Holocene Vegetation Changes Research Articles

The role of climate, forest fires and human population size in Holocene vegetation dynamics in Fennoscandia

AbstractQuestionsWe investigated the changing role of climate, forest fires and human population size in the broad‐scale compositional changes in Holocene vegetation dynamics before and after the onset of farming in Sweden (at 6,000 cal yr BP) and in Finland (at 4,000 cal yr BP).LocationSouthern and central Sweden, SW and SE Finland.MethodsHolocene regional plant abundances were reconstructed using the REVEALS model on selected fossil pollen records from lakes. The relative importance of climate, fires and human population size on changes in vegetation composition was assessed using variation partitioning. Past climate variable was derived from the LOVECLIM climate model. Fire variable was reconstructed from sedimentary charcoal records. Estimated trend in human population size was based on the temporal distribution of archaeological radiocarbon dates.ResultsClimate explains the highest proportion of variation in vegetation composition during the whole study period in Sweden (10,000–4,000 cal yr BP) and in Finland (10,000–1,000 cal yr BP), and during the pre‐agricultural period. In general, fires explain a relatively low proportion of variation. Human population size has significant effect on vegetation dynamics after the onset of farming and explains the highest variation in vegetation in S Sweden and SW Finland.ConclusionsMesolithic hunter‐gatherer populations did not significantly affect vegetation composition in Fennoscandia, and climate was the main driver of changes at that time. Agricultural communities, however, had greater effect on vegetation dynamics, and the role of human population size became a more important factor during the late Holocene. Our results demonstrate that climate can be considered the main driver of long‐term vegetation dynamics in Fennoscandia. However, in some regions the influence of human population size on Holocene vegetation changes exceeded that of climate and has a longevity dating to the early Neolithic.

Late Pleistocene and Holocene vegetation changes in the arid Andean piedmont of central Argentina inferred from sediment stable carbon isotopes and C/N ratios

Stable carbon isotopes (δ13C) and C/N ratios in bulk sediment organic matter were used as indicators of C3 and C4 vegetation functional types in a ca. 40-kyr, predominantly alluvial sequence from Arroyo La Estacada (33.5°S, 69°W), in the arid Andean piedmont of central Argentina. Although this sediment sequence has the potential to contribute to the knowledge of past vegetation dynamics through its fossil pollen content, previous palynological studies provided scant information, constrained to a few time windows, because of poor pollen preservation. Stable isotope results for the total organic carbon fraction from three lithostratigraphic sections that span the past 40 kyr show shifts in δ13C values associated with changing contributions from C3 and C4 plants, suggesting this proxy can detect changes in these photosynthetic pathways, representative of Patagonian (and/or stream-margin macrophytes) and Monte phytogeographic units, respectively. Climate and local factors, such as water availability in the basin, are inferred to be the main drivers of vegetation dynamics. The C/N ratios in a few samples show that organic matter sources are algae and/or phytoplankton, not solely C3 or C4 vascular plants. The δ13C record from Arroyo La Estacada is an example of how isotope geochemistry has overcome difficulties with pollen analysis and provided a reliable tool to investigate past vegetation changes in regions where C3 and C4 are differentially distributed. Isotope values during part of the Marine Isotope Stage 3 (MIS 3) and the middle Holocene are attributed to C3 local floodplain plants, with a little or no contribution from regional terrestrial (Patagonian) vegetation. Late Pleistocene–Holocene palaeosols have a clear C4 signal of terrestrial dominance, representative of Monte plants, thus, carbon isotope values constrain the early times of MIS 1, particularly where the Pleistocene–Holocene transition occurs. A mixture of C3 and C4 values during the last 3–4 kyr is indicative of environmental conditions that prevail in the region today.

