Submitted to: Western North American Naturalist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/5/2001
Publication Date: 10/20/2002
Citation: RIEGEL, G.M., SVEJCAR, A.J., BUSSE, M.D. DOES THE PRESENCE OF WYETHIA MOLLIS AFFECT GROWTH OF PINUS JEFFREYI SEEDLINGS?. WESTERN NORTH AMERICAN NATURALIST. 2002. 62(2):141-150. 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. Allelopathic chemicals 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 dominated by the shrubs Arctostaphylos patula, Ceanothus prostratus, C. Velutinus, and Purshia tridentata, and a late-seral site dominated by mature Pinus. These sites were compared for nutrient content, Pinus seedling growth capacity, and microbial population size. Soil (0-33 cm) from the early-seral site had the lowest C, microbial biomass, and fungal and bacterial populations. There were no consistent trends in soil nutrient content among sites. The early-seral site had the lowest soil Ca and Mg contents but also had a lower C/N ratio and more than twofold greater P content than either the mid- or late-seral site. Pinus seedling growth and foliar nutrient concentrations were compared at 3 harvest dates, (220, 314, and 417 days after germination) in a greenhouse bioassay. The treatment design was a 3 x 2 factorial with soil from each of the 3 sites either with or without Pinus seedlings. Pots without seedlings were used as controls to assess the effects of seedlings on microbial biomass. Seedling growth in the early-seral soil was initially suppressed in comparison to growth in the mid-seral soil, but by the final harvest total seedling weight was similar between these 2 treatments. The most obvious treatment effect was a reduction in growth for seedlings planted in late-seral soil, probably due to a nutrient imbalance in the soil. Seedlings grown in late-seral soil had Fe and Al levels that were nearly twice those of seedlings grown in early-and mid-seral soils. Microbial biomass followed a temporal pattern similar to that found for seedling growth. Differences in microbial biomass between the early- and mid-seral soils, although initially large, were not detected by the final harvest. We interpret these results to indicate that allelopathy or soil nutrient deficiencies resulting from the present of Wyethia are unlikely to be responsible for limited growth of Pinus seedlings in Wyethia-dominated stands.