Disturbance history of an old-growth sub-alpine Picea abies stand in the Bohemian Forest, Czech Republic
What historical natural disturbances have shaped the structure and development of an old-growth, sub-alpine Picea abies forest? Are large-scale, high-severity disturbances (similar to the recent windthrow and bark beetle outbreaks in the region) within the historical range of variability for this forest ecosystem? Can past disturbances explain the previously described gradient in stand structure that had been attributed to an elevation gradient?
Šumava National Park (the Bohemian Forest) of the southwest Czech Republic.
We reconstructed the site's disturbance history using dendroecological methods in a 20-ha study plot, established to span an elevation gradient. Growth patterns of 400 increment cores were screened for: (1) abrupt increases in radial growth indicating mortality of a former canopy tree and (2) rapid early growth rates indicating establishment in a former canopy gap.
Spatial and temporal patterns of canopy accession varied markedly over the 20-ha study area, resulting in disturbance pulses that corresponded to an elevation gradient. On the lower slope of the plot, the majority of the trees reached the canopy during two pulses (1770–1800 and 1820–1840), while most trees on the upper slope accessed the canopy in one pulse (1840–1860). Historically documented windstorms roughly coincide with peaks in our disturbance reconstruction.
Our study provides strong evidence that these forests were historically shaped by infrequent, moderate- to high-severity natural disturbances. Our methods, however, could not definitively identify the agent(s) of these disturbances. Nevertheless, the recent mid-1990s windstorm and the ensuing spruce bark beetle outbreak may provide an analogue for past disturbance, as the duration and severity of these events could easily explain past patterns of growth response and recruitment in our results. Thus, it seems reasonable to assume the interaction of windstorms and bark beetles seen in the contemporary landscape has occurred historically. Finally, our results suggest that the previously documented elevation gradient in forest structure may not be related to elevation per se (lower temperatures and shorter growing season) but rather to changes in disturbance severity mediated by elevation.