Soil Quality on the Tongass National Forest Results of compaction studies conducted on the Kuiu Stream Restoration access trails

Transcription

1 Soil Quality on the Tongass National Forest Results of compaction studies conducted on the Kuiu Stream Restoration access trails Technical Contacts: Dennis J Landwehr and Jacquie Foss Tongass Forest Soil Scientists November, 2014 i

2 Table of Contents Subject Page Introduction and Background 1 Previous soil compaction studies on the Tongass 1 Purpose and Need 2 Objectives 2 Study Area and Soil Conditions 2 Methods 4 Site selection 4 Sampling Methods 5 Lab Methods 5 Statistical Methods 5 Results and Discussion 5 Soils Sampled 6 Effectiveness of puncheon matting 7 Summary 8 Literature Cited 9 ii

3 Introduction and Background Stream restoration on the Tongass National Forest often requires placement of large woody debris in the stream channel, along stream banks, and on the floodplain. The process involves identifying a source of large woody debris, harvesting the trees, transporting them to the stream and placing them in strategic location to simulate natural stream structural components. Transporting the logs or logs with root-wads attached is often accomplished with heavy lift helicopters. When heavy lift helicopters are not available or too expensive the large woody debris is moved to the stream by ground based equipment. The North Kuiu Island stream restoration project required about 1,200 pieces of wood. Most of this wood was moved to the stream with ground-based equipment. To move the wood to the stream six access trails approximately 10 meters wide were cut through 40 year old young-growth. The wide trails were needed for the machines to skid or swing the wood to the stream, similar to a shovel yarding scenario. Unlike shovel yarding where 1 or 2 passes are made over a shovel trail, the number of passes required to move wood to the stream for the Kuiu project was typically more than 50 with some trails receiving more than 100 passes (Whitacre 2014). Ground-based equipment operating on wet soil conditions can result in soil compaction. Soil compaction can lead to a decline in the productivity of a site. Fine textured soils are more susceptible to soil compaction than coarse textured soils. The access trails created for the North Kuiu Stream Restoration Project crossed alluvial soils belonging to the Tonowek and Tuxekan Soil Series. These alluvial soils are typically well drained and coarse-loamy in texture (silt loam or fine sandy loam surface textures), often free of gravels in the upper part, moist year round, and can be considered somewhat susceptible to soil compaction when wet. The Tonowek soils are younger soils (Entisols) and typically occur in a floodplain location. The Tuxekan soils are older more developed soils (Spodosols) and typically occur on a terrace or stable spot on a floodplain. Previous soil compaction studies on the Tongass Previous soil compaction studies on the Tongass have found detrimental soil compaction on individual sites but not over extensive areas of skid trails (Alexander 1990, Foss and Landwehr 2006, Landwehr and Silkworth 2011, and Landwehr et. al. 2012). Alexander (1990) collected soil core samples from shovel trails and undisturbed areas on the North Irish Timber Sale Unit 1. The soils belonged to the Kupreanof soil series. Five core samples from shovel tracked areas were compared to five samples from undisturbed areas. Alexander found no statistical difference in soil bulk density between tracked areas and un-tracked areas. Alexander reported bulk densities ranging from 0.44 to 0.75 grams/cc. Foss and Landwehr (2006) collected 120 bulk density (core and compliant cavity methods) samples on temporary roads, shovel trails and undisturbed areas on the Situk and Coyak Timber sales on the Yakutat Ranger District. Detrimental soil compaction was identified on the temporary road in one harvest unit on the Situk Timber Sale. The temporary road also qualified as a detrimental soil displacement. Foss and Landwehr went on to explain that the core samples provided consistent results while the compliant cavity samples provided more variable results and thus must be interpreted with care. Foss and Landwehr did not find detrimental compaction associated with trails made by the use of ground-based equipment on silty soils. Landwehr and Silkworth (2011) collected and analyzed 60 core samples taken on skid trails created to remove timber from the Commercial Thin study sites on Prince of Wales Island. One of the Commercial Thin plots was on alluvial fan soils in the Maybeso Experimental Forest. The other two plots were located on soils underlain by limestone in the Naukati plots. Landwehr and Silkworth chose these plots because they contained loamy and coarse-loamy soils and in the case of the Naukati plots contained limestone soils with a slightly higher clay content and thus more susceptible to compaction. One of the Naukati plots was also chosen because the skid trail accessed 3 plots and received more passes that any of the 1

