The Gramineae family, commonly known as grasses, represents a phenomenal success story in the plant kingdom. Their near-ubiquitous presence across the globe, dominating diverse ecosystems and boasting remarkable species richness, underscores their ecological prowess. This success can be attributed to their exceptional ability to colonize new territories, persist in challenging environments, and even transform landscapes – a concept we term the “Viking syndrome.” This remarkable capacity stems from a strategic combination of traits that facilitate long-distance dispersal, efficient establishment, ecological adaptability, resilience to disturbances, and the power to reshape environments through fire regimes and interactions with mammalian herbivores.
Several functional traits underpin the dispersal capabilities of Gramineae Family Grasses. Wind dispersal (anemochory), animal dispersal via fur (epizoochory), and animal dispersal via ingestion (endozoochory) are key mechanisms, often facilitated by the spikelet structure, particularly the awned lemma, which acts as an effective dispersal unit. The secret to rapid establishment may lie in the precocious embryo and substantial starch reserves within grass seeds. These resources likely fuel the exceptionally short generation times observed in many grass species, allowing for quick colonization and adaptation. Furthermore, genetic bottlenecks that often arise post-establishment are potentially mitigated by wind pollination, promoting gene flow, and the widespread occurrence of polyploidy, coupled with gametic self-incompatibility, enhancing genetic diversity and adaptability within grass populations.
The ecological competitiveness of Gramineae family grasses is evident in their dominance across a vast spectrum of environmental conditions tolerated by flowering plants. This adaptability is enhanced by the evolution of both C3 and C4 photosynthetic pathways, enabling grasses to thrive in varying light and temperature regimes. Well-developed frost tolerance in numerous grass lineages further expands their geographical range, while a sympodial growth form has allowed for the diversification into both annual and perennial life strategies, optimizing survival in diverse habitats. Crucially, the absence of significant investment in woody tissue (except in bamboos) and the presence of persistent buds located at or below ground level provide remarkable resilience to repeated defoliation events, whether caused by fire, frost, drought, or grazing herbivores.
The dominance of Gramineae family grasses is further amplified by their ability to modify their environment through interactions with herbivores and fire. Grasslands often support high biomass and diversity of mammalian herbivores due to the palatable and productive nature of many grasses. Conversely, grasses themselves influence fire regimes. Many species possess a suite of architectural and functional traits that promote frequent fires, including a tufted growth form that allows for air circulation and rapid drying, and the presence of tannin-like substances in their leaves, which slow down decomposition and contribute to fuel load. These fire feedbacks often reinforce grass dominance, leading to the creation and maintenance of open ecosystems like savannas and grasslands.
Tracing the evolutionary history of these traits across the phylogeny of Poales, the order encompassing grasses and their relatives, reveals a gradual accumulation of advantageous characteristics since the emergence of monocots in the mid-Cretaceous period. While the sympodial growth form is an ancestral monocot trait, the tillering habit, resulting in the characteristic tufted growth of many grasses, likely evolved within the Gramineae family itself. Similarly, the ovary structure and spikelet dispersal units with awned lemmas are innovations that arose within the grass lineage. Frost tolerance and C4 photosynthesis appeared later in grass evolution, emerging in the late Palaeogene. The production of tannins, associated with fire-prone savannas, was probably among the most recent significant trait to evolve in grasses.
Palaeobotanical evidence supports a multi-phase narrative of grass success. From their origin in the late Cretaceous, grasses initially formed occasional tropical grassland patches in the later Palaeogene. The early to middle Miocene saw the expansion of extensive C3 grassy woodlands, followed by the dramatic proliferation of tropical C4 grass savannas and grasslands in the Pliocene. C3 steppe grasslands further expanded during the Pleistocene glacial periods. Today’s grasslands are heavily reliant on strongly seasonal climates, making these vital ecosystems particularly vulnerable to the impacts of ongoing climate change. Understanding the evolutionary and ecological drivers of Gramineae family grass success is crucial for predicting and mitigating the effects of global environmental change on these critical biomes.
