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 Effects of Mechanical Treatments on Florida Scrub
by Marcia Rickey

[With permission, from Saving Our Scrub, Volume 3, Number 2, December 2004; a newsletter from the U.S. Fish and Wildlife Service's  North Florida Field Office.]  --  PDF file

Most Saving Our Scrub readers are by now familiar with the many benefits of prescribed fire to Florida scrub. Most land managers are familiar with the difficulties of prescribing fire, such as changing weather, timing, or avoiding nearby property. Homeowners can be resistant to prescribed burns adjacent to their property for fear of escape. However, without fire, fuels build-up, posing increasing threats to people and property. Mechanical treatments such as logging, mowing and roller chopping are being used across Florida as alternatives to burning or in combination with burning. Few studies have been done in Florida scrub (and fewer on the Lake Wales Ridge) to determine how these mechanical treatments affect fire intensity, vegetation dynamics, and population viability of endangered species.

Despite the increasing application of mechanical treatments, we still don’t have a good answer to the key question: Are mechanical treatments a good way to get the ecological benefits of fire without the risks, or are will they cause unintended harmful effects to vegetation and rare species?

The Archbold Biological Station Plant Lab has three ongoing studies on fire and mechanical treatments: "Log and Burn," "Mow and Burn," and "Saw and Burn." The Log and Burn project takes place at the Lake Wales Ridge Forest, Arbuckle Tract. The Mow and Burn project occurs on two FFWCC sites, Lake Placid and Lake Apthorpe Scrubs. The Saw and Burn project is being done at the Lake Wales Ridge National Wildlife Refuge, Carter Creek South Tract. These projects share the common goal of evaluating the efficacy of mechanical treatments as either surrogates or pre-treatments for fire. All research sites support many listed endemic plant species.

Our approach is to combine mechanical treatment and fire in four combinations, collect detailed pre-treatment data and contrast treatment effects for 5 years. Our emphasis is on evaluating each mechanical treatment, both as surrogate and pre-treatment, in relation to fire-only.

Log and Burn
In oak-rosemary scrub we placed randomly located research plots in each of four treatment blocks (log only, burn only, log and burn, and control). We have completed pre-treatment and 1 and 2 years post-treatment sampling. All treatments were successful in removing the tree and subcanopy layers and reducing shrub cover. Bare sand levels were quite high, especially in logged areas used for skidding. Initial results suggest that logging treatments created initially higher levels of some native weedy species than the burn-only treatment. There has been little recruitment of listed species, perhaps due to dry weather in 1999-2001.

Mow and Burn
This project uses four experimental treatments: control, mow only, burn only, and mow and burn. The sites were dominated by oak-rosemary scrub and were sampled with belt transects within 1 ha treatment areas. Lake Apthorpe was mowed and burned in the same year (1999); however Lake Placid was mowed in 2000 but could not be burned until 2001 because of weather conditions. We have completed pre-treatment and 1 and 2 years post-treatment sampling. After two years, we have seen strong treatment effects at both sites. All treatments were effective in reducing shrub heights relative to the control. Increases in bare sand, important to many listed species, occurred only in the burn only treatment at Lake Placid, where burning took place a year after mowing. Bare sand increased in the burn only and mow and burn at Lake Apthorpe. Densities of Paronychia chartacea increased only in the burn-only treatment, although the increase was highly variable.

Saw and Burn
Three treatments were applied (burn only, sub-canopy felling followed by burning [saw and burn], and a control), each with twenty-four replicates. We have sampled the plots annually since 2001. Eleven of 24 community plots within the burn-only treatment that did not burn were excluded from post burn surveys. Data from this project is currently being analyzed for differences between treatments over time.

Conclusions
Our preliminary data suggest that mechanical treatments alone do not have the same effects as fire and should be avoided. Mechanical treatments used as a pre-treatment or in addition to fire can mimic some of the effects of fire, especially if burning closely follows the mechanical treatment. All projects will be re-sampled 5 years after treatment and at that time we will be able to make strong conclusions about using mechanical treatments. Managers must know if mechanical treatments alone can mimic the effects of burning for scrub plants, or if they must be done in tandem with burning, or if the effects of mechanical treatments with or without burning will be different than burning alone.

Thanks to the DOF, FFWCC, TNC, and the USFWS for cooperation on these projects.

Marcia Rickey is a Research Assistant in Archbold's Plant Ecology Lab.

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Rickey, M. 14 December 2004. ©  Archbold Biological Station, December 2004 Webmaster, Fred E. Lohrer  send e-mail