Abstract
Disturbances such as fire, grazing, windthrows or pathogens play an important role in long-term ecosystem dynamics. Mountain ecosystems of central Europe, such as spruce-dominant forests, experience a variety of disturbances that are threatening their survival.
Therefore, long-term observations of ecosystem disturbance dynamics may provide a better understanding of natural vs. anthropogenic disturbance ecology within spruce forests, which may be crucial for their future conservation. We developed a new method of quantifying overall disturbance by linking pollen records with plant indicator values for disturbance.
Here we use pollen, macrofossil and charcoal records from two sedimentary basins in the Bohemian Forest (Prášilské jezero and Stará jímka), Czech-German borderland, one representing a regional signal and another more local development. Pollen spectra from both well dated pollen records served for calculation of mean disturbance frequency, which was calculated by assigning each pollen to plant taxon with a disturbance index derived for central-European flora.
Reconstructed levels of disturbance frequency were then compared to fire activity (frequency) inferred from contiguous macrocharcoal analysis, and then interpreted along with vegetation changes inferred from pollen and macrofossil analysis. Surrounding vegetation was moreover interpreted quantitatively by application of Landscape Reconstruction Algorithm (using both regional and local scales, REVEALS, LOVE).
Our results reflect similar trends in both pollen records showing continuously increasing disturbance frequency over the entire Holocene. Distinct disturbance events in the early Holocene are associated with increased fire frequency.
The local vegetation during this time was mostly dominated by pine, but with increasing spruce after 8000 cal. BP.
For this period we interpret fire as the main disturbance agent probably fueled by drier and warmer- than-present climate. The period between 6500 and 3500 cal.
BP is marked by quite stable disturbance frequency (ca. once in 50 years), with slightly increasing trend at the local site. This period is also marked by the lowest fire frequency.
A profound increase in disturbance frequency between 3000 and 2000 cal BP is not accompanied by increased fire suggesting human impact or other disturbance factors playing the crucial role. The last millennium is defined by highest disturbance frequency accompanied by increased fire activity.
This regime is most probably triggered by increasing human occupancy in the study area. Our methods introduces another dimension in disturbance regime reconstructions by quantifying other than fire disturbance.
We also suggest that disturbances were inherent part of spruce forests development in the past.