Coastal dune vegetation and pollen representation in south Buenos Aires Province, Argentina

Aim To establish the relationship between coastal dune vegetation and its pollen representation as an aid to interpret Holocene vegetation dynamics and environmental changes from pollen assemblages. Location The study area is situated on the temperate Atlantic coast of south Buenos Aires Province, Argentina (c. 39° S and 61°20′ W). Methods The vegetation of the active dune area adjacent to the beach was described on the basis of its floristic composition from 25 plots. Classification of the vegetation into distinct zones was carried out by cluster analysis. Surface samples were collected from each vegetation stand and analysed for their pollen composition. Pollen percentage data were analysed using principal components analysis in order to investigate the degree to which the different vegetation units can be distinguished by their pollen spectra. Pollen–vegetation relationships for selected taxa were explored using simple scatter plots and indices of association, under‐ and over‐representation. Indices of floristic diversity and palynological richness were used to assess the representation of the vegetation in the pollen spectra. Results and conclusions Five vegetation zones are defined on the basis of species composition and their quantitative variation: back shore, mobile dunes, slacks, semi‐fixed and fixed dunes. Pollen assemblages from back shore, mobile dunes and slacks are clearly differentiated from semi‐fixed and fixed dunes. Pollen assemblages differ considerably from the associated vegetation composition. Major discrepancies are caused by large differences in pollen and vegetation proportion of Hyalis argentea and Discaria americana. There is a considerable proportion of non‐local pollen in every spectrum. Pollen representation in the coastal dunes at Monte Hermoso is influenced by differences in pollen production, dispersal and preservation of individual taxa as well as by the spatial distribution of the vegetation, the topography of the dune system and the wind pattern. The pollen–vegetation relationship established in this study has important implication for understanding and interpreting fossil pollen records from coastal dune environments.     

Experimental comparison of competition and facilitation in alpine communities varying in productivity

Question. Competitive and facilitative interactions among plant species in different abiotic environments potentially link productivity, vegetation structure, species composition and functional diversity. We investigated these interactions among four alpine communities along an environmental productivity gradient in a generally harsh climate. We hypothesised that the importance of competition would be higher in more productive sites. Location. Mt. M. Khatipara (43°27′N, 41°41′E, altitude 2750 m), NW Caucasus, Russia. Communities ranged from low‐productivity alpine lichen heath (ALH) and snowbed communities (SBC), to intermediate productivity Festuca grassland (FVG), and high‐productivity Geranium‐Hedysarum meadow (GHM). Methods. We quantified the relative influence of competition and facilitation on community structure by expressing biomass of target species within each natural community proportionally to biomass of the species in a “null community” with experimental release from interspecific competition by removing all other species (for 6 years). An overall index of change in community composition due to interspecific interactions was calculated as the sum of absolute or proportional differences of the component species. Results. Species responses to neighbour removal ranged from positive to neutral. There was no evidence of facilitation among the selected dominant species. As expected, competition was generally most important in the most productive alpine community (GHM). The intermediate position for low‐productivity communities of stressful environments (ALH, SBC) and the last position of intermediately productive FVG were unexpected. Conclusions. Our results appear to support the Fretwell‐Oksanen hypothesis in that competition in communities of intermediate productivity was less intense than in low‐ or high‐productive communities. However, the zero net effect of competition and facilitation in FVG might be the result of abiotic stress due to strong sun exposure and high soil temperatures after neighbour removal. Thus, non‐linear relationships between soil fertility, productivity and different abiotic stresses may also determine the balance between competition and facilitation.     

The late Quaternary vegetation history of the south‐central highlands of Victoria,Australia. II. Sites below 900 m

The late Quaternary vegetation communities of the south‐central highlands of Victoria are constructed from analyses of pollen and charcoal, and macroscopic plant remains preserved in Sphagnum bogs. The sites, located in eucalypt forest or woodland, form an altitudinal sequence with the component Eucalyptus species varying with altitude and with small pockets of Nothofagus cunninghamii (Hook.) Oerst. in close proximity to the higher sites. The record from the sites above 900 m covers the last 32 000 years, and the record from the lower sites extends from at least 12 000 BP . Around 32 000 BP the region was predominantly covered by a mosaic of alpine feldmark and herbfield, with small patches of Eucalyptus and Nothofagus woodland close to sea level when summer temperatures were probably 5°C lower than present. Lowest values, probably 7°–8°C below present, occurred between 19 800 and 16 900 BP , when alpine communities were most widespread and much of the Central Highlands was treeless. Around 12 000 BP alpine taxa disappeared or were greatly reduced, first at the lower sites. There was an associated rise in the treeline with the movement upslope of Nothofagus and eucalypt forest as a result of a general increase in temperature and probably effective precipitation. By 6000 BP wet eucalypt forest and Nothofagus reached their maximum postglacial extent at all sites, possibly related to a further increase in temperature, at least 2°C lower than present, and higher effective precipitation. A continuing increase in temperature, or an increase in continentality, and a decrease in effective precipitation led to increased fire hazard and retraction of rainforest and wet sclerophyll or tall open forest toward present‐day values. Nothofagus disappeared from the sites below 900 m. The activities of humans pose further threats to remaining forest communities. The record of vegetation and environmental change derived from the local and regional picture from eight sites reinforces and complements that from the individual sites. For example, combining the records overcomes to some extent taphonomic problems such as the effect of streams that flow close to all sites, and other limitations including problems of dating, poor preservation and variable sedimentation rates.     

