u.s. department of the interior u.s. geological survey december 2007 simulated impacts of cyclic...

U.S. Department of the InteriorU.S. Geological Survey

December 2007

Simulated Impacts of Cyclic FireOn Ecosystem Carbon and Nitrogen Cycles at Fort BenningShuguang Liu1, Shuqing Zhao2, Larry Tieszen3, Donald Imm4 and Harold Balbach5

1 Science Applications International Corporation, Contractor to U.S. Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS), Sioux Falls, SD 57198. Work performed under USGS contract 03CRCN0001. Email: [email protected]

2 Earth Resources Technology, Contractor to USGS EROS, Sioux Falls, SD 571983 USGS EROS, Sioux Falls, SD 571984 Savannah River Ecology Laboratory and Univ. of Georgia, BLDG. 5889, Harmony Church Complex, 1st Div. Rd., Fort

Benning, GA 319055 US Army ERDC-CERL, PO Box 9005, Champaign, IL 61826-9005

MethodsThe EDCM Model and Parameterization

EDCM is an ecosystem model capable of simulating C and N cycles in various ecosystems under the impacts of management practices, disturbances, and climate change (Liu et al., 2003, Figure 2). Major C and N cycling processes and pools are shown in Figure 2.

In EDCM, the impacts of prescribed fires on C and N cycles are simulated using a canopy woody layer and an understory layer. Certain fractions of aboveground biomass and nutrients can be consumed by fire (returned to the forest floor or emitted to the atmosphere) according to burning intensity. N2 fixation, expressed as g N fixed per g C fixed, can occur in both layers. N competition between the canopy and understory is largely controlled by the canopy coverage.

In this study, we simulate C and N cycles at relatively infertile longleaf pine sites at Fort Benning. Key model parameters including C:N ratios of plant tissues, soils, atmospheric N deposition rate, symbiotic and nonsymbiotic N fixation, and C allocations among plant parts were either measured from previous studies or taken from literature review. Meteorological data (monthly precipitation and temperature) were from on-installation weather stations.

Modeling Experimental DesignThree levels of fire frequency, fire intensity, symbiotic N2 fixation, and nonsymbiotic N2 fixation were used to assess the impacts of cyclic prescribed burning on C and N dynamics (Table 1). Combinations of fire frequency, fire intensity, and symbiotic and nonsymbiotic N inputs were generated using factorial experimental design (PROC FACTEX; SAS, 2004). Atmospheric N deposition (0.35 g N/m2/yr) obtained from the National Atmospheric Nitrogen Deposition Program (2004) in the vicinity of Fort Benning was used for all model simulations. The significance of differences between the levels of each factor was determined using ANOVA (PROC GLM; SAS, 2004) with α ≤ 0.05.

Figure 1. Geographic location and topography of Fort Benning.

Table 1. Experimental design for the combinations of fire frequency, fire intensity, and symbiotic and nonsymbiotic nitrogen inputs. The “normal” fire cycle at Fort Benning is 3 years.

Figure 2. Flowchart of the EDCM model.

IntroductionPrescribed fire, a common forest management practice throughout the Southeast, has been used at Fort Benning for two purposes. First, prescribed fire is required to maintain and restore fire-adapted pine communities that are a critical habitat for the Federally endangered red-cockaded woodpecker (Picoides borealis). Second, it removes understory, which facilitates military training access. However, the long-term impacts of fire management on carbon (C) and nitrogen (N) cycles and ecosystem sustainability have not been quantified. Here, we used the Erosion-Deposition-Carbon Model (EDCM) to simulate long-term ecosystem C and N dynamics under the impacts of different combinations of fire frequency, fire intensity, symbiotic and nonsymbiotic N inputs, and atmospheric N deposition.


December 2007

Results

• Highest total N inputs occurred under four combinations of high frequency, high intensity, and high symbiotic N2 fixation (Figure 4a1). With the increase of fire frequency and intensity, total N input increased (Figure 4a2 and a3). These suggest that a negative feedback between herbaceous legume (N replenishment) and fire (N loss) did exist, consistent with field observations (Hiers et al., 2003).

