From ice age to modern: a record of landscape change in an Andean cloud forest

Aim To investigate the palaeoecological changes associated with the last ice age, subsequent deglaciation and human occupation of the central Andes. Location Lake Pacucha, Peruvian Andes (13°36′26″ S, 73°19′42″ W; 3095 m elevation). Methods Vegetation assemblages were reconstructed for the last 24 cal. kyr bp (thousand calibrated ¹⁴C years before present), based on pollen analysis of sediments from Lake Pacucha. An age model was established using ¹⁴C accelerator mass spectrometry dates on bulk sediment. Fossil pollen and sedimentological analyses followed standard methodologies. Results Puna brava replaced the Andean forest at the elevation of Lake Pacucha at the Last Glacial Maximum (LGM). Deglaciation proceeded rapidly after 16 cal. kyr bp , and near‐modern vegetation was established by c. 14 cal. kyr bp . The deglacial was marked by the range expansion of forest taxa as grassland taxa receded in importance. The mid‐Holocene was marked by a lowered lake level but relatively unchanged vegetation. Quinoa and maize pollen were found in the latter half of the Holocene. Main conclusions Temperatures were about 7–8 °C colder than present at this site during the LGM. The pattern of vegetation change was suggestive of microrefugial expansion rather than simple upslope migration. The mid‐Holocene droughts were interrupted by rainfall events sufficiently frequent to allow vegetation to survive largely unchanged, despite lowering of the lake level. Human activity at the lake included a 5500‐year history of quinoa cultivation and 3000 years of maize cultivation.     

Elevational variation in regional vegetation responses to late‐glacial climate changes in the Carpathians

Aim We used fossil records to explore patterns of change in vegetation composition, turnover and diversity along an elevational gradient during the late‐glacial to early Holocene, and to locate the elevations most sensitive to past climate changes. Location Romania. Methods Changes in the late‐glacial vegetation communities were inferred from seven published pollen records distributed within the main vegetation belts of the Romanian Carpathians, at elevations from 275 to 1840 m. Principal components analysis, detrended canonical correspondence analysis (DCCA) and rarefaction analysis were undertaken on these data. Results DCCA indicates that compositional change is strongest (SD 1.2, c. 70%) at the late‐glacial/Holocene transition (c. 11,500 cal. yr bp ), but significant shifts also occur at c. 14,700, c. 13,800 and c. 12,700 cal. yr bp (SD 0.4–0.8, 25–50%). Palynological turnover is greater for mid‐elevation records (730–1100 m) than at low and high elevations. Intervals of greater palynological richness occur between c. 13,800 and 12,500 cal. yr bp and after 11,500 cal. yr bp , and intervals of lower richness occur before c. 14,000 cal. yr bp and between c. 12,900 and 11,500 cal. yr bp . Main conclusions Variations in species composition during repeated climate changes of the late‐glacial suggest that community composition at a given time was not only a result of the environmental conditions of that period, but also the legacy of previous cumulative recruitment and extirpation events. Turnover estimates suggest that mid‐elevations have been the most sensitive to climate change during the late‐glacial and early Holocene. Palynological richness estimates show a less clear elevational pattern and no evidence for a greater sensitivity of this measure of biodiversity at high elevations to past climate change. However, results may have been affected by taxa with high pollen productivity and distance dispersability. Our finding concurs with other palaeoecological and local‐scale modelling studies in suggesting that small populations have survived in favourable microhabitats embedded within larger unsuitable areas during the late‐glacial, features not captured by broad‐scale model predictions.     

