Experiment 133 examines effects of different frequencies and patterns in long-term prescribed burning programs on vegetation structure, composition, productivity and nutrient cycling in upland oak savanna and woodland vegetation, and investigates possible influences of fire on resource availability (nutrients, water, and light) and net primary productivity. In addition to these objectives, Experiment 133 aims to restore and maintain the historically important savanna and open woodland ecosystems.
Experiment 133 includes 20 sites, called burn compartments, which collectively cover more than 300 ha of land in and around Cedar Creek. The individual compartments range in size between 2.4 and 30ha, and in burning frequency between complete suppression and fires every 4 out of 5 years. All burn units share similar topography and soils (Typic and Alfic Udipsamment). The woodlands may have experienced some selective logging and grazing, but minimal agriculture.
The basis for the study is an ongoing, experimental prescribed burning program begun in 1964 at Cedar Creek, and a similar program operating since 1962 on the adjacent Helen Allison Savanna property (owned by The Nature Conservancy). Experiment 133 represents a continuation and combination of Experiment 015 and Experiment 094. Fire was suppressed in this area 1940 - 1964, but records suggest that before that (1910 - 1940) there was a fire about every five years (Pierce 1954).
Vegetation Structure
Periodic fires at medium to high frequency were found to gradually kill mature trees and suppress sapling recruitment, but responses differ by tree species (Peterson and Reich 2001). Unburned communities became increasingly dominated by woody plants whereas the frequently burned communities were increasingly dominated by herbaceous plants, especially grasses (Fig. 1, Peterson 1998) Repeated burning also increased the percentage of woody canopy openness (Reich 2001).
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Fig. 1: Fire frequency effects on understory plant cover, by functional group. Fitted curves indicate a significant linear or quadratic relationship between fire frequency and percent cover. Error bars indicate standard errors around the means. (From Peterson 1998) |
Aboveground biomass, most of which was wood, declined sharply and nonlinearly with increasing fire frequency and decreasing tree canopy dominance. Total foliage biomass (woody plus herbaceous) declined markedly along the same gradients. In contrast to foliage biomass, fine root biomass exhibited the opposite pattern, being greatest in the frequently burned, herbaceous-dominated stands. (Fig. 2, Peterson 2001)
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Fig. 2: Total aboveground biomass, total canopy foliage biomass, fine root biomass (0-1 m depth), and fine root biomass as a fraction of total fine tissue biomass (fine roots/[fine root + foliage]) for 20 savanna and woodland stands, in relation to fire frequency (r2 = 0.66, 0.52, 0.44, and 0.71, respectively) and percentage of woody canopy openness (r2 = 0.78, 0.79, 0.63, and 0.83, respectively). All relationships are at P < 0.001. (From Peterson et al. 2001) |
Net Primary Productivity
Total ANPP was highest in the unburned, tree-dominated stands and decreased with fire frequency and %WCO, following the trend of both components of woody plant ANPP, foliage and wood NPP. In contrast herbaceous ANPP increased across the same gradient. The tight scaling with %WCO suggests a key role for light availability in controlling grass ANPP (Fig. 3, Reich et al. 2001).
Aboveground biomass, most of which was wood, declined sharply and nonlinearly with increasing fire frequency and decreasing tree canopy dominance. Total foliage biomass (woody plus herbaceous) declined markedly along the same gradients. In contrast to foliage biomass, fine root biomass exhibited the opposite pattern, being greatest in the frequently burned, herbaceous-dominated stands. (Fig. 2, Peterson 2001)
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Fig. 3: Total aboveground net primary production (ANPP) and its components, aboveground wood NPP, woody foliage NPP, and herbaceous ANPP, in relation to fire frequency (r2 = 0.59, 0.57, 0.58, and 0.56, respectively) and percentage of woody canopy openness (r2 = 0.83, 0.76, 0.94, and 0.95, respectively) for 20 woodland and savanna stands. All relationships are significant at P < 0.001. (From Reich et al. 2001) |
Nitrogen Availability and Cycling
Total canopy N, annual litterfall N cycling, soil net N mineralization and LAI (leaf area index) all decreased, usually nonlinearly, with increasing %WCO and fire frequency, although better or equal correlations were seen for %WCO with all variables.
