These authors contributed equally to this work.
Forest steppes are dynamic ecosystems, highly susceptible to changes in
climate, disturbances 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, fire and human impact,
and the timing of its transition into a cultural forest steppe. We used
multi-proxy analyses (pollen,
Projected changes in climate and increasing human environmental impacts are generating global concern about the functioning of ecosystems as well as the provision of ecosystem services (IPCC, 2014; IPBES, 2019). Lowland ecosystems (mesic and steppic grasslands, woodlands, etc.) provide an array of provisioning (e.g. crops, grazed areas and wood) and regulating services (e.g. soil protection; European Environmental Agency, 2016). However, in comparison to the mountainous areas of central-eastern Europe, lowland ecosystems, especially steppic grasslands, have been more strongly impacted by human activities (Magyari et al., 2010; Tonkov et al., 2014; Feurdean et al., 2015; Kuneš et al., 2015; Novenko et al., 2016; Shumilovskikh et al., 2018, 2019; Jamrichová et al., 2019; Vincze et al., 2019; Cleary et al., 2019; Gumnior et al., 2020). This partly reflects the lowlands' deeper and more fertile soils and the greater accessibility of the terrain, which have promoted extensive agro-pastoral activities and human settlement. Lowlands also include more frequent ecotones, i.e. woodland and grassland borders, which are naturally more sensitive to climate change (Bohn et al., 2003).
Potential natural vegetation cover in Europe showing the extent of the European forest steppe region (after Bohn, 2003; © BfN, Bundesamt für Naturschutz), the location of the study site and the other published records used for comparisons. 1: Lake Oltina (study site); 2: Lake Vracov; 3: Sarló-hát; 4: Lake Stiucii; 5: Durankulak 2; 6: Durankulak 3; 7: Dovjok; 8: Kardashinski; 9: Sudzha; 10: Selikhovo; 11: Istochek; 12: Podkosmovo. For references see Table 3.
According to Bohn et al. (2003), the potential natural vegetation (PNV) in
the study area, the easternmost part of the lower Danube Plain, also known
as the southern Dobrogea Plateau, is forest steppe, i.e. woodland patches
within a matrix of graminoid- and forb-dominated communities. It borders
steppe grasslands i.e. treeless vegetation cover dominated by graminoids
and forbs to the east. Forest steppe and steppe vegetation extend over 6000 km
along an east–west gradient across Eurasia (Fig. 1) under climate conditions
delimited by a ca. 2-month long late-summer drought for the forest steppe
zone and a 4–6 month one for the steppe (Walter, 1974). However, according to
the management plan for the region (Planul de Management, 2016), there are
currently few patches of natural steppe vegetation preserved in the southern
Dobrogea Plateau, as most have become ruderal steppe. Since the lower Danube
Plain represents one of the oldest areas of continuous human occupation from
the Neolithic onwards, i.e. 8000 cal yr BP (
Palaeoecological records provide a way to assess the former natural
vegetation of a region and the legacy of natural disturbances and
anthropogenic impacts on landscapes (Willis and Birks, 2006). However, due
to the dry climate of the lower Danube Plain, very few palaeoecological
archives are available to document past natural vegetation types. This
leaves many open questions regarding the temporal dynamics of vegetation
composition and drivers of changes in this region and how its vegetation
composition compares to other forest steppe areas in central-eastern Europe
(Magyari et al., 2010; Feurdean et al., 2015; Kuneš et al., 2015),
southeastern Europe (Tonkov et al., 2014; Marinova and Atanassova, 2006) and the Eastern European Plain (Novenko et al., 2016; Shumilovskikh et al., 2018). Most of
the archaeological and loess deposits in the region are devoid of an
absolute chronology, have poor lithological context and lack favourable
pollen preservation (Tomescu, 2000). Nevertheless, based on these fragmentary
records, it appears that steppe may have covered the landscapes of this
region in the Early Holocene, while forest steppe vegetation may have
expanded during a moist phase of the Mid Holocene (Feurdean et al., 2014;
Tomescu, 2000; Wunderlich et al., 2012; Hansen et al., 2015). In
addition, models of deforestation rates, using a scenario that accounts for
population history and technological advances, suggest that the extent of
deforestation in the lower Danube basin has increased continuously since
4000 cal yr BP (Kaplan et al., 2009; Giosan et al., 2012). Indirect evidence
for the Holocene persistence of steppe grasslands in this region comes from
the genetic investigation of steppe species including
Here, we explore the long-term vegetation dynamics of the lower Danube
Plain's landscape and the competing driving forces (climate, fire and
anthropogenic impact). More specifically, we address the following research
questions:
Is forest steppe the natural vegetation type of the lower Danube Plain under
climatic conditions similar to those in the present? Has the tree cover been more extensive or dominated by other tree taxa in
the past? When did this area undergo the most marked land cover and land use changes,
and was this transformation continuous in time?
