排序方式： 共有21条查询结果，搜索用时 109 毫秒
1.
A growing body of evidence implies that the concept of 'treeless tundra' in eastern and northern Europe fails to explain the rapidity of Lateglacial and postglacial tree population dynamics of the region, yet the knowledge of the geographic locations and shifting of tree populations is fragmentary. Pollen, stomata and plant macrofossil stratigraphies from Lake Kurjanovas in the poorly studied eastern Baltic region provide improved knowledge of ranges of north‐eastern European trees during the Lateglacial and subsequent plant population responses to the abrupt climatic changes of the Lateglacial/Holocene transition. The results prove the Lateglacial presence of tree populations (Betula, Pinus and Picea) in the eastern Baltic region. Particularly relevant is the stomatal and plant macrofossil evidence showing the local presence of reproductive Picea populations during the Younger Dryas stadial at 12 900–11 700 cal. a BP, occurring along with Dryas octopetala and arctic herbs, indicating semi‐open vegetation. The spread of Pinus–Betula forest at ca. 14 400 cal. a BP, the rise of Picea at ca. 12 800 cal. a BP and the re‐establishment of Pinus–Betula forest at ca. 11 700 cal. a BP within a span of centuries further suggest strikingly rapid, climate‐driven ecosystem changes rather than gradual plant succession on a newly deglaciated land. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
2.
Long-term changes in summer phytoplankton communities of the open northern Baltic Sea 总被引：16，自引：0，他引：16
Sanna Suikkanen Maria Laamanen Maija Huttunen 《Estuarine, Coastal and Shelf Science》2007,71(3-4):580-592
Changes in the biomass and species composition of phytoplankton may reflect major shifts in environmental conditions. We investigated relationships between the late summer biomass of different phytoplankton taxa and environmental factors, and their long-term (1979–2003) trends in two areas of the Baltic Sea, the northern Baltic proper (NBP) and the Gulf of Finland (GF), with statistical analyses. An increasing trend was found in late summer temperature and chlorophyll a of the surface water layer (0–10 m) in both areas. There was also a significant decrease in summer salinity and an increase in winter dissolved inorganic nitrogen to phosphorus (DIN:DIP) ratio in the NBP, as well as increases in winter DIN concentrations and DIN:SiO4 ratio in the GF. Simultaneously, the biomass of chrysophytes and chlorophytes increased in both areas. In the NBP, also the biomass of dinophytes increased and that of euglenophytes decreased, whereas in the GF, cyanobacteria increased and cryptophytes decreased. Redundancy analysis (RDA) indicated that summer temperature and winter DIN concentration were the most important factors with respect to changes in the phytoplankton community structure. Thus, the phytoplankton communities seem to reflect both hydrographic changes and the ongoing eutrophication process in the northern Baltic Sea. 相似文献
3.
Importance of climate,forest fires and human population size in the Holocene boreal forest composition change in northern Europe 
下载免费PDF全文
下载免费PDF全文 Niina Kuosmanen Heikki Seppä Teija Alenius Richard H. W. Bradshaw Jennifer l. Clear Ludmila Filimonova Maija Heikkilä Hans Renssen Miikka Tallavaara Triin Reitalu 《Boreas: An International Journal of Quaternary Research》2016,45(4):688-702
The relative importance of climate, forest fires and human population size on long‐term boreal forest composition were statistically investigated at regional and local scales in Fennoscandia. We employ pollen data from lakes, reflecting regional vegetation, and small forest hollows, reflecting local vegetation, from Russia, Finland and Sweden to reconstruct the long‐term forest composition. As potential drivers of the Holocene forest dynamics we consider climate, generated from a climate model and oxygen isotope data, past forest fires generated from sedimentary charcoal data and human population size derived from radiocarbon dated archaeological findings. We apply the statistical method of variation partitioning to assess the relative importance of these environmental variables on long‐term boreal forest composition. The results show that climate is the main driver of the changes in Holocene boreal forest composition at the regional scale. However, at the local scale the role of climate is relatively small. In general, the importance of forest fires is low both at regional and local scales. The fact that both climate and forest fires explain relatively small proportions of variation in long‐term boreal vegetation in small forest hollow records demonstrates the complexity of factors affecting stand‐scale forest dynamics. The relative importance of human population size was low in both the prehistorical and the historical time periods. However, this is the first time that this type of data has been used to statistically assess the importance of human population size on boreal vegetation and the spatial representativeness of the data may cause bias to the analysis. 相似文献
4.
