2006 HFQLG SOIL MONITORING REPORT
Prepared by: Dave McComb, Soil Scientist &
Randy Westmoreland, HFQLG Soil Monitoring Leader
Reviewed by: Jim Baldwin, PSW Statistician, Brent Roath, Regional
Soil Scientist & Colin Dillingham, HFQLG Monitoring Coordinator
May 24, 2007
This report summarizes soil monitoring data collected on the Lassen, Plumas and
Sierraville District of the Tahoe National Forests between June and August 2006 as a part
of the Herger - Feinstein Quincy Library Group (HFQLG) Forest Recovery Act Pilot
Project. The soil monitoring is required by the HFQLG EIS to answer the question:
Question 6) Do Activities meet Soil Quality Standards?
The soil quality standards and guidelines for soil in the HFQLG forests are found in the
Land and Resource Management Plan (Forest Plan) for each forest.
The definitions, thresholds and indicators in FSH 2509.18 - SOIL MANAGEMENT
HANDBOOK, R5 Supplement No. 2509.18-95-1 were utilized to provide a consistent
method to measure soil condition for this and past years. The 1995 handbook supplement
defines detrimental soil disturbance when the resulting condition exceeds the threshold
values. Recent science (Powers et.al. 2005) raises questions whether exceeding the
threshold value given in the FSH supplement for total soil porosity actually constitutes a
significant change in soil productivity for all soils in general. While the use of the
thresholds and definitions in the handbook supplement provides a consistent method to
measure soil properties, the interpretation of the significance of the monitoring findings
needs to take in to account this recent science.
Whenever heavy ground based equipment is used to perform resource management
activities (such as DFPZ construction) the potential for adverse impacts to soil quality
exists. This includes compaction, disturbance and displacement of soil, or a loss of
ground cover and large woody debris. The individual forest plans set standards and
guidelines for compaction, ground cover, and large woody debris to keep these impacts
within acceptable limits in the context of the benefits accrued from managing the land.
Although not a part of the soil quality analysis standards, the HFQLG soil monitoring
includes recording the level of disturbance and displacement of soil. From 2001 to 2004
planned treatment units were sampled pre-treatment to document existing conditions.
Post-treatment monitoring of units began in 2004 and continued through 2006.
In the 2006 field season an additional 13 units were sampled for pre- and post-treatment
differences, bringing the total to 55 units available for analysis. Harvest areas were
sampled by transects; landing areas were excluded. For details on sampling protocols see
the sampling methodology in the HFQLG monitoring plan. Preliminary statistical
analyses were completed for all units with soil compaction data. Although further
analysis will be completed in the future for both compaction as well as other soil
condition indicators, this report only includes the analyses completed to date.
Harvest areas were sampled by transects; landing areas were excluded. For details on
sampling protocols see the sampling methodology in the HFQLG monitoring plan.
2006 Data - Overview of Soil Conditions Before and After Treatment
The monitoring indicates that legacy compaction is commonplace. A significant increase
in compaction was found in group selection units that did not receive subsoiling
mitigation. The significance of compaction, in regard to long term soil productivity,
needs further evaluation. All of the units (including group selections) met the
recommended thresholds in the Forest Plan soil quality standards for soil cover (50%).
Evidence or observation of increased soil erosion was minimal. Soil displacement was
well within acceptable standards. The percent area with soil disturbance increased
compared to pre-treatment monitoring, but appears to be acceptable within the normal
range of controlled logging activities. Large woody material decreased from levels
observed during pre-treatment monitoring, but met standards and guidelines, which
recommend 3-5 large logs per acre as determined on a project-by-project basis.
Soil Porosity
Soil compaction (loss of soil porosity) has been viewed as a major factor affecting soil
productivity. Compacted soil has lower water infiltration rates, can have higher or lower
water holding capacity (depending on soil texture), and increases in soil strength that can
restrict root growth. Standards and Guidelines within the Forest Plans for the Lassen and
Tahoe National Forests limit detrimental soil compaction to no more than 15 percent of
an activity area excluding the transportation system. Standards and Guidelines within the
Plumas National Forest Plan allow no more than 15 percent of an activity area to be
dedicated to skid trail and landings. Activity areas are typically defined as harvest units.
