Population size effects on germination, growth and symbiotic nitrogen fixation in an actinorhizal plant at the edge of its range

Abstract

Populations at the edge of their range are known to have decreased fitness. However, the factors that cause this and the specific plant characters involved are not well understood. To determine whether symbiotic interactions play a role in altering fitness, I examined population age, seed viability, and nodulation in relation to population size of Ceanothus herbaceous Raf. at the northern limit of its range. Tap roots from three populations were grown in each of the three population’s soils to examine the specificity between C. herbaceous and Frankia . Population size ranged from 14 to 7300 individuals. There was a positive exponential relationship between population size, seed germination, and the proportion of plants forming nodules in the field. Populations varied in age by 26 years and age was not related to size. Recruitment into these populations was episodic and was positively correlated to growing season temperatures. Nodulation rates in the growth experiment were similar to those in the field, and plants showed much greater growth when nodulated. Plants from the largest population showed the greatest growth when infected with Frankia from their own soil. These plants also had significantly higher nitrogen-fixing activity per total plant mass than all other plant–soil combinations, suggesting that the plants from this population show some degree of specificity to the Frankia they are normally found with. Plants from the largest population grown in soil from other populations showed lower nitrogen-fixing activity per nodule mass than any other plant–soil combination. There was a positive relationship between nodule mass allocation and plant growth, a trend not seen in other nitrogen-fixing symbioses where nodule allocation is much higher. These results illustrate the importance of considering both the regeneration and growth phases when evaluating the effects of small population size on plant performance