The Impact of Air Pollution on Ponderosa Pine Growth in the San Bernardino National Forest

Keywords: Pollution, Tree Growth, Southern California
Session Type: Virtual Paper
Day: Sunday
Session Start / End Time: 4/11/2021 08:00 AM (Pacific Time (US & Canada)) - 4/11/2021 09:15 AM (Pacific Time (US & Canada))
Room: Virtual 26

Abstract

The ponderosa pine (Pinus Ponderosa) is a climate sensitive tree species dominant in the mixed conifer stands of the San Bernardino Mountains of California. However, their close proximity to the city of Los Angeles has resulted in extremely high levels of air pollution. Nitrogen deposition, resulting from nitrous oxides emitted from incomplete combustion of fossil fuels, has been observed in this region since the 1980’s. 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. 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 chronology is generated from 34 tree cores taken near Crestline, CA. 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 NO2 pollution during the growing year exhibit primary controls on growth (r2-adj = 0.65, p<0.001). N deposition therefore appears to have a strong and direct positive impact on ponderosa pine growth in this region. However, these results should be interpreted cautiously, given that ozone, a known stressor to ponderosa pine trees, and NO2 are also highly correlated (r = 0.84, p<0.05). 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.