Paul Henne | United States Geological Survey (original) (raw)

Papers by Paul Henne

Little is known about the vegetation and fire history of Sardinia, and especially the long-term h... more 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, orbitallyinduced 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.

The New phytologist, 2014

Climate refugia, locations where taxa survive periods of regionally adverse climate, are thought ... more Climate refugia, locations where taxa survive periods of regionally adverse climate, are thought to be critical for maintaining biodiversity through the glacial-interglacial climate changes of the Quaternary. A critical research need is to better integrate and reconcile the three major lines of evidence used to infer the existence of past refugia - fossil records, species distribution models and phylogeographic surveys - in order to characterize the complex spatiotemporal trajectories of species and populations in and out of refugia. Here we review the complementary strengths, limitations and new advances for these three approaches. We provide case studies to illustrate their combined application, and point the way towards new opportunities for synthesizing these disparate lines of evidence. Case studies with European beech, Qinghai spruce and Douglas-fir illustrate how the combination of these three approaches successfully resolves complex species histories not attainable from any ...

The Mediterranean Basin is the region of Europe most vulnerable to negative climate-change impact... more The Mediterranean Basin is the region of Europe most vulnerable to negative climate-change impacts, including forest decline, increased wildfire, and biodiversity loss. Because humans have affected Mediterranean ecosystems for millennia, it is unclear whether the region's native ecosystems were more resilient to climate change than current ecosystems, and whether they would provide sustainable management options if restored. We simulated vegetation with the LandClim model, using present-day climate as well as future climate-change scenarios, in three representative areas that encompass a broad range of Mediterranean conditions and vegetation types. Sedimentary pollen records that document now-extinct forests help to validate the simulations. Forests modeled under present climate closely resemble the extinct forests when human disturbance is limited; under future scenarios, characterized by increased temperatures and decreased precipitation, extinct forests are projected to re-emerge. When combined with modeling, paleoecological evidence reveals the potential of native vegetation to re-establish under current and future climate conditions, and provides a template for novel management strategies to maintain forest productivity and biodiversity in a warmer and drier future.

Global Change Biology 20(5) Volume 20, 1512–1526, May 2014

Mountain vegetation is strongly affected by temperature and is expected to shift upwards with cli... more Mountain vegetation is strongly affected by temperature and is expected to shift upwards with climate change. Dynamic vegetation models are often used to assess the impact of climate on vegetation and model output can be compared with paleobotanical data as a reality check. Recent paleoecological studies have revealed regional variation in the upward shift of timberlines in the Northern and Central European Alps in response to rapid warming at the Younger Dryas/Preboreal transition ca. 11 700 years ago, probably caused by a climatic gradient across the Alps. This contrasts with previous studies that successfully simulated the early Holocene afforestation in the (warmer) Central Alps with a chironomid-inferred temperature reconstruction from the (colder) Northern Alps. We use LandClim, a dynamic landscape vegetation model to simulate mountain forests under different temperature, soil and precipitation scenarios around Iffigsee (2065 m a.s.l.) a lake in the Northwestern Swiss Alps, and compare the model output with the paleobotanical records. The model clearly overestimates the upward shift of timberline in a climate scenario that applies chironomid-inferred July-temperature anomalies to all months. However, forest establishment at 9800 cal. BP at Iffigsee is successfully simulated with lower moisture availability and monthly temperatures corrected for stronger seasonality during the early Holocene. The model-data comparison reveals a contraction in the realized niche of Abies alba due to the prominent role of anthropogenic disturbance after ca. 5000 cal. BP, which has important implications for species distribution models (SDMs) that rely on equilibrium with climate and niche stability. Under future climate projections, LandClim indicates a rapid upward shift of mountain vegetation belts by ca. 500 m and treeline positions of ca. 2500 m a.s.l. by the end of this century. Resulting biodiversity losses in the alpine vegetation belt might be mitigated with low-impact pastoralism to preserve species-rich alpine meadows.

