|Interactions between forestry practices and the physical and biological processes occurring in small streams are numerous and potentially complex. Our previous research involved the assessment of short-term (1 - 2 years) abiotic (stream temperature, dissolved oxygen, suspended sediment, and pool habitat) and rainbow trout (Oncorhynchus mykiss) density, standing crop biomass, distribution, and condition responses of a small, S3 lake-headed stream in north-central British Columbia to a novel streamside logging treatment whereby only mature timber within the riparian zone was removed. The current research was implemented to assess the medium-term (6 years) post-logging responses in the same stream, and to compare these with the short-term results and with an unharvested, lake-headed control stream. Pre-logging monitoring took place in 1997, whereas short-term data were collected in 1998-1999 and medium-term data in 2004. The lake-headed streams were naturally warmer than neighbouring headwater streams, and despite reductions of ~ 50% in canopy cover summertime stream temperature increases were relatively modest in the short-term (averaging < 1.5 °C with respect to daily mean, maximum, minimum, fluctuations, and downstream cooling). However, the greatest increases in temperature (averaging 2-5 °C) were observed 6 years after our logging treatment was applied when the canopy had recovered to ~ 83% of pre-logging levels. This phenomenon may be related to 2004 being the warmest of the four summers during which monitoring took place, as well as to the quality of the riparian cover (which shifted from a primarily tall, coniferous canopy prior to logging to a thick, low deciduous undergrowth in 2004). Post-logging summer suspended sediment levels remained low (< 1 NTU), and average summer dissolved oxygen (DO) concentrations in both streams remained above 8 mgL-1 (the level required for salmonids to function without impairment) in all four years. The highest DO levels coincided with the warmest stream temperatures, and may reflect the influence of primary productivity occurring in the headwater lakes. Although the number of pools remained relatively stable, reductions in pool area and volume of approximately the same magnitude were observed in both the treatment and control streams in 1999 and 2004, and were thus not likely a result of our logging treatment. The rainbow trout population age structures also remained relatively stable during the pre- and post-logging periods, with > 90% of the populations in both streams comprised of young-of-the-year (YOY) and age 1-2 trout. Trout densities and standing crop biomass were variable across years, with both streams experiencing increases in 1999 and reductions in 1998 and 2004, and with the treatment stream showing the most extreme responses. The number of trout per cubic meter of pool volume increased in both streams during the short and medium-term, with the treatment stream experiencing the greatest increases. Trout distribution was aggregated in the treatment stream, whereas dispersal patterns in the control stream were aggregated as well as randomly and uniformly distributed. Lastly, YOY condition (a surrogate for growth) was significantly greater in the treatment stream during 1999 when compared to the control stream, which we attribute to earlier emergence from stream gravels and a thermal environment that was more conducive to growth. By contrast, juvenile trout (ages 1 - 4) condition was significantly lower in the treatment stream during 1998 and 1999. This may have been a result of increased crowding leading to greater competition for food resources, lower flows affecting invertebrate drift, and temperatures approaching the lethal limit for this species, all of which were likely part of naturally fluctuating cycles and not due to our logging treatment. Overall, our data suggest that lake-headed streams exhibit different DO and temperature responses to streamside clear-cut logging when compared to headwater streams. Small, lake-headed streams may consequently be able to support a more aggressive level of streamside harvesting involving the removal of most commercial timber within the riparian zone while retaining deciduous vegetation and non-commercial as well as some commercial trees. Furthermore, the similarity in the temporal patterns of our stream habitat and rainbow trout response variables between the treatment and control streams suggest that the observed short and medium-term post-logging changes may be related more to inter-annual environmental fluctuations and to the natural differences inherent among the streams than a response to our logging treatment. Lastly, our results underscore the potentially complex interactions among environmental and stream ecosystem physical and biological characteristics, as well as their natural variability, and highlight the need for multi-disciplinary and long-term research to better understand the impacts of streamside clear-cut logging.|
Scott G. Hinch.