The transformation of the forest steppe in the lower Danube Plain of south-eastern Europe : 6000 years of vegetation and land use dynamics

Forest steppes are dynamic ecosystems, highly susceptible to changes in climate and land use. Here we examine the Holocene history of the European forest steppe ecotone in the Lower Danube Plain to better understand its sensitivity to climate fluctuations and human impact, and the timing of its transition into a cultural forest steppe. We used multi-proxy analyses (pollen, n-alkane, coprophilous fungi, charcoal, and geochemistry) of a 6000-year sequence from Lake Oltina (SE Romania), combined with a REVEALS model of quantitative vegetation cover. We found the greatest tree cover, composed of xerothermic (Carpinus orientalis and Quercus) and temperate (Carpinus betulus, Tilia, Ulmus and Fraxinus) tree taxa between 6000 and 2500 cal yr BP. Maximum tree cover (~ 50 %) occurred between 4200 and 2500 cal yr BP at a time of wetter climatic conditions. Compared to other European forest steppe areas, the dominance of Carpinus orientalis represents the most distinct feature of the woodland's composition during that time. Forest loss was under way by 2500 yr BP (Iron Age) with REVEALS estimates indicating a fall to ~ 20 % tree cover from the mid-Holocene forest maximum linked to clearance for agriculture, while climate conditions remained wet. Biomass burning increased markedly at 2500 cal yr BP suggesting that fire was regularly used as a management tool until 1000 cal yr BP when woody vegetation became scarce. A sparse tree cover, with only weak signs of forest recovery, then became a permanent characteristic of the Lower Danube Plain, highlighting recurring anthropogenic pressure. The timing of anthropogenic ecosystem transformation here (2500 cal yr BP) was in between that in central eastern (between 3700 and 3000 cal yr BP) and eastern (after 2000 cal yr BP) Europe. Our study is the first quantitative land cover estimate at the forest steppe ecotone in south eastern Europe spanning 6000 years and provides critical empirical evidence that the present-day forest steppe/woodlands reflects the potential natural vegetation in this region under current climate conditions. This study also highlights the potential of n-alkane indices for vegetation reconstruction, particularly in dry regions where pollen is poorly preserved

Recent fire regime in the southern boreal forests of western Siberia is unprecedented in the last five millennia

Wildfires in Siberia are documented to have increased in frequency and severity over recent decades. However, in the absence of long-term records, it is unclear how far and why this trend deviates from centennial to millennial scale variability. Here we reconstruct past patterns of fire frequency and fire type, and explore how the fire-related traits of boreal species and plant functional types (PFTs) determine ecosystem responses to changing fire regimes. We use charcoal-based reconstructions of the fire regime in combination with a pollen-based assessment of vegetation composition in two boreal forest peat profiles from Plotnikovo Mire in western Siberia that span the last 2400 and 5000 years. We found moderate levels of biomass burning between 5000 and 4000 cal yr BP. Biomass burning and fire severity i.e., more biomass burning per fire episode, increased over the last 1500 cal yr BP associated with the dominance of fire invaders (Betula). Conversely, between 4000 and 1500 cal yr BP lower biomass burning, with perhaps fire types affecting mostly litter and understorey vegetation, coincided with the dominance of fire resisters (e.g., Pinus sylvestris, P. sibirica, Larix) intermixed with a considerable number of fire avoiders (e.g., Abies sibirica and Picea obovata). This long-term perspective shows that the current fire regime commenced 1500 years ago and deviates from the trends observed over the last 5000 years. This deviation is linked to a combination of climate conditions conducive to fire, the amount and composition of woody fuels, and land use changes. Although pines reacted more sensitively to increased fire severity, the fire avoider Picea obovata appears to be much more vulnerable to both frequent, severe fires than Abies sibirica. We anticipate that climatically driven changes in fire weather, with frequent warm and dry spells, and anthropogenic land use intensification will heighten fire severity and their impact, driving vegetation composition towards Betula species to the detriment of Picea obovata. This study also shows that charcoal morphotypes can provide useful information on fuel and fire type, and that, although all macro-charcoal size classes reliably indicate local-scale fires, the larger fraction (>300–500 μm) allows the identification of on-site fire episodes. We recommend multi-site palaeo-fire reconstructions in boreal peatlands to adequately reflect the influence of localised peat moisture content, and vegetation composition and structure variability on the small-scale heterogeneity of fire type and spread. This will ensure that trends in fire regime dynamics are representative and not limited to the local scale. We also propose future directions in fire research that can be tested using fossil records of fire, climate and vegetation ideally in an interdisciplinary approach

High mountain region of the Northern Romanian Carpathians responded sensitively to Holocene climate and land use changes : A multi-proxy analysis

A high-altitude lake sediment sequence (Buhăiescu Mare, 1918 m a.s.l.) in the subalpine zone of the Rodna Mountains was analysed through a multi-proxy approach to determine the sensitivity of high mountain habitats to climate, fire and land use changes. The early Holocene regional forests were dominated by Pinus (sylvestris and mugo) and replaced by Picea abies from 9800 cal. yr BP. After an extended hiatus in the profile (c. 9800–4200 cal. yr BP), probably because of the physical removal of sediments through avalanche or high-flow events, P. abies, Abies alba and Fagus sylvatica forests developed after 4200 cal. yr BP. The timberline and treeline reacted sensitively to past changes in climate and human impact. The site was probably situated above the treeline throughout most of the investigated period. However, a treeline ecotone or krummholz zone may have sporadically reached the lake’s elevation in the early Holocene. A decline in timberline and treeline elevation was noted during the last 1200 years, and more evidently over the past 200 years, with replacement by subalpine shrubs (Alnus viridis) and alpine herbaceous communities. Because these vegetation changes were associated with an increased prevalence of pollen-based anthropogenic indicators, charcoal particles and abiotic indicators, human-induced fires and clearance and resultant erosion inputs to the lake are implied. Effects of current warming on the altitude range of trees are not yet visible, probably because land use has more strongly contributed to changes in land cover than the climate fluctuations of the last millennium in the Rodna Mountains

Climate variability and associated vegetation response throughout Central and Eastern Europe (CEE) between 60 and 8 ka

Abstract Records of past climate variability and associated vegetation response exist in various regions throughout Central and Eastern Europe (CEE). To date, there has been no coherent synthesis of the existing palaeo-records. During an INTIMATE meeting (Cluj Napoca, Romania) focused on identifying CEE paleo-records, it was decided to address this gap by presenting the palaeo-community with a compilation of high-quality climatic and vegetation records for the past 60–8 ka. The compilation should also serve as a reference point for the use in the modelling community working towards the INTIMATE project goals, and in data-model inter-comparison studies. This paper is therefore a compilation of up to date, best available quantitative and semi-quantitative records of past climate and biotic response from CEE covering this period. It first presents the proxy and archive used. Speleothems and loess mainly provide the evidences available for the 60–20 ka interval, whereas pollen records provide the main source of information for the Lateglacial and Holocene. It then examines the temporal and spatial patterns of climate variability inferred from different proxies, the temporal and spatial magnitude of the vegetation responses inferred from pollen records and highlights differences and similarities between proxies and sub-regions and the possible mechanisms behind this variability. Finally, it identifies weakness in the proxies and archives and their geographical distribution. This exercise also provides an opportunity to reflect on the status of research in the area and to identify future critical areas and subjects of research