The Ecology And Site Relationships Of Shrubs In Southeastern Michigan (forest, Understory).

Little information is available on the ecology and site relationships of shrubs except in areas where they dominate the landscape. The available information is typically based on subjective field observations. The major objectives of this study were to (1) objectively determine the site relationships of shrubs in southeastern Michigan relative to soil moisture, soil nutrients, and photosynthetically active radiation (PAR), (2) assess the site indicator value of individual shrubs, and (3) examine differences and similarities in site relationships among species. Relatively undisturbed forest ecosystems were identified and sampled throughout southeastern Michigan. Coverage data on vegetation, including 62 shrub species, were collected using belt transects. Physiographic variables of aspect, slope, and topographic position were recorded, and the soil was described and sampled. PAR was measured along transects oriented parallel to the vegetation transects. Soil was analyzed in the laboratory for particle-size distribution, pH, % organic matter, total N, and concentration of extractable Ca, K, Mg, and P. Ecosystems were classified relative to moisture, nutrient, and PAR gradients and grouped into 1 of 3 moisture classes and 1 of 5 nutrient classes. The gradients and classifications were corroborated by using the results of principal components and canonical variates analyses. The Kruskal-Wallis test, Mann-Whitney U statistic, and tests based on the binomial distribution were used to classify ecosystems into 1 of 5 groups along the PAR gradient. The same nonparametric tests were used to examine the distribution of species relative to the three gradients. Rank correlation was used to test for similarities and dissimilarities among species distributions. Twenty-five shrub species had a statistically significant relationship relative to at least one gradient. Gaylussacia baccata and Rubus flagellaris are examples of species indicating dry ecosystems. They also indicate high PAR levels. Viburnum acerifolium and Euonymus obovata were most abundant in dry-mesic to mesic ecosystems. Viburnum acerifolium was indicative of intermediate PAR levels, whereas Euonymus obovata and Lindera benzoin were indicative of the deepest shade. Species such as Gaylussacia baccata and Juniperus communis were indicative of very infertile ecosystems, whereas Cornus alternifolia and E. obovata indicated fertile conditions. Prunus virginiana was ubiquitous but had its highest coverage in dry-mesic fertile ecosystems with high PAR levels. Finally, species with similar distributions relative to the gradients were used to form 6 ecological shrub groups.