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.     

Charcoal analysis, a method to study vegetation and climate of the Holocene: The case of Lago di Pergusa (Sicily, Italy)

The direct comparison between microcharcoal and pollen data from the Holocene sediment core of Lago di Pergusa (central Sicily, Italy) led us to investigate the linkages between fire, vegetation, and climate in the Mediterranean Basin. The role of human populations in shaping the environment of the last millennia was closely examined as well. Pollen and charcoal were extracted using a standard pollen methodology and both identified from the same pollen slides. At Lago di Pergusa the importance of fire in maintaining and favouring a Mediterranean vegetation, a basic concept in ecology, seems to be inconsistent, as important fires took place in the open environments with xeric vegetation at the beginning of the Holocene well before the expansion of evergreen vegetation. On the contrary some big fires seem to have happened in the period (around 8000 years BP) in which the greatest wetness, the thickest forest canopy, and the most important expansion of submontane vegetation of the record are found. A probable explanation of this phenomenon can be found in the precipitation regime, in the increased available biomass and in the resulting increase in combustible material, indicating also that the highest humidity achieved in central Sicily during the Holocene was not strong enough to prevent fires. During this period drops in arboreal pollen concentration match in minima in the charcoal curves. Fires were practically absent during the Neolithic and Eneolithic periods and not responsible for the general trend towards aridification identified in the pollen record, at times in which prehistoric populations are generally believed to have caused forest clearance by burning. Important and frequent burnings are documented for the Bronze and Iron Ages and probably not all of them were fires of living trees, but burning of wood for producing metals, cooking or heating. The occurrence of a fire around 3700 years BP temporarily caused a minor decrease of arboreal pollen concentration. In the open landscapes of the two last millennia an almost continuous sign of regional burnings is recognized, confirming the historical evidence of an intense land use in central Sicily at least since Roman times.     

Mid- and late-Holocene vegetation and fire history at Biviere di Gela, a coastal lake in southern Sicily, Italy

The vegetation and fire history of few coastal sites has been investigated in the Mediterranean region so far. We present the first paleoecological reconstruction from coastal Sicily, the largest island in the Mediterranean Sea. We analysed pollen and charcoal in the sediments of Biviere di Gela, a lake (lagoon) on the south coast of Sicily. Our data suggest that the area became afforested after a marine transgression at ca. 7200 cal b.p. (5250 b.c.). Build-up of forest and shrublands took ca. 200–300 years, mainly with the deciduous trees Quercus, Ostrya and Fraxinus. Juniperus expanded ca. 6900 cal b.p. (4950 b.c.), but declined again 6600 cal b.p. (4650 b.c.). Afterwards, evergreen trees (Q. ilex-type and Olea) became dominant in the forest and Pistacia shrublands were established. Forest and shrubland reached a maximum ca. 7000–5000 cal b.p. (5050–3050 b.c.); subsequently forest declined in response to human impact, which was probably exacerbated by a general trend towards a more arid climate. During the Neolithic, fire was used to open the landscape, significantly reducing several arboreal taxa (Q. ilex, Fraxinus, Juniperus) and promoting herbs and shrubs (Achillea, Cichorioideae, Brassicaceae, Ephedra). Final forest disruption occurred around 2600 cal b.p. (650 b.c.) with the onset of the historically documented Greek colonization. We conclude that the open maquis and garrigue vegetation of today is primarily the consequence of intensive land-use over millennia. Under natural or near-natural conditions arboreal taxa such as Q. ilex, Olea and Pistacia would be far more important than they are today, even under the hot and rather dry coastal conditions of southern Sicily.     

