Natural regeneration of shade-tolerant Abies alba Mill. in gradients of stand species compositions: Limitation by seed availability or safe microsites?

This paper analyses the effect of selected site and stand variables on the natural regeneration of the late-successional species Abies alba Mill. in a sequence of stands with increasing partition of three other tree species: light-demanding early-successional Pinus sylvestris (L.), moderately shade-tolerant Picea abies (L. Karst.), and shade-tolerant late-successional Fagus silvatica (L.). The data used originated from inventories carried out in publicly owned forests in southern Poland on a total of 2262 stands in a 30,700 ha area. First, the effect of geographic location, altitude, slope, aspect, site trophism, stand age, stand volume and proportion of admixed species on the occurrence of A. alba regeneration was analysed using generalised linear models (GLZ). The empirical associations derived from these models were then compared with the expected outcomes for two scenarios of seedling over-dispersion given a decreasing number of trees producing seeds and a neutral effect of admixed species. The highest regeneration coverage was found in pure A. alba stands. In the mixed stands, regeneration coverage decreased with a decreasing proportion of A. alba, although at a lower rate than might be expected given a neutral effect of admixed species. The survival rate modelled in close to mono-specific stands (i.e., those with 90%) of P. abies, F. sylvatica and P. sylvestris was 3.0-8.5 times higher than in pure A. alba stands. Nevertheless, the reduced seed availability resulting from a lower number of seed producers in mixed stands might not be fully balanced by more advantageous microsite conditions. In stands with a high percentage of the early-successional species P. sylvestris, regenerative conditions were no more advantageous than in stands with a high percentage of late-successional species. The higher modelled survival rate of A. alba regeneration in stands of late-successional P. abies and F. sylvatica than in stands of con-specific trees may be viewed as an increase-when-rare mechanism. The question remains, however, of whether it is an intrinsically anthropogenic mechanism (e.g., increased herbivory pressure) or a natural mechanism capable of maintaining the mixed species composition of climax communities undisturbed by man.