The phenological seasons of Poland in the warming world

Abstract

The aim of the study is to examine the impact of climate change on plant phenology and the shift of phenological seasons. This paper considers phenophases of wild-growing plants in Poland at 36 observation stations over the past nearly 80 years. The analysis was conducted for two 15-year periods: a reference period (1946–1960) and a recent period (2007–2021), during which rapid global warming has been observed. This study applies the phenological calendar method. The results indicate shifts in the onset of successive phenological seasons in the recent period compared to the reference period, as well as in the duration of individual seasons. The most significant changes include (i) a shortening of winter due to an earlier onset of very early spring (by an average of 10 days across Poland); (ii) an advancement of subsequent seasons from early spring to early autumn, ranging from nearly 4 to more than 10 days, with the smallest shift observed for early autumn); (iii) a delay in the autumn onset by approximately 4 days; and (iv) a stable timing of winter onset throughout the entire study period. With regard to seasons length, in addition to the shortening of winter, an extension of the warmest phenological seasons was observed, with the greatest increase of 8 days for early autumn. Due to its location in the temperate transitional climate zone, Poland is characterized by high weather variability, which is reflected in substantial interannual variability in both the onset and duration of successive phenological seasons.

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Introduction

Phenological research (mainly phytophenological ones) is used in many natural sciences and has broad practical applications. It aids in studying the geographical environment, particularly in recognizing and understanding the interactions between living organisms and their environment, as well as understanding ecological processes (Walther et al. 2002).

Studies examining the relationship between the annual plant development cycle on Earth and elements of the natural environment have shown that climate, specifically variables such as air temperature, day length, sunlight, and precipitation/humidity, have the greatest influence on the seasonality of plant development and on its spatial and temporal variability. Plants continuously respond to all weather phenomena, not just to a single variable (e.g., temperature). They respond to climate change in the long term. Therefore, phytophenological observations serve a valuable complement to direct meteorological measurements, and their results play a key role in detecting climate change and its impacts (Kalvãne et al. 2009; Ovaskainen et al. 2013; Menzel et al. 2020). Thus, despite limited data from phenological observation sites, this approach is gaining importance and complements climate studies by highlighting seasonal variability (Sparks and Menzel 2002).

The foundation of all phenological analyses and practical applications of phenology is the concept of the phenological calendar—a concept proposed by Ihne (1895) and later developed by researchers such as Lieth (1974) or DeFila (1992). The phenological calendar describes natural (biological) seasonality by listing the annual sequence of phenological phases, including their start dates, durations, and intervals between them. It enables the identification of homogeneous periods within the yearly cycle, which are defined as phenological seasons. The phenological division appears to be the most neutral among the various ways of dividing the year into seasons (others include thermal, calendar, or astronomical divisions). It is considered neutral because humans do not directly influence how living organisms respond to environmental conditions, including climatic conditions (at least under conditions uncontrolled by technology).

Thus, considering the diversity of biological processes and the complexity of factors influencing them, Ihne (1895) proposed a division of seven phenological seasons for Central Europe: very early spring, early spring, spring, early summer, summer, early autumn, and autumn. The eighth (non-phenological) season is winter, i.e. the period of plant dormancy. The onset of phenological seasons is determined based on the start dates of specific developmental phases in selected indicator plants. The observed phases include leaf development, flowering, ripening, the dispersal of fruits and seeds, leaf colur change, and leaf fall. Indicator species are selected based on the characteristics of a specific region. They were chosen by Łastowski (1951) for the territory of Poland. The beginning of winter is determined based on temperature data.

Phenological observations enable the tracking of variability in the onset of individual plant development phases and changes in the duration of the phenological seasons in a specific area. However, the climate is not constant. Globally and regionally, climate change, which has been observed since the late 20th century, has impacted the natural environment, including the plant kingdom. So, in the context of climate change, analyses of phenological seasons - their shifts and characteristics are becoming increasingly important. Although some scientists, e.g., Cerlini et al. (2022), argue that existing data series from phenological observations should be treated with caution due to their relatively short duration), examples in the literature demonstrate the undeniable impact of climate change on ecosystems’ seasonality (Walkovszky 1998; Menzel et al. 2020).

