Changes in Vegetation Period Length in Slovakia under the Conditions of Climate Change for 1931–2110

Abstract

The global mean near-surface temperature between 2012 and 2021 was 1.11 to 1.14 °C warmer than the pre-industrial level. This makes it the warmest period on record. The aim of this article was to investigate vegetation period changes (onset and termination of the temperature T ≥ 5 °C, T ≥ 10 °C, and T ≥ 15 °C) due to climate change from the average air temperature for the periods 1931–1961, 1961–1991, and 1991–2020 for 24 stations in Slovakia and forecast the length of vegetation periods for the periods 2021–2050, 2051–2080, and 2081–2110. The number of days with these characteristic temperatures was used as an input dataset, from which map outputs were generated in ArcGIS software. Spatial analysis of the vegetation periods in the past, present, and future showed an earlier start of the vegetation period in spring and a later ending in autumn during the last 30 years. The maximum duration of the vegetation period will expand from the south to the north of Slovakia. Future scenarios showed an extension of the vegetation period duration. On the other hand, this potential advantage for crop cultivation is limited by a lack of arable land in the north of Slovakia and by a lack of precipitation in the south of Slovakia.

1. Introduction

Worldwide climate changes will have major impacts on the ecological, social, economic, and political aspects of human society in the next few decades [1]. Extreme temperature events have significantly increased and affected various areas of the globe [2,3,4]. The latest findings about the changing climate, soil degradation, water scarcity, and greenhouse gas emissions have brought major challenges to sustainable agriculture [5]. Earth has warmed by 1.2 °C ± 0.1 °C since the pre-industrial period [6]. Temperatures have increased by an average of 1.2 °C during the last century in the European continent. This is an increase in temperature of about 0.45 °C over the last three decades. This trend showed that the increase in the average temperature was 0.1 °C every 10 years over the last century all over the Europe. The temperature increase has doubled in the last 30 years [7].

According to global climate forecasts published by the World Meteorological Organization (WMO) on 9 July 2020, the average annual global temperature was expected to be at least 1 °C higher each year in the next years (2020–2024) than in the pre-industrial period (average global temperature in the period 1850–1900) and it is very likely to be in the range of 0.91–1.59 °C. The scenarios of the 12 global climate models also indicated that there is approximately a 20% chance that the global temperature anomaly will exceed 1.5 °C in at least 1 year in the period 2020–2024 [8]. The Met Office’s latest climate projections showed that the increase in the global temperature will continue in the coming years, with a 48% probability that at least one of the years between 2022 and 2026 will reach a temperature anomaly of 1.5 °C or more compared to the pre-industrial average [9]. Some aspects of climate change could bring benefits for agriculture, for example, longer vegetation periods and warmer temperatures. However, climate change has negative impacts on food production due to increased water deficits [10].

Global warming in Slovakia has resulted in an increased average annual air temperature of 1.1 °C over the last 100 years (1901–2000). This fact is based on observations from the observatory in Hurbanovo, which were started in 1871. The warmest 12 years have been recorded since the early 1990s. Concurrently, atmospheric precipitation has decreased by an average of 5.6 % (1901–2000) [11]. In Slovakia, there will be a faster onset of warm and dry weather in the spring. Another expected demonstration of climate change in Slovakia is the increase in the daily maximum and minimum air temperature [12]. By 2050, a significant increase in the number of summer and tropical days is expected, in addition to a decrease in the number of frost and ice days [13]. Due to the higher air temperature, the rate of evaporation from the Earth’s surface will also increase, which will create conditions for a longer droughts throughout Slovakia, especially in the southern part [12].

The aim of this article was to determine this change by calculating the vegetation period from the average temperature for the years 1931–1961, 1961–1991, and 1991–2020 at 24 stations in Slovakia and forecast the length of the vegetation period for the periods 2021–2050, 2051–2080, and 2081–2110. The average rainfall was also evaluated for these periods (1931–2020) at the same stations.

