Annual rings in native and introduced forbs of lower Michigan, U.S.A.

Abstract

Almost nothing is known about the presence of annual growth rings in North American perennial herbs and, correspondingly, the age of forbs is also largely unknown. In this study we sampled established individuals of dicotyledonous perennial herbs (60 species) in different habitats in lower Michigan (U.S.A.) and analyzed the main roots for the presence of growth rings in the secondary xylem. Two thirds of the species showed clearly or relatively clearly demarcated growth rings in the root xylem that are most likely annual rings. The anatomical patterns contributing to the delineation of growth rings in the root xylem differed widely, particularly among plant families. These included variations in vessel diameter and vessel density and differences in the presence and extent of tangential bands of fibres or lignified parenchyma, respectively. Among introduced species and in disturbed habitats (meadows, ruderal sites), clearly demarcated growth rings were found more frequently than among native species and in semi-natural and natural habitats (old fields, prairie remnants, open sandy vegetation). While most of the sampled plants were young (2 to 3 years old), the age distribution in the whole sample was relatively wide (2- to 16-year-old plants). Older individuals ([Formula: see text]6 years old) were mainly found in semi-natural and natural habitats. Results indicate that the analysis of annual rings in the root xylem of perennial herbs (herb-chronology) may be widely used as a reconstructive method in plant ecology over extensive geographic areas with seasonal temperate climate. Research into plant invasions may particularly profit from a high proportion of forb species in disturbed habitats that show clearly demarcated annual rings.Key words: age determination, anatomical patterns, annual rings, perennial herbs, population ecology, secondary root xylem.