5 Les/Wood, Vol. 69, No. 1, June 2020 1 INT R ODUC TION 1 UVOD Phenology is the study of cyclic and seasonal natural phenomena, especially in relation to climate and plant (or animal) life. In forest trees, we often observe the phenology of leaf development, wood and phloem production, cambium activity, or a com- bination of these (e.g. Prislan et al., 2013b; Žust, 2015; Gričar et al., 2017). Leaf phenology of forest trees is often studied in European beech (Fagus sylvatica) (e.g., Dittmar & Elling 2006; Donnelly et UDK 630*181.8:630:176.1 Fagus sylvatica Original scientific article / Izvirni znanstveni članek Received / Prispelo: 27. 5. 2020 Accepted / Sprejeto: 7. 6. 2020 Vol. 69, No. 1, 5-19 DOI: https://doi.org/10.26614/les-wood.2020.v69n01a07 Abstract / Izvleček Abstract: In spring of 2020 we observed leaf phenology in mature European beech (Fagus sylvatica) trees in Tivoli, Rožnik and Šišenski hrib Landscape Park in Ljubljana, Slovenia (46.05°N, 14.49°E, 310 m a. s. l.). A group of 11 trees was selected for daily monitoring of leaf development. We recorded seven phases from dormant buds, through leaf unfolding till development of mature leaves. At the same time, photos were taken to illustrate the leaf development. First developing leaves were observed on 4 April in one tree. General leaf unfolding, as defined by International Phe- nological Gardens (IPG), was observed in different trees between 7 and 25 April 2020. The occurrence and duration of individual phases of leaf development showed great variation within and between trees. General leaf unfolding of the tree included in the long-term monitoring program of the Environmental Agency of the Republic of Slovenia (ARSO) occurred on DOY 105 (14 April 2020), which is 4 days earlier than long-term average of the same tree/location, and this is ascribed to above average temperatures in the winter of 2019/2020. Keywords: phenology, leaf development, variability, European beech (Fagus sylvatica), Slovenia Izvleček: V Krajinskem parku Tivoli, Rožnik in Šišenski hrib v Ljubljani (46,05°S, 14,49°V, 310 m n. m.) smo spomladi leta 2020 opazovali fenologijo razvoja listov odraslih dreves navadne bukve (Fagus sylvatica). Izbrali smo skupino enajstih dreves za dnevno opazovanje, beleženje in fotografiranje razvoja listov. Razlikovali smo sedem faz od mirujo- čih brstov, preko različnih faz olistanja do zrelih listov. Prve razvijajoče se liste smo na enem drevesu opazili 4. aprila 2020. Fazo splošnega olistanja, kot jo določa metodologija Mednarodnih fenoloških vrtov, smo pri različnih drevesih beležili od 7. do 25. aprila 2020. Pri nastopu in trajanju različnih faz razvoja listov smo zabeležili velike razlike znotraj posameznega drevesa in med drevesi. Splošno olistanje pri drevesu, ki ga dolgotrajno opazuje Agencija RS za okolje (ARSO), je nastopilo na zaporedni dan 105 (14. aprila 2020), kar je 4 dni prej kot kaže dolgoletno povprečje za isto drevo/lokacijo. Slednje pripisujemo nadpovprečno visokim temperaturam v zimi 2019/2020. Ključne besede: fenologija, razvoj listov, variabilnost, bukev (Fagus sylvatica), Slovenija PHENOL OG Y OF LE AF DEVEL OPMENT IN EUR OPE AN BEE CH (FAGUS SYLVATICA) ON A SITE IN LJUBLJANA, SLOVENIA IN 2020 FENOL OGIJ A RAZV O J A LIS T O V NA V ADNE BUKVE (FAGUS SYLVATICA) NA RAS TIŠČU V L JUBL J ANI V LETU 2020 Nina Škrk 1* , Zalika Črepinšek 2 , Katarina Čufar 1 1 Univerza v Ljubljani, Biotehniška fakulteta, Oddelek za lesar- stvo, Jamnikarjeva 101, 1000 Ljubljana, SLO * e-mail: nina.skrk@bf.uni-lj.si 2 Univerza v Ljubljani, Biotehniška fakulteta, Oddelek za agro- nomijo, Jamnikarjeva 101, 1000 Ljubljana, SLO al., 2006; Vitasse et al., 2011; 2013; Wenden et al., 2019). It can be basically divided into early or spring phenology and autumn phenology. Spring phenolo- gy involves processes related to the interruption of winter dormancy and phases of leaf development from the dormant bud to fully developed and func- tional mature leaves (Meier, 1997). Leaf unfolding is one of the most visible signs of the reactivation of a tree’s physiological activity after the winter dorman- cy (Žust, 2015). It is synchronised with reactivation of cambium and onset of wood and phloem forma- tion, which are not visible unless we extract tissues from the tree for monitoring under a microscope (Čufar et al., 2008; Prislan et al., 2013a; 2013b). Common beech is the most common tree spe- cies in Slovenia and represents one third (32.6% in 6 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Fenologija razvoja listov navadne bukve (Fagus sylvatica) na rastišču v Ljubljani v letu 2020 2018) of the growing stock in the country (Poročilo Zavoda…, 2018). A recent national forest invento- ry by the Slovenian Forestry Institute showed that 208 million beech trees grow in Slovenia (Inventura gozda, 2018). Beech is one of twelve tree species included in the Slovenian National Phenological Network of the Environmental Agency of the Republic of Slove- nia (ARSO) within the Ministry of the Environment and Spatial Planning, which has conducted system- atic monitoring since 1951 (Žust, 2015). The Slove- nian National Phenological Network is also a