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Fires release carbon to the atmosphere through the combustion of organic material. Some of these greenhouse gases are returned to the landscape as biomass re-grows subsequent to fires. The net change of carbon contained in vegetation relative to pre-fire levels depends on the time since burning and the type of vegetation that grows back. Thus, permanence of carbon sequestered in forest biomass depends on the timescale in question, fire severity, the expected or assumed fire return interval, and other ecological factors that influence plant succession post-fire. The time for a forest to develop following a high severity wildfire can be several centuries, but fires that burn at lower severities may be able to replace biomass lost to fire in decadal timescales. The differing productivities of forests and their attendant regrowth rates, coupled with the characteristic fire return interval of each forest type make it difficult to determine whether fires actually cause a net emission or sequestration of carbon at multi decadal timescales. Recent research has shown that at broad scales and over the course of many decades, frequent fire appears to select for forest stands with larger diameter trees that store a greater volume of carbon per unit area. In addition, forests with larger diameter trees of fire resistant species have complex structure which often includes a high height-to-live-crown, making them less susceptible to catastrophic stand-replacing crown fires, and thus promoting long-term carbon storage. We seek to examine three questions using long-term plot data from Yosemite and Sequoia Kings Canyon National Parks:
1) Are such broad scale results applicable at the smaller scales of time and space relevant to individual fires and the people who manage them? 2) Do changes in fire management strategies that return Sierra Nevada forests to more frequent and lower intensity fire regimes have the co-benefit of increasing the carbon sequestration capacity of these forests? 3) Are the broad-scale results applicable to every forest type in Sierra Nevada?
We are also examining the annual incremental changes in carbon sequestration in the 10 – 25 years since fire, and thereby resolve some of the ambiguity present in the static approach (above). Fire monitoring plots will be revisited, and short increment cores will be selected from trees of representative species and size classes. We will sample approximately 2000 increment cores to determine the past 50 years of annual tree growth. We anticipate that increment cores will generally be <10cm in length, with some being 10-30 cm. The increment cores will be used to examine growth rates before and after fire. The relative rates of carbon sequestration for pre-fire and post-fire years will be calculated using BIOPAK as in the static approach. However, the more accurate measurement of ring width will allow more accurate calculations of carbon sequestered by each tree in each of the 50 years of the study period. We will then be able to confirm the change in carbon with respect to the occurrence of fire and estimate the number of years required for trees remaining on the site to sequester (either in their own biomass or in litter) the amount of carbon released in the fire. Carbon sequestration can also be examined with respect to a combination of fire and climate variables. The dynamic approach will also allow direct examination of growth release following fire. In particular, increases in annual increment of large-diameter trees following fire will be interpreted as improving the probability of survival of those large-diameter trees in the near term.
Two increment cores will be taken from approximately 2000 trees representing a cross section of vegetation types, species, size classes and estimated ages. Increment cores will be mounted, sanded, and dated, with annual increment calculated by WINDENDRO, and cross-dating verified with COFECHA. A set of allometric equations from BIOPAK will be selected based on species, size and landscape position. For the subset of plots where both pre-fire and post-fire measurements exist, we will calculate the net change in carbon for the fire, including changes in all above-ground biomass components (duff, litter, woody debris, shrubs, and trees). This will enable changes in above-ground carbon sequestration to be partitioned by category. Below-ground carbon will be inferred from published information associated with stand structure.
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everywhere |
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Climate Matters |
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The Sierra Crest and Mt. Whitney |
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Carbon Sequestration |