Silvicultural efficiency of cultivation regimes for artificial pine stands in the Northern Steppe of Ukraine

Abstract

Introduction

The optimal intensity and periodicity of thinning during tending fellings, as well as the resulting stand density, remain subjects of debate in silviculture. The effect of heavy thinning – particularly during tending – on the growth, resilience, and productivity of planted pine stands have not been sufficiently studied. The aim of the study was to assess the effect of thinning intensity and frequency during tending fellings on the growth, productivity, resilience, and differentiation of planted pine stands in the Northern Steppe of Ukraine, using the Izyum steppe forest as a case study.

Materials and Methods

The experiment was initiated in 1983 in the Chervonooskil Forestry (compartment 66) of the Izium Forestry Management Unit, Branch “Slobozhanskyi Forest Office” of the State Specialized Forest Enterprise “Forests of Ukraine”. The study was conducted in 31-year-old planted pine stands growing in fresh relatively poor forest site conditions. The experimental design included one linear-selective cleaning (weeding) at 17 years of age, one thinning at 31 years, and one thinning at 50 years. Mensuration characteristics, stand health condition, and tree growth classes were assessed using standard forestry methods. Stand resistance to wind, ice, and wet snow damage was evaluated using the relative slenderness ratio (H/D). The maximum (boundary) H/D value for dominant trees, which indicating stand resistance, ranged from 80 to 110. Statistical analysis was performed using analysis of variance (ANOVA).

Results

In 57-year-old stands subjected to different cultivation regimes, stand density ranged from 1,818 to 670 stems ha^-1. Mean diameter varied from 16.5 to 23.4 cm, mean height from 18.0 to 20.9 m, stand basal area from 38.7 to 27.0 m² ha^-1, and stand volume from 371 to 259 m³ ha^-1. Relative density of stocking ranged from 0.9 to 0.6, and the relative slenderness ratio from 116 to 89. The dominant Kraft class was II, and stand health was assessed as “weakened” (І_с = 2.0). Within 7 years after thinning, current volume increment ranged from 8.2 to 4.6 m³ ha^-1·year^-1, while mortality in the densest stands reached 9.4–15.0 m³ ha^-1. As stand density decreased, the number of trees in Kraft classes I and II ranged from 512 to 301 stems ha^-1. Correspondingly, mean diameter was 18.7–23.5 cm, mean height was 19.5–20.9 m, the basal area was 25–30 m² ha^-1, and the stand volume ranged from 267 to 271 m³ ha^-1.

Conclusions

In planted pine stands of the Northern Steppe of Ukraine, the number of thinning operations can be reduced by increasing thinning intensity, provided that stand density and resilience are carefully controlled at all development stages. Less frequent but more intensive thinning reduces mechanical interventions and promotes faster formation of resilient and productive stands. However, thinning intensity must be adapted to actual stand conditions to avoid excessive reduction in resistance to windthrow, snow damage and pest outbreaks. Moderate thinning (18% of stand volume) and heavy thinning (28–30%), reducing stand density to 1,600–1,200 stems?ha^-1, increased mean diameter by 5–34% and mean height by 6–11%, and enhanced resistance to physical stress, as indicated by a reduction in the relative slenderness ratio (H/D) to 96–101, without a significant decrease in total stand volume. Moderate and heavy thinning (22–28% of stand volume), reducing stand density to approximately 1,000–700 stems ha^-1, resulted in a 33–42% increase in mean diameter and further improvement in stand resilience (H/D reduced to 89–96), while maintaining total stand volume over the long term. The results of the study should be considered when developing regulatory guidelines for the management of planted pine forests.

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