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211.
Plant macrofossils from 33 rodent middens sampled at three sites between 2910 and 3150 m elevation in the main canyon of the Río Salado, northern Chile, yield a unique record of vegetation and climate over the past 22,000 cal yr BP. Presence of low-elevation Prepuna taxa throughout the record suggests that mean annual temperature never cooled by more than 5°C and may have been near-modern at 16,270 cal yr BP. Displacements in the lower limits of Andean steppe and Puna taxa indicate that mean annual rainfall was twice modern at 17,520–16,270 cal yr BP. This pluvial event coincides with infilling of paleolake Tauca on the Bolivian Altiplano, increased ENSO activity inferred from a marine core near Lima, abrupt deglaciation in southern Chile, and Heinrich Event 1. Moderate to large increases in precipitation also occurred at 11,770–9550 (Central Atacama Pluvial Event), 7330–6720, 3490–2320 and at 800 cal yr BP. Desiccation occurred at 14,180, 8910–8640, and 4865 cal yr BP. Compared to other midden sites in the region, early Holocene desiccation seems to have happened progressively earlier farther south. Emerging trends from the cumulative midden record in the central Atacama agree at millennial timescales with improved paleolake chronologies for the Bolivian Altiplano, implying common forcing through changes in equatorial Pacific sea-surface temperature gradients. 相似文献
212.
This study explored the impacts of sustained heavy grazing in six vegetation types across an altitudinal and rainfall gradient in the Kamiesberg mountain range in Namaqualand. The study was carried out across the fence separating the Leliefontein communal area and surrounding privately owned farms. The communal area has been stocked over decades at approximately twice the government recommended stocking rate, while adjacent privately owned farms have generally adhered to recommended rates. Plant community data were collected from 66, 0.1 ha modified Whittaker plots and analysed for diversity and compositional changes. Consideration of community-wide responses through NMDS ordination showed that heavy grazing did not result in the dominance of a few wide-spread, weedy species in communal areas. Species richness at the 0.1 ha scale was also not affected by different land use practices. However, there was a significant compositional shift away from large woody and succulent shrubs, and an associated increase in dwarf shrubs and herbaceous perennial plants on the communal areas. This shift was only evident on the sandy lowland habitats, while a reduction in perennial grass was recorded in the rocky upland habitats on the communal areas. Compositional shifts towards smaller and more ephemeral species in the communal area are indicative of a system more closely dependent on rainfall. This has implications for people's livelihoods in the region, particularly in light of predicted climate change. 相似文献
213.
We propose a two-dimensional Lagrangian analytical solution for relating source strength and concentration profiles within
and above a plant canopy. The new solution describes passive scalar dispersion under conditions of local advection through
a fetch correction function in a one-dimensional Lagrangian analytical dispersion model. The model is capable of predicting
absolute concentration profiles of passive scalars for different fetches for situations in which the reference concentration
is known or the background concentration is available. Tests of the model showed good agreement with measurements from field
and wind-tunnel experiments. 相似文献
214.
Pollen and plant macrofossil analyses of lake sediments from Anthony Island in the southern Queen Charlotte Islands (Haida Gwaii), British Columbia, reveal 1800 yr of relatively stable temperate rainforest vegetation. Cupressaceae (cedar) pollen percentages and accumulation rates decline about 1000 cal yr BP, coincident with occupation of the island by Haida peoples, who use Thuja plicata (western red cedar) almost exclusively for house construction, dugout canoes, monumental poles, and many other items. Anthropogenic disturbance offers the most likely explanation for the decline of T. plicata. 相似文献
215.
Rodent middens from ice-rich loess deposits are important new paleoenvironmental archives for Eastern Beringia. Plant macrofossils recovered from three middens associated with Dawson tephra (ca. 24,000 14C yr B.P.) at two sites in Yukon Territory include diverse graminoids, forbs, and mosses. These data suggest substantial local scale floristic and habitat diversity in valley settings, including steppe-tundra on well-drained soils, moist streamside meadows, and hydric habitats. Fossil arctic ground squirrel burrows and nesting sites indicate that permafrost active layers were thicker during Pleistocene glacial periods than at present on north-facing slopes. 相似文献
216.
217.
Coral samples collected at the outlet and the inlet of the Third Nuclear Power Plant were measured for the δ18O and δ13C, indicating the surrounding seawater temperature and physiological/biochemical reactions during coral skeletal growth. The
results show that with higher seawater temperature in summer, the values of δ18O are lower but those for δ13C are higher; however, the δ18O extremes lag six weeks behind the δ13C extremes because the highest water temperature lags the maximum number of sunshine hours. Records of the extreme high temperature
observed in the northern South China Sea between Oct. 1987 and Sep. 1988, and the extraordinary low temperature from Oct.
1992 to Feb. 1993 are also preserved in the coralline isotope signals. However, when the water temperature becomes too high
the coral growth seems to stop. 相似文献
218.
