The piñon pine-juniper woodland of the Western United States has been an important case study for examining processes involved in drought-induced tree mortality and its aftermath. In New Mexico, mortality of pinon pine has been particularly high. Many of the affected ecosystems have a limited organic soil horizon that is underlain by a compacted caliche layer at shallow depth. It is unknown to what extent the caliche limits the development of root systems or whether the caliche is a critical part of the rhizosphere that supports tree transpiration during drought. It is further unknown whether the two co-dominant species differentiate in terms of root distribution. As an initial step in a comprehensive analysis of the ecohydrology of this system, we excavated roots to a depth of 1.5 m along three 10-m trenches that each traversed from a woody plant clusters into an open patch. Roots were sampled in blocks of 30 cm width, 10 cm depth and 10 cm height. Roots > 0.8 mm were separated by species and the length of roots up to 5 mm diameter were determined through image analysis.
Results/Conclusions
The typical soil profile at the field site was composed of organic soil to a depth of 40 cm, a transition zone of relatively loose caliche from 40 to 100 cm and compacted caliche further down below. For both species combined, 93% of the total sampled root length was in the top 100 cm. The fine root length (diameter 0.8-2.0 mm) was consistently higher for piñon pine than juniper. Fine root densities were greater under woody plant clusters than in the open, but for juniper the difference was smaller. Piñon pine had a greater percentage of fine roots in the organic soil, ca. 78% compared to 68% for juniper. Few roots were observed at 150 cm, but at this depth more juniper than pinon pine roots were observed. These results suggest water stored in hard caliche is predominantly available to juniper, who can tolerate extremely low water potentials and may use this reservoir to survive extreme drought. Piñon pine trees appear to be more specialized on exploiting relatively shallow soil moisture, which may cause a more rapid onset of drought stress and a longer time spent in negative carbon balance.