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Chemistry and Long-Term Decomposition of Roots of Douglas-Fir Grown under Elevated Atmospheric Carbon Dioxide and Warming Conditions

^a Biology Dep., Univ. of Illinois at Springfield, One University Plaza, Springfield, IL 62703
^b U.S. Environmental Protection Agency, National Health and Environmental Effects Research Lab., Western Ecology Div., 200 SW 35th St., Corvallis, OR 97333
^c Dep. of Forest Science, Oregon State Univ., 321 Richardson Hall, Corvallis, OR 97331
^d School of Forestry and Wildlife Sciences, Auburn Univ., AL 36849
^e Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun Town, Mengla County, Yunnan Province, 666303, P.R. China

^* Corresponding author (hchen40{at}uis.edu).

Received for publication May 25, 2007. Elevated atmospheric CO₂ concentrations and warming may affect the quality of litters of forest plants and their subsequent decomposition in ecosystems, thereby potentially affecting the global carbon cycle. However, few data on root tissues are available to test this feedback to the atmosphere. In this study, we used fine (diameter 2 mm) and small (2–10 mm) roots of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings that were grown for 4 yr in a 2 x 2 factorial experiment: ambient or elevated (+ 180 ppm) atmospheric CO₂ concentrations, and ambient or elevated (+3.8°C) atmospheric temperature. Exposure to elevated CO₂ significantly increased water-soluble extractives concentration (%WSE), but had little effect on the concentration of N, cellulose, and lignin of roots. Elevated temperature had no effect on substrate quality except for increasing %WSE and decreasing the %lignin content of fine roots. No significant interaction was found between CO₂ and temperature treatments on substrate quality, except for %WSE of the fine roots. Short-term ( 9 mo) root decomposition in the field indicated that the roots from the ambient CO₂ and ambient temperature treatment had the slowest rate. However, over a longer period of incubation (9–36 mo) the influence of initial substrate quality on root decomposition diminished. Instead, the location of the field incubation sites exhibited significant control on decomposition. Roots at the warmer, low elevation site decomposed significantly faster than the ones at the cooler, high elevation site. This study indicates that short-term decomposition and long-term responses are not similar. It also suggests that increasing atmospheric CO₂ had little effect on the carbon storage of Douglas-fir old-growth forests of the Pacific Northwest.

Abbreviations: ACAT, ambient CO₂ and ambient temperature • ACET, ambient CO₂ and elevated temperature • ECAT, elevated CO₂ and ambient temperature • ECET, elevated CO₂ and elevated temperature