Incorporating belowground traits: avenues towards a whole-tree perspective on performance

Tree performance depends on the coordinated functioning of interdependent leaves, stems and (mycorrhizal) roots. Integrating plant organs and their traits, therefore, provides a more complete understanding of tree performance than studying organs in isolation. Until recently, our limited understanding of root traits impeded such a whole-tree perspective on performance, but recent developments in root ecology provide new impetuses for integrating the below- and aboveground. Here, we identify two key avenues to further develop a whole-tree perspective on performance and highlight the conceptual and practical challenges and opportunities involved in including the belowground. First, traits of individual roots need to be scaled up to the root system as a whole to determine belowground functioning, e.g. total soil water and nutrient uptake, and hence performance. Second, above- and belowground plant organs need to be mechanistically connected to account for how they functionally interact and to investigate their combined impacts on tree performance. We further identify mycorrhizal symbiosis as the next frontier and emphasize several courses of actions to incorporate these symbionts in whole-tree frameworks. By scaling up and mechanistically integrating (mycorrhizal) roots as argued here, the belowground can be better represented in whole-tree conceptual and mechanistic models; ultimately, this will improve our estimates of not only the functioning and performance of individual trees, but also the processes and responses to environmental change of the communities and ecosystems they are part of.