part of the International Phenological Gardens (IPG), which has monitored genetically identical trees (clones) on ca. 90 localities all over Europe since 1957, with large latitudinal gradient, from Portugal ~41°N to Norway ~63°N (The international…, 2020; Chmielewski et al., 2013). Observation of geneti- cally identical trees limits the bias in leaf phenol- ogy caused by possible genetic variation, which is known to affect this (Kraj & Sztorc, 2009). Long- term phenological data are a rich source of infor- mation on the response of trees to climate change (e.g., Menzel et al., 2006; Fu et al., 2019). The leaf phenology monitoring of beech by ARSO records general leaf unfolding, defined as the phase BBCH11 when 50% of the leaves have unfold- ed completely (Meier, 1997; ARSO, 2020). In Slove- nia it is considered that the leafing of beech marks the real beginning of spring, while the ripened fruits indicate the beginning of true autumn (Žust, 2015). General leaf yellowing (BBCH94), which oc- curs when more than half of the leaves turn yellow in autumn, and leaf fall (BBCH93-10% of leaves fell down from trees to the ground to BBCH97-100% of leaves fell down from trees to the ground) are also monitored (Žust, 2015; Lukasova et al., 2019). Spring phenology and the mechanisms driving leaf development are generally better understood than the autumn ones. Beech has thinly pointed leaf buds that are already visible on the tree in the autumn before the tree enters winter dormancy, when growth is inhibited even if seemingly favour- able environmental conditions (a warm winter) oc- cur. Leaf development can start in spring when the buds are able to respond to warmer forcing tem- peratures (Caffarra & Donnelly, 2011; Wenden et al., 2020). However, this is only possible if the tree previously experienced adequate chilling and if the photoperiod is sufficiently long (Vitasse & Basler, 2013; Dantec et al., 2014). Such requirements are species and site specific (Wenden et al., 2020). A study based on the leaf unfolding and leaf colouring data for beech obtained by ARSO from 47 localities all over Slovenia (altitudes 55 to 1,050 m a.s.l.), in the period 1955-2007 showed that the average day of the year (DOY) of the leaf unfold- ing varied from 14 April (DOY 104) to 13 May (DOY 133) (Čufar et al., 2012), and was delayed for 2.6 days as the altitude rose 100 m. Year-to-year vari- ation of leaf unfolding was mainly driven by March and April temperatures, whereas the temperatures in March had greater effect at lower elevations and those in the April at higher elevations. During 1955-2007, March and April temperatures showed a slightly positive trend and leaf unfolding occurred 1.52 days earlier per decade at 1,000 m a.s.l. and 0.67 days earlier at 500 m a.s.l., which showed that climate change particularly affects phenological be- haviour at higher altitudes. However, leaf colouring occurred from 2 October till 29 October and was earlier at higher altitudes (1.9 days earlier for every 100 m). Colouring proved to be positively correlat- ed to August and September temperatures, where- as the long-term trends and relation to altitude were not statistically significant (Čufar et al., 2012). It was also shown that leaf unfolding approximately agrees with the onset of wood production by the cambium, although the climatic drivers of both processes are different (Čufar et al., 2008). Varia- bility in leaf phenology generally cannot be directly correlated with variations in radial tree-ring growth (Čufar et al., 2015), and the extension of the grow- ing season due to climate change does not neces- sarily increase tree-ring width (Kolar et al., 2016). As organization and implementation of ground observations is demanding, it has been discussed whether ground monitoring of leaf phenology and the condition of leaves could be replaced by other techniques, like modelling (e.g., Vilhar et al., 2018) or satellite imagery (e.g., Allevato et al., 2019; Lukasova et al., 2019). Research into beech cano- py parameters all over Slovenia between 2001 and 2017 based on analyses of crown features derived from the satellite Moderate Resolution Imaging Spectroradiometer (MODIS) and calculating the remotely sensed Enhanced Vegetation Index (EVI) shows that the resolution and frequency of satellite 7 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Phenology of leaf development in European beech (Fagus sylvatica) on a site in Ljubljana, Slovenia in 2020 images hamper the precise reconstruction of leaf phenology. However, it is possible to reconstruct spatio-temporal leaf and canopy damage due to extreme weather events like heat waves, ice storms and spring frosts recorded in changes of the EVI (Decuyper et al., 2020). Any of the abovementioned methodologies needs to take into account the great variability of phenology, which varies within and among trees and sites. However, this is relatively difficult as we lack precise observations on representative numbers of trees on a daily scale (e.g., Lukasova et al., 2019). The aim of this study was to select beech trees on a site in Ljubljana, Slovenia, and in the spring of 2020 monitor