Not So Barren
A high-pitched trill penetrates the sky above the Silas Little Experimental Forest in Burlington County, N.J. These “bird calls” are the transmitter signals of a sonic detecting and ranging system, an instrument mounted on a tower used to measure wind speed and detect excessive turbulence up to 600 meters high.
This instrument is part of a cooperative project between the USDA Forest Service, NASA, the New Jersey Forest Fire Service, and Rutgers University funded through the National Fire Plan to provide real-time fire weather information, produce maps of forest structure and fuel loading, and enhance fire danger predictions for the Pinelands in South New Jersey.
Perhaps misnamed the “Pine Barrens,” these forests comprise 1.1 million acres of moderately productive pine and oak trees growing on sandy, nutrient-poor soils. Interestingly, wood growth, the traditional measure of forest productivity, is low relative to the production of leaves and twigs, the fine fuels that can carry a fire. The dominant pitch pine and shortleaf pine can produce new branches from mainstems near the ground following fire, termed epicormic buds, an ability shared by a small minority of the 36 species of pines in North America. Most of the oaks and shrubs also readily resprout following fire. The result? Rapid fuel accumulation over coarse-grained, well-drained soils — and one of the most flammable forests in the eastern United States.
Since at least colonial times, large wildfires have occurred in the Pinelands. Today, margins of the Pinelands are now densely populated, key transportation corridors occur to the east and west, and adjacent urban areas have significant air quality problems, making wildfires a major concern. To address the needs of the New Jersey Forest Fire Service and other local fire managers, the Forest Service has embarked on a multi-disciplinary research program. To date, we have:
- Installed a network of six new fire weather towers;
- Mapped forest structure, fuel loading, and quantified fuel reduction during prescribed fire treatments;
- Developed a new fuel moisture index based on measurements of forest energy balance; and
- Evaluated predictions of a fire weather model run at high resolution for the Pinelands.
We are integrating these data in GIS-based fire risk maps that are specific for the Pinelands while using a framework that can be adapted to many forests on the Atlantic Coastal Plain. This approach focuses on providing fire managers with accurate estimates of fuel loadings and near — real-time predictions of fuel moisture contents.
The six new fire weather stations in the Pinelands are equipped with a full suite of meteorological sensors to measure fire weather variables. Near — real-time data is available to fire managers over the Internet through the Office of the New Jersey State Climatologist at http://climate.rutgers.edu/stateclim/. During wildfire events and large prescribed fires, we use a mobile trailer outfitted with a second SODAR to measure fire weather variables and atmospheric turbulence.
We also are using vegetation maps derived from remote sensing and light detecting and ranging measurements of stand height and canopy density made by helicopter. This is being done in collaboration with Dr. Ross Nelson of NASA, Forest Inventory and Analysis census data, fuel photoseries plots, and our intensive forest inventories around the towers to obtain accurate estimates of fuel loads for approximately 500,000 acres of the Pinelands in public ownership.
At the landscape scale, height is positively correlated with forest biomass across all sites. However, this relationship is much better for wetland forests than for upland forests that are of critical wildfire concern, mostly because of high variability in tree basal area due to past disturbances in upland stands. Within single stands, height is positively correlated with biomass in most upland forest types, with the notable exception of intensively managed stands.
Unfortunately, overall stand characteristics aren't always sufficient to evaluate hazardous fuels. LIDAR data also can be used to evaluate the arrangement of branches and foliage within the forest to detect the presence of ladder fuels, which can increase the chance of understory fires moving up to the canopy and becoming crown fires. These data are integrated in GIS layers to produce maps of forest structure and fuel loading across the Pinelands.
During prescribed fire season in the spring, we evaluate fuel reduction treatments in collaboration with the New Jersey Forest Fire Service and other local fire managers by sampling fuel depths and mass on the forest floor pre- and post-prescribed fire.
Prescribed fires typically range from 200 to 850 acres per fire, and a target of more than 30,000 acres are treated per year. On average, one to three years of fuel accumulation on the forest floor is removed in a single prescribed fire, and biomass of understory vegetation is reduced considerably. These measurements also contribute to an understanding of the tradeoffs between hazardous fuel reduction and carbon sequestration by forests in the Pinelands.
A sub-set of the fire weather towers are instrumented to measure forest energy balance above the canopy. By comparing absorbed solar radiation and heat re-radiation by forest canopies, termed sensible heat flux, a live fuel moisture index can be calculated.
When drought occurs and soil moisture is limiting, midday values of the ratio of sensible heat to absorbed radiation exceeds 0.5, thus greater than 50% of absorbed radiation is released as heat. The canopy is relatively hot and dry at this time, a condition favorable to the occurrence of crown fires. In contrast, sensible heat flux is relatively low when precipitation is abundant, because a much larger portion of the absorbed radiation is used in evapotranspiration of water from the forest canopy. These forest energy balance measurements characterize 1-5km
We are currently testing the usefulness of this method across a network of flux towers from Florida to Wisconsin making similar measurements. For examples, visit the Ameriflux network at http://public.ornl.gov/ameriflux/.
We are using the network of fire weather towers and the SODARS to test and refine regional-scale fire weather predictions made by the Eastern Area Modeling Consortium using the MM5 mesoscale atmospheric model (For more on the EAMC, visit www.ncrs.fs.fed.us/eamc). The MM5 model has been run twice daily in real time since summer 2002 to produce fire weather predictions at high resolution for the Great Lakes and Mid-Atlantic regions.
When compared to our network of fire weather towers, the MM5 captures weather events and trends well, but it sometimes underpredicts daily maxima and minima values of key fire weather variables such as air temperature and relative humidity. Overall, correlations for MM5 for key fire weather variables are highly significant. Thus, the MM5 is a valuable tool for predicting severe fire conditions and can be linked to a variety of products such as fuel moisture models, fire behavior models such as FARSITE, and fuel prescription models such as CONSUME.
The fire weather information and accurate fuel loading maps will be used to produce near — real-time predictions of fuel moisture contents using a GIS database, and will be available to fire managers over the internet. Once completely validated, the live fuel moisture index based on forest energy balance measurements will also be available for selected sites. Coupling these approaches to fire weather predictions made with the MM5 model will allow advance notice of severe fire weather conditions and pinpoint areas of hazardous fuel accumulations.
These new tools will enhance the ability of fire managers to predict and respond to wildfires in the Pinelands.
The Forest Service's Experimental Forests are dedicated to long-term research on ecosystem processes, silviculture and forest management options, wildlife habitat characteristics, and forest growth and development. Eight experimental forests are administered by the Northeastern Research Station, including the Silas Little forest. On several of the experimental forests, research and monitoring have been carried out for many decades and have produced long-term, irreplaceable data sets that are valuable in environmental science. Many scientists from the Northeastern Research Station and cooperating scientists from many educational institutions, government agencies, and foundations conduct research on these experimental forests in the Northeast.
Kenneth L. Clark John Hom and Nick Skowronski Silas Little Experimental Forest Northern Global Change Program USDA Forest Service New Lisbon, N.J.
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