Late Holocene changes in vegetation and atmospheric circulation at Lake Uddelermeer (The Netherlands) reconstructed using lipid biomarkers and compound‐specific δD analysis

ABSTRACTWe reconstructed middle to late Holocene changes in atmospheric circulation patterns and vegetation in north‐west Europe by applying novel geochemical techniques to the sediment record of Lake Uddelermeer (The Netherlands). A comparison of higher plant‐derived leaf wax n‐alkane distributions archived in the lake sediments with those in living plant material, combined with palynological analysis, indicates that the vegetation immediately surrounding the lake became more open at 3150 cal a BP, while the regional vegetation responded more gradually and ∼650 years later. Our record of the hydrogen isotopic composition of plant leaf waxes (δDwax) shows a deuterium enrichment starting from 3500 cal a BP, which we interpret as a change in atmospheric circulation. A similar δDwax record from nearby Meerfelder Maar (Germany) shows an opposite trend around this time, which could be explained by a change in sea‐level pressure resembling a negative North Atlantic Oscillation phase. This could account for depleted δD values of precipitation at Meerfelder Maar, while confounding factors related to the more maritime position of Uddelermeer cause the opposite shift there.

Perception of Residents about Urban Vegetation: A Comparative Study of Planned Versus Semi-Planned Cities of Islamabad and Rawalpindi, Pakistan

The present study compared opinions of south Asian planned (Islamabad) versus semi-planned (Rawalpindi) urban residents, regarding urban vegetation (ecological capital) and its usefulness. These urban areas known as twin cities lie in close vicinity but their contrasting contextual setting makes them a suitable case study. The locals were asked questions related to the importance of urban vegetation, changes in it over a period of time and resultant impacts (positive/negative). A majority (90%) of respondents opined that urban vegetation is beneficial while another big proportion (69.20%) believed vegetation cover changed over time and recent changes in urban vegetation were viewed negatively (55.80%). Statistical analysis revealed that respondents form both cities significantly differ in their point of view regarding ecological capital: usefulness of vegetation (p<0.02); urban vegetation cover changes (p<0.02) and its impacts (p<0.01). The study concluded that planning/contextual settings influence human perception about natural capital in urban settings. While, the people were equivocal in perceiving the vegetation change; the negative impacts were more felt by the people living in semi-planned area than the people living in the planned areas.

Trajectories of change in Mediterranean Holocene vegetation through classification of pollen data

Quantification of vegetation cover from pollen analysis has been a goal of palynologists since the advent of the method in 1916 by the great Lennart von Post. Pollen-based research projects are becoming increasingly ambitious in scale, and the emergence of spatially extensive open-access datasets, advanced methods and computer power has facilitated sub-continental analysis of Holocene pollen data. This paper presents results of one such study, focussing on the Mediterranean basin. Pollen data from 105 fossil sequences have been extracted from the European Pollen database, harmonised by both taxonomy and chronologies, and subjected to a hierarchical agglomerative clustering method to synthesise the dataset into 16 main groupings. A particular focus of analysis was to describe the common transitions from one group to another to understand pathways of Holocene vegetation change in the Mediterranean. Two pollen-based indices of human impact (OJC: Oleaceae, Juglans, Castanea; API: anthropogenic pollen indicators) have been used to infer the degree of human modification of vegetation within each pollen grouping. Pollen-inferred cluster groups that are interpreted as representing more natural vegetation states show a restricted number of pathways of change. A set of cluster groups were identified that closely resemble anthropogenically-disturbed vegetation, and might be considered anthromes (anthopogenic biomes). These clusters show a very wide set of potential pathways, implying that all potential vegetation communities identified through this analysis have been altered in response to land exploitation and transformation by human societies in combination with other factors, such as climatic change. Future work to explain these ecosystem pathways will require developing complementary datasets from the social sciences and humanities (archaeology and historical sources), along with synthesis of the climatic records from the region.