2. Materials and Methods

2.1. Study Area

The field of interest of this study is the Slovak Republic (Figure 1). This country is located in central Europe within the northern latitudes from 47°44′21″ up to 49°36′48″ and eastern longitudes from 16°50′56″ up to 22°33′53″ [7,13]. The area of Slovakia is 49,035 km² and the surface is characterized by significant diversity and represents several geographical types. From the lowlands in the south, the country passes through a range of hills and highlands to the mountains, represented in the north. About two-thirds of the territory is the Carpathian Mountains. One-third of the country’s territory belongs to the Pannonian Basin, within which there are 3 areas in Slovakia: Záhorská Lowland in the west, Danube Lowland in the southwest, and Východoslovenská Lowland in the southeast [14].

The country is located in a temperate climate zone with a regular seasonal cycle. Slovakia has various climates relative to its area because of the variations in altitude, significant terrain segmentation, the variety of natural conditions in its territory, and different conditions for air circulation in the individual regions [15]. The shape of the territory in the west-east direction determines the differences in the temperature and precipitation conditions of western and eastern Slovakia. Based on long-term air temperature measurements, the warmest area is the Danube Lowland. The average air temperature is −1 to −2 °C in January, and 18 to 21 °C in July. The annual average is 9 to 11 °C. In the area of Východoslovenská Lowland, the air temperature is, on average, slightly lower. In the basins and valleys of rivers, the average annual air temperature reaches values in the range of 6 to 8 °C while in the highest basins, it is less than 6 °C. In Slovakia, the average annual rainfall varies from less than 500 mm (lowlands) to approximately 2000 mm in the High Tatras mountains [16].

Figure 1. Locality of the Slovak Republic (red) in Europe. Reprinted with permission from Ref. [17] 2011, Tubs.

Figure 1. Locality of the Slovak Republic (red) in Europe. Reprinted with permission from Ref. [17] 2011, Tubs.

2.2. Data Analysis

For this study, monthly air temperatures in the periods 1931–1960, 1961–1990, and 1991–2020 at 24 meteorological stations in Slovakia were analyzed (Figure 2). Datasets for the period 1931–1960 were obtained from Agroclimatic conditions of the Czechoslovak Socialist Republic [18]; for the period 1961–1990, from Practical Biometeorology [19]; and for the current period 1991–2020, from Slovak Hydrometeorological Institute (SHMI). Precipitation was compared according to the seasons for individual periods. The reference value was the period 1931–1960 and the periods 1961–1990 and 1991–2020 were compared with the reference period.

2.2.1. Length of the Vegetation Period—Calculation

Determination of the onset and termination of temperatures ≥5 °C, ≥10 °C, and ≥15 °C is closely related to many physiological and biological processes in nature [14,19]. The onset and termination of temperatures ≥5 °C determines a significant vegetation period because this temperature activates physiological processes in plants. At this temperature, the growth and development of vegetation begins in spring and ends at this temperature in autumn. The onset and termination of temperatures ≥10 °C determines the main vegetation period, and the onset and termination of temperatures ≥15 °C determines the vegetation in summer [19].

The vegetation periods were calculated for each meteorological station according to the following formulas [14]:

$$\mathrm{onset} \mathrm{of} \mathrm{temperatures}: {\mathrm{r}}_{\mathrm{v}}=\mathrm{R}\frac{{\mathrm{T}}_{\mathrm{n}}-{ \mathrm{T}}_2}{{\mathrm{T}}_1-{\mathrm{T}}_2} \hspace{1em}(\mathrm{days})$$

(1)

$$\mathrm{termination} \mathrm{of} \mathrm{temperatures}: {\mathrm{r}}_{\mathrm{p}}=\mathrm{R}\frac{{\mathrm{T}}_1-{ \mathrm{T}}_{\mathrm{u}}}{{\mathrm{T}}_1-{\mathrm{T}}_2} \hspace{1em}(\mathrm{days})$$

(2)

where:

• T_n—onset temperature (°C);
• T_u—termination temperature (°C);
• T₁—the nearest average monthly temperature above the onset or termination of temperature (°C);
• T₂—the nearest average monthly temperature below the onset or termination of temperature (°C);
• R—the difference in days between the middle of the months with the average temperature T₂ and the average temperature T₁, which is expressed as an average number R = 30;
• r_v—difference in days between the middle of the month with temperature T₂ and the date of onset of temperature T_n;
• r_p—difference in days between the middle of the month with temperature T₂ and the date of termination of temperature T_u.

An example of the calculation is shown in Figure S1.