Holocene vegetation and climate change from near Lake Pedder, south-west Tasmania, Australia

Aim To use surface pollen and vegetation relationships to aid the interpretation of a Holocene pollen record. Location South‐west Tasmania, Australia. Methods A survey was undertaken of surface‐pollen samples from the major regional vegetation types: alpine, rain forest and moorland. Relationships between vegetation type and surface‐pollen representation were analysed using twinspan classification and ordination. A core was retrieved from moorland vegetation, and interpretation of the fossil pollen sequence was aided using relationships detected in our surface‐pollen analysis. Results Regional vegetation types are reflected in the pollen rain of south‐west Tasmania, despite the over‐representation of important rain forest tree species in samples from non‐forest sites. twinspan classification of the surface‐pollen samples identified the following indicator pollen taxa for each vegetation type: Astelia alpina (alpine); Lagarostrobos franklinii (rain forest); Leptospermum and Melaleuca (moorland). Detrended correspondence analysis of the surface‐pollen samples clearly separates samples from each vegetation type. Correlation of the ordination axes with environmental data identified a dominant temperature/altitudinal gradient in the surface‐pollen data (R = 0.852/0.844). Application of the results of the surface‐pollen analysis to the fossil sequence revealed that fire‐promoted moorland has dominated the local environment around the core site for the entire Holocene. Changes in fossil pollen composition also suggest that temperatures increased through the Late Glacial to peak in the mid‐Holocene and declined thereafter, a trend consistent with other sites in the region. Main conclusions Pollen spectra can successfully be used to predict local vegetation in south‐west Tasmania. At least this part of inland south‐west Tasmania has remained forest‐free throughout the Holocene, conflicting with the dominant palaeoecological paradigm of a mid‐Holocene dominated by rain forest. A comparison with pollen records from moorland vegetation across the region suggests that fire‐promoted moorland has dominated the landscape since the Late Glacial. We suggest that burning by people through the Late Glacial (if not earlier) facilitated the spread of moorland throughout the region, greatly restricting the expansion of rain forest. The continued influence of fire throughout the Holocene in this perennially wet landscape argues for a revision of the dominant human‐occupation model that depicts an abandonment of the interior of south‐west Tasmania in the Late Glacial in response to the expansion of rain forest.     

The late Quaternary vegetation history of the south-central highlands of Victoria,Australia. I. Sites above 900 m

Abstract The late Quaternary vegetation communities of the south-central highlands of Victoria are reconstructed from analyses of pollen and charcoal, and associated environmental conditions derived from the record of Nothofagus cunninghamii and alpine and sclerophyll taxa preserved in four subalpine Sphagnum bogs. The highest site occurs amid Eucalyptus paudflora woodland, the two intermediate sites are surrounded by Eucalyptus delegatensis forest and the lowest by a mixed forest of E. delegatensis/Eucalyptus regnans. Small pockets of N. cunninghamii occur within the eucalypt forests, and in close proximity to all four sites. Around 32 000 BP the vegetation consisted of a mosaic of alpine feldmark and herbfield, with small scattered groves of Nothofagus and Eucalyptus well below 1100m. Summer temperatures were probably 5°C lower than present with lowest values, probably 7° to 8°C below present, possibly between 17 000 and 13 500 BP, at which time alpine communities reached their greatest extent and much of the Central Highlands was treeless. After ca 13 500 BP herbaceous alpine taxa disappeared and there was an associated movement upslope of Nothofagus and tall open forest taxa to their maximum post-glacial extent, as temperatures and effective precipitation increased, ca 6000 BP. The retraction of cool temperate rainforest and wet sclerophyll or tall open forest towards present day values indicates lower effective precipitation, generally rising temperatures and increased fire hazard. More recently, European activities have increased the stress on the remaining forests. The study of four sites has demonstrated die importance of analysing a number of sites within a given area in order to overcome the limitations imposed by sites which were sub-optimal due to one or more factors including poor preservation, problems of dating, variable sedimentation rates, and the influence of streams which flow close to all sites. While die local environment varies between sites, and some vegetation changes are successional, this study shows that the local records complement one another, to some extent reinforcing die regional picture of vegetation and environmental change.     

Pioneer vegetation on glacier forelands in southern Norway: emerging communities?

