Factors affecting admixed pedunculate oak growth under heavy browsing by deer: benefits from inter- and intraspecific neighbourhoods

Abstract

One of the most important ecosystem processes, especially in temperate and boreal forests, is herbivore browsing. In this study, we tested five hypotheses: (H1) oak tree density would positively affect their height; (H2) despite severe browsing pressure, the height of oaks would increase with the age after planting; (H3) oaks growing among the planted pines would be taller than those among the sown ones; (H4) the growth of single oak saplings or of oak nests would be positively affected by the height of the pines growing in the immediate neighbourhood of the oaks; and (H5) the oak height would be affected by the location within a nest and oaks growing in the central zone would be taller than the others. The study was conducted in the Piska Forest in the north-eastern part of Poland. We established three variants of experiment. In the first variant, oaks were planted individually. In the second and third variants, oaks were planted in small nests. The second and third variants differed in the spacing of the nests: 8 × 8 m and 6 × 6 m, respectively. The division into the sub-variants referred to type of pine regeneration. In the first sub-variant, Scots pine was planted. In the second, pine was sown. In general, oaks growing within nests were significantly taller than those planted individually in the Scots pine rows. Despite heavy browsing, oak height consistently increased during the course of the study. Oak height also depended on the type of pine regeneration, and the mean height of oaks surrounded by planted pines was significantly higher than that of oaks surrounded by sown pines (which, in average, was shorter). That was because the planted pines were taller than the sown ones. We found that Spearman’s correlation coefficients were positive and significant between pine and oak heights in variants 2 and 3. Growth of oaks did not depend on their location within a nest. The results of the regeneration method of oak (single vs nest and few vs many nests) are modified by browsing. The best method found in the heavy browsing circumstances was planting oaks in many nests within successful pine regeneration (providing the oaks with protection against the browsing). The pine will provide the most efficient protection if regenerated a few years before the oaks.