Holocene vegetation history of the Sahel: pollen, sedimentological and geochemical data from Jikariya Lake, north-eastern Nigeria

Aim This study aims to separate regional and local controls on Holocene vegetation development and examine how well pollen records reflect climate change in a semi‐arid region. The relative importance of climate and human activity as agents of vegetation change in the Sahel during the late Holocene is also considered. Location Jikariya Lake, an inter‐dune depression in the Manga Grasslands of north‐eastern Nigeria. Methods Pollen and charcoal were used to provide a record of Holocene vegetation history. Palaeoclimate and hydrological changes were reconstructed from sedimentary and geochemical data. Regional and local influences were separated by comparing the evidence obtained from Jikariya Lake with previously published data from the Manga Grasslands. Results The Manga Grasslands experienced a prolonged wet period during the early and mid‐Holocene, during which swamp forest vegetation with Guinean affinities (Alchornea, Syzygium, Uapaca) occupied the inter‐dune depressions. However, variation in the pollen records between sites suggests that their establishment was dependent on conditions being locally favourable, rather than being directly coupled to regional climate. The pollen records from the Manga Grasslands are more consistent in suggesting the colonization of the dunefields by trees associated with Sudanian savanna (Combretaceae, Detarium) c. 8700 cal. yr bp . The Jikariya Lake pollen data are in accordance with the sedimentological and geochemical data from the region in indicating that the onset of arid conditions occurred progressively during the late Holocene (from c. 4700 cal. yr bp ). Abrupt changes in pollen stratigraphy, recorded at other Manga Grasslands sites 3500 cal. yr bp , appear to be the product of the local passing of ecological thresholds. The dunefield vegetation (Sahelian savanna) appears to have been resilient to (or at least palynologically silent regarding) to the climatic variability of the late Holocene. Main conclusions While climate appears to have been the primary control on vegetation development in the Manga Grasslands during the Holocene, local conditions (particularly depression size and sand influx) had a strong influence on the timing of pollen stratigraphic changes. Anthropogenic influences are difficult to detect, even during the late Holocene.     

Brazilian Páramos I. An introduction to the physical environment and vegetation of the campos de altitude

Aim The campos de altitude are a series of cool-humid, grass-dominated formations restricted to the highest summits of the southeastern Brazilian Highlands. Relatively little is known of the ecology, biogeography, or developmental history of this archipelago of mountaintop formations. This contribution presents an overview of our present state of knowledge with respect to the past and present physical environment and vegetation of the campos de altitude. The aim is (1) to introduce an international audience to the natural history of these diminutive yet important ecosystems, and (2) to provide the background for a series of forthcoming contributions that will treat the ecology of the campos de altitude and explore physical and biotic relationships between these ‘Brazilian páramos’ and similar formations in the high mountains of equatorial South and Central America. Location Beginning at altitudes of 1800–2000 m, the campos de altitude are found atop the highest summits of the main ranges of the southeastern Brazilian Highlands, between the states of Santa Catarina and Minas Gerais/Espírito Santo. Methods Drawing upon both original data and previously published results, this contribution reviews what we presently know about the physical environment and vegetation of the campos de altitude, in the context of geographic setting, geology, palaeoclimatology and palaeobotany, modern climate, modern vegetation, and conservation. Results and main conclusions Uplift of the southeastern Brazilian ranges to altitudes high enough to support orographic grasslands may have occurred as early as the middle- to late-Tertiary; pollen records show that campos de altitude have been extant on southeastern summits at least since the Late Pleistocene. The present-day climate of the campos de altitude is cool and (per)humid, but patterns of rainfall, temperature, and frost are distinctly seasonal. Although the flora of the campos de altitude is highly diverse and characterized by a high degree of endemism, the campos de altitude maintain strong floristic similarities to equatorial alpine formations of the Andean and Central American Cordillera; these similarities also extend to climate, soils, and vegetation physiognomy. Anthropogenic fires and grazing are widespread in the campos de altitude and probably contribute significantly to the modern structure of vegetation communities.     

Holocene vegetation and water level history in two bogs of the Cordillera de Talamanca, Costa Rica

Pollen records of Holocene sediment cores from the Costa Rican Cordillera de Talamanca (La Chonta bog, 2310 m and La Trinidad bog, 2700 m) show the postglacial development of the montane oak forest zone from ca. 9500 to 1500 yr BP. During the early Holocene (ca. 9500–700 yr BP), alder vegetation covered the La Chonta and La Trinidad bogs and their adjacent hills. The upper forest line is inferred to be at 2800–3000 m elevation. A Podocarpus-Quercus forest characterised the middle Holocene (ca. 7000–4500 yr BP). The upper forest line is located at >3000 m reaching the present-day altitudinal distribution. A Quercus forest characterised the late Holocene (ca. 4500–1500 yr BP). Compared to modern conditions, the early Holocene has similar average temperatures, but the moisture level was probably higher. Pollen evidence for the late Holocene indicates drier environmental conditions than today. In order to improve the paleoecological interpretation, we described the local vegetation and used moss samples as pollen traps at both montane bogs along strong soil moisture gradients.The Netherlands Centre for Geo-ecological Research, ICG     

