Average health condition index of European ash (Fraxinus excelsior L.) in inspected stands of the State Enterprise “Trostyanetske Forest Economy” is 2 points. European ash mortality was the highest in Krasnyanske forestry (0.8 % and 3.5 % of recently died trees and the trees died over a year ago respectively). The highest proportion of healthy European ash trees were inspected in Makivske and Krasnyanske forestries (85.3 % and 50.1 % respectively). The trees of European ash of the 1st category of health condition can be reliably distinguished from the trees of other categories of health condition. A total score of “0” for all parameters of health condition (defoliation, dieback, and epicormic shoots occurrence) reliably points the healthy tree. The trees can be assigned to the 1st category of health condition with defoliation, caused by insects, up to 50 %, with up to 10 % dry branches and single epicormic shoots. Multiple epicormic shoots are characteristic for the trees of the 3rd category of health condition. Ambiguous results in distinguishing of European ash trees of the 2nd and the 3rd, the 3rd and the 4th categories of health condition confirm the need for improvement of its assessment and annual monitoring the forest health at key plots.

The age structure of European ash stands in the forest fund of ten forest enterprises from Sumy, Kharkiv and Poltava regions was analyzed considering forest site conditions, stand origin, site index and ash proportion in the forest composition. The average age of ash stands is 68 years for the Left-Bank Forest-Steppe. On average from 46.1 % to 81.6% of European ash stands survive up to the V age class. The average age of ash stands is the highest in the forest-steppe part of Sumy Region in D2 and D3, in Kharkiv Region in D1, and in Poltava Region in C2 and C3. The average age is the highest (75 years) in artificial seed stands in Kharkiv Region, in natural seed (86 years) and vegetative (77 years) origin in Sumy Region. The survival of pure and almost pure ash stands is the highest in Kharkiv Region and the lowest in Poltava Region. In the legislatively adopted maturity age (VIII age class) survival of European ash with its proportion of 80–100 % is 26.5 %, 13.1 % and 3.2 % in Kharkiv, Sumy and Poltava Regions respectively. Correction of the main felling age in forests that are possible for exploitation is suggested considering the possible loss of timber quality as a result of forest damage by different causes.

The health condition of European ash (Fraxinus excelsior) stands in Ukraine has become worse since 2006. Firstly, in 2011 an alien invasive pathogenic fungus Hymenoscyphus fraxineus was identified in the eastern part of the country and, subsequently, its presence was confirmed in the western and central parts. The aims of our research were to evaluate the health of ash trees and identify the main causes of ash decline in different regions of Ukraine with emphasis on ash dieback and its association with collar rots.    Results showed that since 2013 the number of trees with ash dieback symptoms has been gradually increasing, reaching up to 92 % in 2018. Total mortality due to ash dieback was up to 9 % in 2018. Disease intensity remains high in northern and central Ukraine comparing with the east. Branch dieback, collar rots, epicormic shoots and bacterial disease of ash occurred more often in the eastern region, some symptoms were observed simultaneously. Ash bark beetle galleries, as well as foliage browsing insects, were found mostly in weakened and/or dying trees.    It was indicated that collar rots significantly increase the mortality of ash trees. Armillaria spp. fungi were found to be frequently associated with ash dieback on living stems and fallen trees in 2017, causing high rates of mortality in the northern and central regions. For further ash conservation and breeding programmes, resistant trees in severely damaged regions should be selected to preserve genetic diversity in ash populations.                                                       Keywords: Fraxinus excelsior, ash dieback, collar rot, bacterial disease, epicormic shoots, Hymenoscyphus fraxineus, Armillaria spp.

Дендрохронологічними методами досліджено динаміку товщини шарів ранньої, пізньої та річної деревини Fraxinus excelsior L. і реакцію радіального приросту на зміни клімату у вологому субору Лівобережного Лісостепу. Встановлено роки мінімального приросту (1975, 1987, 1999, 2000, 2012), зумовленого посухами, холодними та теплими зимами й аномальними ранньовесняними температурами. Роки максимального приросту (1973, 1980, 1996 та 2004) характеризуються сприятливим співвідношенням тепла та вологи. Встановлено кореляційні залежності між товщиною шарів ранньої, пізньої та річної деревини і кліматичними чинниками. Внаслідок посушливої погоди 1999-2000 рр. відбулося зрідження насадження, що спричинило значну диференціацію дерев різних категорій санітарного стану за величинами шарів річної деревини. Відхилення від норми температури та опадів на 30 % та більше як у більшу, так і в меншу сторону, спричиняє депресію радіального приросту ясена. Упродовж 1975-1995 рр. радіальний приріст ясена обмежували високі температури вегетаційного періоду, а у наступні 1995-2016 рр. – березневі, квітневі та зимові температури. Встановлено, що опади вересня та грудня попереднього року позитивно вплинули на приріст у 1975-1995 рр. Для 1996-2016 рр. визначено зворотний зв'язок між річним радіальним приростом і липневими опадами. Кількість істотних зв'язків між кліматичними показниками і радіальним приростом не збільшилася у другому періоді, що свідчить про стабільний стан лісостану.

The incidence of ash dieback in the forest-steppe part of Sumy and Kharkiv Regions was analysed taking into account the type of forest site conditions, the stand age, relative stocking density, site index class and the proportion of European ash in the stand composition. Ash dieback is quite widespread in inspected stands of the Left-Bank Forest-Steppe of Sumy and Kharkiv Regions. The disease spreads similarly in natural and artificial stands. It was more often revealed in the fresh fertile forest site conditions, however, such forest site conditions are the most spread and the most favourable for European ash. It was found that ash dieback incidence tends to grow with a stand aging and lasts up to the stand’s age of 80. It can be explained by the development of stem and collar rots which occur later. Ash dieback incidence is the highest in the stands with 40–70 % of ash in their composition, at a relative density of stocking over 0.5 in the stands of different site index classes.