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Effects of Management on Carbon Sequestration in Forest Biomass in Southeast Alaska
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The Tongass National Forest (Tongass) is the largest national forest and largest area of old-growth forest in the United States. Spatial geographic informa- tion system data for the Tongass were combined with forest inventory data to estimate and map total carbon stock in the Tongass; the result was 2.8±0.5PgC,or8%of the total carbon in the forests of the conterminous USA and 0.25% of the carbon in global forest vegetation and soils. Cumulative net carbon loss from the Tongass due to management of the forest for the period 1900–95 was estimated at 6.4–17.2 Tg C. Using our spatially explicit data for carbon stock and net flux, we modeled the potential effect of five management regimes on future net carbon flux. Estimates of net carbon flux were sensitive to projections of the rate of carbon accumulation in second-growth forests and to the amount of carbon left in standing biomass after harvest. Projections of net carbon flux in the Tongass range from 0.33 Tg C annual sequestration to 2.3 Tg C annual emission for the period 1995–2095. For the period 1995–2195, net flux estimates range from 0.19 Tg C annual sequestra- tion to 1.6 Tg C annual emission. If all timber harvesting in the Tongass were halted from 1995 to 2095, the economic value of the net carbon sequestered during the 100-year hiatus, assuming $20/Mg C, would be $4 to $7 million/y (1995 US dollars). If a prohibition on logging were extended to 2195, the annual economic value of the carbon sequestered would be largely unaffected ($3 to $6 million/y). The potential annual economic value of carbon sequestration with management maxi- mizing carbon storage in the Tongass is comparable to revenue from annual timber sales historically authorized for the forest.
Key words: carbon sequestration; geographic information system; climate change; forest management; Alaska.
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Attributing physical and biological impacts to anthropogenic climate change
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Significant changes in physical and biological systems are occurring on all continents and in most oceans, with a concentration of available data in Europe and North America. Most of these changes are in the direction expected with warming temperature. Here we show that these changes in natural systems since at least 1970 are occurring in regions of observed temperature increases, and that these temperature increases at continental scales cannot be explained by natural climate variations alone. Given the conclusions from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report that most of the observed increase in global average temperatures since the mid-twentieth century is very likely to be due to the observed increase in anthropogenic greenhouse gas concentrations, and furthermore that it is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent except Antarctica, we conclude that anthropogenic climate change is having a significant impact on physical and biological systems globally and in some continents.
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Atmospheric CO2 forces abrupt vegetation shifts locally, but not globally
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It is possible that anthropogenic climate change will drive the Earth system into a qualitatively different state1. Although different types of uncertainty limit our capacity to assess this risk 2, Earth system scientists are particularly concerned about tipping elements, large-scale components of the Earth system that can be switched into qualitatively different states by small perturbations. Despite growing evidence that tipping elements exist in the climate system1,3, whether large-scale vegetation systems can tip into alternative states is poorly understood4. Here we show that tropical grassland, savanna and forest ecosystems, areas large enough to have powerful impacts on the Earth system, are likely to shift to alternative states. Specifically, we show that increasing atmospheric CO2 concentration will force transitions to vegetation states characterized by higher biomass and/or woody-plant dominance. The timing of these critical transitions varies as a result of between-site variance in the rate of temperature increase, as well as a dependence on stochastic variation in fire severity and rainfall. We further show that the locations of bistable vegetation zones (zones where alternative vegetation states can exist) will shift as climate changes. We conclude that even though large-scale directional regime shifts in terrestrial ecosystems are likely, asynchrony in the timing of these shifts may serve to dampen, but not nullify, the shock that these changes may represent to the Earth system.
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Area–heterogeneity tradeoff and the diversity of ecological communities
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For more than 50 y ecologists have believed that spatial heterogeneity in habitat conditions promotes species richness by increasing opportunities for niche partitioning. However, a recent stochastic model combining the main elements of niche theory and island biogeography theory suggests that environmental heterogeneity has a general unimodal rather than a positive effect on species richness. This result was explained by an inherent tradeoff between environmental heterogeneity and the amount of suitable area available for individual species: for a given area, as heterogeneity increases, the amount of effective area available for individual species decreases, thereby reducing population sizes and increasing the likelihood of stochastic extinctions. Here we provide a comprehensive evaluation of this hypothesis. First we analyze an extensive database of breeding bird distribution in Catalonia and show that patterns of species richness, species abundance, and extinction rates are consistent with the predictions of the area–heterogeneity tradeoff and its proposed mechanisms. We then perform a metaanalysis of heterogeneity–diversity relationships in 54 published datasets and show that empirical data better fit the unimodal pattern predicted by the area–heterogeneity tradeoff than the positive pattern predicted by classic niche theory. Simulations in which species may have variable niche widths along a continuous environmental gradient are consistent with all empirical findings. The area–heterogeneity tradeoff brings a unique perspective to current theories of species diversity and has important implications for biodiversity conservation.