Stability in the patterns of long‐term development and growth of the Canadian spruce–moss forest

Aim The spruce–moss forest is the main forest ecosystem of the North American boreal forest. We used stand structure and fire data to examine the long‐term development and growth of the spruce–moss ecosystem. We evaluate the stability of the forest with time and the conditions needed for the continuing regeneration, growth and re‐establishment of black spruce (Picea mariana) trees. Location The study area occurs in Québec, Canada, and extends from 70°00′ to 72°00′ W and 47°30′ to 56°00′ N. Methods A spatial inventory of spruce–moss forest stands was performed along 34 transects. Nineteen spruce–moss forests were selected. A 500 m² quadrat at each site was used for radiocarbon and tree‐ring dating of time since last fire (TSLF). Size structure and tree regeneration in each stand were described based on diameter distribution of the dominant and co‐dominant tree species [black spruce and balsam fir (Abies balsamea)]. Results The TSLF of the studied forests ranges from 118 to 4870 cal. yr bp . Forests < 325 cal. yr bp are dominated by trees of the first post‐fire cohort and are not yet at equilibrium, whereas older forests show a reverse‐J diameter distribution typical of mature, old‐growth stands. The younger forests display faster height and radial growth‐rate patterns than the older forests, due to factors associated with long‐term forest development. Each of the stands examined established after severe fires that consumed all the soil organic material. Main conclusions Spruce–moss forests are able to self‐regenerate after fires that consume the organic layer, thus allowing seed regeneration at the soil surface. In the absence of fire the forests can remain in an equilibrium state. Once the forests mature, tree productivity eventually levels off and becomes stable. Further proof of the enduring stability of these forests, in between fire periods, lies in the ages of the stands. Stands with a TSLF of 325–4870 cal. yr bp all exhibited the same stand structure, tree growth rates and species characteristics. In the absence of fire, the spruce–moss forests are able to maintain themselves for thousands of years with no apparent degradation or change in forest type.     

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.     

Vegetation and fire history of coastal north-eastern Sardinia (Italy) under changing Holocene climates and land use

Little is known about the vegetation and fire history of Sardinia, and especially the long-term history of the thermo-Mediterranean belt that encompasses its entire coastal lowlands. A new sedimentary record from a coastal lake based on pollen, spores, macrofossils and microscopic charcoal analysis is used to reconstruct the vegetation and fire history in north-eastern Sardinia. During the mid-Holocene (c. 8,100–5,300 cal bp), the vegetation around Stagno di Sa Curcurica was characterised by dense Erica scoparia and E. arborea stands, which were favoured by high fire activity. Fire incidence declined and evergreen broadleaved forests of Quercus ilex expanded at the beginning of the late Holocene. We relate the observed vegetation and fire dynamics to climatic change, specifically moister and cooler summers and drier and milder winters after 5,300 cal bp. Agricultural activities occurred since the Neolithic and intensified after c. 7,000 cal bp. Around 2,750 cal bp, a further decline of fire incidence and Erica communities occurred, while Quercus ilex expanded and open-land communities became more abundant. This vegetation shift coincided with the historically documented beginning of Phoenician period, which was followed by Punic and Roman civilizations in Sardinia. The vegetational change at around 2,750 cal bp was possibly advantaged by a further shift to moister and cooler summers and drier and milder winters. Triggers for climate changes at 5,300 and 2,750 cal bp may have been gradual, orbitally-induced changes in summer and winter insolation, as well as centennial-scale atmospheric reorganizations. Open evergreen broadleaved forests persisted until the twentieth century, when they were partly substituted by widespread artificial pine plantations. Our results imply that highly flammable Erica vegetation, as reconstructed for the mid-Holocene, could re-emerge as a dominant vegetation type due to increasing drought and fire, as anticipated under global change conditions.     

