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Climatic Periods (climatic + period)
Selected AbstractsAn empirical model of carbon fluxes in Russian tundraGLOBAL CHANGE BIOLOGY, Issue 2 2001Dmitri G. Zamolodchikov Summary This study presents an empirical model based on a GIS approach, which was constructed to estimate the large-scale carbon fluxes over the entire Russian tundra zone. The model has four main blocks: (i) the computer map of tundra landscapes; (ii) data base of long-term weather records; (iii) the submodel of phytomass seasonal dynamics; and (iv) the submodel of carbon fluxes. The model uses exclusively original in situ diurnal CO2 flux chamber measurements (423 sample plots) conducted during six field seasons (1993,98). The research sites represent the main tundra biome landscapes (arctic, typical, south shrub and mountain tundras) in the latitudinal diapason of 65,74°N and longitudinal profile of 63°E,172°W. The greatest possible diversity of major ecosystem types within the different landscapes was investigated. The majority of the phytomass data used was obtained from the same sample plots. The submodel of carbon fluxes has two dependent [GPP, Gross Respiration (GR)] and several input variables (air temperature, PAR, aboveground phytomass components). The model demonstrates a good correspondence with other independent regional and biome estimates and carbon flux seasonal patterns. The annual GPP of Russian tundra zone for the area of 235 × 106 ha was estimated as ,485.8 ± 34.6 × 106 tC, GR as +474.2 ± 35.0 × 106 tC, and NF as ,11.6 ± 40.8 × 106 tC, which possibly corresponds to an equilibrium state of carbon balance during the climatic period studied (the first half of the 20th century). The results advocate that simple regression-based models are useful for extrapolating carbon fluxes from small to large spatial scales. [source] Salinization of a Fresh Palaeo-Ground Water Resource by Enhanced RechargeGROUND WATER, Issue 1 2003F.W. Leaney Deterioration of fresh ground water resources caused by salinization is a growing issue in many arid and semi-arid parts of the world. We discuss here the incipient salinization of a 104 km2 area of fresh ground water (<3000 mg/L) in the semiarid Murray Basin of Australia caused by widespread changes in land use. Ground water 14C concentrations and unsaturated zone Cl soil water inventories indicate that the low salinity ground water originated mainly from palaeo-recharge during wet climatic periods more than 20,000 years ago. However, much of the soil water in the 20 to 60 m thick unsaturated zone throughout the area is generally saline (> 15,000 mg/L) because of relatively high evapotranspiration during the predominantly semiarid climate of the last 20,000 years. Widespread clearing of native vegetation over the last 100 years and replacement with crops and pastures leads to enhancement of recharge rates that progressively displace the saline soil-water from the unsaturated zone into the ground water. To quantify the impact of this new hydrologic regime, a one-dimensional model that simulates projected ground water salinities as a function of depth to ground water, recharge rates, and soil water salt inventory was developed. Results from the model suggest that, in some areas, the ground water salinity within the top 10 m of the water table is likely to increase by a factor of 2 to 6 during the next 100 years. Ground water quality will therefore potentially degrade beyond the point of usefulness well before extraction of the ground water exhausts the resource. [source] Late-glacial and Holocene climatic effects on fire and vegetation dynamics at the prairie,forest ecotone in south-central MinnesotaJOURNAL OF ECOLOGY, Issue 5 2003Philip Camill Summary 1Treeline ecotones, such as the prairie,forest boundary, represent climatically sensitive regions where the relative abundance of vegetation types is controlled by complex interactions between climate and local factors. Responses of vegetation and fire to climate change may be tightly linked as a result of strong feedbacks among fuel production, vegetation structure and fire frequency/severity, but the importance of these feedbacks for controlling the stability of this ecotone is unclear. 2In this study, we examined the prairie,forest ecotone in south-central Minnesota using two lake sediment cores to reconstruct independent records of climate, vegetation and fire over the past 12 500 years. Using pollen, charcoal, sediment magnetic analyses and LOI properties, we investigated whether fires were controlled directly by climate or indirectly by fuel production. 3Sediment magnetic and LOI data suggest four broad climatic periods occurring c. 11 350,8250 BP (cool/humid), c. 8250,4250 BP (warm/dry), c. 4250,2450 BP (warm/humid), and c. 2450,0 BP (cool/humid), indicating that, since the mid-Holocene, climate has shifted towards wetter conditions favouring greater in-lake production and fuel production on the landscape. 4The area surrounding both lakes was characterized by boreal forest c. 12 500,10 000 BP, changing to an Ulmus-Ostrya forest c. 10 000,9000 BP, changing to a community dominated by prairie (Poaceae-Ambrosia-Artemisia) and deciduous forest taxa c. 8000,4250 BP, and finally shifting to a Quercus -dominated woodland/savanna beginning c. 4250,3000 BP. 5Charcoal influx increased from an average of 0.11,0.62 mm2 cm,2 year,1 during the early Holocene forest period (c. 11 350,8250 BP) to 1.71,3.36 mm2 cm,2 year,1 during the period of prairie expansion (c. 8250,4250 BP) and again increased to 4.18,4.90 mm2 cm,2 year,1 at the start of the woodland/savanna period (c. 4250 BP). 6As a result of the influence of climate on community composition and fuel productivity, changes in fire severity may be the result and not the cause of shifts in vegetation. [source] Threshold wind velocity as an index of soil susceptibility to wind erosion under variable climatic conditionsLAND DEGRADATION AND DEVELOPMENT, Issue 1 2009Laura A. de Oro Abstract Wind erosion starts when the threshold wind velocity (µt) is exceeded. We evaluated the sensitivity of µt to determine the wind erosion susceptibility of soils under variable climatic conditions. Three years field data were used to calculate µt by means of the equation µt,=,, - , ,,1 (,), where , is the mean wind speed (m,s,1), , the , standard deviation (m,s,1), , the saltation activity and , the standard normal distribution function of ,. Saltation activity was measured with a piezoelectric sensor (Sensit). Results showed that , of the whole studied period (3·41 m,s,1) was lower than µt (7·53,m,s,1), therefore, wind erosion was produced mainly by wind gusts. The µt values ordered in the sequence: Winter (6·10 m,s,1),<,Spring (8·22,m,s,1),=,Summer (8·28,m,s,1),<,Autumn (26·48,m,s,1). Higher µt values were related to higher air humidity and lower wind speeds and temperatures. The µt values did not agree with the erosion amounts of each season, which ordered as follows: Summer (12·88,t ha,1),>,Spring (3·11,t ha,1),=,Winter (0·17,t ha,1),=,Autumn (no erosion). Low µt and erosion amounts of Winter were produced by a scarce number of gusts during eroding storms. We concluded that µt is useful as an index of soil susceptibility to wind erosion of different climatic periods. The use of a unique µt value in wind erosion prediction models can lead to erroneous wind erosion calculations. Copyright © 2008 John Wiley & Sons, Ltd. [source] | ||||||||||