different phases of their leaf devel- opment daily, from dormant buds to the develop- ment of mature leaves, to establish a photo-gallery of temporal variability of leaf development, and to relate the leaf development to air temperature and precipitation. Finally, general leaf unfolding in individ- ual trees in 2020 was compared with long-term data (1951-2020) for beech in Ljubljana collected by ARSO. 2 MA TERIA LS AND METH ODS 2 MA TERIA L IN MET ODE 2.1 S TUD Y SIT E AND TREE S 2.1 OP AZ O V ANA PL OSKEV IN DREVE S A The selected study area was Tivoli, Rožnik and Šiška Hill Landscape Park in Ljubljana which was declared a Natural Site of Special Interest in 1984 (Odlok o razglasitvi…, 2020). Forest management is allowed in the park. The stand with selected trees has 428 m 3 /ha of average growing stock, and the predominant forest community is Blechno – Fage- tum clamagrostidetosum. These forests are site-sil- vicultural class suburban forests on acidic beech habitats. The prevailing tree species is beech (Fagus sylvatica) (49%), followed by sessile oak (Quercus petraea) (20%), spruce (Picea abies) (11%), large- leaved lime (Tilia platyphyllos) (11%) and other de- ciduous trees (Pregledovalnik …, 2020). At the forest foothills, along a transect of about 1.6 km (Figure 1), we selected adult domi- nant or codominant beech trees with diameters at breast height of 50 – 100 cm and heights over 30 m. We observed the trees on three locations: (1) trees 11-19 (where the first number indicates the location and the second the tree number) growing on the slope, along the pathway Pod Turnom, near the water reservoir, (2) one tree number 20 grow- ing in the vicinity of trees 11-19 with the earliest leaf unfolding, and (3) one beech number 30, which grows near the Cekin Mansion (National Museum of Contemporary History) (Figure 1, Table 1). Trees 11-20 grow in the abovementioned beech habitats, while the beech 30 grows in the park and is included in the long-term monitoring of the Slovenian National Phenological Network (Fa- gus sylvatica Hardegsen, identification number of the plant 221, year of planting 1969, origin Germa- ny) of the Environmental Agency of the Republic of Slovenia (ARSO) within the Ministry of the Environ- ment and Spatial Planning as a part of the Interna- tional Phenological Gardens of Europe. Figure 1. Locations of selected trees (red dots) in Tivoli, Rožnik and Šišenski hrib Landscape Park in Ljubljana, Slovenia. Slika 1. Lokacije izbranih dreves (rdeče pike) v Krajin- skem parku Tivoli, Rožnik in Šišenski hrib v Ljubljani. 8 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Fenologija razvoja listov navadne bukve (Fagus sylvatica) na rastišču v Ljubljani v letu 2020 2.2 FIE LD OBSER V A TIONS OF LE AF PHENOL OG Y 2.2 TERENSKA OP AZ O V ANJ A RAZV O J A LIS T O V In the period from 4 till 27 April 2020, we visited and photographed the selected trees and recorded the phenological phases daily (Figure 2, Table 2), which indicated the progress of leaf development. In May 2020, we visited the trees at weekly intervals, because significant changes related to phenological phases were no longer happening on a daily basis. The observations were carried out in agreement with the criteria of the World Meteorological Organization (Guidelines …, 2009). Table 2. Descriptions of the main phenological phases of leaf development in beech. Preglednica 2. Opis glavnih fenoloških faz razvoja listov pri bukvi. Tree numbers La titude Longitude Altitude Številke dreves Zemljepisna širina Zemljepisna dolžina Nadmorska višina 11-19 20 30 46.052585° 46.052697° 46.058247° 14.490125° 14.489115° 14.495366° 316 m 315 m 307 m Table 1. Geographical coordinates of the observed trees Preglednica 1. Geografske koordinate opazovanih dreves Phase / Faza Descrip tion Opis faze BBCH00 A buds dormant speči popki BBCH02 B buds swollen popki napeti (nabrekli) BBCH07 C buds swollen, brown scales open, and the green colour of the developing leaves appears popki nabrekli, rjave luske se razprejo in pojavi se zelena barva razvijajočih se listov BBCH09 D buds swollen and open popki nabrekli in odprti BBCH10 E leaves partly unfold- ed, the petiole is not visible listi delno razviti, vidna listna ploskev, listni pecelj ni viden BBCH11 F leaves unfolded, 10% of leaves have final shape, but not final size and colour, the petiole is visible listi razviti, listna plo- skev je odprta, 10 % listov je značilne oblike, ni še končne velikosti in barve, viden je listni pecelj BBCH19 G mature leaves, final size and colour zreli listi, končne veli- kosti in barve Figure 2. Phenological phases of leaf development in beech from (A) dormant bud to (G) mature leaves. Slika 2. Fenološke faze razvoja listov bukve od (A) spečih popkov do (G) zrelih listov. 