Small-area coexistence of vernal forest herbs: does functional similarity of plants matter? [North Central States, USA]

Deciduous forest herb communities in early spring contain spatially overlapping populations with seasonal differences in shoot persistence and function. Shoots of most species expand simultaneously and occupy the same layer at maturity, i.e., leaves are all about the same height. Though annual shoot growth is seasonally truncated, individuals of most species may grow for many years in relatively undisturbed sites. These observations suggested that interference between plants, particularly between long-lived, phenologically similar taxa (e.g., perennials with spring-ephemeral shoots), could be important in influencing community composition in places where gross disturbance of surface soil has not recently occurred. However, an extensive study of small-area coexistence of early-spring herbs in old, relatively undisturbed forest fragments yielded little evidence of interference between any combination of taxa, including taxa that are very similar with respect to phenological function. Rather, the presence and amounts of plants (cover) appeared to be distributed independently within stands at a scale of observation as small as 10 x 20 cm (determined to be the smallest areas that, on average, hold at least 2 species of mature vascular plants). This was true whether plants were classified as species, genera, or as phenological growth forms. The fact that plants also appeared to be independent at a somewhat larger scale, 50 x 100 cm (a unit sufficiently large that most plants are not neighbors but small enough that the same environment is shared) indicates that the fine-grained (10 x 20 cm) correlations were probably not biased by systematic differences in species response to environmental variation within stands (sample areas, in fact, were preselected to avoid obvious environmental heterogeneity).