Ecological Monographs 83(4), 2013, pp. 419–439, Nov 2013

Paleoecology can provide valuable insights into the ecology of species that complement observatio... more Paleoecology can provide valuable insights into the ecology of species that complement observation and experiment-based assessments of climate impact dynamics. New paleoecological records (e.g., pollen, macrofossils) from the Italian Peninsula suggest a much wider climatic niche of the important European tree species Abies alba (silver fir) than observed in its present spatial range. To explore this discrepancy between current and past distribution of the species, we analyzed climatic data (temperature, precipitation, frost, humidity, sunshine) and vegetation-independent paleoclimatic reconstructions (e.g., lake levels, chironomids) and use global coupled carbon-cycle climate (NCAR CSM1.4) and dynamic vegetation (LandClim) modeling. The combined evidence suggests that during the mid-Holocene (6000 years ago), prior to humanization of vegetation, A. alba formed forests under conditions that exceeded the modern (1961–1990) upper temperature limit of the species by 5–7°C (July means). Annual precipitation during this natural period was comparable to today (>700–800 mm), with drier summers and wetter winters. In the meso-Mediterranean to sub-Mediterranean forests A. alba co-occurred with thermophilous taxa such as Quercus ilex, Q. pubescens, Olea europaea, Phillyrea, Arbutus, Cistus, Tilia, Ulmus, Acer, Hedera helix, Ilex aquifolium, Taxus, and Vitis. Results from the last interglacial (ca. 130 000–115 000 BP), when human impact was negligible, corroborate the Holocene evidence. Thermophilous Mediterranean A. alba stands became extinct during the last 5000 years when land-use pressure and specifically excessive anthropogenic fire and browsing disturbance increased. Our results imply that the ecology of this key European tree species is not yet well understood. On the basis of the reconstructed realized climatic niche of the species, we anticipate that the future geographic range of A. alba may not contract regardless of migration success, even if climate should become significantly warmer than today with summer temperatures increasing by up to 5–7°C, as long as precipitation does not fall below 700–800 mm/yr, and anthropogenic disturbance (e.g., fire, browsing) does not become excessive. Our finding contradicts recent studies that projected range contractions under global-warming scenarios, but did not factor how millennia of human impacts reduced the realized climatic niche of A. alba.

A new sedimentary sequence from Lago di Venere on Pantelleria Island, located in the Strait of Si... more A new sedimentary sequence from Lago di Venere on Pantelleria Island, located in the Strait of Sicily between Tunisia and Sicily was recovered. The lake is located in the coastal infra-Mediterranean vegetation belt at 2 m a.s.l. Pollen, charcoal and sedimentological analyses are used to explore linkages among vegetation, fire and climate at a decadal scale over the past 1200 years. A dry period from AD 800 to 1000 that corresponds to the ‘Medieval Warm Period’ (WMP) is inferred from sedimentological analysis. The high content of carbonate recorded in this period suggests a dry phase, when the ratio of evaporation/precipitation was high. During this period the island was dominated by thermophilous and drought-tolerant taxa, such as Quercus ilex, Olea, Pistacia and Juniperus. A marked shift in the sediment properties is recorded at AD 1000, when carbonate content became very low suggesting wetter conditions until AD 1850–1900. Broadly, this period coincides with the ‘Little Ice Age’ (LIA), which was characterized by wetter and colder conditions in Europe. During this time rather mesic conifers (i.e. Pinus pinaster), shrubs and herbs (e.g. Erica arborea and Selaginella denticulata) expanded, whereas more drought-adapted species (e.g. Q. ilex) declined. Charcoal data suggest enhanced fire activity during the LIA probably as a consequence of anthropogenic burning and/or more flammable fuel (e.g. resinous Pinus biomass). The last century was characterized by a shift to high carbonate content, indicating a change towards drier conditions, and re-expansion of Q. ilex and Olea. The post-LIA warming is in agreement with historical documents and meteorological time series. Vegetation dynamics were co-determined by agricultural activities on the island. Anthropogenic indicators (e.g. Cerealia-type, Sporormiella) reveal the importance of crops and grazing on the island. Our pollen data suggest that extensive logging caused the local extinction of deciduous Quercus pubescens around AD 1750.