Late Holocene vegetation and climate change on the southeastern Tibetan Plateau: Implications for the Indian Summer Monsoon and links to the Indian Ocean Dipole

The Indian Summer Monsoon (ISM) is one of the most important climate systems, whose variability and driving mechanisms are of broad interest for academic and societal communities. Here, we present a well-dated high-resolution pollen analysis from a 4.82-m long sediment core taken from Basomtso, in the southeastern Tibetan Plateau (TP), which depicts the regional climate changes of the past millennium. Our results show that subalpine coniferous forest was dominant around Basomtso from ca. 867 to ca. 750 cal. yr BP, indicating a warm and semi-humid climate. The timberline in the study area significantly decreased from ca. 750 to ca. 100 cal. yr BP, and a cold climate, corresponding to the Little Ice Age (LIA) prevailed. Since ca. 100 cal. yr BP, the vegetation type changed to forest-meadow with rising temperatures and moisture. Ordination analysis reveals that the migration of vegetation was dominated by regional temperatures and then by moisture. Further comparisons between the Basomtso pollen record and the regional temperature reconstructions underscore the relevance of the Basomtso record from the southeastern TP for regional and global climatologies. Our pollen based moisture reconstruction demonstrates the strong multicentennial-scale link to ISM variability, providing solid evidence for the increase of monsoonal strengths over the past four centuries. Spectral analysis indicates the potential influence of solar forcing. However, a closer relationship has been observed between multicentennial ISM variations and Indian Ocean sea surface temperature anomalies (SSTs), suggesting that the variations in monsoonal precipitation over the southeastern TP are probably driven by the Indian Ocean Dipole on the multicentennial scale.

Legacies of Historical Human Activities in Arctic Woody Plant Dynamics

Recent changes in arctic vegetation might not be driven by climate change alone. Legacies of human activities have received little attention as a contributing factor. We examine the extent to which traditional human activities (hunting, herding, fire, wood extraction, and agriculture) have had lasting effects on arctic woody plant communities and therefore might continue to affect biome-wide responses to climate change. Evidence suggests that legacies are likely to be evident across meters to hundreds of kilometers and for decades, centuries, and millennia. The evidence, however, is currently sparse, and we highlight the potential to develop systematic assessments through a circumarctic collaboratory consisting of a network of interdisciplinary field sites, standardized protocols, participatory research, and new approaches. We suggest that human activities should be brought into consideration to increase our understanding of arctic vegetation dynamics in general and to assess woody plant responses to climate change in particular.

Vegetational and agricultural dynamics at Burgäschisee (Swiss Plateau) recorded for 18,700 years by multi-proxy evidence from partly varved sediments

Little is known about the timing and the vegetation dynamics shortly after the Last Glacial Maximum (LGM) on the Swiss Plateau 19,000–15,000 cal bp. Subsequent Late Glacial and Holocene vegetation changes are better known; however, it is unclear if the few available palynological and macrofossil records are able to capture the entire vegetation variability of the region. A new palaeoecological multi-proxy study using pollen, spores, charcoal and X-ray fluorescence (XRF) from Burgaschisee (Swiss Plateau, 465 m a.s.l.) is applied to reconstruct vegetation, fire and land use for the past 19,000 cal years. Steppe tundra vegetation established at c. 18,700 cal bp only c. 300 years after the end of the LGM and deglaciation. A shift from steppe tundra (Artemisia, Helianthemum) to shrub tundra (Betula nana, Salix, Juniperus) with sporadic tree Betula stands occurred around 16,000 cal bp, most likely in response to climate warming after the end of Heinrich event 1. Abundant spores of coprophilous fungi (Sporormiella, Cercophora) may reflect the presence of Pleistocene large herbivores (e.g. Mammuthus primigenius, Bison bonasus, Rangifer tarandus). Afforestation started more than 2,000 years later with Juniperus and tree Betula around 14,500 cal bp. Mixed Betula and Pinus sylvestris forests persisted until 10,800 cal bp, when mixed elm forests expanded into the region in response to climate warming. Around 8,200 cal bp, mesophilous Fagus sylvatica and Abies alba partly replaced more heliophilous species in the forests, when climate became less continental and more moist. Pollen of Cerealia, Plantago lanceolata and other crops and weeds suggest that agricultural activities became significant during the Neolithic around 6,500 cal bp (4550 cal bc). Archaeological findings from Neolithic pile dwellings around 5,950 cal bp (4000 cal bc) indicate local settlements around the lake. The lake sediments are laminated for most of the last c. 6,800 years. With two independent proxies (XRF and pollen), we can demonstrate that these laminations are annual, suggesting short-term mixing of the lake water due to a more open landscape in response to land use. Our study shows that the annually laminated (varved) sediments from Burgaschisee have a great potential for high-resolution multi-proxy analyses covering the past c. 6,800 years. They can provide accurate ages of cultural phases that might be compared with dendrochronologically dated evidence from lake dwellings.

