## Abstract

**Introduction**

The state of root systems of Scots pine (*Pinus sylvestris *L.) trees infected with Heterobasidion annosum (Fr.) Bref. has been studied mainly to evaluate the condition of the trees. Despite the fact that the root rot affects primarily the root system of trees, little emphasis has been placed on the changes in its structure during the disease development.

The aim of our research was to reveal the peculiarities of the structure of root systems of Scots pine trees affected by annosum root rot to varying degrees in a particular decline center.

**Materials and Methods**

The features root system structure in pine stands affected by annosum root rot were studied in pure pine plantation of IV age class (Kharkiv region). Established sample plot (201 trees) included centers of tree decline and sites outside the centers. All the trees at the plot were felled and measured. The root systems of these trees were excavated to a depth of 1.0 m within a radius of 1.0 m around a stem. In the excavated sites, the diameters of all roots were measured in the distance of 5 cm from their branches and the degree of damage was determined.

The condition of the root system and the degree of damage were determined by identifying the percentage of the cross-sectional area that was not damaged by root rot from the total cross-sectional area of all accounted roots.

**Results**

The study indicated that the greatest number of trees with unaffected roots (19.7 %) grow in a relatively dense group in the part of the stand that has not been disturbed by the disease in the north-west of the plot. The most affected trees (12.4 %) grow mainly in the south-eastern part of the sample plot.

Trees with the initial stage of the disease (33.3 %, degree of damage is 1–10 %) are mainly adjacent to the part of healthy trees. The trees with the degree of damage of 21–30 % are adjacent to the edge of the active foci of root rot (9.5 % of the trees). There are trees with the degree of damage of 10–20 % between these two parts (9. 5%). Clear boundaries between the parts of the stand with a different degree of damage were not found because the progression of the focus is depended on the different individual tree resistance to the disease.

It was found that the degree of the root rot damage does not depend on architectonics and size of the root system. With the disease progression, the cross-sectional area of roots decreases both for the entire root system and for its components, mostly for horizontal roots, and, to a lesser extent, for the taproots and deeply descending roots. The maximum decrease is observed at the degree of damage of 21–30 %. The average cross-sectional area of roots for the trees with 31–40 % degree of damage, especially of taproots, slightly increased compared to less affected trees due to the mortality of trees poorly developed root systems in the active part of the disease focus.

In the extremely affected trees (the degree of damage is higher than 40 %) the trend to further increase in cross-sectional area of horizontal roots is detected, while the cross-sectional area of vertical roots, especially taproots, is reduced. The found trends suggest that trees with well-developed taproots are more resistant to the disease on the edge of the gap in active part of the decline center; in open space of the gap, the trees with vigorous horizontal roots are.

Reducing the cross-sectional area of the roots for trees with the degree of damage of 21–30 % occurs mainly in tall trees because of their inadaptability to open space. Some increase in the average cross-sectional area of roots for 14.6–18.0 m height trees with severely damaged roots is explained by dying of trees with smaller root systems. Trees with vigorous root systems are more resistant to the disease.

The dependence of the cross-sectional area of roots on their degree of damage is caused by the position of the tree relative to a sparse area inside the gaps. So the most critical position for the tree in the decline center is the edge of the gap where the roots of most trees (the degree of damage is 21–30 %) are affected by annosum root rot.

Since the main factor that determines the shape and mechanical resistance of a tree is wind, the dynamic pressure increases towards the open space of the gap resulting root rot damage of trees with almost two times less cross-sectional area of taproot system as compared to intact trees. Increasing damage of roots leads to a gradual dying of trees with underdeveloped taproots, cross-sectional area of which for the trees with the degree of damage of 31–40 % is slightly higher compared to the unaffected trees. The damage over 40 % of roots is critical for the tree condition. In such degree of damage, trees, cross-sectional area of which roots is 30 % smaller than of healthy trees, remain in open space of the gap.

The cross-sectional area of horizontal roots that are adapted to the static load gradually decreases as the degree of damage increase, reaching the minimum values at 21–30 % damage degree. As the disease progresses (the degree of damage is 31–40 % or more), the trees that are in the sparse area of the gaps the cross-sectional area of horizontal roots slightly increases as the load on the horizontal roots also increases in these conditions. High trees (1^{st} Craft class) are particularly exposed to dynamic pressure in the sparse area inside the decline focus, especially on the edge of the gap at the degree of damage of 21–30 % where most of these trees die.

**Conclusions**

In the foci of root rot disease in pure pine plantations of IV age class, with the progression of the disease, the sum of the cross-sectional areas of roots decreases both for the entire root system and for its components, most substantially for horizontal. The maximal decrease was observed on the edge of the decline gaps. In the initial stage of the disease, the pathogen damages trees with weak taproots. The trees with vigorous taproots remain resistant against annosum root rot in the active part of the disease focus and with well-developed horizontal roots, at open space of the gaps. In the sparse areas of the disease foci, the high trees are exposed to the maximum dynamic pressure.

**3 Figs., 1 Table, 10 Refs.**