Temporal dynamics of steppe plant communities

Global climate change affects the conditions of ecosystems. However, the nature of changes induced by climatic factors remains unknown due to the complex nature of climatic transformations. The global trend of temperature increase is associated with an increase in precipitation and changes in its rhythm. The dynamics of plant communities under the influence of climate occurs against the background of natural successional phenomena. The aim of the study is to develop methodological approaches in order to identify aspects of vegetation variability that are caused by global climate change and give them an ecological interpretation. Geobotanical descriptions of vegetation in steppe ecosystems were carried out over the last 20 years. Exactly in this period dramatic climatic changes were observed, which allows to compare climatic and vegetation trends. For ecological interpretation the results of ordination of plant communities were explained with the help of phytoindication scales. Plant communities exhibit dynamics that are driven by endogenous and exogenous causes. These aspects of community dynamics were differentiated using the ordination procedure as different ordination axes. The axes that reflect endogenous dynamics were described using temporal variables. The axes that reflect exogenous dynamics were described using phytoindication scales. The phytoindication scales made it possible to assess the ecological directions of plant community transformation under the influence of global climate change. The transformation of the plant community under the influence of global climate change is inconsistent with the concepts of desertification. The revealed temporal patterns of the plant community have a complex and multidirectional trajectory. The plant community studied over the twenty-year investigation period exhibited a dynamic that is a superposition of two processes that are driven by the temperature and precipitation. The temperature trend is directional and reflects the tendency of global warming. This trend is accompanied by an increase in species richness and projective cover of the plant community. At the same time, thermophilicity and continentality of the community increase against the decrease of soil trophic status, acidity increase and soil carbonation decrease. Obviously, along with the mineralization of organic matter, the soil decarbonization can also be the cause of increased carbon dioxide emission into the atmosphere as a result of global warming. The trend, which is regulated by precipitation, is oscillatory. This trend changes the regime of moisture, light and cryoclimate of the plant community.


Introduction
The main feature of today's climate is increased global warming since the second half of the 1970s [1].The global air temperature is predicted to increase steadily through the end of this century due to continued increases in greenhouse gases such as CO 2 , and changes in land use such as deforestation of natural forests [2].Due to global climate change, which affects the transformation of regional climate and the value of individual meteorological variables, the 1254 (2023) 012022 IOP Publishing doi:10.1088/1755-1315/1254/1/012022 2 average monthly air temperature in Ukraine over the past decades differs from the values of the climatic norm.The air temperature has become higher for most months and for the year as a whole, only in September, November and December it decreased insignificantly [3].The amount of atmospheric precipitation for the territory of Ukraine has not changed significantly, but the nature and intensity of precipitation has changed markedly [3].The variability of precipitation dynamics affects the productivity of plant communities [4][5][6].Recently, the number of cases where half or a month's worth of precipitation falls in a few hours has increased [3].The forms of ecosystem transformation as a consequence of global climate change are accelerated desertification [7,8].The change of water regime in steppe ecosystems strongly affects ecosystem functioning, productivity and photosynthetic capacity [9][10][11][12].The intensification and increased frequency of extreme climate change events, including high summer temperatures and increased precipitation variability, threaten sustainable development in semiarid and arid regions through both biophysical and socioeconomic factors [13].The stability of terrestrial ecosystems will be further threatened by the increasing frequency and severity of extreme climatic events [14,15].An increase in air temperature and uneven distribution of precipitation, which is stormy, localized in the warm season and does not provide an effective accumulation of moisture in the soil, can lead to an increase in the frequency and intensity of droughts.Changes in precipitation frequency, intensity, and pulse size, as well as seasonal changes in precipitation, affect arid ecosystem functions, including carbon flux [16][17][18][19], water exchange, and plant physiological status [20].There are also changes in the values of extreme (maximum and minimum) temperatures.The minimum temperature has increased in almost all months and in the whole year.In the secular course of the maximum temperature in winter months, especially in January, there was a tendency to its growth.In summer months and for the year as a whole, the trend of maximum temperature change in the trend is insignificant, but in recent years, the maximum temperature increases.The particularly affected areas include grassland and desert ecosystems of semi-arid and arid areas [21][22][23].A loss of biodiversity is an obvious consequence of climate change [22,24].The warming is evolving according to positive feedback dynamics.The combination of heat waves and variability in precipitation dynamics affect carbon exchange [25].The global climate change is initiated by an increase in atmospheric concentrations of greenhouse gases, but the warming continuously stimulates the emission of carbon dioxide into the atmosphere [26][27][28].An increase in air temperature leads to soil evaporation [8,[29][30][31].The intensification of heat waves and abnormal precipitation led to the functional and structural degradation of particularly sensitive terrestrial ecosystems.The heat waves affect the intensity of plant growth [32][33][34].Consequently, a water deficit occurs, further exacerbating ecological vulnerability and sensitivity and reducing resilience to rapid degradation in steppe regions [35,36].An increase in precipitation in the steppe zone can improve the carbon balance, thereby mitigating the negative effects of a warming climate [37,38].The warming climate and increased precipitation variability can regulate the function and structure of ecosystems in steppe ecosystems.The transition from vegetation dominated by grasses to vegetation dominated by shrubs or tree species can be predicted [24].The most important indicators of terrestrial ecosystem function are the primary production of aboveground plants and the activity of soil microorganisms [39].The activity of soil microorganisms, which transform the biomass created by plants, determines the intensity of carbon sequestration and carbon dioxide emission into the atmosphere.The precipitation quantity affects the condition of the soil biota.Biomass, activity and composition of the microbial community are sensitive to the effects of environmental factors [40][41][42].The water balance and temperature regime affect the functional state of soil biota [39,[43][44][45].The biomass of soil biota decreases with warming [46][47][48], but increases with increased precipitation [49].Estimating the balance of carbon, water, and energy in terrestrial ecosystems under conditions of climate change is possible only if the importance of microbial activity in ecosystem processes is elucidated [50].