Forests near the Mediterranean coast have been shaped by millennia of human disturbance. Conseque... more Forests near the Mediterranean coast have been shaped by millennia of human disturbance. Consequently, ecological studies relying on modern observations or historical records may have difficulty assessing natural vegetation dynamics under current and future climate. We combined a sedimentary pollen record from Lago di Massacciucoli, Tuscany, Italy with simulations from the LandClim dynamic vegetation model to determine what vegetation preceded intense human disturbance, how past changes in vegetation relate to fire and browsing, and the potential of an extinct vegetation type under present climate. We simulated vegetation dynamics near Lago di Massaciucoli for the last 7,000 years using a local chironomid-inferred temperature reconstruction with combinations of three fire regimes (small infrequent, large infrequent, small frequent) and three browsing intensities (no browsing, light browsing, and moderate browsing), and compared model output to pollen data. Simulations with low disturbance support pollen-inferred evidence for a mixed forest dominated by Quercus ilex (a Mediterranean species) and Abies alba (a montane species). Whereas pollen data record the collapse of A. alba after 6000 cal yr bp, simulated populations expanded with declining summer temperatures during the late Holocene. Simulations with increased fire and browsing are consistent with evidence for expansion by deciduous species after A. alba collapsed. According to our combined paleo-environmental and modeling evidence, mixed Q. ilex and A. alba forests remain possible with current climate and limited disturbance, and provide a viable management objective for ecosystems near the Mediterranean coast and in regions that are expected to experience a mediterranean-type climate in the future.

High-resolution pollen, macrofossil and charcoal data, combined with accelerator mass spectrometr... more High-resolution pollen, macrofossil and charcoal data, combined with accelerator mass spectrometry 14C dating and multivariate analysis, were used to reconstruct Holocene vegetation and fire dynamics at Urio Quattrocchi, a small lake in the supra-mediterranean belt in the Nebrodi Mountains of Sicily (Italy). The data suggest that after 10 000 cal a BP increasing moisture availability supported closed forests with deciduous (Quercus cerris, Fagus sylvatica and Fraxinus spp.) and evergreen (Quercus ilex) species. Species-rich closed forest persisted until 6850 cal a BP, when Neolithic activities caused a forest decline and affected plant diversity. Secondary forest with abundant Ilex aquifolium recovered between 6650 and 6000 cal a BP, indicating moist conditions. From 5000 cal a BP, agriculture and pastoralism led to the currently fragmented landscape with sparse deciduous forests (Quercus cerris). The study suggests that evergreen broadleaved species were more important at elevations above 1000 m a.s.l. before ca. 5000 cal a BP than subsequently, which might reflect less human impact or warmer-than-today climatic conditions between 10 000 and 5000 cal a BP. Despite land use since Neolithic times, deciduous supra-mediterranean forests were never completely displaced from the Nebrodi Mountains, because of favourable moist conditions that persisted throughout the Holocene. Reconstructed vegetation dynamics document the absence of any pronounced mid- or late-Holocene ‘aridification’ trend at the site, an issue which is controversially debated in Italy and the Mediterranean region.