9 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Phenology of leaf development in European beech (Fagus sylvatica) on a site in Ljubljana, Slovenia in 2020 2.3 CLIMA TIC AND PHENOL OGICAL D A T A F OR C OMP ARISON 2.3 KLI MA T SK I IN FENOL OŠKI POD A TKI Z A PRIMERJ A V O To study the weather conditions during the ob- served leaf development we used the daily climatic data, minimum and maximum temperatures and sums of precipitation for Ljubljana for the period 1951-2020 obtained from the on-line meteorologi- cal archive ARSO METEO. Current leaf development was compared with long-term phenological data, i.e. the data of general leaf unfolding for Ljubljana for the peri- od 1951-2019 obtained from the database of the Slovenian National Phenological Network of ARSO (Žust, 2015). 3 RE SUL T S AND DISCUSS ION 3 RE ZUL T A TI IN RAZPRA V A 3.1 D Y NAMICS OF LE AF DEVEL OPMENT IN SPRING 2020 3.1 DINAMIKA RAZV O J A LIS T O V SPOMLADI 2020 On the first day of observation, 4 April 2020 (DOY 95), we inspected all mature dominant and co-dominant beech trees along the 1.6 km long pathway to select the trees for observation (Figure 1, 3). Tree number 20 already had partly unfold- ed leaves (phase E), whereas all other trees had dormant or swelling buds (phases A, B) and showed no leaf emergence yet (Figures 4, 5, 6). The crown of tree 30 (included in the IPG) still had abundant dry leaves from the previous year. Figure 3. Study site and trees: (A) Tivoli, Rožnik forest as seen from Ljubljana Castle on 3 May 2020 (DOY 124) with the locations of the monitored trees, and (B) the observed trees before general leaf unfolding. Trees 11-20 belong to a beech forest site, while tree 30 grows in the park and is a clone (Fagus sylvatica Hardegsen) included in the International Phenological Gardens. Slika 3. Opazovano območje in drevesa: (A) gozd Tivoli Rožnik, fotografiran z Ljubljanskega gradu 3. maja 2020 (DOY 124) z mesti opazovanih dreves in (B) drevesa pred splošnim olistanjem. Drevesa 11–20 rastejo v pretežno bukovem gozdu, drevo 30 pa v parkovnem delu in je kot klon (Fagus sylvatica Hardegsen) vklju- čeno v Mednarodni fenološki vrt. 10 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Fenologija razvoja listov navadne bukve (Fagus sylvatica) na rastišču v Ljubljani v letu 2020 Tree 12 already showed numerous buds which started to open (phase C) on 5 April 2020 (DOY 96); phase C was observed in tree 30 on 7 April 2020 (DOY 98), and in trees 13, 19, and 14 on 10, 11 and 16 April (DOY 101, 102, 107), respectively. Phase C was quickly followed by the first leaf emergence (phase E) and unfolding (F). General leaf unfolding (F*), as defined by IPG (when 50% of the leaves have unfolded completely) was first observed in tree 20 (7 April, DOY 98), slightly later in tree 12 (9 April, DOY 100), and in tree 30 on 14 April (DOY 105). In tree 19 general leaf unfolding appeared much later, on 22 April (DOY 113), and in tree 14 on 25 April (DOY 116). The first mature leaves, phase G, were observed on 24 April (DOY 115) in tree 20. In all other trees the leaves remained light green and soft till 27 April 2020, the last day of daily mon- itoring. However, monitoring of the trees on 3 May (DOY 124) showed that the colour of the leaves was dark green in most of the trees (Figure 3A). The de- scribed phases were documented on photos of the entire trees that were taken every day (Figures 5, 6). Besides tree 20, which had the earliest leaf development (Figures 5, 6), it is worth mentioning tree 30, which is included in the long-term mon- itoring within IPG (Figures 5, 6, 7), and for which ARSO has long-term data on leaf unfolding (Figure 9). In this tree general leaf unfolding occurred on 14 April (DOY 105) (Figure 7). The latest leaf unfolding was observed in trees 14 and 19 (Figure 3B, 5, 6, 8). As tree 14 grows in the group, it was difficult to follow its leaf develop- ment after the neighbouring trees unfolded their leaves. Therefore, we put special attention to the nearby tree 19 which is solitary and also showed late leaf unfolding, with a general leaf unfolding date of 22 April (DOY 113) (Figure 8). The upper part of the crown also developed numerous male flowers, which possibly affected late leaf flushing. 3.3 WE A THER SITU A TION BE F ORE AND DURING L E AF UNF OLDING 3.3 VREMENS KE RAZMERE PRED IN MED OLIS T ANJE M Weather situation after the leaf fall of previ- ous season was characterized by a warm winter 2019/2020 in Slovenia and Ljubljana with above av- erage temperatures and smaller amount of precip- itation compared to long-term data (Cegnar 2019; 2020a; b and Figure 9). In winter 2019/2020 there were no days with maximum daily temperatures below 0°C. January and February 2020 were also very dry (Figure 9). The warm winter was followed by a warm early spring. The average March temperature in Ljublja- na was 7.2°C, the average minimum daily temper- ature was 2.3°C, and the amount of precipitation Legend / Legenda Figure 4. Phenological phases of most typical trees, showing different timing and dynamics of leaf development; A-G indicate leaf development phases (see Figure 2). Phase F* in the frame indicates the day of general leaf unfolding when 50% of the leaves have unfolded completely. “Date” indicates the day in April 2020 and DOY indicates the day of the year. Slika 4. Fenološke faze najbolj tipičnih dreves, ki prikazujejo različen čas nastopa posameznih faz in različno dinamiko razvoja listov; oznake A-G označujejo faze razvoja listov (glej sliko 2). Faza F* v okviru nakazuje dan splošnega olistanja za opazovano drevo, ko je bilo 50 % listov popolnoma razvitih. Datum predstavlja dan v aprilu 2020 in DOY predstavlja zaporedni dan v letu. 11 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Phenology of leaf development in European beech (Fagus sylvatica) on a site in Ljubljana, Slovenia in 2020 Figure 5. Phenology of leaf development of observed beech trees on chosen days of the year (DOY) - dates. Slika 5. Fenologija razvoja listov opazovanih bukev na izbrane zaporedne dneve v letu (DOY) - datume. 