UC Davis Department of Plant Sciences


Project 1: Riparian Ecosystem response to timber harvesting for the purpose of restoring aspen  

Contact Person: Dr. Kenneth W. Tate

Participants: Dr. Bobette Jones, Monika Krupa, Coye Burnett, Dave Burton, Shannon Cler, Betsy Huang, Sarah Hubert, Melanie McFarland, Tom Rickman, Yukako Sado, Alfred Vazquez, Natalie Wegner, and the Quincy Library Group

Get USFS Aspen Final Report(11.5mb)>>   Get USFS Aspen Final Report (3.9mb)>>

Project Background & Description




Trembling aspen (Populus tremuloides michx.) occurs in the montane zone of California’s Sierra Nevada/Cascade range. In the West, aspen is considered a keystone species providing crucial habitat to support a high diversity of local and landscape plant, animal, and insect species. Because of the importance of aspen, there is currently growing concern about declines in the health and distribution of aspen stands across this region. Much of this decline is caused by the lengthening of fire intervals through fire suppression, as well as by historic and current heavy grazing of aspen suckers, which lead to the encroachment of aspen stands by conifers.

Recent studies have found that the use of commercial timber harvest techniques to remove conifers is an effective treatment for stimulating aspen when combined with controls on ungulate herbivory. The broad-scale implementation of conifer removal treatments in this region is of concern because many aspen stands are associated with riparian areas, which can be severely impaired by logging activities



  The management challenge we are facing is to design and implement prescriptive conifer removal strategies sufficient for restoration of encroached riparian aspen stands with minimal short-term and no long-term negative impacts on riparian resources. To this end, we monitored the soil, streamwater, and macroinvertebrate impacts of four conifer removal treatments in Lassen National Forest from 2003 through 2010.    

This study examined the riparian ecosystem impacts of 2 different management-scale conifer removal projects:

1.Pine-Bogard Project (located in the Eagle Lake Ranger District, on the eastern side of Lassen National Forest, at the confluence of Pine and Bogard Creeks ) which consisted of 3 conifer removal treatments:
    i.Phase 1 in January 2004
    ii.Phase 2 in August 2005
    iii.Phase 3 in January 2008

2.Bailey Project (located in the Hat Creek Ranger district on the western side of Lassen National Forest, in Brokeoff Meadow) which consisted of 1 treatment implemented in August 2006.

Specifically, we performed water quality, stream temperature, solar radiation, canopy cover, aquatic macroinvertebrate, soil bulk density, and soil moisture monitoring, both before and after the implementation of each of the 4 conifer removal treatments.


Unhealthy Pine Creek riparian aspen stands encroached by conifers and without recruitment.
  Figure 1. Timeline of Pine-Bogard Project and Bailey Project
treatment implementation.

May 2005

Left: Pine Creek aspen stand treated during Phase 1 conifer removal. (photo May, 2005) Below Left: Load transport from Pine-Bogard Phase 1 treatment. Below Right: Aspen stand north of sample stations BO4 and BO6 on Bogard Creek, treated during Phase 1. Left side illustrates post treatment, right side illustrates initial conifer encroachment level. Bogard Creek lies ~ 30m to the right of treatment boundary. (photo May, 2005)





Study Design



Sampling and monitoring were carried out from May through September each year from 2003 through 2010.
We measured the following parameters:

i.   stream and air temperature – every ½ hour
ii.  stream discharge and chemistry, including nitrate as N (NO3-N), ammonium as N (NH4-N), phosphate as P (PO4-P),
     sulfate as S (SO4-S), potassium (K), total suspended solids (TSS), and turbidity ~ every 2 weeks
iii.  soil moisture ~ every 2 weeks (excluding Phase 1 treatment)
iv.  soil bulk density – before and after each treatment (excluding Phase 1 treatment)
v.   stream canopy cover and solar radiation inputs – before and after each treatment
vi.  aquatic macroinvertebrates – before and after each treatment





Grab Sample

  Stream Flow   Solar Radiation   Soil Moisture   Soil Bulk Denisty  


Soil moisture and bulk density were measured in treatment and reference aspen stands (Figs. 3 – 4). Treatment stands are aspen stands in which conifer removal took place, while reference stands are untreated aspen stands. Streamwater parameters and aquatic macroinvertebrates, were measured at stream monitoring stations (Figs. 3 – 4). Canopy cover and solar radiation measurements were made along treated reaches, that is, the stream reaches located between the upstream and downstream stations for each treatment (Figs. 3 – 4; Table 1).