Research aim and objectives
Grasslands provide many critical benefits to humans, including forage for livestock, food, biodiversity, carbon storage, and recreation [51].Studies of the long-term dynamics of plant communities in steppe ecosystems are largely absent, and such studies are urgently needed to assess ecosystem responses and feedbacks to climate change [52].
The aim of the study is to develop methodological approaches in order to identify aspects of vegetation variability that are caused by global climate change and give them an ecological interpretation.

Material and methods
The Stone Graves reserve is located on the border between Donetsk and Zaporozhye regions, near the village of Nazarovka of Mariupol district on the Priazovsky upland, in the upper reaches of the Karatysh river, which is a tributary of the Berda river.The Stone Graves reserve has an area of about 400 hectares, of which almost 300 hectares are part of the Donetsk region and 100 hectares are part of the Zaporozhye region.The most part of the reserve area, about 200 hectares, falls on powerful granite rocky outcrops of Azov-Podolsky crystalline massif, towering above the surrounding steppe, which is the largest intrusive area in Donbass and Azov regions [53].The rock outcrops are represented by the Western and Eastern ridges with absolute height of separate tops up to 100 m.According to landscape-geochemical zoning, the landscapes of the Stone Graves reserve are localized on eluvio-deluvium of crystalline rocks.According to the generic classification, these are loess uplands tending to erosion processes, so the migration of chemical elements here is weak and occurs when soils are washed away.The landscapes of the territory are characterised as self-purifying.The reserve is located in an area of elevated plains, where upward and downward radial migration of chemical elements takes place.The accumulation elements are Mn, Co, Sn, and the removal elements are Pb, Zn, Ni.Soil-forming rocks are clay and clayey sediments and loamy rubbly formations on eruptive and metamorphic rocks.The soils of the Stone Graves reserve are typical sparse humus black earths on loess-like loam and weathering products of crystalline rocks, which are the most productive along the bottom of the depression between two parallel ridges of rocks.Soil thickness decreases along the rock slopes along with disappearance of loess rocks and increase of area of weathering products of crystalline rocks to the day surface.The bottom of the basin between the ridges is dissected by gullies [54].
The stationary plot No.1 was created on July 14, 2000 to study the phenological development of steppe vegetation.The plot is located on the slope of 3°of eastern exposition along the upper edge of the amphitral catchment of the central gully of the inter-ridge trough at its headwaters.The geographic coordinates of the area center are N 47.310°E 37.076°.The elevation is 179 m above sea level.Vascular plant species lists were recorded for each 10 m × 10 m (the area is 100 m 2 ) sampling polygon along with a visual assessment of species coverage using a Braun-Blanquet scale [55].The projective cover of plant species was measured at soil level.Plant taxonomic names follow the Euro+Med Plantbase resource (https://europlusmed.org/).Meteorological data obtained from NOAA climate data using the rnoaa package [56] for a language and environment for statistical computing R [57].Based on the data obtained, indicators such as average spring temperature (figure 1, a), average temperature of the coldest month of the year (figure 1, b), and the amount of precipitation during the spring period of the year (figure 1, c) were calculated.The range scales values according to Y. Didukh [58] were used for phytoindication.Further, for phytoindication of environmental factors, we used the ideal indicator method of G. Buzuk [59].Statistical calculations were carried out using the software Statistica 12.0 [60].

