Fifty-years of ecological change in Wisconsin forest communities

Jump to

Northern Wisconsin Upland Forests (orange)

Floodplain and Hardwood Swamp Forests (yellow)

click for a higher quality .pdf document

Many conservation efforts focus on maintaining species diversity. The conservation of large continuous blocks of forested habitat however, does not necessarily ensure that the species within the forests are protected from loss. In fact, forests are experiences decreases in native species, increases in exotic and invasive species, increases in homogenization, shifts in tree species competition, and the failure of some tree species to regenerate. Maintaining plant diversity is important because plant diversity supports the species in other trophic levels and contributes to the resilience and resistance's of the community to perturbations.

John T. Curtis, about 1957 in the greenhouse at Univeristy of Wisconsin, Madison

But changes in plant diversity remain poorly documents and understood. The best studies to demonstrate ecological change in forests use baseline data collect in the past. Without baseline data, losses in forest communities may go unnoticed. The majority of studies that incorporate baseline data demonstrate substantial species loss. These studies are often limited in scope because the baseline data exists only from one or a few sites. We are uniquely position to examine 50 years of change across hundreds of forested sites in Wisconsin because of a unique and valuable data set. John Curtis and his colleagues at the University of Wisconsin conducted vegetation surveys at 1,000 terrestrial sites in Wisconsin, much of this work was published in the Vegetation of Wisconsin (1950). Since 2000 we have been systematically recensusing these sites and to date have returned to about 250 sites.

Northern Wisconsin Upland Forests

Wiegmann, S.M. and D.M. Waller. 2006. Fifty years of change in northern upland forest understories: Identity and traits of "winners" and "loser" plant species. Biological Conservation. 129:109-123.

Taxus canadensis (Canada Yew) thriving in Apostle Islands, June 2005

Resurveys of plant communities provide valuable information on changes in species composition over time and clues about how species respond to environmental change. We report results from resurveys of 62 upland forest stands in northern Wisconsin and the western Upper Peninsula of Michigan fort survey around 1950. We identify plant species that have significantly increased or decreased in frequency in 1m2 quadrats (winners and losers) and evaluate the traits that distinguish these groups. Twenty-one winner species increased across sites (by25-400%), while 21 loser species decreased (by21-95%). Winners include both common native species and five invading exotics. Many are grasses or sedges are abiotically pollinated or dispersed (e.g., Carex, which increased 286%). Losers are mostly rarer native forbs that rely on animals for pollination and/or dispersal. Losers appear sensitive to desiccation, anthropogenic disturbance, and/or herbivory by white-tailed deer (e.g., Streptopus roseus, which decreased 73%). Declines in lower species are heterogeneous and stochastic across sites where as winners are more uniform and deterministic. Increases in common widespread native species account for most of the community change we observe across sites. The fact that winners resist or tolerate deer herbivory while many losers are sensitive to deer suggests that deer may be a key driver of the shifts we observe in these forests.

Rooney, T.P., S.M. Wiegmann, D.A. Rogers, and D.M. Waller. 2004. Biotic impoverishment and homogenization in unfragmented forest understory communities. Conservation Biology 18: 787-798.

Ecological change is often hard to document because of a lack of reliable baseline data. Several recent then-versus-now surveys of temperate forest and grassland communities demonstrate losses of local plant species, but most are based on data from a single site. We resurveyed understory communities in 62 upland forest stands in northern Wisconsin (U. S. A.) for which quantitative baseline data exist from 50 years ago. These stands are within a largely unfragmented region but vary in species composition and successional stage. We collected data on changes in (1) total and native species richness, (2) the ratio of exotic to native species, (3) the relative abundance of habitat generalists, and (4) community similarity among sites. We also compared how these rates of change varied over time. Over the past 50 years, native species density declined an average of 18.5% at the 20-m2 scale, whereas the ratio of exotic species to native species increased at 80% of all sites. Habitat generalists increased, and habitat specialists declined, accounting in part for an 8.7% rise in average similarity in species composition among sites. Most of these changes cannot be related to succession, habitat loss, or invasion by exotic species. Areas without deer hunting showed the greatest declines in native species density, with parks and research natural areas faring no better than unprotected stands. Animal-pollinated and animal-dispersed species also declined, particularly at unhunted sites. These results demonstrate the power of quantitative multi stand data for assessing ecological change and identify overabundant deer as a key driver of community change. Because maintaining forest habitats alone fails to preserve plant diversity at local scales, local biotic simplification seems likely to continue in the region unless active efforts are taken to protect diversity.

Rooney, T. P., D. A. Rogers, S. M. Wiegmann and D. M. Waller. 2004. "Monitoring non-native plant invasions over 50 years in Wisconsin forests." Weed Technology. 18: 1266-1268.

