ancestral ecology of angiosperms: emerging perspectives from extant basal lineages

The early ecophysiological trajectories of angiosperm evolution remain uncertain. This hinders resolution of the possible ecological and physiological catalysts that set the stage for the emergence of our modern flora. One approach for generating hypotheses on the early ecology of angiosperms is to consider the distribution of functional traits among lineages that diverge near the root of the extant angiosperm phylogenetic tree. Using recent phylogenetic results to choose lineages for comparative work, we review and expand on previous studies that examine the ecology and physiology of basal angiosperm lineages. Phylogenetic mapping of functional traits reveals that the basal lineages, Amborella, Austrobaileyales, and some Chloranthaceae, share ecological and physiological traits linked to shady, disturbed, and possibly wet habitats. Traits include low and easily light-saturated photosynthetic rates, absence of leaf palisade cells, guttation resulting from root pressure, as well as small seed sizes. Some Chloranthaceae, such as Ascarina and Hedyosmum, however, recruit seedlings in forest light gaps and other sunny, disturbed habitats. In contrast, the early-diverging water lily clade (Nymphaeales) possesses ecophysiological features linked to aquatic, often sunny habitats, such as absence of a vascular cambium and floating leaves with high photosynthetic rates and palisade mesophyll. On the basis of the phylogenetic distribution of functional traits, we hypothesize that the earliest angiosperms were woody and grew in dimly lit, disturbed forest understory habitats and/or shady stream-side settings. The significance of the hypothesis that angiosperms evolved in shady, disturbed habitats for the origin of their distinctive features, e.g., xylem vessels and carpels, and for patterns of their early diversification is discussed. Finally, we examine how the emerging ecological picture of early angiosperms drawn from living basal lineages can be tested with the fossil record.