This study is built on a pollen-based quantitative vegetation reconstruction
(REVEALS model; Regional Estimates of Vegetation Abundance from Large Sites), along with records of long-chain higher-plant wax
Lake Oltina (44
The main potential vegetation types of the region include forest, forest
steppe and calciphile steppe (Bohn at al., 2003). The main forest types are:
thermophilous mixed deciduous broadleaf forests (subtypes G21, G22 and G34
according to Bohn et al., 2003), mesophytic deciduous broadleaf forests
(subtypes F49 and F67), forest steppe (subtype L13) and steppe (subtype M5).
Additionally, alluvial forests (subtypes U19 and U20), halophytic vegetation
(P33) and tall-reed vegetation and sedge swamps (R1) prevail (Bohn et al.,
2003). CORINE (Coordination of Information on the Environment) Land Cover data (2012) indicate that the present land cover
within a 20 km radius of the lake comprises ca. 65 % arable land and
orchards, 19 % steppe and semi-natural grassland, and 16 % deciduous
forest (S1; Grindean et al., 2019). Important tree species in forests within
this radius, and thus the most relevant for the pollen source area, are
Sediment cores were extracted with a Livingstone piston corer (1 m long and 5 cm diameter) from the central part of the lake (1.8 m water depth) in spring 2016. The less consolidated sediment at the surface (36 cm) had previously been retrieved with a gravity corer in 2014. A lithostratigraphic description was made according to changes in texture, colour, magnetic susceptibility and the organic carbon content (loss on ignition; LOI).
Volume magnetic susceptibility (
Age–depth model for Lake Oltina using a Bayesian approach (for more details, see Appendix A1). Accumulation: acc.; memory: mem.
AMS (accelerator mass spectrometry)
The chronology was established based on 18 AMS (accelerator mass spectrometry)
Pollen productivity estimates relative to Poaceae and their respective fall speeds used in the REVEALS model.
To determine the past vegetation cover we used pollen analysis on samples of
1 cm
To determine the source of organic matter and the predominant vegetation
type (Eglinton and Calvin, 1967; Ficken et al., 2000; Diefendorf et al., 2015), we measured the concentration of higher-plant-derived
In this study, we calculated the ratio of straight-chain
Integrative diagram showing in-lake ecosystem and catchment
changes. Lake properties: volume magnetic susceptibly, organic matter
content and detrital elements (Zr and Fe
The
To determine past disturbance by fire, macroscopic charcoal particles were
counted in samples of 2 cm
The lithology of the core at Lake Oltina showed little variability
throughout the profile and comprises clay, gyttja clay and sandy clay. The
age–depth model indicates a rather constant sediment accumulation rate with
a mean of 5 yr cm
Raw pollen percentages and estimated regional vegetation cover based on the REVEALS model for 27 taxa including trees, shrubs and herbs at Lake Oltina. Horizontal lines denote the timing of the most important changes in the vegetation assemblages. NAP: non-arboreal pollen; CONISS: stratigraphically constrained cluster analysis.