Comparisons of the before and after treatment soil porosity status were made for 55 Units
for various types of treatments and subsoiled status. Listed in Table 1 below are the
mean differences (“after” minus “before”) of percent of tile-spade measurements that
were determined to exhibit soil compaction along with 95% confidence limits, standard
errors of the mean, and the t-value and P-value for the hypothesis test of no change.
Only the Group select treatment showed a statistically significant increase in soil
compaction (P-value = 0.03942 which in the table is rounded to 0.04) however, the 95%
confidence interval is rather large: (1.32%, 32.36%). Note that the variance in the data
set is very large, and when all units are included in one dataset, there is a normal
distribution around no change (Figure 1).
Table 1. Summary of change in mean percentages of tile-spade measurements that were determined
to exhibit soil compaction along with 95% confidence intervals, the number of units (N), and the
standard error of the mean difference for various types of treatment and subsoiled status.
95% Confidence
Interval for Mean
Type
Subsoiled N
No
42
All
Yes
13
Group
No
5
Select
Yes
6
Masticate
No
2
Mechanical
No
33
Thinning
Yes
7
No
No
2
treatment
Mean
Difference Lower
2.59
-1.92
-6.00
-14.70
16.84
1.32
0.33
-7.70
5.00
-37.35
1.15
-3.85
-11.43
-27.21
-11.67
-96.37
Upper
7.11
2.70
32.36
8.36
47.35
6.15
4.35
73.04
tPvalue value
1.16 0.25
-1.50 0.16
3.01 0.04
0.11 0.92
1.50 0.37
0.47 0.64
-1.77 0.13
-1.75
0.33
Std.
Err.
2.24
3.99
5.59
3.12
3.33
2.46
6.45
6.67
In addition, the individual pre- and post-treatment means and confidence intervals of the
percent of tile-spade measurements that were determined to exhibit soil compaction give
an idea as to the level of compaction and the precision of those estimates as found in
Table 2 below:
Table 2. Mean percentages of tile-spade measurements that were determined to exhibit soil
compaction along with 95% confidence and prediction intervals, the number of units (N), and the
standard error of the mean. (N/A for “Not applicable” is given for the collection of all pre-treatment
units in the “Subsoiled after Treatment?” column.)
Type
Subsoiled
after
Treatment?
N/A
No
All
Yes
Group
Select
Masticate
Mechanical
Thinning
No
treatment
No
Yes
No
No
Yes
No
Time
Period
Pre
Pre
Post
Pre
Post
Pre
Post
Pre
Post
Pre
Post
Pre
Post
Pre
Post
Pre
Post
95% Confidence
Interval for
Mean
N
55
42
13
5
6
2
33
7
2
Mean
21.15
19.93
22.52
25.09
19.08
8.87
25.71
11.57
11.90
12.50
17.50
22.30
23.45
36.67
25.24
15.83
4.17
Lower
15.40
13.73
17.06
9.52
10.14
0.00
13.45
1.25
0.62
0.00
0.00
14.81
16.82
8.82
10.74
0.00
0.00
Upper
26.90
26.13
27.98
40.65
28.03
21.97
37.98
21.90
23.19
100.00
100.00
29.79
30.08
64.51
39.74
100.00
57.11
Std.
Err.
2.87
3.07
2.70
7.14
4.10
4.72
4.42
4.02
4.39
12.50
9.17
3.68
3.26
11.38
5.93
10.83
4.17
P-value
For
H0:
Mean=15%
0.0367
0.1160
0.0081
0.1833
0.3393
0.2639
0.0723
0.4325
0.5123
0.8743
0.8305
0.0558
0.0141
0.1056
0.1348
0.9511
0.2338
95% Prediction
Interval for
Individual
Units
Lower
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Upper
63.80
60.09
57.91
81.19
51.32
38.17
53.13
36.86
39.55
100.00
100.00
65.34
61.55
100.00
63.60
100.00
79.04
Note that the 95% confidence intervals for the mean characterize how well the mean is
estimated. Also, for example, the estimate of 23.45% for post-treatment mechanical
thinning units does not mean that all units were above the 15% threshold. This is simply
the estimate for the mean of a collection of units. The prediction intervals are used to
predict the range of individual unit values to be observed 95% of the time. P-values must
be less than 0.05 to be statistically significant.