Continental evaporation is a significant and dynamic flux within the atmospheric water budget, bu... more Continental evaporation is a significant and dynamic flux within the atmospheric water budget, but few methods provide robust observational constraints on the large-scale hydroclimatological and hydroecological impacts of this ‘recycled-water' flux. We demonstrate a geospatial analysis that provides such information, using stable isotope data to map the distribution of recycled water in shallow aquifers downwind from Lake Michigan. The δ2H and δ18O values of groundwater in the study region decrease from south to north, as expected based on meridional gradients in climate and precipitation isotope ratios. In contrast, deuterium excess (d = δ2H − 8 × δ18O) values exhibit a significant zonal gradient and finer-scale spatially patterned variation. Local d maxima occur in the northwest and southwest corners of the Lower Peninsula of Michigan, where ‘lake-effect' precipitation events are abundant. We apply a published model that describes the effect of recycling from lakes on atmospheric vapor d values to estimate that up to 32% of recharge into individual aquifers may be derived from recycled Lake Michigan water. Applying the model to geostatistical surfaces representing mean d values, we estimate that between 10% and 18% of the vapor evaporated from Lake Michigan is re-precipitated within downwind areas of the Lake Michigan drainage basin. Our approach provides previously unavailable observational constraints on regional land-atmosphere water fluxes in the Great Lakes Basin and elucidates patterns in recycled-water fluxes that may influence the biogeography of the region. As new instruments and networks facilitate enhanced spatial monitoring of environmental water isotopes, similar analyses can be widely applied to calibrate and validate water cycle models and improve projections of regional hydroecological change involving the coupled lake-atmosphere-land system.

Few examples of natural forest remain near the Mediterranean coast. Therefore, it is difficult to... more Few examples of natural forest remain near the Mediterranean coast. Therefore, it is difficult to study how coastal forests respond to climatic change or their resilience to human impact. We developed a new sedimentary record of Holocene vegetation and fire history at Lago Preola, a coastal lake in southwestern Sicily (Italy). In order to verify the existence of forest at large scale on the coast, we compare pollen from Lago Preola, a medium-sized lake (33 ha), to Gorgo Basso, a small lake (3 ha) located nearby with the aim of separating local from extra-local vegetation dynamics through time using pollen percentages and influx. We then compare Lago Preola pollen to the record from Biviere di Gela, a large lagoon (120 ha) situated 160 km to the east in southern Sicily, to examine differences in vegetation dynamics between the two coastal areas during the Holocene. Lake-level reconstructions and ostracode analyses from Lago Preola provide vegetation-independent evidence of climate change, and help to disentangle human and climatic impacts on vegetation. Pollen data indicate Pistacia-dominated shrublands replaced open grasslands in the region surrounding Lago Preola by 9500 cal yr BP. This change coincided with rising lake levels and the development of an ostracode fauna typical of fresh waters. Evergreen forest dominated by Quercus ilex and Olea europaea started to expand by 7000 cal BP and consolidated at 6500 cal yr BP, when lake levels were near their Holocene high. Similarities between pollen from Lago Preola and Gorgo Basso demonstrate that forest was the dominant vegetation type in coastal Sicily during the middle Holocene at both regional and local scales, and even developed in the drier climatic setting around Biviere di Gela. Lake levels fell at Lago Preola after 7000 cal yr BP, with a strong decline accompanied by increasing salinity after 4500 cal yr BP. However, no transition in vegetation matched these inferred hydrological changes. Instead, forests persisted in the surrounding region until 2200 cal BP when human disturbance intensified. We propose that different climatic factors control lake levels and vegetation in coastal Mediterranean ecosystems. Whereas lake levels are most sensitive to the abundance of winter precipitation, coastal forests depend on spring precipitation and are limited by the length of summer drought. Moisture availability remained suitable for evergreen forests in coastal Sicily during the late Holocene, and humans, not a drier climate drove the regional forest decline.

Journal of …

Aim Forest communities in the European Central Alps are highly sensitive to climatic change. Pal... more Aim

Forest communities in the European Central Alps are highly sensitive to climatic change. Palaeobotanical studies have demonstrated that forests rapidly expanded upslope during Holocene warm intervals and contracted when temperatures fell. However, temperature alone cannot account for important changes in tree species abundance. For example, population expansion by Norway spruce (Picea abies), a dominant subalpine species, lagged suitable temperatures by about 3000 years in eastern and by 6000 years in western Switzerland. We hypothesize that spruce expansion was delayed by limited water availability in weakly developed soils and/or by drier-than-present climatic conditions.