2.2.2. Length of the Vegetation Period—Prediction

The future scenarios of the vegetation periods were based on the assumption of the development of climate change and a linear trend of temperature increase according to the Paris Agreement in 2015 [9,20]. For each meteorological station, the maximum duration of the vegetation period was calculated by the mathematical function linear trendline. The linear trendline equation uses the least squares methods to seek the slope and intercept coefficients such that [21]:

$$\mathrm{y}=\mathrm{bx}+\mathrm{a}$$

(3)

where:

• b—slope of the trendline;
• a—y-intercept, which is the expected mean value of y when all x variables are equal to 0.

According to this equation, probable trends were estimated individually for the time horizons of 2021–2050, 2051–2080, and 2081–2110. These data were used as an input dataset for the creation of the map outputs.

2.3. Map Processing in ArcGIS

In ArcGIS Desktop v.10.2.2. (ESRI, Redlands, CA, USA), data (point data, i.e., location of meteorological stations and separate number of days for temperatures ≥5 °C, ≥10 °C, and ≥15 °C) from the meteorological stations (Figure 2) were loaded and transformed to a point vector model (*.shp) in the S-JTSK coordinate system [7,22]. To obtain data between stations, the interpolation methods provided in ArcGIS software Topo to Raster were used. This tool is an interpolation method that was created especially for the design of hydrologically correct digital elevation models (DEMs). The basis of the ANUDEM (Australian National University, Canberra, Australia) program was created by Michael Hutchinson [23,24]. The interpolation procedure was designed to take advantage of the types of input data commonly available and the known characteristics of elevation surfaces. This method uses an iterative finite difference interpolation technique. It is optimized to have the computational efficiency of local interpolation methods, such as inverse distance weighted (IDW) interpolation, without losing the surface continuity of global interpolation methods, such as Kriging and Spline. It is essentially a discretized thin plate spline technique [23,25,26,27,28,29]. The raster output cell size (resolution) was set to 100 m.

For the creation of the map outputs, it was necessary to separate the land on which the vegetation periods were determined from the forest area. To separate “Forests and seminatural areas” from the rest of the area, CORINE Land Cover (CLC) was used (

Table 1. CORINE Land Cover CLC 2018 parameters [30].

Satellite Data	Sentinel-2 and Landsat-8 for gap filling
Time consistency	2017–2018
Geometric accuracy, satellite data	≤10 m (Sentinel-2)
Min. mapping unit/width	25 ha/100 m
Geometric accuracy, CLC	better than 100 m
Thematic accuracy, CLC	≥85%
Change mapping (CHA)	boundary displacement min.100 m;
	all changes ≥ 5 ha are to be mapped
Thematic accuracy, CHA	≥85%
Production time	1.5 years
Documentation	standard metadata
Access to the data (CLC, CHA) *	free access for all users
Number of countries involved	39

* https://land.copernicus.eu/pan-european/corine-land-cover (accessed on 19 July 2022).

). CLC is the oldest dataset of the Copernicus Land Monitoring Service (CLMS). CLC was specified to standardize data collection related to land in Europe to support environmental policy development. The nomenclature includes 44 classes in 5 main land cover/use groups: artificial surfaces, agriculture, forests and seminatural areas, wetlands, and water. The results of the CLC inventories can be downloaded from the CLMS site free of charge for all users for any use [30].

3. Results

3.1. Great Vegetation Period

The great vegetation period during 1931–1960 (Figure 3a) in the north of Slovakia (station Oravská Lesná) lasted for 179 days from 21 April until 16 October. In the south of Slovakia (station Hurbanovo), the period with temperatures T ≥ 5 °C lasted 244 days from 17 March until 15 November. The great vegetation period also lasted a comparatively long time in the years 1961–1990 (Figure 3b), from 183 days in the north to 244 days in the south. Change is visible in the last 30 years (Figure 3c). The onset of the great vegetation period has shifted in the north to 192 days from 14 April until 22 October and to 257 days from 10 March until 21 November in the south, which is an extension of 13 days in the period 1991–2020.

With the current trend of temperature development, this extension of the vegetation period is expected, especially in the northern regions of Slovakia. We assume that in the period 2021–2050 (Figure 3d), the vegetation period will be extended to 198 days in the north and 261 days in the south. In the period 2051–2080 (Figure 3e), the vegetation period will be 203 days in the north and 269 days in the south. For the last period 2081–2110 (Figure 3f), we assume an increase in the vegetation period to 208 days in the north and 276 days in the south. Compared to the period 1931–1960, this is an increase of approximately 30 days.

