A landscape-level assessment of understory diversity in upland forests of North-Central Wisconsin, USA

The study measured landscape level diversity of the understory plants of ma ture, upland forests in north-central Wisconsin USA. Habitat types were use d to segregate the landscape along a moisture-nutrient gradient. Forty site s that had closed canopies, had been undisturbed for at least 20 years, and were at least 8 ha in size were used. The percent cover of groundlayer spe cies was ocularly estimated in 12-18 randomly located, one meter square plo ts in June and August, 1995. Shrub cover was estimated by the line intercep t method. Alpha, beta and gamma diversity were determined for early and lat e summer periods separately. Gamma diversity was quantified using a new met hod, affinity analysis, which generates a list of modal and outlier sites a nd calculates mosaic diversity, a measure of landscape complexity. Generall y, communities in the middle of the moisture-nutrient gradient were modal, whereas those at the mesic end of the gradient were outlier. Mosaic diversi ty values were very similar for early summer and late summer (2.88 +/-0.04, 2.95 +/-0.03, respectively), but was much higher for both periods combined (3.95 +/-0.07). Whittaker's Index (beta diversity) revealed varying rates of species turnover along presumed moisture and nutrient gradients, whereas species densities and richness were relatively constant among habitat type s. A one-way analysis of variance of Shannon-Weaver values found no signifi cant differences among habitat types (p greater than or equal to0.05). Regi onal diversity mainly resulted from high beta values which appears to be pr imarily a function of the moisture gradient. The other factors influencing compositional differences among sites are variation in site history, especi ally disturbance, with niche partitioning and differences in seed dispersal capacity having a minor influence. The affinity analysis method indicated that sampling once per season is inadequate, and that many types of sites a re modal. This method for estimating gamma (landscape) diversity shows cons iderable promise, but information on the processes that produce outlier sit es is needed to fully understand and use the results of this method.