This study aims to verify the research hypothesis that climate change resulting from global warming leads to modifications in the phenology of plants, which also means changing the phenological seasons. Thus, this research aims to identify the shifts in phenological seasons in Poland. An attempt is also being made to develop a comprehensive phenological calendar. Thus, this study will analyze phenological seasons during two 15-year periods: the historical period of 1946–1960 and the present period of 2007–2021. The time span covered by the analysis is as long as possible, given the available data. These two 15-year-long data series (due to their length) provide only a preliminary and approximate assessment of changes in the timing of individual phenological seasons. However, their comparison can offer a clearer understanding of climate change over time. The Yearbooks of Phenological Observations from 1946 to 1960 contain highly valuable observational data, enriching our understanding of climatic conditions before the period characterized by rapid global warming. As this material has not been made available to a broader group of researchers, this research will enhance knowledge of Poland’s climate change, including its temporal and spatial variability. At the same time, it will help fill Poland’s blank spot on the map of European phenology.

Methodology

Data

The article examines the variability of the phenological seasons over the past nearly 80 years, based on phenological events recorded during two 15-year observation periods, the historical period 1946–1960 and the present period 2007–2021, at 36 observation stations. An attempt is also being made to develop a phenological calendar for Poland, covering both the past and present periods.

Figure 1 illustrates the locations of 36 observation posts selected for analysis, which are part of the current network of phenological observations, along with their counterparts from 1946 to 1960. The former data are sourced from the Yearbooks of Phenological Observations, which the authors digitized for this study. The latter originates from the database of the Institute of Meteorology and Water Management – National Research Institute (IMGW-PIB). In order to present the thermal conditions during the periods under study and to verify the phenological database available in the printed yearbooks, time series of mean daily air temperature from 1951 to 2021 at 28 meteorological stations across Poland were used. Daily temperature data are available from 1951.This data is also sourced from the Institute’s database.

Fig. 1.

Phenological observation posts 2007–2021 () and their counterparts from 1946–1960 ()

The two 15-year periods covered by the analysis are the longest possible series due to data availability. Phenological observations have been conducted regularly within the meteorological stations network of the IMGW-PIB (formerly National Hydrology-Meteorology Institute) since 1946. Between 1946 and 1960, the Institute published an annual Yearbook of Phenological Observations. However, the number of observation stations gradually decreased over time, and in 1992, the observations were permanently suspended. Since 1961, these yearbooks have not been published, and the observational material for the following period is unavailable. In 2007, the Institute resumed phenological observations. They are now collected and processed.

Phenological observations from the 1940s and 1950s were typically conducted in different locations than those in recent years. Sometimes, indicator species disappear over time in certain locations due to various circumstances, such as changes in land use. The study aims to analyze ongoing climate changes from a regional perspective rather than focusing on changes in specific locations. To achieve this, older observation sites were matched to current stations. The base locations were the stations where observations are currently conducted. To ensure the best possible matching, an area with a 30 km diameter (buffer) was generated around each current station to locate the nearest historical stations within this range. If multiple stations were found within a given buffer (which is often the case), the selection was further refined based on the quantity/completeness of the data series, and the optimal alignment of the phenological pattern with the air temperature pattern at the nearest meteorological station. Only stations that recorded a specific phenological occurrence in at least 10 of the 15 observed years were included. Each event was analyzed separately, as not all stations, both past and present, have conducted and continue to conduct the full range of observations.

Methods

This study applies the phenological calendar method, based on the methodology developed at the Institute, including the selection of plant species and their phenophases serving as indicators of phenological seasons (see the IMGW-PIB website: https://agrometeo.imgw.pl/fenologia/fenologiczne_pory_roku). However, in the case of early spring and autumn, this approach is not fully applicable because between 1947 and 1960, (i) common dandelion was observed in early spring at only a few stations and/or the observation series was incomplete, and (ii) no observations of lime leaf yellowing and leaf fall were made in autumn. Hence, to ensure consistent criteria for determining subsequent seasons in both study periods, and thus enable comparison, the spring observations of common dandelion and the autumn observations of linden were excluded from the analysis. Winter (a period of plant dormancy, during which no phenological events occur) was defined based on a thermal criterion. In this study, it begins when the mean temperature drops below zero degrees Celsius.

