The Effect of Understory Shrub Species on the Natural Regeneration of Hyrcanian Mixed Broad-Leaved Forests (Kheyrud, Iran)

Iran’s Hyrcanian forests are part of the temperate forests and have survived since Tertiary. These forests have experienced severe damages in recent decades because of socio-economic issues such as overgrazing. This study aimed at investigating the Nurse-protégé interaction and the impact of understory shrub species on the regeneration of the Hyrcanian main tree species in Kheyrud Forests. Filed sampling was carried out using the random-systematic method. Regeneration and their features under and outside of the canopy of shrub species, as well as some environmental variables were recorded in strip plots. According to the results, the canopy cover percentage of 25-50% provides the best regeneration protection efficiency for the six nurse shrub species. Overall, regeneration density was found to be higher outside the canopy area of the shrub species than beneath their canopy area; however, regeneration under the shrub species caused higher mean height and age. Moreover, the mean cover and the depth of the litter under the shrub canopy were significantly higher than that of outside the canopy area. Although the shrub species allow more durable and complete development of seedlings by providing protection against grazing, suitable conditions for seed germination and seedling growth may not be provided because of high litter depth and insufficient sunlight due to the massive canopy of trees as well as nurse shrubs’ dense foliage (especially, evergreen understory species).