Features of the formation of self-seeded Scots pine forests on abandoned agricultural lands in Eastern Polissia and the northeastern part of the Left-Bank Forest-Steppe of Ukraine

Abstract

Introduction

In the Polissia and Forest-Steppe regions of Ukraine, a substantial area of agricultural land (arable fields, meadows, and pastures) has been withdrawn from agricultural use. As a result, during recent decades, natural regeneration processes have led to the formation of self-seeded forests of various compositions on abandoned lands adjacent to existing forest stands. The natural overgrowth of former agricultural lands with woody vegetation and the formation of self-seeded forests occur through seed dispersal from neighbouring parent stands. The study aimed to identify the characteristics of natural self-afforestation processes on abandoned agricultural lands and to assess the feasibility of preserving self-seeded forests in Eastern Polissia and the northeastern part of the Left-Bank Forest-Steppe of Ukraine.

Materials and Methods

The research was conducted in young pine stands up to 27 years of age that developed on abandoned agricultural lands under fresh relatively poor and relatively fertile site conditions. To determine average stand parameters of naturally regenerated forests, sample plots (SPs) were established using standard forestry methods. Within the SPs, all tree species were recorded individually, and their age, diameter, Kraft class, and health condition category were determined. To assess the spatial distribution of Scots pine natural regeneration depending on location and distance from parent stand edges, transects were laid perpendicular to the forest edge. Accounting plots measuring 5 x 5 m were established at 20 m intervals, within which Scots pine regeneration was inventoried.

Results

On abandoned agricultural lands adjacent to pine forest stands, active natural regeneration under fresh relatively poor and relatively fertile site conditions resulted in the formation of self-seeded forests dominated by Pinus sylvestris L., with admixtures of Betula pendula Roth., Pyrus communis L., and Malus sylvestris (L.) Mill.

Natural Scots pine regeneration extended 100–250 m from the edge of parent stands. Tree distribution was uneven, forming clusters with high stem density.

Analysis of age structure showed that uneven-aged Scots pine stands developed on former agricultural lands. Continuous natural afforestation generally lasted up to 15 years, followed by a period of approximately 5 years with almost no emergence of new pine seedlings. As a result of the prolonged afforestation period, self-seeded forests exhibited heterogeneous distributions of trees by age, diameter, and height.

During the first 10-year period, the growth rate of self-seeded pine stands was somewhat reduced due to strong competition from herbaceous vegetation. In the first age class, medium-density stands of site quality class I were formed, with a growing stock of 22–47 m³·ha^-1 and no signs of weakening (health condition index I_c= 1.24–1.39).

In the second age class, following canopy closure and the formation of typical forest phytocoenosis, Scots pine growth rate increased markedly. At the age of 13–23 years, naturally regenerated pine stands with a birch admixture, growing under fresh relatively poor site conditions on former agricultural lands, corresponded to site quality class I^b. These stands exhibited a relative density of stocking exceeding 1 and a growing stock of 165–275 m³·ha^-1, nearly 2.4 times higher than that of fully stocked pine stands of natural origin. Under such conditions, stand health deteriorated due to increased natural mortality among trees of lower diameter classes.

Conclusions

The most effective natural regeneration occurred within 100–150 m wide strips of abandoned farmland located on the leeward side of prevailing westerly winds adjacent to seed-source pine stands. Under fresh relatively poor and relatively fertile site conditions, the average density of 5–27-year-old self-seeded stands ranged from 2,000 to 2,800 stems·ha^-1, generally corresponding to site quality class I or higher. Growth rates accelerated after canopy closure and the establishment of a typical forest microenvironment. The growing stock of 13-year-old high-density self-seeded pine forests in fresh relatively poor pine site types on abandoned farmland exceeded that of fully stocked naturally regenerated pine stands by nearly 2.4 times. However, excessive stand density may lead to deterioration in tree health, emphasizing the need for timely tending felling to reduce competitive stress, improve growth conditions, and promote the development of productive and stable stands with the desired composition and structure appropriate to the forest site type.

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