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FOLIAR N RESPONSE OF PONDEROSA PINE SEEDLINGS TO ELEVATED CO2 AND O3
Olszyk, D M., D T. Tingey, AND W E. Hogsett. FOLIAR N RESPONSE OF PONDEROSA PINE SEEDLINGS TO ELEVATED CO2 AND O3. Presented at 6th International Symposium on Air Pollution and Global Change, Tsukuba, Japan, October 19-22, 2004.
Interactions between needle N status and exposure to combined CO2 and O3 stresses were studied in Pinus ponderosa seedlings. The seedlings were grown for three years (April 1998 through March 2001) in outdoor chambers in native soils from eastern Oregon, and exposed to ambient or elevated (+ 280 ppm) CO2, with low or elevated O3. Seedlings were not fertilized, but relied on litter and soil processes for N. During the last 2 ? years of the study, abscised leaves were collected ~ biweekly. Green needles on trees were sampled yearly in April, in July and in October at the end of the main leaf abscission periods in 1999 and 2000, and at final harvest. The pattern in total needle N on an area basis (Narea) over the course of the 3 years was indicative of a soil limited in N. Narea was greatest in the leaves emerging during the study's first season (1998 age class), reaching > 1 mg N cm-2 by July 1999; and then declining to 0.2 mg N cm-2 by July 2000. The 1999 needle age class had less initial N than the 1998 age class (~ 0.7 mg N cm-2) which declined to ~0.4 mg N cm-2 by March 2001. The 2000 age class had <0.4 mg N cm-2 N at emergence and through March 2001. Elevated CO2 had little effect on Narea. However O3 affected all three age classes, especially the 1999 needles on trees where O3 reduced Narea. Neither CO2 nor O3 affected Narea retranslocation or resorption. Elevated CO2, but not O3, increased needle abscission, likely due to increased leaf production early during the study when Narea was high. By the end of the study when Narea was low, neither CO2 nor O3 significantly affected total leaf biomass.
This study indicated that soil N fertility in an ecosystem and resulting needle N status, is a major factor determining tree responses to air pollutants. For Pinus ponderosa seedlings growing under realistic soil N conditions, needle production and abscission increased with elevated CO2 at high Narea but not with low N area. With low needle Narea O3 further decreased Narea but did not affect needle abscission.