Natural disturbance baselines for sustainable forest management in Europe: an index comparing natural and human disturbance frequency, size, and severity

Background/Question/Methods

In Europe, there has long been interest in natural dynamics silviculture, although the majority of forests remain tightly regulated under a variety of production-driven silvicultural systems. These have diminished ecosystem resilience to environmental stress, leading to increasing susceptibility to an array of disturbances. A major barrier has been incomplete understanding of the ranges of variability in disturbance regimes, including frequencies, spatial attributes, and severities. Addressing this constraint, we adapted a “comparability index” that was first developed in the USA to compare natural disturbances and forest management effects. We extended the original concept that included spatial and temporal axes by adding disturbance severity (i.e. tree survivorship or retention) as a third dimension. Working with two dozen scientists from across Europe, we populated the model by compiling published data from 13 countries on disturbance dynamics for four major forest types (spruce, beech, oak, and pine-dominated). Data on harvesting effects by country and forest type for a variety of silvicultural systems were obtained through an expert-based process employing standardized estimation protocol. The data for both natural and harvest disturbances were visualized in three-dimensional plots indicating ranges for frequency, size, and severity. We developed an algorithim to calculate the index values for bivariate comparisons.

Results/Conclusions

We found natural disturbances to be highly variable in size, frequency, and residual structure, but European forest management fails to encompass this complexity. Silviculture in Europe is heavily skewed towards even-aged systems, used predominately (72.9% of management) across the countries assessed. Residual structure proved crucial in the comparison of natural disturbances and silvicultural systems. The comparability index indicated the highest congruence between uneven-aged silvicultural systems and natural disturbance attributes. Even so, uneven-aged practices emulated only a portion of the complexity associated with natural disturbance effects. The remaining silvicultural systems perform poorly in terms of retention as compared to tree survivorship after natural disturbances. We suggest that natural dynamics silviculture can expand Europe’s portfolio of forestry practices, for example where wood production is not the primary objective. These practices are especially relevant to forests managed for structurally complex forest habitats, risk reduction, and a variety of ecosystem services.