Friday, August 6, 2010

PS 102-105: Effects of Differential Brush Competition on Giant Sequoia and Mixed Conifer Associates in a Common Garden Trial

Christopher M. Valness, Humboldt State University


Climate change forecasts for the native range of giant sequoia (Sequoiadendron giganteum (Lindl.) Buchholz) predict reductions in the amount and duration of snowpack and resulting increases in tree mortality.  Giant sequoia is reliant on this snowpack for soil moisture throughout the dry growing season.  With an already limited distribution, dramatic changes may occur to the structure of native giant sequoia groves.  If out-planting is necessary for the conservation of giant sequoia, research on the effects of competition with brush species and associated mixed conifer species of the Sierra Nevada will help guide conservation strategies.  In a common-garden trial established at Foresthill, CA in 1981, north of giant sequoia’s current range, giant sequoia seedlings were planted in mixture with seedlings from local collections of ponderosa pine (Pinus ponderosa), sugar pine (Pinus lambertiana), white fir (Abies concolor), Douglas-fir (Pseudotsuga menziesii) and incense-cedar (Calocedrus decurrens) in six contiguous blocks.  Two blocks were kept free of brush competition, two were interplanted with local manzanita species, and two were interplanted with local Ceanothus species.  Volume growth of the conifer species was assessed and mixed effects modeling applied to test for significance of species, treatment, edge effect and their interactions.


Ponderosa pine had the greatest mean volume growth, while giant sequoia, generally regarded as the fastest growing Sierra conifer, was fourth also behind sugar pine and Douglas-fir.  Volume growth of conifers differed significantly between tree species (p<0.000) and brush treatments (p=0.01). Brush effects varied across species and significantly impacted growth of both pine species (p<0.000), Douglas-fir (p=0.03), and, marginally, giant sequoia (p=0.09).  All conifers performed better on bare ground than either brush treatment, except Douglas-fir, which performed favorably in the presence of manzanita.  Though volume growth for giant sequoia was greater on bare ground than Ceanothus, statistical tests indicated no significant difference between the two treatments.  Whether this is an indication that giant sequoia may perform favorably in the presence of Ceanothus under different site conditions must await further study.  Recommendations would be for planting of giant sequoia in fields cleared of brush.  Giant sequoia’s poor volume growth at Foresthill may be an indication that on drier, lower elevation sites ponderosa pine, sugar pine, and Douglas-fir will have a competitive advantage.