New published research: Duff removal may speed restoration in fire-suppressed wetlands

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A small pine flatwoods wetland from the Florida Panhandle – Houston Chandler

Effective management of natural systems is a challenging endeavor, especially given the almost ubiquitous landscape changes occurring at an often alarming rate in ecosystems around the world. In the southeastern U.S., managing the small fragments of remaining longleaf pine forest requires extensive efforts to simulate natural disturbances that historically occurred through regular wildfires. Managers most frequently manage longleaf pine systems with the application of prescribed fire, but there are a variety of other management techniques that can also be applied in these systems. For example, one of the main responses of vegetation communities to an absence of fire is the proliferation of woody vegetation (see below photos). Woody vegetation dramatically increases the canopy cover, shading out herbaceous vegetation on the forest floor and reducing the available habitat for many species. Such woody growth can be removed using mechanical and herbicide treatments, which can rapidly improve vegetation quality. However, that is often only part of the work needed to restore an ecosystem after decades of neglect and poor management.

A longleaf pine embedded wetland with herbaceous vegetation on the wetland edge (fire maintained) and woody vegetation in the interior of the wetland (fire suppressed). – Houston Chandler
A small patch of relatively open wetland habitat surrounded by impenetrable walls of woody vegetation. – Houston Chandler

In wetlands embedded within longleaf pine forests, a secondary effect of dense woody vegetation is the creation of a thick organic duff layer in wetland basins. This accumulation of leaves and other organic material (just like a flowerbed) decomposes slowly and prevents herbaceous vegetation from sprouting even if the shrub layer is removed and a seed bank is present. Furthermore, duff layers can slow or prevent prescribes fires burning through wetland basins (unless basins are burned under the driest conditions). Managers are also often reluctant to attempt to remove duff layers using fire because fires can smolder in duff layers for long periods, producing large amounts of smoke that can have negative consequences. Overall, this highlights how interconnected habitat changes can make complete ecosystem restoration a challenging endeavor.

Example of a wetland with a thick duff layer. Dead vegetation has been manually treated to reduce canopy cover within the wetland. – Houston Chandler

In an attempt to better understand the effects of duff layer establishment on vegetation sprouting and regrowth, we performed a simple experiment in several wetlands on Eglin Air Force Base, working with collaborators at Virginia Tech and the Florida Fish and Wildlife Conservation Commission. Across two wetlands, we established 40 1 x 1 m experimental plots (20 per wetland). In half of the plots, we raked the duff layer down to mineral soil (a depth of 2–12 cm). Plots were positioned such that each raked plot had a directly adjacent unraked plot. After approximately seven months, we returned to the plots and counted the vegetation that had sprouted. We quantified the total number of individual plants, identified plants to the lowest possible taxonomic level, and broadly grouped plants into a ‘woody’ or ‘herbaceous’ category.

Example of raked (left) and unraked (right) plots established in wetlands. – Brandon Rincon

After seven months, we found a wide variety of plant species growing within our experimental plots (see the full publication for a complete list of plants). The raking treatment had a clear and significant effect on the number of plant stems sprouting within the plots. Overall, there were almost 30 times as many stems in raked plots when compared to unraked plots, and there was significantly more herbaceous vegetation growing in raked plots. Furthermore, we found that species composition was different, both across treatments and between the two wetlands included in the study.

The number of woody and herbaceous stems in raked and unraked plots across two wetlands. – George Brooks
Visualization (non-metric multidimensional scaling) of differences in plant community composition across treatments and wetlands. Each point represents a sampling plot, and points that are farther apart have plant community compositions that are more different than points closer together. – George Brooks

This small-scale experiment has potential implications for how we think about and manage degraded wetlands in longleaf pine systems. Managers have often been frustrated by the slow response of herbaceous vegetation to wetland restoration efforts. In severely degraded wetlands, removal of the shrub layer may only be the beginning of needed management actions to restore habitat on a timeframe relevant to most restoration projects (Martin and Kirkman 2009; Gorman et al. 2013). There are two important additional considerations for this work. First, like many restoration activities, removal of the duff layer is labor intensive, and additional efforts should be made to find the most efficient techniques. Second, long-term maintenance of restored sites is a necessity. Managers should have a clear plan for maintaining vegetation communities in restored wetlands after initial treatments are completed (whether through prescribed fire, additional vegetation treatment, or a combination of the two). Ultimately, restoration of degraded ecosystems is a challenging and long-term commitment, and this work highlights the importance of applying a holistic approach that uses multiple techniques to address various aspects of habitat improvement.

This work was published in the journal Wetlands. The full publication can be found here and on our publication page.

 

Literature Cited

Gorman, T.A., C.A. Haas, and J.G. Himes. 2013. Evaluating methods to restore amphibian habitat in fire-suppressed pine flatwoods wetlands. Fire Ecology 9:96–109.

Martin, K.L., and L.K. Kirkman. 2009. Management of ecological thresholds to re-establish disturbance‐maintained herbaceous wetlands of the south‐eastern USA. Journal of Applied Ecology 46:906–914.