The dynamics of forest understories in eastern North America have only been described in generalities, for early successional stages and in response to a few types of disturbance. A much deeper and more complete picture of the natural changes that occur as forests mature is needed to address conservation concerns, to address impacts of novel agents (e.g., exotic earthworms) and to pinpoint human induced effects. In the early 1990’s the Chequamegon National Forest, northern WI, established a series of monitoring locations to address these needs and others. Approximately one-third of these forests that had been manipulated since 1980, and most are 60-90 yr old. A subset of these were re-measured in 1996, 2003 and 2008. I use these data, primarily the first and last inventory periods, to assess the compositional stability of the understory layer in several common forest types. The two primary research questions are: 1) How much change occurs over 17 yr in mature, upland forests, and 2) how has forest management affected this dynamic? A variety of possible ‘drivers’ were assessed to explain differences. The methods included fixed area plots for snags (1991), saplings and seedlings, the herb layer, deer browsing and earthworm biomass. Variable-radius sampling was used for the overstory and snags (2008). Coarse woody debris and canopy closure were sampled by line transect. The primary analytical approaches were paired, 2-sample tests, regression and ordination.
Results/Conclusions
The results included expected as well as novel changes. Average cover fluctuated widely in the “Control” [unmanaged] forests, but less so in the Managed forests. The difference between 1991 and 2008 was highly significant (p=.004). The average richness of the forests increased by more than 5 species over time (p=.004), despite a steadily increasing amount of overstory basal area. Contrary to expectations, the average cover of all Carex spp. and the cover of exotics decreased more than 50% (p < .01). We found no relationship between a deer browsing index and understory richness or cover. We noted a tentative indication of a negative effect of earthworm abundance on understory richness and Carex cover. Ordinations indicated three important trends: 1) Control and Managed forests have converged over the 17-yr period, 2) Understory assemblages were relatively stable from 1991-2008, and 3) Distinct groupings based on overstory composition were evident at both periods. Further examination of the understory assemblage will be conducted with ‘mean rank shift’ and ‘proportional persistence’ analyses.
