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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. 

Abstract Trees can differ enormously in their crown architectural traits, such as the scaling relationships between tree height, crown width and stem diameter. Yet despite the importance of crown architecture in shaping the structure and function of terrestrial ecosystems, we lack a complete picture of what drives this incredible diversity in crown shapes. Using data from 374,888 globally distributed trees, we explore how climate, disturbance, competition, functional traits, and evolutionary history constrain the height and crown width scaling relationships of 1914 tree species. We find that variation in height–diameter scaling relationships is primarily controlled by water availability and light competition. Conversely, crown width is predominantly shaped by exposure to wind and fire, while also covarying with functional traits related to mechanical stability and photosynthesis. Additionally, we identify several plant lineages with highly distinctive stem and crown forms, such as the exceedingly slender dipterocarps of Southeast Asia, or the extremely wide crowns of legume trees in African savannas. Our study charts the global spectrum of tree crown architecture and pinpoints the processes that shape the 3D structure of woody ecosystems.