Changes in Ponderosa Pine Site Productivity following Removal of Understory Vegetation

Competition from understory vegetation for water and nutrients can limit productivity of young forest stands. Less is known of the effect of understory vegetation on long-term stand growth or soil organic properties. The effect of understory vegetation on periodic annual increments (PAIs) of basal area, height, and volume for ponderosa pine (Pinus ponderosa Doug].) in central Oregon at 4 or 5-yr intervals was examined for a 35-yr period. Soil C, N, and microbial biomass C (MBC) were also quantified after 32 and 35 yr with and without understory vegetation on a sandy loam pumice soil (Xeric Vitricryand). Five tree spacings, ranging from 2.0 to 8.0 m (1542469 trees ha), in combination with two understory treatments (understory vegetation present or continuously absent) were installed in 1959. Total understory vegetation cover averaged 3570 between 1959 and 1994 for treatments with understory vegetation present, and was dominated by three shrub species: antelope bitterbrush [Purshia tridentata (Pursh) DC.], greenleaf manzanita (Arctostaphylos patula Green), and snowbrush (Ceanothus velutinus Dougl. ex Hook.). Covariance analyses of PAIs for each successive interval were performed using appropriate stand parameters at the start of each interval as covariates. Tree growth was reduced by competing understory vegetation during the first 12 to 20 yr only; understory vegetation did not reduce the adjusted PAIs during the last 15 yr. Soil C and N were measured incrementally to a depth of 24 cm. Presence of understory vegetation resulted in greater C and N in the O horizon and upper 4 to 12 cm of mineral soil. Seasonal MBC, measured at 14-d intervals from May to November, was greater when understory vegetation was present. The results suggest that understory vegetation plays an important role in maintaining soil quality. W ILDFIRE SUPPRESSION began shortly after the turn of the century in ponderosa pine forests of central Oregon. Prior to that time, most stands experienced light, periodic ground fires (Bork, 1985), effectively controlling the understory vegetation and creating open, shrub-depauperate conditions. By the 1940s, the absence of fire resulted in stands with a dense understory of suppressed pine and a well-developed shrub component consisting primarily of antelope bitterbrush, greenleaf manzanita, and snowbrush. This large-scale change in stand structure and understory vegetation composition sparked concern for the potential reduction in tree vigor and growth due to increased competition for soil water (Van Sickle and Hickman, 1959) and nutrients. Competition from understory vegetation retards the early development of ponderosa pine (Barrett, 1982; Oliver, 1990; Oren et al., 1987). Barrett (1982) reported large reductions in height and diameter growth of pine saplings when a natural understory was allowed to develop, and Oliver (1990) found similar losses in volume due to understory shrubs in a 20-yr-old plantation. The complexity of factors responsible for these growth deUSDA-Forest Service, Silviculture Lab., 2400 Washington Ave., Redding, CA 96001. Received 1 Feb. 1995. *Corresponding author. Published in Soil Sci. Soc. Am. J. 60:1614-1621 (1996). clines was not clearly defined in either study, yet limitations caused by increased competition for water and nutrients are implicit (Barrett and Youngberg, 1965; Riegel et al., 1991, 1992; Nambiar and Sands, 1993). Contrary to their role as competitors for soil resources, shrub species common to central Oregon may enhance long-term site productivity by their ability to supply fixed C and N. Both snowbrush and bitterbrush are actinorhizal shrubs, capable of fixing atmospheric N2 in symbiosis with species of the soil bacteria Frankia (Delwiche et al., 1965; Webster et al., 1967). Nitrogen accretion of about 100kg ha -l y r has been credited to snowbrush in mixed conifer stands in the Oregon Cascades (Youngberg and Wollum, 1976; McNabb and Cromack, 1983). Less is known of N2-fixing potential of bitterbrush, although its inconsistency in modulation (Righetti and Munns, 1982) and low nitrogenase activity (Dalton and Zobell, 1977) suggest a minor role in site N accretion. A long-term study was established at the Pringle Falls Experimental Forest in 1959 to answer three questions: (i) Can suppressed ponderosa pine saplings develop into crop trees when released by overstory removal and thinning? (ii) How do tree and stand growth rates vary with tree spacing? (iii) How does the shrub-dominated understory vegetation influence development of thinned, second-growth stands? We examined the effect of understory vegetation on growth of ponderosa pine during a 35-yr period at Pringle Falls and report differences in soil N, C, and microbial biomass C due to the presence of understory vegetation. MATERIALS AND METHODS