Ecological restoration has been widely proposed as a tool to promote the recovery of degraded ecosystems. In this context, understanding which functional traits are able to predict tree growth in planting initiatives is a pivotal question. Although the mechanisms that drive plant performance across species have been well documented for aboveground organs, understanding the belowground mechanisms still requires detailed measurements. Even more, evidence for coordinated above and belowground trait effects on growth is scarce, and it remains poorly understood how these traits together determine tree growth.

We evaluated which traits (leaf, wood, and root) or functional strategies are more important in predicting plant performance. Specifically we ask: when considered alone, are belowground traits more critical than aboveground traits for predicting plant performance in degraded lands? And, are ecological strategies integrating foliar, wood, and root traits in a multidimensional space able to predict plant growth better than traits alone?.

We planted 40 individuals of 12 tree species used for forest restoration in the Colombian Andes (480 individuals in total) and measured plant growth and survival for a year. After that time, we harvested 120 individuals and measured two foliar traits, two wood traits, and eight root traits. We related plant performance and trait values through linear mixed models.

When evaluated alone, seven of the 12 traits measured significantly affected the species' growth, three of which were related to the root system. Traits related to thin leaves, high stem water content, and a large proportion of secondary roots showed the highest growth rate. In fact, the proportion of secondary roots was the trait that best explained plant growth. These results showed that the performance of species under degraded environments depends on traits associated with the acquisition of limited resources, such as water and nutrients. We found that plant species did not coordinate their above and belowground traits according to plant resource economics, when evaluated together. The species enhanced their growth rates by combining acquisitive traits aboveground with conservative traits belowground; this result emphasizes the call of recent studies for a new, multidimensional trait space that includes these different belowground strategies.