Location

We examine the impact of reduced moisture availability on forest dynamics using a combined dynamic modelling/palaeoecological approach at two high-elevational lakes in the Swiss Central Alps.

Methods

We simulate Holocene vegetation dynamics with the LandClim model in landscapes surrounding the two lakes and validate the model output by comparison with palaeobotanical reconstructions from the same sites. We evaluate the impact of shallow soils on vegetation dynamics at these sites by varying soil water-holding capacity (i.e. bucket size) and precipitation abundance in model scenarios.

Results

Simulations with modern soil conditions and precipitation abundance matched reconstructed vegetation dynamics near the tree line, where temperature limits plant growth, but simulated abundant spruce during the entire Holocene. Spruce was absent only in simulations with a maximum bucket size of less than 7 cm, or when precipitation was reduced by at least 60%. In exploratory simulations of future conditions with average temperatures raised by 4 degrees C, the low water-holding capacity of shallow alpine soils, not temperature, determined the upper elevational limit of spruce.

Main conclusions

Spruce expanded in the Central Alps only after soils developed sufficient water-holding capacity and precipitation neared its modern abundance. Soil development will probably constrain the future response of tree species to warmer conditions (e.g. upslope migrations), as it did in the past.

Journal of …, Jan 1, 2010

1. We use high-resolution records of macroscopic charcoal and plant remains from sediments of a l... more 1. We use high-resolution records of macroscopic charcoal and plant remains from sediments of a lake in the Swiss Alps (Gouille Rion, 2343 m a.s.l.) to reconstruct local fire variability and vegetation dynamics over the last 12 000 years.

2. Species response to fire variability and to summer temperature was obtained by combining regression analyses between contiguous series of plant macrofossils, macroscopic charcoal and an available reconstruction of past summer temperature.

3. With a dynamic landscape vegetation model (LandClim), we simulated fire regimes using two levels of ignition frequency and moisture availability to disentangle the role of climate vs. humans on fire occurrence. The simulation results show that human disturbance was relevant in controlling the fire variability and are in agreement with pollen evidence of human impact from previous studies from Gouille Rion.

4. Our results show that fire is a natural disturbance agent in the tree line ecotone. Biomass availability controlled the fire regime until increased land use and anthropogenic fire during the past 4000 years changed species composition and vegetation structure close to the tree line.

5. Important species at the tree line ecotone such as Pinus cembra greatly benefitted from periods with temperature above the modern mean July temperature, if anthropogenic fire disturbance was not too severe, such as during the Bronze Age ( c. 4000 cal. years bp).

6. When mean July temperatures were lower than modern mean July values, Juniperus nana and Larix decidua were at an advantage over P. cembra. With increasing anthropogenic fire, open lands with J. nana replaced L. decidua and P. cembra forest stands.

7. Synthesis. Fire activity was low to moderate during the early and mid-Holocene. Intensified land use coupled with fire occurrence since the Bronze Age ( c. 4000 cal. years bp) had a larger impact on species composition near the tree line than climate change. Although climate change will alter vegetation composition, future dynamics of mountain forests will be co-determined by anthropogenic fire. For example, high fire variability may impede upslope establishment of forests in response to climatic warming as expected for this century, with serious implications for forest diversity.

