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Palaeoenvironments of the Medieval warm period in the “Kaluzhskiye Zaseki” Nature Reserve (Central European Russia)

This paper is focused on environment conditions during the Medieval warm period and human impact on the broadleaved forests in central European Russia. The study area is located in the “Kaluzhskiye Zaseki” Nature Reserve, one of the unique remnants of primary Eastern European broadleaved forests. Here, we present new high-resolution pollen and plant macrofossil records and radiocarbon dating from the sediment sequences in the outcrop of the right bank of Vytebet’ River (a tributary of Oka River) in the northern part of the Reserve. The obtained results have shown no clear evidence of vegetation changes in the region in response to the climatic amelioration during the Medieval time. The pronounced changes of forest ecosystems were caused by anthropogenic disturbance since the 13th century AD.


Introduction
A great number of palaeoclimatic reconstructions based on various proxy records and modelling approaches have shown that climate in the Medieval time was as warm as, or warmer than, it is today [1 -6]. Therefore, detailed studies of the Medieval Climate Anomaly (MCA, ca. 900-1350 AD; e.g.,) [2,3] is important for understanding vegetation response to current climate changes, including increasing air temperature, changing precipitation patterns and increasing frequency of extreme weather events The present study is focused on the reconstruction of environmental conditions in the "Kaluzhskiye Zaseki" Nature Reserve, located in the north-western part of the Mid-Russian Uplands. These forests, known as Zaseki, are a part of the formerly continuous forest belt along the south-western borders of Muscovy that was created in the 14th-16th centuries AD for the protection of state borders against raids of nomads from steppe [8]. Forest ecosystems in this area are unique remnants of primary broadleaved forests in central European Russia. Modern dynamics of plant cover, soils and human activity in the Reserve were intensively investigated during the last decades [9,10].
The previous studies of the Holocene vegetation and climate changes in this region demonstrated that mixed broadleaved forests expanded about 8000 cal yr BP, and remained dominant, until nowadays, in the study area [11]; these mixed broadleaved forests were destroyed outside of the "Kaluzhskiye Zaseki" Nature Reserve for the last few centuries by human impact [12]. However, studies of the Medieval time were very fragmentary due to low sampling resolution for this period in the peat cores and, probably, due to loss of part of the peat profiles after a fire. In this study, we reconstruct environmental changes for the Medieval warm period using high resolution pollen and plant macrofossils analysis from sediment sequences in the outcrop of the right bank of Vytebet' River (a tributary of Oka River) in the northern part of the "Kaluzhskiye Zaseki" Nature Reserve. Analyses of these paleoenvironmental data shed new light on vegetation changes in this region during MCA, and allow us to examine the human influence on forests.

Study area
The "Kaluzhskiye Zaseki" State Nature Reserve is located in the Ul'anovo district of the Kaluga region in the north-western part of the Mid-Russian Uplands in the Upper Oka River basin (figure 1). The landscape is a hilly plain of about 150-250 m above sea level dissected by gullies and ravines. Pre-Quaternary deposits are represented by Early Cretaceous clays, sands and sandstones. The Quaternary sediment sequences consist of the Early-Middle Pleistocene till of the Don Glacial complex, Middle Pleistocene fluvio-glacial sediments and Late Pleistocene loess-paleosol sequences [13]. The total thickness of Quaternary deposits varies from 10 to 30 m. The climate of the study area is temperate and moderately continental with relatively cold winters and warm summers. The mean July and January temperatures are +17.5°C and -10.1°C respectively; the mean annual temperature is +4.8°C. Annual precipitation varies from 360 to 1000 mm with a mean value of 630 mm [14].
The area is covered in broadleaved forests formed by Quercus robur, Fraxinus excelsior, Tilia cordata, Ulmus glabra, Acer platanoides and A. campestre, with an admixture of Populus tremula and Betula verrucosa [10]. In the shrub layer, Corylus avellana is abundant in the forest understorey; The 5th International Conference "Ecosystem dynamics in the Holocene" IOP Conf. Series: Earth and Environmental Science 438 (2020) 012020 IOP Publishing doi:10.1088/1755-1315/438/1/012020 3 Euonymus europaea, E. verrucosa, Lonicera xylosteum and Padus avium are common. "Kaluzhskiye Zaseki" Reserve is located close to the southern border of the Picea geographical range [15]. Picea-Pinus forests with an admixture of broadleaved tree species and Corylus avellana occur in wet habitats with sandy soils.
The study site is located on the floodplain of the right side of the Vytebet' river valley. The following sediment sequences, 200 cm long, were revealed during the exposure of the riverbank (figure 2, 3, 4). The upper part of the outcrop (up to a depth of 116 cm) is represented by a soil layer and alluvial loam, affected by biogenic processes. At a depth of 116-150 cm, an interbedding of peat and powdery loam is revealed; the thickness of the layers varies from several mm to 5 cm. Remains of woody plants (seeds, nutlets, bark, etc.) were abundant in this part of the section. A horizon of high decomposed peat with thin interlayers (up to 1 mm) of powdery loam occur at a depth of 150-170 cm. A low decomposed peat is exposed between depths of 170 and 188 cm. The remains of well-preserved leaves of broadleaved trees were found in this layer. The peat is underlain by alluvial deposits (silt with clay).

