Population dynamics of four understorey shrub species during a 7-yr period in a primary beech forest

Shrubs persist in the understorey layer of forests throughout their lives, while tall trees remain there only during the juvenile stage and then grow into the canopy layer. Thus demographic parameters (recruitment-, mortality-, and growth-rates) of shrub species are expected to differ from those of tall tree species. We investigated aspects of the demography of four dominant deciduous-shrub species (Viburnum furcatum, Lindera umbellata var. membranacea, Magaolia salicifolia, and Hydrangea paniculata) in Fagus crenata forests at the beginning and at the end of a 7-yr period in a 1-ha permanent plot. For each species, the number of stems changed little (within ± 10%) during the study period, while total basal area increased markedly (11.7 - 33.8%), because (1) new stems continuously recruited by vegetative growth replaced the substantial number of dead stems, and (2) vegetative stems grew vigorously, probably due to nutrient support from parents. The results indicate that these four understorey shrub species have high ability to maintain their population size in the shaded forest understorey. While in each species more than half of the dead stems were standing dead, a substantial proportion of the dead stems (9.0 - 38.5%) showed signs of mechanical damage, such as stem breakage and suppression by fallen branches or trees. Snow pressure that resulted in decumbent stems was also an important mortality agent for V. furcatum (20.7%) and L. umbellata var. membranacea (5.6%). Probability of damage was constant across all DBH-classes for all study species. In each species, newly recruited stems and dead stems were spatially aggregated, largely due to habits of vegetative growth and mechanical damage, respectively. This study revealed that several demographic traits, resulting from recruitment by vegetative growth and death by mechanical damage, were shrub-species specific and markedly different from those of tall tree species.