B. D. Amiro

Ecosystem carbon dioxide fluxes after disturbance in forests of North America

Disturbances are important for renewal of North American forests. Here we summarize more than 180 site years of eddy covariance measurements of carbon dioxide flux made at forest chronosequences in North America. The disturbances included stand-replacing fire (Alaska, Arizona, Manitoba, and Saskatchewan) and harvest (British Columbia, Florida, New Brunswick, Oregon, Quebec, Saskatchewan, and Wisconsin) events, insect infestations (gypsy moth, forest tent caterpillar, and mountain pine beetle), Hurricane Wilma, and silvicultural thinning (Arizona, California, and New Brunswick). Net ecosystem production (NEP) showed a carbon loss from all ecosystems following a stand-replacing disturbance, becoming a carbon sink by 20 years for all ecosystems and by 10 years for most. Maximum carbon losses following disturbance (g C m−2y−1) ranged from 1270 in Florida to 200 in boreal ecosystems. Similarly, for forests less than 100 years old, maximum uptake (g C m−2y−1) was 1180 in Florida mangroves and 210 in boreal ecosystems. More temperate forests had intermediate fluxes. Boreal ecosystems were relatively time invariant after 20 years, whereas western ecosystems tended to increase in carbon gain over time. This was driven mostly by gross photosynthetic production (GPP) because total ecosystem respiration (ER) and heterotrophic respiration were relatively invariant with age. GPP/ER was as low as 0.2 immediately following stand-replacing disturbance reaching a constant value of 1.2 after 20 years. NEP following insect defoliations and silvicultural thinning showed lesser changes than stand-replacing events, with decreases in the year of disturbance followed by rapid recovery. NEP decreased in a mangrove ecosystem following Hurricane Wilma because of a decrease in GPP and an increase in ER.

Fire, climate change, carbon and fuel management in the Canadian boreal forest

This paper was presented at the conference ‘Integrating spatial technologies and ecological principles for a new age in fire management’, Boise, Idaho, USA, June 1999 Fire is the dominant stand-renewing disturbance through much of the Canadian boreal forest, with large high-intensity crown fires being common. From 1 to 3 million ha have burned on average during the past 80 years, with 6 years in the past two decades experiencing more than 4 million ha burned. A large-fire database that maps forest fires greater than 200 ha in area in Canada is being developed to catalogue historical fires. However, analyses using a regional climate model suggest that a changing climate caused by increasing greenhouse gases may alter fire weather, contributing to an increased area burned in the future. Direct carbon emissions from fire (combustion) are estimated to average 27 Tg carbon year–1 for 1959–1999 in Canada. Post-fire decomposition may be of a similar magnitude, and the regenerating forest has a different carbon sink strength. Measurements indicate that there is a net carbon release (source) by the forest immediately after the fire before vegetation is re-established. Daytime downward carbon fluxes over a burned forest take 1–3 decades to recover to those of a mature forest, but the annual carbon balance has not yet been measured. There is a potential positive feedback to global climate change, with anthropogenic greenhouse gases stimulating fire activity through weather changes, with fire releasing more carbon while the regenerating forest is a smaller carbon sink. However, changes in fuel type need to be considered in this scenario since fire spreads more slowly through younger deciduous forests. Proactive fuel management is evaluated as a potential mechanism to reduce area burned. However, it is difficult to envisage that such treatments could be employed successfully at the national scale, at least over the next few decades, because of the large scale of treatments required and ecological issues related to forest fragmentation and biodiversity.

