*4.3. Drought Risk for Beech in the Polish Carpathians*

The forest drought indices used in the study are based on air temperature and precipitation (EQ, AI and FAI) or on air temperature only (MT). They were calculated for all stations except Kasprowy Wierch, as that station is located far above the tree line. For EQ, FAI and AI, no statistically significant trend in the study period was found for any station, neither by regression analysis nor with the Mann–Kendall test (Appendix B). The indices show high spatial and temporal variability (Figure 9). The Carpathian foreland and foothills, together with basins located about 200–400 m a.s.l., are the areas where, in each decade, there are years with forest drought conditions, and at some stations (i.e., in Katowice, Kraków, Nowy S ˛acz—TPZ 4) an increasing tendency can be observed. In the remaining part of the study area, drought conditions do not occur at all or they occur only sporadically.

**Figure 8.** Percentage of dry months according to SPEI (SPEI ≤ −0.8) for 1-, 3- and 6-monthly timescales, for the whole year (**a**) and for the subperiods May–October (**b**) and November–April (**c**) in the decades of the study period. Stations are ordered following the concept of TPZ explained in Section 4.1. Explanation of numbers on axis x: 1—1991–2000, 1 month; 2—2001–2010, 1 month; 3—2011–2020, 1 month; 4—1991–2000, 3 months; 5—2001–2010, 3 months; 6—2011–2020, 3 months; 7—1991–2000, 6 months; 8—2001–2010, 6 months; 9—2011–2020 6 months.

**Figure 9.** The number of years in specific decades when forest drought conditions unfavorable for beech occurred at the stations studied: (**a**) EQ > 30; (**b**) FAI > 4.75; (**c**) AI < 35. Stations are ordered following the concept of TPZ explained in Section 4.1.

Unlike the indices shown in Figure 9, MT has a statistically significant increasing trend at all stations (*p* < 0.05) at the rate from 0.4 to 0.5 per decade; only for Limanowa did the value reach 0.6 per 10 years. The Mann–Kendall test confirms those results (Appendix B). The threshold value of 18 ◦C, above which the conditions for beech are estimated to be unfavorable, was crossed for the first time in 2002 at all stations in the zone of the Carpathian foreland and foothills, together with basins located about 200–400 m a.s.l. (TPZ 4), except for Bielsko-Biała. In the second decade, the value was crossed in three years in the zone mentioned, and in the third decade for five years. In 2018, in Katowice and Kraków, the index value reached 19 ◦C (Figure 10).

**Figure 10.** Values of the Mayr Tetratherm Index for particular TPZs and stations in the period 1991–2020, together with linear trends. See Appendix D for regression equations and the values of R2. Stations are ordered following the concept of TPZ explained in Section 4.1.

#### **5. Discussion**

The analyses presented above, based on the application of various indices, show that atmospheric drought risk has increased in the study area, and the main reason is increasing air temperature. It is recommended to study the drought issue with a variety of methods as in many studies, solely precipitation-based indices show only minor changes in drought

occurrence, whereas other indices that consider evapotranspiration indicate a significant increase in the area under drought [52]. The results presented for air temperature and precipitation series confirm earlier findings for the Polish Carpathians that there has been significant warming in the area, particularly over recent decades. Climate change is most evident in the foothills; however, it is the highest summits that have experienced the most intensive increases in temperature during the recent period. Precipitation does not demonstrate any substantial trend and has high year-to-year variability. The distribution of annual temperature provides evidence of the upward shift of vertical climate zones in the Polish Carpathians, reaching approximately 350 m, on average, which indicates further ecological consequences [53]. The absence of statistically significant trends in precipitation is in accordance with the results for Czechia, Slovakia and Austria [54] where the main driver of drought is an increase in the evaporative demand of the atmosphere, driven by higher temperatures and global radiation with limited changes to precipitation totals. However, the observed drying trends were most pronounced there during the April−September period and at lower elevations. Conversely, the majority of stations above 1000 m exhibited a significant wetting trend for both the summer and winter (October−March) half-years. The part of the Carpathians included in that study is located on the southern side of the Western Carpathian chain, while the Polish Carpathians, considered in the present paper, are located on the northern side. Therefore, the climatic factors, for example, atmospheric circulation impacts are different for these regions and this is then visible in the values for climatic elements. Conditions for areas above 1000 m a.s.l. can be estimated on the northern side with the data from Kasprowy Wierch only, and they do not show a wetting trend but an increase in drought risk. For stations located at lower altitudes, according to indices based on precipitation only, drought frequency was highest in the warm half-year in the first decade only, while in the last decade it was most frequent in the cold half-year. However, the Selianinov index, based on both air temperature and precipitation, and calculated for the warm half-year, showed that the last decade experienced drought much more often than previous decades, which is in accordance with the results presented in [54]. SPEI is also based on both air temperature and precipitation, but it has been calculated for the whole year, and as shown in Figure 8, the frequency of dry months increased a lot in the last decade all over the study area. The increase was larger in the cold half-year than in the warm one; at some stations, it was more than double in comparison to previous decades. In the study for the whole Carpathian region [5], there are no differences among the mountains and lowlands shown but the general trend confirms an increase in drought frequency. For the Alpine region [55], drought impacts were studied and turned out to be most pronounced in the warm half-year, while the high-altitude region showed this effect the most. The Polish Carpathians can also be compared to the remainder of Poland. Areas classified as dry increased their surface area from 13% in the period 1931–1960 to 20% in the 30-year period of 1971–2000, and 46% in the 30-year period of 1981–2010. However, the northern and western regions of Poland are more endangered by drought than the mountains where the precipitation is always higher [56].