Mid-Late Holocene Vegetational History and Land-Use Dynamics in County Monaghan, Northeastern Ireland—The Palynological Record of Lough Muckno

We conducted high-resolution palynological analysis on a sediment core obtained from Lough Muckno, County Monaghan, Ireland. The results presented represent the first paleoecological account of Mid-Late Holocene vegetational change and land-use dynamics in the study region. Human activity and agriculture is first recorded during the Early Neolithic (ca. 3870–3500 B.C.). After a period of undiscernible human activity of ∼900 years, farming resumes during the Early Bronze Age (ca. 2600 B.C.). Henceforth, human presence on the landscape is constant with fluctuating levels of intensity. During the Bronze Age, anthropogenic activity is most pronounced during ca. 2000–1750 B.C. and ca. 1500–1300 B.C. followed by a phase of reduced intensity in the Late Bronze Age (ca. 1000–650 B.C.). Farming activity increases during the Iron Age and is disrupted with the onset of a period of rapid woodland regeneration from ca. 200 B.C. to A.D. 200. During the prehistorical period agriculture has a strong focus on pastoral grazing with a limited arable component. An upsurge in agricultural activity is recorded in the historical period from ca. A.D. 400 in which a mixed agricultural economy placing more emphasis on cereal-crop cultivation is adopted. Arable farming attains its maximum levels ca. A.D. 990–1140. Evidence of farming disruptions in the pollen record may reflect of a period of local “conflict” during the Viking Age/Medieval period (ca. A.D. 800–1190). We explore the characterizing features of the pollen assemblage of this large lake system and its use in reconstructing past cultural landscape change.

Vascular plants as ecological indicators of metals in alpine vegetation (Karkonosze, SW Poland).

Calluna vulgaris, Carex rigida, Deschampsia flexuosa, Nardus stricta and Vaccinium myrtillus are abundant in the vegetation of mountainous areas in Northern and Central Europe. Knowledge of their ability to accumulate increased amounts of metals could be useful in the evaluation of environmental pollution in the alpine tundra of high mountains. Additionally, this investigation may contribute to understanding the rate and direction of recent vegetation change in Karkonosze and similar types of environments. Our investigation revealed that Carex rigida, C. vulgaris and V. myrtillus contain excessive Mn concentrations in shoots with the highest BF for this element compared to the BFs of other elements. C. rigida, with Cu, Mn and Zn concentrations exceeding the toxicity thresholds for plants, seems to be the best metal phytoaccumulator for Nardus stricta grasslands Carici (rigidae)-Nardetum (CrN) and alpine heathlands Carici (rigidae)-Festucetum airoidis (CrFa) associations in the Karkonosze. Based on relevant BFs >1, it can be stated that the following plant available metals were transferred to shoots: Cu, Mn and Ni by C. vulgaris; Cd, Cu, Mn, Ni and Zn by C. rigida; Cd, Cu, Mn, Ni and Zn by D. flexuosa; Cu, Mn, Ni and Zn by N. stricta and Cu, Mn and Zn by V. myrtillus.

