Combined application of computer tomography and light microscopy for analysis of conductive xylem area in coarse roots of European beech and Norway spruce

Axial water transport in trees is mainly determined by the gradient of negative water pressure and the structure of conductive xylem elements (i.e. conduits) connecting the fine roots with the foliage. There is still an essential lack of knowledge concerning the relationship between wood structure and hydraulic properties, especially of coarse roots. To this end, the study aimed (1) to work out a novel approach, based on the combination of computer tomography (CT) and light microscopy (LM), for determining the cumulative cross-sectional lumen area of conduits involved in the water transport of coarse roots in European beech (Fagus sylvatica) and Norway spruce (Picea abies) and (2) to demonstrate its adequacy in quantifying the functional relationship between sapwood anatomy and ascending water mass flow in the xylem. The cross-sectional sapwood area of coarse roots was assessed through CT. The cumulative cross-sectional lumen area of conduits in the sapwood (i.e. the lumen area of conductive conduits) was measured by LM in combination with interactive image analysis. The new approach was developed with coarse roots of both the tree species growing in a 60-year-old mixed forest in Bavaria, Germany. The combination of the two methods unveiled spruce to possess a distinct sapwood/heartwood boundary in small-diameter roots, whereas such roots of beech reflected a gradual transition zone; only large-diameter roots displayed a distinct boundary in beech. Additionally, the cumulative lumen area of conductive conduits was found to be approximately 12% of the total coarse root cross-sectional area in both the tree species. The new approach of measuring the conductive lumen area of coarse-root conduits yielded levels of specific sap flow (i.e. axial conductivity) that substantially differed from those derived from commonly applied methods, which were based on sap flow per unit of total cross-sectional root area or xylem cross-sectional area of individual roots. The combination of CT and LM will facilitate functional comparisons of woody roots differing in diameter and of tree species of different anatomical xylem structure.