Silva Balcanica, 20(3)/2019

DENDROCHRONOLOGY OF 358-YEARS-OLD EUROPEAN BEECH (FAGUS SYLVATICA L.) FOREST STAND FROM TREE-LINE ZONE OF THE BALKAN RANGE MOUNTAIN, BULGARIA

Dimitar Petrov Dimitrov Forest Research Institute - Sofia, Bulgarian Academy of Sciences

Abstract

The impact of globally changing climate conditions on monodominant tree-line beech forests in the Balkan Mts., Bulgaria, was studied through the application of the dendrochronological method. Nineteen European beech (Fagus sylvatica L.) cross-section stem discs from an elevation of 1550 m a. s. l. in the Etropole Region were dated and measured. It was established that the age of the sampled beech trees varied from 177 to 358 years. Increment series were calibrated to temperatures and precipitations, using climate data from the hydrometeorological station Petrohan, situated at approximately the same elevation in the mountains as the sampled plot. The applied multiple regression analysis showed that the influence of the temperatures in the boundary zone of the vertical distribution of the European beech in Balkan Mts. is of higher significance for the formation of the tree-ring width (R²=0.56), as compared to the precipitations (R²=0.36). Application of response function analysis indicated that the high temperatures during the summer months June and July have negative influence on the annual radial increment. High temperatures in May and August have positive effect on tree-ring formation.

Key words: dendrochronology, European beech, Fagus sylvatica L., climatology, tree-line zone, Balkan Range Mountains

INRODUCTION

According to UN-ECE/FAO (2000), 16.7% of the forest territories in Bulgaria are covered by European beech (Fagus sylvatica L.). Apart from the other mountains in Bulgaria (Rila, Pirin, Rodope, Vitosha, Sredna gora, Osogovo, etc. Mountains) European beech is a widespread species in the Balkan Mountains and forms vast monodominant forest stands. The distribution of the beech forests on the northern slopes of the Balkan Mts. starts from elevations as low as 400 m a.s.l. and reaches up to the mountain tree-line which varies from 1500 to 1600 m a.s.l.. Due to extensive cuttings in 70ies and 80ies of the 20 th century, considerable part of the old-growth beech forests have been turned into young beech stands. Nevertheless, the primary old-growth beech forest exists and is still very well preserved in core areas (forest reserves) in the Central Balkan National Park or in State Forestry Units situated predominantly at high elevations on steep and difficult to reach locations in the mountains. Recently, even high-elevation century-old beach forests are reachable for harvesting out of the protected forest areas using cable logging timber extraction techniques. Such cuttings provide opportunities for dendrochronology field sampling of the fallen beech trees.

19 At the end of 20th and the beginning of 21st centuries, several studies concerning beech dendrochronology were published in Bulgaria. The main goal of the publications was to study “climate - annual increment” relationship of beech forests in the West (Dimitrov et al., 2003; Mirtchev, 2012) and Central (Georgiev, Raev, 1983; Mirtchev et al., 2003) Balkan Mountains. Despite these publications, the full elevation spectrum of beech forests was not studied. Special focus on high mountain stands is necessary in order to expand our knowledge on beech reaction to climate factors in the tree-line zone. The main purpose of this study is to build a tree-ring chronology of a high- elevation old-growth beech forest and to explore the driving climate factors for tree-ring formation in Fagus sylvatica L. from the tree-line zone in the Balkan Range Mountains.

