Наукові праці Лісівничої академії наук України (Oct 2018)
The role of biotic factors in spruce decline in the Ukrainian Carpathians
Abstract
Derived spruce forests have been planted on the territory of the Ukrainian Carpathians as the results of silvicultural activities instead of the native Silver fir-beech and oak forests. First signs of spruce forests decline and their degradation were observed in the last 10-15 years. The forest-pathological survey of forest plantations combined with analyzing of forest enterprises databases in Carpathian region was used for determining biotic factors influence on the degradation of spruce forests. Spruce forests mortality has emerged as the result of the interaction of three groups of factors constituting the epidemiologic triad: 1) sensitive derived spruce forests including their origin, genotype, age, and function; 2) environmental factors (natural and anthropogenic); 3) phylophagous insects and diseases. The interaction of biotic factors has the cumulative impact and decline process is generally, linked with abiotic and anthropogenic factors. Under their influence, the functioning of the soil biotic component deteriorates, e.g. the development of mycorrhiza of the fine roots decreases from 8 to 33% in declined spruce forests whereas from 75 to 91% of fine roots in native spruce-beech and fir forests are covered with mycorrhiza. The number of invertebrates in the soil also grows in declined spruce forests. Root rot pathogens have also activated their pathogenicity in examined spruce forests and 79-84% of spruce trees have found affected by root rot. Moreover, the infected root rot areas in the mountain spruce forests in Lviv region increased from 100-400 ha in the 80s of the 20th century to more than 20,000 ha in 2010-2012. It’s commonly detected that xylotrophic macromycetes can influence and weak the health conditions of spruce trees and, among them, 26 species can inhabit living trees with a slight degree of decline. In particular, cambio- and xylophagous insects attacks may gradually accelerate the decline process, especially dominated "true" weevils species (Curculionidae): 25 Scolytinae species, 2 Entiminae species, and 5 Molytinae species. The longhorn beetles (Cerambycidae, 28 species) is the second largest and predominant family of cambio- and xylophagous insects found in declined spruce forests. The "mass attack" of bark beetle (mainly Ips typographus) also considerably increased from small areas of several ha in the 80s of the 20th century to 1,9 thousand ha in 2009. Nowadays, Ips beetle populations fluctuate year-to-year depending on the prevalence of stresscausing conditions in the spruce forest. During “normal” years, beetle populations tend to decline because reasonably healthy trees are better able to resist beetle attacks. Since 2010, the large areas of declined spruce forests have been cut in the Lviv mountainous region which slightly reduced the bark beetle populations in the region thereby improving overall tree and stand vigour. The mechanisms of regulation the number of bark beetle populations were also characterized in the study. Multi-species entomophagy and insectivorous vertebrate animals are not able effectively and quickly to eat the growth of the number of bark beetle populations and optimize their balance especially in case of massive attacking of trees in particular, weakened middle-aged mature and overmature tree stands. Specialized entomophagous species are abundantly concentrated in the areas of "mass attack" the Ips beetles with an increase number of their populations. However, the key role and significance of entomophagous insects in the regulation of the bark beetle outbreaks is inadequate: with the massive growth of bark beetle populations. Based on the study, results the small numbers (only a few percent) of larva under the bark can be found attacked with parasites and our findings indicate that the predators cannot be relied upon to stop outbreaks. Normal populations of bark beetles can also be kept in check by woodpeckers and other insect eating birds, parasitic wasps, and predatory beetles. But woodpeckers often chip away outer bark to expose larvae and adult beetles of entomophagous insects reducing natural controlling agents in declined forests. The role of entomopathogenic microorganisms (bacteria, fungi, etc.) for regulation of bark beetle outbreaks other cambio- and xylophagous species is insignificant. Beside this, epizootics of Ips beetle populations are most actively revealed at deteriorating conditions of their development, especially, when other mechanisms of regulation included to that process, e.g. internal competition, weather conditions, etc. Entomopathogenic fungi can also play the role in regulating of the sheer number of beetles, in particular Beauveria species. However, further researches are requested to estimate the effectiveness of these fungi. Intraspecies competition for food is the main regulator of the bark beetle populations and cambio- and xylophagous insects. The spreading of the sheer number of Ips beetles begins when almost all trees are available to be infested, e.g. the live inner bark and outer layer of sapwood have dried or many of trees have killed. The search of susceptible spruce trees (even individual trees) can start in the nearby mixed fir forests but fir tree stands are not usually successfully attacked and not provide good bark beetle food and habitat. After the attenuation of Ips beetle outbreaks, their populations can be kept only in the special "reservations", e.g. on some weakened or fallen by the wind trees or, in the foci of root rot. Under outbreak conditions, the bark beetle populations can develop from 8-9 to over 20 years. The causes of the mass spruce decline are complicated and not easy regulated by humans. The main determinants indicating the deterioration of spruce stands and decline of derivate forests depend on negative effects of weather conditions (high temperatures, absence of precipitation in the spring and early summer) and forestry activities, which resulted in massive planting of spruce in unappropriated conditions. Biotic factors, such as changes in the functioning of the soil biotic components, root and stem rot, development of cambio- and xylophagous insects accelerate and intensify decline process in the spruce forests. The strategies for preventing damage and controlling the spread of Ips beetles and other cambio- and xylophagous insects essentially involve promoting tree vigor, reducing the amount of vulnerable host material within the stand and with timely conducting sanitary measures in the foci of root rot can sufficiently save the rest of spruce forests and slow down spruce decline in the region.
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