Granitoid rock samples from the assumed center of the Keurusselkä impact site were subjected to a systematic study of fluid‐inclusion compositions and densities in various microstructures of the shocked quartz. The results are consistent with the following impact‐induced model of formation. After cessation of all major regional tectonic activity and advanced erosional uplift of the Fennoscandian shield, a meteorite impact (approximately 1.1 Ga) caused the formation of planar fractures (PFs) and planar deformation features (PDFs) and the migration of shock‐liberated metamorphic fluid (CO2 ± H2O) to the glass in the PDFs. Postimpact annealing of the PDFs led to the formation of CO2 (±H2O) fluid‐inclusion decorated PDFs. The scarce fluid‐inclusion implosion textures (IPs) suggest a shock pressure of 7.6–10 GPa. The postimpact pressure release and associated heating initiated hydrothermal activity that caused re‐opening of some PFs and their partial filling by moderate‐salinity/high temperature (>200 °C) H2O (+ chlorite + quartz) and moderate‐density CO2. The youngest postimpact endogenic sub‐ and nonplanar microfractures (MFs) are characterized by low‐density CO2 and low‐salinity/low‐temperature (<200 °C) H2O. 相似文献
5.
Gry Mine Berg Patricia M. Glibert Niels O. G. Jørgensen Maija Balode Ingrida Purina 《Estuaries and Coasts》2001,24(2):204-214
Concentrations and rates of uptake of dissolved organic nitrogen (DON, free amino acids, and urea) and inorganic nitrogen (DIN, nitrate, and ammonium) were measured along two transects in the Gulf of Riga, a sub-basin of the Baltic Sea, during May and July 1996. Concentrations of total dissolved nitrogen (TDN) were 23±3 μg-at N 1−1 in the northern region (mouth) and 41±5 μg-at N 1−1 in the southern region (head) of the Gulf. Rates of nitrogen uptake, determined with15N-labeled substrates, reflected differences in TDN concentration between the regions. In May, uptake of DIN+DON measured 0.17 and 0.43 μg-at N 1−1 h−1 in the northern and southern parts of the Gulf, respectively. In July, DIN+DON uptake measured 0.38 and 0.68 μg-at N 1−1 h−1 in the north and south, respectively. Most of the variability in total nitrogen flux between the northern and southern regions was due to heterogeneity of DON utilization. Uptake of urea and dissolved free amino acid were up to 6 and 3 times greater in the south compared to the north. As evidenced by size-fractionation, plankton size structure appeared to play a role in the uptake of DON. The community in the southern part was largely composed of cells <5 μm, while up to 67% of the community in the northern part was composed of cells >5 μm. Our results indicate that DON was a major source of nitrogen to phytoplankton, particularly in the southern part of the Gulf. 相似文献
6.
7.
A new approach has been arrived at for determining the linear unbiased minimum variance estimate of ore reserves, or the Kriging. The approach rests heavily on geometric intuition, which reduces the estimation to the simple task of determining an orthogonal projection. First, the problem is presented and solved in ordinary three-dimensional space. The result is stated in matrix formulation, lending itself to a simple generalization in higher dimensions. Then, the general problem is stated in random-variable notation and solved via the results derived. Finally, an illustrative example is given to demonstrate the new approach. 相似文献
8.
Karen Heikkilä 《Singapore journal of tropical geography》2014,35(3):362-381
The forest realm of the Gòòl‐War watershed of southeast Perak, Malaysia holds much socio‐cultural significance to the Semai Orang Asli communities living along these rivers. Notwithstanding its present‐day location within the Bukit Tapah Forest Reserve, the forest continues to be where Semai conceptions of history and lore are encoded in the geographical particulars of the landscape. A thriving forest‐dependent lifestyle maintains and transmits Semai place knowledge, observed in this research through people's command of ancestral place‐names. This paper provides an introduction to Semai toponymy and considers the implications of indigenous place‐names on the protection of indigenous homelands and ways of life. 相似文献
9.
Maija Taka Juha Aalto Miska Luoto 《Geografiska Annaler: Series A, Physical Geography》2015,97(4):819-834
It is widely known that intensive land use generally decreases stream water quality, but the influence of watershed physiography is relatively poorly understood. Since management planning has to take into account the protection of water quality, the current status of stream water must be identified. The potential effects of land use and watershed physiography variables on water quality were studied in an extensive set of 83 watersheds in the Helsinki region, Finland, covering wide land‐use intensity gradient. The aims of this study were to test if the geographical information of watershed land‐use data can be used to model the stream water quality, and to examine whether the spatial water quality models are improved after including predictors of watershed physiography to the land‐use model. Water quality variables were related to watershed predictors by utilizing generalized additive models and linear mixed models, and the independent effect of the variables was investigated using a hierarchical partitioning approach. While land use turned out to be the most influential factor explaining water quality, all models improved significantly after incorporating the watershed characteristics, such as topography and soil. These results were consistent across three modelling techniques. This study, with its novel approach to examine the impacts of several watershed physiographic characteristics on urban stream water quality in northern Europe, demonstrates that spatial land use and watershed physiography data can be used as cost‐efficient predictors in stream water quality models. 相似文献
10.