SUMMARYExperiments were conducted to test the relative ability of mycorrhiza! and non-mycorrhizal Douglas-fir [Pseudotsuga menziesit (Mirb,) Franco] seedlings to tolerate and recover from drought conditions, using reduction in CO, fixation as an overall indicator of piant moisture stress. Seedlings were watered daily or conditioned to cyclic drying and re-wetting of the soil. Net photosynthetic rates of mycorrhizal and non-mycorrhizal seedlings watered daily did not differ Significantly; however, drought-stressed mycorrhizal seedlings fixed COj at a rate ten times that of non-mycorrhizal seedlings. Total leaf water potentials of mycorrhizal plants were lower (more negative) than those of non-mycorrhizal plants but they recovered more rapidly.Non-mycorrhizal seedlings and seedhngs inoculated w-ith four ectomycorrhizal fungus species were allowed to become desiccated, then were rewatered and compared for their abiiitv-to tolerate and recover from drought. Seedlings inoculated with Rhizopogon vinicolor were less affected by drought than any of the other mycorrhiza! or non-mycorrhizal treatments. Net photosynthetic rate of /i/ii>o/>ogon-inocu!ated seedlings 24 h following re-w-atering was seven times that of non-mycorrhizal seedlings. The transpiration rate of Rkizopogon-\nocu\ated seedlings was low before desiccation, declined rapidly during the drought period and, after re-watering, quickly resumed a rate higher than that for other treatments, INTRODI'CTION In southern Oregon and northern California, container-grown or bare-root conifer seedlings are planted on reforestation sites in the winter and spring. First-year mortality of these seedlings can approach 100 "o on harsh sites (Herman, 1965), Plant moisture stress is probably the single most innportant cause in southern

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The objective of this work was to evaluate several mathematical models for describing N mineralization in three arctic tundra soils (Histic Pergelic Cryaquepts) as functions of time and temperature. Intact soil cores were collected in the field and incubated in a laboratory at 5, 15, 25, and 35 °C for 11 weeks. The exponential equation fit the data well at 35 °C but was generally unsuited for describing N mineralization at lower temperatures. The parabolic equation better fit the N mineralization data than the exponential equation except in a few cases where the data appeared to be approaching an asymptote. The parameters of the parabolic equation did not follow simple trends with respect to temperature, however, which made interpolating between experimental temperatures difficult. A second‐order polynomial equation with respect to both time and temperature was evaluated. This empirical equation gave R2 >0.98 in all but one case. The major advantages of the empirical, polynomial equation are that it assumes little about the nature of the N mineralization process, it is flexible, and it is accurate. The temperature data did not follow the Arrhenius relationship nor have a simple overall Q10 value because the mineralization rates at different temperatures did not follow similar trends with respect to time. Only a small fraction of the N that is potentially mineralizable is likely to be mineralized under the prevailing cold temperatures of these arctic soils.

Marion, G. M., Miller, P. C. and Black, C. H. 1987. Competition for tracer '^N in tussock tundra ecosystems. -Holarctic Ecology 10: 230-234.The objectives of this study were to assess the roles of plant species, time, and site on competition for tracer 'N (without carrier) in tussock tundra ecosystems. Six experimental sites were located in northern Alaska. After one year across the experimental sites, the recovery of "N by litter (11.3-16.3%) and mosses (5.4-16.4%) was significantly greater than for aboveground vascular plants (2.6-5.0%). "N recoveries by tundra vascular plants (2.6-5.0%) were low when compared to forest trees (9-25%) whieh suggest that competition for nitrogen is particularly severe in these eolddominated tundra ecosystems. There were no significant differences among sites in ' *N recoveries by vascular plants, by mosses, or by litter. There was a statistically significant decline in '^N recovery with time for Vaeeinium vitis-idaea and Eriophorum vaginatum between the second and third year. The shallow rooted Vaeeinium vitisideae was more highly labeled than the deep rooted £riophorum vaginatum. Nearness to the source of the applied 'N played a critical role in competition for surface applied nitrogen.