Thesis (Ph. D.)--University of Arizona, 1995.

This study is the geochemical examination of mineral weathering and its path from hinterland, through sediment deposition and pedogenesis, to its dissolution and eventual uptake into plants or precipitation as carbonate minerals. The three papers examine the rate and character of carbonate and silicate mineral weathering over a wide range of climatic and tectonic regimes, time periods, and lithologies, and focus on very different questions. Examination of the 87Sr/86Sr ratios of architectural ponderosa pine in Chaco Canyon, New Mexico confirms a societally complex style of timber procurement from the 10th to the 12th centuries. In El Malpais National Monument, New Mexico, we measured the 87Sr/86Sr ratios in local bedrock and soils and compared them to the leaf/wood cellulose of four conifers (Pinus ponderosa, Pinus edulis, Juniperus monosperma, Juniperus scopulorum), a deciduous tree (Populus tremuloides), three shrubs (Chrysothamus nauseosus, Fallugia paradoxa, Rhus trilobata), and an annual grass (Bouteloua gracilis) and a lichen (Xanthoparmelia lineola). We found that plant 87Sr/86Sr ratios covaried with variations in plant physiognomy, life history, and rooting depth. In addition, the proportion of atmospheric dust and bedrock mineral contributions to soil water 87Sr/86Sr ratios varied predictably with landscape age and bedrock lithology. On the Himalayan floodplain, soils and paleosol silicate weathering intensities were measured along a climatic transect and through time. Overall, carbonate weathering dominates floodplain weathering. But, periods of more intense silicate weathering between 9 - 2 Ma, identified in soil profile and in the 87Sr/86Sr ratios of pedogenic carbonates, appear to be driven by changes in tectonic, rather than climatic, regime. All three papers are good examples of how 87Sr/86Sr isotopic tracer studies can shed light on pedogenic formation rates and internal processes. The complexity of each system warns against generalizations based on just one locale, one species or lithology, or a few isotopic ratios.

My dissertation research addressed woodland dynamics and dendroclimatology on the volcanic badlands of western New Mexico. The research was intended to complement previous studies by: (1) assessing vegetation structure and composition dynamics at El Malpais National Monument between 1948–2010 using repeat photography; (2) improving knowledge of the influence of climate and land use on vegetation dynamics at El Malpais National Monument; (3) providing a unique tree-ring data set from Rocky Mountain juniper growing on the malpais; (4) elucidating relationships between Pacific teleconnections and radial growth in Rocky Mountain juniper; and (5) improving understanding of the dynamic nature of climate in the Southwest. I used tree-ring data from the interior of the Bandera Lava Flow and repeat-photography sequences from a nearby location at the edge of the flow to assess vegetation changes at two ecologically different locations on the malpais. I concluded that noticeable vegetation changes occurred during the 20th and early 21st centuries at the periphery of the Bandera Lava Flow. Vegetation changes at the lava-substrate interface could be linked to human activity, resource management, and drought. I also sampled Rocky Mountain junipers on a lava flow in Cibola National Forest to produce a multi-century tree-ring chronology. The data set is the first Rocky Mountain juniper chronology produced in New Mexico and is one of few conifer chronologies from the Southwest with a significant temperature-growth relationship. Dendroclimatic analyses identified growth relationships with monthly mean temperature, monthly total precipitation, monthly PDSI, and local water year precipitation. Trees appeared most sensitive to climate factors that influence and indicate moisture availability during dry periods of the growing season. Tree-ring data indicated positive relationships between SSTs in the El Niño 3.4 region of the central Pacific Ocean and Rocky Mountain junipers on the malpais. Positive PDO-growth relationships during the cool months prior to current growing season further suggest a link between SSTs in the Pacific Ocean and trees on the badlands. Positive relationships between monthly PNA index values and annual radial growth may result from the large distances between the malpais and PNA centers of activity.

The purpose of this research was to: (1) reconstruct climate for the malpais region from long-lived trees and remnant wood; (2) reconstruct the fire history of forests in the malpais; and (3) investigate short-term and long-term relationships between wildfire and climate. To reconstruct climate, I calibrated a 2,129 year long tree-ring chronology (136 BC - AD 1992) with annual rainfall (previous July to current July). Since AD 100, seven major long-term trends in rainfall occurred. Above normal rainfall occurred during AD 81-257, 521-660, 1024-1398 and 179 1- 1992, while below normal rainfall occurred during AD 258-520, 661-1023 and 1399- 1790. The prolonged drought from AD 258-520 was unsurpassed in its intensity, while rainfall during the most recent 200 years has exceeded any since AD 660. The reconstruction of long-term climate trends confirmed the general sequence of environmental change over the last 2,000 years for the southern Colorado Plateau. To reconstruct past fire occurrences, 217 fire-scarred trees were collected from nine sites representing the major habitat types of the malpais and dendrochronologically dated. Fire frequency was highest at sites on cinder cones and on the highly-weathered basalt flows (ca. once every five years), and lowest on the isolated kipukas and on the Hoya de Cibola Lava Flow (once every 10-12 years). Fire frequency decreased along a north to south gradient, reflecting changing vegetation properties. Combined information revealed fire occurred once every two years, while more widespread fires occurred once every 2.5 years. Fires were largely asynchronous between sites, suggesting the malpais landscape effectively hinders fire spread. Past fire history at El Malpais was characterized by four temporally distinct periods: (1) FH-1 (prior to 1782): high fire frequency, patchy fires, throughout the growing season; (2) FH-2 (1795 - 1880): longer fire intervals, widespread fires, mostly early season fires; (3) FH-3 (1893 - 1939): even longer intervals, decreased widespread fires; (4) FH-4 (1940 - 1992): longest fire-free periods during the last 600 years. The increase in rainfall and the simultaneous change in fire regimes ca. 1790 was likely related to an increase in summer monsoonal rainfall due to changes in hemispheric circulation patterns. The decrease in fire spread ca. 1880 was most likely due to intense sheep grazing, while the change ca. 1940 reflects greater efficiency in fire suppression techniques. The presettlement fire regime emphasizes that the current absence of fire in the monument exceeds the historical range of variability established for the presettlement period. Unless effects of past humanrelated disturbances are mitigated, fire regimes of El Malpais will continue to favor high-intensity, catastrophic fires.