Results
The 79 plant species were found within the study area during the study period (table 1).The number of species in the community varied between 51 (in 2007) and 66 species (2014 and 2015).The number of species increased over time.There was a positive correlation (r = 0.77, p < 0.001) between the number of species in the community and the order of years.The regression analysis revealed that the average rate of increase in the number of species in the community was 0.62 species per year.The projective cover of the community ranged from 70 to 85%.The projective cover showed an increasing trend.A statistically significant correlation (r = 0.42, p = 0.05) was found between the projective vegetation cover and the order of years.The projective cover increased on average by 0.4% with each year.2).This indicator did not show a stable linear trend of variability over time, as indicated by the lack of statistically significant correlation with the order of years (r = -0.04,p = 0.85).It depended on spring precipitation (r = 0.64, p = 0.001) and on precipitation in the preceding year, which can be described by regression equation: Hd = 32.0+0.00754Prec spr + 0.00178 -Prec prev (R 2 adj = 0.47, p < 0.001), where Prec spr is the total spring precipitation in current year; Prec prev is the total precipitation in preceding year.The amount of precipitation that the region receives on average is sufficient to accumulate significant water reserves, if the soil properties allow it.The soils on which the community under study has formed have a very low water-holding capacity, so the plant species that make up the community are highly xerophilous.The moisture contrast regime was characterized by indices that were in the range of 5.47-6.24.The moisture contrast regime exhibited a steady linear trend of decreasing indices over time, as indicated by a negative statistically significant correlation with the order of years (r = -0.49,p = 0.02).The soil acidity was characterized by phytoindicator scores that ranged from 9.18-10.49,which corresponded to a soil acidity pH of 6.8-7.2.The acidity decreased linearly with time (r = -0.69,p < 0.001).It should also be noted that acidity was negatively correlated with mean spring temperature (r = -0.47,p = 0.027).The overall salinity regime was characterized by phytoindication scores, which were in the range of 6.8-8.34,which corresponded to a salt content of 0.031-0.060% in the soil solution.The overall salinity regime showed an increasing trend with time (r = 0.68, p = 0.001) and was also positively correlated with mean spring temperature (r = 0.62, p = 0.001).The content of carbonate in soil was characterized by phytoindicator indices, which were in the range 11.22-11.73,which corresponded to the content of carbonate in soil 8.6-11.3%.The carbonate content in soil showed a decreasing trend with time (r = -0.71,p = 0.001), and was also negatively correlated with mean spring temperature (r = -0.62,p = 0.001).The soil nitrogen content was characterized by phytoindicator indices, which were in the range 2.68-3.98,which corresponded to the content of plant-available nitrogen in the soil of 0.044-0.11%.The soil nitrogen content showed a decreasing trend with time (r = -0.87,p = 0.001), and also negatively correlated with mean spring temperature (r = -0.74,p = 0.001).The soil aeration regime was characterised by phytoindication indices, which were in the range 4.96-5.71,corresponding to an aeration porosity of 65-77%.This index was stationary over time.
The thermal climate was characterised by phytoindicator indices that ranged from 9.63-11.29,which corresponded to a radiative balance of 2016-2250 MJ m −2 year −1 .The radiation balance estimate showed an increasing trend with time (r = 0.66, p = 0.001) and was also positively correlated with mean spring temperature (r = 0.65, p = 0.001).The ombroclimate was characterised by phytoindicator indices that ranged from 9.25-10.46,corresponding to a difference between precipitation and evaporation of -722 --485 mm.The ombroclimate assessment was positively correlated with the average temperature of the coldest month (r = 0.41, p = 0.05).The continentality was characterised by phytoindication indices, which were in the range 11.87-13.15,corresponding to a continentality index of 159-172%.The continentality score showed an increasing trend with time (r = 0.75, p = 0.001).The phytoindication of the cryo-climate score corresponds to the average temperature of the coldest month -6 --3°C and correlates positively with these meteorological indices measured instrumentally (r = 0.51, p = 0.016).Lighting regime was characterised by scores of 8.87-8.96.The lighting regime was stationary during the study period.
The principal component analysis identified two components, which together describe 53.1% of the variation in the original variables (figure 3).The principal component 1 describes a trend of increasing number of species in the community, thermal regime, soil salinity and continentality over time and decreasing indicators of plant nitrogen nutrition, carbonation and acidity.The principal component 2 reflects a positive effect of precipitation and temperature in the coldest month of the year on moistening, ombrorheme and cryoregime and a negative effect on aeration and lighting regimes.
