Submitted to: Great Basin Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2000
Publication Date: N/A
Interpretive Summary: In the Sierra Mountains of northern California, regeneration of Jeffrey pine can be spotty. In particular, regeneration is difficult on sites dominated by the herbaceous species mules-ear. Reestablishment of Jeffrey pine is important to landowners that use these forested rangelands for multiple economic benefits. There has been research which suggests that as shrubs move onto a site, the soil becomes more hospitable for pine seedlings. However, in this greenhouse study we did not find compelling evidence that changes in soil microbiology or soil chemistry under mules-ear plants would limit longer term growth of Jeffrey pine seedlings. Our results suggest that higher temperatures at the soil surface or increased competition for soil water are more likely explanations for poor Jeffrey pine establishment on mules-ear sites.
Technical Abstract: Regeneration of Pinus jeffreyi in the Sierra Nevada is often limited on sites dominated by Wyethia mollis. Allelopathy and competition for soil moisture have been suggested as possible mechanisms for limiting regeneration. We tested the hypothesis that soil chemical and microbial properties from sites in different stages of succession influence seedling growth of Pinus jeffreyi. Soil was collected from an early seral site dominated by Wyethia mollis, a mid-seral site dominated by shrubs, Arctostaphylos patula, Ceanothus prostratus, C. velutinus, and Purshia tridentata, and late seral site dominated by mature Pinus. Soil (0-33cm) from the early seral site had the lowest C, microbial biomass and fungal and bacterial populations. The early seral site had the lowest soil Ca and Mg contents but also had a lower C/N ratio and more than two-fold higher P content than either the mid- or late seral site. Pinus seedling growth and foliar nutrient concentrations were compared at three harvest dates (220,314,& 417 days after germination) in a greenhouse bioassay. Fresh litter from each site was added monthly to the soil surface. Seedling growth in the early seral soil was initially suppressed in comparison to growth in the mid-seral soil. Total seedling weight was similar between these two treatments by the final harvest however. The most obvious treatment effect was reduction in growth for seedlings planted in late seral soil. Seedlings grown in late seral soil had Fe & Al levels that were nearly twice those of seedlings grown in early & mid-seral soils. Our interpretation of the results is that it is unlikely that allelopathy or soil nutrient deficiencies resulting from the presence of Wyethia are responsible for limited growth of Pinus seedlings.