This is the first of three tree-fall gap studies conducted at the H.J. Andrews Experimental Forest addressing the effects of tree-fall gaps on forest soil characteristics. The second (Gap2) 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 gaps. The final study (Gap3) expanded the number of variables studied and sampling intensity. By using the same grid system as Dr. Andy Gray in his vegetation survey work, the Gap3 study provided the most definitive connection between above-ground vegetation and associated below-ground processes.
Soil properties in 10, 20, 30, and 50 meter tree-fall gaps were compared with soils in the surrounding old-growth Douglas-fir forest by sampling at 1 meter intervals along transects running north and south through the gaps. These transects extended one radius into the surrounding forest. This study was designed to determine if carbon cycling within these gaps were different from those in the soils of surrounding undisturbed forests. If there were differences, she wanted to determine what sized gap was required to show an effect. 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.
Robert P. Griffiths
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.