Grasslands are also affected by ongoing climate change [63][64][65].Anthropogenic impacts result in significant changes in environmental factors that affect the composition and functioning of plant communities [66].Identifying the causes that drive changes in the structure of plant communities is critical because the composition of plant communities affects important ecosystem functions and services [67,68].Fluctuations are short-term changes in the structure of plant communities on the scale of several years [69,70].In contrast to succession, these changes are characterized by the absence of long-term trends in the structure of communities in one direction or another, the possibility of returning to a state close to the initial one, and the absence of significant changes in the floristic composition.Most often fluctuations are considered as changes occurring within one solar cycle (about 11 years) [71].Fluctuations can be caused by various factors.The fluctuations can be ecotypic, anthropic, zoogenic, phytocyclic, phytoparasitic [72].The ecotopic fluctuations are caused by variability in meteorological and hydrological conditions [73,74].Ecotope changes can cause a chain of different mechanisms of impact on vegetation cover.Drought is associated with higher temperatures and increased salinity in arid regions [75][76][77], and can also cause mass development of locusts that consume a significant portion of organic matter [78].Drought has the greatest impact on plant communities developing on sparse soils [79][80][81].
The potential to interpret and predict the response of plant communities to global change is complicated by many factors, such as the type of driver of global change and the ecological context [82][83][84].The phytoindication of the moisture content indicates that conditions within the study area are favourable for peroxerophytes and xerophytes.These conditions are characteristic of steppe herbaceous communities on rocky ground.In conditions of precipitation deficit on stony soils petrophytic communities are formed which specificity consists in high level of species and syntaxonomy endemism [53].The precipitation levels observed in the study area can allow for the formation of significantly larger water reserves.However, the unfavourable water-physical properties of soils formed on rocks do not allow for the accumulation of water in larger quantities.As a result, the plant community is formed by species that are able to tolerate water deficits.The phytoindication assessment of moisture depends on the precipitation conditions of the current and previous year.The ecological structure of the plant community also changes according Figure 2: Dynamics of soil environmental factors assessed by phytoindication: Hd is the plant available soil water content, %; fH is the index of variability of damping, which varies from 0 to 0.5; Rc is the soil acidity (pH); Sl is the salt content in the soil solution (mg/l); Ca is the carbonate content in soil (%); Nt is the content in soil plant accessible forms of nitrogen (%); Ae is the aeration porosity (%). to precipitation trends.These changes have a fluctuational character, as they do not show a stable directional trend.The level of soil moisture is naturally inversely correlated with soil aeration conditions.Accordingly, the level of soil aeration does not show a directional trend.Rainfall levels in the spring are coordinated with the temperature of the coldest month of the year.The warmer the winter in a given year, the better the soil moisture conditions will be.This relationship is the reason for the correlation of phytoindication estimates of moisture, ombroclimate and cryoclimate.This relationship is the reason for the correlation between the phytoindication estimates of cryoclimate and instrumental temperature measurements.Higher temperatures in winter months and higher precipitation contribute to lower light levels in the plant community.Obviously, these factors contribute to an increase in the phytomass of the plant community and thus a decrease in its illumination.Once again, the variation in precipitation and temperature during winter time did not show a directional temporal trend.lags of 1, 13 and 14 years (positive autocorrelation) and 6-7 years (negative autocorrelation).The presence of an oscillatory process is fully consistent with the notion of plant community dynamics as a fluctuation.The species composition of plants in a dry grassland in Central Europe showed marked dynamics from year to year in response to weather conditions in the previous two years.These changes in the community are undirected and have contributed to the stability of this community, which has not changed significantly over the past 90 years.However, increased drought frequency due to ongoing climate change may lead to directional changes with the expansion of ruderal species [69].
Environmental changes and resource availability are key mechanisms that support biodiversity [85][86][87].Weather fluctuations prevent competitive exclusion between species cooccurring at a site and, at the same time, allow species with different strategies and from different climatic regions to co-exist at the edge of their ecological niches [61].Differences in response to climatic extremes between species with different ecological strategies can lead to non-equilibrium coexistence due to stochastic fluctuations between years [88].When weather conditions are favourable for a certain type of species strategy, other species may be reduced and confined to small areas partly because of direct response to the environment and partly because of competition.As soon as the weather shifts to a state favourable to other types of strategies, species with those strategies spread out again, compensating for their losses in the previous period of time [85].This is particularly true for dry grasslands on shallow, stony soils with low water retention, which are characterised by considerable species turnover in response to weather fluctuations, but remain stable in the long term [89].