The indicator plants used in the analysis are: common hazel (Corylus avellana L.), common coltsfoot (Tussilago farfara L.), silver birch (Betula pendula Roth), bird cherry (Prunus padus L.), horse-chestnut tree (Aesculus hippocastanum L.), lilac (Syringa vulgaris), black locust (Robinia pseudoacacia), small-leaved lime (Tilia cordata Mill.), and common heather (Calluna vulgaris). Specifically, the study uses annual datasets of 12 phenological events (phenophases) from 1946 to 1960 and 2007–2021 to define seven phenological seasons (Table 1). The term “flowering” is defined as the beginning of the flower development, observed when 10% of the flowers are open. The term “leafing” refers to the moment when, in various places on the plant, both halves of the leaf blade spread out to take the form of a normal leaf. The term “fruit ripening” is used to denote the stage at which at least 50% of the fruit is mature. “Leaf color change” and “leaf fall” occur when they apply to more than half of the leaves. It should be noted that the determination of each phenophase always refers to the plant population surrounding the observation station, rather than to a single plant. When two phenological events determine the onset of a season, its start date is calculated as the arithmetic mean of the corresponding occurrence dates.

Table 1.

The phenological events indicators

Season	Very early spring	Early spring	Spring	Early summer	Summer	Early autumn	Autumn
Phenophase	hazel flowering	birch leafing	horse-chestnut flowering	black locust flowering	lime flowering	ripening of horse- chestnut fruit	birch leaf color change
	coltsfoot flowering	bird cherry flowering	lilac flowering			heather flowering	birch leaf fall

Unfortunately, data on individual occurrences do not cover the entire country. Moreover, the number of such occurrences varies depending on the season. To define two seasons (early summer and summer), only one phenological occurrence is needed, whereas for the remaining five, two occurrences at the exact location are required. This significantly limits the amount of available data. Assembling the entire phenological calendar, i.e. the whole sequence of subsequent phenological seasons, was only possible for 10 stations.

This study examines the dates of individual phenological events, including the beginning and end of each phenological season, as well as their duration in each year and across the analyzed 15-year periods. Phenological seasons are discussed within the framework of a so-called “normal year” (for both 15-year periods) and in years characterized by extreme phenological conditions (i.e., weather conditions).

To ensure the reliability and consistency of the analyzed time series from 1946 to 1960, the Standard Normal Homogeneity Test (SNHT) was applied to detect potential inhomogeneities in the data. The SNHT computes the T-statistic for each potential breakpoint in the series, and values exceeding the critical threshold indicate significant inhomogeneities. In this study, due to the scale of the data, T values below 0.6 indicated homogeneous data, while higher values suggested potential breakpoints. At the same time, data quality was verified by comparing the dates of specific occurrences with the course of average air temperature at the nearest synoptic station. Phenological stations that failed the test and/or those with obviously erroneous observations (e.g., when a phenological event indicating the beginning of vegetation was recorded while the average air temperature for the preceding 20 days was below zero) were excluded.

To evaluate whether the differences in the timing of the start of the phenological seasons between the historical period (1946–1960) and the present period (2007–2021) were statistically significant, Welch’s t-test was applied for each location with available data from both periods. The test was selected for its robustness to unequal variances and sample sizes.

In this study, the sequence of phenophases is presented in relation to prevailing thermal conditions, described using monthly average temperatures from 1951 to 2021. The monthly temperatures of both studied periods were compared. In order to determine the direction and statistical significance of long-term changes in thermal conditions, a trend analysis was performed via parametric regression and the Mann–Kendall nonparametric test. The statistical significance was assessed at α = 0.05. The strength of the trends over the multi-year period was determined using the slope coefficients of the linear regression.

QGIS software version 3.20, with the SAGA plugin was used to illustrate the spatial distribution of the occurrence dates of phenological seasons.

Results

The onset of phenological seasons

The temporal variability

Table 2 presents the mean and the absolute extreme onset dates of the seven phenological seasons and winter. A comparison of the two 15-year periods indicates an advancement in six out of the eight seasons. Of the eight seasons analyzed, in seven cases the absolute earliest arrivals occurred in the period 2007–2021 (summer being the exception). In turn, the latest seasons always occurred in the years 1947–1960.

Table 2.

The onset of phenological seasons in Poland in 1946–1960 and 2007–2021 [day number of the year (DoY)]. Data source: the IMGW-PIB database and the Yearbooks of Phenological Observations

