Acta Silvae et Ligni 115 (2018), 21-28 21 Izvirni znanstveni članek / Original scientific paper RESIN YIELD OF Pinus nigra AND Pinus sylvestris IN THE SLOVENIAN KARST DONOS SMOLE ČRNEGA BORA (Pinus nigra) IN RDEČEGA BORA (Pinus sylvestris) NA KRASU V SLOVENIJI Domen GAJŠEK1, Miha BRECELJ2, Kristjan JARNI3, Robert BRUS4 (1) Goričica 24, 3230 Šentjur, Slovenia; domen_g@yahoo.co.uk (2) Zapuže 43c, 5270 Ajdovščina, Slovenia; miha.brecelj@gmail.com (3) Biotechnical Faculty, Department of Forestry and Renewable Forest Resources; kristjan.jarni@bf.uni-lj.si (4) Biotechnical Faculty, Department of Forestry and Renewable Forest Resources; robert.brus@bf.uni-lj.si ABSTRACT The aim of our research was to study the impact of various environmental factors on the resin production of pines in the Slo- venian Karst. Five plots were established – three in Pinus nigra (Arnold) stands and two in Pinus sylvestris (L.) stands. On each plot, the 19-20 most vigorous dominant or codominant trees with a minimum diameter at breast height (DBH) of 20 cm were selected and their resin yield analysed in 2012. Resin yield in P. nigra was considerably higher than that in P. sylvestris. The average resin yield per tree during the study period of 102 days was 1.144 kg for P. nigra and 0.612 for P. sylvestris. There were substantial differences in resin yield among individual trees in the study period: 0.336-2.487 kg for P. nigra and 0.249-1.270 kg for P. sylvestris. The resin yield in P. nigra was considerably higher for the trees with larger DBH, while this was not the case in P. sylvestris. Tree species was the most important factor in resin yield. Increased precipitation resulted in higher resin yields on most plots, whereas better site productivity positively affected resin yield on all P. nigra plots but not on P. sylvestris plots. Key words: black pine, Scots pine, resin production, resin yield, Slovenian Karst IZVLEČEK Namen naše raziskave je bil proučiti vpliv različnih dejavnikov na proizvodnjo borove smole na Krasu v Sloveniji. Za ta namen smo določili 5 raziskovalnih ploskev, in sicer tri ploskve v sestojih črnega bora (Pinus nigra Arnold) in dve v sestojih rdečega bora (Pinus sylvestris L.). Proizvodnjo smole smo spremljali v letu 2012, ko smo na vsaki ploskvi izbrali 19 – 20 dominantnih ali kodominantnih dreves z minimalnim prsnim premerom 20 cm. Analiza je pokazala, da je proizvodnja smole pri črnem boru značilno večja kot pri rdečem boru. Povprečna količina smole na drevo v času periodičnega spremljanja 102 dni je pri črnem boru dosegala 1,144 kg in pri rdečem boru 0,612 kg. Variabilnost v proizvodnji smole je bila znotraj vrste velika, pri črnem boru med 0,336 – 2,487 kg in pri rdečem med 0,249 – 1,270 kg. Pri črnem boru je bila količina proizvedene smole v pozitivni povezavi z debelino drevesa, medtem ko pri rdečem boru te odvisnosti nismo potrditi. Na količino smole pri črnem boru pozi- tivno vpliva produktivnost rastišča. Ugotovili smo, da na smoljenje črnega bora pozitivno vpliva količina padavin, medtem ko višje dnevne temperature dnevni donos smole zmanjšujejo. Ključne besede: črni bor, rdeči bor, smolarjenje, donos smole, Kras GDK 174.7:284.1(497.4 Kras)(045)=163.6 Prispelo / Received: 14. 2. 2018 DOI 10.20315/ASetL.115.2 Sprejeto / Accepted: 30. 5. 2018 1 UVOD 1 INTRODUCTION Resin tapping dates back to Gallo-Roman times and began in Gascony, France. The practice was prospe- rous until the beginning of the world economic crisis of 1929, after which the demand for resin started to decline (Piškorić, 1992). This time-honoured and labo- ur-intensive practice is performed by incising, i.e. da- maging, the outer layers of the bark of a pine or some other conifer in order to collect the resin or sap. The two main components of resin are turpentine and gum rosin. Rosin is widely used to produce adhesives, pa- per sizing agents, printing inks, detergents, etc., while turpentine is usually the raw material for varnishes, perfume, disinfectants, cleaning agents, etc. (Wang et al., 2006). The total world production of resin products has remained fairly stable since the 1960s and reached its maximum in 2007, when production was approxima- tely 1,050,000 tonnes of gum rosin and 170,000 ton- nes of turpentine. The most valued type of turpentine, Iberian turpentine, is of a very high quality and by 15- 20% (and up to 50%) more expensive than turpentine