Invariable hydraulics and rapidly tapering cross-sectional areas of lateral roots reduce water transport capacity to half at the crown periphery of mature Scots pine

Dynamics of soil water extraction reflect soil texture and moisture, and the three-dimensional architecture and hydraulics of root systems. Lateral roots, placing fine roots at less exploited distances from trees, may become the primary source of water during rainless periods. Yet, little is known about their capacity to deliver water absorbed from their distal reach. Cross-sectional area-specific saturated hydraulic conductivity (Ksat) and vulnerability to cavitation were estimated along lateral roots of 22- and 80-year-old (YO) Pinus sylvestris on sandy soils. Assessing the capacity to deliver water, the cross-sectional area of lateral roots (CSAL) was estimated along roots based on derived allometric relationships, expanding the scope of inference using published data on 13, 30 and 50YO trees. The results were used to estimate the capacity of roots to deliver water from different distances under homogeneous soil conditions. Excluding the portion of CSAL contributing to mechanical support in the 22YO stand, Ksat was invariable along roots in both stands, suggesting that the capacity to deliver water is proportional to the sum of CSAL across annuli surrounding trees. In the 80YO stand, for which all necessary data were collected, CSAL drops to 50% at 2 − 3 m from stems, coinciding with the periphery of crown projections. Within the average reach of roots in the 80YO stand (8.13 ± 2.81 m), the ground area to the point representing 50% of water transport capacity of lateral roots comprises only ∼13% of the total area tapped by these roots, suggesting lower water extraction in distal locations under similar soil moisture. Soil water depletion likely ripples between rain events from the area under crowns to inter-crown gaps, making lateral roots progressively more important as drought durations increase. One-dimensional rendering of soil moisture availability will fail to capture the effects of these dynamics on canopy conductance and photosynthesis.

Duke Scholars

Author Ram Oren Earth and Climate Sciences