Effects of Timber Harvest on Stream Benthic Communities and Soil Communities in Northeastern Oregon
Logging can have various impacts on soil and water ecosystems. Many aspects of logging can be detrimental to the environment including the use of heavy machinery, removal of trees and their supporting root systems and the extensive use of roads. These actions affect soil ecosystems by causing soil compaction, an increase in sediment yield and erosion and a decrease in organic matter or leaf litter which can result in more extreme temperature and moisture fluctuations in the soil. Impacts on land can be connected to impacts on aquatic systems. Logging can affect water systems by increasing sediment load, decreasing coarse particulate organic matter and removing canopy cover which results in more extreme temperature fluctuations and a shift from a heterotrophic community to an autotrophic community.
This thesis studied the effects of logging on water and soil ecosystems at three sites in the Umatilla National Forest in Northeastern Oregon. Samples were collected in July, before logging had occurred, and then in October, after logging. The diversity of the soil and aquatic macroinvertebrate communities was evaluated with the Simpson and Shannon diversity indices and the % EPT (Ephemeroptera, Plecoptera, Trichoptera) before and after logging. Diversity among the benthic macroinvertebrates significantly decreased at one site; however, this site was 0.75 km from the logging units and the decrease in diversity was most likely due to seasonal variation. The site closest to the logging units had the lowest Shannon and Simpson diversity indices as well as the lowest % EPT. This site received large amounts of sediment runoff from the logging units and the nearby road. The low indices at this site may be linked to the logging disturbance.
Two of the three sites contradicted the river continuum concept (RCC) presenting high percentages of scrapers in first/second order streams in October (Vannote et al. 1980). This might be explained by the loss of canopy cover due to the autumn leaf fall. The site closest to the logging units supported the RCC by presenting a majority of shredders with no scrapers present. Stone and Wallace (1998) found the same trend in other streams after logging due to the increase in leaf litter from the fallen trees.
For the soil macroinvertebrates, community diversity decreased significantly at five of the nine sites after logging had occurred. The three sites closest to U. S. Forest Service roads all had a significant decrease in diversity between July and October. The decline in community diversity could be due to logging activity but seasonal variation must also be taken into consideration.
This study looked at short-term effects of logging but did not investigate the long-term effects. Growns and Davis (1991) found that benthic macroinvertebrate communities could be affected by logging for up to eight years after the harvest, but also found that some communities can recover within months, after the spring rains had ceased. Great variability exists as to how long stream and soil ecosystems are affected once site disturbance ends. This study indicated that some short-term effects such as the loss of species diversity occurred; however, seasonal variation may explain differences amongst sites and across time. Perhaps, the analysis of additional sites in the affected area and comparison with undisturbed sites might provide more information. An analysis of undisturbed sites could give an indication of seasonal variation within macroinvertebrate populations versus changes due to the disturbance. Robinson et al. (2000) and others have found that macroinvertebrates are persistent from year to year in stream ecosystems in terms of community composition. Thus, if July samples were taken again at all sites, they would give a real indication of community changes due to the logging disturbance while minimizing seasonal variation factors.
This study indicated that disturbances on land can lead to changes in nearby aquatic systems. More research needs to examine the extent of the changes and the negative impacts. As concern grows for a variety of fish species, the health of streams needs to become a priority for the U. S. Forest Service, thus when logging proposals are developed, the U. S. Forest Service needs to take into account all nearby aquatic systems and adjust the proposals to maintain the health of the streams. Buffers are a first step in this protection. Many U. S. Forest Service logging proposals require buffers of vegetation to be left between the logging units and nearby streams. These buffers help filter sediment from the timber harvests and decrease sediment pollution. As long as humans use paper and lumber products, logging will need to continue; however, to ensure complete stream health on U. S. Forest Service lands, logging should be strictly limited with extensive research assessing the potential impacts for any proposed logging activity.
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