Question: How variable is the pioneer plant community on glacier forelands in southern Norway, both in terms of species composition and geographical distribution? Location: The Jotunheim and Jostedalsbreen regions of southern Norway (61°‐62°N, 6°‐9°E). Methods: The relative frequencies of vascular plant species were recorded in the pioneer zones of 43 glacier forelands, with an altitudinal range of 80‐1860 m (boreal to high alpine) and an east‐west range of 100 km. Classification and ordination techniques were used to search for evidence of consistently recurring communities, variability along a continuum or stochasticity. Results: Mean variability in species composition between all glacier forelands sampled was 65% (Sørensen dissimilarity). Poa alpina, Oxyria digyna, Deschampsia alpina and Festuca ovina had the highest frequency, occurring on over 80% of forelands. Non‐metric multidimensional scaling did not reveal clear divisions between groups of sites, but cluster analysis, multi‐response permutation procedures and indicator species analysis suggested two sub‐communities: the Saxifraga cespitosa‐Trisetum spicatum sub‐community is restricted to forelands above 1100 m in the Jotunheim region; whereas the D. alpina‐O. digyna sub‐community has a wider altitudinal range of 80‐1780 m. Variance partitioning indicated that altitude alone accounts for 24%, distance east for 18%, and the component shared by altitude and distance east for 17% of the variance in species composition. Conclusions: At the broadest scale, pioneer vegetation on the glacier forelands can be viewed as a single P. alpina‐O. digyna community of predominantly wind‐ and water‐dispersed perennials. However, this community shows a high degree of variability, with dominant species missing from a number of sites, and is poorly structured, suggesting a degree of stochasticity. Furthermore, the pioneer vegetation can be dissected within a continuum of variation to produce two emerging sub‐communities, reflecting the influence of environmental factors and, possibly, early successional development within the pioneer zone. Variance partitioning indicates that altitudinal and continentality gradients are important in accounting for a significant proportion of the variability within this dataset.     

Late quaternary vegetation dynamics and human impact on Alexander Selkirk Island, Chile

Aim To reconstruct the history of vegetation and environments using pollen, charcoal and sediment analysis, and to identify the timing and nature of climate change and human impact on the vegetation of a remote Pacific island. Location Cerro de Los Inocentes, 1000 m above sea level, Alexander Selkirk Island (33°45′S, 80°45′W), Chile. The westernmost island of the Juan Fernandez Archipelago, south‐east Pacific Ocean. Methods A 150‐cm long sediment core comprising 87 cm dark brown peat overlying 63 cm of yellow grey clay was extracted from a shallow depression on the southern slopes of Cerro de Los Inocentes. Pollen, charcoal, sediment and accelerator mass spectrometry radiocarbon analyses were used to construct a record of vegetation change through time. Numerical analysis of multispecies data allowed the classification of fossil assemblages into distinct pollen zones. Results Pollen and spores are preserved throughout the sediment with high concentrations coinciding with the beginning of organic sediment accumulation at around 8000 ¹⁴C yr BP. Prior to 8000 ¹⁴C yr BP, the deposition of clays, presumably from upslope erosion, occurred in a landscape sparsely vegetated by grasses, ferns and Pernettya rigida heath, including several plants that are only found 100–200 m above the site today (Zone CI‐1). After 8000 ¹⁴C yr BP, a P. rigida heath was the dominant vegetation (Zone CI‐2). A shift to a wet heath–shrubland (Zone CI‐3) occurred at 6000 ¹⁴C yr BP and was followed by a transition to a treefern–shrubland mosaic accompanied by periodic burning (Zone CI‐4) after 4500 ¹⁴C yr BP. The impact of human occupation is evident in Zone CI‐5 at 450 ¹⁴C yr BP with the loss of forest species, increased burning and invasion of the exotic plant Rumex. Main conclusions The pollen and charcoal record provides the first evidence of vegetation changes spanning at least the last 8000 ¹⁴C yr BP from the high altitude environment of Alexander Selkirk Island. Prior to 8000 ¹⁴C yr BP, the altitudinal ranges of different plant species may have been suppressed by a cooler and drier climate. Increasing precipitation and temperatures at the end of the last glacial period may have mobilized exposed sediments in a sparsely vegetated upland environment, altering local drainage patterns, eventually leading to slope stabilization and deposition of organic detritus under an increasing density of heath and shrub vegetation. The subalpine heath–shrubland persisted until 4500 ¹⁴C yr BP when first evidence for sustained burning is found in association with the establishment of a more open treefern–shrubland vegetation pattern. In the absence of human occupation at this time, the influence of increased climatic variability associated with more frequent El Niño‐Southern Oscillation events during the mid to late Holocene is considered one of the main driving forces behind increased vegetation disturbance during this period. The record provides evidence that island vegetation patterns have been highly dynamic over millennial to decadal time‐scales and that the flora has persisted through periods of rapid and major climate change. This changed with the discovery of the island by European explorers in the late sixteenth century and the subsequent introduction of goats and exploitation and burning of forests, which resulted in the progressive destruction of native vegetation and the invasion of introduced plants. There is evidence that reduced burning and control of the goat population within the last 50 years has resulted in marginal recovery of some high altitude native plant species.     