from other parts of the world. However, its use is cur- rently limited by its low production (La resina, 2009). China is presently the largest producer of gum rosin 22 Gajšek D., Brecelj M., Jarni K., Brus R.: Resin yield of Pinus nigra and Pinus sylvestris in the Slovenian Karst globally with more than 70% of the total production, followed by Latin America (notably Brazil) with 10% and Indonesia with 7% (Cunningham, 2009). The demand for resin derivatives in Europe is gro- wing, but European production amounts to less than 10% of its consumption. The largest European produ- cers are France, Portugal, Spain and Greece, while the largest consumers are Germany and the Netherlands, followed by Spain and France (La resina, 2009). Ho- wever, resin tapping in Europe has declined conside- rably as a consequence of the introduction of synthe- tic resins and low-price resin from countries such as China and Brazil. In Spain, the number of resin tappers has decreased considerably in recent years, though in some areas, such as Segovia, resin tapping has been su- ccessfully preserved (La resina, 2009). Several pine species are used for resin tapping wor- ldwide: Pinus massoniana Lamb. in China; P. elliottii En- gelm. in Brazil, Argentina and South Africa; P. oocarpa Schiede ex Schltdl. in Mexico and Honduras; P. merkusii Jungh. & de Vriese in Indonesia and Vietnam; P. roxbur- ghii Sarg. in India and Pakistan; P. caribaea Morelet in Venezuela; and P. radiata D. Don. in Kenya (Coppen and Hone, 1995). In Spain, France and Portugal, the main resin-tapping pine species is P. pinaster Aiton, while in Central Europe and in the Adriatic region, resin ta- pping has traditionally been carried out on Pinus nigra Arnold and Pinus sylvestris L. (Bojanin, 1967). In Slovenia, the first serious attempts at resin ta- pping started in the Primorska region in 1938, where resin tapping of both P. nigra and P. sylvestris began according to the French method. In 1946, the first re- sin-tapping experiments were carried out in Prekmurje and Dravsko polje using the German method, and from there, this method continued to spread into the Karst region. In that period, resin tapping in Slovenia increa- sed every year. The annual resin yield was 84.5 tonnes in 1947, 119.3 tonnes in 1948, 105 tonnes in 1949, 140 tonnes in 1950 and 148 tonnes in 1951 and 1952 (Kia- uta, 1953). At that time, P. sylvestris was mainly used for resin tapping along with the short-term modified Ger- man method (also named the Chorin-Finowtal method) (Pejoski, 1952). The seasonal resin yield per individual tree in Slovenia at that time was 1.06 kg for P. sylvestris and 1.33 kg for P. nigra (Pejoski, 1953). Near the town of Sežana, the reported average seasonal resin yield per single tree was 1.4 kg for P. nigra (Simić, 1953). For comparison, the average seasonal resin yield per single tree in similar conditions and for the same species on the Croatian island of Brač was 0.58 kg at that time. At the present time, large areas of once desolate Slovenian Karst are covered by plantations of predomi- nately P. nigra. They cover a surface area of more than 16,500 ha (Diaci et al., 2014) and are considered ecolo- gically and mechanically unstable. They are threatened by fire and fungal diseases, and their wood producti- vity is low. An important goal of forest management plans is the gradual transformation of these stands into ecologically more stable broadleaved forests with improved productivity. However, alongside timber production provided through the final cutting, it is also possible to generate additional income from non-tim- ber forest products. In this respect, resin tapping in the Slovenian Karst region, once well established but now completely abandoned, is an option worth considering by a forest owner in the years prior to the final cut of a pine plantation. The main goals of our study were to determine the total amount and differences in resin yield between P. nigra and P. sylvestris as well as the influence of enviro- nmental factors, site productivity and tree diameter at breast height (DBH) on total resin production. 