Philip P. Johnson, Scott Kobal, Wendy Leonard, and Emily S. Minor, July 2023

RESEARCH BRIEF 47

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MANAGEMENT IMPLICATIONS

Richness of native herbaceous species and the number of species gained in suburban forests are greater in burned sites compared to unburned sites.

The amount of forested habitat (but not its configuration) surrounding  plots was positively related to current herbaceous richness and species gains over 30 years.

Results suggest that white-tailed deer (and their management) may play a large role in suburban forest herbaceous richness.

In this study, authors used existing data to examine thirty years of change in the herbaceous plant community of suburban forest preserves in DuPage County, Illinois, part of the Chicago metropolitan area. They investigated species richness, species gains, and species losses in relation to previous prescribed burns and analyzed the amount and configuration of forested habitat in the surrounding landscape. In addition, they examined the spatial scales at which the landscape factors have the greatest effect.

The authors used existing data collected between 1986 and 2016 by the Forest Preserve District of DuPage County at approximately 5-year intervals from 34 permanently marked plots in 16 forest preserves. Of the 34 plots, the authors classified 28 as burned if they had burned at least once (burn status); most were burned every 3–5 years. The authors analyzed the suite of native herbs (forbs, grasses, ferns, sedges, and non-woody vines) identified in each survey to examine the effects of time, burn status, and their interaction on herb richness. They also compared the species list of the first and most recent survey of each plot to determine species gained and lost over 30 years.

The herbaceous understory of a fire-managed site in DuPage County, IL, USA. (Photo: Philip Johnson)

Surrounding forest habitat was extracted from an intersection of the National Land Cover Database forest classes with parcels in the Chicago Metropolitan Agency for Planning 2013 land use inventory dataset classified as conservation areas. This excluded forested areas not considered natural areas, such as residential areas with substantial canopy cover. The authors calculated the amount and configuration (degree of fragmentation) of these habitat areas surrounding the plots at 37 nested spatial scales from 0.3 to 4 km (~0.2 – 2.5 miles) using 0.1 km (~0.06 mile) increments.

Analyses focused on 1) whether richness changed over time and if change was associated with burn status; 2) whether current richness or species turnover are best explained by burn status, the amount of surrounding habitat, habitat configuration, or a combination of these; and 3) at which spatial scale habitat amount, configuration, or both, affect current species richness or turnover over 30 years.

(A) Current native herb richness in long-term monitoring sites managed with prescribed burns or not burned over 30 years in suburban forests of DuPage County. (B) Number of native herb species gained over ~30 years in long-term monitoring sites that were burned and unburned. (C) Number of native herb species lost over ~30 years in burned and unburned sites. Brackets and p-values refer to prescribed burn management having an effect on the observed response. Prescribed burn management had no effect on the number of native herb species lost over ~30 years.

Native herb richness increased over time in burned plots but not in unburned plots. A mixture of 93 shade tolerant and intolerant species were gained exclusively in burned sites, and two species were gained solely in unburned sites. Species richness and gains were greater in burned plots compared to unburned plots, but species losses over 30 years did not differ (see figure below). Species richness and gains, but not species losses, were positively related to the amount of forest habitat within 0.9 km (~0.6 mile) of a plot, but the configuration of forest habitat was not a predictor of any response variable at any scale.

The native species gained at burned sites could result from increased microsite heterogeneity, which is important for understory diversity, but the authors caution that increased native herb richness should not be interpreted as restoration. These forests, like others in suburban and urban areas, contain many non-native plant species, especially shrubs. Additionally, more research is needed to determine whether increased richness indicates dispersal from offsite sources, increased detection of native herbs, or release from the existing seed bank.

The authors suggest that seed dispersal by white-tailed deer may explain the observed spatial scale of species richness and gains in plots. Particular species gained in the plots such as Canada goldenrod (Solidago canadensis), and several sedges (Carex spp.) are known to be dispersed by white-tailed deer; however, deer herbivory can suppress both woody and herbaceous species and spread non-native and invasive species. Any potentially positive effect of deer on herb richness in suburban forests likely depends on intensive deer management.

The authors note the challenges of analyzing data not collected in a controlled experiment, but they point out that similar data exist across the US and are currently underutilized. Funding shortages typically limit agency monitoring activities, posing difficulties in analysis of long-term effects. This is especially true for understory herbs, which were not included in the public land survey and lack that historic baseline. It may be possible to supplement existing datasets with free, publicly collected, citizen science data.