Holocene vegetation and climate change on the Colorado Great Plains,USA, and the invasion of Colorado piñon (Pinus edulis)

Aim To reconstruct the last c. 7000 years of vegetation and climate change in an unusual region of modern Great Plains grassland and scarp woodland in south‐east Colorado (USA), and to determine the late Holocene biogeography of Colorado piñon (Pinus edulis) at its easternmost extent, using a series of radiocarbon‐dated packrat (Neotoma sp.) middens. Location The West Carrizo Canyon drains the Chaquaqua Plateau, a plateau that projects into the western extent of the southern Great Plains grasslands in south‐eastern Colorado, USA. Elevations of the study sites are 1448 to 1525 m a.s.l. Today the plateau is mostly Juniperus scopulorum–P. edulis woodland. Methods Plant macrofossils and pollen assemblages were analysed from 11 ¹⁴C‐dated packrat middens. Ages ranged from 5990 yr bp (6839 cal. yr bp ) to 280 yr bp (485 cal. yr bp ). Results The results presented here provide information on the establishment and expansion of Juniperus–P. edulis woodland at its eastern limits. The analysis of both plant macrofossils and pollen from the 11 middens documents changes in plant communities over the last 7000 years, and the establishment of P. edulis at its easternmost limit. Though very minor amounts of P. edulis pollen occur as early as the middle Holocene, plant macrofossils were only recovered in middens dating after c. 480 cal. yr bp . Main conclusions Originally, midden research suggested a late glacial refuge to the north‐east of the Carrizo Canyon site, and a middle Holocene expansion of P. edulis. Results reported here are consistent with a late Holocene expansion, here at its eastern limits, but noted elsewhere at its northern and north‐eastern limits. In general, this late Holocene expansion is consistent with pollen data from sediments in Colorado and New Mexico, and suggests that P. edulis is still expanding its range at its present extremes. This has implications for further extension of its range due to changing climatic conditions in the future.     

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.     

Vegetation dynamics in central Africa since 18,000 yr BP: pollen records from the interlacustrine highlands of Burundi, Rwanda and western Uganda

A standardized analysis of palaeoecological data, in the form of six pollen sequences and forty- four radiocarbon ages, has permitted a region-wide reconstruction of Late Quaternary vegetation dynamics for the interlacustrine highlands of central Africa.
A landscape widely dominated by ericaceous scrub and grasslands, but also supporting sparse patches of open-canopied montane forest, possibly in those areas with a topography most favourable to the growth of trees, is indicated for the last glacial maximum of 18,000 yr bp . Major expansions in the extent of upper altitudinal forms of montane forest occurred from around 12,500 yr bp , while lower moist montane forest—the expected climax for much of the region today—did not become prominent until 11,000 yr bp to 10,000 yr bp . From the palaeoecological evidence at least, it appears that the major east Central forest refuge, proposed by some workers on the basis of current species' distribution patterns, did not extend to the eastern flanks of the Albertine Rift.
A late glacial–early Holocene transition is only fully chronicled in two of the sites. However, it appears that the expansion of lower montane forest had a time-transgressive pattern across the region, and was not simply from low to high altitude. The composition of forests during the early Holocene appears to have been different to that in the later stages of the present interglacial, as taxa presently associated with wetter and/or more open forest types were much more common. Pollen data also indicate that higher altitude parts of the interlacustrine highlands were more attractive to the earliest (possibly Bantu-speaking) farmers and metal-workers. There is evidence of wide-spread forest clearance around the beginning of the present millennium, possibly as a result of substantial changes in socio-economic conditions, and patterns of settlement, associated with the onset of the Late Iron Age.     