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Fig. Total ecosystem leaf area index, annual net N mineralization rate, total annual aboveground litter N, and total canopy N, in relation to fire frequency (r2 = 0.57, 0.65, 0.63, and 0.48, respectively) and percentage of woody canopy openness (r2 = 0.77, 0.69, 0.80, and 0.86, respectively) for 20 woodland and savanna stands. All relationships are significant at P < 0.001. (From Reich et al 2001) |
While in situ net N mineralization was reduced with increased fire frequency overall, this reduction was less extreme in oak-dominated patches than in grass-dominated patches. Greater net N mineralization in oak-dominated patches occurred despite greater N losses through volatilization and leaching, likely because of higher plant litter N concentration. (Dijkstra et al. 2005)
Litter N dynamics suggest that the microbial decomposer community is limited by low substrate N when it is decomposing litter. As the initial nitrogen content of litter decreased, more N was immobilized into it by the decomposer community from the surrounding soil (Fig. 4, Hernandez et al. 2008).
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Fig. 4: N dynamics in decomposing litter in sites with high, medium, and control fire frequency. Values over 1.9 indicate the litter N content increased above its initial value. Symbols represent the fire frequency of the site of litter origin: open triangle indicates high burn, open square indicates medium burn, and dark filled circle indicates control. Values are mean +- SE. (From Hernandez et al. 2008) |
The combined effects of slower rates of decomposition and increased N immobilization associated with increased fire frequency could result in positive feedback, reinforcing low N availability in frequently burned sites (Fig. 5, Hernandez and Hobbie 2008). Fire promotes N losses through volatilization, decreasing soil N availability in frequently burned sites. Sites with low N availability have decreased oak litter N content which results in decreased rates of decomposition and increased rates of N immobilization. N that is immobilized into litter is taken up by microbes from the soil N pool, further decreasing soil N availability and creating a positive feedback. When a fire does occur, the litter burned has a relatively higher N content than it did initially, potentially causing even greater declines in soil N over time through increased volatilization.
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Fig. 5: A positive feedback of decreased soil N availability due to fire is caused by the immobilization of soil N into low-N litter by microbes (a), and increased N losses when fire burns litter that has increased N content due to immobilization (b). (From Hernandez and Hobbie 2008) |
Carbon Storage
Total ecosystem carbon was highly dependent on fire treatment. The fire suppression treatment had 90% more total ecosystem C than the high-frequency fire treatment and 67% more than the moderate treatment (Fig. 6, Tilman et al. 2000)
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Fig. 6: Total stores of carbon (as mean C + 1 SE) in all forms (summed, on an areal basis, for soil plus fine roots, coarse roots, forest floor litter, coarse woody debris, trees, and other vegetation) in the three fire treatments at Cedar Creek Natural History Area. The eight fire-suppressed plots had a mean fire frequency of 0.025 per yr, the four moderate fire frequency plots had a fire frequency of 0.24 per yr, and the seven high fire frequency plots had a mean fire frequency of 0.62 per yr. Results for an ANOVA (df = 2, 16) of the effects of the fire frequency treatments on total ecosystem carbon are shown. Means sharing the same lowercase letter do not differ significantly (P>0.05) based on Student-Newman-Keuls multiple comparison test. (From Tilman et al. 2000) |
The C storage was mainly caused by increased tree mass following fire suppression. This increase corresponded with a shift from savanna in high fire frequency treatments to shrub woodland at moderate fire frequenies, to a more closed canopy oak forest with fire suppression (Fig. 7, Tilman et al. 2000). The growth and spread of a single species, pin oak, accounts for 1.3 mg/ha/yr of C accumulation, ~70% of the annual rate of accumulation.
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Fig. 7: Responses (mean + 1 SE) of all six carbon store components to fire frequency. Results of an ANOVA (df = 2,16) for the dependence of each component on fire frequency are given. Means sharing the same lowercase letter do not differ significantly (P > 0.05), based on Student-Newman-Keuls multiple comparison test. Total tree carbon (A), coarse root carbon (B), and forest floor carbon (D) were all significantly greater (P < 0.01) in suppressed than in moderate or high fire frequencies. Shrub carbon was greater at moderate fire frequencies. The other components were not significantly dependent on fire frequency (P > 0.1). (From Tilman et al. 2000) |
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