Other ecological factors and species richness of the community show a consistent directional trend of change over time.Climate change is considered a leading threat to biodiversity because it may increase the rate of species extinction.In addition to extinctions, the distribution of species is predicted to shift as a result of variations in temperature and precipitation around the world [90].Climate change is also expected to increase weather fluctuations within and between successive years.Extreme climate events and increased seasonal variations have already been reported around the world [91].This trend is driven by directional changes in temperature regime and is described by principal component 1.After extracting the linear trend, principal component 1 did not exhibit an oscillatory component.Over the study period, the species richness of the community and projective cover increased together with the thermoclimate, continentality and salinity of the soil solution.Also such indicators as the level of nitrogen compounds supply to plants, acidity, moisture regime variability and soil carbonate content decreased throughout the study period.Variations in the availability of resources, such as atmospheric carbon dioxide, nitrogen, and precipitation patterns, can have significant consequences for the structure of plant communities [92].
Biotic processes, such as shifts in competitive dominance or susceptibility to herbivory, and abiotic factors, such as environmental filtration, affect the richness and composition of plant communities at local scales [93][94][95].The conditions of soil moisture variability are favourable for hemicontrastophobes, which are characteristic of ecotopes with moderately uneven moistening of the root-containing soil layer, which is completely soaked by precipitation only in some rainy seasons.The steppe zone of Ukraine is generally characterised by uneven precipitation and rainfall that is of a torrential nature.Such meteorological features lead to an extremely uneven soil moisture regime against a background of water scarcity.
Community composition and change in richness varies from place to place depending on the environment, as well as fluctuations in weather conditions, combined with inherent population and community dynamics over time [96].Such dynamics can best be understood with detailed long-term observations at stationary plots [69,97,98].The phytoindication method estimates soil acidity as being in the pH range of 6.8-7.2.This agrees with the results obtained experimentally.The acidity of soils in the reserve is pH7.0 and ranges from pH5.9 to 7.6 [54].This result confirms the high accuracy of phytoindication assessments of environmental factors, which can be achieved by studying multi-species plant communities.It is legitimate that the decrease in soil acidity was accompanied by a decrease in soil carbonate content.It should be noted that carbonates are an important carbon reserve in the soil.The decrease of carbonate content in the soil due to global warming is an important source of its supply to the atmosphere.It is obvious that decarbonisation of soil is accompanied by transition of insoluble calcium and magnesium salts into soluble forms that is accompanied by increase in indicators of mineralisation of a soil solution.The mineralisation of the soil solution is not significant, so this factor cannot act as a limiting factor.Increased environmental stress can have various effects on plant community composition, either shifting or increasing niche availability.The repeated removal of plant material by haying can increase species richness by increasing light availability and creating favorable conditions for species that can tolerate aboveground removal.Increased drought or temperature stress can reduce the species richness of plants because many species will not be able to persist under these new conditions [99,100].
Empirical data and theoretical evidence suggest that the response of ecosystem functions attenuates as the number of simultaneously introduced factors increases due to the leveling out of positive and negative effects on functions such as productivity and nutrient cycling [101,102].Adding resources (e.g., nutrients) is predicted to reduce plant species richness and change plant community composition due to alterations in competitive interactions between species for remaining limiting resources (e.g., water or light) [103,104].The nitrate content of the reserve's soils is 179 µg/g and varies between 93 and 348 µg/g [54].A decrease in the supply of soil nutrients, mainly nitrogen compounds, may be a factor in stimulating an increase in the species diversity of the community.Obviously, an increase in temperature leads to an equilibrium shift in the processes of mineralisation and humification of organic matter in the soil towards mineralisation, which leads to a decrease in nutrient reserves.In turn, this transformation initiates a decrease in soil acidity and decarbonization as a consequence of increased soil respiration and destruction of soil carbonates.The combined effect of multiple transforming IOP Publishing doi:10.1088/1755-1315/1254/1/01202213 factors causes a stronger effect on the plant community [105,106].

