Abstract
Background: To understand the theory of forest cycle and reveal the relationship between forest gaps and forest structure, species composition and biodiversity, we need to study the spatial structure of forest gaps. However, the complexity of natural secondary forest structure makes it difficult to quantify the spatial structure characteristics of gaps in large areas by field measurement. In this study, aerial orthophotos, and light detection and ranging (LiDAR), were used as data sources. The experimental area was Maoer Mountain Forest Farm, a typical natural secondary forest in northeastern China, and we used the investigation data of forest resources as reference material. We extracted 1343 forest gaps by manual digitization combined with canopy height model correction. The spatial characteristics of the extracted gaps were quantified from the spatial characteristics (area, shape complexity index), spatial heterogeneity (gap height diversity index) and spatial distribution characteristics (Clark–Evans index) of individual gaps.Results: In the three types of natural secondary forest, the frequency distribution of gap area showed a negative exponential distribution: 90% of the gap area was less than 100 m2. As forests aged, the proportion of gap area decreased from young forest to near-mature forest, and increased from near-mature forest to over-mature forest. The maximum frequency range of shape index changed from 1.2 to 1.4 for young forest, middle age forest and near-mature forest; and which is from 1.4 to 1.6 for mature forest and over-mature forest. The gap height diversity index increased from young forest to near-mature forest, decreased when the forest was mature and increased when it was over-mature. The spatial pattern of forest gaps was mainly random. The proportion of random distribution increased from young forest to middle-aged forest, decreased from near-mature forest to mature forest and increased from over-mature forest.Conclusions: Most of the gaps in the natural secondary forest were small and medium-sized; the shapes were complex; the internal spatial heterogeneity was high; and the gaps were mostly randomly distributed. Use of aerial orthophoto and canopy height model sets was efficient and reliable in quantifying the spatial characteristics of forest gaps, and can replace the time-consuming (and usually field-based) measurement of their subjective spatial characteristics.
Spatial structure of the vertical layers in a subtropical secondary forest 57 years after clear-cutting