	Mean [DoY]			The earliest onsets			The latest onsets
Season	1946–1960	2007–2021	diff	DoY	date	station	DoY	date	station
Very early spring	84.1	73.7	-10.4	37	06.02.2008	Leszno	120	29.04.1951	Krypy
Early spring	121.4	112.5	-8.9	87.5	27.03.2014	Opole	143	22.05.1956	Ustka
Spring	134.6	125.9	-8.7	110.5	19.04.2014	Opole	163.5	11.06.1955	Wygoda
Early summer	158.0	147.1	-10.9	123	02.05.2018	Opole, Sandomierz	191	09.07.1952	Dzikowiec
Summer	181.6	173.5	-8.1	141	20.05.1948	Częstochowa	210	28.07.1955	Dzikowiec
Early autumn	247.8	244.1	-3.7	213.5	31.07.2007	Chojnice	281	07.10.1957	Żupawa
Autumn	288.1	292.3	4.2	243	30.08.2019	Kozienice	324.5	19.11.1958	Żupawa
Winter	321.5	321.3	-0.2	276	02.10.2016	Hel	387	21.01.1951	Hel

The greatest average advancement was observed for very early spring (about 10 days) and early summer (11 days). During the 1946–1960 period, these seasons began on average on March 25 and June 6, respectively. In the more recent period (2007–2021), they began on March 14 and May 27, depending on the location. The onset of vegetation (meaning the beginning of the very early spring) accelerated at 25 out of 26 studied stations. The acceleration of the growing season was not observed only on the coast. The Welch’s t-test was applied to assess whether the observed differences in the mean onset of the very early spring between the years 1947–1960 and 2007–2021 were statistically significant at the 0.05 level, indicating significance in 8 out of the 26 analyzed locations, with the most pronounced change occurring in Zielona Góra. Other stations with statistically significant shifts in the beginning of the very early spring are located in western, southwestern, and southern Poland. Regarding early summer, the t-Welch’s t-test showed that, among the 32 analyzed stations, 17 locations exhibited statistically significant differences (p < 0.05).

On average, the onset of winter proved to be the most stable, showing no change between the two periods. One phenological season – autumn – nowadays begins later than it did from 1946 to 1960. Before 1960, phenological autumn began on October 15, while in recent years it starts, on average, 4 days later, i.e., on October 19. The results of the t-Welch’s t-test showed that, among the 29 analyzed stations, 10 locations exhibited statistically significant differences in the onset of the autumn (p < 0.05). In seven of these, a delay in the autumn onset was observed, while in three, it occurred earlier (i.e. the arrival of autumn was accelerated).

The shifts in the onset of successive seasons described above represent averages for Poland. To obtain a more complete picture of the changes taking place, it is necessary to account for regional differences and for the variability in the timing of seasonal onsets between individual years. Analyses indicate considerable year-to-year variability in the beginnings of the seasons. This variability is particularly pronounced in the first half of the year. From very early spring to summer, adjacent seasons overlap from one year to the next. It is often the case that while some areas are still experiencing very early spring in one year, early spring, or even spring has already arrived in the same period of another year, e.g., 2013 vs. 2018 (see Fig. 2).

Fig. 2.

Phenological season’s onset in all studied locations in 1946–1960 vs. 2007–2021 in [DoY]. Lines represent the mean value. Numbers above 365 indicate cases when winter begins in January.

Source: own study

Both before 1960 and after 2007, individual seasons can be identified that appear to contradict the above findings. There have also been years characterized by exceptionally large shifts in the timing of certain seasons and/or by shifts occurring in directions not necessarily consistent with the overall trends. Examples include the exceptionally early onset of spring in 1953 and the delayed onset in 2021. Interannual variability is also reflected in the differences in the timing of the passage of a given season across Poland. Depending on weather conditions, early summer may sometimes occur almost simultaneously throughout the entire country, whereas in other years its progression may take more than two months (see Fig. 3a and b). Similar situations have occurred in both historical and recent periods and may affect any phenological season.

Fig. 3.

The onset of early summer in 2010 (a), 2021 (b) and 1953 (c).

Source: own study

The spatial variability

Regional variability in the onset of phenological seasons exists. For example, the overall picture of a stable winter (no shift in mean appearance between periods) comprises, at present, areas showing both slight delays and advances in its onset (in a 50/50 ratio). However, while no significant delays are observed in the west and central regions, the northeast, east, and south tend to experience slight delays in the onset of winter.

Similarly, presently the delay in the onset of phenological autumn is observed across most of the country (at 21 of the 29 stations analyzed) and is on average 4 days. The greatest delay is visible in the west, southwest and south of Poland. It appears that the easternmost regions of Poland remain under a stronger influence of continental air masses from the east than other areas, which is reflected in the earlier onset of colder conditions in autumn and winter. Consequently, changes in the timing of the onset of cold seasons in these regions are minimal (Fig. 4a).

Fig. 4.

The onset of autumn (a) and early spring (b) in 1946–1960 vs. 2007–2021.

