Innovative soil conditioners and mulches for forest restoration in semiarid conditions in northeast Spain

Restoring degraded ecosystems is a global challenge. Wherever applicable, forest restoration is one of the most effective tools for reversing degradation processes and enhancing multiple ecosystem services. In Mediterranean semiarid conditions the main limiting factor for tree establishment is the low and irregular precipitation regime, which has a particularly harmful effect in areas where the soil has a poor water retention capacity. We tested, alone and combined, two types of cost-effective and locally applied plantation techniques that aim to promote early seedling establishment: i) various mulches including biodegradable and reusable prototypes and commercial models; ii) two soil conditioners with water-superabsorbing polymers in their formulation, one of which includes a new polyacrylamide-free polymer, which was tested at various doses. In a three-year study we examined their effects on Pinus halepensis performance (survival, shoot and root growth and tree water status) and on soil moisture on a north-facing and a south-facing slope in Mequinenza, NE Spain. The use of mulches led to slight increases in seedling growth and soil moisture compared to untreated seedlings, without great differences between the models tested. Therefore the new prototypes can be considered as suitable alternatives to commercially available ones. On the other hand, the new soil conditioner led to much clearer positive effects. Compared to untreated seedlings, the new soil conditioner improved seedling survival, root and shoot growth and water status, as well as soil moisture. The benefits of the new soil conditioner were highest when applied at doses of 40 or 80 g per seedling. We found that this new formulation achieved similar performance as the commercially available one. Combining mulches and soil conditioners resulted in additive outcomes, rather than in synergistic ones. We conclude that in conditions limited by low precipitation and coarse textured soils the use of small mulches does not seem a priority technique, in contrast with the application of soil conditioners, which seems an effective option for enhancing early seedling performance.