4 other skid trails. Landwehr and Silkworth did not find detrimental compaction as a whole, but noted that individual sample pairs suggested portions of the skid trails may be compacted. Landwehr and Silkworth reported bulk density values ranging from 0.25 to 1.05 grams/cc. They also noted that a couple of samples had lower bulk density due to organic matter in the samples. Those samples were not used in the analysis. Landwehr et. al. (2012) collected 40 bulk density core samples at 10 sites (two depths) on and off 50 year old tractor skid trails on limestone soils on Kosciusko Island. Landwehr et. al did not find a significant increase in bulk density at the shallow depth (7.62 to cm) but did find a statistically significant increase (90 percent confidence) at the deeper depth (15.24 to cm). The increase was not enough to classify the soils as detrimentally compacted and Landwehr noted that on the skid trails the samples may have come from deeper soil horizons where natural soil bulk density is often higher. Landwehr et. al. stated that these findings combined with other findings support the statement that most Tongass soils are not susceptible to soil compaction from ground-based equipment under normal forestry practices. Purpose and Need Given the high number of equipment passes on the North Kuiu Stream Restoration project access trails there is a need to verify that the soils under those trails are not detrimentally compacted. The number of equipment passes experienced on the restoration access trails are well beyond the number of passes expected under typical shovel yarding or ground-based felling operation but may be similar to the number of passes expected on a primary tractor skid trail. Objectives The main objective of this monitoring is to determine if detrimental soil compaction occurred as result of numerous passes by ground-based equipment used to deliver large woody debris to Saginaw Creek. Study Area and Soil Conditions The west fork of Saginaw Creek is a glaciated U-shaped valley. Precipitation ranges from 2,000 to 3,000 mm per year Detrimental Soil compaction - soil bulk density is increased more than 15 percent over the undisturbed levels. (FSM 2554). - Bulk density is an indicator of how well plant roots are able to extend into the soil. The National Soil Survey Handbook provides root-restriction initiation values and root restriction for soils based on soil particle size class. and mean annual temperature is about 7 degrees Celsius. Bedrock in the area is typically volcanic and has yielded deep, well drained soils on the lower valley footslopes. The valley bottom soils traversed by the access trails consisted of the floodplain and terrace soils mentioned previously and colluvial footslope soils belonging to the Kwatahein and Mitkof soil series. Sampling was confined to the Tonowek and Tuxekan soil series. Both soils consist of deep alluvial deposits with mixed minerology. The 1968 harvest of this stand used a combination of cable yarding systems and tractor yarding systems. Tractor yarding was generally limited to the alluvial soils. Access Trail 1 (Sample sites FP1 and T1) followed a skid trail created for the original timber harvest. A small portion of Access Trail 5 (Sample site FP4) crossed slopes over 20 percent gradient. All sample sites were on slopes less than 20 percent gradient. The study area consists of six equipment trails used to access stream restorations sites on the west fork and mainstem of Saginaw Creek on northern Kuiu Island in southeast Alaska. Figure 1 shows the planned access trails and restoration site locations in the Saginaw Watershed. The as built trail locations are slightly different and are shown in Figure 2. There are six trails totaling about 945 meters in length and averaging about 10 meters in width. The longest trail is 235 meters. The number of passes was estimated at 80 for access trail two (40 round trips) and over 300 (160 round trips) for access Trail 6. See Table 1 and Whitacre (2014). 2

5 Table 1. Planned access trails and estimated number of equipment passes (based on Whitacre 2014 and final trail layout. Planned Puncheon trail ID As built trail number Estimated maximum number of equipment round trips Trail length (meters) WF Saginaw PT #1 Access trail WF Saginaw PT #2 Access trail WF Saginaw PT #3 Access trail WF Saginaw PT #4 and #5 Access trail WF Saginaw PT #6 and #7 Access trail Mainstem Saginaw Access trail Figure 1. A 2008 aerial photo of the west fork of Saginaw Creek and the proposed access trails and stream restoration sites (from Whitacre 2014). 3

6 The access trails were constructed through a 46 year old young-growth stand. Some of the young-growth timber was large enough to be used in the stream restoration project. The smaller pieces of wood were used to create a puncheon mat for the equipment to operate on and for the logs to slide over. The puncheon mat was fluffed after the stream restoration work was completed. Fluffing involved using the equipment to pick up portions of the puncheon mat and then drop it in place. The goal of fluffing is to break up dense portions of the puncheon mat, thereby providing a better seedbed for the desired vegetation. The desired vegetation in this case is conifer (western hemlock and Sitka spruce trees). Figure 2. Bulk density sample sites (green dots) and as built access trails (red lines) in the Saginaw Creek Watershed, Kuiu Island. Methods Site selection Site selection was not random. Bulk density sample points were taken at sites where a soil scientist using hand tools could reasonably remove the woody debris puncheon trail and access intact mineral soil under the puncheon trail. Undisturbed samples were extracted from undisturbed forest as close as possible to the access trail sample point. The bulk density sampling was focused on areas where compaction would be most likely to occur. The silty alluvial soils were more likely to compact than the coarser textured upland soils. In addition, the floodplain soils often have water tables high in the soil profile, thus making them 4

