Phylogenetic constraint, absent life forms, and preadapted alien plants: A prescription for biological invasions

Widely separated plant communities supported by similar physical environments can display much similarity in their life form spectra even though these communities are often floristically unrelated. Such structural similarity reveals the strength of the physical environment in natural selection, particularly for the height, position, and duration of the perennating bud-the underlying premise of Raunkiaer's life form system. But similarities among communities in life form spectra can be limited in part by phylogenetic constraints within the native flora. Phylogenetic constraint has been forcefully and repeatedly demonstrated with the arrival into new ranges of species with novel life forms that have become invasive (i. e., alien species with life forms novel to a recipient community have often become persistent, prolific, and even dominant). Communities invaded by species with a heretofore unrepresented life form are exceptionally diverse, both geographically and environmentally, and include coastal foredunes, intertidal mud flats, riparian sites, temperate grasslands, temperate marshes, and subtropical and temperate forests. The frequency with which alien trees (Acacia nilotica, Myrica pigra, Pinus taeda, Sapium sebiferum) have invaded native grasslands and other treeless communities calls into question the degree to which intrinsic physical limitations compared with phylogenetic constraints have influenced the current extent of these communities worldwide. In effect, invasions by species representing novel life forms in their new ranges have become ongoing, unintentional experiments for detecting phylogenetic constraint in any flora. Identification of native communities at risk of plant invasion may improve through closer examination of their life form spectra for opportunities for the establishment of previously absent life forms.