12 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Fenologija razvoja listov navadne bukve (Fagus sylvatica) na rastišču v Ljubljani v letu 2020 Figure 6. Phenology of leaf development of observed beech trees on chosen days of the year (DOY) - dates. Slika 6. Fenologija razvoja listov opazovanih bukev na izbrane zaporedne dneve v letu (DOY) - datume. 13 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Phenology of leaf development in European beech (Fagus sylvatica) on a site in Ljubljana, Slovenia in 2020 Figure 7. Tree 30, included in the International Phenological Gardens: leaf development between 9 April (DOY 100) and 15 April (DOY 106), with 14 April 2020 (DOY 105) considered as the day of general leaf un- folding (for detailed daily photos see Supplement https://repozitorij.uni-lj.si/IzpisGradiva.php?id=116807). Slika 7. Drevo 30, vključeno v Mednarodne fenološke vrtove: razvoj listov med 9. aprilom (DOY 100) in 15. aprilom 2020 (DOY 106); 14. april 2020 (DOY 105) je dan splošnega olistanja za to drevo (za podrobne dnevne slike glej Dodatek https://repozitorij.uni-lj.si/IzpisGradiva.php?id=116807). Figure 8. Tree 19 with late leaf unfolding: its position on the site, and detailed views of the crown on 19, 22 and 26 April 2020 (DOY 110, 113 and 117) (see Figure 4). Slika 8. Drevo 19 s poznim olistanjem - položaj drevesa in podroben pogled na krošnjo 19., 22. in 26. aprila 2020 (DOY 110, 113 in 117) (glej sliko 4). 14 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Fenologija razvoja listov navadne bukve (Fagus sylvatica) na rastišču v Ljubljani v letu 2020 was 105 mm. Compared to the long-term average, March 2020 was generally warmer and had less precipitation, only in the period between 22 and 26 March was the weather cold and temperatures below the long-term average. In total, Ljubljana had eight frost days in March, when the minimum daily temperature was below 0°C. Agrometeoro- logical conditions in March were characterised by premature flowering of early stone fruit trees, while the frost between 22 and 26 March caused damage to apricots and peaches throughout Slo- venia. The lack of rainfall in winter and early spring caused drought conditions, which at the end of March affected the growth of plants in the area of Ljubljana (Žust, 2020). April, when the leaf development was ob- served, was characterised by warm, sunny and dry weather. The average monthly temperature in Lju- bljana in April was 12.9°C, and the minimum dai- ly temperatures ranged between -3°C and 12.3°C. The maximum daily temperatures were between 9.3°C and 25.9°C. The amount of precipitation was only 25.6 mm, with just three rainy days (Figure 9). Compared to the long-term average April 2020 was 2.1°C warmer and had only 26% of the long-term average amount of precipitation. April had four cold days and 292 hours (70% more than the long-term average) of sunshine. Agrometeor- ological conditions in April were characterised by a long drought period which had a negative im- pact on agriculture. 3.4 LE AF DEVEL OPMENT PHENOL OG Y IN THE LIGHT OF L ONG-TERM OBSER V A TIONS 3.4 FEN OL OGIJ A RAZV O J A LIS T O V V L UČI DO L GOLETNIH OP AZ O V ANJ As mentioned before, tree 30 (the number 30 is assigned to this tree only for the purpose of this study) is a clone (Fagus sylvatica Hard- egsen) which is a reference tree for Ljubljana monitored in the framework of the Slovenian National Phenological Network included in the IPG and is one of a series of genetically identical trees that are planted all over Europe (The Inter - national…, 2020). Leaf unfolding in Ljubljana has varied over time, with the long-term (1951-2019) average day of leaf unfolding being DOY 109 (19 April when note a leap year). The earliest leaf un- folding was observed on DOY 95 (5 April 1990), and the latest on DOY 125 (5 May 1958) (Figure 10). In Ljubljana we can observe a trend towards an earlier general leaf unfolding date (Figure 10), which is however not statistically significant (Ču- far et al., 2012). General leaf unfolding of tree 30 was observed on DOY 105 (14 April 2020), and this is thus four days earlier than the long-term average and can be ascribed to the generally warm winter. Figure 9. Minimum and maximum daily temperatures (T) and precipitation (P) in winter 2019/2020 and average minimum and maximum temperatures for the period 2000–2018. Slika 9. Minimalne in maksimalne dnevne temperature (T) ter padavine (P) v zimi 2019/20 in povprečne minimalne ter maksimalne temperature za obdobje 2000–2018. 