7 more susceptible to compaction by ground-based equipment. Figure 2 shows access trail and bulk density sample site location. Sampling methods The Tongass methods for identifying soil compaction include bulk density sampling with either the core method or the compliant cavity method (USDA 2004). Due to difficulties in achieving consistent results with the compliant cavity method, the core method is preferred and was used for all samples on this project. The Regional Soil Quality Standards describe a 15 percent increase in bulk density as an indicator of detrimental compaction. The Tongass protocol (Landwehr 2009) requires soil core samples taken on and off skid trails with a minimum of 5 sample sites per plot. Each sample site includes 4 bulk density core samples (76.2mm by 69.8mm) two on the access trail and two off the access trail. One sample is taken at the 3 to 6 inch depth (7.62cm to 15.24cm), and one at the 6 to 9 inch depth (15.24cm to 22.86cm). Comparable samples were taken in close proximity off the skid trails at the same depths and in the same or similar soil horizons. Our goal for monitoring bulk density on this project was 5 sites from floodplain soils and five sites from terrace soils. Both soils have similar textures and bulk density samples should be comparable between the two soil series. Lab methods In the lab all samples were oven dried at 105 degrees Celsius for at least 96 hours. Samples were weighed, then sieved with the fine earth and coarse fragments weighed separately. Coarse fragment volume was measured via displacement in water in a graduated cylinder. Coarse fragment volume was subtracted from the sampler volume to obtain fine earth fraction volume. Fine earth bulk density values are recorded as grams per cubic centimeter. Statistical methods Statistical analysis of bulk density involved paired T-tests with 90 percent confidence intervals. Significance is achieved when precision goals (standard error less than 10 percent of the mean) are met and confidence intervals do not overlap. The samples were sorted by landform (floodplain versus terrace) and depth (shallow versus deep). The results from undisturbed samples were compared to results from access trail samples to determine a significant difference and 15 percent increase. Results were also compared with published root restriction initiation values and root restriction values (USDA 2005). Results and Discussion Forty bulk density core samples were taken from 10 sites on alluvial soils on five access trails used for the North Kuiu Stream restoration project. Four of the sites were on floodplain soils and six of the sites were on slightly elevated terrace soils. Samples were collected from two depth classes in an undisturbed area and two depth classes under the access trail at each site. Table 1 displays the statistics for the aggregated 40 core samples sorted by depth, the samples grouped by floodplain, terrace, and all samples sorted by skid trail versus undisturbed and sample depth. Our desired level of precision for paired T-tests is achieved when the standard error for the sample group is less than 10 percent of the sample mean. Our test for precision was met for all but two sample groups (Table 1). The bulk density data does not show a significant increase soil density between skid trails and undisturbed areas at 90 percent confidence (Table 2). The analysis also does not show a significant difference in bulk density when the samples are sorted in floodplain or terrace soils. Floodplain soils appear to be more susceptible to soil compaction based on the slightly higher bulk density values for soil samples on access trails on floodplains (Table 2), possibly due to higher moisture contents at the time the access trails were used. Another possible factor is that there was likely less conifer puncheon available on the floodplains than on the terrace sites as evidenced by the alder plant associations typically found on the floodplain sites. We did not measure puncheon depth or type as part of 5