Density
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Histogram of % Compaction Change
-40
-20
0
20
40
% Compaction Change
Figure 1. Frequency Distribution differences in pre-post compaction measurements
Explanation of findings and trends
The desired condition is that detrimental soil compaction is limited to no more than 15
percent of an activity area. Although the mean pre-existing compaction value is over
threshold (statistically significant; P = 0.04), the estimate does not indicate that all units
were above the 15% threshold. Note that there is a very wide prediction interval for the
amount of compaction for individual units. Further analysis of each individual unit is
warranted and planned in an effort to determine which units are statistically over the
threshold.
The comparison of pre- and post-sampling shows that the overall trend is no significant
change. However, when groups of units were analyzed, there was an increase in the
extent of compacted ground in group select units that were not subsoiled (statistically
significant; P = 0.04). The silvicultural prescription, the location of trees to be removed,
and the kinds of logging equipment used has changed from the pre-treatment situation to
the latest entry being monitored and is likely partially responsible for the increase in
compaction. It is important to note that most units that exceed the threshold posttreatment start with a significant amount of legacy compaction from previous treatments.
Even where current equipment operations are well controlled, cumulative effects
including legacy compaction may exceed the standard.
While the sample post-treatment mean amount of compaction is less than the pretreatment mean (which is expected if subsoiling acts as a mitigation measure to reduce
compaction), the decrease in compaction was not statistically significant (P = 0.16).
Prevention of compaction should be considered the highest priority. Use subsoiling as a
second best management choice where it is appropriate based upon the area management
objectives, the particular soil characteristics, and other factors.
Significance of the Findings for Soil Porosity (Compaction)
Recent findings on compaction effects on total biomass productivity (soil productivity)
(Powers and others 2005) indicate that for soils with texture classes grouped into “sandy”
(coarse sandy loams or sandier) declines in total biomass productivity are not expected.
On soils grouped as “loamy”, compaction did not appear to significantly decrease or
increase total biomass productivity. On soils grouped as “clayey” (such as clay loams or
more clay), total biomass productivity declined when compacted.
Soils monitored during the 2006 period can be characterized as being in either the
“sandy” or “loamy” soil texture groupings. Soil textures on 11 units would be classed as
“sandy” and soil textures on 4 units would be classed as “loamy”. None were classified
as “clayey”. The remaining 40 units in the sampling pool are still unclassified. Collecting
soil texture information on these units is planned in 2007. So in regard to overall
significance, it appears that although some units do not meet the 15% standard for
compaction, a decrease in soil productivity (total biomass productivity) would not be
expected unless they occur on “clayey” soils. Additional analysis will be completed.
There were some unexpected results in the compaction data. The monitoring data
appears to show a decrease in compaction on some units that were not subsoiled. Several
factors could contribute to these results:
•
Although GPS is used to locate post-treatment transects in the same approximate
location as the pre-treatment transects, the post-treatment transects are not in the
exact same locations. Sampling a different line across the landscape will result in
some random variation in results.
•
Compaction is a continuous variable. Still the protocol for compaction sampling
requires the crew to judge detrimental compaction based on soil structure and
other field properties, and to judge if it is above or below a threshold. A smaller
set of core samples is collected, taken to the laboratory for measurement, and used
to calibrate the sampling crew. Variation in samplers and their skills can lead to
some variability in results.
Additional 2006 Post Treatment Soil Condition Indicators
Eleven thinning units, three group selection units and two mastication units on the
Lassen, Plumas and the Sierraville District of the Tahoe National Forests were evaluated
for post-treatment soil conditions in 2006. The following data has not been evaluated
statistically and there remains the question whether the sampling intensity accurately
represents the units. A cumulative analysis of all units will be conducted, but this
statistical analysis has not been completed. Construction of confidence intervals could
help determine the adequacy of the sampling. In addition to the soil porosity
(compaction) indicator described above; soil ground cover, soil displacement, soil
disturbance and large down wood were also considered.
