Effects of soil temperature on the carbon exchange of taiga seedlings.: II. Photosynthesis, respiration, and conductance

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Potted seedlings of Poputustremutoides Michx., Populusbalsamifera L., Alnuscrispa (Ait.) Pursh., and Betulapapyrifera Marsh., hardwoods of the taiga of interior Alaska, were placed under soil-temperature treatments of 5, 15, and 25 °C with plant tops under a growth-room regime of 20-h day length and day–night air temperatures of 25 and 20 °C, respectively. Photosynthesis showed a differential soil-temperature effect among the species. Maximum photosynthetic rates of 9.5 mg CO 2 •h −1 •dm −2 were observed for P. tremuloides at 25 °C soil and 25 °C air temperature; 9.3 mg CO 2 •h −1 •dm −2 for P. balsamifera at the same temperature combination; and 7.8 mg CO 2 •h −1 •dm −2 for A. crispa at 25 °C air temperature and the lowest soil-temperature treatment, 5 °C. The maximum photosynthetic rate observed for B. papyrifera was 4.9 mg CO 2 •h −1 •dm −2 at 25 °C soil temperature and 20 °C air temperature; low values being due to nonsaturating light conditions. Both leaf dark respiration and conductance were reduced by the 5 °C soil-temperature treatment at one or more air temperatures in all species except A. crispa which was unaffected by cold soils. A cold-soil reduction of conductance was most pronounced at air temperatures above 20 °C.