Research Objectives

Anthropogenic nitrogen deposition is expanding globally as human populations and activities increase. Elevated N impacts ecosystems by causing changes in N cycling fire, and hydrology, and large-scale shifts in vegetation composition.

Our overall goal is to test complex systems theory by comparing different ecosystems under N deposition and plant invasions, some undergoing irreversible changes, and others stable but showing initial changes. Variable thresholds and non-linear dynamics for different vegetation types will be modeled and measured by examining N inputs, vegetation productivity, and response to N. Irreversible change is also expressed as an emergent property at the landscape scale by fire dynamics of invasive grasses, in that the limiting step for fire shifts from a cumulative process of slow fuel accumulation by shrubs, to rapid, high-production flash-fuel (cured grasses).

Our objectives are to determine the thresholds of N deposition for vegetation change, and how N and C cycling are affected. We will use a combination of N fertilization experiments and vegetation and diversity measurements in four vegetation types along N deposition gradients; reanalysis of historic vegetation plots and remote sensing; measurements of N cycling to determine rates of N supply, N accumulation, and loss; examination of feedbacks of exotic annual grass invasion on N cycling; measurement and modeling of atmospheric N inputs to determine past, present, and future N loading; and use a fire model and a landscape transition model to show how elevated N and precipitation affect exotic grass biomass, the fire cycle, and vegetation change. We will compare possible impacts associated with different development strategies considered by local planning officials. Thus our research will examine complex linkages among human activities and population growth, urban to regional scale air quality and meteorology and resulting ecosystem forcing.
 

Hypotheses related to N emissions, N deposition, and N atmospheric modeling

1. Atmospheric concentrations of N pollutants and O3 in four different ecosystems in southern California will show pronounced spatial and seasonal variation.
2. The chemical partioning, atmospheric lifetime and rates of deposition of N compounds will exhibit interannual shifts due to changes in anthropogenic emissions of VOC and N species as well as changes in the ambient ratios of VOC to N.
3. High NOx and VOC emissions in the 1970s produced very different rates and distributions of N deposition from the present day; emissions will be lower in 2018 but will be deposited at greater distances downwind from urban source areas.

Hypotheses related to soil N and C turnover and hydrologic losses

1. Nitrogen mineralization and nitrification rates in all four vegetation types will increase with N deposition and N amendments.
2. Enhanced nitrification in N enriched sites will result in significantly greater pools of nitrate in soil. Nitrate will accumulate to very high levels in all four vegetation types during dry periods and will lead to large hydrologic nitrate export from forest and chaparral systems, less from arid desert and CSS.
3. N deposition reduces C storage capacity and reduces SOM. This is caused by increased turnover and increased fire frequency.
4. Differences in soil hydraulic properties and climatic forcing lead to different moisture regimes and different levels of inorganic N in soils and thus different susceptibilities to plant invasions.
5. Plant invasions in turn change biogeochemical and hydrologic conditions that maintain the long-term change of vegetation to the invasive species.
6. Leaching losses of N will follow the pattern forest > chaparral > CSS > desert. Hypotheses for Hypotheses on N impacts on vegetation and thresholds of change
7. N deposition has variable impacts on different vegetation types, creating a complex mosaic of responses along N deposition gradients at the landscape level.
8. N deposition and wet periods act together to promote invasive plant species and fires.
9. Exotic grass accumulation of 0.5-1.0 t/ha standing biomass is a threshold that changes the fire cycle and promotes a permanent transition to exotic grass domination.
10. The elevated productivity of exotic grasses negatively impacts native species richness.