Figure 2. Summary of 2006 monitoring data for the 11 thinning units before and after
treatment is shown in the graph below.
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
ov
er
So
il
C
rb
an
c
e
t
en
D
is
tu
C
om
pa
ct
io
n
Pre Treatment
Post Treatment
D
is
pl
ac
em
Percent of Area
Pre Treatment versus Post Treatment Conditions (Average
of 11 Thinning Units)
Soil Conditions
Soil Ground Cover – Cover of duff & litter, vegetation, large woody debris or rock.
Soil Ground Cover – this is a composite of two recommended thresholds from the R5 soil
quality analysis standards: the effective soil cover (ESC) for erosion prevention and the
organic matter threshold for fine OM. The ESC standard is site specific and the OM
standard (regional threshold) is for a minimum of 50% fine OM, preferably undisturbed.
Soil cover, on average, decreased after logging, from 93 percent to 83 percent in the
thinning units. Ten units showed a decrease and only one had an increase in soil cover.
This was to be expected as normal felling and skidding operations tend to displace duff
and litter. The thresholds in the individual forest plan soil quality analysis standards
generally recommend a minimum of a 50 percent ground cover to prevent erosion. All of
the thinning units and group selection units met the standard this year.
Post Treatment Soil Ground Cover
Percent of Units
100%
80%
Group Selects
60%
Thinning Units
40%
20%
0%
Met Soil Quality Std
(50% cover)
> 75% Soil Cover
Soil Displacement – Soil has been moved from its original location, resulting in
loss of topsoil.
Although not a part of the soil quality standards, this monitoring includes a metric for soil
displacement.
Overall, the level of soil displacement measured after the latest management treatment
was low. Soil displacement from the HFQLG treatments does not appear to be a
significant problem for soil quality. A summary of the soil displacement data shows an
anomalous result; two units showed an increase in displacement after treatment, seven
showed a decrease and two units had no change. On the average, soil displacement in the
thinning units stayed the same (6% soil displacement) after logging.
Post Treatment Detrimental Soil Displacement
Percent of Units
100%
80%
60%
Group Selects
40%
Thinning Units
20%
0%
None
1-10%
> 10%
Percentage of Sample Points in a Unit with
Displaced Soil
Soil Disturbance – Any indication of soil disturbance (e.g., loss of duff and litter or
evidence of past equipment operation).
Although not a part of the soil quality standards, this monitoring includes a metric for soil
disturbance.
Of the 11 thinning units sampled, 10 had an increase in soil disturbance and one showed
a decrease. The one unit showing a decrease in soil disturbance is probably an artifact of
sampling design or slight differences in interpretation rather than an actual decrease. The
data is more consistent when it is averaged across all units. When averaged, soil
disturbance increased after logging activities from 14 percent to 32 percent as expected.
Any change in the natural condition is recorded as disturbance. There is no forest plan
standard or guideline for disturbance.
Post Treatment Soil Disturbance
Percent of Units
120%
100%
80%
Group Selects
Thinning Units
60%
40%
20%
0%
None
1-25%
> 25%
Percentage of Sample Points in a Unit That
Show Soil Disturbance
Group Selection Units
Of the 3 group selection units
monitored:
- All 3 units (100%) had more
than 25% soil disturbance.
Thinning Units
Of the 11 thinning units
monitored:
- No units (0%) lacked soil
disturbance.
- 2 units (18%) had some level
of soil disturbance (1-25%).
- 9 units (82%) had more than
25% soil disturbance.
Large Woody Material – Down logs at least 20” in diameter and 10’ long.
Large down woody debris decreased from an average of 9 logs/acre to 4 logs/acre. Many
of the units had been burned as part of the follow up fuels treatment and this eliminated
many of the large down logs. In general HFQLG treatments decrease the number of large
down logs. It does not eliminate them. Forest plan guidelines recommend 3-5 large logs
per acre as determined on a project-by-project basis.