• Fires cyclically stimulate net N mineralization by returning some plant N into the soils (Figure 4). Infrequent fire-induced “pulse” input of available N into soil has been observed widely and a number of hypotheses have been proposed to explain the mechanisms (e.g., Blair, 1997).

Simulated Impacts of Cyclic Fire

NoLowIntermediateHigh

Figure 3. Impacts of fire frequency, fire intensity, symbiotic and nonsymbiotic N inputs on net primary production (NPP) of ecosystem (a), tree canopy (b), and understory (c). Values within colored squares with different capital letters denote significant differences between levels of treatment. The color of squares corresponds to the level of treatment and matches the line color.

Figure 4. Impacts of fire frequency, fire intensity, symbiotic and nonsymbiotic N inputs on total N input (a) and net N mineralization (b). Total N input includes symbiotic and nonsymbiotic N2 fixation and atmospheric N deposition. Legends are the same as in Figure 3.

More than 100 output variables can potentially be examined to study the impacts of cyclic prescribed fire on C and N cycles. In the following, we will concentrate on some of the key variables of C and N fluxes and stocks. Because it is difficult to compare and visualize all 160 simulated scenarios, we averaged and presented the results according to treatment levels.

Net Primary Productivity (NPP)NPP varied greatly among different combinations of scenarios (see Figure 3 a1, b1, and c1), indicating that fire management practices and how ecosystem responds to fire, specifically biologic N2 fixation rates, have a dramatic impact on the end state of ecosystem NPP. Maximum ecosystem NPP was realized under the combination of no fire with high level of both symbiotic and nonsymbiotic N2 fixation rates. However, because fire is a key factor that can sustain the longleaf pine ecosystem, the above scenario is unlikely to exist in reality. With the presence of fire, the best NPP was realized under low frequency and low intensity fire with high N inputs. Lower ecosystem NPP generally occurs with higher fire frequency and intensity (higher nutrient loss) and lower N inputs. The worst case for ecosystem NPP was high fire frequency and intensity without biological N inputs. The average conditions of different treatment levels and their differences can be seen in Figure 3.

Nitrogen Inputs and Net N Mineralization

A B C DA B C CA B C DA B C D

A B C C A B C DA B C D A B C D

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December 2007

ResultsCarbon Stocks and Change


• Ecosystem C stock (sum of biomass C, litter and soil organic carbon (SOC)) varied between 100 and 300 Mg/ha at the end of simulation (Figure 5a1). It was adversely affected by fire. With the increase of fire frequency or fire intensity, the accumulation of ecosystem C was significantly decreased. The average accumulation rates varied about 0.25 to 1.25 Mg/ha/yr. Nevertheless, it should be recognized that under high frequency and high intensity of fire without biological N input, the ecosystem acted as a C source (i.e., released C into the atmosphere).

• Although ecosystem C stocks declined under certain individual cases (Figure 5a1), on average they did not decline for any of the treatment levels (Figure 5a2–a5).

• SOC (sum of carbon in humus, particulate matters, microbes, and fungi) changed from about 17 Mg/ha at the start of simulation to 10 ~ 33 Mg/ha at the end of simulation (Figure 5b1), strongly depending on fire regimes and N input. Cyclic fire with low frequency or low intensity promoted the accumulation of SOC over time. In contrast, intermediate and high frequency fire or intermediate and high intensity fire caused a significant reduction in SOC over time, suggesting longleaf pine soil acted as a C source under the impacts of relatively high frequency fire or high intensity fire. Similar to ecosystem NPP, the rapidly increasing trend of SOC without fire was due to the biological N2 fixation and atmospheric N deposition. SOC significantly increased over time with the increase of symbiotic and nonsymbiotic N input levels, suggesting biological N2 fixation played a critical role in offsetting the N loss from fire (Figure 5b).

