Value of large Quercus robur fallen logs in enhancing the species diversity of vascular plants in an old-growth mesic broad-leaved forest in the Central Russian Upland

Abstract Large old trees and decaying logs are key-retained structures for the retention forestry approach, but quantitative estimations of the role of fallen logs in the overall diversity and composition of forest vascular species are few and controversial. Particularly for mesic deciduous forests only a small number of such estimates are known. Our study aimed to partly fill this gap using data collected in the Reserve Kaluzhskie Zaseki where large Quercus robur individuals of more than 300 years old and fallen logs are frequent. We found 44 fallen oak logs supporting vascular plants within a 200-ha forest tract. The vegetation in plots of 1 m2 located on the fallen logs and on the forest floor adjacent to the logs was surveyed together with vegetation in fifteen 10 × 10 m plots randomly placed in patches without large fallen logs. NMDS and PERMANOVA were applied to examine differences in composition between fallen logs and forest floor. Generalized linear models were used to examine the relationships between environmental variables and species richness. Additive diversity partitioning was performed to examine differences in species richness and to estimate the contribution of the fallen logs to the general diversity in vascular species. In total, 71 vascular plant species in 108 1 m2-plots were identified. Twenty-five and 16 species were detected only on fallen logs and on the forest floor, respectively. More than 60% of the studied logs were in early stages of decomposition; their mean diameter was 75.4 ± 25.6 cm. Fallen logs significantly affected species richness and composition in terms of both woody seedlings and herbaceous species. The diameter of logs significantly and positively affected species diversity and composition. Large logs with cracked bark at the first two stages of decay were places of successful vascular plant establishment and growth. The abundance and frequency of tree seedlings were higher on the logs than on the forest floor and seven out of ten tree species appeared to be significant indicator species for the log-plots. Illumination varied over the observed sites and directly influenced the richness of herbaceous species and the overall species composition, but had no effect on woody species richness. We conclude that retention of large fallen logs can be an effective tool for maintaining and restoring the vascular species richness and composition in mesic broad-leaved forests.