Sperry’s packing rule affects the spatial proximity but not clustering of xylem conduits: the case of Fagus sylvatica L.

What is it about?

Sperry’s packing rule predicts the optimum packing of xylem conduits in woody plants, where the frequency of xylem conduits varies approximately inversely with the square of the conduit radius. By analysing xylem vessel features in a long-term anatomical time series of F. sylvatica L. taken at different stem heights we challenged the hypothesis of an ontogenetic effect on the vessels’ spatial distribution inside tree-rings. Our results indicated that the probability of finding a greater number of vessels within the annual tree-ring increases with the distance from the stem base, and the distance between neighbouring vessels is reduced. Although there is an increase in the intensity and proximity among vessels, we demonstrated that no patterns of aggregation between vessels existed. Rather, point pattern profiles clearly highlighted a lack of aggregation of vessels in the face of a regular spatial distribution of vessels in the growth rings along with stems. Thus, increasing the frequency of vessels at decreasing distance to the tree apex in diffuse-porous wood, the grouping will not occur merely because of packing. We argued that although the importance of vessel clustering has been recognized by many authors as a valid compromise to maximize hydraulic efficiency within the xylem network, our results sustain the hypothesis that the clustering of vessels may be affected more by the environmental variability (e.g., climate) rather than by ontogenetic constraints.

Featured Image