A 3-dimensional model of Pinus edulis and Juniperus monosperma root distributions in New Mexico: implications for soil water dynamics

Aims (1) To develop a 3D root distribution model for piñon-juniper woodland using only tree species, sizes and locations as input. (2) To interpret a two-year time series of soil moisture relative to root distributions. Methods The study was conducted in a piñon ( Pinus edulis (Englem.)) -juniper ( Juniperus monosperma (Englem.) Sarg.) woodland in New Mexico. We extracted roots from 720 soil blocks (30 cm × 10 cm × 10 cm) cut from the walls of three 10-m long and 1.5-m deep trenches. Roots were sorted by species and diameter class. Distribution models were developed for the dry weight of roots ≤5 mm in diameter. Soil water content and water potentials were measured in soil profiles under tree cluster and canopy gaps for 2 years, including a protracted dry-down period. Results Piñon had twice the root dry mass of juniper, similar to the ratio of canopy projection areas. Root densities were ca. 50% lower in soils under canopy gaps compared to tree clusters and the species ratio did not significantly differ between clusters and gaps. Piñon root density declined faster with soil depth and distance from the stem compared to juniper. A hard caliche layer at 60–80 cm soil depth had no apparent effect on the already low root density at that depth. Overall, the models explained 66% (piñon) and 54% (juniper) of the spatial variation in root density at the scale of sampled soil blocks. During an 8-month dry period, soil moisture declined faster in regions of higher root density: in shallow soil and under tree clusters. This left a reserve of plant-available soil water in the deep soil under canopy gaps. Conclusions Horizontal variation in root density in these open woodlands is predictable and an important component of the dynamic interactions between plant and soil. Under wet and dry conditions, soil water content is substantially different under tree clusters and canopy gaps.