Reducing soil compaction after thinning work in agroforestry plantations

Afforestation of marginal farmland with fast-growing tree species is a cost-effective way to produce wood fiber for industrial and energy use. The final harvest is often performed with terrain chipping, in order achieve high productivity and minimum-cost supply. Several machine manufacturers have developed new chipper models, specifically designed for this practice in agroforestry plantations. Soil impacts, particularly soil compaction, represent a concern in such practice. This study evaluated the impact of terrain chipping work on soil compaction in agroforestry plantations. This study tested two different options for the chipper, and namely: a new all-road chipper specifically designed for agroforestry plantations (evaluated under two tire-pressure configurations) and capable of both cross-country and road traffic, and a high-mobility chipper truck. In contrast, one option was tested for the chip shuttle, which consisted in a farm tractor equipped with a two-axle trailer. The four treatments were tested on three different sites, representing the most common soil conditions encountered in the new agroforestry plantations. Soil texture ranged from loam to sandy loam. The occurrence of soil compaction in the upper 30 cm of the soil was evaluated with two different methods: bulk density determination and penetration resistance sampling. Measurements were conducted in the machine tracks after a single pass. The two methods seemed to give contradictory results, especially in two of the three test sites. In general bulk density seemed more sensitive to changes than penetration resistance after a single passage. Bulk density measurements showed that the all-road chipper at low tire pressure and the chipper truck caused soil compaction at almost all sites. In contrast, the all-road chipper at standard tire pressure and the chip shuttle were much gentler on the soil. The limited increments found in this study are likely dependant on a relatively high soil density before traffic. The higher impact of the all-road chipper after reducing tire pressure is contrary to expectations, probably due to the use of relatively stiff tires that may deny the benefits of deflation. This study indicates that wellorganized terrain chipping may not cause such high soil compaction levels as to jeopardize tree root development and growth, despite the heavy weight of the equipment used for this task. In that regard, the specially-designed all-road chipper performs better than the chipper truck.