This was the second 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 present study compares 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 third study expanded the number of variables studied and sampling intensity. By using the same grid system as Dr. Andy Gray in his vegetation survey work, we were able to relate below-ground processes with above-ground vegetation.
Soil properties in eight, 7 year-old tree-fall gaps were compared with soils in the surrounding old-growth Douglas-fir forest. Soil characteristics were measured along two transects; one running north and south and the other east and west. This study was an extension of one done two years earlier by Shirley King (see Gap1 - study code SP017). In that study, there were significant differences in soil properties not only between soils collected in and out of gaps but also by orientation within the gap. More specifically, soils in the north end of the larger gaps were significantly different from those in the south.
This study was designed confirm the previous findings that soils within gaps were different than those in the surrounding forest. In addition, we wanted to determine if there were also E/W differences. If there were none, then we could conclude that microclimate gradients were effecting these soils because the microclimate gradient along E/W transects should be much less than that found along N/S transects. We chose to measure soil characteristics at 2-meter intervals using this same basic design used by Shirley King in the Gap1 study. E/W and N/S transects were established in all of the gaps that were studied in Gap1 with the transects extending one radius into the surrounding forest.
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 characteristics, one must consider how both vegetative and microclimatic gradients influence soil properties. This study was designed to assist and differentiating between these effects.