Comparison of turbulence statistics within three boreal forest canopies

Three-dimensional sonic anemometers were used to measure velocities and temperatures within three natural boreal forest canopies. Vertical profiles of atmospheric turbulence statistics for a black spruce forest, a jack pine forest, and a trembling aspen forest, all located in southeastern Manitoba, were plotted and compared. The canopy structures were quite different, with total leaf-area indices of 2, 4 and 10, for the pine, aspen, and spruce forests, respectively.The profiles of the first and second moments differed among the canopies, where velocities decreased more rapidly in the top portions of the denser canopies. The velocity distributions were skewed and kurtotic within all canopies, and showed some differences among the canopies. Eulerian time scale profiles were generally similar among the canopies, and the vertical and streamwise time scale profiles were almost mirror images of each other. Eulerian length scale profiles showed some differences among canopies caused by differences in the velocity profiles. Ratios of vertical-to-horizontal time and length scales had a maximum in mid-canopy.Shear stress profiles were similar in the top parts of all canopies, and upward momentum fluxes were occasionally observed within the canopy trunk space. Countergradient heat fluxes were also observed sometimes. The countergradient fluxes and the skewed, kurtotic velocity distributions indicate the contribution of intermittent, large-scale eddies that are important for energy and mass transfer within canopies.

Future emissions from Canadian boreal forest fires

New estimates of greenhouse gas emissions from Canadian forest fires were calculated based on a revised model for fuel consumption, using both the fire fuel load and the Drought Code of the Canadian Forest Fire Weather Index System. This model was applied to future climate scenarios of 2×CO2 and 3×CO2 environments using the Canadian Global Climate Model. Total forest floor fuel consumption for six boreal ecozones was estimated at 60, 80, and 117 Tg dry biomass for the 1×CO2, 2×CO2, and 3×CO2 scenarios, respectively. These ecozones cover the boreal and taiga regions and account for about 86% of the total fire consumption for Canada. Almost all of the increase in fuel consumption for future climates is caused by an increase in the area burned. The effect of more severe fuel consumption density (kilograms of fuel consumed per square metre) is relatively small, ranging from 0% to 18%, depending on the ecozone. The emissions of greenhouse gases from all Canadian fires are estimated to increase from about 162 T...

Model comparisons for estimating carbon emissions from North American wildland fire

Research activities focused on estimating the direct emissions of carbon from wildland fires across North America are reviewed as part of the North American Carbon Program disturbance synthesis. A comparison of methods to estimate the loss of carbon from the terrestrial biosphere to the atmosphere from wildland fires is presented. Published studies on emissions from recent and historic time periods and five specific cases are summarized, and new emissions estimates are made using contemporary methods for a set of specific fire events. Results from as many as six terrestrial models are compared. We find that methods generally produce similar results within each case, but estimates vary based on site location, vegetation (fuel) type, and fire weather. Area normalized emissions range from 0.23 kg C m−2 for shrubland sites in southern California/NW Mexico to as high as 6.0 kg C m−2 in northern conifer forests. Total emissions range from 0.23 to 1.6 Tg C for a set of 2003 fires in chaparral-dominated landscapes of California to 3.9 to 6.2 Tg C in the dense conifer forests of western Oregon. While the results from models do not always agree, variations can be attributed to differences in model assumptions and methods, including the treatment of canopy consumption and methods to account for changes in fuel moisture, one of the main drivers of variability in fire emissions. From our review and synthesis, we identify key uncertainties and areas of improvement for understanding the magnitude and spatial-temporal patterns of pyrogenic carbon emissions across North America.

Drag coefficients and turbulence spectra within three boreal forest canopies

Atmospheric turbulence was measured within a black spruce forest, a jack pine forest, and a trembling aspen forest, located in southeastern Manitoba, Canada. Drag coefficients (Cd) varied little with height within the pine and aspen canopies, but showed some height dependence within the dense spruce canopy. A constant Cdof 0.15, with the measured momentum flux and velocity profiles, gave good estimates of leaf-area-index (LAI) profiles for the pine and aspen canopies, but underestimated LAI for the spruce canopy.Velocity spectra were scaled using the Eulerian integral time scales and showed a substantial inertial subrange above the canopies. In the bottom part of the canopies, the streamwise and cross-stream spectra showed rapid energy loss whereas the vertical spectra showed an apparent energy gain, in the region where the inertial subrange is expected. The temperature spectra showed an inertial subrange with the expected -2/3 slope at all heights. Cospectra of momentum and heat flux had slopes of about -1 in much of the inertial subrange. Possible mechanisms to explain some of the spectral features are discussed.