Materials and methods
The sediment sequence in the exposure of the right riverbank of the Vytebet' river was found during field campaign of 2017. The organic reach deposits at depths of 122-188 cm were sampled and studied by pollen and plant macrofossil analyses.
The chronology of the section is based on two AMS radiocarbon dates of plant residuals taking from the depths of 138 cm and 187 cm (table 1). Radiocarbon dating was performed in the CCU "Laboratory of radiocarbon dating and electronic microscopy" of the Institute of Geography of the Russian Academy of Sciences (Moscow, Russia). The 14C dates were calibrated with Calib 7.1 software and the Intcal13 calibration dataset [16]. Age-depth model was constrained by linear interpolation between midpoints of radiocarbon dates. 14 samples for plant macrofossil analysis with a volume of 300 cm 3 were washed on sieves with 0.25 mm meshes [17]. The obtained organic residue was studied under a stereomicroscope Altami SM-T, and carpological remains (subfossil seeds and fruits) were picked out. Identification of plant remains was carried out in accordance with Katz et al. [18] and the reference collection of the Institute of Geography of the Russian Academy of Sciences. The results of carpological analysis are presented as the number of plant remains in the sample ( figure 5).
Additional studies of remains of bryophytes, bark fragments, stems, leaves, and epidermis of vegetative and reproductive organs were performed in samples with volumes of 5-10 cm³. Probes were rinsed with water onto sieves with mesh (0.25 mm). The plant remains were studied using a Motic BA210 microscope at 200 magnification. The plant remains were identified in accordance with Katz et al. [19] and Dombrovskaya et al. [20]. As this type of plant remains were scarce in studied sediment, we indicated only their presence on the plant macrofossil diagram (figure 5).
The samples for pollen analysis (2 cm 3 ) were prepared in accordance with Moore et al. [21]. The samples were heated for 7 min in 10% KOH to remove humic materials. The residue was then sieved over a 200 μm mesh, followed by acetolysis in a water bath for 5 min to dissolve the cellulose. Pollen was identified in accordance with Reille [22] and reference collection of the Institute of Geography, RAS. Calculation of pollen percentages was based on the total terrestrial pollen sumarboreal pollen (AP) plus non-arboreal pollen (NAP) plus spores. A minimum of 500 pollen grains per sample were counted. Pollen and plant macrofossil diagrams were constructed using the Tilia software version 2.6.1.

Plant macrofossil analysis
According to the results of plant macrofossil analysis, three LCAZ (Local Carpological Assemblage Zone) were determined in the studied sediment sequence.  In the group of aquatic plants, the occurrence of Stratiotes aloides seeds was noted. The diversity of the macroremains of wetland and aquatic species is increasing. This group is represented by the fruits of Sparganium emersum, Alisma plantago-aquatica and Sagittaria sagittifolia, and abundant spines of Stratiotes aloides.
In the same interval, bark of Salix and Alnus, epidermis of Carex and spikes of Nymphaea sp. were identified.
Composition of the plant macrofossil assemblages of the LCAZ 3 (depth 150 -140 cm) is similar to that in LCAZ 2; however a number of remains in the LCAZ 3 is lower. The fragments of Salix bark, epidermis of Scirpus sylvaticus, idioblasts of Nymphaea sp., and remains of brown mosses (incl. Polytrichum commune Hedw.) were identified.