15 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Phenology of leaf development in European beech (Fagus sylvatica) on a site in Ljubljana, Slovenia in 2020 4 CONCLUSIONS 4 Z AKL JUČKI Long-term phenological observations of beech carried out by ARSO throughout Slovenia provide a date for general leaf unfolding for an individual location. For beech in Ljubljana, the 70-year (1951- 2020) set of leaf unfolding data shows that it occurs on average on DOY 109 (19 April when not a leap year), with a 30-day range between DOY 95 and DOY 125. In this study, we investigated seven spring phe- nological phases of beech leaf development in one location in Ljubljana. We observed the trees with the earliest and latest leaf unfolding (on a 1.6 km long transect) and obtained information on the variability of the occurrence of individual pheno- logical phases among the trees. The observed trees showed general leaf unfolding between 7 and 25 April 2020. There were large differences among the trees even if they grow very close to each other. In the reference beech, for which we have long-term data, general leaf unfolding occurred on DOY 105 (14 April). This is four days earlier than the long- term (1951-2020) average of the data collected by ARSO for the same tree/location. Earlier leaf unfolding of beech is in line with observations of ARSO in other plant species, which in sprig of 2020 generally showed up to one week earlier phenolog- ical phases than the long-term average, which is as- cribed to the above-average warm winter and early spring (Cegnar, 2020c; Žust, 2020). This study helped us to obtain basic informa- tion on phenological variability among the trees on the same microsite, which is, among other things, important to better understand tree physiology and cambial activity with wood and phloem formation. 5 SU MMAR Y 5 PO VZETEK Fenologija (iz grške besede φαινομαι, pha- inomai - pojaviti se) je veja ekologije, ki proučuje zakonitosti periodičnih pojavov v razvojnem ciklu rastlin in živali ter ugotavlja njihovo odvisnost od sezonskih in letnih nihanj podnebnih dejavnikov okolja. Primera tega sta čas razvoja listov in cvetov pri rastlinah ter prvega opaženega prihoda ptic se- livk na določenem območju. Pri gozdnih drevesih najpogosteje opazujemo fenologijo razvoja listov in iglic. Med najbolj razi- skovanimi gozdnimi drevesi je navadna bukev (Fa- gus sylvatica). Posebej zanimiva je zgodnja (spo- mladanska) fenologija, ki proučuje faze razvoja listov od listnih brstov (popkov) v mirujočem stanju do nastanka zrelega, polno delujočega lista (slika 2, preglednica 2). Olistanje je med najbolj očitni- Figure 10. General leaf unfolding of beech in Ljubljana observed by ARSO in the period 1951-2020 and the corresponding average (dotted line). DOY - day of the year. Slika 10. Dan olistanja bukve v Ljubljani, ki ga je spremljal ARSO v obdobju 1951-2020 in pripadajoče pov- prečje (črtkana črta). DOY – zaporedni dan v letu. 16 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Fenologija razvoja listov navadne bukve (Fagus sylvatica) na rastišču v Ljubljani v letu 2020 mi zunanjimi znaki fiziološke aktivnosti drevesa po zimskem mirovanju. Povezano je tudi s procesi re- aktivacije kambija ter nastajanja lesa in skorje, ki na zunaj niso vidni in jih lahko spremljamo samo, če iz drevesa odvzamemo tkiva in jih ustrezno pripravi- mo za proučevanje pod mikroskopom (Čufar et al., 2008; Prislan et al., 2013a; 2013b). Navadna bukev je v Sloveniji najbolj zastopana drevesna vrsta in predstavlja tretjino (32,6 %) lesne zaloge (Poročilo Zavoda …, 2018). Ob nedavni in- venturi gozda so ugotovili, da v Sloveniji raste 208 milijonov bukovih dreves (Inventura gozda, 2018). Olistanje bukve naj bi označevalo začetek prave po- mladi, cvetenje, ki je nekoliko kasnejše, pa glavno obdobje prave pomladi; dozoreli plodovi nato ozna- njajo začetek prave jeseni (Žust, 2015). Listno fenologijo bukve že od leta 1951 sis- tematično spremlja Agencija Republike Slovenije za okolje (ARSO). Na reprezentativnih drevesih po Sloveniji spremljajo naslednje faze: (1) olistanje (BBCH11), ki nastopi, ko se prvi listi izvijejo iz ovoj- ne zaščitne luske, listna ploskev je odprta, značilne oblike, ni pa še prave velikosti, viden je listni pecelj. Mladi listi so porasli s številnimi dlačicami, ki kasne- je odpadejo, (2) splošno rumenenje listja (BBCH94), ki nastopi, ko je več kot polovica listov jesensko po- rumenelih ter (3) odpadanje listov. ARSO je vklju- čen tudi v aktivnosti mednarodne mreže fenoloških vrtov (The International .., 2020; Chmielewski et al., 2013; Žust, 2015) in v tem okviru spremlja feno- logijo bukovih klonov, ki so nasajeni po botaničnih vrtovih Evrope, primer tega klona (Fagus sylvatica Hardegsen) raste tudi v Ljubljani na območju parka Tivoli (drevo 30 v naši raziskavi). Dragoceni dolgoletni podatki listne fenologije bukve so med drugim tudi bogat vir informacij pri spremljanju odziva dreves na podnebne spremem- be. Študija splošnega olistanja in rumenenja bukve na osnovi podatkov ARSO iz 47 krajev po vsej Slo- veniji (nadmorske višine od 55 do 1.050 m), je za obdobje 1955–2007 pokazala, da olistanje nastopi od 13. aprila do 13. maja (Čufar et al., 2012) in se zakasni za 2,6 dni, če se nadmorska višina poveča za 100 m. Na