Interaction of gap age and microsite type for the regeneration of Picea abies

To assess the influence of gap age and microsite type within a near-natural montane Norway spruce stand at the Harz National Park in Germany, we tested the following hypotheses: (1) The relationship between Picea abies regeneration and gap age is represented by an optimum curve. (2) Within gaps, tree regeneration mainly occurs on gap-induced microsites. (3) The contribution of specific microsites to regeneration changes with gap age. We randomly established 90 permanent plots stratified into three gap age classes as well as undisturbed forest to assess the density of gap-induced microsites (logs, stumps, root plates) and of microsites not related to disturbance (forest floor, moss-covered rocks) and recorded relative light conditions, spruce density and biometric variables. We found significant differences in spruce regeneration between microsites as well as between undisturbed and disturbed plots. Sapling density was only related to microsite type, with the highest density being encountered on logs with 2.3 individuals/0.25 m(2), in contrast to moss-covered rocks with 1.8 individuals/0.25 m(2). Logs also ranked, together with root plates, highest in terms of sapling mortality. A larger density of young saplings on gap-induced microsites was compensated for by a higher mortality rate, probably due to self-thinning. In contrast, survival was highest on ordinary microsites, i.e. intact ground. Biometric variables differed significantly between microsites as individuals on ordinary microsites were taller than individuals on gap-induced microsites (94.21 +/- 137.09 cm versus 39.86 +/- 50.45 cm) and had more whorls (12.32 +/- 10.55 versus 7.73 +/- 6.79). Significant interactions between gap age class and microsite type were only evident in growth rates. In conclusion, although gap-induced microsites enhance spruce establishment, their role in long-term regeneration has been widely overestimated when compared to the relevance of the ordinary microsites, in particular as the latter are permanent and do not depend on disturbances. (C) 2010 Elsevier B.V. All rights reserved.