Conclusion
The plant community studied over the twenty-year investigation period exhibited a dynamic that is a superposition of two processes that are driven by the temperature and precipitation.The temperature trend is directional and reflects the tendency of global warming.This trend is accompanied by an increase in species richness and projective cover of the plant community.At the same time, thermophilicity and continentality of the community increase against the decrease of soil trophic status, acidity increase and soil carbonation decrease.Obviously, along with the mineralization of organic matter, the soil decarbonization can also be the cause of increased carbon dioxide emission into the atmosphere as a result of global warming.The trend, which is regulated by precipitation, is oscillatory.This trend changes the regime of moisture, light and cryoclimate of the plant community.

Figure 1 :
Figure 1: Dynamics of meteorological parameters during the study period 2000-2021: (a) average temperature during the spring period, °C (March-May); (b) the average temperature of the coldest month of the year, °C (January or February); (c) total spring precipitation (March-May), mm.

Figure 3 :
Figure 3: Dynamics of climatic factors assessed by phytoindication: Tm is the radiation balance (m −2 year −1 ); Om is the humidity index determined as a difference between annual precipitation quantity and evaporation (mm); Kn is the index of continentality (%); Cr is the mean temperature of the coldest month of the year (C); Lc is the light regime in 9-grade scale.

Figure 4 :
Figure 4: Principal component analysis of variation in phytoindicator estimates of environmental factors, species richness, projective cover of the plant community, average temperature of the coldest month of the year, average temperature and amount of precipitation in spring.

Table 1 :
Species diversity and abundance of the plant community (projective cover, %; + is signed that species presents).

Table 1 -continued from previous page
+The phytoindication score of soil water regime ranged from 3.69 to 3.98 (table 2), corresponding to plant available moisture of 32-33% (figure

Table 2 :
Descriptive statistics of phytoindicator estimates of environmental factors.