How different thinning can improve carbon sequestration, carbon stock and mechanical stability in peri-urban mixed forest stands: a study case in Mediterranean environment.

Peri-urban plantations - artificial forests located near urban areas - in the Mediterranean context are often degraded due to the combined effect of human inactivity and climate changes. Degraded peri-urban forests provide fewer ecosystem services and have reduced biodiversity compared to natural and semi-natural forests.

Silvicultural practices – such as thinning, pruning, weeding, planting – can increase the amount of carbon stored in trees and forests. Thinning can also create more growing space for new trees, resulting in higher carbon sequestration. Additionally, thinning in forests can increase tree mechanical stability and reduce the forest fires risk and, consequently, the related large amounts of carbon released into the atmosphere.

While the main trend of the process is well known, the magnitude can vary significantly according to the climate, the starting condition of the stand, and the natural and human disturbances. All these causes can impact the payback time of carbon stocks. Payback time refers to the time span for the carbon recovering by remaining trees after thinning intervention.

In this study case, we report the results of a silvicultural trial in a mixed peri-urban degraded plantation after 6 years from thinning. Three different silvicultural treatments were compared: a) moderate thinning from below (-20% of current biomass) representing the typical silvicultural treatment of Italian Apennine and considered the traditional scenario; b) intense selective thinning (-30% of current biomass) representing the innovative scenario and c) no management considered the business-as-usual scenario). We also projected the growth to estimate the payback time in recovering harvested carbon stock.

The results show that the more intense thinning has a positive impact on carbon sequestration in the following years, confirming literature results. Besides, the estimated payback time was a) of about 7 years for recovering (in both thinning approaches) the harvested volume; b) of about 8 years for innovative thinning overcoming traditional one; c) of about 12 years for innovative thinning overcoming the control option; d) of about 17 years for traditional thinning overcoming the control option. Finally, we also observed a significant tree mechanical stability increasing from no management option to both thinning options after 2 years. After 6 years, we observed an additional increase of stability for the stands treated with the innovative thinning, while for stands treated with traditional thinning the difference with business as usual reduced until losing its significance.