The experimental design and site descriptions have been published (Gray and Spies, 1996, 1997) but are summarized below. Eight gaps ranging in size from 10 to 50 m were used to determine how large tree-fall gaps influence below-ground properties along different microclimatic gradients. These gaps were created in the fall of 1990 at a site located 44 15 N, 122 15 W at an elevation of 900 m at the H.J. Andrews Experimental Forest in the Central Oregon Cascade Mountains. This study was conducted 7 years after gap formation. This site and the rational for this long-term study have been described by Gray and Spies (1996). Cores (4.7 x 10 cm) were collected every 2 m along transects which extended one radius length into the surrounding forest.
Gray, A.N., and T.A. Spies. 1996. Gap size, within-gap position, and canopy structure effects on seedling establishment of conifer species in forest canopy gaps. Journal of Ecology 84: 635-645.
Gray, A.N., and T.A. Spies. 1997. Microsite controls on tree seedling establishment in conifer forest canopy gaps. Ecology 78:2458-2473.
The following measurements were made in the field: litter depth, mineral soil respiration, ambient light, soil temperature and the relative abundance of ectomycorrhizal mat. Field (mineral soil) respiration rates were measured with a nondispersive, infrared CO2 analyzer (Li-Cor, LI-6200). Measurements were made over a period of 1 min after the chamber gas reached ambient CO2 concentration. The instrument was calibrated on site against a known standard at each location. A Q10 adjustment was made for ambient soil temperature. Soil temperature was measured by electronic thermometers calibrated at 0 degrees C with ice water. The temperature probes were inserted into the mineral soil to a depth of 10 cm. Light was measured with the Li-Cor photometer.
The distribution of ectomycorrhizal mats was determined visually in the field by inspecting the relative abundance of mats in 4.7 x 10 cm cores. This approach has been used successfully in the past to document ectomycorrhizal mat distribution patterns in coniferous forests of the Pacific Northwest (Griffiths et al. 1996).
In preparation for laboratory analyses, all soils were sieved through a 2-mm sieve. Soil moisture was determined by drying duplicate 10 g field-moist sieved soils at 100 degrees C for at least 8 h. The percent soil moisture was calculated by dividing the difference between wet and dry samples and dividing that number by the dry wt., which was then multiplied by 100. Soil organic matter was measured by loss-on-ignition at 550 degrees C for 6 h after oven drying at 100 degrees C.
Duplicate denitrification potential measurements were made using a method by Groffman and Tiedje (1989) as modified by us (Griffiths et al., 1998). Each reaction vessel (25-mL Erlenmeyer flask) contained 5 g of less than 2 mm, field-moist soil. Flasks were sealed with rubber serum bottle stoppers and purged with Ar to displace O2 in the headspace gas. After purging with Ar, 2 mL of a 1 mM solution of glucose and NO3- was added to each flask. Flasks were subsequently incubated at 25 degrees C for 1 h. This preincubation period was used because previous time-series experiments showed a lag in N2O production during this period. The same experiments have shown linear N2O production rates during the following 2-4 h (unpublished data). After the preincubation period, 0.5 mL of headspace gas was removed from the reaction vessel and injected into a gas chromatograph (GC) fitted with an electron capture detector (Hewlett Packard model 5890 GC, connected to a Hewlett Packard model 3396 integrator). The integrator was calibrated by the external calibration method with known gas standards. A second headspace N2O analysis was made after an additional 2-h incubation at 25 degrees C. The net N2O released over this 2-h period was used to estimate N2O production rates.
Duplicate laboratory respiration measurements were made on field-moist, sieved soils (4 g dry weight). These rates represent the basal respiration rate for soil microorganisms. Soils were brought to 75% moisture content by the addition of enough sterile deionized water to equal 3 g water per 25-mL Erlenmeyer flask. Once sealed with serum bottle stoppers, the flasks were incubated at 24 degrees C for 14 days after which headspace CO2 concentrations were measured using gas chromatography. This was a measure of labile soil carbon. The same GC and integrator as were used for this assay as that used to measure N2O, but in this case a flame ionization detector and a methanizer in series were used.
Beta-glucosidase activity was determined by the spectrophotometric assay of Tabatabai and Bremmer (1969), as modified by Zou et al. (1992). One mL of 10 mM p-nitrophenyl b-D glucopyranoside substrate was added to duplicate 1-mL subsamples containing a soil slurry (1 gdw in 1 mL deionized H2O). The tubes were shaken and then placed with duplicate controls without substrate in a 30 degrees C water bath for 2 h. After incubating, 1 mL of 10 mM p-nitrophenyl b-D glucopyranoside was added to the controls, and all reactions were immediately stopped by the addition of 2 mL of 0.1 M tris[hydroxymethyl]aminomethane at pH 12.0. The mixtures were centrifuged for 5 min at 500 x g. From the supernatant, 0.2 mL was diluted with 2.0 mL deionized water. The optical density was measured at 410 nm, and a standard curve was prepared from 0.02 to 1.0 micro-mol/mL p-nitrophenol (pNP). Live root biomass was estimated from dried (8 h at 100 C) 4.8 x 10 cm cores. The roots were removed by hand and weighed.
Tabatabai, M. A., and J. A. Bremner. (1969) Use of p-nitrophenyl phosphate for assay of soil phosphatase activity. Soil Biology & Biochemistry 1,301 - 307.
Zou, X., D. Binkley, and K. G. Doxtader. (1992) A new method for estimating gross phosphorus mineralization and immobilization rates in soils. Plant & Soil 147, 243
Griffiths, R.P., Homann, P.S., and Riley, R. (1998) Denitrification enzyme activity of Douglas-fir and red alder forest soils of the Pacific Northwest. Soil Biology and Biochemistry 30, 1147-1157.
Groffman, P.M., and Tiedje, J.M. 1989. Denitrification in north temperate forest soils: relationships between denitrification and environmental factors at the landscape scale. Soil Biology & Biochemistry 21, 621-626.
The following are publications describing past work in these gaps:
Gray, A.N., and T.A. Spies. 1995. Water content measurement in decayed wood and forest soils using time domain reflectometry. Canadian Journal of Forest Research 25: 376-385.
McCune, B. 1994. Using epiphyte litter to estimate epiphyte biomass. Bryologist 97: 396-401.
Van Pelt, Robert, and Jerry F. Franklin. 1999. Response of understory trees to experimental gaps in old-growth Douglas-fir forests. Ecological Applications 9: 504-512.
Vogt, K.A., D.A. Publicover, J. Bloomfield, J.M. Perez, D.J. Vogt and W.L. Silver. 1993. Belowground responses as indicators of environmental change. Environmental and Experimental Botany 33:189-205.
Vogt, K.A., D.J. Vogt, H. Asbjornsen, and R.A. Dahlgren. 1995. Roots, nutrients and their relationship to spatial patterns. In: pp. 113-123 (L.O. Nilsson, R.F. Huttle and U.T. Johansson, eds.) Nutrient Uptake and Cycling in Forest Ecosystems. Developments in Plant and Soil Science Vol. 62. Kluwer Academic Publishers, Dordrecht, Boston.
Gray, A. N. 1995. Tree seedling establishment on heterogenous microsites in Douglas-fir forest canopy gaps. Dissertation. Oregon State University, Corvallis, OR .
St. Pierre, E.A. 2000. Effects of canopy gaps in Douglas-fir forests and resource gradients on fecundity and growth of understory herbs. Dissertation. Oregon State University, Corvallis, OR.
