In the course of its long life, a tree confronts environmental conditions that range from natural variation in local weather or regional climate to large scale alteration of the earth’s atmosphere. Forest ecosystems are modified and potentially degraded by an array of anthropogenic enterprises, not the least of which is air pollution. Environmental change can alter ecosystem patterns and processes, particularly when effects accumulate over the long term or multiple factors interact. My dissertation research examines two key aspects of forest ecosystem dynamics in response to altered environmental conditions over the long term. First, I examine mortality, and in particular standing dead trees, one of the predominant physical consequences of forest ecosystem stress. This work quantifies the decay patterns of six common species of California’s mixed conifer forests, revealing the role of standing dead trees in forest carbon budgets. Next, my research examines influences on the growth and vitality of live trees in the southern Sierra Nevada, a forested region impacted by chronic ozone pollution. This work encompasses the regional patterns of ecosystem exposure to ozone pollution, long term monitoring of ozone-induced injury to ponderosa and Jeffrey pine trees (Pinus ponderosa and Pinus Jeffreyi), and a description of tree growth responses to pollution in light of their simultaneous responses to climate.
Forest mortality is always an important part of ecosystem processes, but in recent years, elevated mortality rates have increased the relative abundance of dead trees in forests across the Western United States. Though the importance of woody debris to ecosystem processes is clear, the structural and biogeochemical contributions of standing dead trees remain largely unknown. The first chapter of my dissertation characterizes the decay patterns and carbon density of standing dead trees in Sierra Nevada mixed conifer forests, examining traits in six dominant species. I used a dimensional analysis to describe the patterns of wood density, carbon concentration, and net carbon density. As decay class advanced, trees showed a progressively lower density and a small increase in carbon concentration. Net carbon density of the most decayed standing dead trees was only 60% that of live trees. The key characteristics that determined these patterns were species, surface to volume ratio, and relative position within each tree. Decay while standing and estimation of deadwood biomass in large scale inventories also have repercussions in greenhouse gas accounting. When the measured changes in carbon density were applied to standing dead carbon stock estimates for California mixed conifer forests, the decay-adjusted estimates were 18% (3.66-3.74 teragrams) lower than estimates that did not incorporate change due to decay.
In the second and third chapters, I focus on anthropogenic ozone pollution, a major stressor in southern Sierra Nevada forests. Ozone poses a risk to ecosystems worldwide because of its damaging effects on plant tissues and the carbon fixation they carry out. Ozone is a secondary pollutant formed by the reaction of nitrogen oxides and oxygen in the presence of sunlight and heat. Elevated tropospheric ozone has impacted parts of southern California, the San Joaquin Valley, and the southern Sierra Nevada for more than 40 years. This field-based research relies on data collected in Sequoia and Kings Canyon National Parks and on the Sierra National Forest.
Chapter two investigates the connections between ozone exposure and injury to trees. The tools of this study were a long term air quality monitoring network across a regional gradient of ozone concentration and repeat measures of pollution injury in ponderosa and Jeffrey pines. I used these measures to quantify trends in ozone concentration, assess patterns in ozone-caused foliar injury, and understand tree demographic responses to ozone exposure. Since region-wide observations began in 1991, air quality has improved, but across much of the mixed conifer forest, ozone exposure is still high enough to cause permanent damage to ecosystems. Chlorotic mottle, the key symptom of pollution injury in ponderosa and Jeffrey pines, continues to provide evidence of physiological impacts to trees but has also incrementally declined in recent years. Because growth is a leading indicator of tree vitality and forest ecosystem condition, in this study I also remeasured tree diameters to determine the long term relative growth rates of individuals exposed to ozone pollution. Relative to asymptomatic trees, typical ozone-injured trees from the most polluted sites had growth reduced by up to 24%. Over the 20-year study survival of damaged trees was lowest at high pollution levels, but within the range of rates in similar forests. The pollution-injured pines that make up southern Sierra Nevada forests today clearly have the capacity for recovery, but will continue to bear a legacy of anthropogenic impacts.
In the third chapter, I examine how Sierra Nevada forest ecosystems respond to climatic conditions and chronic ozone pollution, both individually and interactively. The gradient of pollution exposure on the western slope of the southern Sierra Nevada enabled a comparison of annual tree growth under very low to severe summer ozone levels, across sites with shared climatic conditions. I used the Jeffrey pine tree ring record to characterize growth as shaped by these conditions. First, I found that the temperature and precipitation of the preceding winter and summer have an important influence on annual growth. Building on this understanding of climatic dependency, analysis showed that trees exposed to elevated ozone had slower annual growth rates than their counterparts in relatively unpolluted locations. Annual growth rates in severely polluted sites were 8.4-23% lower than predicted growth under conditions that meet current air quality standards. Although the isolated effects of both ozone and water limitation are negative, an antagonistic interaction between these environmental factors was also apparent. As predicted in earlier research, high summer temperatures limited the negative growth impacts of ozone pollution. The likely mechanism for this interaction amongst stressors is stomatal closure, which prevents uptake of ozone into the leaf. These growth losses, attributable to a chronic anthropogenic stressor and modified by prevailing environmental conditions, may facilitate further change in forest processes.
