Many morphological, physiological and ecological traits of trees scale with diameter, shaping the structure and function of forest ecosystems. Understanding the mechanistic basis for such scaling relationships is key to understanding forests globally and their role in Earth's changing climate system. Here, we evaluate theoretical predictions for the scaling of nine variables in a mixed-age temperate deciduous forest (CTFS-ForestGEO forest dynamics plot at the Smithsonian Conservation Biology Institute, Virginia, USA) and compare observed scaling parameters to those from other forests world-wide. We examine fifteen species and various environmental conditions. Structural, physiological and ecological traits of trees scaled with stem diameter in a manner that was sometimes consistent with existing theoretical predictions – more commonly with those predicting a range of scaling values than a single universal scaling value. Scaling relationships were variable among species, reflecting substantive ecological differences. Scaling relationships varied considerably with environmental conditions. For instance, the scaling of sap flux density varied with atmospheric moisture demand, and herbivore browsing dramatically influenced stem abundance scaling. Thus, stand-level, time-averaged scaling relationships (e.g., the scaling of diameter growth) are underlain by a diversity of species-level scaling relationships that can vary substantially with fluctuating environmental conditions. In order to use scaling theory to accurately characterize forest ecosystems and predict their responses to global change, it will be critical to develop a more nuanced understanding of both the forces that constrain stand-level scaling and the complexity of scaling variation across species and environmental conditions.