olistanje vplivajo predvsem temperature v marcu (na nižjih nadmorskih višinah) in v aprilu (na višjih legah). V času 1955–2007 so temperature v marcu in aprilu naraščale, kar je vplivalo na ved- no zgodnejše olistanje, to je za 1,52 dni na desetle- tje na 1000 m n. m. in za 0,67 dni na 500 m n. m. Slednje kaže, da podnebne spremembe vplivajo na fenološko vedenje bukve predvsem na višjih nad- morskih višinah. Splošno obarvanje listov je v istem obdobju nastopilo od 2. do 29. oktobra in se prej pojavi na višjih nadmorskih višinah (1,9 dni prej za vsakih 100 m). Rumenenje listov je pozitivno pove- zano s temperaturami v avgustu in septembru, ven- dar dolgoročni trendi in spreminjanje z nadmorsko višino tu niso statistično značilni (Čufar et al., 2012). Pokazalo se je tudi, da se čas olistanja približno uje- ma z začetkom delovanja kambija in nastajanja lesa, čeprav omenjene procese vodijo različne klimatske spremenljivke (Čufar et al., 2008). Fenologije listov na splošno niso mogli povezati z variiranjem širin branik (Čufar et al., 2015), prav tako se podaljšanje rastne sezone v splošnem ne odraža v širinah branik (Kolar et al., 2015). Zaradi zahtevne organizacije fenoloških opazo- vanj se v zadnjem času razpravlja, kako bi jih lah- ko nadgradili z modeliranjem (Vilhar et al., 2018) ali s pomočjo satelitskih posnetkov (npr., Allevato et al., 2019; Lukasova et al., 2019). Nedavne raz- iskave bukve v Sloveniji, kjer so med leti 2001 in 2017 z analizo multispektralnih posnetkov satelitov MODIS in z izračunom vegetacijskega indeksa EVI spremljali spremembe zelene biomase v krošnjah dreves, so pokazale, da ločljivost satelitskih slik pra- viloma še ne omogoča spremljanja fenologije listov, zabeležiti pa je bilo mogoče poškodbe listov/kroš- enj zaradi izjemnih vremenski dogodkov in ujm, kot so vročinskih valovi, žledolomi in pozebe (Decuyper et al., 2020). Na rezultate omenjenih raziskav vpliva velika variabilnost fenoloških faz istega drevesa, med drevesi in med rastišči, različne metode, kot so te- rensko opazovanje ali slikanje, pa na različne na- čine obvladujejo problem variabilnosti (Lukasova et al., 2019). Zato smo se odločili, da v času ra- zvoja listov v aprilu 2020 vsakodnevno fotografsko spremljamo razvoj listov pri skupini dreves na ob- močju Krajinskega parka Tivoli, Rožnik in Šišenski hrib v Ljubljani v Sloveniji (slika 1) ter predstavimo časovno variabilnost razvoja listov za posamezno drevo in med drevesi. Ob 1,6 km dolgi poti ob vznožju hriba smo pregledali bukova drevesa in izbrali najzgodnejše drevo (drevo 20), skupino dreves z različno dinami- ko olistanja (drevesa 11-19) in drevo 30 (klon Fagus sylvatica Hardegsen), ki ga ARSO spremlja v okviru 17 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Phenology of leaf development in European beech (Fagus sylvatica) on a site in Ljubljana, Slovenia in 2020 mednarodne mreže fenoloških vrtov IPG ob podpo- ri Mestne občine Ljubljana (Identifikacijska številka rastline: 221, leto sajenja: 1969, izvor: Nemčija) (sli- ki 1, 3). V času od 4. do 27. aprila 2020 smo dnevno fotografirali izbrana drevesa in popisali najbolj zna- čilno fenološko fazo razvoja listov (slika 2). V maju 2020 smo drevesa obiskovali v tedenskih intervalih. Opazovanja so potekala po kriterijih, ki jih je izdala agrometeorološka komisija pri WMO (Guidelines …, 2009) (slika 2, preglednica 2). Pridobljene podatke smo ovrednotili tudi z vi- dika dolgoletnih meteoroloških podatkov in fenolo- ških opazovanj bukve v Ljubljani. Prvi dan opazovanja, 4. aprila 2020 (dan v le- tu=DOY 95) je drevo 20 že imelo liste v fazi razpira- nja (faza E), medtem ko so bili brsti pri vseh drugih drevesih v mirujočem stanju ali v fazi nabrekanja (fazi A, B), listi pa še niso pogledali iz lusk (slike 3, 4, 5). Krošnja drevesa 30 je imela brste v mirujočem stanju, na vejah pa še veliko suhega listja iz prej- šnjega leta. Že 5. aprila 2020 (DOY 96) so se brsti na dre- vesu 12 začeli odpirati (faza C), medtem ko je bila faza C pri drevesu 30 opažena 7. aprila 2020 (DOY 98), pri drevesih 13, 19 in 14 pa po vrsti 10., 11. in 16. aprila (DOY 101, 102, 107). Fazi C je hitro sledil prvi razvoj listov (faza E) in olistanje (F), ko so bili listi popolnoma razprti. Splošno olistanje (F*) po definiciji Mednarodnih fenoloških vrtov, ko je oli- stane 50 % krošnje, smo najprej opazili pri drevesu 20 (7. aprila, DOY 98), nekoliko kasneje pri drevesu 12 (9. aprila, DOY 100), pri drevesu 30 pa 14. apri- la (DOY 105). Pri drevesu 19 se je faza F* pojavila veliko kasneje, 22. aprila (DOY 113), pri drevesu 14 pa 25. aprila (DOY 116). Prve zrele temno zelene in čvrste liste, ki so značilni za fazo G, smo opazili 24. aprila (DOY 115) na drevesu 20. Na vseh drugih drevesih so listi ostali svetlo zeleni in mehki do 27. aprila 2020, ko smo zaključili vsakodnevno opazo- vanje. Ob naslednjem obhodu, 3. maja (DOY 124), pa je bila barva listov pri večini dreves temno zele- na (slika 2A). Opisane faze so bile dokumentirane na fotografijah celotnih dreves, posnetih vsak dan (sliki 5, 6). Poleg drevesa 20, ki je imelo najzgodnejši ra- zvoj listov (sliki 5, 6), smo fotografsko predstavili tudi drevo 30, za katerega ima