Evidence that branches of evergreen angiosperm and coniferous trees differ in hydraulic conductance but not in Huber values

Abstract

The hydraulic efficiency conferred by vessels is regarded as one of the key innovations explaining the historical rise of the angiosperms at the expense of the gymnosperms. Few studies, however, have compared the structure and function of xylem and their relationships with foliage traits in evergreen representatives of both groups. We measured sapwood cross-sectional area, conduit diameters, hydraulic conductance, and leaf area of fine branches (2.5–7.5 mm diameter) of five conifers and eight evergreen angiosperm trees in evergreen temperate forests in south-central Chile. Conductance of both lineages was higher at Los Lleuques, a warm temperate site with strong Mediterranean influence, than in a cool temperate rain forest at Puyehue. At a common sapwood cross-sectional area, angiosperm branches at both sites had greater hydraulic conductance (G) than conifers, but similar leaf areas. Branch conductance normalized by subtended leaf area (GL) at both sites was, therefore, higher in angiosperms than in conifers. Hydraulically weighted mean conduit diameters were much larger in angiosperms than in conifers, although this difference was less marked at Puyehue, the cooler of the two sites. Conduits of the vesselless rain forest angiosperm Drimys winteri J.R. & G. Forst were wider than those of coniferous associates, although narrower than angiosperm vessels. However, GL of D. winteri was within the range of values measured for vesselbearing angiosperms at the same site. The observed differences in xylem structure and function correlate with evidence that evergreen angiosperms have higher average stomatal conductance and photosynthetic capacity than their coniferous associates in southern temperate forests. Comparisons of conifers and angiosperm branches thus suggest that the superior capacity of angiosperm conduits is attributable to the development of higher gas-exchange rates per unit leaf area, rather than to a more extensive leaf area. Results also suggest that the tracheary elements of some vesselless angiosperms differ in width and hydraulic efficiency from conifer tracheids.