Frequency and Diversity of Woody Plants Within Gaps Following the Hurricane Disturbance

Background and Objectives: Knowledge of the diversity and structure of forest stands is a fundamental prerequisite for understanding and managing natural ecosystems. The purpose of this study is to determine the composition, frequency and diversity of woody species in the canopy gaps (caused by the storm) and compare it with the control stands (without the storm). The forest stands mixed with abundance and different composition at the level of 7 districts in 15 replications were selected by completely random method.
Materials and Methods: Based on this, mixed forest stands with different frequency and composition at the level of 7 plots in 15 replications were selected as completely random. The studied area was Darab-Kola Sari forest series, Mazandaran province. In order to analyze the data, the plots were divided into four categories in terms of area: Small (less than 200 m²), medium (200 to 500 m²), large (500 to 1000 m²), and very large (more than 1000 m²). Within each gap, the specifications of regeneration and height of woody species were calculated and recorded by establishing microplots (3×3 m). Diversity of woody species was investigated using diversity indices (Simpson, Shannon-Wiener and Brillouin), richness (Margalef and Menhinick) and evenness (Pitt and Hill).
Results: Sixteen tree species were identified within the gaps (created by storm) and control stands in the study area. The highest frequency of regeneration occurred inside the gaps resulting from the storm, and their frequency increased with the increase of the gap size. Results showed that there was only a significant difference between gap and control stands of plant species (at 5% level) and the highest frequency was observed inside the gaps (1458±249 in gap vs. 879±138 in control stand). Also, with increasing the gap size, the diversity and frequency indices increased (5 seedlings in small gap to 4767 seedlings in very large gap). Among the diversity indices, Simpson had the highest values. Peat index in gaps (0.100) and in the control stands (0.130) showed the lowest value among evenness indices.
Conclusion: Natural disasters, including windstorms, are phenomena which their causes are well understood, but their incidence is usually unpredictable. Storms can cause a lot of damage to forest stands, and the frequency of damage is proportional to their severity. Results of this study showed that storm occurrence can have positive and significant effects on the forest structure, which by the correct forecasting and management of the forest, some steps can be taken to improve the vegetation cover. A combination of wind intensity and pre-storm characteristics of stand structure can play an important role in determining tree diversity dynamics because immediately after the storm, tree mortality is high and causes changes in plant composition and diversity. Considering the occupancy of broadleaf mixed stands in the Darabkola region, it is suggested to pay more attention to woody species with deep roots, the crown length to trunk length ratio and also be resistant to strong winds.