An estimate of carbon emissions from 2004 wildfires across Alaskan Yukon River Basin.

BACKGROUND: Wildfires are an increasingly important component of the forces that drive the global carbon (C) cycle and climate change as progressive warming is expected in boreal areas. This study estimated C emissions from the wildfires across the Alaskan Yukon River Basin in 2004. We spatially related the firescars to land cover types and defined the C fractions of aboveground biomass and the ground layer (referring to the top 15 cm organic soil layer only in this paper) consumed in association with land cover types, soil drainage classes, and the C stocks in the ground layer. RESULTS: The fires led to a burned area of 26,500 km2 and resulted in the total C emission of 81.1 +/- 13.6 Tg (Tg, Teragram; 1 Tg = 1012 g) or 3.1 +/- 0.7 kg C m-2 burned. Of the total C emission, about 73% and 27% could be attributed to the consumption of the ground layer and aboveground biomass, respectively. CONCLUSION: The predominant contribution of the ground layer to the total C emission implies the importance of ground fuel management to the control of wildfires and mitigation of C emissions. The magnitude of the total C emission depends on fire extent, while the C loss in kg C m-2 burned is affected strongly by the ground layer and soil drainage condition. The significant reduction in the ground layer by large fires may result in profound impacts on boreal ecosystem services with an increase in feedbacks between wildfires and climate change.

Comparative water use and nitrogen relationships in a mistletoe and its host.

The impact of the xylem-tapping mistletoe Phoradendron juniperinum on the nitrogen and water relations of its host Juniperus osteosperma was investigated under natural field conditions. Leaf conductance, leaf water potential, and leaf Kjeldahl nitrogen contents were followed through the growing season on mistletoes, infected junipers (separating infected from uninfected stems) and uninfected junipers. Infected trees experienced lower leaf water potentials than uninfected trees and also had lower leaf conductances and lower leaf nitrogen contents. Infected juniper stems had higher conductances than uninfected stems. Mistletoes had higher leaf nitrogen contents than their hosts and much of this nitrogen appeared as arginine, a potential nitrogen storage compound. Photosynthetic rates (per unit leaf area) were significantly higher in junipers than in the mistletoe, and higher in the uninfected than infected junipers. Water use efficiencies as estimated by carbon isotope ratios were significantly lower in mistletoes than in their hosts. Increased mistletoe infestation appeared to increase absolute water use efficiency of both host and mistletoe.

The distribution of C3 and C4 grasses and carbon isotope discrimination along an altitudinal and moisture gradient in Kenya.

More than 500 species of the Poaceae are found in Kenya, East Africa. Eighteen of twenty-seven tribes are exclusively (except the Paniceae and Danthonieae) of the C3 photosynthetic type. A floristic analysis of low altitude grasslands suggests that nearly all species at these low altitudes are of the C4 photosynthetic type. At high altitudes, however, nearly all grasses are of the C3 photosynthetic type. Open grassland vegetation was sampled along a transect from arid low altitude sites to the top of Mt. Kenya in an attempt to document the general distributions of the photosynthetic types.The major tribes illustrated three general patterns of distribution. The C4 tribes Chlorideae, Eragrosteae, Sporoboleae, and Aristideae were abundant at low altitudes (or low indices of available soil moisture). The Paniceae and Andropogoneae were also exclusively C4 but were more common at intermediate altitudes. The C3 tribes Aveneae, Festuceae, and Agrostideae were found only at high altitudes. In these open grasslands there were no C3 species below 2,000 m and no C4 species above 3,000 m. The variation in δ13C of the live grass vegetation with altitude confirms these distributional patterns and suggests a sharp transition zone between these two photosynthetic types. The photosynthetic type accounts for broad distributions within the Poaceae but these distributions are further modified by characteristics which may be inherent in the tribal groups. Ecological and paleoecological significance of these patterns of distribution are discussed.

Use of δ13C values to determine vegetation selectivity in East African herbivores.

The quantitative plant species composition of the rumen contents of a large number of individuals from eight East African herbivores was determined by direct visual analysis. All plant species were classified as either C3 or C4, and an estimated δ13C for the rumen sample was calculated. This estimated value was compared to a measured value determined directly from rumen subsample. The two methods of determining quantitative C3 and C4 composition differed by less than 1%, and the isotopic analysis has the advantage of being rapid and totally objective.The isotopic analysis allowed us to differentiate between grazers and browsers and to determine the quantitative dependence of each animal on C3 and C4 photosynthetic types. Kongoni, wildebeest, cattle, and sheep were nearly pure grazers on the Athi Kapiti Plains; and the Grant's gazelle were predominantly browsers. Thompson's gazelle, goast and impala were intermediate. The species most dependent upon browse showed a marked and rapid shift to grass within a few days following rain. This isotopic method may have general utility in the study of East African ecology.

Aboveground biomass allocation, leaf growth, and photosynthesis patterns in tundra plant forms in arctic Alaska.

Tundra plant growth forms can generally be characterized as consisting predominantly of low-growing perennial grasses and sedges, perennial herbaceous forbs, dwarf deciduous shrubs, and dwarf evergreen shrubs. Gross aboveground carbon allocation, leaf growth, and photosynthesis pattern studies were initiated to develop a quantitative understanding of the functional importance of these particular tundra growth forms. Photosynthetic capacities of 13 species were determined under standardized exposure conditions using a14CO2 field system and ranged between 5 and 47 mg CO2·g dry wt-1·h-1. These results, in conjunction with detailed leaf growth determinations, support the generalization that species with an evergreen growth form have lower photosynthetic capacities than species with a perennial graminoid, forb, or deciduous shrub growth form. However, these low photosynthetic capacities in evergreen shrubs are associated with relatively extended leaf longevities. Conversely, deciduous shrub forms exhibited high photosynthetic capacities, but were offset by relatively short leaf longevity periods. The perennial grasses, sedges, and forbs showed patterns intermediate to these. As a result, it appears that among tundra species of different growth form, photosynthetic capacity is inversely related to leaf longevity.