MATERIALS AND METHODS Sampling area We studied beech trees in a high-elevation harvesting field of an old-growth, non- managed forest on the territory of the Etropole State Forestry Unit, Balkan Mountains, Bulgaria. Using a chainsaw, nineteen cross-sections were cut from the base of the trunks (40 – 50 cm above ground) from 19 beech trees. The elevation of the sampled monodominant beech stand is 1550 m above sea level and the geographical coordinates of the location are: 42°45’46.95” N; 23°59’23.10” E. The studied terrain is facing north. Climate peculiarities According to the Environmental Stratification of Europe (Metzgeret at al., 2005), the study area belongs to the continental environmental zone. We used temperature and precipitation data from the nearest mountain local climate station “Petrohan hut” in order to assess the climate conditions and to describe the climate-growth relationship of dendro series of Fagus sylvatica. The elevation of the station is 1480 m above sea level and it is located at about 80 km air distance north-west from the study area in the Balkan Mountains. The climate diagram for the region of “Petrohan hut” (Fig. 1) showed that the annual precipitation amounted to over 1200 mm and the mean annual temperature was 4.6°C. During four months of the year – December, January, February and March, the mean monthly temperature was below 0°C. The maximum of the precipitations was recorded in late spring and at the beginning of the summer (May, June), while the rainfall minimum was in late summer and early autumn (August, September). Chronology building The cross sections of all sampled beech trees were visually dated and crossdated following the rules of Stokes and Smiley (1968). Statistical validation of visual dating was performed using COFECHA software (Holmes, 1983). Tree-rings width was measured (one radii per tree) with LINTAB 5 measuring station, utilised under the TSAP Win program (Rinn, 2005). Individual raw chronologies were detrended by applying cubic smoothing splines with a 50% frequency response cut-off at 38 years (Cook, Kairiukstis, 1990). Index curves of all sampled trees were built as a deviation of measured tree-ring

20 Fig. 1. Walter, Lieth (1973) climate diagram for the “Petrohan hut” climate station, Balkan Mountains, Bulgaria values to the calculated values obtained after fitting of cubic smoothing spline model to the raw chronology. Mean chronology building and calculation of basic statistical parameters of the tree-ring series were performed with ARSTAN (Cook at al., 2006). Climate-growth relationship The signal strength quality of the series was assessed by means of 0.85 threshold of the expressed population signal (EPS) (Wigley et al., 1984). The “climate – growth” relationships were analysed using response function analyses (Fritts, 1976; Cook, Kairiukstis, 1990). For the multiple regression calculations, climate data for the period 1951-2000 were used.

RESULTS AND DISCUSSION Characteristic of the chronology The length of the tree-ring mean chronology of Fagus sylvatica L. at a high elevation in an old-growth forest in the Balkan Mountains was 358 years and covered the period from 1647 to 2004 (Fig 2. A, C). The age of the trees constructing the chronology varied in a wide range. Only three out of the total number of 19 beech trees were more than 300 years old. Although the youngest tree, included in the mean chronology, was twice as young as the oldest one, it was still more than one century old (177 years) and covered the period 1827-2004. For Bulgaria, this is the oldest beech chronology published to this moment and is one of the oldest in Europe. Piovesan et al. (2003) reported the longest tree- ring series of 486 years for the period 1516-2001 for the high-elevation old-growth beech forest in central Italy. Even older (more than 358 years) beech trees were found within the sampled materials but they were not included in the mean chronology. This was because of the uncertainty in dating them due to unknown number of missing rings “formed” between

21 the very narrow, compression wood (or dense wood) tree-rings in the area around the pit of the trunk during the first 100 or more years of the tree life. Therefore, tree-rings from dense wood were omitted from the sample measurements. Similar observations and pattern of work were published by Biondi (1993), Piovesan at al. (2003) and others. Beside this, an unavoidable obstacle to determine the exact age of the beech trees is the rotten central part of some of the cross-sections due to fungi causing wood decay. The century-old trees in the old-growth forests have reduced vitality status and naturally decaying processes are evident in the internal area of the logs. In that case it is not possible to distinguish tree-rings because the inner part of the xylem is destroyed by fungi. In that case only tree rings in between the area of the bark and the beginning of the destroyed xylem of the trees were measured. The statistical parameters of the “Etropole” mean row chronology were as follows: standard deviation - 0.596; mean sensitivity - 0.394; mean width - 1.107 mm; maximum value of EPS – 0.89. The highest annual increment was measured for 1951 and was 2.01 mm and the narrowest tree-ring was formed in 1702 and was 0.2 mm wide. Climate-growth relations In accordance with the definition for contemporary climate following the IPCC (2014), the calibration process of the mean chronology was performed for the climate window 1951-2000. The EPS signal above 0.85 was obtained for all 19 constituted chronologies after 1875 (Fig. 3). The tree response to climate variation was quantified based on the index chronologies (Fig. 2 B). In total, 30 predictor values were used to quantify the dependant value of the annual increment. Fifteen values for the mean monthly precipitation and 15 temperature values, including the monthly amount of rainfall and the average monthly temperature from July of the preceding year to September of the current growing season, have been used.

