Effects of black locust and black pine on extremely degraded sites 60 years after afforestation - a case study of the Grdelica Gorge (southeastern Serbia)

The selection of tree species can affect the success of afforestation in the rehabilitation of degraded forest sites and forest restoration. In general, black locust (Robinia pseudoacacia L.) and black pine (Pinus nigra Arnold.) represent the most commonly used species in the afforestation of soils that have been degraded by erosion. As far as the extent of the ameliorative effects of black locust and black pine are concerned, it was found that they may play an important role in the selection of species for the afforestation of extremely degraded sites. This study is aimed at determining the potential of black locust and black pine to affect several soil properties, erosion control and C stock, thus creating favourable site conditions for the restoration of previous forest vegetation. This research was conducted in the Grdelica Gorge in south east Serbia, where eight sample plots with an average size of 0.47 ha were established 60 years ago on terrain afforested with black locust and black pine. In each sample plot, we measured the diameter at breast height of all black locust and black pine trees, and the height of 10 black locust and 10 black pine trees in each diameter class. In addition, samples of mineral soil (from depths of 0-5, 5-10 and 10-20 cm) were taken at 4 randomly selected soil profiles in each sample plot, and 8 samples of litter (30 × 30 cm) were also collected. Additionally, laboratory analyses of the physical and chemical properties of the soil and litter were performed in 2 replicates. The obtained results showed that: (1) at the 0-5 cm depth, there was no statistically significant difference in the reaction of the soil solution, although a significant difference in the reaction of the soil solution between the soils under the two species was observed at soil depths greater than 5 cm; (2) there was a significantly higher N content under black locust in the 0-5 cm soil layer; (3) the reduction of soil loss under black locust is statistically significant in all observation periods; (4) black pine is more efficient in C storage. Our results demonstrate that black locust has the potential to improve soil properties and reduce soil loss caused by erosion, while its favourable impact does not decrease over time, making it more suitable for afforestation on degraded land in the examined area.