A Re-Evaluation of Long-Term Flux Measurement Techniques Part I: Averaging and Coordinate Rotation 总被引:11,自引:16,他引:11
J. J. Finnigan R. Clement Y. Malhi R. Leuning H.A. Cleugh 《Boundary-Layer Meteorology》2003,107(1):1-48
Experience of long term flux measurements over tall canopiesduring the last two decades has revealed that the eddy flux of sensible plus latentheat is typically 30% smaller than the available radiant energy flux. This failureto close the energy balance is less common close to the surface over short roughnessbut is still sometimes seen, especially in complex topography. These observationscast doubt on the results obtained from long term flux studies where daily and annualnet ecosystem exchange is usually the small difference between large positive andnegative fluxes over 24 h. In this paper we investigate this problem by examiningsome fundamental assumptions entailed in analysis of surface exchange by the eddyflux method.In particular, we clarify the form and use of the scalar conservation equation thatunderlies this analysis and we examine the links between averaging period androtation of coordinates in the situation where coordinates are aligned with thewind vector. We show that rotating coordinates so that the x axis is alignedwith the mean wind vector has the effect of high pass filtering the scalar covariance,¯wc, such that contributions to the aerodynamic flux from atmosphericmotions with periods longer than the averaging period are lost while those of shorterperiod are distorted.We compare the effect of computing surface exchange by averaging many shortperiods, in each of which the coordinates are rotated so that the mean verticalvelocity is zero (the method currently adopted in most long-term flux studies),with analysis in long-term coordinates and show a systematic underestimationof surface exchange in the former case. This is illustrated with data from threelong-term forest field sites where underestimations of sensible and latent heatfluxes of 10–15% averaged over many days are seen.Crucial factors determining the loss of flux are the averaging period T, themeasurement height and the content of the scalar cospectrum at periods longerthan T. The properties of this cospectrum over tall canopies in both homogeneousand complex terrain are illustrated by measurements at our three sites and we see thatover tall canopies on flat ground in convectiveconditions, or on hilly sites in near neutralflow, the scalar cospectra have much more low frequency contentthan classical surface-layerspectral forms would predict. We believe that the filtering of this low frequencycovariance by the averaging-rotation operations in common use is a large contributoryfactor to the failure to close the energy balance over tall canopies. 相似文献
219.
Lucia Wick Jacqueline F.N. van Leeuwen Willem O. van der Knaap André F. Lotter 《Journal of Paleolimnology》2003,30(3):261-272
Pollen and plant macrofossils were analysed at Sägistalsee (1935 m asl), a small lake near timber-line in the Swiss Northern Alps. Open forests with Pinus cembra and Abies alba covered the catchment during the early Holocene (9000–6300 cal. BP), suggesting subcontinental climate conditions. After the expansion of Picea abies between 6300 and 6000 cal. BP the subalpine forest became denser and the tree-line reached its maximum elevation at around 2260 m asl. Charcoal fragments in the macrofossil record indicate the beginning of Late-Neolithic human impact at ca. 4400 cal. BP, followed by a extensive deforestation and lowering of the forest-limit in the catchment of Sägistalsee at 3700 cal. BP (Bronze Age). Continuous human activity, combined with a more oceanic climate during the later Holocene, led to the local extinction of Pinus cembra and Abies alba and favoured the mass expansion of Picea and Alnus viridis in the subalpine area of the Northern Alps. The periods before 6300 and after 3700 cal. BP are characterised by high erosion activity in the lake's catchment, whereas during the phase of dense Picea-Pinus cembra-Abies forests (6300–3700 cal. BP) soils were stable and sediment-accumulation rates in the lake were low. Due to decreasing land-use at higher altitudes during the Roman occupation and the Migration period, forests spread beween ca. 2000 and 1500 cal. BP, before human impact increased again in the early Middle Ages. Recent reforestation due to land-use changes in the 20th century is recorded in the top sediments. Pollen-inferred July temperature and annual precipitation suggest a trend to cooler and more oceanic climate starting at about 5500 cal. BP. 相似文献
220.
Kaarina Sarmaja-Korjonen Seija Kultti Nadia Solovieva Minna Väliranta 《Journal of Paleolimnology》2003,30(4):415-426
Mid-Holocene changes in vegetation, palaeohydrology and climate were investigated from the sediments of Lake Vankavad in the northern taiga of the Usa Basin, NE European Russia, through the analysis of pollen, plant macrofossils, Cladocera and diatoms. Lake Vankavad was probably formed at ca. 5000 BP (ca. 5600 cal. BP) and initially it was shallow with a littoral cladoceran fauna. Macrofossil and pollen results suggest that dense Betula-Picea forests grew in the vicinity and the shore was close to the sampling point. At ca. 4600 BP (ca. 5400 cal. BP) the water level rose coincident with the decrease in the density and area of forests, probably caused by cooling climate and accelerated spread of mires. There was also a further rise in the water level at ca. 3500 BP (ca. 3800 cal. BP). The initiation of the lake, followed by two periods of rising water-level, as well as the increase in mire formation, was a consequence of a rise in groundwater level. This probably reflects lower evapotranspiration in a cooling mid-Holocene climate and/or higher precipitation in the lowland area. Also the decreased forest density and area may have contributed to the lower evapotranspiration. It is also possible that permafrost aggradation or changes in peat ecosystems might have affected the hydrological conditions in the area. 相似文献