Brazilian Páramos IV. Phytogeography of the campos de altitude

Aim This contribution treats the phytogeography of the contemporary campos de altitude flora, with a focus on patterns at the level of genus. Comparative analysis using data from 17 other sites in Latin America is used to describe phytogeographical patterns at the continental scale. Results are combined with those of previous publications to shed light on the biogeographical origins of contemporary floristic patterns in the high mountains of south‐east Brazil. Location The campos de altitude are a series of cool‐humid, mountaintop grass‐ and shrublands found above elevations of 1800–2000 m in south‐east Brazil, within the biome of the Atlantic Forest. Methods Vascular floras are compiled for the three best‐known campos de altitude sites, and for 17 other highland and lowland locations in Latin America. Floras are binned into phytogeographical groups based on centres of diversity/origin. Floristic and geographical distances are calculated for all location‐pairs; Mantel tests are used to test for relationships between patterns in geographical distance, and floristic and climatic similarity. Multivariate statistics are carried out on the similarity matrices for all genera, and for each phytogeographical group. Predominant life‐forms, pollination and dispersal syndromes are determined for each genus in the campos de altitude flora, and proportional comparisons are made between phytogeographical groups. Supporting evidence from previously published literature is used to interpret analytical results. Results Two‐thirds of the genera in the campos de altitude are of tropical ancestry; the remainder are of temperate‐zone or cosmopolitan ancestry. Most campos de altitude genera are phanerophytes and hemicryptophytes, insect pollinated, and wind or gravity dispersed, but there are significant differences in the distribution of these traits among phytogeographical groups. The campos de altitude show stronger floristic similarities with other Brazilian mountain sites and distant Andean sites than with nearby low‐ and middle‐elevation sites; these similarities are best explained by climatic similarities. Floristic similarities among sites for temperate genera are better explained by ‘sinuous’ distance (e.g. measured along the spines of mountain ranges) than by direct distance; similarities in tropical genera are more related to direct distance. Different phytogeographical groups appear to be responding to different climatic signals. Main conclusions Many taxa currently living at the summits of the south‐east Brazilian Highlands trace their ancestry to temperate latitudes. Patterns of endemism and diversity in the south‐east Brazilian mountains point to climatically driven allopatry as a principal mechanism for speciation. The tropical component of the campos de altitude flora is primarily derived from drier, highland environments of the Brazilian interior; the temperate component rises in importance with elevation, but never reaches the levels seen in the tropical Andes. Most temperate taxa in the campos de altitude appear to have arrived via migration through favourable habitat rather than by recent, long‐distance dispersal. At least 11% of the plant species in the campos de altitude study sites are directly shared with the Andes. Palynofloras show that the campos de altitude have significantly contracted over the past 10,000 years, as regional temperatures have warmed and become more humid.     

Estimating past floristic diversity in montane regions from macrofossil assemblages

The relationship between the diversity of higher plant macrofossils in surface sediments of lakes and the surrounding vegetation is examined in two mountain regions; Grødalen in central Norway and the south-east Cairngorms in Scotland. Two lake sediment cores from each area were also analysed to examine vegetation history and to estimate changes in biodiversity through the Holocene. The diversity of present day vegetation in each region was estimated using both quadrat data and classified satellite images of the study areas. The mean surface sample macrofossil representation of species recorded in quadrats collected within 250 m of the lakes was c. 17%. This figure drops to only c. 2% when the satellite imagery of the same area is used to provide a maximal species list. The macrofossil data from the Norwegian cores show that deglaciation in this region occurred earlier on the mountain summit than in the valley and that the maximum tree line elevation was during the interval 9100–4400 ¹⁴C yr BP . In the Cairngorms the maximum tree line elevation was prior to c. 4500 ¹⁴C yr BP . The changes in higher plant diversity recorded at these sites through the Holocene show that c. 4000 ¹⁴C yr BP the reduction in the tree line resulted in decreased β-diversity at higher altitudes but an increase at the lower altitude as the forest cover opened up. Under conditions of climatic warming it is likely areas that come to lie below the tree line will experience reduced diversity and that a permanent loss of biodiversity would result from a severe reduction in the area above the tree line.     