Fig. 2. Chronology of a high-elevation old-growth forest in Etropole forest district, Balkan Mountains, Bulgaria. Explanation notes: A – mean chronology of tree-ring width (TRW); B – mean standardised chronology; C – number of trees

22 Fig. 3. Variation of EPS signal among the years

Fig. 4. Regression coefficients (β) of multiple regression analyses for Etropole high-elevation old- growth forest, Balkan Mountains, Bulgaria

The coefficients of determination2 R obtained from the multiple regression analyses, which were conducted separately over the temperature and over the precipitation predictors, showed that the temperatures had higher significance for the formation of the tree-rings widths (R² = 0.56), compared to precipitations (R² = 0.36) in high-elevation old-growth beech forests, Balkan Mountains. The regression coefficients were used to describe the strength and the direction of the “climate- growth” relationships (Fig. 4). The climate conditions during the spring months April and May control the start of the vegetation season. April temperatures affected negatively beech xylem formation. Possible explanation for this could be searched in two directions. First, high temperature in April could result in sudden snow melting and losses of soil water supply due to big above-ground water flow of the melted water. Second, high temperatures could create conditions for early leaf bursts which are sensitive to the late frost events (Dittmar, Elling,

23 2006). The positive correlation with May temperatures is connected with the successful beginning of the vegetation season. In the summer, Fagus sylvatica have negative reaction to the high temperatures and positive to the high precipitations in June and July. This observation is in line with the studies revealing the negative reaction of beech to temperatures and the positive reaction to precipitation in the summer periods within the wide distribution area of beech forests in Europe (Roibu et al., 2017; Cufar et al., 2008; Di Filippo et al., 2007; Jump et al., 2006; Piovesan et al., 2005; Dittmar et al., 2003; Dimitrov et al., 2003; Rozas, 2001; Biondi, 1993). The regression coefficients for the previous July show negative sensitivity of the beech to the high temperatures in the Etropole high-elevation forest. Similar negative response to the temperatures of previous July was observed throughout Europe (Di Filippo et al., 2007; Kern, Popa, 2007; Dittmar et al., 2003; Rozas, 2001). Lebourgeois et al. (2005) explains that pattern with the depletion of carbohydrate reserves due to high rates of evapotranspiration, which is difficult to be accumulated during the next year. It has been demonstrated that beech trees need more than one year to overcome severe summer drought stress (Bolte et al., 2007; Granier et al., 2007) The specific regional behaviour of the studied high-elevation stand in Balkan Mountains is evident from its positive reaction to temperature in August and negative to precipitation in August. This finding is in direct connection with the end of the vegetation season, which at this elevation could be stopped due to cool temperatures in August or prolonged by high temperatures during the same month. Favourable conditions in August lead to the formation of wide tree-rings of Fagus sylvatica L. in the high-elevation forest in the State Forestry Unit Etropole, Balkan Mountains. Monthly winter precipitation for December, January, February and March have positive correlations with tree-ring width. At high elevations, European beech likely has bigger dependence on winter precipitations than on summer rainfalls. Respectively, beech forests at high elevations may suffer from winter water deficit (winter drought).

CONCLUSIONS

A 358-year (from 1647 to 2004) tree-ring chronology of high-elevation Fagus sylvatica L. old-growth forest in the Balkan Mountains was built. Dendroclimatological analysis revealed a general climate response pattern of high-elevation European beech stand: negative relationship with summer temperatures - June and July; positive with spring and late summer temperatures - May and August and positive with winter precipitations – December, January, February and March. The temperatures have higher significance for the formation of the tree-rings widths based on the coefficient of determination R² = 0.56, as compared to precipitations (R² = 0.36).

24 REFERENCES

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