Source: own study

With respect to multi-year averages, certain directional patterns of the arrival of the seasons can be identified. This is particularly evident for the onset of spring phenophases. They often occur earlier in western and south-western Poland along the Oder River valley (see Fig. 4b as an example). However, the onset of a given season may occur from different directions, depending on the prevailing air masses’ advection. For instance, in early summer 1953, it advanced from the west, whereas in 2010, it arrived from the east (see Fig. 3a vs. 3c).

The climate-moderating effect of the Baltic Sea exists. As a result, along the coast and in the adjacent regions, many phenological seasons arrive later than in other parts of Poland. During the warmer half of the year, particularly when mean air temperatures increase, the Baltic Sea cools the adjacent land areas. Conversely, during the onset of typically low winter temperatures, the proximity of the Baltic Sea exerts a moderating and warming effect.

The duration of the phenological seasons

The demonstrated shifts in the onset of successive phenological seasons also lead to changes in their duration. Based on the average dates of onset of individual seasons in Poland in both 15-year periods, the mean duration of each season was calculated (Table 3). Three seasons are shortened (autumn, winter and spring), while the remaining five (very early spring, early spring, early summer, summer, early autumn) are lengthened. The most pronounced changes in duration were observed for winter and early autumn: winter shortened by 10.2 days, whereas early autumn lengthened by 7.9 days compared with the historical period. The smallest changes in duration were found for the three phenophases of spring, with early spring showing virtually no change in length between the analyzed periods.

Table 3.

The mean for Poland duration of the phenological seasons in [days]. Source: own study

Season	1946-1960	2007-2021
Very early spring	37.3	38.8
Early spring	13.2	13.3
Spring	23.5	21.2
Early summer	23.5	26.4
Summer	66.3	70.6
Early autumn	40.3	48.2
Autumn	33.5	29.1
Winter	127.6	117.4

The phenological calendar

In order to establish the phenological calendar, a complete set of phenological events for all seasons is needed. The absence of even a single event excludes a given station from the analysis. Thus, a complete phenological calendar could be compiled for only 10 observation stations. It, unfortunately, affects spatial coverage, resulting in a considerably lower density of observation points in some areas. At the same time, these stations do not represent all geographic regions. For this reason, the authors decided not to average the results or extrapolate them to the entire country. Instead, the phenological calendar for three stations located along the southwest-northeast axis, namely Opole, Płock, and Suwałki, is presented (Fig. 5). Along the west-east axis, there is a gradation in the dominant influence of air masses, from areas influenced by oceanic masses from the west to those largely influenced by continental masses from the east. Simultaneously, along the axis, there is a gradation in mean annual temperature, from the warmest to the coldest areas.

Fig. 5.

The phenological calendar for three stations along the southwest-northeast axis: Opole-Myślina (a), Płock-Opatówiec (b) and Suwałki-Bargłów (c).

Source: Own study

In Opole, in southwestern Poland, the onset of very early spring now occurs approximately 15 days earlier on average. Subsequent phenophases of spring also begin earlier. Taken together, these phases now last 11.5 days longer than in the historical period, largely due to the lengthening of spring itself (from 15.8 days before 1960 to 28.5 days at present). Early summer now begins only about 3 days earlier, as does early autumn. However, early autumn is currently 11.4 days longer, resulting in an 8-day delay in the onset of autumn. At the same time, autumn is 8 days shorter than it was before 1960, and therefore gives way to winter on the same calendar date in both periods. The duration of winter has shortened by almost 15 days, primarily due to the earlier onset of a very early spring (Fig. 5a).

In Płock, in central Poland, the onset of very early spring now occurs about six days earlier; however, all spring phases taken together last approximately 4 days longer than in the earlier period. Consequently, early summer begins more than 10 days earlier. Summer advances by more than 20 days, mainly due to the shortening of the early summer season. Early autumn now begins 18 days earlier and has lengthened substantially, increasing from 23.2 to 61.3 days. In contrast, autumn has shortened from 48.1 days in the earlier period to 28.7 days at present and begins with a 20-day delay. The onset date of winter remains nearly unchanged between the two analysed periods, while its duration has shortened by more than 6 days (Fig. 5b).

In Suwałki, in northeastern Poland, changes in the onset dates and durations of successive seasons are relatively small. All seasons have advances, with the exception of early autumn. The delay in autumn onset is minor, amounting to 2 days. The most pronounced changes in seasonal duration concern the summer. The entire summer period (early summer and summer combined) has lengthened by more than 11 days, nearly 7 of which are attributable to the proper summer season. Summer phenological phases have both advanced and lengthened at the expense of the spring phases (Fig. 5c).