Holocene environmental changes in the Horqin desert revealed by OSL dating and δ13C analyses of paleosols

The Horqin desert in northeastern China is located on the climatically-sensitive margin of the East Asian summer monsoon front and the paleosols in the desert potentially provide a record of environmental changes in the region. During the last two decades several paleoenvironmental studies have increased our knowledge of past shifts in the desert-grassland border in the deserts of northeastern China. However, studies of Holocene vegetation changes with high-quality age data are scarce. Here, we present the results of optically stimulated luminescence dating and analyses of magnetic susceptibility, total organic carbon and the stable carbon isotope composition of organic matter, and inferred C4 biomass, of the paleosol sequences of the Tuquan and Mutougou sections in the Horqin desert and utilize them to document Holocene environmental changes in the region. The results reveal that the East Asian summer monsoon precipitation gradually increased and previously unstable dunes were semi-stabilized from ∼11 to 9 ka. The semiarid conditions were then replaced by the more humid environment of the Holocene climatic optimum from ∼9 to 5 ka. During the Holocene climatic optimum, the paleosols in the Tuquan and Mutougou sections developed, and the percentage of C4 plants increased and the dunes in the Horqin desert were extensively stabilized. Subsequently, alternation of eolian sand mobilization and paleosol development occurred, indicating that the environment became cold and dry, with temporary wet and warm stages. Estimates of C4 biomass from the two sections demonstrate that the favorable growing conditions for C4 plants extended to at least 45°N in North China during the Holocene. We conclude that Holocene environmental changes in the Horqin desert were controlled by movements of the East Asian summer monsoon rainfall belt on a millennial time scale, and that human activity also played a significant role in environmental change during the late Holocene.

Climatic and anthropogenic forcing of prehistorical vegetation succession and fire dynamics in the Lago di Como area (N-Italy, Insubria)

Combined pollen, charcoal and modeling evidence from the Insubria Region suggests that fire was a major driver of late Holocene vegetation change. However, the extent and timing of fire response dynamics are not clear yet. We use lacustrine sediments from Lago di Como (N-Italy, S-Alps) to assess if the reconstructed vegetation and fire dynamics were relevant at large scales and if they coincided in time with those observed at smaller sites. The lake, due to its size (142 km2) and economic potential, was very attractive for early land use and human presence in this area is well documented since ca. 10,000 yrs ago (Mesolithic). We used pollen, plant macrofossils and charcoal to reconstruct the vegetation composition and fire activity. During the Younger Dryas and the Early Holocene until ca. 8000 cal BP natural dynamics prevailed. Subsequently, land use and slash-and-burn activities increased at the Mesolithic-Neolithic transition and became widespread around ca. 6500 cal BP. Microscopic charcoal and numerical analyses demonstrate that anthropogenic fires had a determinant influence on long-term vegetation dynamics at regional scales in Insubria. Microscopic charcoal and pollen and spores indicative of land use show that human pressure intensified after ca. 5300 cal yr BP and even more since ca. 4300 cal yr BP. Our results suggest that important species which disappeared or were strongly reduced by land use and fire (e.g. Abies alba, Tilia, Ulmus) will potentially reestablish in the Lago di Como area and elsewhere in Insubria, if land abandonment initiated in the 1950s will continue.

Biome changes in Asia since the mid-Holocene – an analysis of different transient Earth system model simulations

Abstract. The large variety of atmospheric circulation systems affecting the eastern Asian climate is reflected by the complex Asian vegetation distribution. Particularly in the transition zones of these circulation systems, vegetation is supposed to be very sensitive to climate change. Since proxy records are scarce, hitherto a mechanistic understanding of the past spatio-temporal climate–vegetation relationship is lacking. To assess the Holocene vegetation change and to obtain an ensemble of potential mid-Holocene biome distributions for eastern Asia, we forced the diagnostic biome model BIOME4 with climate anomalies of different transient Holocene climate simulations performed in coupled atmosphere–ocean(–vegetation) models. The simulated biome changes are compared with pollen-based biome records for different key regions.In all simulations, substantial biome shifts during the last 6000 years are confined to the high northern latitudes and the monsoon–westerly wind transition zone, but the temporal evolution and amplitude of change strongly depend on the climate forcing. Large parts of the southern tundra are replaced by taiga during the mid-Holocene due to a warmer growing season and the boreal treeline in northern Asia is shifted northward by approx. 4° in the ensemble mean, ranging from 1.5 to 6° in the individual simulations, respectively. This simulated treeline shift is in agreement with pollen-based reconstructions from northern Siberia. The desert fraction in the transition zone is reduced by 21 % during the mid-Holocene compared to pre-industrial due to enhanced precipitation. The desert–steppe margin is shifted westward by 5° (1–9° in the individual simulations). The forest biomes are expanded north-westward by 2°, ranging from 0 to 4° in the single simulations. These results corroborate pollen-based reconstructions indicating an extended forest area in north-central China during the mid-Holocene. According to the model, the forest-to-non-forest and steppe-to-desert changes in the climate transition zones are spatially not uniform and not linear since the mid-Holocene.

