Is Mechanized Harvesting of Shrubs Grown for Energy Purposes Environmentally Sustainable

Mechanized harvesting of shrub formations as part of sustainable forest management not only helps reduce the risk of forest fires in Mediterranean environments but also provides economic benefit from the extracted biomass, contributing to the development of the bioeconomy. However, these mechanized operations require an environmental impact assessment to identify the short-term impacts, both positive and negative. This is especially important in the Mediterranean basin given the specific climatic conditions which exist. In this study, the environmental impact of mechanized shrub harvesting is analyzed in relation to (i) changes in biodiversity and in the presence and growth of species; (ii) physical and chemical properties of the soil; and (iii) changes in forest fire risk. For this purpose, a pre-harvest inventory was conducted and post-harvest monitoring schedules of 1- and 2-year durations were established in three characteristic Mediterranean shrubland formations located in the northern-central area of the Iberian Peninsula. Our results reveal that the recovery rates in biodiversity indices after harvesting were very high, with values ranging from 30 to 70% depending on the site. Two years after harvesting, the species coverage was similar to the pre-harvest scenario in some locations, although not with regards to height, the ericaceous species being those with the greatest sprouting capacity. Significant changes in the physical and chemical properties of soils were also observed. In this regard, negative impacts such as soil compaction or slight acidification were identified at some sites. However, positive effects were also found such as an increment in carbon and nitrogen content after harvesting, along with increased litter quantity a year from the clearing operation due to biomass residue left on the ground after harvesting. Furthermore, mechanical harvesting effectively modified fire behavior in all the shrub formations 2 years after clearing, with a notable reduction in fire risk at all the studied sites.