Long-term forest fire ecology and dynamics in southern Switzerland

1 Pollen and charcoal analysis at two lakes in southern Switzerland revealed that fire has had a prominent role in changing the woodland composition of this area for more than 7000 years.
2 The sediment of Lago di Origlio for the period between 5100 and 3100  bc cal. was sampled continuously with a time interval of about 10 years. Peaks of charcoal particles were significantly correlated with repeated declines in pollen of Abies , Hedera , Tilia , Ulmus , Fraxinus excelsior t., Fagus and Vitis and with increases in Alnus glutinosa t., shrubs (e.g. Corylus , Salix and Sambucus nigra t.) and several herbaceous species. The final disappearance of the lowland Abies alba stands at around 3150  bc cal. may be an example of a fire-caused local extinction of a fire-intolerant species.
3 Forest fires tended to diminish pollen diversity. The charcoal peaks were preceded by pollen types indicating human activity. Charcoal minima occurred during periods of cold humid climate, when fire susceptibility would be reduced.
4 An increase of forest fires at about 2100  bc cal. severely reduced the remaining fire-sensitive plants: the mixed-oak forest was replaced by a fire-tolerant alder–oak forest. The very strong increase of charcoal influx, and the marked presence of anthropogenic indicators, point to principally anthropogenic causes.
5 We suggest that without anthropogenic disturbances Abies alba would still form lowland forests together with various deciduous broadleaved tree taxa.     

云贵高原抚仙湖近13300年的花粉/炭屑记录*

抚仙湖是云贵高原著名的断陷深水湖,其沉积物蕴藏着流域地质历史时期丰富的环境信息。对钻取自该湖的900cm 湖泊沉积物岩芯进行花粉/炭屑分析及花粉数据的主成分分析表明,抚仙湖流域的植被、气候与火灾在过去的13 300年经历了5个阶段的变化：(1)13 300—10 400cal．a BP,植被以松林为主,伴有山地暗针叶林和常绿阔叶林,表明该时期气候较为冷湿,森林火灾多发,在后期随着温度和湿度的降低,森林火灾愈加频繁。(2)10 400—5 700cal．a BP,松林收缩,常绿阔叶林扩张,出现一定数量的落叶阔叶林,显示该时期气候偏暖偏干;此阶段早期随着气候变暖变干森林火灾的发生延续上阶段高发的状态,直到9 500cal．a BP后随着湿度的增加森林火灾明显减少。(3)5 700—1 800cal．a BP,松林变化较小,常绿/落叶阔叶林比重增大,首次出现了暖热性的枫香林,显示该时期暖湿的气候特征,火灾发生频率低。(4)1 800—500cal．a BP,松林扩张,阔叶林收缩,本阶段后期草本植被比重开始增加,显示该时期气候相对冷干,森林火灾发生频率较高。(5)500cal．a BP至今,松林收缩,落叶阔叶树种增多,草本植物花粉明显增多,显示该时期气候温凉偏干,森林火灾发生频率降低。     

The mid-Holocene extinction of silver fir (Abies alba) in the Southern Alps: a consequence of forest fires? Palaeobotanical records and forest simulations

Pollen records suggest that Abies alba played a dominating role in both the montane and lowland forests at the border of the Southern Alps between ca. 8500 and 5700 years ago. Two major declines in fir, at about 7300–7000 cal b.p. and at ca. 6000 cal b.p., followed by the local extinction of the species are characteristic of the area below ca. 1000 m a.s.l. In order to test the impact of fire on the population dynamics of silver fir, a dynamic model (DisCForm) with a fire module was applied to simulate the early- and mid-Holocene forest development. Simulation outputs based on different fire scenarios were compared with the pollen record from Lago di Annone (226 m a.s.l.). The marked Abies decreases shown in the pollen record can be simulated with very intensive fire scenarios, but they do not result in an extinction of silver fir in the model. Low charcoal influx values related to the Abies declines in the palaeobotanical record suggest that fire was not the only reason for the extinction of silver fir. Human impact, as well as Holocene climatic changes leading to temporary moisture deficits and reduced adaptability due to low genetic variation may have had a significant impact on the Abies forests.     

