This is the third and final study in a series of tree-fall gap studies conducted at the HJA addressing the effects of tree-fall gaps on forest soil characteristics. The first looked at the effects of gap size on changes in soil carbon cycling within the gap along N-S transects. The second compared the effects of gaps on soil properties along both N-S and E-W transects to better differentiate between microclimate and vegetation effects within the gaps. The current study expands the number of variables studied and sampling intensity. By using the same grid system as Dr. Gray in his vegetation survey work, we are able to relate below-ground processes with above-ground vegetation.
Soil properties in two large 9 year-old tree-fall gaps were compared with soils in the surrounding old-growth Douglas-fir forest by intensive sampling of a circular grid that extended 12 m into the forest. This study was designed compare below-ground soil properties with above-ground vegetation and coarse woody debris distribution patterns using three-dimensional response surfaces and to compare soil properties in and outside the gap. To accomplish this goal, samples were collected along a grid already established by Gray, A.N., and Spies, T.A. (1996) designed to study vegetative succession in tree-fall gaps of varying sizes. We chose to measure soil characteristics at 4-meter intervals using the Gray/Spies grid design. The sample grid was essentially a circle centered within the gap. The sample grid was expanded 12 meters into the surrounding forest so that comparisons could be made between soils within the gap and those in the forest. During the same summer that the soils work was done, Dr. Gray and his students conducted studies of vegetation and coarse woody debris distribution patterns within these same gaps, generating GIS data layers which could be used to directly compare with our soils data.
Robert P. Griffiths, Thomas A. Spies
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.