Modern pollen samples from alpine vegetation on the Tibetan Plateau

• 1 A set of 316 modern surface pollen samples, sampling all the alpine vegetation types that occur on the Tibetan Plateau, has been compiled and analysed. Between 82 and 92% of the pollen present in these samples is derived from only 28 major taxa. These 28 taxa include examples of both tree (AP) and herb (NAP) pollen types.
• 2 Most of the modern surface pollen samples accurately reflect the composition of the modern vegetation in the sampling region. However, airborne dust‐trap pollen samples do not provide a reliable assessment of the modern vegetation. Dust‐trap samples contain much higher percentages of tree pollen than non‐dust‐trap samples, and many of the taxa present are exotic. In the extremely windy environments of the Tibetan Plateau, contamination of dust‐trap samples by long‐distance transport of exotic pollen is a serious problem.
• 3 The most characteristic vegetation types present on the Tibetan Plateau are alpine meadows, steppe and desert. Non‐arboreal pollen (NAP) therefore dominates the pollen samples in most regions. Percentages of arboreal pollen (AP) are high in samples from the southern and eastern Tibetan Plateau, where alpine forests are an important component of the vegetation. The relative importance of forest and non‐forest vegetation across the Plateau clearly follows climatic gradients: forests occur on the southern and eastern margins of the Plateau, supported by the penetration of moisture‐bearing airmasses associated with the Indian and Pacific summer monsoons; open, treeless vegetation is dominant in the interior and northern margins of the Plateau, far from these moisture sources.
• 4 The different types of non‐forest vegetation are characterized by different modern pollen assemblages. Thus, alpine deserts are characterized by high percentages of Chenopodiaceae and Artemisia, with Ephedra and Nitraria. Alpine meadows are characterized by high percentages of Cyperaceae and Artemisia, with Ranunculaceae and Polygonaceae. Alpine steppe is characterized by high abundances of Artemisia, with Compositae, Cruciferae and Chenopodiaceae. Although Artemisia is a common component of all non‐forest vegetation types on the Tibetan Plateau, the presence of other taxa makes it possible to discriminate between the different vegetation types.
• 5 The good agreement between modern vegetation and modern surface pollen samples across the Tibetan Plateau provides a measure of the reliability of using pollen data to reconstruct past vegetation patterns in non‐forested areas.

     

Modern pollen–vegetation relationships along transects on the Whangapoua Estuary, Great Barrier Island, northern New Zealand

Aims To quantify pollen–vegetation relationships from saline to freshwater in an estuarine gradient from surface samples of the modern pollen rain, to allow more accurate interpretations of the stratigraphic palynological record. Location Whangapoua Estuary, Great Barrier Island, northern New Zealand. Methods Six transects were laid out along a vegetation sequence running from estuarine mud to freshwater swamp. Along these transect lines, 108 plots were sampled for vegetation and surface sediments from wet sand, mud, plant litter or moss (sand and mud sites are inundated by most tides, other sites less frequently). All sediment samples were analysed for pollen. The relationships between plant species frequency and pollen representation were examined at a community scale using twinspan and ordination analyses, and for individual species using fidelity and dispersibility indices, regression and box‐plot analyses. Results The quantitative relationships between source taxon vegetation frequency and its pollen representation varied between species due to differential pollen production and dispersal. twinspan of the surface pollen samples suggests five vegetation types: (A) mangrove (Avicennia marina); (C) Leptocarpus similis salt meadow; (D) Baumea sedges; (E) Leptospermum shrubland; and (F) Typha/Cordyline swamp forest. The (B) Juncus kraussii community is not represented palynologically owing to the destruction of its delicate pollen grains during acetolysis of samples. Detrended correspondence analysis places these communities on an estuarine‐to‐freshwater gradient. However, pollen assemblages at the seaward end of the salinity gradient are less clearly representative of the associated vegetation than those at the landward end, probably because the open vegetation at the former allows the influx of wind‐ and water‐dispersed pollen from surrounding vegetation. Main conclusions The vegetation pattern (zonation) at Whangapoua is reflected in the pollen rain. When the long‐distance and over‐represented pollen types are excluded, five out of six of the broad vegetation communities can be identified by their pollen spectra. Species with high fidelity and low‐to‐moderate dispersibility indices can be used to identify the vegetation types in the sedimentary sequences. The more open vegetation types at the ‘marine end’ of the sequence tend to be ‘overwhelmed’ by regional pollen, but the nature of the sediments and the presence of discriminatory species (e.g. A. marina, Plagianthus divaricatus, Cordyline australis), even in small amounts, will allow correct identification of the local vegetation represented in sedimentary palynological sequences. A box‐plot analysis indicates that the pollen and spore types A. marina (mangroves), Sarcocornia quinqueflora (salt meadow), P. divaricatus (sedges), Gleichenia (shrubland) and C. australis (swamp forest) are highly discriminatory in relation to vegetation type. These discriminatory palynomorphs help with the interpretation of stratigraphic pollen studies. However, salt marsh vegetation communities in the sediments must be interpreted with caution as the marine sediments are easily affected by erosion, bioturbation and tidal inundation effects.     

