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1.
Research on grazing effects usually compares the same sites through time or grazed and ungrazed sites over the same time period. Both approaches are complicated in arid environments where grazing can have a long undocumented history and landscapes can be spatially heterogenous. This work employs both approaches simultaneously by comparing grazed and ungrazed samples through both time and space using fossil plant macrofossils and pollen from packrat middens. A series of 27 middens, spanning from 995 yr BP to the present, were collected from Glen Canyon in southeastern Utah, USA. These middens detail vegetation change just prior to, and following, the historical introduction of domesticated grazers and also compares assemblages from nearby ungrazable mesas. Pre-grazing middens, and modern middens from ungrazed areas, record more native grasses, native herbs, and native shrubs such as Rhus trilobata, Amelanchier utahensis, and Shepherdia rotundifolia than modern middens from grazed areas. Ordinations demonstrate that site-to-site variability is more important than any temporal changes, making selection of comparable grazed versus ungrazed study treatments difficult. But within similar sites, the changes through time show that grazing lowered the number of taxa recorded, and lessened the pre-existing site differences, homogenizing the resultant plant associations in this desert grassland. 相似文献
2.
Benjamin M. Rau Dale W. Johnson Bruce A. Roundy Richard F. Miller Todd G. Caldwell Annmarie Lucchesi 《Journal of Arid Environments》2011,75(9):827-835
Vegetation changes associated with climate shifts and anthropogenic disturbance can have major impacts on biogeochemical cycling and soils. Much of the Great Basin, U.S. is currently dominated by sagebrush (Artemisia tridentate (Rydb.) Boivin) ecosystems. Sagebrush ecosystems are increasingly influenced by pinyon (Pinus monophylla Torr. & Frém and Pinus edulis Engelm.) and juniper (Juniperus osteosperma Torr. and Juniperus occidentalis Hook.) expansion. Some scientists and policy makers believe that increasing woodland cover in the intermountain western U.S. offers the possibility of increased organic carbon (OC) storage on the landscape; however, little is currently known about the distribution of OC on these landscapes, or the role that nitrogen (N) plays in OC retention. We quantified the relationship between tree cover, belowground OC, and total below ground N in expansion woodlands at 13 sites in Utah, Oregon, Idaho, California, and Nevada, USA. One hundred and twenty nine soil cores were taken using a mechanically driven diamond tipped core drill to a depth of 90 cm. Soil, coarse fragments, and coarse roots were analyzed for OC and total N. Woodland expansion influenced the vertical distribution of root OC by increasing 15-30 cm root OC by 2.6 Mg ha−1 and root N by 0.04 Mg ha−1. Root OC and N increased through the entire profile by 3.8 and 0.06 Mg ha−1 respectively. Woodland expansion influenced the vertical distribution of soil OC by increasing surface soil (0-15 cm) OC by 2.2 Mg ha−1. Woodland expansion also caused a 1.3 Mg ha−1 decrease in coarse fragment associated OC from 75-90 cm. Our data suggests that woodland expansion into sagebrush ecosystems has limited potential to store additional belowground OC, and must be weighed against the risk of increased wildfire and exotic grass invasion. 相似文献
3.
甘肃平凉地区约80万年以来的植被与气候变迁* 总被引：6，自引：1，他引：6
将该地约80万年以来的植被发展和气候变化历史划分为14个大的阶段.其中距今约80-78万年、20-14万年、10-1万年三个阶段的植被主要为荒漠草原,气候干冷;距今约78-64、55-46、14-10万年三个阶段植被主要为落叶阔林,气候温暖湿润;其它阶段的植被和气候处于上述二者之间.全新世早、中期,本区气候较现代温湿,晚期趋于温凉半干旱. 相似文献
4.
Much of the Great Basin, U.S. is currently dominated by sagebrush (Artemisia tridentate ssp. (Rydb.) Boivin) ecosystems. At intermediate elevations, sagebrush ecosystems are increasingly influenced by pinyon (Pinus monophylla Torr. & Frém.) and juniper (Juniperus osteosperma Torr.) expansion. Some scientists and policy makers believe that increasing woodland cover in the intermountain western US will create new carbon storage on the landscape; however, little is currently known about the distribution of carbon on these landscapes. This is especially true of below ground pools. Our objectives were to quantify the spatial distribution of soil carbon in expansion woodlands, and to determine prescribed fire's effect on soil C and N. We looked at two treatments (control and burn), three microsites (undertree, undershrub, and interspace), and four soil depths (0–8, 8–23, 23–38, and 38–52 cm). The study was conducted over a six year period with one year pre-fire and five years post-fire data. Results for both carbon and nitrogen were similar, indicating the close relationship between the two elements in this ecosystem. Undershrub microsites had higher soil C and N concentrations than interspace and undertree microsites; however, under tree microsites had higher C:N ratio than interspace and undershrub microsites. Carbon and nitrogen concentration tended to decrease with increasing depth at both control and burn sites. Prescribed burning caused immediate increases in surface soil C and N concentration, but over intermediate to longer periods of time no statistically detectable change in soil C or N content occurred from burning. 相似文献
5.