The cluster analysis applied on the pollen record indicated three major
periods of change in land cover and vegetation openness over the last 6000 years: open temperate and xerothermic deciduous broadleaf forest between
6000 and 4200 cal yr BP, the maximum extent of broadleaf tree cover between
4200 and 2500 cal yr BP, and the expansion of grassland between 2500 cal yr BP and the present (Fig. 4). Results from the REVEALS model suggest that
landscape openness was ca. 10 %–15 % greater than the estimates derived from
the raw pollen data. Overall, REVEALS indicate a greater proportion of
The REVEALS estimate of tree cover fluctuated around 40 %, compared to
Tree cover increased to its maximum extent in the profile (fluctuating
around 55 %) and was mostly represented by
The tree cover declined abruptly to ca. 20 %; this was most evident for
The (C
Comparative summary of the percentage vegetation cover estimates
based on the REVEALS model and raw pollen percentages at Lake Oltina. Open
land cover includes all non-arboreal-pollen types, mostly indicators of
pastures and grasslands. The Cerealia group includes
The pollen-based quantitative land cover reconstruction shows an average
tree cover of 40 % between 6000 and 4200 cal yr BP and a tree cover
maximum of 50 % between 4200 and 2500 cal yr BP in the surroundings of
Lake Oltina (Figs. 4 and 5). In a pollen-based biome reconstruction, such a
proportion of trees is likely to be indicative of a forest steppe or open-woodland type (Marinova et al., 2018). This woodland consists of tree taxa
of xerothermic character including
On a regional scale, the 6000–4200 cal yr BP interval was characterized by
contrasting climate conditions north and south of 45
Macro-charcoal-based reconstruction of biomass burning and thus disturbance by fire was low between 4200 and 2500 cal yr BP at the time of greater forest cover and wetter climate conditions (Fig. 3). This fire–climate–vegetation relationship is typical for a temperate environment but contrasts with the pattern found in environments with low vegetation productivity, where increased moisture tends to enhance vegetation productivity and therefore fuel availability (Pausas and Ribeiro 2013; Feurdean et al., 2020). Disturbance by herbivores, as inferred from the abundance of coprophilous spores, showed moderate values at the beginning of the record but declined during the interval of tree cover increase (Fig. 3). This may point to some impact by herbivores on the degree of forest openness, i.e. increased tree cover with a decline in grazing activity. However, given the large size of the study lake, the distance from the lakeshore to the coring point might have limited the transportation of these spores and have an impact on how representative their presence might be (Baker et al., 2013).
Tree cover dropped from 50 % at 2500 cal yr BP to
The timing of forest loss coincides with the peak number of archaeological
finds in the Iron Age but is pre-dated by the abundant archaeological finds
of the Bronze Age, i.e. 4000 cal yr BP (Fig. 5). Interestingly, the typology
of the houses in this area (i.e. small houses three-quarters buried in the
ground with one-quarter above ground comprising mud brick walls, roofed
with straw or reeds typical during the past 3000 years) also reflects the
limited availability of timber for building (Ailincăi, 2009). Historical
records show that, due to its smaller size,
In-lake and catchment changes are also apparent in the Lake Oltina around
the onset of forest loss. A slight increase in the Fe
Arboreal-pollen percentages illustrating temporal trends in deforestation in three different sub-regions along a west–east transect across the European forest steppe region. For the location of individual sites, see Fig. 1 and Table 3.
Compilation of palaeoecological records along a west–east transect of the European forest steppe. Cz: Czech Republic; Hu: Hungary; RO: Romania; BG: Bulgaria; UA: Ukraine; RU: Russia.
Our quantitative record of vegetation cover indicates a higher-than-present
tree cover across the landscape of the eastern lower Danube Plain between
6000 and 4200 cal yr BP with an absolute maximum of 50 % (60 % raw
pollen percentages) between 4200 and 2500 cal yr BP (Fig. 6). The
composition and structure of Mid to Late Holocene vegetation of the eastern
lower Danube Plain resembled, to a large degree, that of other European
forest steppe areas, although particularities also exist. Whilst in central-eastern European forest steppe
Mid Holocene landscape openness near Lake Oltina (ca. 45 % raw pollen
percentages) appears to fall in between that of the steppe in southeastern
Bulgaria (60 %–80 %; Tonkov et al., 2014), the forest steppe of Ukraine
(25 %; Kremenetski et al., 1995, 1999) and that of the Eastern European
Plain (20 %–50 %; Shumilovskikh et al., 2018). However, landscape openness
was greater than in other forest steppe sites from central-eastern Europe,
i.e. Romania (Feurdean et al., 2015; Tantau et al., 2006), Hungary (Willis
et al., 1997; Magyari et al., 2010), the Czech Republic and Slovakia
(Pokorny et al., 2015; Hajkova et al., 2013; Kuneš et al., 2015),
where it varied between 10 % and 35 % (Fig. 6). The composition of
herbaceous plant cover included grasses (Poaceae) and a diversity of forbs
thriving on a wide variety of habitats ranging from dry and saline soils
(
The comparison of pollen records from the European forest steppe shows a west-to-east gradient in the timing and magnitude of deforestation (Fig. 6). For example, the timing of major anthropogenic ecosystem transformation in the lower Danube Plain from about 2500 cal yr BP falls in between that of other records in lowland areas in central-eastern and southeastern Europe, where it generally occurred after 3000 cal yr BP (Fig. 6). However, this is earlier than on the Eastern European Plain, where it mostly occurred after 2000 cal yr BP. On the Thracian Plain, southeastern Bulgaria, anthropogenic deforestation was, however, noted already from 4000 cal yr BP (Connor et al., 2013). The anthropogenically driven opening up of the forest steppe soon reached a similar extent as today in most regions, which then remained open until the present day, although climate conditions could have allowed for the recovery of tree cover. Notably, however, the study region is increasingly confronted by desertification (European Environmental Agency, 2016). Given its dry character, the conversion of forests to cropland may have acted as a positive feedback to the warm and dry climate, enhancing evaporation and altering the moisture balance, further contributing to the tendency towards the aridization of Lower Danube landscapes. Ongoing climate change (warmer temperatures and a decline in precipitation), coupled with agricultural intensification, will probably exacerbate the process of desertification.