Holocene mountain forest changes in central Mediterranean: Soil charcoal data from the Sila Massif (Calabria, southern Italy)

Soil charcoal analysis was performed on soil profiles excavated in the area of Cecita Lake on the Sila Massif (Calabria, southern Italy). Charcoals identification, combined with a good number of radiocarbon dating and the pedological analysis of the soils, allowed drawing a detailed history of the Holocene vegetation changes in the area. Five periods characterised by high concentration of radiocarbon dates – mirroring increased fire activity – are documented at ca. 10400-9000 BP, 8200-7200 BP, 5100-3700 BP, 3300-2400 BP and 600 BP to the present. Furthermore, a severe environmental change took place between the Middle Bronze Age and the Final Bronze Age/Iron Age (ca. 3600 to 3000 BP), when the mixed deciduous oak-silver fir woodland, which dominated the landscape until the Early Bronze Age, declined and was substituted by a Calabrian pine forest or by an open landscape. Available climate, pollen and historical/archaeological data are taken into account to investigate the possible causes of the detected environmental change and of the periods of increased fire activity. If climate seems to play a role in the fire regime during the first part of the Holocene, human action seems to be the triggering factor in the environmental change and the fire regime during the Late Holocene.

Driving forces of recent vegetation changes in the Sahel: Lessons learned from regional and local level analyses

A wide range of environmental and societal issues such as food security policy implementation requires accurate information on biomass productivity and its underlying drivers at both regional and local scales. While many studies in West Africa are conducted with coarse resolution earth observation data, few have tried to relate vegetation trends to explanatory factors, as is generally done in land use and land cover change (LULCC) studies at finer scales. In this study we proposed to make a bridge between vegetation trend analysis and LULCC studies to improve the understanding of the various factors that influence the biomass production changes observed in satellite time series (using integrated Normalized Difference Vegetation Index [NDVI] as a proxy). The study was conducted in two steps. In the first step we analyzed MODIS NDVI linear trends together with TRMM growing season rainfall over the Sahel region from 2000 to 2015. A classification scheme was proposed that enables better specification of the relative role of the main drivers of biomass production dynamics. We found that 16% of the Sahel is re-greening—but found strong evidence that rainfall is not the only important driver of biomass increase. Moreover, a decrease found in 5% of the Sahel can be chiefly attributed to factors other than rainfall (88%). In the second step, we focused on the “Degré Carré de Niamey” site in Niger. Here, the observed biomass trends were analyzed in relation to land cover changes and a set of potential drivers of LULCC using the Random Forest algorithm. We observed negative trends (29% of the Niger site area) mainly in tiger bush areas located on lateritic plateaus, which are particularly prone to pressures from overgrazing and overlogging. The significant role of accessibility factors in biomass production trends was also highlighted. Our methodological framework may be used to highlight changing areas and their major drivers to identify target areas for more detailed studies. Finer-scale assessments of the long-term vulnerability of populations can then be made to substantiate food security management policies.