Holocene altitudinal shifts in vegetation belts and environmental changes in the Sierra Madre Occidental, Northwestern Mexico, based on modern and fossil pollen data

A modern pollen rain study was performed in a 300 km-long altitudinal transect (~ 28° N latitude) from 300 to 2300 m elevation. The higher elevation modern communities: epithermal oak–pines, pine–oak forest, pine forest, and mixed conifer forest were easy to distinguish from their pollen content. In contrast, lower elevation subtropical communities: thornscrub and tropical deciduous forest were difficult to separate, because they share many pollen taxa. Nevertheless we identify high frequencies of Bursera laxiflora as an important component of the tropical deciduous forest.Additionally, fossil pollen was analyzed at three sites located between 1700 and 1950 m altitude at ~ 28° latitude north in the Sierra Madre Occidental of northwestern Mexico. The sites were in pine–oak (Pinus–Quercus), pine, and mixed-conifer forests respectively. Shifts in the altitudinal distribution of vegetation belts were recorded for the last 12,849 cal yr BP, and climate changes were inferred. The lowest site (pine–oak forest) was surrounded by pine forest between 12,849 and 11,900 cal yr BP, suggesting a cold and relatively dry Younger Dryas period. The early Holocene was also cold but wetter, with mixed conifer forest with Abies (fir) growing at the same site, at 1700 m elevation, 300 m lower than today. After 9200 cal yr BP, a change to warmer/drier conditions caused fir migration to higher elevations and the expansion of Quercus at 1700 m. At 5600 cal yr BP Abies was growing above 1800 m and Picea (spruce) that is absent today, was recorded at 1950 m elevation. Fir and spruce disappeared from the 1950 m site and reached their present distribution (scattered, above 2000 m) after 1000 cal yr BP; we infer an episodic Holocene migration rate to higher elevations for Abies of 23.8 m/1000 cal yr and for Picea of 39.2 m/1000 cal yr. The late Holocene reflects frequent climate oscillations, with variations in the representation of forest trees. A tendency towards an openness of the forest is recorded for the last 2000 yrs, possibly reflecting human activities along with short-term climate change.     

Vegetation strategy of <Emphasis Type="Italic">Vellozia crinita</Emphasis> (Velloziaceae)

Plant communities in which the herb layer is dominated by Vellozia crinita were surveyed in seven mountain ranges with a complex comprising open herbaceous or savanna-like montane vegetation overlying quartzite and sand, locally known as “campos rupestres” in the State of Minas Gerais, southeastern Brazil, by means of 92 vegetation relevés (totaling approx. 4629 m²). Vellozia crinita displayed a growth strategy which is rare among the Velloziaceae south of the Espinhaço mountain chain.While most species of Vellozia are phanerophytes with erect caudices and grow exclusively on or among rock outcrops, V. crinita is a caespitose chamaephyte with a profusely branched, repent caudex which grows under the ground or very slightly exposed. It forms cushions which stabilize convex mounds of colluvial sand, thus occupying a distinct niche in the “campos rupestres”. One association, two subassociations and two variants of the white-sand vegetation component are described as new.     

Late Holocene vegetation, fire, climate and upper forest line dynamics in the Podocarpus National Park, southeastern Ecuador

Late Holocene vegetation, fire, climate and upper forest line dynamics were studied based on detailed pollen and charcoal analyses. Two sediment cores, from the Rabadilla de Vaca mire (RVM) and the Valle Peque?o bog (VP), with an age of about 2100 and 1630 cal yrs b.p., respectively, were taken at the modern upper forest line in the Parque Nacional Podocarpus (Podocarpus National Park) in southeastern Ecuador. The two pollen records reflect relatively stable vegetation with slight changes in floral composition during the recorded period. Changes of the proportion between subpáramo and páramo vegetation are related to lower and higher frequency of fires. The RVM records show that the upper forest line moved to a higher elevation between 1630 and 880 cal yrs b.p., stabilising after 310 cal yrs b.p. Human impact is suggested by a high fire frequency, mainly between 1800–1600 and 880–310 cal yrs b.p. The VP records indicate no marked changes in the upper forest line. The charcoal records suggest an increased human impact from 230 cal yrs b.p. to the present. The results indicate that high fire frequency is an important factor in reducing the expansion of subpáramo vegetation and upper montane rainforest and in favouring the distribution of grass páramo. Since there is a clear correlation between fire and vegetation dynamics, it is difficult to detect how far climate change also played a significant role in upper forest line changes during the late Holocene.     

