Determining the contribution of the diversity of understory plant species in broadleaf and coniferous trees reforestation (case study: Colet forest of Mazandaran)

Introduction: Species diversity is a combination of two interrelated components of species richness and evenness. The first component is the species richness, which is related to the number of species in the sampling unit; The second component is evenness, which refers to the distribution of species in the site. One of the important problems of these indicators is not determining the contribution of these two components in the diversity index value. An effective method that has been developed for this purpose is called SHE. The aim of this study was to investigate the contribution of the diversity of understory species in four natural stands, Acer velutinum Boiss, Alnus subcordata C. A. M. and Cupressus sempervirens. var horizontalis afforestation in the Colet forest.Material and methods: In this research, 40 square plots with an area of 400 square meters, were selectively used for sampling in the study area. Then, the list of all herbaceous species with their cover percentage was recorded by Vandermarel criteria. Woody species were counted in each sample plot. The grass and wood cover data were imported and analyzed in Excel and PAST software for analyzing of richness (number of species, Margalf richness and Menhenick richness), evenness (Shannon-Wiener), and diversity (Simpson and Shannon-Wiener). The normality and homogeneity of the data were evaluated using Kolmogorov-Smirnov and Leven test, respectively. SHE analysis was used to determine the contribution of each component of richness and evenness.Results and discussion: The results of comparing the mean of different diversity indices related to herbaceous species showed that the four studied stands (natural, Acer velutinum Boiss, Alnus subcordata C. A. M. and Cupressus sempervirens. var horizontalis) have significant differences in terms of richness (Margalef and Menhinick) (P <0.01). The results of the classification of plants in terms of biological form by the Raunkiaer method also showed that hemicryptophytes have the highest abundance in the study area. Also, the SHE analysis of richness and evenness components for woody regeneration species in the Colet area showed that the evenness of the trend was constant among the four stands and the richness index in the Cupressus sempervirens and natural stands was fluctuating and had an increasing trend. In the Alnus subcordata stand, the evenness and richness indices had the same contribution in determining the diversity of herbaceous species and woody regeneration in this region. In addition, the SHE curve showed that the richness curve (LnS) and the uniformity (Ln E) do not change with each other, and the change in the richness curve is greater than the evenness. This indicates a greater contribution of richness than evenness in the contribution of determining the diversity of data related to herbaceous species and woody regeneration. In fact, this showed that with the increase in the number of samples, only rare species were being added to the list of herbaceous and woody species.Conclusion: In general, the results of the present study showed that the diversity of herbaceous species due  to species composition in the natural stand had the highest value in this region. Also, richness and diversity of woody plants in natural stand and broadleaves reforestation stands were higher than coniferous reforestation. Therefore, reforestation with broadleaves species has a positive effect on the diversity in the region. Also, the effective factor on the diversity indices in the four stands was the richness component (Ln S), which means that severe changes and fluctuations in plant species and their composition was observed.