The long-term ecology of the lost forests of La Laguna, Tenerife (Canary Islands)

Aim We report the first analysis of the long‐term ecology of Tenerife, in order to establish a pre‐colonization base‐line and to assess the effect of human activity and the role of climatic variation on vegetation during the Late Holocene. Location A former lake bed in the city of La Laguna (Tenerife, Canary Islands, Spain). Methods A sedimentary sequence of over 2 m was obtained from the former lake bed. Fossil pollen and microfossil charcoal concentrations were analysed. Radiocarbon dating of the sequence indicates that it spans approximately the last 4700 years. The pollen diagram was zoned using optimal splitting within psimpoll 4.25. Results Three pollen zones were differentiated: (1) in Zone L1 (c. 4700–2900 cal. yr bp ) a mixed forest was dominated by Quercus, Carpinus, Myrica and Pinus; (2) in Zone L2 (c. 2900–2000 cal. yr bp ) the laurel forest taxa increased, while Pinus, Juniperus and Phoenix declined; and (3) Zone L3 (c. 2000–400 cal. yr bp ) was characterized by the decline of Carpinus and Quercus and the abundance of laurel forest taxa (e.g. Myrica). Neither Carpinus nor Quercus was hitherto considered to be native to the Canary Islands. Their decline started c. 2000 years ago, coinciding with microfossil charcoal evidence of increased burning and with archaeological evidence for the first human settlement on Tenerife. Main conclusions Between c. 4700 and 2000 cal. yr bp , the composition of the forest in the valley of La Laguna was very different from what it is at present. In particular, Quercus and Carpinus appear to have been significant components, alongside components of the present‐day laurel forest, and the native pine (Pinus canariensis) forest and thermophilous woodland were also more prevalent in the region (but probably not within the lake basin itself) until 3000 cal. yr bp . The subsequent decline of Quercus and Carpinus led to the establishment of the present laurel forest in the region and a shift to more open vegetation types. These changes indicate that the aboriginal inhabitants of the islands, the Guanches, had a far more profound impact on the vegetation of Tenerife than hitherto realized.     

Late‐glacial and Holocene vegetation,climate and fire dynamics in the Serra dos Órgãos,Rio de Janeiro State,southeastern Brazil

We present a high‐resolution pollen and charcoal record of a 218 cm long sediment core from the Serra dos Órgãos, a subrange of the coastal Serra do Mar, located at 2130 m altitude in campos de altitude (high elevation grass‐ and shrubland) vegetation near Rio de Janeiro in southeastern Brazil to reconstruct past vegetation, climate and fire dynamics. Based on seven AMS ¹⁴C ages, the record represents at least the last 10 450 ¹⁴C yr bp (12 380 cal years bp ), The uppermost region was naturally covered by campos de altitude throughout the recorded period. Diverse montane Atlantic rain forest (ARF) occurred close to the studied peat bog at the end of the Late‐glacial period. There is evidence of small Araucaria angustifolia populations in the study area as late as the early Holocene, after which point the species apparently became locally extinct. Between 10 380 and 10 170 ¹⁴C yr bp (12 310–11 810 cal yr bp ), the extent of campos de altitude was markedly reduced as montane ARF shifted rapidly upward to higher elevations, reflecting a very wet and warm period (temperatures similar to or warmer than present day) at the end of the Younger Dryas (YD) chronozone. This is in opposition to the broadly documented YD cooling in the northern Hemisphere. Reduced cross‐equatorial heat transport and movement of the Intertropical Convergence Zone over northeastern Brazil may explain the YD warming. Markedly extended campos de altitude vegetation indicates dry climatic conditions until about 4910 ¹⁴C yr bp (5640 cal yr bp ). Later, wetter conditions are indicated by reduced high elevation grassland and the extension of ARF into higher elevation. Fire frequency was high during the early Holocene but decreased markedly after about 7020 ¹⁴C yr bp (7850 cal yr bp ).     