8 this monitoring and field observations and photographs do not suggest a difference in puncheon density at any of the sites. Table 2. Fine earth bulk density values for each depth class on and off access trails. Data shown for all 10 sites (40 samples), floodplain sites (16 samples) and terrace sites (24 samples) sorted by depth class, Location Access trail or Depth Class Mean Fine earth bulk density Standard Standard error expressed as 90 percent confidence Significantly Different Undisturbed (centimeters) (grams/cc) error % of mean interval All sites n=10 Undisturbed 7.6 to % 0.06 Access trail 7.6 to % 0.09 No Undisturbed 15.2 to % 0.07 Access trail 15.2 to % 0.11 No Floodplain soils n=4 Undisturbed 7.6 to % 0.09 Access trail 7.6 to % 0.13 No Undisturbed 15.2 to % 0.09 Skid trail 15.2 to % 0.21 No Terrace soils n=6 Undisturbed 7.6 to % 0.09 Skid trail 7.6 to % 0.13 No Undisturbed 15.2 to % 0.10 Skid trail 15.2 to % 0.09 No When looking at the bulk density values for the 4 floodplain sites we noticed three sites with much higher bulk density than the fourth site. If the fourth site is considered an outlier and dropped from the analysis the bulk density value from the three remaining floodplain sites showed a significant difference with a 34 percent increase in bulk density at the 7.62 to 15.24cm depth (data not shown). At the deeper depth the bulk density values for the three sites were not significantly different. Two of the four sites had bulk density values that were significantly different from the undisturbed sites with a 47 percent increase in bulk density at the to depth (data not shown). The samples sizes are very small but it appears that three of the floodplain sites meet the definition for detrimental soil compaction at the 7.62 to 15.24cm depth. Two the floodplain samples meet the definition of detrimental soil compaction at the to depth. The floodplain soils were generally wetter than the terrace soils and a water table was encountered at two of the floodplain sites. Soils sampled Thirty-eight of the 40 total bulk density samples came from BC horizons. The O, E and Bh horizons (where present) were typically severely altered by puncheon placement and pressing into the soil under the weight of equipment and logs. We made a point to sample similar horizons as bulk density naturally increases with soil depth in these soils. We believe we have very comparable soils for bulk density analysis. Soil texture was silt loam or fine sandy loam in 33 of the samples. The remaining 7 samples were gravelly sandy loams. The soils fall into the coarse-loamy particle size class family. We also took care to avoid organic material in our sampling. The BC horizons have very low levels of organic material. The National Soil Survey handbook (2005) has published root restriction initiation values and root restricting values for soils based on soil texture class. For the textures we found the root restriction initiation value begins at 1.60 grams/cc or higher and the root limiting value is 1.79 grams/cc or higher. None of our samples approach the root restriction initiation values or root limiting values. The highest bulk density values we documented were 1.08 grams/cc. 6

9 The bulk density values reported here are similar to the bulk density values found for similar Tongass soils by Alexander (1990), Foss and Landwehr (2006), Landwehr and Silkworth (2011) and Landwehr et. al. (2012). To date about 270 bulk density samples have been collected and analyzed on 5 different projects and other than a few individual sites detrimental soil compaction (15 percent increase over natural) has not been identified. All bulk density values from these 270 samples have fallen below the root restriction initiation values identified in the National Soil Survey Handbook (2005). There are several reasons for this finding. First is the inherent low natural bulk density values of Tongass soils due to the relatively coarse textures (coarse-loamy particle size class) and low clay content. Clayey soils are generally more susceptible to soil compaction than sandy or silty soils. Secondly, most Tongass soils have thick duff layers and high organic matter content in the upper layers. Organic matter has very low bulk density. Thirdly, Tongass forested soils experience relatively high rates of soil churning due to windthrow which loosens soils, increases porosity and reduces bulk density. Lastly, the vegetation on these soils experience rapid growth and rooting of trees and other vegetation which increases soil porosity and reduces bulk density. Effectiveness of puncheon matting The use of puncheon to support equipment and logs was also very effective at maintaining soil porosity and avoiding soil compaction at most of the sites we sampled. Figure 3 shows two pictures of the puncheon trails monitored in this project. Even after fluffing, both pictures show soils covered with a thick layer of puncheon material including cull logs and limbs. The puncheon effectively spreads the weight of equipment out over a very large surface area, thus reducing the chance for soil compaction to occur. Observations on the six access trails used for this project suggest mineral soil was rarely exposed on the surface of the access trails. To access the mineral soil for sampling slash had to be removed at most of our sample sites on the access trails. Off the access trails the duff layer ranged from 5 to 40 centimeters thick. Figure 3. Access trail at sample point FP1 (left). Access trail at sample Point FP3(right). Slash penetration and depth and density of slash over the soil surface is a concern identified by Landwehr and Silkworth (2011). They were concerned that dense slash on the soil surface (not fluffed) may inhibit soil respiration and conifer growth. On the north Kuiu stream restoration project puncheon was fluffed on all access trails. The result is a reduction in dense slash on the soil surface. Fluffing did not remove all slash from the mineral soils and under the puncheon we still found slash had penetrated the upper soil horizons, often to a depth to 25 or 30 centimeters (Figure 4). The structure of the organic duff layer and E and/or upper Bh horizons (where present) is severely disturbed. This is generally not considered a detrimental soil condition and the mixing may increase soil productivity by stimulating increased microbial activity and subsequent release of nutrients. 7