If general findings are to be drawn, the results indicate that: (i) winter has shortened across all study areas, in the southwest the most; (ii) the shortening of winter is associated with the earlier onset of very early spring; (iii) the duration of spring phenophases has increased in the west and south-west, while toward the north-east it becomes similar in both analyzed periods; (iv) both summer phases as well as early autumn have advanced and lengthened; early autumn in the central part of the country has lengthened the most; (v) autumn is currently shorter and delayed across most regions (except for the north-eastern margins, where early autumn is delayed whereas autumn is not).

The thermal conditions

The period between 1951 and 2021 is characterized by a successive increase in average temperatures, both daily and monthly. For the period 1951–2021, statistically significant increasing trends in monthly temperature (according to the Mann-Kendall test and linear regression, both at the α = 0.05 level) were observed at 25 analyzed meteorological stations from February to September. Significant warming from February to August was detected at the next two stations. The Lesko station (in the southeastern part of the country) was the only exception, with statistically significant (α = 0.05) increases in mean monthly temperatures observed only from March to July. In general, the further west, the more months are characterized by a significant increase in average monthly temperature. Regarding the magnitude of the warming trends, the strongest increases were observed in three months: February, July, and August. The rates vary by region (with the biggest in the West) and amount to 0.50–0.60 °C/10 years, 0.40–0.50 °C/10 years, and 0.45–0.52 °C/10 years, respectively. Table 4 presents the mean monthly temperature in 3 locations: Opole, Płock and Suwałki (corresponding to the 3.3. section), for the period 1951–1960 vs. 2007–2021, and the temperature change per 10 years from 1951 to 2021.

Table 4.

The mean temperature in Opole, Płock and Suwałki in 1951–1960 vs. 2007–2021, and the temperature change (1951–2021) per 10 years. Source: own study

Month	Opole			Płock			Suwałki
	1951-1960 [°C]	2007-2021 [°C]	change/10yy 1951-2021	1951-1960 [°C]	2007-2021 [°C]	change/10yy 1951-2021	1951-1960 [°C]	2007-2021 [°C]	change/10yy 1951-2021
January	-1.4	-0.3	0.397	-2.2	-1.4	0.323	-4.5	-3.7	0.396
February	-2.7	0.8	0.532	-3.5	-0.5	0.528	-6.1	-2.8	0.635
March	1.5	4.4	0.415	0.9	3.6	0.405	-2.8	1.4	0.611
April	6.8	9.8	0.391	6.6	9.0	0.320	4.8	7.4	0.402
May	12.1	14.2	0.397	12.2	13.9	0.260	10.8	13.0	0.306
June	16.2	18.7	0.432	16.7	18.0	0.172	15.4	17.1	0.268
July	17.9	20.3	0.505	18.4	19.7	0.276	17.0	18.7	0.384
August	17.2	19.7	0.489	17.3	19.0	0.359	16.1	17.6	0.342
September	13.1	14.5	0.201	12.8	13.9	0.148	11.2	12.6	0.171
October	8.1	9.4	0.253	7.7	8.6	0.173	6.4	6.9	0.126
November	3.1	5.6	0.327	2.8	4.6	0.211	1.1	3.2	0.282
December	1.1	1.4	0.225	0.6	0.7	0.203	-1.9	1.0	0.268

Statistically significant trends in bold

Discussions

Our research points at progressive changes in the phenological timing of indicator plant species in Poland. These changes are reflected in shifts in the timing of successive phenological seasons during the recent period (2001–2021) compared with the earlier reference period (1946–1960), as well as changes in the duration of individual phenological seasons. The most significant changes include a shortening of winter resulting from an earlier onset of very early spring, an advancement of subsequent seasons up to early autumn, a delayed onset of autumn, and a temporally stable onset of winter persisting over multiple decades. With regard to the seasons’ length, in addition to the shortening of winter, an extension of the warmest seasons, i.e., namely early summer, summer, and early autumn, has been observed. Parallel studies conducted in Poland and across Europe corroborate these findings.

Based on the monthly temperature analysis conducted in the present study, the most rapid temperature increases occur before the onset of vegetation. These changes drive earlier starts to the growing season. In fact, air temperature during the two to three months preceding a phenological phase acts as a primary environmental cue, determining the progression of phenological events (e.g., Vogel 2022).