Fire regimes in north-eastern Cambodian monsoonal forests, with a 9300-year sediment charcoal record

Aim To provide insights concerning changes in fire regime in north‐eastern Cambodia over the course of the Holocene, and discuss implications of these long‐term data for fire management in the present day. Location Southern Ratanakiri Province, north‐eastern Cambodia. The lake sites sampled here are embedded in a mosaic of mostly open, strongly deciduous dipterocarp forest, with patches of riparian, semi‐evergreen and evergreen forests. Methods Background information on the environmental and cultural setting comes from informal and semi‐structured interviews of local villagers to determine present‐day burning patterns and customs. Primary data come from analysis of changes in charcoal concentration within sediments from small, closed basin lakes. Charcoal data are compared with changes in pollen and sediment physical characteristics, and to present‐day local customs, to infer or speculate on changes in human use of fire. Results Interviews with local people reveal two general types of human‐induced fires, one type for swidden cultivation in denser forests, the other type for clearance of ground layer vegetation in more open forests. A 9300‐year sediment record of microscopic charcoal deposition shows strongest fire activity ending by 8000 years ago, and the remainder of the early Holocene reflecting a strong summer monsoon and low fire activity. Beginning c. 5500 years ago, forest disturbance and fire activity increased. A subtle change in the record at c. 3500 years ago and more marked change at c. 2500 years ago suggest that fire frequency, and maybe human control over fire, became more important during that period and continuing up to the present. Main conclusions With this type of empirical data from only one site, it is impossible to make accurate conclusions about long‐term human impacts from burning. However, this record does show that present‐day charcoal input from fire activity is among the lowest for the last 9300 years. Considered together with other changes in the record and with present‐day customs, there is a suggestion that anthropogenic fire is an adaptation to the monsoonal environment, and may be conservative of forest cover in open forest formations. This long‐term perspective on the role of indigenous land‐use customs in landscape evolution should be considered in forest management and biological conservation, as it differs significantly from the traditional rationale for policies of fire suppression in tropical forests.     

The vegetational history of the northern Apennines, Italy: information from three new sequences and a review of Regional vegetational change

Abstract. The results of pollen analysis and radiocarbon dating are presented from three northern Apennine sequences; Lago Padule in northern Tuscany and Lago Pratignano and Ospitale in the Emilia-Romagnan Apennines. This is the first detailed pollen stratigraphic information from sites in Emilia-Romagna and north Tuscany and extends eastwards the area from which information on vegetational history is available. The sequence from Lago Padule is one of the most complete Holocene records known from the northern Apennine region. Lago Pratignano has the deepest sequence of organic sediments (1544 cm) and the fastest rates of sediment accumulation (up to 28 cm per 100 years) providing the highest resolution record for the mid to late Holocene periods in the region. High rates of sedimentation have also occurred at Ospitale where organic sediments are 780 cm deep and began to form at approximately 5500 bp. The record from Lago Padule is compared with Holocene records from two nearby sites and a series of Regional Pollen Assemblage Zones is defined for the eastern area of the northern Apennines. The main features of vegetational change identified are: (i) a‘pioneer’phase of rapid forest development during the early Holocene followed by the establishment of an upper forest belt dominated by Abies, and a belt of mixed deciduous forest at lower altitudes; (ii) the appearance and rapid expansion of Fagus between approximately 5200 bp and 2900 bp forming a mixed Abies-Fagus association in the upper forest belt; and (iii) the overall reduction of forest cover, and dominance of Fagus in the arboreal vegetation from around 2900 bp. The scheme of Regional PAZs for the eastern area is used as a framework for the review of pollen stratigraphic information and radiocarbon dates from other sites in the northern Apennine region. Characteristics of pollen records which can be identified in sequences from across the region are identified and the chronology of similar changes in pollen stratigraphy is examined. The scheme of four regional PAZs for the Holocene period is shown to be valid for the entire northern Apennine region. The examination of a series of pollen records shows that Fagus appeared earlier in the western than the eastern part of the region during the mid Holocene and became dominant in the northern Apennine forests post 3000 bp. The impact of anthropogenic activity and climatic change on the spread and development of Fagus are discussed. The palynological evidence which is now available from the northern Apennines is compared with information for the region shown in the‘European pollen maps’of Huntley & Birks (1983). This study provides an updated review of the representation of different tree taxa in Holocene pollen records from the northern Apennines and illustrates the role of the northern Apennines as a refugium for trees during the Wurmian Lateglacial.     

