Occurrence of Woods with a Gradation in Vessel Diameter Across a Ring

The occurrence of a range of vessel diameters across a ring greater than × 5 (referred to here as graduated-porous) is not completely equivalent to ring-porosity as defined in the IAWA features list. Trees with graduated porous wood occur mainly in the mid-latitudes. A limiting factor relating to temperature (winter temperature extremes below -40° C) defines the northern occurrence of these trees, preventing them from being an important element in the boreal forest. Towards the equator, numbers fall off steeply where winter temperatures stay above freezing. In the mid-latitudes, where these trees are most prevalent (40-100% of the tree flora), percentages vary inversely with precipitation amount, a relations hip that has potential applicability in interpreting fossil assemblages. Occurrence of this character is consistent with its interpretation as an adaptation allowing high conduction rates early in the growing season that is particularly favoured in drier climates of the mid-latitudes. Trees with graduated-porous wood occur in the tropics, where they appear to be most common in dry-climate areas. Some tropical woods that display a range of vessel diameters but have narrow and wide vessels arranged in a random pattern (rather than showing a radial sequence from wide to narrow) may serve as indicators of climates that are seasonal with respect to precipitation but not temperature. Well-founded climate estimates based on fossil wood depend upon understanding more about the way wood is preserved in the fossil record; graduated-porosity (or ring-porosity) is itself a character that may influence likelihood of preservation.