2 METHODS 2 METODE Five plots were established near the town of Sežana in the Karst region of Slovenia. Of these five plots, three were placed in P. nigra stands and two in P. sylvestris stands (Fig. 1). On each plot, the 19-20 most vigorous dominant or codominant trees were selected. For each selected tree, DBH, GPS coordinates and tree vigour were measured or assessed. According to Smith et al. (1997), the size of the crown and its density indica- te vigour; therefore, four crown vigour classes were used: 1 – full vigour, 2 – good to fair vigour, 3 – fair to poor vigour, and 4 – very poor vigour. All trees inclu- ded in the research sample belonged to either vigour class 1 or 2. The altitude and plant association of each plot were also assessed. The predominant association on the plots was Seslerio-Ostryetum, followed by Sesle- rio autumnalis-Quercetum petraeae (Table 1). The amount of harvested resin was correlated with environmental data from the nearby Godnje meteoro- logical station (45.75530 N, 13.839775 E). The station is located at an altitude of 316 m and, on average, 3 kilometres from individual research plots. Meteorolo- gical data of individual days (minimum, maximum and average air temperature 2 m above the ground; preci- pitation; and sun duration) were used to calculate ave- rages of individual tapping periods (3-6 days). These mean values were correlated with the resin yield in the corresponding period (25 tapping periods in total). Mean values of climatic parameters are shown in table 2. Acta Silvae et Ligni 115 (2018), 21-28 23 Based on our preliminary tests from 2011 that sho- wed virtually negligible resin yields before mid-June, we started incising the trees in mid-June 2012 and en- ded at the end of September 2012 when the weather cooled. We used the Slovenian method, which is al- most identical to the German method, in which 1.5-2 cm wide incisions are made in 8-10-day intervals. The main difference is that the incision width is smaller in the Slovenian method (0.5-1.5 cm). The method was further adapted with the use of tools – instead of the traditional, bent resin-tapping knife, a straight carpen- try chisel was used. This enabled more effective work, particularly with knotty wood and thick bark. At the bottom of every wound’s vertical channel, we carved out a bed for a pot that was then nailed to the stem. After removing the thicker parts of the rhytidome, cle- aned bark was stripped off in the shape of a triangle and one wound per tree was incised. The wound co- vered about 40 per cent of the tree perimeter (Fig. 2). It should be noted that no stimulating paste was used in the experimental process. The harvest cycle ranged between 3 and 6 days. One of the reasons for choosing a short tapping period was that the obtained resin was also chemically analysed (data not shown). The resin was stored more quickly and thus the evaporation of turpentine was reduced. In each cycle, resin was col- lected and weighed and new incisions were carved simultaneously. The resin was collected with a spoon and stored in containers. Weighing was carried out with a Gorenje KT05NS kitchen scale with an accuracy of one gram. To study the impact of site conditions on resin yield, the site productivity of the plots was assessed. Five do- minant or codominant trees from each plot were cored with an increment borer to determine their age. Prior to analysis, the cores were prepared with established Fig. 1: Locations of the studied plots Slika 1: Lokacije raziskovalnih ploskev Plot Ploskev No. of trees Št. dreves Plant association Rastlinska združba Coordinates Koordinate Altitude Nadmorska višina Distance to meteo. station Razdalja do meteo. postaje P. nigra 1 19 Seslerio-Ostryetum N 45.78489ºE 13.85494º 290 m 3.5 km P. nigra 2 19 Seslerio-Ostryetum N 45.78177ºE 13.85765º 290 m 3.3 km P. nigra 3 20 Seslerio autumnalis-Quercetum petraeae N 45.75681ºE 13.80976º 263 m 2.3 km P. sylvestris 1 20 Seslerio-Ostryetum N 45.78053ºE 13.85376º 280 m 3.0 km P. sylvestris 2 19 Seslerio autumnalis-Quercetum petraeae N 45.75396ºE 13.80308º 273 m 2.9 km Table 1: Main characteristics of the studied plots Preglednica 1: Glavne značilnosti raziskovalnih ploskev 24 Gajšek D., Brecelj M., Jarni K., Brus R.: Resin yield of Pinus nigra and Pinus sylvestris in the Slovenian Karst dendrochronological methods (Stokes and Smiley, 1968). The counting of annual rings was performed with a Nikon SMZ80 stereoscopic microscope with 10× magnification. For describing the site productivity of P. nigra plots, site classes (Gatzojannis, 1999) were used and a site index (Halaj et al., 1987) for P. sylvestris plots. To assess the correlation between resin yield and the various environmental factors (air temperature, amount of precipitation and sun duration), Pearson’s correlation was used. When considering the potential impact of tree species and the diameter of the tree on resin yield, analysis of covariance (ANCOVA) was con- trived, where ‘tree species’ was a fixed factor and dia- meter at breast height (DBH) was used as a covariate. All computations were performed with Microsoft Excel 2013 and IBM SPSS Statistics 25.0 software. Table 2: Climatic variables from the nearest meteorologi- cal station at Godnje (approx. 