Canopy gap characteristics and regeneration patterns in the Białowieża forest based on remote sensing data and field measurements

Abstract

Forest dynamics is driven by the formation of gaps, especially in temperate forests. A potentially useful tool for identifying canopy gaps at different spatial scales is remote sensing. Our study used two types of data: (1) high spatial resolution Canopy Height Model (CHM) to detect canopy gaps in the forest and (2) field data to quantify natural and artificial regeneration within the gaps. The study aimed to: (i) measure the characteristics of canopy gaps; (ii) identify the regeneration patterns within gaps; (iii) identify the factors influencing tree species regeneration within gaps. We analysed 313 tree canopy gaps in the Polish part of Białowieża Forest (ca. 620 km2), east Poland. Variability in the mean gap area was very high, ranging from 20.2 to 8693.8 m2. The number of gaps with an area>1000 m2 was the largest. The total area of gaps was highest in coniferous stands, where the biggest median area was also found. Gap size did not affect regeneration density in the Białowieża Forest, except for birch and hornbeam. The density of less light-demanding tree species in the gaps was greater than light-demanding ones. Birch regenerated in all gap sizes, and its density increased with gap area. Oak seedlings dominated in small gaps but conditions in the 'matrix of small gaps' was sufficient for short oak saplings. Hornbeam regeneration was highest in the gaps surveyed, especially the density of short saplings. Seedlings of hornbeam were present in 32% of gaps, but saplings were present in as many as 56% of gaps. Spruce regeneration was less numerous than hornbeam regeneration. Birch regeneration was the most numerous in short sapling stage. We analyzed 31 variables from different categories to identify the factors influencing tree species regeneration within gaps. The most essential variables for the birch and hornbeam regeneration were those containing supporting information, especially the percentage of the specific tree species covering the land around the gap. The variables based on gap geometry (size, shape, direction, elongation) were less important than variables from other categories. We found the positive influence of browsing on the density of hornbeam saplings. However, the high density of ungulates and their browsing pressure hinder birch regeneration. Intensive browsing by ungulates seems to be the most important factor in changing species composition and forest regeneration structure in gaps.