Stephen Mitchell

Forest fuel reduction alters fire severity and long‐term carbon storage in three Pacific Northwest ecosystems

Two forest management objectives being debated in the context of federally managed landscapes in the U.S. Pacific Northwest involve a perceived trade-off between fire restoration and carbon sequestration. The former strategy would reduce fuel (and therefore C) that has accumulated through a century of fire suppression and exclusion which has led to extreme fire risk in some areas. The latter strategy would manage forests for enhanced C sequestration as a method of reducing atmospheric CO2 and associated threats from global climate change. We explored the trade-off between these two strategies by employing a forest ecosystem simulation model, STANDCARB, to examine the effects of fuel reduction on fire severity and the resulting long-term C dynamics among three Pacific Northwest ecosystems: the east Cascades ponderosa pine forests, the west Cascades western hemlock-Douglas-fir forests, and the Coast Range western hemlock-Sitka spruce forests. Our simulations indicate that fuel reduction treatments in these ecosystems consistently reduced fire severity. However, reducing the fraction by which C is lost in a wildfire requires the removal of a much greater amount of C, since most of the C stored in forest biomass (stem wood, branches, coarse woody debris) remains unconsumed even by high-severity wildfires. For this reason, all of the fuel reduction treatments simulated for the west Cascades and Coast Range ecosystems as well as most of the treatments simulated for the east Cascades resulted in a reduced mean stand C storage. One suggested method of compensating for such losses in C storage is to utilize C harvested in fuel reduction treatments as biofuels. Our analysis indicates that this will not be an effective strategy in the west Cascades and Coast Range over the next 100 years. We suggest that forest management plans aimed solely at ameliorating increases in atmospheric CO2 should forgo fuel reduction treatments in these ecosystems, with the possible exception of some east Cascades ponderosa pine stands with uncharacteristic levels of understory fuel accumulation. Balancing a demand for maximal landscape C storage with the demand for reduced wildfire severity will likely require treatments to be applied strategically throughout the landscape rather than indiscriminately treating all stands.

Carbon debt and carbon sequestration parity in forest bioenergy production

The capacity for forests to aid in climate change mitigation efforts is substantial but will ultimately depend on their management. If forests remain unharvested, they can further mitigate the increases in atmospheric CO2 that result from fossil fuel combustion and deforestation. Alternatively, they can be harvested for bioenergy production and serve as a substitute for fossil fuels, though such a practice could reduce terrestrial C storage and thereby increase atmospheric CO2 concentrations in the near‐term. Here, we used an ecosystem simulation model to ascertain the effectiveness of using forest bioenergy as a substitute for fossil fuels, drawing from a broad range of land‐use histories, harvesting regimes, ecosystem characteristics, and bioenergy conversion efficiencies. Results demonstrate that the times required for bioenergy substitutions to repay the C Debt incurred from biomass harvest are usually much shorter (< 100 years) than the time required for bioenergy production to substitute the amount of C that would be stored if the forest were left unharvested entirely, a point we refer to as C Sequestration Parity. The effectiveness of substituting woody bioenergy for fossil fuels is highly dependent on the factors that determine bioenergy conversion efficiency, such as the C emissions released during the harvest, transport, and firing of woody biomass. Consideration of the frequency and intensity of biomass harvests should also be given; performing total harvests (clear‐cutting) at high‐frequency may produce more bioenergy than less intensive harvesting regimes but may decrease C storage and thereby prolong the time required to achieve C Sequestration Parity.

Can fuel‐reduction treatments really increase forest carbon storage in the western US by reducing future fire emissions?

It has been suggested that thinning trees and other fuel-reduction practices aimed at reducing the probability of high-severity forest fire are consistent with efforts to keep carbon (C) sequestered in terrestrial pools, and that such practices should therefore be rewarded rather than penalized in C-accounting schemes. By evaluating how fuel treatments, wildfire, and their interactions affect forest C stocks across a wide range of spatial and temporal scales, we conclude that this is extremely unlikely. Our review reveals high C losses associated with fuel treatment, only modest differences in the combustive losses associated with high-severity fire and the low-severity fire that fuel treatment is meant to encourage, and a low likelihood that treated forests will be exposed to fire. Although fuel-reduction treatments may be necessary to restore historical functionality to fire-suppressed ecosystems, we found little credible evidence that such efforts have the added benefit of increasing terrestrial C stocks.

Processes influencing model-data mismatch in drought-stressed, fire-disturbed, eddy flux sites

Semiarid forests are very sensitive to climatic change and among the most difficult ecosystems to accurately model. We tested the performance of the Biome-BGC model against eddy flux data taken from young (years 2004-2008), mature (years 2002-2008), and old-growth (year 2000) ponderosa pine stands at Metolius, Oregon, and subsequently examined several potential causes for model-data mismatch. We used the Generalized Likelihood Uncertainty Estimation methodology, which involved 500,000 model runs for each stand (1,500,000 total). Each simulation was run with randomly generated parameter values from a uniform distribution based on published parameter ranges, resulting in modeled estimates of net ecosystem CO2 exchange (NEE) that were compared to measured eddy flux data. Simulations for the young stand exhibited the highest level of performance, though they overestimated ecosystem C accumulation (-NEE) 99% of the time. Among the simulations for the mature and old-growth stands, 100% and 99% of the simulations underestimated ecosystem C accumulation. One obvious area of model-data mismatch is soil moisture, which was overestimated by the model in the young and old-growth stands yet underestimated in the mature stand. However, modeled estimates of soil water content and associated water deficits did not appear to be the primary cause of model-data mismatch; our analysis indicated that gross primary production can be accurately modeled even if soil moisture content is not. Instead, difficulties in adequately modeling ecosystem respiration, mainly autotrophic respiration, appeared to be the fundamental cause of model-data mismatch.