3 km from the research plots) for the period from 17 June to 28 September 2012 Preglednica 2: Meteorološki parametri območja raziskave, pridobljeni z bližnje meteorološke postaje Godnje (odd- aljenost 3 km od raziskovalnih ploskev), za obdobje med 17. junijem in 28. septembrom 2012 Mean values of tapping periods Povp. vrednost med obhodi Total period Celotno obdobje Average air temp. (° C) Povprečna temp. zraka (° C) 22.45 / Max. air temp. (° C) Maks. temp. zraka (° C) 29.26 / Min. air temp. (° C) Min. temp. zraka (° C) 16.98 / Precipitation (mm) Padavine (mm) 9.16 229 Sun duration (h) Sončno obsevanje (h) 40.73 1018.2 Data from the archives of the Slovenian Environmental Agency (ARSO, 2018) Fig. 2: Initially carved triangle 14 days after bark removal (photo: Brecelj, 2011) Slika 2: Smolina v obliki trikotnika 14 dni po odstranitvi sko- rje (foto: Brecelj, 2011) Acta Silvae et Ligni 115 (2018), 21-28 25 3 RESULTS 3 REZULTATI The maximum resin yield of a single tree in the stu- dy period of 102 days was 2.487 kg for a P. nigra tree on plot P. nigra 3, where the best site conditions (site class 1) were recorded. The minimum resin yield produced per tree was 0.249 kg for a P. sylvestris tree on plot P. sylvestris 2. While the average resin yield was very si- milar on both P. sylvestris plots, it was more variable on the P. nigra plots (Table 3). The analysis of covariance showed that the covari- ate, diameter at breast height, was significantly related to resin yield, F = 17.10, p < 0.001 (Table 4). Moreover, the value of b for the covariate (b = 23.3, p < 0.001) means that resin yield increases with DBH. However, further analysis showed that resin yield was significan- tly related to DBH only in P. nigra (Pearson’s correlati- on: 0.462**) and not in P. sylvestris (Pearson’s correla- tion: 0.089 n.s.) (Figure 4). There was also a significant effect of ‘tree species’ on resin yield after controlling for the effect of DBH, F = 18.00, p < 0.001 (Table 4). The average resin yield in the study period was considera- bly higher for P. nigra (1.144 kg/tree) compared to P. sylvestris (0.612 kg/tree) (see Table 3 and Figure 3). The resin yield correlated with most of the studied environmental factors on P. nigra plots, while on P. syl- vestris plots no such correlation was detected (Table 5). The correlation between resin yield and average daily air temperature was statistically significant (P<0.01) for the two plots with the highest resin yield – P. nigra 2 and 3. For these two plots, there was also a statistically significant correlation between resin yield and average, maximum and minimum air temperatures (Table 5). All of these correlations were negative, which means that resin yield decreased with increasing air temperatu- re (the average air temperature during our study was 22.5 °C on all plots, with the maximum temperature Table 3: Average (x̅), maximum (max.) and minimum (min.) resin yield per tree for all plots during the study period (102 days) Preglednica 3: Aritmetična sredina (x̅), maksimum (max.) in minimum (min.) donosa smole na drevo po posameznih ploskvah v obdobju periodičnega spremljanja 102 dni x̅ (kg) Max. (kg) Min. (kg) DBH (cm)Prsni premer (cm) Site productivity Produktivnost rastišča P. nigra 1 0.942 1.923 0.421 39.95±9.70 Site class 3 (A) P. nigra 2 1.116 2.434 0.336 32.84±7.06 Site class 2 (A) P. nigra 3 1.361 2.487 0.597 39.45±5.78 Site class 1 (A) P. sylvestris 1 0.632 1.270 0.283 31.25±6.95 SI 25 (B) P. sylvestris 2 0.590 1.139 0.249 30.63±2.79 SI 25 (B) P. nigra total 1.144 2.487 0.336 37.41±8.19 / P. sylvestris total 0.612 1.270 0.249 30.94±5.29 / A: Gatzojannis, 1999; B: Halaj et al., 1987 Source of variation Vir variacije F Diameter at breast height Prsni premer 17.10*** Tree species Drevesna vrsta 18.00*** n.s. P>0.05; * 0.01