Post‐glacial patterns in vegetation dynamics in Romania: homogenization or differentiation?

Aim This paper examines eight fossil pollen datasets from Romania with the aim of exploring regional and elevational patterns in site similarity throughout the Holocene. In particular, we aim to determine whether there are clear intervals of homogenization/differentiation and to ascertain the potential driving factors. Location Romania. Methods Qualitative (pollen diagrams) and numerical methods including principal components analysis and Bray–Curtis similarity analyses were used. Results We found strong variability in the past vegetation dynamics during the Holocene. Bray–Curtis similarity analyses show large fluctuations in vegetation similarity and distinct periods of homogenization and differentiation throughout the Holocene. The magnitude and length of these periods appear quite variable in time, but the significant ones can be delimited as follows: (1) differentiation between 11,250 and 11,000 cal. yr bp , 10,000 and 9750 cal. yr bp , 6000 and 5750 cal. yr bp , 2500 and 2250 cal. yr bp , and especially over the last 200 years; and (2) homogenization between 9750 and 9500 cal. yr bp , and 2750 and 2500 cal. yr bp , with more stable periods between 9000 and 7750 cal. yr bp , 4750 and 3500 cal. yr bp , and 2000 and 1000 cal. yr bp . Main conclusions First, periods of biotic homogenization that occurred before significant anthropogenic impact on vegetation demonstrate that not all homogenization is a product of anthropogenic change: it can also be driven by natural causes. In fact, recent human impact (over the last 200 years) appears to have resulted in increased regional differentiation and not in homogenization – a result that contradicts most studies based on more modern, short‐term records. Second, both abiotic (climate and disturbance) and biotic factors are likely drivers of intervals of differentiation and homogenization. We suggest that differentiation may be triggered primarily by climate changes and disturbances (mostly natural pre‐2500 cal. yr bp and human‐induced thereafter), whereas homogenization may be driven predominantly by biotic interactions (e.g. immigration and interspecific competition). Third, this long‐term study raises awareness that assessments of pattern in vegetation homogenization/differentiation may depend on the specific time period and length of investigation. Long‐term investigations through multiple generations are likely to yield particularly useful information on the mechanisms and effects of biotic homogenization.     

Pollen‐based biomes for Beringia 18,000, 6000 and 0 14C yr bp†

The objective biomization method developed by Prentice et al. (1996) for Europe was extended using modern pollen samples from Beringia and then applied to fossil pollen data to reconstruct palaeovegetation patterns at 6000 and 18,000 ¹⁴C yr bp . The predicted modern distribution of tundra, taiga and cool conifer forests in Alaska and north‐western Canada generally corresponds well to actual vegetation patterns, although sites in regions characterized today by a mosaic of forest and tundra vegetation tend to be preferentially assigned to tundra. Siberian larch forests are delimited less well, probably due to the extreme under‐representation of Larix in pollen spectra. The biome distribution across Beringia at 6000 ¹⁴C yr bp was broadly similar to today, with little change in the northern forest limit, except for a possible northward advance in the Mackenzie delta region. The western forest limit in Alaska was probably east of its modern position. At 18,000 ¹⁴C yr bp the whole of Beringia was covered by tundra. However, the importance of the various plant functional types varied from site to site, supporting the idea that the vegetation cover was a mosaic of different tundra types.     

