Hydraulic anatomy and function of trees : basics and critical developments

The study of hydraulic anatomy and function of trees has a long tradition. Motivated by current and projected changes in water availability, the field of tree hydraulics is experiencing a bout of activity. Significant progress has been made in understanding how water transport in trees is organized, how it integrates with other physiological processes, and what it takes for it to malfunction. Many of these advances have been possible, as fields of research that have traditionally been separate are merging, for example, wood anatomy and plant ecophysiology. These developments have led to the creation of powerful and novel approaches that help to answer longstanding questions relating to fundamental aspects of fluid transport in plants and how plants survive in the face of environmental stresses such as drought and freezing. The increased capacities to visualize the ultrastructures of wood in ever-greater detail and the ability for in-situ visualization of long-distance fluid transport have contributed to this progress. This review provides an entry point to the vast and continuously increasing knowledge of how water transport in trees is organized. It scales from cells to tissue anatomy to whole-tree physiology and landscape ecology. Known mechanisms and novel conceptual theories around how trees die as well as ways to prevent this fate are summarized. High-priority research questions for the coming years are formulated