• The biomass C stocks of understory were quite small relative to tree biomass C. Understory aboveground C decreased with the increase of fire frequency or fire intensity (Figure 5c1–c3). Under high frequency and high intensity fire scenarios, the understory biomass C was severely suppressed (Figure 5c1), indicating that those scenarios are not likely to occur in reality because ground level fuel is inadequate to propagate fire. Intermediate and high level symbiotic N inputs enhanced the C stock of understory (Figure 5c4 and c5). No significant difference in understory biomass C was observed between intermediate and high symbiotic N input scenarios (Figure 5c4 and d4), indicating that N fixed by understory is used to not only support itself but also provide essential N to other components of the N-limited ecosystem (e.g., trees). At the same time, understory biomass C is also strongly constrained by cyclic burning and tree canopy closure.

• Understory belowground C increased with the increase of fire frequency or fire intensity, indicating that understory adapted to cyclic fire by storing biomass C in belowground structures. Compared to understory aboveground C stock, the impacts of symbiotic N2 fixation on understory belowground C was more evident. Different levels of nonsymbiotic N2 fixation had no effect on understory belowground C stock (Figure 5d5).

Figure 5. Impacts of fire frequency, fire intensity, symbiotic and nonsymbiotic N inputs on forest ecosystem C stock (a), soil organic carbon (SOC) (b), understory aboveground C stock (c), and understory belowground C stock (d). Legends are the same as in Figure 3.




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December 2007



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ResultsNitrogen Stocks and Change

The N cycle in EDCM is tightly coupled with the C cycle (Figure 2). By comparing Figure 5 with Figure 6, it can be seen many of the trends of N stocks were similar to those of C stocks. Total ecosystem N and Soil organic nitrogen (SON) can be replenished or depleted in the future depending on fire management regimes and total N input from various sources.

Acknowledgments The project (SI-1642) is supported by the Strategic Environmental Research and Development Program (SERDP). Valuable comments, suggestions, and logistic support from J. Hall and L. Mulkey are greatly appreciated.

ReferencesBlair JM. 1997. Ecology 78: 2359-2368.DiStefano JF and Gholz HL. 1989. Forest Science 35: 863-869.Hendricks JJ and Boring LR. 1999. Forest Ecology and Management 113: 167–177.Hiers JK, Mitchell RJ, Boring LR, Hendricks JJ, and Wyatt R. 2003. New Phytologist 157: 327–338. Lajeunesse SD, Dilustro JJ, Sharitz RR, and Collins BS. 2006. American Journal of Botany 93: 84-93.Liu SG, Bliss N, Sundquist E and Huntington TG. 2003. Global Biogeochemical Cycles 17 1074:

doi:10.1029/2002GB002010.



Figure 6. Impacts of fire frequency, fire intensity, symbiotic and nonsymbiotic N inputs on forest ecosystem N stock (a), soil organic nitrogen (SON) (b), understory aboveground N stock (c), and understory belowground N stock (d). Legends are the same as in Figure 3.

SummaryModel simulations indicated that cyclic prescribed fire had significant impacts on long-term equilibrium states of C and N fluxes and stocks, ecosystem productivity, and ecosystem sustainability at Fort Benning. How fire regime affects C and N cycles and how the ecosystem responds to fire appears to depend strongly on biological N2 fixation rates. For instance, annual high intensity fire without symbiotic and nonsymbiotic N inputs presented a challenge for maintaining ecosystem sustainability, whereas infrequent low intensity fire with high symbiotic and nonsymbiotic N inputs resulted in C sequestration and cyclic “pulse” input of N into the ecosystem. Our results suggest that the level of N input, closely related to fire regime, must be considered in the prediction of the effects of cyclic prescribed fire. To predict the end states of ecosystems, future research should emphasize the quantification of N input from symbiotic and nonsymbiotic processes under various fire management practices at Fort Benning and other similar southeastern pine forest systems. To facilitate the restoration of longleaf pine ecosystems on these naturally nutrient-poor and severely degraded soils (due to historical agricultural practices), practices that enhance the presence, diversity, and abundance of legume species should be promoted.

u.s. department of the interior u.s. geological survey december 2007 simulated impacts of cyclic...

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