ARSO dolgoletne podatke (slike 5, 6, 7) in je v letu 2020 olistalo 14. aprila (DOY 105) (slika 7). Nazadnje sta se olistali drevesi 14 in 19 (slike 3B, 5, 6, 8). Ker drevo 14 raste v skupini, je bilo po oli- stanju sosednjih dreves pri njem težko slediti razvoju listov, zato smo bolj podrobno predstavili olistanje drevesa 19, ki raste v neposredni bližini in ni utesnje- no. Drevo 19 je olistalo 22. aprila (DOY 113) (slika 8). Zgornji del krošnje je razvil številne moške cvetove, kar je morda dodatno vplivalo na pozno olistanje. V prispevku diskutiramo tudi o vremenskih raz- merah pred in med olistanjem (slika 9). Cikel razvoja listov se začne že v letu pred olistanjem, saj se tipični zašiljeni brsti oblikujejo že do zaključka predhodne sezone, za olistanje spomladi pa mora drevo skozi ustrezno dolgo obdobje mrzlega vremena, dovolj dolgo dolžino dneva in ustrezno toplo vreme pred olistanjem (Vitasse & Basler, 2013; Dantec et al., 2014; Wenden et al., 2020). Zima 2019/2020 je bila nadpovprečno topla, kar je veljalo tako za novem- ber in december 2019, ter januar in februar 2020 (Cegnar 2019; 2020a; 2020b). Razen novembra, ki je bil nadpovprečno moker, so bili ostali zimski meseci nadpovprečno suhi (Cegnar, 2019; 2020a; 2020b). Tudi marec je bil toplejši in bolj suh kot dol- goletno povprečje 1981–2010, v času od 22. do 26. marca pa se je močno ohladilo. V Ljubljani je bila povprečna marčevska temperatura 7,2 °C, povpreč- na najnižja dnevna temperatura pa je bila 2,3 °C, kar je 0,7 oz. 0,5 °C nad dolgoletnim povprečjem. V Ljubljani je bilo v marcu 8 hladnih dni, ko se najniž- ja dnevna temperatura spusti pod ledišče (Cegnar, 2020c). Fenološki razvoj pri večini rastlin je bil na začetku pomladi 2020 bolj zgoden kot običajno. V marcu je po Sloveniji predčasno zacvetelo koščičas- to sadje, v hladnem obdobju med 22. in 26. marcem pa je prišlo do obsežne pozebe. Dve pozebi so po Sloveniji zabeležili tudi 8. in 15. aprila (ARSO, 2020). Temperature v Ljubljani so bile v obeh hladnih ob- dobjih v aprilu 2020 nekoliko višje kot po Sloveniji, zato učinkov obeh pozeb na bukev (v občutljivi fazi razvoja listov) v Ljubljani nismo zabeležili. V tej študiji smo spremljali sedem faz razvo- ja listov pri bukvah na eni lokaciji, pri čemer smo skušali zajeti tudi drevesi z najzgodnejšim in najpo- znejšim olistanjem in s tem pridobiti informacije o variabilnosti pojava posameznih faz med drevesi. Splošno olistanje je pri različnih drevesih nastopilo med 7. in 25. aprilom 2020, olistanje pri bukvi, za katero imamo dolgoletne podatke o datumu olista- nja, pa je nastopilo 14. aprila (DOY 105), kar je 4 18 Les/Wood, Vol. 69, No. 1, June 2020 Škrk, N., Črepinšek, Z., & Čufar, K.: Fenologija razvoja listov navadne bukve (Fagus sylvatica) na rastišču v Ljubljani v letu 2020 dni prej od dolgoletnega povprečja (DOY 109) (slika 10). Pri najzgodnejši bukvi na opazovanem transek- tu, za katero nimamo dolgoletnih podatkov, pa je olistanje nastopilo že 7. aprila (DOY 98). K zgodnje- mu pojavu fenoloških faz pri bukvi je pripomogla nadpovprečno topla zima in zgodnja pomlad, kar je bilo opazno tudi pri drugih rastlinah, ki jih spremlja ARSO (Žust, 2020). V tej študiji smo pridobili infor- macije o variabilnosti listne fenologije, kar nam bo med drugim pomagalo bolje razumeti fiziologijo gozdnega drevja, vključno z delovanjem kambija ter nastajanjem lesa in skorje. SUPPLEMENT DOD A TEK The supplement related to this article is availa- ble online in the Repository of the University of Lju- bljana (RUL) and can be accessed through https:// repozitorij.uni-lj.si/IzpisGradiva.php?id=116807, and cited as Škrk et al. (2020). It contains daily photos (period 5 - 27 April and 2 May 2020) of leaf development in European beech (Fagus sylvatica) (tree 30 in this study) which is as a plant number 221 included in the long-term monitoring of the Slovenian National Phenological Network of the Environmental Agency of the Re- public of Slovenia (ARSO) within the Ministry of the Environment and Spatial Planning as a part of the International Phenological Gardens of Europe. Dodatek, povezan s tem člankom, je prosto dostopen na spletu v Repozitoriju Univerze v Lju- bljani (RUL). Dostop do njega je možen preko po- vezave https://repozitorij.uni-lj.si/IzpisGradiva. php?id=116807 in se ga citira kot Škrk et al. (2020). Dodatek vsebuje dnevne fotografije (obdobje od 5. do 27. aprila in 2. maja 2020) razvoja listov pri na- vadni bukvi Fagus sylvatica (drevo številka 30 v tej študiji), ki je kot drevo z identifikacijsko številko 221 vključeno v dolgoletno spremljanje v okviru Sloven- ske nacionalne fenološke mreže Agencije Republike Slovenije za okolje (ARSO) Ministrstva za okolje in prostor Republike Slovenije, ki je vključen v aktiv- nosti Mednarodnih fenoloških vrtov Evrope. A CKNO WLEDGEMENT S ZAHVALA The study was supported by the Slovenian Research Agency (program P4-0015 and P4-0085 and young researchers’ program). Climatic and long-term phenological data were provided by the Environmental Agency of the Republic of Slovenia (ARSO) within the Ministry of the Environment and Spatial Planning. 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