3.2. Main Vegetation Period

The main vegetation period in 1931–1960 (Figure 4a) at the Oravská Lesná station lasted 120 days (from 20 May until 16 September). In the south, in Hurbanovo, this period (temperatures T ≥ 10 °C) was 187 days (12 April until 15 October). During the period 1961–1990 (Figure 4b), the number of days of the vegetation period almost did not change. Compared to the years 1931–1960, there were differences of ±3 days at the individual stations, except for the Prievidza and Nitra stations, where this increase was by 6 days. A significant increase occurred in the period 1991–2020 (Figure 4c). In the north, the vegetation period was extended to 129 days (12 May–17 September) and to 200 days in the south (4 April–20 October), which is an increase from 9 to 13 days.

The predicted development of the main vegetation period for the years 2021–2050 (Figure 4d) showed an increase from 12 to 18 days: 132 days in the north to 205 days in the south. For the years 2051–2080 (Figure 4e), the assumption of the vegetation period is from 136 to 212 days, and from 141 to 218 days for the years 2081–2110 (Figure 4f). This represents an increase compared to the years 1931–1960 of 12 days in the north and 31 days in the south of Slovakia.

3.3. Vegetation Summer

The vegetation summer lasted 55 days in the years 1931–1960 (Figure 5a) at the Poprad station from 25 June until 18 August. In the south of Slovakia, it was 133 days from 13 May until 22 September at Bratislava Airport station. In the years 1961–1990 (Figure 5b), the vegetation summer was the shortest at the northern stations Liesek (25 days), Poprad (33 days), and Červený Kláštor (51 days). The longest vegetation period was recorded for the southern stations Hurbanovo (130 days), Nitra (128 days), and Bratislava Airport (126 days). The lower number of days of the vegetation summer in the period 1961–1990 was influenced by the colder decade of 1971–1980. During the periods of 1931–1960 and 1961–1990, the vegetation summer did not occur at the Telgárt and Oravská Lesná stations. At these stations, an increase from 0 to almost 57 days (Figure 5c) was recorded over the last 30 years (1991–2020).

The prediction for the period 2021–2050 (Figure 5d) indicates an extension of the vegetation period from 55 days in the north to 140 days in the south. In the years 2051–2080 (Figure 5e), the vegetation summer will extend from 58 to 148 days. The assumption for the years 2081–2110 (Figure 5f) is that the length of the vegetation summer will be 61 days in the north (Liesek station) and 153 days in the south (Hurbanovo station). Compared to the years 1931–1960, there is an increase of 20 days in the south and up to 61 days in the north. In the north, this increase is so extreme because there was no vegetation summer here in 1931–1960. If we compare the period 2080–2110 with the years 1991–2020, when the vegetation summer appeared in the north, this increase is about 10 days.

As vegetation periods are becoming longer, the temporal distribution of precipitation is also changing, with more frequent dry periods. In the period 1931–2020, there were several dry periods in Slovakia in most locations in Slovakia. These periods occurred for varying lengths of time during the 1930s and 1940s. Dry periods also occurred in the years 1964, 1973, 1974, 1992, 2000, 2003, 2006, 2007, 2011, 2012, 2016, and 2020.

The analyzed results of the precipitation in the years 1931–1960, 1961–1990, and 1991–2020 clearly show that the distribution of precipitation is changing (

Table 2. Average amount of precipitation in spring, summer, autumn, and winter in the periods 1931–1960, 1961–1990, and 1991–2020.