Late-glacial and Holocene palaeovegetation zonal reconstruction for central and north-central North America

Aim The purpose of this study is to develop palaeovegetation zonation models for central and north‐central North America, based on late‐Quaternary and Holocene pollen stratigraphic data (n = 246 sites). A secondary purpose was to evaluate an hypothesis ( Strong & Hills, 2003 ) to explain the disjunct distribution of species in western Alberta. Location Hudson Bay‐Lake Michigan to the Rocky Mountains region, north of 36° N to the Arctic Ocean (c. 70° N). Methods Pollen profiles spanning 40 years of palaeoecological research in North America were extracted from published and unpublished archival sources. Individual profiles were subdivided into 1000‐year increments based on the assumption of a constant sedimentation rate between stratigraphic dates (e.g. surface sediments, radiocarbon ¹⁴C dates, tephra layers). The pollen composition among profiles was standardized to 54 commonly recognized taxa, with percentage composition within each stratigraphic sample prorated to 100% prior to analysis. Near‐surface sediments from these profiles were included as analogues of modern vegetation. Cluster analysis was used as a guide to the classification of 2356 temporal stratigraphic samples, which resulted in the recognition of 16 pollen groups. These groups were summarized in terms of their pollen composition, mapped, and used in combination with terrain information and an ecological knowledge of the study area to construct six physiognomically‐based palaeovegetation zonation models at 2000‐year intervals from 14,000 to 4000 yr bp (radiocarbon years before present). Results The 14,000 yr bp model placed Boreal and Cordilleran Forests proximal to the southern glacial front, whereas Arctic tundra dominated the Yukon Territory–Alaska ice‐free zone. Pollen and macrofossil evidence suggests that this Boreal Forest zone contained a mixture of coniferous and deciduous tree species. Grassland was postulated immediately south of the forest zone, with its northern extreme near 49° N latitude in the Alberta–Montana border area. Separation of the Laurentide and Cordilleran glacial fronts about 12,000 yr bp initiated the northward advance of Boreal Forests into western Canada. By the end of the Hypsithermal at about 6000 yr bp , Boreal Forests occurred near the Arctic Ocean, and Grassland and Aspen Parkland zones may have extended to 54° N and 59° N latitude in Alberta, respectively. Between 6000 and 4000 yr bp , a 5° and 1° latitudinal southward shift of the northern Boreal Forest and Grassland/Aspen Parkland boundaries occurred, respectively, near their contemporary positions with corresponding expansions of the Subarctic and Arctic zones. Modern Canadian Cordilleran Forests along the eastern slopes of the Rocky Mountains were interpreted as originating from the north‐central Montana–south‐western Alberta area. Jack pine (Pinus banksiana Lamb.), a common Boreal Forest species, appears to have entered central Canada via the north side of Lake Superior after 11,000 yr bp . Main conclusions Modern vegetation in central Canada evolved from biomes located in the northern USA during the late‐Quaternary. The Boreal Forest biome contained the same arboreal taxa as the modern vegetation, except it lacked jack pine. The proposed regional palaeovegetation models support the hypothesis of Strong & Hills (2003) , but new independent palaeoecological data will be needed for a proper evaluation.     

Vegetation patterns,plant distribution and life forms across the alpine zone in southern Tierra del Fuego,Argentina

The alpine zone is examined at meso‐ and microscales in southern Tierra del Fuego (54°49′S), where the full zone is expressed. Mesoscale patterns were studied on opposing aspects, and microscale patterns were studied on a series of solifluction terraces, in a hanging valley overlooking the Beagle Channel. Plant cover and life form data were collected within 50‐m altitudinal bands on north and south aspects and comprehensive plant lists were compiled for each band. Topography and associated surface cover were recorded on the terraces. Six alpine plant communities, in lower and upper floristic zones, were differentiated with multivariate analyses and significantly related to five ecological factors. Equivalent communities were separated by approximately 185 m altitude on opposing aspects, which related to a soil temperature difference of approximately 3.0°C. The richness (and range) of 80 local vascular taxa (18.6% of the regional flora), decreased with increasing altitude (6.6 per 100 m); however, richness differed significantly with aspect (north: 5.6, south: 7.5). Upper altitudinal limits (approximately 1250 m a.s.l.), were associated with a midsummer isotherm of approximately 1.7°C. Chamaephytes and hemicryptophytes dominated throughout but the tall tussock form was conspicuously absent. Reasons for this are discussed in the context of the Nothofagus treeline, which conformed to a midsummer isotherm of only approximately 6.0°C. Such patterns are at variance with those found in the oceanic subantarctic islands, other oceanic perhumid temperate mountain regions and tropical high mountains. However, the microscale pattern of fines, pebbles, stones and rock across the active solifluction terraces, with dense vegetation on their steep risers, had a clear affinity with that of other subantarctic regions. Inferences that alpine systems of the Southern Hemisphere are necessarily equivalent to those at similar northern latitudes are cautioned against. Likewise, such comparisons within the Southern Hemisphere may also be invalid.     

