In this paper, the summer climate of 1991 in North China is simulated by using the high-resolution regional climate model (RegCM2) and the effects of the grassland desertification on summer drought in the central and ...In this paper, the summer climate of 1991 in North China is simulated by using the high-resolution regional climate model (RegCM2) and the effects of the grassland desertification on summer drought in the central and the northern parts of North China as well as Mongolia are studied. It shows that the regional climate model essentially catches the characteristics on distribution and seasonal variation of the precipitation that keep good agreement with the observation. The desertification makes precipitation in the central part of North China during its flood period decrease obviously in July. The border of the precipitation or the soil moisture reduction in the desertification region extends about one latitude southeastward and beyond the southeast edge of the desertification. Thus, vegetation in the border region approaches desertification further. However, there appears evident difference of variation of precipitation over the whole desertification region. The grassland desertification greatly changes the transfers of fluxes between land and atmosphere. The secondary circulation or secondary circulation cells in the desertification region are excited and as a result moisture transport is changed. The variation of flux transfers between land and atmosphere as well as the vertical motion of atmosphere is closely related to that of precipitation.展开更多
Grassland cover and composition respond to climate and have undoubtedly changed during the Holocene, but quantitative reconstructions from fossil pollen have been vague about spatial scale and taxon-specific cover. He...Grassland cover and composition respond to climate and have undoubtedly changed during the Holocene, but quantitative reconstructions from fossil pollen have been vague about spatial scale and taxon-specific cover. Here, we estimate the relevant source area of pollen for sedimentary basins approximately 50 m in radius, and we report pollen productivity estimates for 12 plant taxa in the tallgrass prairies of central North America. Both relevant source area of pollen and pollen productivity estimates were calculated via the Extended R-Value Model. To obtain these estimates, we collected and quantified the pollen found in surface sediment samples from 24 ponds across the study area. Vegetation was surveyed in the field in a 100 m radius around each pond, and vegetation maps from the Kansas Gap Analysis Project (GAP) were used to a radius of 2 km. Pollen fall speeds were calculated according to Stoke’s Law. Pollen assemblages from basins approximately 50 m in radius have a relevant source area of 1060 m in this grassland landscape. Pollen productivity estimates range from 0.02 to over 30 among the 12 taxa: Artemisia, Ambrosia, Asteraceae, Chenopodiaceae, Cornus, Fabaceae, Juniperus, Maclura, Poaceae, Populus, Quercus, and Salix. Woody taxa generally have higher pollen productivity than herbaceous taxa (except for Chenopodiaceae and Ambrosia).展开更多
文摘In this paper, the summer climate of 1991 in North China is simulated by using the high-resolution regional climate model (RegCM2) and the effects of the grassland desertification on summer drought in the central and the northern parts of North China as well as Mongolia are studied. It shows that the regional climate model essentially catches the characteristics on distribution and seasonal variation of the precipitation that keep good agreement with the observation. The desertification makes precipitation in the central part of North China during its flood period decrease obviously in July. The border of the precipitation or the soil moisture reduction in the desertification region extends about one latitude southeastward and beyond the southeast edge of the desertification. Thus, vegetation in the border region approaches desertification further. However, there appears evident difference of variation of precipitation over the whole desertification region. The grassland desertification greatly changes the transfers of fluxes between land and atmosphere. The secondary circulation or secondary circulation cells in the desertification region are excited and as a result moisture transport is changed. The variation of flux transfers between land and atmosphere as well as the vertical motion of atmosphere is closely related to that of precipitation.
文摘Grassland cover and composition respond to climate and have undoubtedly changed during the Holocene, but quantitative reconstructions from fossil pollen have been vague about spatial scale and taxon-specific cover. Here, we estimate the relevant source area of pollen for sedimentary basins approximately 50 m in radius, and we report pollen productivity estimates for 12 plant taxa in the tallgrass prairies of central North America. Both relevant source area of pollen and pollen productivity estimates were calculated via the Extended R-Value Model. To obtain these estimates, we collected and quantified the pollen found in surface sediment samples from 24 ponds across the study area. Vegetation was surveyed in the field in a 100 m radius around each pond, and vegetation maps from the Kansas Gap Analysis Project (GAP) were used to a radius of 2 km. Pollen fall speeds were calculated according to Stoke’s Law. Pollen assemblages from basins approximately 50 m in radius have a relevant source area of 1060 m in this grassland landscape. Pollen productivity estimates range from 0.02 to over 30 among the 12 taxa: Artemisia, Ambrosia, Asteraceae, Chenopodiaceae, Cornus, Fabaceae, Juniperus, Maclura, Poaceae, Populus, Quercus, and Salix. Woody taxa generally have higher pollen productivity than herbaceous taxa (except for Chenopodiaceae and Ambrosia).
