Tree-fall gaps are known to play an important role in the formation and maintenance of old-growth forest structure and forest biodiversity. Prior research has focused on above-ground vegetative succession and population dynamics and little is known about changes occurring below-ground as vegetation becomes reestablished. The interplay between gap microclimatic gradients and both vegetation and the below-ground component of the ecosystem is potentially complex. Thus to understand how gaps influence forest floor characteristics, one must consider both above and below-ground components. This study was designed to make these connections.
As expected, soil temperature and moisture were both higher in gaps. Soil respiration, labile carbon concentrations, and litter depth were all lower in the gaps as the result of lower net primary productivity (NPP). The lower Â-glucosidase activities seen in the gaps, probably reflecting lower microbial activities in response to lower carbon cycling rates. Denitrification potentials were, however, almost twice that in the adjacent old-growth forest suggesting that there was more mineralized N available to denitrifying microorganisms in the gap than in the forest. This pattern also suggests that even 9 years after the gap was formed, it had not been colonized by sufficient root and mycorrhizal biomass to act as an effective sink for mineralize N. The low concentration of ectomycorrhizal mats may be symptomatic of this condition.
The experimental design and site descriptions have been published (Gray and Spies, 1996, 1997) but are summarized below. Two 50 m gaps were used to determine how large tree-fall gaps influence below-ground properties. 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 9 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 in a sampling grid with nodes every 4 m. Sampling was done throughout the grid within the gap and 12 m all directions into the surrounding old-growth forest. In this way we could compare gap and non-gap influences on the soils. In all, 269 locations were sampled in each of these grids.
Gray, A.N., and Spies, T.A. (1996) Gap size, within-gap position and canopy structure effects on conifer seedling establishment. Journal of Ecology 84, 635-645.
Gray, A.N., and Spies, T.A. (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 ectomycorrhizal mat characteristics.
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 degreesC 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. Two distinct mat types were scored: (1) mats similar to those of the genus Hysterangium and (2) mats similar to those of the genus Gautieria. 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 wet wt., which was then multiplied by 100.
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 1mM 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 before a headspace CO2 measurement was made. 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. This was a measure of labile soil carbon.
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). Duplicate aliquots and controls were run for all samples.
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.
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
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.
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.
McCune, B. 1993. Gradients in epiphyte biomass in three Pseudotsuga-Tsuga forests of different ages in western Oregon and Washington. Bryologist 96: 405-411.
McCune, B. 1994. Using epiphyte litter to estimate epiphyte biomass. Bryologist 97: 396-401.
Van Pelt, R., and M.P. North. 1996. Analyzing canopy structure in Pacific Northwest old-growth forests with a stand-scale crown model. Northwest Science 70(special issue): 15-30.
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.
Van Pelt, Robert, and Jerry F. Franklin. 2000. Influence of canopy structure on the understory environment in tall, old-growth conifer forests. Canadian Journal of Forest Research 30: 1231-1245.
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. Plant and Soil 169: 113-123.
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.
Gitzen, R.A. 1999. The Effects of Experimental Canopy Gaps on Small Mammal Communities in the Southern Washington Cascades. M.S. Thesis. University of Washington, Seattle, WA.
Gray, A. N. 1995. Tree seedling establishment on heterogenous microsites in Douglas-fir forest canopy gaps. Dissertation. Oregon State University, Corvallis, OR .
London, S.G. 1999. Spatial Distribution of Understory Vegetation in Tree Canopy Gaps of the Pacific Northwest. M.S. Thesis. 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.
Van Pelt, R. 1995. Understory Tree Response to Canopy Gaps in Old-growth Douglas-fir Forests of the Pacific Northwest. Dissertation. University of Washington, Seattle, WA.
References
Griffiths, R. P., Bradshaw, G. A., Marks, B., and G. W. Lienkaemper. 1996. Spatial distribution of ectomycorrhizal mats in coniferous forests of the Pacific Northwest, USA. Plant and Soil 180:147-158.
Keeney, D.R., and Bremner, J.M. 1966. Comparison and evaluation of laboratory methods of obtaining an index of soil nitrogen availability. Agronomical Journal 58, 498-503.
Keeney, D.R., and Nelson, D.W. 1982. Nitrogen-inorganic forms. In Methods of soil analysis. Edited by A.L. Page, R.H. Miller, and D.R. Keeney. American Society of Agronomy, Madison, Wis. pp. 643-698.
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