Human impact on an island ecosystem: pollen data from Sandoy, Faroe Islands

Aim To investigate the form and dynamics of ecosystems on an isolated island in the North Atlantic before human settlement in the first millennium ad , and the effects of human activities thereafter. Location The island of Sandoy, Faroes (61°50′ N, 6°45′ W). Methods Two sequences of lake sediments and one of peat were studied using pollen analysis and sedimentological techniques. Age models were constructed on the basis of radiocarbon dating and, in one case, tephrochronology. The data were analysed statistically and compared with existing data from the region. Results The pollen data indicate that early Holocene vegetation consisted of fell‐field communities probably growing on raw, skeletal soils. These communities gave way to grass‐ and sedge‐dominated communities, which in turn were largely replaced by dwarf shrub‐dominated blanket mire communities well before the first arrival of humans. There is evidence for episodic soil erosion, particularly in the uplands. Changes in the records attributable to human impact are minor in comparison with many other situations in the North Atlantic margins, and with certain published sequences from elsewhere in the Faroes. They include: (1) the appearance of cereal pollen and charcoal, (2) an expansion of ruderal taxa, (3) a decline in certain taxa, notably Juniperus communis and Filipendula ulmaria, and (4) a renewed increase in rates of upland soil erosion. The reliability of palaeoecological inferences drawn from these sites, and more generally from sites in similar unforested situations, is discussed. Main conclusions The subdued amplitude of palynological and sedimentological responses to settlement at these sites can be explained partly in terms of their location and partly in terms of the sensitivity of different parts of the ecosystem to human activities. This study is important in establishing that the imposition of people on the pristine environment of Sandoy, while far from negligible, especially in the immediate vicinity of early farms and at high altitudes, had relatively little ecological impact in many parts of the landscape.     

Vegetation patterns at the alpine treeline ecotone: the influence of tree cover on abrupt change in species composition of alpine communities

Aims: The upper elevation limit of forest vegetation in mountain ranges (the alpine treeline ecotone) is expected to be highly sensitive to global change. Treeline shifts and/or ecotone afforestation could cause fragmentation and loss of alpine habitat, and are expected to trigger considerable alterations in alpine vegetation. We performed an analysis of vegetation structure at the treeline ecotone to evaluate whether distribution of the tree population determines the spatial pattern of vegetation (species composition and diversity) across the transition from subalpine forest to alpine vegetation. Location: Iberian eastern range of the Pyrenees. Methods: We studied 12 alpine Pinus uncinata treeline ecotones. Rectangular plots ranging from 940 to 1900 m² were placed along the forest‐alpine vegetation transition, from closed forest to the treeless alpine area. To determine community structure and species distribution in the treeline ecotone, species variation along the forest‐alpine vegetation transition was sampled using relevés of 0.5 m² set every 2 m along the length of each plot. Fuzzy C‐means clustering was performed to assess the transitional status of the relevés in terms of species composition. The relation of P. uncinata canopy cover to spatial pattern of vegetation was evaluated using continuous wavelet transform analysis. Results: Vegetation analyses revealed a large degree of uniformity of the subalpine forest between all treeline ecotone areas studied. In contrast, the vegetation mosaic found upslope displayed great variation between sites and was characterized by abrupt changes in plant community across the treeline ecotone. Plant richness and diversity significantly increased across the ecotone, but tree cover and diversity boundaries were not spatially coincident. Conclusions: Our results revealed that no intermediate communities, in terms of species composition, are present in the treeline ecotone. Ecotone vegetation reflected both bedrock type and fine‐scale heterogeneity at ground level, thereby reinforcing the importance of microenvironmental conditions for alpine community composition. Tree cover did not appear to be the principal driver of alpine community changes across the treeline ecotone. Microenvironmental heterogeneity, together with effects of past climatic and land‐use changes on ecotone vegetation, may weaken the expected correlation between species distribution and vegetation structure.     

Increasing deterministic control of primary succession on Mount St. Helens,Washington

Question: Does the relative importance of stochastic and deterministic factors change during primary succession? Location: Small depressions (potholes) located on Mount St. Helens, Washington (46°13′51″N, 122°09′10″W, 1290 m). Methods: Pothole vegetation was described in 1993, 1997 and from 2001 to 2008. Explanatory variables included location and elevation (spatial factors), soil factors and Lupinus lepidus cover from prior years (a fertility surrogate). RDA assessed species‐variable relationships. DCA calculated β diversity and within‐year heterogeneity. Flexible sorting classified the vegetation. Species composition, richness, cover, H′ and evenness were also calculated. Results: Vegetation cover increased through 2001, and then fluctuated due to changes in L. lepidus cover. Richness peaked in 2005, after which pioneer species began to decline as persistent evergreen species increased. The six CTs recognized in 2008 were more scattered than were the six different CTs from 2001. DCA demonstrated that woody and rhizomatous species increased as pothole vegetation became less variable. RDA revealed weak spatial relationships in 1993, 1997 and 2001; thereafter, environmental and biological factors became important. The species‐explanatory data relationship increased during this study from 10.2% to 36.0%, leaving 64.0% of the variation unexplained. Conclusions: This is the first temporal study to demonstrate that deterministic control of vegetation development increases during succession. Pothole vegetation has converged somewhat due to deterministic factors, but the initial effects of chance, local disturbances and history remain large and may prevent strong convergence.     

