Mechanized thinning of walnut plantations established on ex-arable land

The study compares two product strategies—chips versus firewood—and two technological levels—manual versus mechanized—as applied to the thinning of walnut agroforestry plantations, established on ex-arable land. Such plantations are widespread all across Europe, and their establishment was subsidized under the provisions of EU Directive 2080/90, and of regional grant schemes. Field test data were used to build a simple deterministic model for estimating thinning productivity and cost under varying work and economic conditions. This model can assist prospective users when checking the profitability of an operation, or when assessing the competitiveness of alternative options. Removing nurse alder from young walnut plantations yields between 25 and 50 t of fresh biomass per hectare and is crucial to the good development of the stand. The study shows that the removal of nurse alder from walnut plantations is economically viable, and it can also offer some profits if stand and market conditions are favourable. As a rule, the average DBH of removal trees should not be smaller than 12 cm. Best results are obtained with mechanized harvesting, which does not seem to cause heavier stand and soil damage than manual harvesting. Manual harvesting is preferable only if the annual utilization of machinery is very low, and in this case it should be geared to firewood production. The manual whole-tree harvesting (WTH) method used in this study offers the lowest performance under all conditions, and should be replaced with some other manual alternative to WTH. Mechanized WTH offers a significant cost reduction over mechanized short-wood (SWS) harvesting, but this difference is still rather limited: hence, other parameters come into play when deciding what system to apply, and namely product price and mass output. In this respect, one also has to consider the cost of managing the harvesting residue, and that of fertilizing, if soil nutrient depletion is to be feared.