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Abstract
Introduction
Spruce forest stands in Novgorod-Severskiy Polissya (Ukraine) are mainly secondary and grow in fresh and moist fairly fertile site types. Half of them are young stands. In conditions of temperate continental climate in Novgorod-Seversky Polissya, droughts occur periodically. They negatively affect the health of the forest stands, causing their weakening and dieback. Pure planted secondary spruce stands have low biological stability and are especially sensitive to drought conditions.
Therefore, forestry activities in spruce stands in the southernmost limit of Picea abies in Novgorod-Severskiy Polissya should take into account the periodicity of drought periods, which increase the risk of weakening and massive dieback of spruce forests. The large areas of young planted spruce stands in Novgorod-Seversky Polissya require the appropriate forestry measures. They should ensure the reduced terms for obtaining technically qualitative wood together with preserving and improving biological stability. Further, maturity plantings are expediently removed by final clear felling followed by regeneration of indigenous stands according to forest types.
Materials and Methods
The research conducted in planted 23-year-old secondary spruce stands, which grow in fresh fairly fertile sites in Novgorod-Severskiy Polissya. Permanent sample plots (PPP) were established using generally accepted techniques and in accordance with the regulatory documents. To calculate parameters of stands, normative reference materials were used. The condition of the stands was determined by the health of the trees and by mortality indicators. Mortality of the trees was estimated by the levels of absolute mortality, relative mortality and mortality gradient.
Results
The scheme of the experiment on a permanent object provided tending felling of the combined harvesting method for growing various quantities of best target trees of Norway spruce (600, 800 and 1000 stems per ha) and the control (without the tending felling). Trees were cut only around the target trees. The trees that hindered the normal growth of the target trees were removed including trees with low-quality tapering trunks from the upper canopy layer, suppressing the best trees with their broad crowns. In addition, a production low thinning was conducted.
As a result of cutting, the stock of obtained timber ranged from 13.0 to 22.4 m3 per ha. The relative stocking of thinned spruce stands decreased to 0.8–1.1 in sections that should ensure their biological stability. Norway spruce has a lateral root system, and after reducing the relative stocking of high-density forest stands below 0.8, there is a high probability of their damage by wind and favorable conditions for the spread of light and heat-loving bark beetles in weakened plantations.
The total mortality of spruce trees on the permanent site was the largest in Section 2 (selected to grow 800 stems of target trees per ha), where 14.2% of the stock of timber was removed. Here, in the 3 years after the thinning, 148 trees per ha have died with a stock of 6.0 m3 per ha. The Norway spruce trees in section 3 died least of all. Here, the largest number of target trees (1000 pcs. per ha) was selected to grow, and 11.9 % of the stock of timber was removed as a result of thinning. At section 5, where no current forest measures were carried out, the mortality of spruce trees by the stock was the lowest, 1.1 m3 per ha. In general, the rates of mortality in all sections of the stationary for 3 years did not exceed the rates of natural mortality of the dense young spruce stands of Ia site class in the conditions of fresh fairly fertile sites.
In all the affected by Heterobasidion annosum (Fr.) Bref. young spruce trees, blackening and resin exudation in the butt of their trunks were detected. Trees with such signs were in most cases exposed to the wind as a result of the destruction of the root system.
Conclusions
The influence of tending felling on the spruce trees mortality in planted high-density young secondary spruce stands in fresh fairly fertile sites is determined. It has been established that the mortality of trees increases with increasing intensity of thinning, but does not exceed the indices of natural mortality. Dieback of trees predominantly occurs the next year after the thinning. Tending felling in the secondary planted high-density spruce stands are advisable to carry out a weak intensity, not allowing a stand stocking decrease below 0.8 and retaining a share of viable trees of associate species in the composition of the stand to enhance its biological stability. Spruce trees in with blackening and resin exudation in the butt are advisable to design to the felling, as they are affected by Heterobasidion annosum (Fr.) Bref. Spruce trees in sites bordering with clearings and open spaces should not be tended to and the gap after the removal of trees should not be made.
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