Nitrogen limitation and microbial diversity at the treeline

Tree growth limitation at treeline has mainly been studied in terms of carbon limitation while effects and mechanisms of potential nitrogen (N) limitation are barely known, especially in the southern hemisphere. We investigated how soil abiotic properties and microbial community structure and composition change from lower to upper sites within three vegetation belts (Nothofagus betuloides and N. pumilio forests, and alpine vegetation) across an elevation gradient (from 0 to 650 m a.s.l.) in Cordillera Darwin, southern Patagonia. Increasing elevation was associated with a decrease in soil N‐NH₄⁺ availability within the N. pumilio and the alpine vegetation belt. Within the alpine vegetation belt, a concurrent increase in the soil C:N ratio was associated with a shift from bacterial‐dominated in lower alpine sites to fungal‐dominated microbial communities in upper alpine sites. Lower forested belts (N. betuloides, N. pumilio) exhibited more complex patterns both in terms of soil properties and microbial communities. Overall, our results concur with recent findings from high‐latitude and altitude ecosystems showing decreased nutrient availability with elevation, leading to fungal‐dominated microbial communities. We suggest that growth limitation at treeline may result, in addition to proximal climatic parameters, from a competition between trees and soil microbial communities for limited soil inorganic N. At higher elevation, soil microbial communities could have comparably greater capacities to uptake soil N than trees, and the shift towards a fungal‐dominated community would favour N immobilization over N mineralization. Though evidences of altered nutrient dynamics in tree and alpine plant tissue with increasing altitude remain needed, we contend that the measured residual low amount of inorganic N available for trees in the soil could participate to the establishment limitation. Finally, our results suggest that responses of soil microbial communities to elevation could be influenced by functional properties of forest communities for instance through variations in litter quality.     

A tale of two green walls: a functional trait approach to assess vegetation establishment on restored steep slopes

Green walls (GWs) have been increasingly recognized as an important restoration technique for steep slopes resulting from quarrying activities or major infrastructure construction projects. In practice, GW irrigation is considered essential, although studies evaluating vegetation establishment under different irrigation regimes are lacking. Besides taxonomic metrics, functional diversity measures were used to compare the diversity and composition of plant communities of two hydroseeded GWs, with different irrigation regimes (irrigated vs. low‐irrigated). The studied GWs were installed in Peneda‐Gerês National Park (NW Portugal) to minimize the visual impact of shotcrete walls, along a road infrastructure, and promote their ecological restoration. Species' cover was recorded 3 years after installation. Species were classified according to their growth form, life form, and life strategy. Despite hydroseeding approximately 57 species, GW vegetation was dominated by spontaneous species, particularly acrocarpous mosses, regardless of irrigation regime. Species and functional richness were higher under irrigated conditions, while no differences were observed for species diversity and functional dispersion. Functional trait composition did not differ between GWs, indicating that both systems potentially provide the same ecosystem functions and services. Our results also suggest that spontaneous species colonizing GWs are highly adapted to local environmental conditions, given their dominance in both irrigated and low‐irrigated communities. Overall, irrigation did not affect the structure and functioning of GW communities, only their redundancy, since more species exhibiting similar traits were established in the irrigated GW. Therefore, our findings suggest that low‐irrigated GWs, hydroseeded with native species, represent a more cost‐effective solution to reduce the ecological impacts of steep slopes.     

Using biodiversity deconstruction to disentangle assembly and diversity dynamics of understorey plants along post‐fire succession in boreal forest

