•Although tree stem growth is ubiquitous with huge ecological implications, it is poorly understood.•Quantification of diel patterns in both water and carbon relations is critical for a complete understanding.•Anatomy and ecophysiology should be combined in new experiments and models to build an integrated theory.•An integrated theory is much needed to understand and predict trends in stem growth as impacted by climate. Impacts of climate on stem growth in trees are studied in anatomical, ecophysiological, and ecological disciplines, but an integrative framework to assess those impacts remains lacking. In this opinion article, we argue that three research efforts are required to provide that integration. First, we need to identify the missing links in diel patterns in stem diameter and stem growth and relate those patterns to the underlying mechanisms that control water and carbon balance. Second, we should focus on the understudied mechanisms responsible for seasonal impacts on such diel patterns. Third, information on stem anatomy and ecophysiology should be integrated in the same experiments and mechanistic plant growth models to capture both diel and seasonal scales.