An earlier onset of vegetation (very early spring) is observed across Europe. It is consistent with an accelerated and lengthened growing season. On a European scale, similar findings have been reported based on phenological, satellite, and climatological studies, e.g., by Menzel and Fabian (1999), Ahas et al. (2000), Stockli and Vidale (2004), Chmielewski et al. (2005), Linderholm et al. (2008), Aalto et al. (2022), Kollo et al. (2023). It is estimated that a 1 °C increase in mean annual air temperature results in a 5-day extension of the growing season (Chmielewski and Rötzer 2002). In Central Europe, hazel begins its pollination phase with a trend value of -0.28 days per year (Scheifinger et al. 2003). In Poland, the acceleration of vegetation was reported by: Chojnacka-Ożga and Ożga (2018), Szyga-Pluta et al. (2023), and Szwed and Holka (2024), etc. Our findings show comparable seasonal variability, expressed as the mean advance in the onset of very early spring, amounting to -10.4 days from 2007 to 2021, relative to 1946–1960.

An advancement in the flowering period of plant species has been identified across Europe, with pronounced differences across biogeographic regions (Templ et al. 2017). Regarding the onset of early spring, the results of this study indicate a shift in the average date of early spring from May 1 before 1960 to April 22 after 2007. These findings are comparable to a detailed analysis of the birch flowering period carried out in the Czech Republic using the growing degree days (GDD) method (Hájková et al. 2020). Other studies of wild plants such as hazel and lime conducted in the Czech Republic have shown that wild species exhibit an acceleration of spring phenophases by an average of 2.5–3.1 days per decade (Bartošová et al. 2025). A study from Germany on the phenophases of sweet cherry, which flowers during this period of the year, described the trend observed for the period 1961–2000, indicating a shift of -2.0 days per decade (Chmielewski et al. 2004).

A species selection similar to that used in the present study was applied in an analysis of several plant species in Hungary, including lilac, black locust, and lime (Szabó et al. 2016). According to Szabó et al. (2016), most of the observed plant species exhibited markedly earlier flowering, with an average rate of 1.9–4.4 days per decade. These findings are consistent with those reported in our study, which showed an acceleration in flowering among species classified as indicators of spring, early summer, and summer. The acceleration we observed ranged from 8.1 to 10.9 days during the period 2007–2021, compared with the period 1946–1960.

Our results concerning the shift in the onset of early summer, determined based on black locust flowering, showed the same direction of change but a stronger negative trend compared to studies conducted in Hungary between 1851 and 1994 (Walkovszky 1998). However, the most recent Hungarian research reported a markedly stronger advancement in the flowering of this species, with a shift of 3.8–9.6 days earlier per decade (Templ et al. 2017).

Summer currently arrives about 8 days earlier compared with the historical period of 1946–1960 based on our study. Other European studies report results similar to those obtained in the present work. For example, a comparable but stronger advancement of summer onset, as determined by lime flowering, has been demonstrated in Hungary, with rates ranging from 2.8 to 4.4 days per decade (Szabó et al. 2016). Studies by Weryszko-Chmielewska et al. (2019) conducted in eastern Poland, demonstrated an advancement of this phenophase by 14 days over an 18-year observation period. Likewise, Templ et al. (2017) reported acceleration of summer onset in Central Europe ranging from 3.2 to 9.5 days per decade.

This study showed an average advancement of early autumn in Poland by 3.7 days and a delay of autumn by 4.2 days. As Chmielewski and Rötzer (2001) argue, the end of the growing season (meaning the onset of autumn) is characterized by low variability across Europe. The stability of autumn temperatures and lower variability during this season are also highlighted in numerous scientific publications on climate, e.g., Szwed and Wasielewska (2024); Chojnacka-Ożga and Ożga (2018). Our findings lead to similar conclusions.

The currently higher summer temperatures (July–August) likely contribute to the delayed onset and prolongation of autumn, as demonstrated in this study. However, extremely high summer temperatures — which can induce physiological stress in plants — have recently occurred with increasing frequency. Cammarano et al. (2022) reported that excessive heat — specifically, when temperatures exceed the plant physiological tolerance threshold — can cause a temporary slowdown in vegetative growth.

According to our research, winter, the period of plant dormancy, consistently begins around November 17, but is currently approximately 10 days shorter than during the earlier reference period (1946–1960). These observations corroborate climatic studies. The progressive warming is most noticeable during the cold season, as evidenced by the shortening of the thermal winter period (Czarnecka and Niezgórska-Lencewicz 2017; Czernecki and Miętus 2015).