Alternative approaches for addressing non-permanence in carbon projects: an application to afforestation and reforestation under the Clean Development Mechanism

Afforestation and reforestation (A/R) projects generate greenhouse gas (GHG) reduction credits by removing carbon dioxide from the atmosphere through biophysical processes and storing it in terrestrial carbon stocks. One feature of A/R activities is the possibility of non-permanence, in which stored carbon is lost though natural or anthropogenic disturbances. The risk of non-permanence is currently addressed in Clean Development Mechanism (CDM) A/R projects through temporary carbon credits. To evaluate other approaches to address reversals and their implications for policy and investment decisions, we assess the performance of multiple policy and accounting mechanisms using a forest ecosystem simulation model parameterized with observational data on natural disturbances (e.g., fire and wind). Our analysis finds that location, project scale, and system dynamics all affect the performance of different risk mechanisms. We also find that there is power in risk diversification. Risk management mechanisms likewise exhibit a range of features and tradeoffs among risk conservatism, economic returns, and other factors. Rather than relying on a single approach, a menu-based system could be developed to provide entities the flexibility to choose among approaches, but care must be taken to avoid issues of adverse selection.

Scale-dependent responses of longleaf pine vegetation to fire frequency and environmental context across two decades

Summary 1. Disturbance is an important driver of plant community structure in many grasslands and woodlands, and alteration of disturbance regimes can have large consequences for species richness and composition. However, the response of vegetation to disturbance may change with environmental context. We resampled a unique, nested permanent vegetation plot data set in the longleaf pine ecosystem of the southeastern USA after 20 years to determine how environmental context and fire frequency jointly influence vegetation change across multiple spatial scales (0.01–1000 m 2 ). 2. The magnitude of vegetation change was quantified using two different, yet complementary metrics of beta-diversity (beta turnover measured as the proportion of species turning over and Bray–Curtis dissimilarity) and by documenting changes in species richness. We used null model analysis to explore whether communities were more dynamic over time at small spatial scales relative to larger scales. 3. Changes in species richness, beta turnover and Bray–Curtis dissimilarity were greatest on silty, frequently burned sites, whereas sandy, less frequently burned sites remained relatively stable. The amount of change detected was scale dependent: species richness increased at larger spatial scales over time, but decreased at the two smallest spatial scales. Null model analysis revealed that beta turnover standardized effect sizes (SES) were negative and significantly different from random expectation at all spatial scales except the smallest. Thus, the magnitude of compositional change across most scales was small, despite substantial changes in species richness across time. We attribute this initial contradiction to the turnover of infrequent, low-abundance species amidst a matrix of dominant grasses. 4. Synthesis. In contrast to previous longleaf pine studies, we found fire frequency to be less important than environmental site conditions in predicting vegetation change. Thus, future work in this ecosystem and in other fire-dependent grasslands and woodlands should consider not only disturbance, but also environmental context. Since species richness and beta-diversity patterns were scale dependent, we recommend sampling vegetation across multiple spatial scales in order to comprehensively quantify changes in community structure over time. We believe this study lays the groundwork for understanding how fire and environmental filtering jointly influence vegetation dynamics across space and time in fire-dependent grasslands and woodlands.

Alternative Approaches to Addressing the Risk of Non-Permanence in Afforestation and Reforestation Projects under the Clean Development Mechanism

The report provides quantitative and qualitative insights into the performance of different non-permanence approaches for consideration of parties. This note summarizes the results of the analysis presented in the report. Besides the existing mechanism for temporary crediting, the study analyzed a range of alternative approaches to addressing non-permanence, including those considered in prior deliberations of the United Nations Framework Convention on Climate Change (UNFCCC). The approaches address the risk of non-permanence in several ways. Under the tonne year accounting, credits are issued for the increments of carbon sequestered corresponding to a defined permanence period, and their quantity depends on the carbon stored in biomass each year of the permanence period. However, this approach has not been implemented by any standard. These approaches are not mutually exclusive, but can be used in tandem with each other.

Chapter 10 – Carbon Dynamics of Mixed- and High-Severity Wildfires: Pyrogenic CO2 Emissions, Postfire Carbon Balance, and Succession

Among the concerns raised by climatic change is the potential for the additional release of carbon dioxide as a result of biomass combustion. Most of the carbon emissions from wildfires are from the combustion of litter, duff, and small woody debris, whereas most, if not all, of the biomass stored in the boles of large trees is not combusted. Consequently, most of the carbon stored in forests remains unconsumed, even by high-severity wildfires. Thus the application of fuel reduction treatments, while sometimes effective in reducing fire severity and carbon emissions, nearly always result in a net reduction in carbon storage. Postfire carbon emissions from the decomposition of fire-killed biomass can continue for decades, but effects of forest regrowth can exceed the losses of carbon from biomass combustion and the decomposition of fire-killed biomass within 5-50 years, depending on the ecosystem.