This project will execute a research strategy to determine the processes underlying tree ring formation, from the individual tree level tree-ring formation to regional patterns of plant growth at the landscape scale. The researcher will also use field and remote sensing observations, combined with field manipulations, to establish the causal relationships of climate variability and tree ring growth. The project builds on an EPSCOR-funded field program called Nev-CAN (Nevada Climate-Ecohydrological Assessment Network), which will provide high temporal and spatial resolution monitoring of atmospheric, soil and vegetation variables using in place instrumentation and remote sensing. The study areas are the Sheep and Snake Ranges in Nevada, where the primary tree vegetation is mountain conifers, including the Bristlecone pine, a tree used extensively in paleoclimate reconstructions using tree rings.
The broader impacts of the proposal include postdoctoral mentoring and an international collaboration with a Canadian group active in this type of work. The study has the potential to improve our understanding of how bristlecone pine, a tree that provides some of the longest and most important paleoclimate records, reacts to climate variability. In addition, the results of this work may contribute to improved risk assessments for Great Basin ecosystems.
Long-term variability of water supply is not well understood. Therefore, estimations of surface precipitation and air temperature derived from tree-ring records have been used to extend records of past climate. Great Basin conifers, such as bristlecone pine, have provided some of these very long records. To better understand what controls the formation of tree rings, we have used cellular (wood anatomy) data. Our studies have taken place in Nevada mountains, where multiple, solar-powered, automated sensors have been installed. This field infrastructure, which requires very large human and financial investments, was already in place. At these NevCAN ("Nevada Climate-ecohydrological Assessment Network") sites we collected micro cores from tree stems to understand how wood is formed during the growing season. This project has jump-started cellular-level dendroclimatology in the western US, and it represents a high-risk / high-payoff project because this kind of measurements have not been conducted before on these conifer species. Wood anatomy and tree-ring characteristics of conifer species were analyzed at a lower and a higher location within a northern and a southern site. At the southern (drier and warmer) sites, trees (Pinus monophylla) had smaller cell lumen, tracheid diameter, and cell wall thickness. Pinus monophylla and Pinus flexilis showed bigger cellular elements at the higher elevations whereas the opposite pattern was found in Picea engelmannii and Pinus longaeva. When all species and sites were pooled together, stem diameter was positively related with earlywood anatomical parameters. Changes in anatomical parameters between species and elevations indicated substantial interplay between temperature and moisture as the limiting factors for tree-ring development in high-elevation conifers. Additional research is being performed to further clarify climatic influences on wood formation. With regard to stem size variations, we found that Pinus monophylla is characterized by an annual cycle with amplitude normally below 1.0 mm, and showing a monotonic steep increase from May to July. Stem shrinkage up to 0.2 mm occurred in late summer, followed by an abrupt expansion of up to 0.5 mm in the fall, at the arrival of the new water year precipitation. Subsequent winter shrinkage and enlargement were less than 0.3mm each. Changes in precipitation regime, which affected the diel phases especially when lasting more than 5–6 h, could substantially influence the dynamics of water depletion and replenishment of this widespread pine species. Other significant results were provided by the development of task-specific software for tree-ring analysis (the R package treeclim), which has been placed in the public domain.
