![]() ![]() However, most of these studies have been conducted in the Sierra Nevada, where ponderosa pine trees dominate the interior forest of that region. Tree ring growth in this seminal species has been found to be sensitive to changes in precipitation, water availability, the El Niño Southern Oscillation, and drought. Lawson) has long been used as a proxy for climate information in the American Southwest. The ponderosa pine ( Pinus ponderosa, Douglas ex C. Chronic exposure to both ozone and nitrogen dioxide may, therefore, have unexpected impacts on tree sensitivity to climate and other stressors in a warming world. However, it is important to note that ozone, a known stressor to ponderosa pine trees, and NO 2 are also highly correlated (r = 0.84, p < 0.05). The direct correlation between NO 2 and tree growth suggests that N deposition has a positive impact on ponderosa pine bole growth in this region. A stepwise multiple regression between the tree ring chronology and various climate and air pollution stressors indicates that drought conditions at the end of the rainy season (March) and NO 2 pollution during the water year (pOct-Sep) exhibit primary controls on growth (r 2-adj = 0.65, p < 0.001). The chronology is generated from 34 tree cores taken near Crestline, CA. Here I use a 160-year-long (1855–2015) ponderosa pine tree ring chronology to examine the annual response of tree growth to both N deposition and climate in this region. The impact of this N deposition on ponderosa pine growth is complex and often obscured by other stressors including climate, bark beetle attack, and tropospheric ozone pollution. Nitrogen (N) deposition, resulting from nitrous oxides emitted from incomplete combustion of fossil fuels, has been recorded in this region since the 1980s. However, the close proximity to the city of Los Angeles has resulted in extremely high levels of air pollution. Lawson) is a climate-sensitive tree species dominant in the mixed conifer stands of the San Bernardino Mountains of California.
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