INFLUENCE OF SOIL-MOISTURE ON THE NODULATION OF POST FIRE SEEDLINGS OF CEANOTHUS GROWING IN THE SANTA-MONICA MOUNTAINS OF SOUTHERN CALIFORNIA

During the dry season (early May through September of 1994), following a fall 1993 wildfire, a survey of seedling nodulation was conducted a t several sites in the Santa Monica Mountains of Southern California. Seedlings of Ceanothus spinosus, C. megacarpus, C. oliganthus, and C. cuneatus were manually excavated. During this period, only 12 of the 1 82 seedlings excavated were nodulated, and all of the nodulated seedli ngs were found in the relatively moist clay soils of a stream bank. No nodules were observed on the 170 seedlings excavated from the drier s ires. An irrigation experiment was established in midsummer to assess whether water stress inhibits nodulation of post-fire Ceanothus seedli ngs. Flour plots with numerous seedlings of C. cuneatus and C. spinosu s were irrigated with distilled water and monitored over a 9-week peri od. There was a significant increase in nodulation frequency, water po tential, stomatal conductance, transpiration, shoot elongation, and ph otosynthetic rate of irrigated seedlings compared with adjacent contro ls. Although these data support the hypothesis that water stress inhib its nodulation, it is unclear whether this is because of an effect of soil moisture on the nodulation capacity of the soils (i.e. on the siz e and physiological state of the soil Frankia population) or to a host plant response to drought which might prevent actinorhizal root infec tion and/or nodule development.