The hydraulic architecture of conifers

Abstract

The water-conducting cells in the xylem of conifers are tracheids. Despite the fact that these conduits are limited in diameter and length, the tracheid-based xylem structure of conifers supports the largest and tallest trees. This is explicable in the light of highly conductive pits and the fact that a large fraction of conifer wood is occupied by conduits owing to the double role of tracheids in transport and mechanical support. In habitats where conduit diameter is constrained by freezing and/or drought stress, tracheid- and vessel-based xylem may exhibit similar xylem area-specific conductivities, and this may help conifers compete with angiosperms. Timberline conifers are exposed to many freeze-thaw cycles during winter; these frost cycles in combination with low water potentials reduce hydraulic conductivity. Conifer stems can often recover hydraulic conductivity in late winter and early spring, even when soils remain frozen. Water for recovery may be taken up via the needles. Conifer needles are marvels in terms of tissue complexity and longevity, and recent studies have explored needle hydraulics and aquaporin function. Picea and Pinus species exhibited considerable potential for acclimating to different environments. However, recent reports of piñon mortality indicate that there are genetically determined limits to drought tolerance. While water loss can be regulated in the short term, the potential of xylem to become more resistant to drought-induced cavitation during development appears to be rather limited. Progress has been made in linking differences in cavitation resistance with pit properties, and future work will likely lead to a better understanding of the cavitation mechanism(s) in conifer xylem. As we learn more about conifer xylem, we come to appreciate both its simplicity and its elegance.