Pinyon and juniper woodland encroachment influence on root density and below-ground carbon and nitrogen

Background/Question/Methods

Vegetation changes associated with climate shifts and anthropogenic disturbance are thought to have major impacts on biogeochemical cycling and soils. Much of the Great Basin, U.S. is currently dominated by sagebrush (Artemisia tridentate ssp.  (Rydb.) Boivin) ecosystems. At intermediate elevations, sagebrush ecosystems are increasingly influenced by pinyon (Pinus monophylla Torr. & Frém and Pinus edulis Engelm.) and juniper (Juniperus osteosperma Torr. And Juniperus occidentalis Hook.) expansion. Some scientists and policy makers believe that increasing woodland cover in the intermountain western U.S. will create new carbon storage on the landscape; however, little is currently known about the distribution of carbon on these landscapes. This is especially true of below ground pools. Our objectives were to quantify the spatial distribution of roots, soil carbon, and nitrogen in expansion woodlands. This study is part of a Joint Fire Sciences funded project called the Sagebrush Treatment Evaluation Project (SageSTEP). The 13 woodland sites sampled for this study can be organized into three regions, each reflecting the dominant tree species involved. The Western Juniper Region has five sites located in Oregon, Idaho, and Northern California,. The Pinyon-Juniper Region has four sites clustered in east-central Nevada, and the Utah Juniper Region consists of four sites in western Utah. The 13 woodland sites span a geographic range of more than 800 km, and represent conditions that vary considerably in elevation, topography, soils, and climate. Each site contained at least three core plots which will be given a fuels reduction treatment. Within each core plot we sampled three sub-plots which represent a different phase of tree encroachment into sagebrush systems. Phase I (shrub-dominated stands), Phase II (shrubs and trees share dominance), and Phase III (tree-dominated stands). Soil cores were taken using a mechanically driven diamond tipped core drill to a depth of 90 cm, or until bedrock or a restrictive layer was encountered. Samples were taken in 15 cm increments, dried, sieved to 2 mm, and roots were separated from rock by flotation. Soil and roots were analyzed for total C and N using a LECOÔ C and N determinator.

Results/Conclusions

Initial results show that increasing tree cover does not necessarily affect total soil C and N. However, there is a large increase in root biomass associated with the transition to phase III woodlands. Over longer periods increased root mass may influence total C and N pools.