Locality	Period	Spring (III.–V.) [mm]	Summer (VI.–VIII.) [mm]	Autumn (XI.–XI.) [mm]	Winter (XII.–II.) [mm]
Bardejov	1931–1960	170	304	183	142
	1961–1990	171	282	162	128
	1991–2020	151	269	148	107
Bratislava Airport	1931–1960	144	184	149	138
	1961–1990	128	181	131	136
	1991–2020	131	182	148	113
Čadca	1931–1960	191	327	192	154
	1961–1990	205	338	197	176
	1991–2020	214	299	212	170
Hurbanovo	1931–1960	136	177	136	120
	1961–1990	122	170	125	108
	1991–2020	130	185	151	105
Košice Airport	1931–1960	139	246	150	95
	1961–1990	146	242	144	90
	1991–2020	135	248	141	89
Nitra	1931–1960	134	184	140	113
	1961–1990	127	179	131	103
	1991–2020	129	178	149	103
Oravská Lesná	1931–1960	242	395	244	217
	1961–1990	242	366	247	240
	1991–2020	268	365	278	272
Piešťany	1931–1960	128	223	144	116
	1961–1990	134	196	136	111
	1991–2020	129	199	149	100
Poprad	1931–1960	135	257	134	85
	1961–1990	144	226	132	77
	1991–2020	149	265	144	79
Prievidza	1931–1960	152	248	156	128
	1961–1990	145	223	148	126
	1991–2020	147	231	167	133
Rimavská Sobota	1931–1960	142	232	153	111
	1961–1990	154	202	139	101
	1991–2020	156	215	155	111
Myjava	1931–1960	152	238	164	124
	1961–1990	151	209	159	152
	1991–2020	157	207	169	153
Vígľaš-Pstruša	1931–1960	143	212	150	115
	1961–1990	145	207	149	105
	1991–2020	148	235	157	107
Liptovský Hrádok	1931–1960	154	268	158	100
	1961–1990	153	242	158	115
	1991–2020	163	266	178	118
Červený Kláštor	1931–1960	166	331	171	107
	1961–1990	171	311	152	114
	1991–2020	203	365	183	114
Rožňava	1931–1960	155	270	160	114
	1961–1990	176	242	147	103
	1991–2020	161	276	160	105
Moldava and Bodvou	1931–1960	142	243	146	104
	1961–1990	158	241	143	101
	1991–2020	143	249	146	94
Trenčín	1931–1960	143	234	159	134
	1961–1990	135	205	144	131
	1991–2020	151	215	162	133
Sliač	1931–1960	158	235	169	143
	1961–1990	155	226	175	146
	1991–2020	161	237	178	139
Liesek	1931–1960	185	323	176	125
	1961–1990	180	322	187	162
	1991–2020	194	337	197	102
Telgárt	1931–1960	193	328	206	149
	1961–1990	220	301	192	147
	1991–2020	221	337	214	126
Plaveč and Popradom	1931–1960	118	255	126	81
	1961–1990	156	276	129	92
	1991–2020	183	326	160	100
Kamenica and Cirochou	1931–1960	133	253	161	120
	1961–1990	168	275	164	117
	1991–2020	165	272	182	116
Somotor	1931–1960	125	215	134	103
	1961–1990	135	204	123	97
	1991–2020	133	190	148	106

). Summers are becoming drier and summer rainfall is shifting to the autumn months. The results indicate that the southern regions are suffering from a greater lack of precipitation in the summer while the northern part of Slovakia appears to have greater precipitation, especially in the last period.

4. Discussion

4.1. Changes in Vegetation Period Duration

Increases in air temperature are shown by long-term measurement step by step. It is expected that the anomaly of the global annual temperature will be 1.1 to 1.7 °C higher than the pre-industrial average compared to the period 2022–2026 [9]. The development of the vegetation period length is one of the significant consequences of climate change. An extension of the vegetation period duration was proven not only by our research but also by others research in the regions of Slovakia and the central European region. According to Burić et al. [31], the number of days with a daily maximum temperature above 25 °C in Montenegro has increased since the 1980s. Unkašević et al. [32] reported the temperature indices in Serbia and the climate has tended to become warmer over the last 61 years. Popov et al.’s [33] analyses showed a decreasing trend in the number of frost days in Bosnia and Herzegovina since the 1990s and particularly since the beginning of the 21st century. According to Sar et al.’s [34] research on the representative concentration pathway scenarios RCP 4.5 and RCP 8.5, the vegetation period in the Inner West Anatolia subregion could extend by 20 to 40 days. Olszewski and Żmudzka [35] analyzed data in Poland and reported an increase in the length of the vegetation period from 1 to 3 days per decade.