《自然地理学》2013,34(4):302-320
Using a 545-year ponderosa pine (Pinus ponderosa) tree-ring chronology, we examine the drought history of central Oregon to: (1) determine the relationship among drought, ENSO (El Niño/Southern Oscillation), and the PDO (Pacific Decadal Oscillation), and (2) compare the climatic sensitivity of ponderosa pine and western juniper (Juniperus occidentalis) to determine their suitability as interchangeable climate proxies. Our climatic reconstruction explained 35% of the variance in historical Palmer's Drought Severity Index (PDSI) values and revealed severe drought periods during the 1480s, 1630s, 1700s, and 1930s. The most sustained drought period in our reconstruction occurred during the 1930s, with the most severe single drought year occurring in 1489. We found a significant (p ≤ .01) but weak relationship between our ponderosa pine chronology and ENSO and the PDO, explaining 9% and 12% of the variation respectively. Both ponderosa pine and western juniper record periods of severe regional drought, but western juniper is more sensitive to regional and seasonal climatic variations, whereas ponderosa pine is more responsive to temperature change. These differences suggest that their substitutability as climate proxies in dendroecological studies is limited. 相似文献
6.
中国西北干旱区有大面积的戈壁地貌,但对其形成过程的研究不多。阿尔金山北缘的沉积物记录着临近戈壁地貌的发育过程。选择甘肃敦煌南湖绿洲附近的山水沟剖面作为研究对象,在独立的光释光测年基础上,通过对比分析粒度特征、磁化率和有机碳同位素值变化,探讨5 ka BP以来阿尔金山北缘戈壁沉积动力过程及其环境控制因素。结果表明：戈壁发育进程具有波动性,戈壁沉积动力在百（千）年时间尺度上受到环境因素制约,在2.67～3.57 ka BP和1.65～1.95 ka BP干热气候条件下,沉积物搬运动力较弱,戈壁发育受限;在4.27～4.84 ka BP时期水量较大,戈壁发育范围较广,属于扩张阶段。这一结果初步揭示了戈壁地貌演化与气候环境变化的联系。 相似文献
7.
Zhang zulu 《地理学报(英文版)》1994,(Z2)
NorthShandOngPlain(NSP)ispartofalluvialplainthatlocatestonorthOftheYellowffiveradjoiningShandongPrOvince.ThedistributionofPaleochanne1sinNSPcontrolsthedeveloPinganddistributionregUlationsofmicrolandscapes,soilandshallowgrOundwaer.I.TheDistrihutiveTendencyofPale0channels1.Distribution0fgroundpale0channelsAneochannelsinNSPtooktheirsourcesmainlyfromn0ttheastHenanandsouthHebeiProvinces.PelethYellowmverg6nerallystretchedfromSWtoNEalongthebountwofShandOng.ThegroundPaeochannelsinNSPc… 相似文献
8.
Deborah L. Elliott-Fisk 《自然地理学》2013,34(4):299-323
The White Mountains, astride the California-Nevada stateline, are the highest and westernmost of the Great Basin ranges. This range was extensively glaciated during the Quaternary Period. Glacial landforms and scattered erratics were identified in the field using primarily morpho-and lithostratigraphic criteria, and mapped on aerial photographs and topographic maps. Topographic characteristics of the glacial deposits were analyzed using standard statistical procedures. A sequence of glacial deposits was identified in terms of six glacial stages, these glaciations named according to type site, and relative ages inferred. With the exception of perched Stage I (early) deposits along the range crest, reconstructed equilibrium-line altitudes and elevation of the glacier termini increase to the present, with glacier length and inferred size decreasing through time. Preliminary data suggest that weathering and pedogenesis are also progressive, though environmental gradients mask some of these distinctions between deposits. It is hypothesized that the White Mountains have a similar glacial chronology to the adjacent Sierra Nevada, but that with the intensification of the Sierran rainshadow during the Quaternary, the extent of glaciation in the White Mountains apparently decreased through time owing to regional tectonic uplift. This had led to the preservation of a more complete sequence of glacial deposits than in the Sierra Nevada, making the delineation of multiple mid-Quaternary events possible in many valleys. Further radiometric and chronometric dating of these deposits is in progress. [Key words: Glaciation, Quaternary, glacial geomorphology, White Mountains, California, Nevada. 相似文献
9.
