Devensian Late-glacial and early Flandrian environmental history of the Vale of Pickering,Yorkshire, England

Pollen, sedimentological and charcoal particle analyses are presented from Devensian Late-glacial and early- to mid-Flandrian deposits from a former lake in the Vale of Pickering, Yorkshire. The combined analytical methods provide evidence for a short-lived climatic deterioration towards the end of the Late-glacial Interstadial, followed by a brief recovery prior to the Loch Lomond Stadial. This deterioration may be correlated with one of the ‘pre-Younger Dryas’ cooling periods identified not only in other pollen sequences from Britain and Europe, but from such diverse sources as Foraminifera from the Norwegian Sea and electrical conductivity measurements from the Greenland ice sheet. Loss-on-ignition and magnetic susceptibility data suggest that the Loch Lomond Stadial was characterised by an initial prolonged temperature decline, followed by a sudden more severe downturn resulting in particularly intense solifluction. Radiocarbon accelerator dating of the early Flandrian marl deposits illustrates the problem of age determination in calcareous lakes, and an estimate of the magnitude of ‘hard water error’ is obtained. The local population expansion of Alnus glutinosa is dated to 7640 ± 85 yr BP, but there is possible evidence for a Late-glacial presence of the tree, the significance of which is discussed in relation to other sites in east Yorkshire. © 1996 John Wiley & Sons, Ltd.     

Fluvial seed dispersal of riparian trees: transport and depositional processes

Fluvial seed dispersal considers both the transport and deposition of seeds where channel geomorphic structures, hydrology and seed dispersal traits contribute to transport times and depositional locations. This study examines the influence of stream flow patterns on fluvial seed dispersal of buoyant white alder (Alnus rhombifolia) seeds by applying a one‐dimensional transport model. Conceptually, the model separates the stream into two components: (i) the main channel where the seeds are transported downstream; and (ii) the transient storage zone where seeds are temporarily detained or deposited on the river bank. Transport processes are characterized by an advection–dispersion equation which is coupled to a transient storage model using an exponential decay term. The model parameters: longitudinal dispersion (D_L), exchange coefficient (α), main channel area (A) and storage zone (A_s) are estimated based on field experiments conducted in a confined, bedrock‐gravel bed river with pool‐riffle morphology located in coastal northern California. The riparian zone is inhabited by Alnus rhombifolia that disperse buoyant seeds in mid‐spring coinciding with the end of the wet, Mediterranean season. Artificial seeds, with similar traits of buoyancy and density to alder seeds, were used to quantify transport times and depositional locations. Preferential deposition resulted in stream reaches with larger A_s, high A_s/A ratios, and faster exchange coefficients corresponding to divergent stream flow (back‐eddies, re‐circulating flow, flow expansions) caused by geomorphic structures such as the ends of bar/riffle features and bends in the stream. The results demonstrate the importance of transient storage for seed transport and depositional processes. Morphological features that increase a channel's complexity create complex flow structures that detain seeds and provide a greater opportunity for deposition to occur. The model provides a simplification of river hydraulics to represent dispersal dynamics and lends itself to further understanding of hydrochory processes and associated population structure. Copyright © 2015 John Wiley & Sons, Ltd.     

云南省中部3种森林土壤含水率、容重和细根重及其垂直分布

根据27个土壤剖面的405个土壤样品,研究云南中部地区华山松林、旱冬瓜林、云南松林的土壤含水率、容重和细根重及其垂直分布特征。结果表明：3种森林100em深土壤的平均含水率分别是（28．79±11．98）％、（24．66±9．89）％、（14．11±5．84）％,差异极显著;平均容重分别是（1．14±0．20）g／cm3、（1．26±0．18）g/cm3、（1．42±0．12）g／cm3,差异极显著;平均细根重分别为（0．1956±0．2144）g／100cm3、（0．0706±0．1066）g／100cm3、（0．1381±0．2989）g／100cm3,在0．05水平上差异显著。华山松林林下土壤的物理性质最好,旱冬瓜林次之,云南松林最差。3种森林里,随着土壤深度增加,土壤含水率增加,土壤容重和土壤细根重减少。华山松林与云南松林平均容重与平均细根重均显著负相关,旱冬瓜林平均含水率与平均容重显著负相关。华山松林下土壤细根量最大,0～20cm层根系占根系总重量的83％。     

Holocene vegetation and climate dynamics of the boreal alpine ecotone of northwestern Fennoscandia

Climate change with respect to summer temperature throughout the Holocene is inferred from oscillations in the local Pinus sylvestris, Alnus incana and Betula pubescens forest‐lines, as recorded by fossil pollen and plant macrofossils in lake sediments at four altitudinal levels. Mt Skrubben (848 m a.s.l.), in Dividalen, was deglaciated down to below 280 m a.s.l. during 10 800–10 300 cal. yr BP. Betula pubescens established 10100 cal. yr BP at 280 m a.s.l. and expanded up to near the summit during the next 700 years. Birch woodland prevailed on the mountain plateau until 3300 cal. yr BP. Local Pinus sylvestris stands are recorded up to 400 m a.s.l. at 8450 cal. yr BP and >548 m a.s.l. about 8160 cal. yr BP. Alnus incana expanded from 400 to nearly 790 m a.s.l. during the period 7900–7600 cal. yr BP. The maximum forest distribution lasted until ca. 6000 cal. yr BP. Marked climatic deteriorations caused lowering of the forest‐lines around 4600 and 3000 cal. yr BP. Reconstruction of the summer temperature indicated mean July temperatures at 400 m a.s.l. of 1.5–3°C above the present during the period of maximum forest expansion, whereas >3°C above the present temperature at 548 m a.s.l. This is in accordance with other regional temperature reconstructions from northern Europe. Copyright © 2008 John Wiley & Sons, Ltd.     

Postglacial history of alder (Alnus glutinosa (L.) Gaertn.) in the British Isles

Data from 92 postglacial pollen sequences are used to map the spread and increase of alder (Alnus glutinosa) across the British Isles between 9000 and 5000 years ago. The spread is found to be patchy and erratic in space and time. Consideration of the habitat requirements and reproductive ecology of alder suggest that it spread within Britain and Ireland after about 10 000 yr BP, when suitable habitat for it was scarce. Alder spread across most of Britain and Ireland early in the postglacial but only increased in abundance as (i) suitable habitat became available through changing sea levels, hydroseral successions, and floodplain development, and as (ii) rare weather events produced the necessary conditions for reproduction. Alder is unique among British and Irish trees in its requirement for a suitable habitat isolated among expanses of unsuitable habitats. Because of this, maps of its postglacial population spread and increase do not show the spatial coherence of maps for other forest tree taxa.