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.     

Predicting the Recovery of <Emphasis Type="Italic">Pinus halepensis</Emphasis> and <Emphasis Type="Italic">Quercus ilex</Emphasis> Forests after a Large Wildfire in Northeastern Spain

Quercus ilex and Pinus halepensis are two of the most common tree species of the western Mediterranean basin. Both species regenerate reliably after fire: P. halepensis colonizes recently disturbed areas by effective seedling recruitment, while Q. ilex resprouts vigorously after disturbances. For this reason, the natural regeneration of these species after fire should ensure the re-establishment of a forest similar to that which existed before the fire. This study analyzes with a simple simulation model whether or not the relative abundance of monospecific and mixed forests of these species in the landscape is altered by fire. We also analyze the topographic factors and the forest structure before the fire that determine the changes in forest composition after fire. This study has been carried out in a large fire that occurred in NE Spain. Overall, 33% of plots changed to another community type, but this probability of change varied considerably among community types before the fire. Monospecific forests of P. halepensis or Q. ilex had a high probability of remaining in their original composition after the fire, whereas the resilience of mixed forests of these two species was quite low. Mixed forests changed for the most part to monospecific P. halepensis or Q. ilex forests. Analysis of several factors determining these changes indicated that only elevation as a significant topographical variable. The effect of fire was to increase the altitudinal differentiation between the two species. P. halepensis forests that changed to mixed or Q. ilex forests were those of highest elevation, while the mixed and Q. ilex plots that changed to P. halepensis forests were those located at the lowest elevations. Concerning structural variables before fire, density of Q. ilex trees before the fire showed a much greater effect than P. halepensis density in determining the post-fire community. Finally, burn severity also influenced the changes observed. For both P. halepensis and Q. ilex forests, plots that changed to another forest type were mainly those that burned more severely. In the case of mixed forests, even low fire severities involved high probabilities of change to monospecific forests.     

Recent climate hiatus revealed dual control by temperature and drought on the stem growth of Mediterranean Quercus ilex

A better understanding of stem growth phenology and its climate drivers would improve projections of the impact of climate change on forest productivity. Under a Mediterranean climate, tree growth is primarily limited by soil water availability during summer, but cold temperatures in winter also prevent tree growth in evergreen forests. In the widespread Mediterranean evergreen tree species Quercus ilex, the duration of stem growth has been shown to predict annual stem increment, and to be limited by winter temperatures on the one hand, and by the summer drought onset on the other hand. We tested how these climatic controls of Q. ilex growth varied with recent climate change by correlating a 40‐year tree ring record and a 30‐year annual diameter inventory against winter temperature, spring precipitation, and simulated growth duration. Our results showed that growth duration was the best predictor of annual tree growth. We predicted that recent climate changes have resulted in earlier growth onset (?10 days) due to winter warming and earlier growth cessation (?26 days) due to earlier drought onset. These climatic trends partly offset one another, as we observed no significant trend of change in tree growth between 1968 and 2008. A moving‐window correlation analysis revealed that in the past, Q. ilex growth was only correlated with water availability, but that since the 2000s, growth suddenly became correlated with winter temperature in addition to spring drought. This change in the climate–growth correlations matches the start of the recent atmospheric warming pause also known as the ‘climate hiatus’. The duration of growth of Q. ilex is thus shortened because winter warming has stopped compensating for increasing drought in the last decade. Decoupled trends in precipitation and temperature, a neglected aspect of climate change, might reduce forest productivity through phenological constraints and have more consequences than climate warming alone.     

