
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