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Carbon debt of Conservation Reserve Program (CRP) grasslands converted to bioenergy production
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Over 13 million ha of former cropland are enrolled in the US Conservation Reserve Program (CRP), providing well-recognized biodiversity, water quality, and carbon (C) sequestration benefits that could be lost on conversion back to agricultural production. Here we provide measurements of the greenhouse gas consequences of converting CRP land to continuous corn, corn–soybean, or perennial grass for biofuel production. No-till soybeans preceded the annual crops and created an initial carbon debt of 10.6 Mg CO2 equivalents (CO2e)·ha−1 that included agronomic inputs, changes in C stocks, altered N2O and CH4 fluxes, and foregone C sequestration less a fossil fuel offset credit. Total debt, which includes future debt created by additional changes in soil C stocks and the loss of substantial future soil C sequestration, can be constrained to 68 Mg CO2e·ha−1 if subsequent crops are under permanent no-till management. If tilled, however, total debt triples to 222 Mg CO2e·ha−1 on account of further soil C loss. Projected C debt repayment periods under no-till management range from 29 to 40 y for corn– soybean and continuous corn, respectively. Under conventional tillage repayment periods are three times longer, from 89 to 123 y, respectively. Alternatively, the direct use of existing CRP grasslands for cellulosic feedstock production would avoid C debt entirely and provide modest climate change mitigation immediately. Incentives for permanent no till and especially permission to harvest CRP biomass for cellulosic biofuel would help to blunt the climate impact of future CRP conversion.
land-use change | renewable energy | carbon balance | agriculture | nitrous oxide
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Effect of habitat area and isolation on fragmented animal populations
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Habitat destruction has driven many once-contiguous animal populations into remnant patches of varying size and isolation. The underlying framework for the conservation of fragmented popu- lations is founded on the principles of island biogeography, wherein the probability of species occurrence in habitat patches varies as a function of patch size and isolation. Despite decades of research, the general importance of patch area and isolation as predictors of species occupancy in fragmented terrestrial systems remains unknown because of a lack of quantitative synthesis. Here, we compile occupancy data from 1,015 bird, mammal, reptile, amphibian, and invertebrate population networks on 6 continents and show that patch area and isolation are surprisingly poor predictors of occupancy for most species. We examine factors such as improper scaling and biases in species representation as expla- nations and find that the type of land cover separating patches most strongly affects the sensitivity of species to patch area and isolation. Our results indicate that patch area and isolation are indeed important factors affecting the occupancy of many species, but properties of the intervening matrix should not be ignored. Improving matrix quality may lead to higher conservation returns than manipulating the size and configuration of remnant patches for many of the species that persist in the aftermath of habitat destruction.
incidence function island biogeography logistic regression metaanalysis occupancy
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Carbon Mitigation by Biofuels or by Saving and Restoring Forests?
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The carbon sequestered by restoring forests is greater than the emissions avoided by the use of the liquid biofuels.
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Biotic Multipliers of Climate Change
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A focus on species interactions may improve predictions of the effects of climate change
on ecosystems.
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Assemblage Time Series Reveal Biodiversity Change but Not Systematic Loss
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The extent to which biodiversity change in local assemblages contributes to global biodiversity
loss is poorly understood. We analyzed 100 time series from biomes across Earth to ask how diversity
within assemblages is changing through time. We quantified patterns of temporal a diversity, measured
as change in local diversity, and temporal b diversity, measured as change in community composition.
Contrary to our expectations, we did not detect systematic loss of a diversity. However, community
composition changed systematically through time, in excess of predictions from null models.
Heterogeneous rates of environmental change, species range shifts associated with climate change,
and biotic homogenization may explain the different patterns of temporal a and b diversity.
Monitoring and understanding change in species composition should be a conservation priority.
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Changes in Climatic Water Balance Drive Downhill Shifts in Plant Species’ Optimum Elevations
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Uphill shifts of species’ distributions in response to historical warming are well documented, which leads
to widespread expectations of continued uphill shifts under future warming. Conversely, downhill shifts
are often considered anomalous and unrelated to climate change. By comparing the altitudinal
distributions of 64 plant species between the 1930s and the present day within California, we show that
climate changes have resulted in a significant downward shift in species’ optimum elevations. This
downhill shift is counter to what would be expected given 20th-century warming but is readily
explained by species’ niche tracking of regional changes in climatic water balance rather than
temperature. Similar downhill shifts can be expected to occur where future climate change scenarios
project increases in water availability that outpace evaporative demand.
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