Vegetation history,climate and human impact over the last 15,000 years at Lago dell’Accesa (Tuscany,Central Italy)

Interdisciplinary studies of the sediments of Lago dell’Accesa started in 2001. We present here results from the palynological study. The pollen diagram provides a record of vegetation and climatic change spanning over 15,000 years. The oldest pollen spectra show a late-glacial steppe vegetation typical of central and southern Italy during this period. The Late-glacial Interstadial, interrupted by two cooling events, is dominated by open deciduous oak forests. The Younger Dryas is represented by 150 cm of sediment and shows the presence of steppic vegetation. The Holocene vegetation is characterised by alternating dominance of deciduous oaks and Quercus ilex. The three zones characterised by Q. ilex are accompanied by peat layers marking lake-level lowering at ca. 8600–7900, 4600–4300 and 3700–2800 cal b.p. Between approximately 9000 and 6000 cal b.p. extensive Abies-forests existed on the Colline Metallifere located 15–20 km to the north and northeast of the lake. Local fir populations may also have existed by the lake. Human impact starts at approximately 8000 cal b.p. during the Neolithic period, and increases at ca. 4300 cal b.p. Castanea and Juglans pollen is recorded from ca. 2800 cal b.p. The impact of the Etruscan settlement near the lakeshore is shown in the increasing values of arable crops, species of secondary forest canopy (Ericaceae, Pinus, Pistacia, Myrtus) and anthropogenic indicators (Chenopodiaceae, Plantago lanceolata, Rumex etc).     

Holocene persistence of wooded steppe in the Great Hungarian Plain

Aim We used a combination of new and previously published palaeoecological data to test three hypotheses: (1) that wooded steppe persisted in the Great Hungarian Plain throughout the Holocene; (2) that wooded steppe and steppe were most extensive between c. 9900 and 8300 cal. yr bp (the ‘Boreal steppe’ period); and (3) that Southern Continental, Pontic and Eastern Sub‐Mediterranean steppe species reached the region during the early Holocene via the ‘Lower Danube Corridor’. Location Sarló‐hát oxbow lake, Hungary and the Eastern European wooded steppe zone. Methods Holocene sediments deposited in the Sarló‐hát oxbow lake were subjected to pollen and microcharcoal analyses. Twelve radiocarbon age estimates were obtained to determine sediment chronology. In addition, previously published palaeoecological data from the Great Hungarian Plain were compiled, analysed and compared with previous studies in other regions of steppe and wooded steppe in eastern Europe. Results Palynological data from two sediment cores extending to c. 11,400 cal. yr bp indicate the persistent dominance of the landscape by temperate deciduous wooded steppe throughout the Holocene, although with varying canopy composition. Warm‐continental steppe grasslands and saline tall‐grass meadows developed on edaphically constrained areas, which remained steppe‐dominated throughout the Holocene. The extent of steppe grasslands did not increase between 9900 and 8300 cal. yr bp . After c. 3100 cal. yr bp , anthropogenic activities led to the development of cultural steppe. Thermophilous steppe species of the Southern Continental, Pontic and Sub‐Mediterranean floristic elements probably reached the Great Hungarian Plain principally via the Lower Danube Corridor during the late glacial interstadial and Holocene. Eurythermic members of these elements, however, probably survived the Last Glacial Maximum in favourable microsites, extending their ranges during the Holocene from these local sources. Main conclusions Our results confirm the Holocene persistence of wooded steppe in the Great Hungarian Plain, disprove the ‘Boreal steppe’ theory, and suggest an Early Holocene period of greater vegetation openness between 11,400 and 9900 cal. yr bp . Evidence for the post‐glacial immigration of south‐eastern steppe elements into the Carpathian Basin is equivocal: the last glacial/interglacial presence of several southern steppe species suggests that the Hungarian Plain hosted suitable habitats for them during warm and cold phases alike.     

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.     

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.