Pollen analysis
The pollen diagram was divided into 4 local pollen assemblage zones (LPAZ), corresponding to the main phases of vegetation changes ( figure 6). In LPAZ 2 (172-156 cm), the proportion of AP increased to 85-90%. Corylus formed a sharp peak up to 50% in the lower part of the zone at a depth of 168 cm. Share of Betula, Tilia, Ulmus and Fraxinus rose to between 166 and 156 cm. Rare pollen of Euonymus was found, so also the pollen of Fagus. Among the NAP group, Asteraceae, Poaceae, Ranunculaceae, Rosaceae, Polygonaceae and Apiaceae were common.
In LPAZ 3 (156-140 cm), pollen assemblages were characterized by a noticeable peak of Quercus (40-60%), whereas all other taxa except for Picea and Pinus (7-10%) were reduced. The composition of herbaceous pollen and spores was similar to their composition in the previous zone.
In , the proportion of Quercus pollen decreased significantly (to 15-17%) in the lower part of the zone (subzone 4a, 128-140 cm), while pollens of Alnus incana and Betula rose to characterized by warmer and relatively drier climate than modern climate (e.g.) [28]. Obviously, such conditions were favorable for the existence of broadleaved forests in the region. The relative abundance of pollens of wood species that require enough soil moisture, such as Tilia and Picea, in the study site does not suggest long and severe summer droughts.
Composition of pollen assemblages from the study site indicates dramatic changes in plant cover between 1000 and 800 cal yr BP. Quercus pollen forms a high peak (up to 60%), while there is a reduction in the occurrence of other broadleaved species. Most likely, this phase of vegetation changes with duration of 180-200 years was caused not by climatic fluctuations but was a result of vegetation successions possibly after fire. The macro-charcoal data from the peatland Mochulya indicate that fire activity increased considerably in the same time [11]. Quercus as a light-loving species benefitted at the collapse of the forest canopy. After regeneration of other broadleaved species, abundance of oak in forest stands and, consequently, the share of its pollen declined.
The pollen records from the study site reveal the beginning of human impact on forest ecosystems. After about 770-750 cal yr BP (in 13th century AD), pollens of anthropogenic indicators (Plantago, Onagraceae, Centaurea) and cultivated cereals appeared. The proportions of Alnus incana and Betula, which grow often as secondary forest stands after clear cutting, increased notably. In studied sediment sequences, these changes in pollen assemblages coincided with transition from high decomposed peat to an interbedding of peat and powdery loam. An increase of clastic materials in the peat could be an indirect evidence of active erosion processes in the Vytebet' river valley after vegetation disturbance. However, high ratio of AP to NAP as an indicator of forest coverage [29,30] and high relative abundance of Tilia, Quercus and Ulmus in pollen assemblages suggest low anthropogenic influence in the vicinity of the study site. In the areas within radii of 40-50 km from the studied section, there are several settlements of the 14th-17th century. Archeological findings of this time are quite common in this part of the Kaluga region [31]. According to pollen data from the peatland Mochulya [11], deforestation and notable anthropogenic transformations of plant cover occurred later, about 500 cal yr. BP. Evidences of human impact on vegetation detected at the study site seem to be one of the waves of this process.

Conclusions
The data obtained allow us to conclude that during the Medieval Warm period, the area of "Kaluzhskiye Zaseki" Nature Reserve was occupied by broadleaved forests formed by Quercus, Tilia, Acer, Fraxinus and Ulmus, with an admixture of Picea. As shown by pollen data from this region, broadleaved forests were widespread in the northwestern part of the Mid Russian Uplands during the Middle and Late Holocene, and despite climatic fluctuations, broadleaved forests persisted in the area. The present highresolution study of the period including the MCA also reveals no clear evidence of vegetation changes in the region in response to the climatic amelioration. The beginning of human impact on forest ecosystems in the Medieval time corresponded to the 13th century, although significant vegetation changes caused by anthropogenic disturbance occurred later.