Vegetation Dynamics in the Upper Guinean Forest Region of West Africa from 2001 to 2015

The Upper Guinea Forest (UGF) region of West Africa is one of the most climatically marginal and human-impacted tropical forest regions in the world. Research on the patterns and drivers of vegetation change is critical for developing strategies to sustain ecosystem services in the region and to understand how climate and land use change will affect other tropical forests around the globe. We compared six spectral indices calculated from the 2001–2015 MODIS optical-infrared reflectance data with manually-interpreted measurements of woody vegetation cover from high resolution imagery. The tasseled cap wetness (TCW) index was found to have the strongest association with woody vegetation cover, whereas greenness indices, such as the enhanced vegetation index (EVI), had relatively weak associations with woody cover. Trends in woody vegetation cover measured with the TCW index were analyzed using Mann–Kendall statistics and were contrasted with trends in vegetation greenness measured with EVI. In the drier West Sudanian Savanna and Guinean Forest-Savanna Mosaic ecoregions, EVI trends were primarily positive, and TCW trends were primarily negative, suggesting that woody vegetation cover was decreasing, while herbaceous vegetation cover is increasing. In the wettest tropical forests in the Western Guinean Lowland Forest ecoregion, declining trends in both TCW and EVI were indicative of widespread forest degradation resulting from human activities. Across all ecoregions, declines in woody cover were less prevalent in protected areas where human activities were restricted. Multiple lines of evidence suggested that human land use and resource extraction, rather than climate trends or short-term climatic anomalies, were the predominant drivers of recent vegetation change in the UGF region of West Africa.

Overview of environmental changes and human colonization in the Balearic Islands (Western Mediterranean) and their impacts on vegetation composition during the Holocene

According to radiometric dates and the current state of research, the Balearic Islands were not colonized by humans prior to c. 4420/4220calyr BP. Therefore, it is possible to know the natural evolution of the landscape of the Balearic Islands for the first two-thirds of the Holocene (c. 10,000 to c. 4300calyr BP). This study aims to improve our understanding of the respective roles of human societies and/or climate in the transformation of vegetation cover during the Late Holocene in this Western Mediterranean archipelago. The results show the importance and control of climate oscillations in the evolution of vegetation throughout the Early and Middle Holocene. Our data clearly show that the transformation of the landscape started before the first human settlements. In Minorca (north-eastern Gymnesian Islands), this upheaval occurred between 5825 and 4675calyr BP (fourth to third millennium BC), while in Majorca (the largest of the Gymnesian Islands) the transition is less well dated, oscillating between 7169 and 2535calyr BP. In the southern Pityusic Islands, observed changes in Ibiza are less pronounced and coincide with the 4.2calkyr BP climate event, synchronous with human colonization. The correlation between forest fires and rapid climate events, as well as the resilience of vegetation until the Middle Ages (tenth century) in Ibiza, suggest that the evolution of climatic conditions is the preponderant parameter for explaining Holocene vegetation changes on these islands.

Recent changes in alpine vegetation differ among plant communities

AbstractAimReports on recent changes in high‐mountain vegetation are mostly based on re‐surveys of mountaintop floras. Summits are very specific habitats, however, and detected trends may not necessarily represent alpine vegetation in general. Here, we analyse re‐samples of three prevalent plant communities in non‐summit alpine habitats (snowbeds, two grassland types).LocationNortheastern European Alps.MethodsIn 2013 we re‐sampled a total of 91 relevés of three plant communities from the 1990s. We compared original and re‐samples in terms of species richness (α‐diversity), pair‐wise compositional dissimilarity (β‐diversity) and species pool size (γ‐diversity). Moreover, we calculated mean ecological indicator values (temperature, moisture, nutrient status and soil acidity) as well as potential plant height for each of the 182 relevés. Differences in species pool sizes between the two sampling campaigns were evaluated by randomization tests based on rarefaction curves. In all other cases, differences were compared using paired t‐tests or Kruskal–Wallis tests.ResultsOf all the community attributes analysed, changes were most consistent with respect to increasing mean species' temperature indicator values. Mean potential plant height also increased slightly in all three communities, but differences were statistically significant only in the case of Carex sempervirens grasslands. Changes in species richness varied among communities, with only the Carex firma grasslands accumulating significantly more species at both the local and regional scales. For the other two plant communities, α‐ and γ‐diversity remained constant, but β‐diversity increased. Indicator values other than temperature either did not change or changed idiosyncratically in the different communities.ConclusionsOur results indicate that some long‐term changes occurred in high mountain grassland and snowbed vegetation of the study area. Consistent changes in mean species' temperature indicator values suggest that climate warming was an important driver. However, trends in biodiversity metrics differed substantially between the studied communities. We conclude that recent increase in species numbers commonly reported from temperate and boreal mountaintops cannot readily be generalized to all temperate and boreal high‐mountain vegetation.