Post Treatment Large Down Woody Material
(Decomposition Classes 1-5)
Percent of Units
120%
100%
80%
Group Selects
60%
Thinning Units
40%
20%
0%
No Down Wood 3 or more logs/ No logs in Class
(all classes)
acre (all
1-3
classes)
Group Selection Units
Thinning Units
Of the 3 group selection units
monitored:
- 3 units (100%) had no large
down wood (all decomposition
classes).
- No unit had 3 or more logs per
acre (all decomposition classes).
- 3 units (100%) had no logs in
decomposition classes 1-3.
- The highest value in a unit was 0
logs per acre.
- The average for all 3 units was 0
logs per acre.
Of the 11 thinning units monitored:
- No units (0%) lacked large down
wood (all decomposition classes).
- 9 units (82%) had 3 or more logs
per acre (all decomposition classes).
- 2 units (18%) had no logs in
decomposition classes 1-3.
- The highest value in a unit was 8
logs per acre.
- The average for all 11 units was
4.3 logs per acre.
Mastication
Mastication is a mechanical method for reducing competing vegetation and thinning
small trees. Typically, a grinding head is mounted on the end of the boom of an
excavator. The excavator then travels through a unit masticating the competing brush and
small trees. The excavator exhibits low ground pressure and leaves a mulch of shredded
debris in its wake. The result is generally low soil disturbance and compaction to the soil
while leaving relatively high soil cover. Two units that had been masticated were
monitored in 2006.
Because the pre-treatment conditions of the two masticated units were drastically
different, the data are presented separately because the average values are not very
meaningful. Both units are pine plantations, but one unit showed no compaction,
displacement or disturbance prior to treatment. The other unit had high levels of legacy
compaction, displacement and disturbance prior to treatment; probably from prep work
when the unit was planted. Although the data for Waters Unit 8D appear to exceed
desirable conditions, this has not yet been evaluated statistically, but it is planned.
Waters 8D
Waters 8E
Before
After
Before
After
Compaction -
25%
27%
Compaction -
0%
8%
Displacement -
23%
35%
Displacement
0%
8%
Disturbance -
28%
43%
Disturbance -
0%
15%
Soil cover -
84%
85%
Soil cover -
100%
88%
Adaptive Management Changes as a result of Monitoring
Finding: Analysis of the soil compaction data indicates that there is a need to increase the
precision of the data. Confidence intervals are too wide to make definitive statements
about the effect of management in some instances.
Management Response: Changes in the sampling protocol are being developed and
implemented during the 2007 evaluations.
Finding: Forty units in the sampling pool do not have soil texture classifications. This is
needed to determine the significance of soil compaction.
Management Response: Collecting soil texture information on these units is planned in
2007.
Finding: Legacy compaction exists above threshold levels and cumulative increased
compaction effects are occurring in group select units. However, negative effects on soil
productivity are not expected for soils with “sandy” or “loamy” soil texture classes.
Subsoiling, although not appropriate in all situations, may reduce compaction.
Management Response: Subsoiling is continuing to occur where appropriate. A review
of subsoiling was conducted by the Regional Soil Scientist in 2006 and recommendations
have been provided. Forest plan standards and guidelines may need to be amended to
allow for compaction for particular soil types where negative impacts to soil productivity
are not expected. Findings from the 10 year results of the Long Term Soil Productivity
Study and other studies will be used to revise the LRMP standards.
Finding: Increases in detrimental compaction are occurring in group selection units, and
possibly in some thinning units. Prevention and/or reduction of detrimental compaction
can be improved by using soil moisture objectives relative to equipment operations to
determine when equipment operations should be permitted.
Management Response: There have been increases in seasonal operation restrictions to
dry soil conditions when soil strength is high for soil textures dominated by loam or clay
particle sizes. Slash has been placed on skid trails during harvest operations in some
project areas in an effort to reduce compaction.
References
R. F. Powers et al. 2005; The North American long-term soil productivity experiment:
Findings from the first decade of research; Forest Ecology and Management 220 (2005)
31-50.