Jennifer M. Galloway R. Timothy Patterson Christine T. Doherty Helen M. Roe 《Journal of Paleolimnology》2007,38(4):569-588
Pollen and diatoms preserved in the radiocarbon dated sediments of Two Frog Lake in the Seymour-Belize Inlet Complex of the
central mainland coast of British Columbia document postglacial climate change. Two Frog Lake was isolated from the sea prior
to 11,040 ± 50 yr BP (13,030 cal. yr BP) when the climate was cool and dry, and open Pinus contorta woodlands covered the landscape. These woodlands were replaced by a mixed conifer forest ca. 10,200 yr BP (ca. 12,300 cal.
yr BP) when the climate became moister. A relatively dry and warm early Holocene climate allowed Pseudotsuga menziesii to migrate northward to this site where it grew with Picea, Tsuga heterophylla and Alnus. The climate became cooler and moister at ca. 8,000 yr BP (ca. 9,200 cal. yr BP), approximately 500–1,000 years prior to
sites located south of Two Frog Lake and on the Queen Charlotte Islands, but contemporary with sites on the northern mainland
coast of British Columbia and south coastal Alaska. Climate heterogeneity in central coastal British Columbia appears to have
occurred on a synoptic scale, suggesting that atmospheric dynamics linked to a variable Aleutian Low pressure system may have
had an important influence on early Holocene climate change in the Seymour-Belize Inlet Complex. The transition to cooler
and moister conditions facilitated the expansion of Cupressaceae and the establishment of a modern-type coastal temperate
rainforest dominated by Cupressaceae and T. heterophylla. This was associated with progressive lake acidification. Diatom changes independent of vegetation change during the late
Holocene are correlative with the mid-Neoglacial period, when cooler temperatures altered diatom communities. 相似文献
10.
Marith C. Reheis Janet L. Slate Constance K. Throckmorton John P. McGeehin rei M. Sarna-Wojcicki & Lori Dengler 《Basin Research》1996,8(3):279-299
Well-dated surface and subsurface deposits in semiarid Fish Lake Valley, Nevada and California, demonstrate that alluvial-fan deposition is strongly associated with the warm dry climate of the last two interglacial intervals, and that fans were stable and (or) incised during the last glaciation. Fan deposition was probably triggered by a change from relatively moist to arid conditions causing a decrease in vegetation cover and increases in flash floods and sediment yield. We think that this scenario applies to most of the other valleys in the southern Basin and Range. Radiocarbon, tephra, and a few thermoluminescence and cosmogenic ages from outcrops throughout Fish Lake Valley and from cores on the Leidy Creek fan yield ages of >100–50 ka and 11–0 ka for the last two periods of alluvial-fan deposition. Mapping, coring and shallow seismic profiling indicate that these periods were synchronous throughout the valley and on the proximal and distal parts of the fans. From 50 to 11 ka, fan deposition ceased, a soil formed on the older alluvium and the axial drainage became active as runoff and stream competence increased. Slow deposition due to sheet flow or aeolian processes locally continued during this interval, producing cumulic soil profiles. The soil was buried by debris-flow sediment beginning at about 11 ka, coincident with the onset of relatively dry and warm conditions in the region. However, ground-water discharge maintained a large freshwater marsh on the valley floor throughout the Holocene. Pulses of deposition during the Holocene are recorded in the marsh and fan deposits; some pulses coincided with periods of or transitions to warm, dry climate indicated by proxy climate records, whereas others may reflect local disturbances associated with volcanism and fires. Within the marsh deposits, much of the clastic material is probably desert loess. In addition, the deposition of coppice dunes within the fan deposits coincides with two dry periods during the late Holocene. 相似文献
11.
