Boosting multifunctionality through adaptive trait-based species addition in ongoing restoration projects

As trait-based restoration practices continue to gain momentum, there is still an absence of effective methods to monitor ongoing restoration and, if necessary, amend species composition to achieve multiple restoration targets. This challenge is even greater in long-term restoration projects, as a result of different techniques and restoration strategies, leading to a heterogeneous landscape with different levels of ecosystem functions (multifunctionality). During the restoration process, it may be necessary to increase multifunctionality, or a particular ecosystem function, either from scratch or beyond what has already been provided by planted species or species established through natural regeneration. However, these aspects remain underexplored in restoration ecology, primarily because of the lack of operational frameworks. Using data from a 40-year ongoing quarry restoration in Portugal, we evaluated current levels of multifunctionality and how to restore or increase drought resistance, fire resilience, pollination, seed dispersal, and vegetation structure. We found that multifunctionality varies significantly across restoration sites within the landscape. Natural regeneration plays a central role in maintaining current levels of multifunctionality, but we demonstrate that it can be considerably increased by trait-based planting of additional individuals-whether of resident or new species-into restored sites. Furthermore, we show that enhanced levels of multifunctionality can be achieved in future restoration sites by using optimized species combinations. Our study provides important insights into the adaptive management of trait-based restoration and provides a framework to achieve multiple objectives in ongoing restoration projects. We expect the proposed framework will enhance both the appeal and practical application of trait-based and functional enrichment approaches in restoration practice.