Click to Enlarge Figure 3a.


Click to Enlarge Figure 3b.


Click to Enlarge Figure 4.







Table 1. Stations immediately upstream and downstream of each treatment area, with the corresponding lengths of treated reaches.


Analysis Approach






In order to evaluate treatment effects on the various parameters measured, the difference (?) between the reference and treatment measurements were compared before versus after treatment using linear mixed effects analysis or graphical and tabular analysis. Fig. 5 (right) illustrates this concept.

The reason for this analysis is that within these systems, both treatment and reference values for a given parameter can be expected to fluctuate over time, primarily in response to the annual climatic variability.  However, if there is no treatment effect (Fig. 5a), then the difference between the reference and treatment values will remain the same (Δ = 1 in both 2003 and 2010), despite changes in both treatment and reference values across years. 

In contrast, if a treatment effect occurs (Fig. 5b), then the difference between reference and treatment values will either increase or decrease relative to pre-treatment years (Δ = 1 in 2003 and Δ = 3 in 2010).

Solar radiation and canopy cover do not fluctuate over time without disturbance, therefore these parameters were analyzed using a simple before versus after treatment comparison along each treated reach.



Figures 5a and b. Conceptual diagram of no treatment effect (5a) and treatment effect (5b) occurring for parameters measured in this study.




Summary of Findings




The major findings of this study are that conifer removal activities:

i.    had no effect on stream chemistry or aquatic macroinvertebrates,
ii.   had no effect on soil bulk density but did cause a significant increase in soil moisture
iii.  decreased canopy cover and increased solar radiation following the Bailey Project and following
     Phase 3 of the Pine-Bogard Project, but did not influence stream temperature.

This information indicates that riparian habitats, aquatic ecosystem functions, and native fish species are not impaired by conifer removal in riparian aspen stands.

A summary of the major findings from this study found below


Water Collection

  Stream Chemistry
Stream nutrient (nitrate-N, ammonium-N, phosphate-P, sulfate-S, and potassium), total suspended solids (TSS), and turbidity levels across all years and sampling locations were consistently below levels of concern for drinking water safety and aquatic ecosystem function. Additionally, all nutrient concentrations showed little variation within creeks and across years. Turbidity and TSS exhibited annual variation, but there was no significant increase in the difference between upstream and downstream turbidity and TSS levels in response to timber harvesting. >>More

Canopy Cover & Solar Radiation
There was no significant change in stream canopy cover or solar radiation in response to Phase 2 conifer removal at Pine and Bogard Creeks. There was a significant decrease in stream canopy cover and increase in solar radiation in response to Phase 3 conifer removal at Pine and Bogard Creeks, and in response to conifer removal at Bailey Creek. >>More


  Stream Temperature
The difference between upstream and downstream temperatures did not increase in response to any of the conifer removal treatments. Stream temperature appeared to be driven primarily by discharge. Maximum stream temperatures were almost always within the optimal temperature range (60.8 to 64.4F) for rainbow trout, which is the native trout species in the region. >>More

Aquatic macroinvertebrate
Aquatic macroinvertebrate metrics indicate high water quality and in-stream habitat conditions across all years and sampling locations, with the highest level of % Tolerant species being 0.6 % at a monitoring station upstream of Pine Creek treatment areas. >>More



  Soil Bulk Density
There was no significant change in soil bulk density in either the 0-6 inch or 6-12 inch depths following conifer removal at Bailey Creek and following Phase 2 and 3 conifer removals at Pine and Bogard Creeks. >>More

Soil Moisture
Soil moisture increased significantly in treatment stands relative to control stands at both the 6 and 18 inch depths following conifer removal at Bailey Creek and following Phase 2 and 3 conifer removals at Pine and Bogard Creeks. The greater retention of soil moisture within treatment transects is most likely the result of the removal of vegetation causing a reduction in evapotranspiration, which sustains high soil moisture levels into the dry season. >>More






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