Several published studies [13,36,37] that focused on Slovakia showed similar conclusions to our study. Valšíková-Frey et al. [36] analyzed predictions for a sooner potential beginning of seeding (25–30 days earlier) and later possible harvest (10 to 15 days) in the year 2075 in Hurbanovo (Slovakia). Changes in the length of the vegetation period for fruit and root vegetables in field conditions were analyzed by Špánik et al. [37]. They forecasted a prolongation of the vegetation period length by about 21–26% for several species of vegetables in Hurbanovo and Liptovský Hrádok in 2075. Changes in temperature development in Slovakia and future predictions were described by Čimo et al. [13]. This author also dealt with changes in the length of the vegetation period of Capsicum annuum, Brassica oleracea var. capitata, Beta vulgaris subsp. Vulgaris, Daucus carota L., and Solanum lycopersicum L. [7,38].

4.2. Consequences of Changing Air Temperature

Climate change leads to growth in the global temperature. This fact have a positive consequence for crop production. On the other hand, taking advantage of this climate change outcome is questionable due to the predicted higher evapotranspiration and water shortages. Climate change also results in more frequent weather extremes such as heat waves. Higher temperatures cause increased mortality, lower productivity, and can damage infrastructure. A shift in the geographical distribution of climate zones is expected [39]. Individual plant species react differently to temperature during their life cycle with phenological reactions, i.e., stages of development. Each species has a specified maximum and minimum range of growing temperatures [40]. As a result of the increased temperatures, plants ripen faster, which changes their developmental stages. This can affect crop rotation and the ability to efficiently and profitably manage fields for farmers. This could lead to double cropping and the use of cover crops. Earlier growth can also affect the habitus of the plant, which does not reach the required one. Accelerated plant growth places pressure on processes in the soil. Water and nutrients stored in the soil do not have enough time to reach the plant at the required cultivation stage. This reduces the production of grains, forages, and fruits and vegetables [41].

Due to the changing climate, European countries are already facing more frequent, severe, and longer-lasting droughts. This is the main barrier to using increasing vegetation period lengths as an advantage. With a global average temperature growth of 3 °C, it is projected that droughts will double [40]. Crop production is highly sensitive to the climate. Correct plant selection is linked to long-term trends in average rainfall and temperature [42]. Agricultural crops have different water needs. A long-lasting drought can reduce crop yields because water and soil moisture decrease and are unavailable for crop growth [43].

As the climate heats up, precipitation patterns change, evaporation increases, glaciers melt, and sea levels rise [39]. Changes in the intensity and amount of rain/snow mix for a location are expected to increase the management of water delivery to plants at the right time through irrigation systems. Extreme rainfall can be as damaging as the occurrence of too little rainfall due to the increase in flooding events, greater erosion, and decreased soil quality [41]. Extreme precipitation (from a short-term to multi-day duration) has shown an increasing trend in recent decades. It is assumed that it will intensify in the future due to global warming. Short-term precipitation extremes are also more sensitive to warming than long-term precipitation extremes [44]. Rain-induced flooding and waterlogging, occurring in many regions of the world, are disasters for farmers, often causing significant damage to crop production [45].

Future scenarios for Slovakia shown an extension of the vegetation period duration. On the other hand, this potential is limited by the lack of arable land in the mountainous areas [7]. These areas have only very-low- or medium-productivity agricultural soils [46].

There will always be some uncertainty in the correlation between climate scenarios and agricultural yields [42]; however, with climate change, it is necessary to change crop production strategies.

5. Conclusions

The aim of this work was to determine the changes in the temperature trends from 1931 to 2020. Maps of the vegetation period duration development were created based on the analysis of the air temperature changes for 2021–2050, 2051–2080, and 2081–2110. Spatial analysis of the vegetation periods in the past, present, and future pointed to:

• A potential earlier start of the vegetation periods in spring and a later ending in autumn in the future;
• The length of the vegetation periods will extend more in the south than in the north of the Slovak Republic;
• A general trend of extension in the regions with the longest maximum vegetation period after the decade 1991–2020; and
• Changes in the distribution of precipitation.

Although the future scenarios showed a possible extension of the length of the vegetation period, in the northern part of Slovakia, this potential is limited by the availability of arable land and is limited by rainfall shortages in the south, which causes a decrease in the soil water content. These results can be used by scientists and, especially, farmers to propose adaptation strategies. However, further research and development of different forecasting models is needed.