Post-glacial changes in spatial patterns of vegetation across southern New England

Aim We analysed lake‐sediment pollen records from eight sites in southern New England to address: (1) regional variation in ecological responses to post‐glacial climatic changes, (2) landscape‐scale vegetational heterogeneity at different times in the past, and (3) environmental and ecological controls on spatial patterns of vegetation. Location The eight study sites are located in southern New England in the states of Massachusetts and Connecticut. The sites span a climatic and vegetational gradient from the lowland areas of eastern Massachusetts and Connecticut to the uplands of north‐central and western Massachusetts. Tsuga canadensis and Fagus grandifolia are abundant in the upland area, while Quercus, Carya and Pinus species have higher abundances in the lowlands. Methods We collected sediment cores from three lakes in eastern and north‐central Massachusetts (Berry East, Blood and Little Royalston Ponds). Pollen records from those sites were compared with previously published pollen data from five other sites. Multivariate data analysis (non‐metric multi‐dimensional scaling) was used to compare the pollen spectra of these sites through time. Results Our analyses revealed a sequence of vegetational responses to climate changes occurring across southern New England during the past 14,000 calibrated radiocarbon years before present (cal yr bp ). Pollen assemblages at all sites were dominated by Picea and Pinus banksiana between 14,000 and 11,500 cal yr bp ; by Pinus strobus from 11,500 to 10,500 cal yr bp ; and by P. strobus and Tsuga between 10,500 and 9500 cal yr bp . At 9500–8000 cal yr bp , however, vegetation composition began to differentiate between lowland and upland sites. Lowland sites had higher percentages of Quercus pollen, whereas Tsuga abundance was higher at the upland sites. This spatial heterogeneity strengthened between 8000 and 5500 cal yr bp , when Fagus became abundant in the uplands and Quercus pollen percentages increased further in the lowland records. The differentiation of upland and lowland vegetation zones remained strong during the mid‐Holocene Tsuga decline (5500–3500 cal yr bp ), but the pattern weakened during the late‐Holocene (3500–300 cal yr bp ) and European‐settlement intervals. Within‐group similarity declined in response to the uneven late‐Holocene expansion of Castanea, while between‐group similarity increased due to homogenization of the regional vegetation by forest clearance and ongoing disturbances. Main conclusions The regional gradient of vegetation composition across southern New England was first established between 9500 and 8000 cal yr bp . The spatial heterogeneity of the vegetation may have arisen at that time in response to the development or strengthening of the regional climatic gradient. Alternatively, the differentiation of upland and lowland vegetation types may have occurred as the climate ameliorated and an increasing number of species arrived in the region, arranging themselves in progressively more complex vegetation patterns across relatively stationary environmental gradients. The emergence of a regional vegetational gradient in southern New England may be a manifestation of the increasing number of species and more finely divided resource gradient.     

Stable forest–savanna mosaic in north‐western Tanzania: local‐scale evidence from δ13C signatures and 14C ages of soil fractions

Aim The spatio‐temporal dynamics of dry evergreen forest patches in the savanna biome of the Kagera region (north‐western Tanzania) are largely unknown owing to a lack of pollen and macrofossil evidence. Our aims were to reconstruct local‐scale shifts of the forest–savanna boundary in order to determine whether the forests have been expanding or retreating on a centennial and millennial time‐scale. Location The Kagera region of north‐western Tanzania, East Africa. Methods The vegetation reconstruction was based on analysing δ¹³C signatures in soils along a transect spanning both C₄ open savanna and C₃ forest vegetation. Furthermore, we fractionated soil organic matter (SOM) according to density and chemical stability to analyse δ¹³C values of soil fractions with distinct radiocarbon ages. Results We found sharp changes in δ¹³C signatures in bulk SOM from the forest to the savanna, within a few metres along the transect. The forest soil profiles carried a persistent C₃‐dominated signature. Radiocarbon dating of the oldest, most recalcitrant forest soil fraction yielded a mean age of 5500 cal. yr bp , demonstrating that the forest has existed since at least the mid‐Holocene. The savanna sites showed a typical C₄ isotopic signature in SOM of topsoils, but subsoils and more recalcitrant SOM fractions also contained signals of C₃ plants. The dense soil fraction (ρ > 1.6 g cm^?3) carrying a pure C₄ label had a mean age of c. 1200 cal. yr bp , indicating the minimum duration of the dominance of grass vegetation on the savanna site. At the forest edge, the older C₄ grass signature of SOM has steadily been replaced by the more negative δ¹³C fingerprint of the forest trees. As this replacement has occurred mainly in the 10‐m‐wide forest–savanna ecotone over the last c. 1200 years, the forest expansion must be very slow and is very likely less than 15 m century^?1. Main conclusions Our results suggest that forest patches in the Kagera savanna landscape are very stable vegetation formations which have persisted for millennia. During the last millennium, they have been expanding very slowly into the surrounding savanna at a rate of less than 15 m century^?1.     

