Mechanical site preparation severity mediates one-year-survival response to summer drought in planted tree seedlings

Abstract

Abstract In face of future climatic conditions, methods to ensure the success of forest plantation in warm and dry conditions are required. Mechanical site preparation (MSP) prior to planting is widely used around the world to enhance seedling establishment success. Our study aimed at identifying, among a set of MSP methods that are used in practical forestry, which methods ensure plantation success under dry weather conditions. We evaluated the combined effects of summer drought (estimated using the Standard Precipitation Index) and MSP severity (estimated using vegetation cover in the immediate seedling vicinity one year after MSP) on 1-year seedling survival. We used a network of 20 experimental sites established in France over a 10-year-period, and where seedlings were planted after various MSP. In all treatments (severe MSP, moderate MSP, no MSP), seedling survival was higher in years with rainy summers than in years with dry summers. In rainy years, both moderate and severe MSP methods slightly improved the seedling survival rate (95%) compared to the unprepared control (88%). In dry years, seedling survival was similar after moderate MSP or with no MSP (50 and 54%, respectively), whereas it was much higher after severe MSP (79%). In practical forestry, severe MSP appears as an option to enhance early seedling survival, especially when summer precipitations are lower than the seasonal average, whereas moderate MSP does not significantly improve seedling survival compared to an unprepared control, in all summer weather conditions.