Two directions of regeneration of post-windthrow pine stands depend on the composition of the undergrowth and the soil environment

Abstract

Alongside fires, windthrows are among the strongest forest disturbances, which is why research on the regeneration of windthrown forests is so important for silviculture. While there have been many studies devoted to the regeneration of windthrown boreal spruce forests, few have dealt with the regeneration of Scots pine forests in the temperate zone.In Pisz Forest (northern Poland), the State Forests administration left 475.6 ha of pine stands disturbed by windthrow for research on spontaneous ecosystem recovery, leaving all broken trees untouched. For the 11-year study (beginning in 2008, six years after the windthrow, when saplings became more abundant), 10 × 10 m research plots were established in pine stands with different classes of severity of windthrow disturbance: severely disturbed stands (canopy cover up to 30%), moderately disturbed stands (canopy cover of 40–60%) and the least disturbed stands (canopy cover of 70–90%). Each year all saplings on the plots were recorded, with measurements of their height, diameter at breast height, and changes in the percentage cover of forest floor vegetation and its species composition. Selected environmental variables were measured as well. Density of pine saplings in the severely disturbed stands was lower than in the moderately and least disturbed stands, while the density of birch saplings was higher in the moderately disturbed stands than in the least disturbed stands. Moss P. schreberi particularly favored spontaneous regeneration of pine saplings, while the nitrophilous grass D. flexuosa, overgrowing entire surfaces, impeded it. Soil cover by a litter layer alone also significantly hampered the emergence of tree saplings.An elevated content of nitrogen in the soil was conducive to rapid coverage of the area by D. flexuosa, while limiting cover by moss P. schreberi. Moss P. schreberi and shrubs V. vitis-idaea preferred soil with a high respiration rate, while grass D. flexuosa avoided it. Two directions of spontaneous regeneration of windthrown stands were shown, depending on cover by grass or mosses and on the initial content of nitrogen in the soil, which provided the basis for indicating two different forest management strategies in windthrown pine forests.