Machinery's Impact on Forest Soil Porosity

The heavy machines used in forest operations not only cause reduction of the total amount of soil pores but also drastically modify their size distribution. The effects in these terms of repeated passages of a tracked skidder and a wheeled one of quite similar size were evaluated in a forest of central Italy. In particular, we focused on macro-aggregates, i.e. the ones of centimetric size, which are crucial for soil functioning and fertility. Empty machines, i.e., without dragging or loading logs, were driven along trails for five round trips (back and forth) with two soil water contents: 11-13%, after a relatively dry period, and 24-25% right after a rain. Soil cores were sampled in the travelled trails and, for comparison, a couple of meters away from them.Bulk density and total porosity were substantially affected by both machines only in the moister soil, while the pore-size distribution changed because of compaction also in the drier soil. The main change the pore system experienced concerned the "coarse storage" pores (the ones from 7 to 50 µm equivalent pore size diameter), which decreased up to more of the 30% in the moist soil. Focusing on just such class of pores, which is the most important for soil water holding capacity, the wheeled machine was slightly more impactful than the tracked one. Overall, the findings of this study highlight the importance of basing on soil moisture for deciding when to perform forest operations and, possibly, with which machine.