10 Figure 4. Soil pit at one of the access trail sites. Note slash penetration to a depth of about 25 centimeters. Summary We monitored soil bulk density on about 1,000 meters of equipment trails used to deliver wood to the west fork and mainstem Saginaw Creeks. The trails average about 10 meters in width and were created by felling 46 year old young-growth trees and laying down a nearly continuous mat of cull logs and slash. Under repeated passes with equipment the cull logs and slash were forced into the soil to a depth of 20 to 30 centimeters, atering the structure of the O, E, and upper B horizons. The puncheon trail was fluffed after stream restoration was complete. Fluffing broke up the dense slash mat and likely loosened the upper layers of soil that the slash had penetrated. Fluffing did not move all of the slash and some slash is still evident in the upper mineral soil horizons. Soil bulk density samples indicate that overall the soils under the access trails are not compacted. Bulk density values from three of the individual sites on floodplain soils did indicate a 34 percent increase in bulk density under the access trails at a depth of 7.62 to centimeters. At the depth of to only two of these sites showed a 47 percent increase in bulk density. The floodplain soils were typically wetter than the terrace soils and a water table was encountered at two of the floodplain sites we sampled. It is likely that soils under a small portion of the shovel trails are detrimentally compacted according to the Region 10 soil quality standards. Similar to other bulk density sampling on the Tongass NF none of the samples we analyzed in this study had bulk density values that exceed published root restriction initiation values or root restriction values for these alluvial soils. There are several reasons for the lack of detrimental soil compaction in most Tongass forest soils. First is the inherent low natural bulk density values of Tongass soils due to relatively low clay content (less than 20 percent clay), the presence of thick duff layers and high organic matter content in the upper layers, and the relatively high rates of soil turbation due to windthrow and rooting of trees and other vegetation. The north Kuiu stream restoration access trails represent equipment trails with many passes of equipment on silt loam and fine sandy loam soils. The trails may be similar to what we can expect to see on a long primary skid trail used to yard timber from a large young-growth stand on similar soils. As the forest 8

11 transitions to young-growth management there is a need to understand the potential effects to soils from skid trails of this type. This monitoring contributes to that understanding. The use of cull logs and slash to blanket the soil under equipment routes appears to be very effective at preventing detrimental soil compaction. This monitoring, combined with other bulk density monitoring on the forest (Alexander 1990, Foss and Landwehr 2006, Landwehr and Silkworth 2011, Landwehr et. al. 2012) support the statement that most Tongass soils are not susceptible to soil compaction from equipment under normal (where a puncheon mat is used) forestry practices. Re-vegetation of the access trails should be monitored after 5 years to determine if the high slash load is negatively affecting re-vegetation by the desired vegetation (in this case conifers). After 10 years growth of the conifers could be monitored to determine if the slash load is affecting growth of the desired vegetation. If tree growth is measured on the access trails it could be compared to growth on similar soils at the root-wad and log harvested site on the Security Bay Road. Literature Cited Alexander, E.B Compaction by Shovel yarder. North Irish timber Sale Unit 1. May 18, Unpublished monitoring report. Foss, J.V. and D.J. Landwehr Soil Quality on the Yakutat ranger District. Monitoring report summarizing soil quality data following salvage logging operations. July pages. Unpublished monitoring report. Landwehr, D.J Soil Quality Monitoring on the Tongass National Forest. Protocols updated from pages. October, Landwehr D.J. and D. Silkworth Soil Quality on the Tongass National Forest. Results of compaction studies conducted on the Experimental Thinnings at Naukati and Maybeso. August pages. Unpublished monitoring report. Landwehr D.J., Foss J. and D. Silkworth Soil Quality Monitoring on the Tongass National Forest. The Tongass Interpretation of the Region 10 Soil Quality Standards. Part 2: Summarizing 4 years of data collection. January pages. Unpublished monitoring report. USDA Forest Service Manual Supplement, 2554 Soil Quality Monitoring. Region 10 Supplement Effective date 5/5/ pages. USDA Soil Survey Laboratory Methods Manual. Soil Survey Investigations Report No. 42. Version 4.0. November Rebecca Burt, Editor. USDA NRCS. U.S. Department of Agriculture, Natural Resources Conservation Service, National Soil Survey Handbook, title 430-VI. [Online] Available: USDA Keys to Soil Taxonomy. Eleventh Edition. Soil Survey Staff, USDA NRCS. Whitacre, H North Kuiu Stream Restoration Project updates. February 6, Unpublished project report. 9