Morphological variability of English oak acorns in oak stands in the forest-steppe zone of the Kharkiv region

Abstract

Introduction

English oak (Quercus robur L.) is characterized by considerable intraspecific variability in morphological traits, growth intensity, and wood quality, which results from the interaction between genetic factors and environmental conditions. The aim of the study was to assess the variability of morphological and biometric parameters and acorn mass of English oak, and to identify the relationship between these indicators and the origin and density of the stand.

Materials and Methods

The morphological variability of acorns was studied in a fresh maple-lime oak forest within the Mokhnachanske Forestry of the State Enterprise “Skrypaivske Educational and Research Forestry Enterprise” (Southern part of Left-Bank Forest-Steppe of Ukraine). The research was carried out using a transect-based sampling method, where 1 x 1 m sample plots were established along each transect. Transects were arranged parallel to each other at a distance of 10 m, while the distance between sample plots was 5 m. On average, 55 sample plots were established within each study area.

Results

Seven morphological forms of acorns were identified: barrel-shaped, oval-cylindrical, elongated-cylindrical, obovate, ovoid, broadly oval, and cylindrical. A high level of individual variability of these morphotypes was recorded, which further substantiates the effectiveness of using morphological indicators in selection and breeding studies. In English oak stands, the oval-cylindrical and cylindrical forms predominated, accounting for 49% and 16%, respectively, of the total number of examined morphotypes. Other forms were distributed as follows: broadly oval – 8%, obovate – 4%, elongated-cylindrical – 4%, barrel-shaped – 12%, and ovoid – 6%. In seed-origin stands, the proportion of elongated acorn forms was 17% higher than in coppice stands, whereas the latter showed an increased proportion of rounded forms. This confirms that the structural composition of acorn morphotypes varies depending on the regeneration method of stands, which should be considered when organizing forest seed sources and selecting plus trees.The highest proportion of elongated acorn forms was recorded in stands with a relative density of 0.6, where they accounted for 83.8%. In stands with densities of 0.7 and 0.8, this indicator was 72.2% and 72.4%, respectively. A reduction in resource scarcity in medium-density stands affects the selectivity of fruiting in individual trees, which determines the morphological heterogeneity of the crop and leads to a redistribution of the proportions of certain acorn morphotypes.

The highest mean mass was recorded for ovoid and barrel-shaped acorns, at 3.6 g and 3.5 g, respectively, whereas the lowest mass was observed for cylindrical and broadly oval forms (1.7 g and 1.9 g, respectively). A strong positive correlation was found between acorn size and mass parameters: length and mass (r = 0.83), diameter and mass (r = 0.91), and diameter and length (r = 0.84).

Conclusions. The obtained data on individual variability in acorn shape confirm the genetic determination of morphological traits and indicate the feasibility of using them as diagnostic criteria for identifying clones of plus trees of English oak. The detected differences in acorn mass correspond to variations in acorn length and diameter and may have practical significance for evaluating seed quality. The high correlation coefficients between linear and mass parameters indicate their close interdependence, allowing acorn diameter to be considered the most informative indicator for rapid selection of viable seed material.

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