Changes in urban forest plant communities after three decades of fragmentation

Background/Question/Methods

Use of historical data can reveal ecological changes that occur on a timescale longer than most researchers are able to devote to an average study.  This project makes use of historical data to track long term changes occurring in forest communities in an urban setting.  While these urban forest fragments have remained uncleared, they are still perturbed by forces that are common in human encroached habitats.  Changes taking place in these communities may not be evident until examined on a broader time scale as the changes are part of an "extinction debt" paid after initial alteration of the habitat (Tilman 1994).  We located 10 forest fragments in the Milwaukee urban area that were surveyed in 1973 by Hoehne for densities and frequencies of plant species in the canopy, shrub layer, and ground layer.  We resurveyed sites by replicating the methods of the previous study so data could be compared directly. We examined changes in Shannon diversity indices that have occurred over the three decade time span as well as shifts in community composition.

Results/Conclusions

Data collected from 7 sites show increases of Acer Sacharrum in the understory, while Fagus grandifolia, Fraxinus spps., Prunus virginiana, Quercus rubra, and Tilia americana have declined at most sites.  Canopy layer species compositions examined at 10 sites were more stable in comparison to the understory over the three decade time period.  Herbaceous plants were more variable in frequency changes across the 10 sites surveyed.  Overall, declines were seen for Actaea pachypoda, Allium tricoccum, Podophyllum peltatum, and Trillium grandiflorum. While Arisaema triphyllum and Circaea quadrisulcata increased in frequency at most sites.  Species richnesses detected  in the present survey were similar to the past survey for some sites, while other sites have suffered severe losses of species. Changes in Shannon diversity indices for the groundlayer also varied across sites, with some sites increasing in diversity while others became more homogenous.  Increases in diversity may be attributed to invasion by exotic species at some sites.  All sites had increases in exotic species, with some having increases greater than 10 percent.  Field surveys of additional sites will allow better detection of overall trends for particular species across sites and identify potential winners and losers in urban forest fragments.