Regeneration of Abies pinsapo within gaps created by Heterobasidion annosum-induced tree mortality in southern Spain

Abstract

We explored the effects of tree-fall gaps caused by Heterobasidion annosum-induced tree death on the regeneration of the relict Abies pinsapo in southern Spain. We analyzed the structure and regeneration of A. pinsapo in 44 circular closed-canopy plots and 11 gaps (mean size 75 m2) located in H. annosum infection foci. We measured adult density, basal area, and height of all tree species with a diameter at breast height greater than 7.5 cm and quantified regeneration abundance. Diameter structures were analyzed using Weibull functions. A. pinsapo comprised 49-81% of total adult tree density (358-1418 trees ha-1) and total basal area (15.70-39.22 m2 ha-1). Density of A. pinsapo juveniles ranged from 129 to 416 individuals ha-1, while that of juveniles of other tree species such as Quercus ilex was much lower. A. pinsapo saplings and seedlings were more abundant within gaps than in closed-canopy sites. A. pinsapo regeneration was related to the tree species composition of the stands adjacent to gaps. The main potential species replacing A. pinsapo in H. annosum-induced gaps were, in decreasing order of importance, Quercus ilex, Q. faginea, and Juniperus oxycedrus. The current stand structure and gap-infilling processes suggest that A. pinsapo regenerates adequately within H. annosum-induced gaps. However, the relative proportion of other coexisting tree species within H. annosum-related gaps will increase substantially over time as compared with closed-canopy areas.