Fire weather index system components for large fires in the Canadian boreal forest

Canadian Fire Weather Index (FWI) System components and head fire intensities were calculated for fires greater than 2 km 2 in size for the boreal and taiga ecozones of Canada from 1959 to 1999. The highest noon- hour values were analysed that occurred during the first 21 days of each of 9333 fires. Depending on ecozone, the means of the FWI System parameters ranged from: fine fuel moisture code (FFMC), 90 to 92 (82 to 96 for individual fires); duff moisture code (DMC), 38 to 78 (10 to 140 for individual fires); drought code (DC), 210 to 372 (50 to 600 for individual fires); and fire weather index, 20 to 33 (5 to 60 for individual fires). Fine fuel moisture code decreased, DMC had a mid-season peak, and DC increased through the fire season. Mean head fire intensities ranged from 10 to 28 MW m −1 in the boreal spruce fuel type, showing that most large fires exhibit crown fire behaviour. Intensities of individual fires can exceed 60 MW m −1 . Most FWI System parameters did not show trends over the 41-year period because of large inter-annual variability. A changing climate is expected to create future weather conditions more conducive to fire throughout much of Canada but clear changes have not yet occurred.

Radiological dose conversion factors for generic non-human biota used for screening potential ecological impacts

Abstract Protection of non-human biota from radionuclides in the environment is an important aspect of many environmental assessments. Biosphere transport models can be used to estimate radionuclide concentrations in plants and animals, and the radiological dose is calculated as the product of concentration and a dose-rate conversion factor (DCF). Here, we calculate and present DCF values for 99 radionuclides that can be used in a generic sense to estimate the dose to a wide variety of plants and animals. DCF values for internally incorporated radionuclides are based on the absorption of all emitted radiations from within the body. DCF values for external exposures include immersion in air, water, soil/sediment and vegetation. We implicitly include the energy from the decay progeny if they have a half-life of less than 1 day, to be consistent with many biosphere transport models. The DCF values can be used for simple screening of potential doses in assessments where a specific target organism cannot be defined.

Footprint climatologies for evapotranspiration in a boreal catchment

Abstract A footprint model was applied to the micrometeorological monitoring of evapotranspiration (ET) in a boreal forest catchment. The footprints were weighted by the measured value of ET to give the footprint that determines the ET flux. The resulting footprint climatology was used to investigate the effect of different averaging times, from a single day to a whole season. The climatology defined the area where 90% of ET originates as a reasonable indicator of the most important part of the landscape sampled. The analysis showed that two 12-m high towers would be needed to fully sample the 90% footprint contour of ET in the 0.5 km 2 catchment study area on a seasonal basis. We conclude that it would be difficult to use micrometeorological techniques to monitor ET without doing a quantitative footprint analysis to define the sampled area.

Burning radionuclide question: What happens to iodine, cesium and chlorine in biomass fires?

Fires can mobilize radionuclides from contaminated biomass through suspension of gases and particles in the atmosphere or solubilization and enrichment of the ash. Field and laboratory burns were conducted to determine the fate of I, Cs and Cl in biomass fires. Straw, wood, peat, dulse (seaweed) and radish plants were combusted with temperatures varying from 160 to 1000 degrees C, representing the normal range of field fire temperatures. Loss to the atmosphere increased with fire temperature and during a typical field fire, 80-90% of the I and Cl, and 40-70% of the Cs was lost to the atmosphere. The remainder was left behind in the ash and was soluble. Typically, the ash was enriched in I by a factor of two to three, with higher enrichments of Cs and lower enrichments of Cl, when compared to the initial fuel concentration during field burns. Most of the I was lost to the atmosphere as a gas. If the elements were radioactive isotopes, such as 129I, 137Cs and 36Cl, fires could cause an increased radiological dose to people through inhalation, exposure to ash, or ingestion of plants because of increased uptake of ash leachate.