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 carbon cycling, one must consider both above and below-ground components.
Soil moisture and temperature were both higher in the gap than in the surrounding forest but SOM, lab respiration and litter depth were both lower in the gap than in the forest. These results suggest that either litter input is lower in the gap than in the surrounding forest and/or that the decomposition rates are lower in the gap. Except for the smallest gap, soil moisture was lower and temperature higher in the northern half of the gap. In the 50 m gaps, laboratory respiration rates, litter depth, and soil organic matter (SOM) were higher in the southern portion of the gap as were total carbon values.
A general description of these gaps and the rational for this long-term study have been described by Gray and Spies (1996). The experimental design and site descriptions have been published (Gray and Spies, 1996, 1997) but are summarized below.
Replicate 10, 20, 30 and 50 gaps as well as one 30 m control gap were used to determine how tree-fall gaps influence soil carbon cycling. These gaps were created in the fall of 1990 at a site located 44 15N, 122 15W at an elevation of 900 m at the H.J. Andrews Experimental Forest in the Central Oregon Cascade Mountains. This study was conducted 5 years after gap formation. North-south transects were established through the center of all gaps and sample nodes were located at 1 m intervals along those transects. These transects ran one radius length into the surrounding forest. These transects were divided into four zones. Two zones were north of the E-W centerline; one in and one out of the gap. The other two zones were south of the E-W centerline; one in and one out of the gap.
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.
Litter depth and soil temperature measurements were made in the field.
Litter depth was measured with a cm ruler and soil temperatures were made in the top 10 cm of soil using a dial thermometer calibrated with ice water. Cores (4.7 x 10 cm) were collected for bulk density measurements. Grab samples were taken to a depth of 10 cm with a trowel for soil for the laboratory measurements of gravimetric soil moisture, laboratory respiration and total N and C.
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°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 mater (SOM) was measured as weight loss after ignition at 550 ºC for 12 h on soils that had been dried at 100°C for 12 h.
Laboratory respiration measurements were made in duplicate on 5 g of field-moist soil brought up to a moisture level of 75% with de-ionized water. The soil sample was added to a 25-mL Erlenmeyer flask. Once sealed with serum bottle stoppers, the flasks were incubated at 15°C for 2 weeks. Headspace CO2 concentration was measured using a gas chromatograph fitted a flame ionization detector and a methanizer in series.
Ten percent of the samples were analyzed for total carbon and nitrogen using a Carlo-Erba® NA Series 2 CNS analyzer. The soils were pulverized and passed through a 60 mesh screen in preparation for analysis. Sample size varied from 8-15 mg.
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.