Aim The study aims to decipher the co‐occurrence of understorey plant assemblages and, accordingly, to identify a set of species groups (diversity deconstruction) to better understand the multiple causal processes underlying post‐fire succession and diversity patterns in boreal forest. Location North‐eastern Canadian boreal forest (49°07′–51°44′ N; 70°13′–65°15′ W). Methods Data on understorey plant communities and habitat factors were collected from 1097 plots. Species co‐occurrence was analysed using null model analysis. We derive species groups (i.e. biodiversity deconstruction) using the strength of pairwise species co‐occurrences after accounting for random expectation under a null model and cluster analyses. We examine the influence of a set of spatiotemporal environmental variables (overstorey composition, time‐since‐fire, spatial location and topography) on richness of species groups using Bayesian model averaging, and their relative influence through hierarchical partitioning of variance. Results Understorey plant assemblages were highly structured, with co‐occurrence‐based classification providing species groups that were coherently aggregated within, but variably segregated between, species groups. Group richness models indicate both common and distinct responses to factors affecting plant succession. For example, Group 2 (e.g. Rhododendron groenlandicum and Cladina rangiferina) showed concurrent contrasting responses to overstorey composition and was strongly segregated from Groups 3 (e.g. Clintonia borealis and Maianthenum canadense) and 4 (e.g. Epilobium angustifolium and Alnus rugosa). Groups 3 and 4 showed partial similarity, but they differed in their response to time‐since‐fire, drainage and latitude, which were more important for Group 1 (e.g. Ptilium crista‐castrensis and Empetrum nigrum). A single successional model based on total richness masked crucial group‐level relationships with factors that we examined, such as latitude. Main conclusions By demonstrating the co‐occurrence structure and linking to causal factors, the results from this study characterize both common and distinct responses of understorey plants to biophysical attributes of sites, and potential interspecific interactions, behind non‐random assemblage structure during post‐fire succession. A biodiversity deconstruction approach could offer a concise and explicit framework to gain a better understanding of the complex assembly of ecological communities during succession.     

THIS ARTICLE HAS BEEN RETRACTED: What caused the mid‐Holocene forest decline on the eastern Tibet‐Qinghai Plateau?

Aim Atmospheric CO₂ concentrations depend, in part, on the amount of biomass locked up in terrestrial vegetation. Information on the causes of a broad‐scale vegetation transition and associated loss of biomass is thus of critical interest for understanding global palaeoclimatic changes. Pollen records from the north‐eastern Tibet‐Qinghai Plateau reveal a dramatic and extensive forest decline beginning c. 6000 cal. yr bp . The aim of this study is to elucidate the causes of this regional‐scale change from high‐biomass forest to low‐biomass steppe on the Tibet‐Qinghai Plateau during the second half of the Holocene. Location Our study focuses on the north‐eastern Tibet‐Qinghai Plateau. Stratigraphical data used are from Qinghai Lake (3200 m a.s.l., 36°32′–37°15′ N, 99°36′–100°47′ E). Methods We apply a modern pollen‐precipitation transfer function from the eastern and north‐eastern Tibet‐Qinghai Plateau to fossil pollen spectra from Qinghai Lake to reconstruct annual precipitation changes during the Holocene. The reconstructions are compared to a stable oxygen‐isotope record from the same sediment core and to results from two transient climate model simulations. Results The pollen‐based precipitation reconstruction covering the Holocene parallels moisture changes inferred from the stable oxygen‐isotope record. Furthermore, these results are in close agreement with simulated model‐based past annual precipitation changes. Main conclusions In the light of these data and the model results, we conclude that it is not necessary to attribute the broad‐scale forest decline to human activity. Climate change as a result of changes in the intensity of the East Asian Summer Monsoon in the mid‐Holocene is the most parsimonious explanation for the widespread forest decline on the Tibet‐Qinghai Plateau. Moreover, climate feedback from a reduced forest cover accentuates increasingly drier conditions in the area, indicating complex vegetation–climate interactions during this major ecological change.     

Reliability of pollen ratios for environmental reconstructions on the Tibetan Plateau

Aim Pollen ratios are widely used to gain palaeovegetation and palaeoclimatic information from fossil pollen spectra, although their applicability has seldom been tested with modern pollen data. I used a data set of 113 lake‐surface sediments from the eastern Tibetan Plateau to test the reliability of several pollen ratios. Location The lake‐surface pollen spectra cover a wide range of vegetation types (temperate desert, temperate steppe, alpine desert, alpine steppe, high‐alpine meadow, sub‐alpine shrub, coniferous and mixed forest) and climatic conditions (mean July temperature, T_July: 4.0–17.4°C; mean annual precipitation, P_ann: 104–670 mm). Methods Lake‐surface sediments were analysed palynologically, and several pollen ratios were calculated. These ratios were interpreted with respect to vegetation and climatic conditions. Results The arboreal pollen sum (AP) was highest in samples from forested areas and was significantly correlated with P_ann (r² = 0.44). In non‐forested areas, samples from large lakes and from lakes surrounded by sparse vegetation had increased AP values, suggesting that AP is a useful vegetation density indicator. Artemisia/Chenopodiaceae (A/C) ratios were lowest in desert areas and were positively correlated with P_ann (r² = 0.25). The aridity pollen index was inappropriate for inferring (palaeo‐)climatic information from samples on the eastern Tibetan Plateau as it had no significant correlation with the environmental factors. Artemisia/Cyperaceae (A/Cy) ratios had a significant correlation with T_July (r² = 0.23), but only a weak correlation with P_ann, which indicates that the A/Cy ratio is applicable as a temperature indicator. Furthermore, it is a valuable tool for the differentiation of high‐alpine meadow from steppe vegetation. Main conclusions AP sum, A/C ratio and A/Cy ratio are useful tools for qualitative and semi‐quantitative palaeoenvironmental reconstruction on the Tibetan Plateau; however, the results obtained should not be interpreted quantitatively.