Quaternary Science Reviews, Jan 1, 2010

Lake-effect snow is an important constraint on ecological and socio-economic systems near the Nor... more Lake-effect snow is an important constraint on ecological and socio-economic systems near the North American Great Lakes Little is known about the Holocene history of lake-effect snowbelts, and it is difficult to decipher how lake-effect snowfall abundance affected ecosystem development. We conducted oxygen-Isotope analysis of calcite in lake-sediment cores from northern Lower Michigan to infer Holocene climatic variation and assess snowbelt development The two lakes experience the same synoptic-scale climatic systems, but only one of them (Huffman Lake) receives a significant amount of lake-effect snow. A 177-cm difference in annual snowfall causes groundwater inflow at Huffman Lake to be O-18-depleted by 2 3 parts per thousand relative to O'Brien Lake. To assess when the lake-effect snowbelt became established, we compared calcite-delta O-18 profiles of the last 11,500 years from these two sites. The chronologies are based on accelerator-mass-spectrometry C-14 ages of 11 and 17 terrestrial-plant samples from Huffman and O'Brien lakes, respectively. The values of delta O-18 are low at both sites from 11,500 to 9500 cal yr BP when the Laurentide Ice Sheet (LIS) exerted a dominant control over the regional climate and provided periodic pulses of meltwater to the Great Lakes basin Carbonate delta O-18 increases by 2.6 parts per thousand at O'Brien Lake and by 1 4 parts per thousand at Huffman Lake between 9500 and 7000 cal yr BP, suggesting a regional decline in the proportion of runoff derived from winter precipitation The Great Lakes snowbelt probably developed between 9500 and 5500 cal yr BP as Inferred from the progressive O-18-depletion at Huffman Lake relative to O'Brien Lake, with the largest increase of lake-effect snow around 7000 cal yr BR Lake-effect snow became possible at this time because of increasing contact between the Great Lakes and frigid arctic air. These changes resulted from enhanced westerly flow over the Great Lakes as the LIS collapsed, and from rapidly rising Great Lakes levels during the Nipissing Transgression. The delta O-18 difference between Huffman and O'Brien lakes declines after 5500 cal yr BP, probably because of a northward shift of the polar vortex that brought increasing winter precipitation to the entire region However, delta O-18 remains depleted at Huffman Lake relative to O'Brien Lake because of the continued production of lake-effect snow.

Journal of Ecology, Jan 1, 2007

1 Recent studies demonstrated the sensitivity of northern forest ecosystems to changes in the amo... more 1 Recent studies demonstrated the sensitivity of northern forest ecosystems to changes in the amount and duration of snow cover at annual to decadal time scales. However, the consequences of snowfall variability remain uncertain for ecological variables operating at longer time scales, especially the distributions of forest communities.

2 The Great Lakes region of North America offers a unique setting to examine the long-term effects of variable snowfall on forest communities. Lake-effect snow produces a three-fold gradient in annual snowfall over tens of kilometres, and dramatic edaphic variations occur among landform types resulting from Quaternary glaciations. We tested the hypothesis that these factors interact to control the distributions of mesic (dominated by Acer saccharum, Tsuga canadensis and Fagus grandifolia) and xeric forests (dominated by Pinus and Quercus spp.) in northern Lower Michigan.

3 We compiled pre-European-settlement vegetation data and overlaid these data with records of climate, water balance and soil, onto Landtype Association polygons in a geographical information system. We then used multivariate adaptive regression splines to model the abundance of mesic vegetation in relation to environmental controls.

4 Snowfall is the most predictive among five variables retained by our model, and it affects model performance 29% more than soil texture, the second most important variable. The abundance of mesic trees is high on fine-textured soils regardless of snowfall, but it increases with snowfall on coarse-textured substrates. Lake-effect snowfall also determines the species composition within mesic forests. The weighted importance of A. saccharum is significantly greater than of T. canadensis or F. grandifolia within the lake-effect snowbelt, whereas T. canadensis is more plentiful outside the snowbelt. These patterns are probably driven by the influence of snowfall on soil moisture, nutrient availability and fire return intervals.

5 Our results imply that a key factor dictating the spatio-temporal patterns of forest communities in the vast region around the Great Lakes is how the lake-effect snowfall regime responds to global change. Snowfall reductions will probably cause a major decrease in the abundance of ecologically and economically important species, such as A. saccharum.