阿尼玛卿山地祁连圆柏径向生长对气候的响应 总被引：2，自引：0，他引：2
通过对阿尼玛卿山地5 个坡面20 个祁连圆柏树轮宽度标准年表的分析, 发现公共区间内各树木年轮宽度标准年表中的信噪比SNR 和样本总解释量EPS 值都较高, 说明年表中都含有较强的环境信息; 树轮宽度年表之间的平均相关为0.35 (大多达到95%的置信程度), 具有较好的区域一致性。聚类分析使树木年轮宽度年表分成东、西两大部分, 而年表的第一主分量显示出: 西部树轮的指数序列明显长于东部即树木生长有自西向东扩展的趋势, 并且东、西区域树木生长变化的特征年变化具有同步性。东、西树轮宽度年表中第一主分量和第二主分量与气候因子的相关和响应都表现出一定的相似性, 但第二主分量对气候因子响应的差异性更显著。西部树木生长PCI 的主要限制因子是温度, 尤其当年春末夏初及前一年秋季温度影响最大, 同样降水也起着重要的作用, 西部树木生长PC2 的主要限制因子是降水量; 而东部树木生长PC1 主要受降水的制约, 第二主分量PC2 的响应分析显示出气候的滞后影响(树木前期生长) 是非常重要的限制因子。 相似文献
12.
查格勒布鲁剖面—晚更新世以来东亚季风进退的地层记录 总被引：3，自引：1，他引：3
本文着重分析了查格勒布鲁剖面沉积物的粒度特征和孢粉组合特征，结合（１４）Ｃ测年、古冰缘现象和沉积物化学元素的ＳｉＯ２／Ａｌ２Ｏ３分子比值变化规律指出，晚更新世以来巴丹吉林沙漠南缘地区的气候条件随全球冰期气候的波动亦经历了由晚更新世早期的相对温湿阶段、晚更新世晚期尤其是末次盛冰期的干冷阶段向全新世温暖期的演化过程。东亚夏季风尾阎在晚更新世早期到达甚至越过本区影响到拐子湖一带，晚更新世晚期本区受冬季风的控制，全新世以来东亚夏季风尾闾又向本区推进，并于全新世大暖期对本区造成明显影响。 相似文献
13.
14.
Rock varnish microlamination dating of late Quaternary geomorphic features in the drylands of western USA 总被引：1，自引：1，他引：0
Varnish microlamination (VML) dating is a correlative age determination technique that can be used to date and correlate various geomorphic features in deserts. In this study, we establish a generalized late Quaternary (i.e., 0–300 ka) varnish layering sequence for the drylands of western USA and tentatively correlate it with the SPECMAP oxygen isotope record. We then use this climatically correlated varnish layering sequence as a correlative dating tool to determine surface exposure ages for late Quaternary geomorphic features in the study region. VML dating of alluvial fan deposits in Death Valley of eastern California indicates that, during the mid to late Pleistocene, 5–15 ky long aggradation events occurred during either wet or dry climatic periods and that major climate shifts between glacial and interglacial conditions may be the pacemaker for alteration of major episodes of fan aggradation. During the Holocene interglacial time, however, 0.5–1 ky long brief episodes of fan deposition may be linked to short periods of relatively wet climate. VML dating of alluvial desert pavements in Death Valley and the Mojave Desert reveals that pavements can be developed rapidly (< 10 ky) during the Holocene (and probably late Pleistocene) in the arid lowlands (< 800 m msl) of these regions; but once formed, they may survive for 74–85 ky or even longer without being significantly disturbed by geomorphic processes operative at the pavement surface. Data from this study also support the currently accepted, “being born at the surface” model of desert pavement formation. VML dating of colluvial boulder deposits on the west slope of Yucca Mountain, southern Nevada, yields a minimum age of 46 ka for the emplacement of these deposits on the slope, suggesting that they were probably formed during the early phase of the last glaciation or before. These results, combined with those from our previous studies, demonstrate that VML dating has great potential to yield numerical age estimates for various late Quaternary geomorphic features in the western USA drylands. 相似文献
15.
新疆博斯腾湖地区全新世以来的孢粉组合与环境 总被引：11，自引：3，他引：8
本文通过对新疆博斯腾湖地区沉积物的孢粉分析,讨论了全新世以来的植被和气候变化的四个阶段,冻结结合其它沉积特征,反映了这一时期湖泊的水面的收缩和扩张的状况和自然环境的演变． 相似文献
16.