The pollen-based vegetation modelling applied here (REVEALS) provides the
first, long-term quantitative reconstruction of land cover changes across
the lower Danube Plain (southeastern Romania) and in southeastern Europe. Enhanced moisture
availability likely led to a more extensive tree cover between 6000 and 2500 cal yr BP and its maximum of 50 % between 4200 and 2500 cal yr BP. This
woodland consisted of tree taxa of xerothermic character including
We also show that both the extension and decline in tree cover determined by
pollen, a well-established proxy for past vegetation change, is also
reflected in the
A chronology for Lake Oltina was established on the basis of 17 AMS
We have attempted to correct for the reservoir effect in the following ways.
Firstly, we compared our youngest radiocarbon date on the shell sample from
30 cm in depth (880 uncal yr BP) with the potential sediment age of recent
samples based on geochemistry, mineral magnetic measurements and a specific
pollen marker; 30 cm geochemical elements (particularly Pb) potentially
associated with regional industrialization (after 1850) show concentrations
above the background levels that would naturally be found, suggesting an
additional anthropogenic input (Fig. A1). Further, mineral magnetic
properties (X) also show an increase from 30 cm that might reflect an
anthropogenic influence on the sediment (Fig. A1). Rose et al. (2009),
Akinyemi et al. (2013) and Hutchinson et al. (2016) note atmospherically derived inputs of trace elements and
heavy metals and mineral magnetic particles in the Romanian
Carpathian Mountains from the start of the
20th century with peaks from the 1950s. Similarly, Begy et al. (2012)
attribute peaks in heavy metals in a lake in the Danube delta to industrial
and traffic pollution from the 1950s. We noted the occurrence of pollen of
Secondly, for older sediments, we compared the radiocarbon date of the
terrestrial macrofossil sample at a depth of 548 cm (3459
AMS
Selected geochemical elements (Pb and Zn), mineral magnetic
measurements (magnetic susceptibility) and pollen (
Full-pollen diagram for Lake Oltina grouped on trees, shrubs herbs, wetland and aquatic taxa as well as coprophilous fungi.
All essential input data can be requested from the corresponding author.
The supplement related to this article is available online at:
AF designed the study. AF, AD, AP and MB performed the
fieldwork. RG, IT and AF performed the pollen analysis. GF and EMN analysed the biomarkers. GF and SMH performed
the geochemistry analysis. AD performed the macro-charcoal analysis. ST, AP and AF performed the
The authors declare that they have no conflict of interest.
We thank the managers at Lake Oltina and Natura 2000 for granting and facilitating access to the lake for sediment sampling. Gabriela Florescu and Eva Niedermeyer thank Ulrich Treffert for laboratory support regarding biomarker analysis. Roxana Grindean thanks Sorina Farcas for granting laboratory access for pollen preparation. We thank Rebecca Kearney for her work on tephrostratigraphy, Mihaly Molnar for the suggestions on the construction of the age model and David, a student assistant, for assistance in the field. Finally, we thank the two reviewers, Simon Connor and Natalie Schroeter, for their constructive comments on the paper.
This research has been supported by the Deutsche Forschungsgemeinschaft (grant nos. FE_1096/4 and FE_1096/6) and the CNCS-UEFISCDI (grant nos. PN-II-RU-TE-2014-4-2445 and PN-III-P4-ID-PCE-2016-0711).This open-access publication was funded by the Goethe University Frankfurt.
This paper was edited by Sönke Zaehle and reviewed by Simon Connor and Natalie Schroeter.