A phylogenetic perspective on the ecology, morphology and physiology of grasses

Grasslands cover a third of the Earth's surface, and have great economic and ecological importance. The finding of multiple independent origins of C4 photosynthesis in grasses has motivated an integration of phylogeny, biogeography, ecology and physiology. Phylogenetic niche conservatism (PNC) is the tendency of closely related species to share similarities in ecological niches, morphological and physiological traits over time. Contrasting relationships between the biogeographical patterns of different grass lineages and rainfall gradients provided early evidence of PNC, but these were confounded with photosynthetic type. Therefore this thesis compared phylogeny and photosynthetic type as explanations of grass ecology, and tested how PNC changes at different spatial scales by investigating habitat water and morphological and physiological traits. Globally, divergence of the two largest C4 grass subfamilies was responsible for phylogenetic signals in both morphological and habitat traits, indicating PNC. Regionally, the Inner Mongolian steppe provided strong climatic gradients to study how environmental filtering and niche partitioning affected community assembly. PNC was supported in this species assembly in both morphological and habitat traits, but was weaker at the habitat scale. A field study along habitat gradients found that niche partitioning existed both among and within communities and the wide niche breadth of most species weakened phylogenetic signals. Morphologically, some traits were more conservative than others, especially stomatal traits. Ecophysiological traits were neither phylogenetically nor habitat dependent. Greenhouse comparison of C4 grasses testified that phylogeny interacted with both C4 subtype and habitat type. Morphologically, only stomatal traits showed phylogenetic signals. Ecophysiologically, leaf hydraulics showed overall associations with leaf structure, and NAD-me species in Chloridoideae had drought tolerant leaf water release characteristics. In conclusion, it is inappropriate to consider all C4 grasses as a single functional type at the global and regional scales since phylogeny explains distribution, morphological and habitat traits. At the community and habitat levels, PNC becomes weaker when phylogeny interacts with both photosynthetic type and environmental factor.EThOS - Electronic Theses Online ServiceGBUnited Kingdo