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Pollen Taxa (pollen + taxa)
Selected AbstractsAnthropogenic and climatic impacts on surface pollen assemblages along a precipitation gradient in north-eastern ChinaGLOBAL ECOLOGY, Issue 5 2010Yun Zhang ABSTRACT Aim, To understand the scenarios of ,anthropogenic biomes' that integrate human and ecological systems, we need to explore the impacts of climate and human disturbance on vegetation in the past and present. Interactions among surface pollen, modern vegetation and human activities along climate and land-use gradients are tested to evaluate the natural and anthropogenic forces shaping the modern vegetation, and hence to aid the reconstruction of vegetation and climate in the past. This in turn will help with future predictions. Location, The North-east China Transect (NECT) in north-eastern China. Methods, We analysed 33 surface pollen samples and 213 quadrats across four vegetation zones along the moisture/land-use gradients of the NECT. Detrended correspondence analysis (DCA) and redundancy analysis (RDA) of 52 pollen taxa and three environmental variables were used to distinguish anthropogenic and climatic factors that affect surface pollen assemblages along the NECT. Results, The 33 surface samples are divided into four pollen zones (forest, meadow steppe, typical steppe and desert steppe) corresponding to major vegetation types in the NECT. Variations in pollen ratios of fern/herb (F/H), Artemisia/Chenopodiaceae (A/C) and arboreal pollen/non-arboreal pollen (AP/NAP) represent the vegetation and precipitation gradient along the NECT. DCA and RDA analyses suggest that surface pollen assemblages are significantly influenced by the precipitation gradient. Changes in the abundance of Chenopodiaceae pollen are related to both human activities and precipitation. Main conclusions, Surface pollen assemblages, fossil pollen records, archaeological evidence and historical documents in northern China show that a large increase of Chenopodiaceae pollen indicates human-caused vegetation degradation in sandy habitats. The A/C ratio is a good indicator of climatic aridity, but should be used in conjunction with multiple proxies of human activities and climate change in the pollen-based reconstruction of anthropogenic biomes. [source] Modern pollen precipitation from an elevational transect in central Jordan and its relationship to vegetationJOURNAL OF BIOGEOGRAPHY, Issue 10 2001Caroline P. Davies Aim To explore the relationship between modern pollen precipitation and vegetation patterns in an arid region of the Middle East. Location Data come from the central Jordan Rift from 1700 m elevation in the highlands to 300 m below sea level in the Dead Sea basin. Methods Modern pollen samples and descriptive vegetation data were collected from twenty-one locations at 100 m elevational intervals from the highest elevations on the eastern side of the rift valley, where woodlands grow, to the lowest elevation desert on earth, characterized by drought and salt tolerant plants. Pollen percentage data from each vegetation zone are compared descriptively and numerically using cluster and Principal Components Analyses (PCA). Results Three major vegetation zones: woodland, shrub steppe, and desert scrub, are identified by their dominant plant species. The widely spaced tree, Quercus calliprinos Webb, defines the Quercus L. woodland that grows above 1500 m elevation. The shrub steppe can be divided into two subzones found between about 1500 and 900 m elevation: Artemisia herba-alba Asso shrub steppe and Artemisia L. shrub steppe with Juniperus phoenica L. Desert scrub dominates the lower elevation landscape with Hammada salicornia (Moq.) Iljin the dominant shrub between 900 and 200 m and Haloxylon persicum Bge. found below 200 m elevation. Pollen spectra reflect the elevational vegetation zones. In particular, Quercus L., Juniperus L. and Tamarix L. pollen are abundant where the trees grow. Highly variable amounts of non-arboreal pollen taxa , primarily Artemisia L. and Chenopodiaceae Vent. , differentiate shrub steppe from desert scrub vegetation. Cluster and PCA of pollen data support the qualitative vegetation zonation. Main conclusions The main vegetation zones along the Jordan Rift from 1700 to ,300 m elevation can be distinguished by their modern pollen precipitation. Open vegetation types, in particular, can be recognized by their pollen spectra. High amounts of Artemisia