Intenzivno sušenja borova na Marjanu primijećeno je 2017. godine, a vjerojatno je započelo 2016. godine pojedinačnim sušenjem stabala alepskog bora. Uzimajući u obzir vrlo ozbiljne razmjere i širenje sušenja stabala u Park šumi Marjan, cilj ovog istraživanja utvrditi je moguće uzroke sušenja alepskog bora s fokusom na determinaciju uzročnika, dobnu i debljinsku strukturu sušaca te utjecaja klime na sušenje. U 2017. godini oboreno je nekoliko stabala zaraženih borova, s kojih su sakupljani uzorci sa simptomima bolesti iglica i grana. Uzorci su se obilježili i transportirali u laboratorij Hrvatskog šumarskog instituta, gdje su inkubirani i pripremani za daljnju analizu. Sakupljao se zaraženi materijal svih dijelova stabla te se evidentirala zaraza. Zaražen materijal inkubiran je u kaveze na kontroliranim uvjetima temperature (20 °C ± 2 °C) te odnosa dana i noći (L : D = 18 : 6). U kavezima su se sakupljale izašle jedinke kukaca, koje su pohranjene u alkoholu ili odmah determinirane. Iz svih dijelova stabala u klima komori izašli su potkornjaci Orthotomicus erosus i Pityogenes calcaratus. Obje vrste spadaju u štetne organizme, ali u radu se, ipak, više pažnje daje mediteranskom potkornjaku, koji je u gradacijskoj fazi te je vjerojatno počeo napadati i zdrava stabla. Na cijelom području Park šume Marjan, pod šumom, položene su primjerne pruge, dijagonalno kroz svaki odjel/odsjek u 2017., 2018. i 2019. godini. Svakom stablu na koje se naišlo izmjeren je promjer i visina, te je vizualnim pregledom i korištenjem dalekozora određena kategorija zdravstvenog stanja, pri čemu su sušci borova raspoređeni u jednu od 5 kategorija stupnja sušenja. Najveći broj potkornjacima zaraženih stabala srednjeg su promjera 27 cm, dok je za cijelu šumu srednji promjer signifikantno veći i iznosi 31 cm. U 2017. godini na Marjanu se sušilo oko 7.500 stabala borova, što predstavlja 13 % svih stabala, a sušenje se nastavlja i u sljedećim godinama, dosežući kumulativno 17 % u 2018. te 23 % u 2019. godini. Sezonska odstupanja temperature i oborina u percentilima ukazuju na višegodišnji pomak normale prema toplijem i sušnijem. Više temperature i ekstremne suše stvaraju kumulativni stres na stabla. S druge strane, kukci su poikilotermne životinje, brzo reagiraju na promjene u temperaturi, što kod vrsta s visokim biološkim potencijalom, poput nekih vrsta potkornjaka, može dovesti do brze populacijske i prostorne ekspanzije, odnosno gradacije. Uočeni fenomen upućuje na zaključak da je uzrok sušenju borova na Marjanu ulančavanje nekoliko nepovoljnih čimbenika (suša, produljenje vegetacije uslijed promjena u klimi), što je rezultiralo gradacijom potkornjaka O. erosus. Nova saznanja proizašla iz ovog istraživanja pokazuju da se mediteranski potkornjak najprije pojavljuje u krošnji, a zatim u deblu i donjim dijelovima stabla. Predlaže se da se donji nezaraženi dijelovi stabla koriste kao lovna. Gradacija potkornjaka O. erosus kakva je zabilježena u ovom istraživanju nikada prije pojave na Marjanu nije evidentirana u Hrvatskoj, te je moguće da će ovaj štetnik, ustraju li aridifi kacijske promjene, postati permanentan problem u mediteranskim šumama.The intensive dieback of pine trees on Marjan was observed in 2017, and it probably began in 2016 with dieback of individual Aleppo pine trees. Given the very serious extent and spread of drying of trees in the Forest Park Marjan, the aim of this study is to determine the possible causes of Aleppo pine dieback with the focus on identification of the cause, the age and the thickness of the dead trees, as well as the effect of the climate on this event. In 2017, several infested pine trees were cut down from which samples of symptomatic needles and branches were collected. Samples were marked and transported to the Croatian Forest Research Institute laboratory, where they were incubated and stored for further analysis. The infested material from all parts of the tree has been collected and infestation was recorded. The infested material was incubated in cages at controlled temperature conditions (20 ° C ± 2 ° C) and day and night regime (L : D = 18 : 6). Thus, the emerging insects were collected from the cages, where upon stored in alcohol or immediately determined. Bark beetles Orthotomicus erosus and Pityogenes calcaratus emerged from all parts of the trees in the climate chamber. Both species are harmful pests, but more attention here is given to the Mediterranean bark beetle, which is in the gradation phase and probably started to attack
healthy trees. In the whole forest area of Marjan sample stripes have been laid out, diagonally throughout each compartment/subcompartment in 2017, 2018 and 2019. To each encountered tree the diameter and height was measured, and a certain category of health condition was determined by visual inspection and using of the binoculars for each pine tree, dividing them into one of the 5 categories of drying stage. The largest number of infected and dried trees are about 25 cm in diameter. In 2017, about 7,500 pine trees were showing symptoms of dieback on Marjan, representing 13% of all trees, and drying continued in the following years, reaching cumulative 17% in 2018 and 23% in 2019. Seasonal deviations of temperature and precipitation in percentile indicate the perennial shift of normal to hotter and drier. Higher temperatures and extreme droughts create cumulative stress on the trees. On the other hand, insects are poikilotherm animals, they react quickly to temperature changes which in species with high biological potential such as some types of bark beetles can lead to rapid population and spatial expansion, i.e. gradation. The observed phenomenon suggests that the cause of the pine trees dieback on Marjan lies in
chaining of the several unfavorable factors (drought, the prolongation of vegetation due to climate changes), resulting in the gradation of the bark beetle O. erosus. Climate changes enabled a higher number of generations and an increase in population that led to the attack of healthy trees and their drying in groups in 2017, i.e. the gradation of bark beetles. New findings emerged from this research show that the Mediterranean bark beetle first appears in the tree crown and then in the trunk and lower parts of the tree. It is suggested that the lower uninfected parts of the tree are used as trap trees. Gradation of bark beetle O. erosus that was observed in this study has never been recorded so far and it is possible that this pest, in case that aridifi cation changes continue, will become a permanent problem in Mediterranean forests