Quaternary Science …, Jan 1, 2009

We used a new sedimentary record to reconstruct the Holocene vegetation and fire history of Gorgo... more We used a new sedimentary record to reconstruct the Holocene vegetation and fire history of Gorgo Basso, a coastal lake in south-western Sicily (Italy). Pollen and charcoal data suggest a fire-prone open grassland near the site until ca 10,000 cal yr BP (8050 cal BC), when Pistacia shrubland expanded and fire activity declined, probably in response to increased moisture availability. Evergreen Olea europaea woods expanded ca 8400 to decline abruptly at 8200 cal yr BP, when climatic conditions became drier at other sites in the Mediterranean region. Around 7000 cal yr BP evergreen broadleaved forests (Quercus ilex, Quercus suber and O. europaea) expanded at the cost of open communities. The expansion of evergreen broadleaved forests was associated with a decline of fire and of local Neolithic (Ficus carica–Cerealia based) agriculture that had initiated ca 500 years earlier. Vegetational, fire and land-use changes ca 7000 cal yr BP were probably caused by increased precipitation that resulted from (insolation-forced) weakening of the monsoon and Hadley circulation ca 8000–6000 cal yr BP. Low fire activity and dense coastal evergreen forests persisted until renewed human activity (probably Greek, respectively Roman colonists) disrupted the forest ca 2700 cal yr BP (750 BC) and 2100 cal yr BP (150 BC) to gain open land for agriculture. The intense use of fire for this purpose induced the expansion of open maquis, garrigue, and grassland-prairie environments (with an increasing abundance of the native palm Chamaerops humilis). Prehistoric land-use phases after the Bronze Age seem synchronous with those at other sites in southern and central Europe, possibly as a result of climatic forcing. Considering the response of vegetation to Holocene climatic variability as well as human impact we conclude that under (semi-)natural conditions evergreen broadleaved Q. ilex–O. europaea (s.l.) forests would still dominate near Gorgo Basso. However, forecasted climate change and aridification may lead to a situation similar to that before 7000 cal yr BP and thus trigger a rapid collapse of the few relict evergreen broadleaved woodlands in coastal Sicily and elsewhere in the southern Mediterranean region.

Numerical models and statistical analysis aid interpretation of fire history from sediment-charco... more Numerical models and statistical analysis aid interpretation of fire history from sediment-charcoal records, allowing inferences into the causes of past fire-regime shifts through quantitative analyses and data-model comparisons.

Climatic warming during the last glacial–interglacial transition (LGIT) was punctuated by reversa... more Climatic warming during the last glacial–interglacial transition (LGIT) was punctuated by reversals to glacial-like conditions. Palaeorecords of ecosystem change can help document the geographical extent of these events and improve our understanding of biotic sensitivity to climatic forcing. To reconstruct ecosystem and climatic variations during the LGIT, we analyzed lake sediments from southwestern Alaska for fossil pollen assemblages, biogenic-silica content (BSiO2%), and organic-carbon content (OC%). Betula shrub tundra replaced herb tundra as the dominant vegetation of the region around 13 600 cal BP (cal BP: 14C calibrated calendar years before present), as inferred from an increase of Betula pollen percentages from << 5% to >> 20% with associated decreases in Cyperaceae, Poaceae, and Artemisia. At c. 13 000 cal BP, a decrease of Betula pollen from 28 to << 5% suggests that shrub tundra reverted to herb tundra. Shrub tundra replaced herb tundra to resume as the dominant vegetation at 11 600 cal BP. Higher OC% and BSiO2% values suggest more stable soils and higher aquatic productivity during shrub-tundra periods than during herb-tundra periods, although pollen changes lagged behind changes in the biogeochemical indicators before c. 13 000 cal BP. Comparison of our palaeoecological data with the ice-core dδ18O record from Greenland reveals strikingly similar patterns from the onset through the termination of the Younger Dryas (YD). This similarity supports the hypothesis that, as in the North Atlantic region, pronounced YD climatic oscillations occurred in the North Pacific region. The rapidity and magnitude of ecological changes at the termination of the YD are consistent with greenhouse experiments and historic photographs demonstrating tundra sensitivity to climatic forcing.

Talks by Paul Henne