中国干旱、半干旱区末次冰期以来气候变化规律 总被引：18，自引：11，他引：7
以临夏塬堡末次冰期中晚期黄土剖面为基础,选用我国干旱、半干旱区末次冰期以来同期异相或同期同相的气候记录与研究剖面进行对比分析后表明：位于西风气流控制的新疆地区全新世气候总体表现为偏干偏冷,晚期较为温暖的特点．其气候波动的幅度和频率较低。而位于季风环流控制的其它地区．全新世气候变化反映了同样的变化规律,即早全新世的升温波动期、中全新世的温暖稳定期和晚全新世的变冷期,其气候波动的幅度和频率较快。而无论是西风气流控制的新疆地区,还是季风环流影响的其它我国干旱、半干旱区,末次冰期气候变化均表现为早晚期的干冷期和中期的温湿期,尽管其干冷和温湿的程度不尽相同。 相似文献
17.
试论西藏全新世古地理的演变 总被引：13，自引：1，他引：12
本文从冰后期以来湖泊的退缩、泥炭沼泽的发育、冰川与冻土的变化、生物的演替以及人类活动范围的扩大与缩小等方面论述西藏全新世古地理环境特征和演变过程,并结合C14的年龄数据,认为全新世时期西藏自然环境的演变可分为早全新世（10,000-7,500年）环境好转阶段、中全新世（7,500-3,000年）环境最宜阶段以及晚全新世（3,000年至现在）环境恶化阶段.而且全新世不同时期古地理环境具有明显的地域差异.现时西藏地区仍在继续上升,高原内部仍将继续向干冷的自然环境变化. 相似文献
18.
湖泊沉积记录的环境演变是全球变化的重要研究领域之一,通过对中国内陆区中30 个湖泊研究成果的总结和梳理,探讨了全新世以来该地区干湿变化的规律和区域分异。通过降水量结合传统分区方法将中国内陆区分为西北干旱区、东亚季风边缘区和青藏高原区。对每个湖泊样点以500 年为时间间隔,以孢粉为主要干湿指标,综合氧同位素、有机质及碳酸盐等,将湖泊干湿状况划分3 个干湿等级（干旱,半湿润,湿润）,建立区域干湿指数。结果表明,中国内陆不同区域全新世可能经历了不同的干湿变化过程：①西北干旱区基本上是早中全新世干旱,中晚全新世相对湿润,但区域差异明显;②东亚季风边缘区早全新世早期干旱,早全新世晚期和中全新世相对湿润,晚全新世干旱;③青藏高原区的湿润时期主要发生在早中全新世,但是不同地区有所不同。对比分析显示：西北干旱区的干湿变化可能受西风环流控制,但在时间和强度上区域内部差异较明显;东亚季风边缘区可能主要受东亚季风控制;青藏高原地区早中全新世的湿润可能与印度季风的增强相关。 相似文献
19.
对新疆伊犁河谷可克达拉剖面(TKP)晚全新世沉积粒度参数及其气候意义分析结果表明:总体为植被较好、输沙势较弱的沉积环境。可以划分为5个气候变化阶段:3.71~3.06 ka B.P.风力由强到弱,气候暖由干到凉湿;3.06~2.78 ka B.P.风力很强、气候暖干;2.78~2.10 ka B.P.风力较弱,气候湿润;2.10~0.5 ka B.P.前期风力振荡频繁,气候较湿润;后期风力减弱、气候冷湿、成壤作用较强;0.50 ka B.P.以来,以相对冷湿为主,最近100 a来风力加强、气候趋于暖干。各阶段起止与全球气候变化基本一致,但受西风环流传递北高纬大西洋区域气候变化的影响,各气候期水热组合呈现典型西风带模式特征。 相似文献
20.
大小兴安岭高位泥炭孢粉纪录及泥炭发育和演替过程研究 总被引：9，自引：2，他引：9
根据３个高位泥炭剖面孢粉分析和＾１４Ｃ测年数据，以及植物残体、微量元素与营养元素等分析相互印证，揭示大小安岭分别５０００ａ Ｂ．Ｐ．和２７００ａ Ｂ．Ｐ．以来孢粉组合特征和植物群与气候演变过程。分析表明大小兴安岭高位泥炭形成了低、中和高位发育阶段的演变，大兴安岭高位泥形成晚于小兴安岭。１３００ａ Ｂ．Ｐ．大小兴安岭普遍发育高位泥炭。该区图强和汤北（９４－Ａ）高位泥炭剖面分别是大小兴安岭高位泥炭标准 相似文献