Josh Sulman, 2005 field crew botanist, sampling on Outer Island

Long-term ecological change is often hard to document without reliable baseline data. We obtained baseline data for and resurveyed 80 forest understory communities in Wisconsin first surveyed 50 yr ago. Sixty-two sites were in the forested region of northern Wisconsin, and 18 were in the agricultural region of southern Wisconsin. For 50 yr, the number of sites with normative plants increased from 4 to 62. Species richness in 1950 had little influence on normative species establishment by 2002, and normative species did not seem to influence rates of native species loss over time.

Return to top

Southern Wisconsin Upland Forests

Rogers, D. A., T. P. Rooney, D. Olson, D. M. Waller. 2008. Shifts in southern Wisconsin forest canopy and understory richness, composition and heterogeneity. Ecology 89(9). 2482-2492.

Although the replacement of oak (Quercus spp.) forests by more shade-tolerant species across Eastern North America is widely appreciated, little is known about its impact on understory species. We re-surveyed the under-and over-story species composition of 94 undeveloped stands in southern Wisconsin in 2002-2004 to assess shifts in canopy and understory richness, composition and heterogeneity relative to 50 years ago. Tree composition has shifted away from oaks (Quercus spp.) towards more mesic and shade-tolerant species (primarily Acer spp.). Most sites (80%) experienced declines in understory native species richness with mean species density declining 25% at the 1 m² scale and 23.1% at the 20 m² scale. Rates of native understory species loss are much greater in late-successional stands with conspicuous declines in remnant savanna species and those with narrow leaves. Initial overstory composition predicts changes in the understory better than changes in the overstory. Despite declines in absolute abundance, woody species have increased 15% in abundance relative to herbaceous species in the understory. Exotic species that occurred in 13 stands in 1949-1950 representing 5.5% of the flora now occur in 76 stands and account for 8.4% of the species present. Gains in exotic species richness and abundance are not closely linked to declines in native species richness or community heterogeneity. Although canopy succession has clearly influenced shifts in forest understory composition and diversity, these results suggest that understory dynamics are becoming decoupled from overstory dynamics as landscape effects start to play an increasing role.

Return to top

Southern Wisconsin Floodplain Forests

See Sarah Johnson's page for more information on this project.

See entire list of lab publications here

Other Wisconsin Plant Communities

While our lab deals primarily with forest communities, other researchers have studied 50 years of change in other Wisconsin communities originally studied by John Curtis and the Plant Ecology Laboratory:

Prairies Mark Leach and Tom Givnish

Dry Prairies Sarah Kraszewski

Northern Wet Mesic Forests Matt Bushman (UW-Stevens Point)

Cedar Glades Jason Mills and Tim Allen

Apostle Islands Forests Sarah Johnson and Erika Mudrak

Birds in S. Wisc Forests Anders Olson with Volker Radeloff

Beech Forests Elodie Seguin, Rachel Collins, David Rogers

Changes in woody species dynamics

Collins, R.J., D.A. Rogers, T.P. Rooney, and D.M. Waller. (2006) 50-years of change in forest tree species diversity and composition in Wisconsin: The role of native and exotic generalists. Ecology Society of America Annual Meeting 5-11 August. Memphis, TN

Many concerned with conserving forests emphasize retaining blocks of forest habitat. Often within intact forests, however, some tree species dramatically decline in abundance because their seedlings fail to recruit into adults. Such losses make clear that to protect forest diversity, we must identify patterns of forest change, even if patterns take decades to detect. Here, we explore 50-years of forest community change in WI. Our goals are to examine changes in woody species diversity and abundance, and to test the hypotheses: (1) Some specialist or sensitive species (e.g., Quercus and Taxus) have declined in abundance while a few generalist species (e.g., Acer rubrum) have increased in abundance, and (2) declines in woody species richness are associated with increases in weedy species abundance. In 1946-1956, John Curtis and colleagues censused the vegetation at 1,000 terrestrial sites in WI and we recensused 240 of these sites in 2000-2005 using more intense sampling. Species presence/absence data were collected in the understory (<0.5 tall) in 20 quadrats (1m2; 1950’s), and in 60-120 quadrats (2000’s). Canopy tree data were collected using a point quarter, random pairs, or fixed area plots at each site. We adjusted for sampling differences in our analyses. Site-level woody species richness declined by 2.23±0.52 species (mean±] SE; P< 0.005; N =60). This loss was due to declines in tree seedling richness (P<0.0001; N=60). Shrub richness did not significantly change (P>0.5; N=48) and vines were too rare to matter (1 or 2 occurrences at 5/60 sites). Interestingly, at the quadrat scale, richness increased (0.35±0.07 species/quadrat; P< 0.005; N=60). This result suggests that quadrat-level increases may be due to local increases in weedy species abundance, which contributed to site-level declines of rare species. Together with other results, we find that forests homogenization was due to increases in native and exotic weedy species and decreases in endemic species.

Funding for the above work was provided by
NSF 99-74041, 023633
USDA NRI 2003 02472

Return to top

____________________________________________________________________________