Participating Students - Alan Talhelm

Alan Talhelm
University of Nevada, Reno
Enrollment Year: 2006
Email Address: atalhelm at
Atmospheric Mentor: Richard Honrath, Michigan Technological University
Biospheric Mentor: Kurt Pregitzer, Michigan Technological University

Research Topics:
Carbon assimilation and gas exchange under long-term simulated nitrogen deposition and episodic ozone

Nitrogen (N) deposition and elevated levels of tropospheric ozone (O3) are closely linked through anthropogenic NOx emissions.  Continued growth in global NOx emission makes it important to understand how increases in O3 and N deposition may impact the global carbon (C) cycle.  This proposed research uses the resources of the PROPHET and Michigan N Gradient projects to understand leaf level gas exchange of CO2 and H2O in mature sugar maple (Acer saccharum) following episodic O3 events and 12 years of simulated N deposition.  N deposition has been suggested to increase terrestrial C sequestration through enhancement of photosynthesis, but this has rarely been tested in long-term experiments.  Forests in the Michigan N Gradient study have shown increased aboveground net primary productivity (ANPP), perhaps indicating enhanced C assimilation.  However, other studies of simulated long-term N deposition have shown reduced canopy-level C assimilation.  Evidence exists for mechanisms such as decreased stomatal conductance and reduced specific activity of Rubisco that might circumvent gains in photosynthesis from higher foliar [N].  These same mechanisms may also reduce plants' vulnerability to damage from episodic O3 events.  Hypotheses for both N deposition and O3 will be tested using canopy measurements of both instantaneous leaf gas exchange and stable isotope ratios of 13C and 18O, using the detection abilities of PROPHET to conduct measurements during both periods of elevated and background O3 concentrations.


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