An evaluation of the Lost World and Vertical Displacement hypotheses in the Chimantá Massif, Venezuelan Guayana

Aim To document the occurrence of vertical displacements of vegetation in the high plateaus of the Venezuelan Guayana (tepuis) over the last c. 6000 years, and to discuss their significance for the origin of their flora, especially the endemism patterns observed in their flat summits. Two hypotheses have been proposed for the origin of the summit flora. One (the Lost World hypothesis) proposes a long history of evolution in isolation from the surrounding plains, while the other (the Vertical Displacement hypothesis) suggests that vertical movements of vegetation during the Pleistocene glacial‐interglacial cycles would have resulted in floristic mixing within the lowlands, and genetic interchange among plateau summits. Location This work has been conducted on the flat summit of the Churí‐tepui, in the Chimantá massif, at 5°15′ Lat. N and 62°01′ Long. W, around 2250 m altitude. Methods Pollen analysis and radiocarbon dating of two peat outcrops, using modern analogue technique and numerical methods for palaeoecological interpretation were used. Results The replacement of a high‐altitude plant community (a paramoid Chimantaea shrubland) by a lower elevation (< 2300 m) Stegolepis meadow, occurred about 2500 years before present (yr bp ). This vegetation change is inferred to have resulted from a regional climatic shift to higher temperature and moisture. A subsequent decrease in temperature and moisture led to the establishment of present conditions after about 1450 yr bp . Main conclusions The highland vegetation of the tepuis responded to climate shifts with vertical displacements, supporting the hypothesis of vertical mixing. However, a physiographical analysis shows that around half of the tepuis would never have been connected by lowlands. Therefore, both hypotheses are needed to explain the origins of the summit flora in the tepuis.     

Evidence for drought and forest declines during the recent megafaunal extinctions in Madagascar

Aim There remains some uncertainty concerning the causes of extinctions of Madagascar’s megafauna. One hypothesis is that they were caused by over‐hunting by humans. A second hypothesis is that their extinction was caused by both environmental change and hunting. This paper systematically addresses the second hypothesis through examination of two new pollen records from south‐eastern Madagascar alongside other published records across the island. Location South‐eastern Madagascar. Methods We reconstructed past vegetation and fire dynamics over the past 6000 years at two sites in south‐eastern Madagascar (Ste‐Luce) using fossil pollen and charcoal contained in sedimentary sequences. We investigated drivers of vegetation changes and how these, in turn, influenced faunal species in the south‐east, using published climatic, archaeological and faunal records. Further, we also used published records to provide a synthesis of environmental changes on the whole island. Results Vegetation reconstructions indicate that the mosaic vegetation in the region of Ste‐Luce was highly dynamic in response to climatic changes. The open woodland, surrounding the littoral forest, transformed into an ericoid grassland between c. 5800 and 5200 cal. yr bp , possibly in response to a moderate drought recorded during this period. The littoral forest was more stable between c. 5100 and 1000 cal. yr bp , with only some minor compositional changes c. 2800 cal. yr bp and between c. 1900 and 1000 cal. yr bp . Significant forest decline, however, is observed at c. 950 cal. yr bp , coinciding with a drought and a marine surge. A comparison of these results with a synthesis of published vegetation records across the island shows asynchronous vegetation changes in response to various droughts during the Holocene, except for the 950 cal. yr bp drought event, with evidence of widespread vegetation transformations and fires across the island. Main conclusions Pronounced climatic desiccation between 1200 and 700 cal. yr bp may have been the slow driver framing and triggering vegetation transformations and decline in megafaunal populations. In addition, hunting by drought‐impacted human inhabitants and competition with newly introduced cattle would have amplified the impacts on megafaunal populations, leading to numerous extinctions in this period.