Small changes in species composition despite stand-replacing bark beetle outbreak in Picea abies mountain forests

Abstract

In the mid-1990s, a Spruce Bark Beetle (Ips typographus L.) outbreak affected the Norway spruce (Picea abies (L.) Karst.) forests in the higher elevations of the Bavarian Forest National Park in Germany. To monitor the effect of this large-scale disturbance on the understory vegetation, a series of permanent plots was established in 1998. Until 2000, most of the trees of the spruce-dominated forests at elevations > 1.100 m a.s.l. died. To explore the long-term vegetation development after the disturbance, we resurveyed these plots in 2010. We hypothesised that as a consequence of the stand-replacing disturbance, (i) species composition would change substantially, (ii) shade-tolerant forest species would be replaced by light-demanding species, and (iii) the expansion of the light-demanding species would homogenize species composition. Comparing the 1998 and 2010 situations, we found that species composition did not change substantially after the dieback of the tree layer, shade-tolerant species survived, light-demanding species increased only marginally, and vegetation heterogeneity did not change. The main reason is the missing forest floor disruption: there is no open space emerging for species to establish when trees die standing. As a consequence, a dramatic change in ecosystem structure (loss of the whole canopy layer) is not necessarily connected with a general change in species composition.