The influence of Holocene vegetation changes on topography and erosion rates: a case study at Walnut Gulch Experimental Watershed, Arizona

Abstract. Quantifying how landscapes have responded and will respond to vegetation changes is an essential goal of geomorphology. The Walnut Gulch Experimental Watershed (WGEW) offers a unique opportunity to quantify the impact of vegetation changes on landscape evolution over geologic timescales. The WGEW is dominated by grasslands at high elevations and shrublands at low elevations. Paleovegetation data suggest that portions of WGEW higher than approximately 1430 m a.s.l. have been grasslands and/or woodlands throughout the late Quaternary, while elevations lower than 1430 m a.s.l. changed from a grassland/woodland to a shrubland ca. 2–4 ka. Elevations below 1430 m a.s.l. have decadal timescale erosion rates approximately 10 times higher, drainage densities approximately 3 times higher, and hillslope-scale relief approximately 3 times lower than elevations above 1430 m. We leverage the abundant geomorphic data collected at WGEW over the past several decades to calibrate a mathematical model that predicts the equilibrium drainage density in shrublands and grasslands/woodlands at WGEW. We use this model to test the hypothesis that the difference in drainage density between the shrublands and grassland/woodlands at WGEW is partly the result of a late Holocene vegetation change in the lower elevations of WGEW, using the upper elevations as a control. Model predictions for the increase in drainage density associated with the shift from grasslands/woodlands to shrublands are consistent with measured values. Using modern erosion rates and the magnitude of relief reduction associated with the transition from grasslands/woodlands to shrublands, we estimate the timing of the grassland-to-shrubland transition in the lower elevations of WGEW to be approximately 3 ka, i.e., broadly consistent with paleovegetation studies. Our results provide support for the hypothesis that common vegetation changes in semi-arid environments (e.g., from grassland to shrubland) can change erosion rates by more than an order of magnitude, with important consequences for landscape morphology.

Holocene vegetation and climate variability between winter and summer rainfall zones of South Africa

To better understand Holocene vegetation and hydrological changes in South Africa, we analyzed pollen and microcharcoal records of two marine sites GeoB8331 and GeoB8323 from the Namaqualand mudbelt offshore the west coast of South Africa covering the last 9900 and 2200 years, respectively. Our data corroborate findings from literature that climate developments apparently contrast between the summer rainfall zone (SRZ) and winter rainfall zone (WRZ) over the last 9900 years, especially during the early and middle Holocene. During the early Holocene (9900–7800 cal. yr BP), a minimum of grass pollen suggests low summer rainfall in the SRZ, and the initial presence of Renosterveld vegetation indicates relatively wet conditions in the WRZ. Toward the middle Holocene (7800–2400 cal. yr BP), a rather moist savanna/grassland rich in grasses suggests higher summer rainfall in the SRZ resulting from increased austral summer insolation and a decline of fynbos vegetation accompanied by an increasing Succulent Karoo vegetation in the WRZ, which possibly suggests a southward shift of the Southern Hemisphere westerlies. During the last 2200 years, a trend toward higher aridity was observed for the SRZ, while the climate in the WRZ remained relatively stable. The ‘Little Ice Age’ (ca. 700–200 cal. yr BP) was rather cool in both rainfall zones and drier in the SRZ while it was wetter in the WRZ.