This study also reveals an extension of the warmest phenological seasons, namely early summer, summer, and early autumn. In the context of previous research, such a result is to be expected. Numerous studies indicate that the course of the seasons and their characteristics are changing in response to global warming, with all seasons becoming warmer. Autumn exhibits the greatest thermal stability, whereas the most pronounced temperature increases are observed in winter and spring, amounting to approximately 0.3 to 0.45 °C per decade over time (Szwed and Wasielewska 2024). Thus, thermal conditions characterized for the warmest phenological seasons persist recently for a longer period. Similar conclusions have been reported by, among others, Owczarek and Filipiak 2016; Ustrnul et al. 2021; and Kejna and Pospieszyńska 2023.

Conclusion

In summary, our research, in line with other Polish and European studies, demonstrates ongoing changes in the timing and duration of individual phenological seasons. The most significant changes include a marked advancement of phenophases in the first half of the year, a slight delay in the onset of autumn, and a stable onset of winter. At the same time, winter is shortening, while the warmest seasons, from early summer to early autumn, are lengthening. However, the above general statements pertain to average conditions. Due to its location in the transitional temperate climate zone, Poland’s climate is characterized by high temporal variability, i.e. weather variability from day to day, season to season, or between years. The transitional nature of Poland’s climate is also evident in the climatic differences between regions.

In conclusion, the observed shifts in phenological observations/phenophases are consistent with climate warming. Long-term changes in average temperatures seem to confirm this. Nevertheless, it should be noted that global climate change does not necessarily provide a comprehensive explanation for all observed changes in plant phenology. Climate change encompasses more than just global changes, changes strictly related to atmospheric physics or ocean thermodynamics. The phenomenon of climate change is experienced every day at a variety of spatial scales. Every change introduced to the environment—a different land cover, a change in land use, or even a single element, such as a small water body or a new road—affects local climatic conditions. If we were unable to modify the climate, even locally, all mitigation and adaptation strategies and actions to climate change would make no sense. Our analysis is regional in scope; the local changes—or those occurring in smaller areas—are taking place across the entire country, both currently (during the present study period of 2007–2021) and in the past (1947–1960). These changes fit into the worldwide pattern of a changing climate, and the observed shifts in plant phenology result from climate change at all scales.

The authors are aware of the study’s numerous shortcomings. Relatively short data series, the selection of observation stations, and particularly the matching of current stations to historical ones may raise additional questions related to the applied methodology. It is possible that, despite meticulous selection of stations, certain components of the results may be erroneous due to non-identical contemporary and historical stations. The short 15-year phenological data series limit the use of statistical methods for trend analysis and, consequently, the ability to draw conclusions about the significance of observed changes in the timing of the season cut-off dates. Nevertheless, in the authors’ opinion, the proposed approach provides a solid basis for selecting series suitable for variability analyses and is generally considered sufficient for phenological studies at a regional scale. Despite the limited data from phenological observation sites, phenological studies are gaining importance and complement climate studies by emphasizing seasonal variability. Therefore, considering the importance of the issue, even if the presented results are treated as illustrative or preliminary, they provide valuable insights into the study on “recognizing climate change”.

The role of phenological research cannot be overstated, not only in complementing and strengthening strictly climatological studies on climate variability (currently focused primarily on climate change), but also due to its economic and social relevance. The potential value of further phenological research is also evident in the context of complex relationships among individual plant and animal species across different ecosystems, as well as for society, not only with regard to food production, but also to human health, for example, through studies on the variability of allergy seasons.

Abbreviations

SNHT

Standard Normal Homogeneity Test

DoY

Day number of the year

Authors’ contributions

Małgorzata Szwed conceived the study, gathered the data, led the analyses, created the figures, and wrote the first draft of the manuscript, which was edited and revised by all authors. Michał Lewicki made the figures and contributed to the interpretation, analyses and discussion of the results. All authors reviewed and approved the final version of the manuscript.

Funding

The authors declare that no additional funds, grants, or other support were received for the preparation of this manuscript, and that no external funding was received for this research.

Data availability

The phenological data that support the findings of this study stem from the database of the Institute of Meteorology and Water Management–National Research Institute (IMGW-PIB), and they are not publicly available. Time series of daily mean temperature are available in the IMGW-PIB Public Data Repository at https://dane.imgw.pl/.

Declarations

Ethics approval

Not applicable.

Consent for participation and publication

Not applicable.

Competing interests

The authors declare no conflicts of interest.