L. pollen distinguish the moister upper elevations where Artemisia L. steppe grows. In contrast, greater amounts of Chenopodiaceae Vent. pollen characterize the drier, lower elevation deserts. [source] Pollen-inferred palaeoclimate reconstructions in mountain areas: problems and perspectives,JOURNAL OF QUATERNARY SCIENCE, Issue 6 2006Elena Ortu Abstract Transfer functions are an efficient tool for the quantitative reconstruction of past climate from low to mid-elevation pollen sites. However, the application of existing methods to high-altitude pollen assemblages frequently leads to unrealistic results. In the aim of understanding the causes of these biases, the standard ,best modern analogue' method has been applied to two high-altitude pollen sequences to provide quantitative climate estimates for the Lateglacial and Holocene periods. Both pollen sequences (Laghi dell'Orgials, 2130,m, SW aspect and Lago delle Fate, 2240,m, E aspect) are located in the subalpine belt, on opposing sides of the St. Anna di Vinadio Valley (Italian Maritime Alps). Different results were obtained from the two sequences. The largest differences occurred in palaeotemperature reconstruction, with notable differences in both the values and trends at each site. These biases may be attributed to: (1) a lack of high elevation ,best modern analogues' in the database of modern samples; (2) the problem of pollen taxa that have multiple climatic significance; (3) problems related to the complexity of mountainous ecosystems, such as the phenomenon of uphill transport of tree pollen by wind. Possible improvements to the reconstruction process are discussed. Copyright © 2006 John Wiley & Sons, Ltd. [source] Pollen-based biome reconstructions for Colombia at 3000, 6000, 9000, 12 000, 15 000 and 18 000 14C yr ago: Late Quaternary tropical vegetation dynamicsJOURNAL OF QUATERNARY SCIENCE, Issue 2 2002Robert Marchant Abstract Colombian biomes are reconstructed at 45 sites from the modern period extending to the Last Glacial Maximum (LGM). The basis for our reconstruction is pollen data assigned to plant functional types and biomes at six 3000-yr intervals. A reconstruction of modern biomes is used to check the treatment of the modern pollen data set against a map of potential vegetation. This allows the biomes reconstructed at past periods to be assessed relative to the modern situation. This process also provides a check on the a priori assignment of pollen taxa to plant functional types and biomes. For the majority of the sites, the pollen data accurately reflect the potential vegetation, even though much of the original vegetation has been transformed by agricultural practices. At 18 000 14C yr BP, a generally cool and dry environment is reflected in biome, assignments of cold mixed forests, cool evergreen forests and cool grassland,shrub; the latter extending to lower altitudes than presently recorded. This signal is strongly recorded at 15 000 and 12 000 14C yr BP, the vegetation at these times also reflecting a relatively cool and dry environment. At 9000 14C yr BP there is a shift to biomes thought to result from slightly cooler environmental conditions. This trend is reversed by 6000 14C yr BP; most sites, within a range of different environmental settings, recording a shift to more xeric biome types. There is an expansion of steppe and cool mixed-forest biomes, replacing tropical dry forest and cool grassland,shrub biomes, respectively. These changes in biome assignments from the modern situation can be interpreted as a biotic response to mid-Holocene climatic aridity. At 3000 14C yr BP the shift is mainly to biomes characteristic of slightly more mesic environmental conditions. There are a number of sites that do not change biome assignment relative to the modern reconstruction, although the affinities that these sites have to a specific biome do change. These ,anomalies' are interpreted on a site-by-site basis. Spatially constant, but differential response of the vegetation to climatic shifts are related to changes in moisture sources and the importance of edaphic controls on the vegetation. The Late Quaternary reconstruction of large-scale vegetation dynamics in Colombia allows an understanding of the environmental controls on these to be developed. In particular, shifts in the character of the main climatic systems that influence Colombian vegetation are described. Copyright © 2002 John Wiley & Sons, Ltd. [source] | ||||||||||