It can be concluded that the present paper shows the frequency and trends of drought in the Polish Carpathians in the most recent 30 years, that is, in the period marked by global warming. The results obtained are in accordance with the outcomes of other studies concerning drought in Central Europe and in the Central European mountains, but they also show some new aspects which have not been analyzed so far. There is no drought frequency variability in the W-E profile. The stations representing the easternmost part of the Polish Carpathians, Koma ´ncza and Lesko, belong to two different TPZs (2 and 3, respectively), and the indices values obtained show that drought risk in that region is similar to that in the remaining part of the study area. However, there is a high impact of local environmental conditions on spatial patterns of precipitation. Unlike the indices based on precipitation only, the index based on both precipitation and air temperature has shown a clear increase in drought risk with decrease in altitude. Additionally, for the highest parts of the mountains represented by Kasprowy Wierch, there are clear indications of increasing

drought risk, so both TPZ 1 and TPZ 4 should be considered most endangered with the increase of drought risk due to ongoing air temperature increase. In the Polish Carpathians, agriculture is not the dominating sector, due to more unfavorable environmental conditions than in the lowland part of the country. Much more important is tourism for which the state of the natural environment, including forests, is one of the key factors. Analyses of the indices describing the conditions for beech forest have shown that the zone of the Carpathian foreland and foothills, together with basins located about 200–400 m a.s.l., is the part of the Polish Carpathians where conditions are already unfavorable and are worsening most rapidly. According to [57], Carpathian ecosystems located in water-limited environments of lowland to foothill areas can be particularly exposed to climate change, and the Tatras are climate change hotspots. As the vertical climate-vegetation zones are shifting due to constant warming, it can be expected that the deterioration of climatic conditions for beech will appear at higher altitudes. Other studies show that in the Carpathian Basin, beech has already reached its xeric limit on many sites [58]. Beech total yield production in the Western Carpathians was recently found to be lower by −11% on average compared to beech forests in Central Europe (Germany) [59]. The extraordinary drought and heat in the summers of 2018 and 2019 have demonstrated the climatic vulnerability of European beech in many parts of its Central European distribution range. At its southern and southeastern range edges, beech is most likely limited by summer drought and probably also by heat [60]. In the high-mountain zone of the Carpathians, populations of cold-adapted species are very vulnerable to climate change, while their habitats tend to shrink. The climate-driven decrease of snow cover often leads to frost damage to vegetation that provides gaps appropriate for the establishment of many rare species [61]. Most probably, many species of conservation concern will irreversibly disappear from the regional flora under the ongoing climate change [62]. The increase of drought risk in the highest parts of the Polish Western Carpathians, shown in the present paper, is another factor that can contribute to those negative processes. In Europe, a strong spatial pattern in the beech growth responses to summer temperature and to drought was found; radial growth of the species generally did not respond to summer drought in Central Europe (Germany, Slovakia and Romania), but it became highly responsive in the Balkan Peninsula (Bosnia and Herzegovina). However, beech shows a wide variety of growth patterns driven by several factors, and beech growth has been declining over the last two decades [63]. The results shown in the present paper suggest that the drought risk has been increasing during the last 30 years in the Polish Western Carpathians, especially in the foothills zone, so we can expect the beech forests growing there to be effects and even decline in the next decades.

It should be mentioned that the trends from observed data might, for two main reasons, not be suitable for extrapolation into the future [64,65]. First, they could be related to climate variability and not too persistent changes over time. Second, an investigated trend depends on the observation period, so it could differ if the observation period was extended.

**Author Contributions:** Conceptualization, A.B., M.K., P.K. and W.K.; methodology, A.B., M.K. and P.K.; software, not applicable; validation, M.K., P.K. and W.K.; formal analysis, A.B., M.K. and P.K.; investigation, A.B., M.K. and P.K.; resources, M.K., P.K. and W.K.; data curation, M.K., P.K.; writing—original draft preparation, A.B., M.K. and P.K.; writing—review and editing, A.B., M.K. and P.K.; visualization, A.B., M.K., P.K.; supervision, A.B.; project administration, not applicable; funding acquisition, not applicable. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors wish to thank the reviewers for their valuable comments and suggestions, and Piotr Sekuła, for the assistance in SPEI calculations and preparation of Figures 2 and 3.

**Conflicts of Interest:** The authors declare no conflict of interest.
