It is generally assumed that tree-rings, and their vessel diameters, are wider in warmer and wetter years. To maintain constant conductance per unit leaf area as trees grow taller, stem vessels should widen from tip to base. But wider vessels are more susceptible to embolism, so taller plants are progressively becoming more vulnerable to drought or cold as they grow. The traditional theory of vessel hydraulic adaptation postulates that vessel diameter is affected by climate, with cold envi...