Research

Effects of slope and ash layer on soil moisture and soil temperature

In August of 2021, I was a part of a team of researchers aiming to survey the effect that elevation had on soil moisture, and soil temperature. We also explored whether the ash layer thickness of a site had an impact on thermal gradients and soil moisture.

San Bernardino National Forest

Introduction

Bear Paw Reserve in the San Bernardino Mountains was affected by a fire that took place in the 2020, named El Dorado. This was the first ever reported fire in Bear Paw Reserve and was reported to take 23 days to achieve 95% containment. Since the start of the fire, the site has been closed off to the public. On August 10th-11th, Bear Paw reserve still showed physical changes caused by the fire (burnt trees, ashes on the ground, less canopy cover).

research questions

Question 1: How does moisture change in response to a slope on a burnt site?

Hypothesis: Higher elevation will have lower soil moisture and vice versa.

Question 2: How does ash layer thickness affect soil moisture?

Hypothesis: The soil with ash retains more water than non-ashy soil, so higher ash layer thickness will have higher soil moisture and vice versa.

Question 3: Is there a thermal gradient response to slope?

Hypothesis: The temperature of the soil decreases with increase of altitude.

methods

Soil samples were collected on August 10-11, 2021 at San Bernardino National Forest near 34°05’25”N and 116°58’02”W
The site was partially burnt in September 2020
A 90m vertical transact was set on a 19 degree slope with 3 samples collected every 30m on a horizontal plane (12 samples total)
Soil temperature at 5 cm and 10 cm depth were recorded using soil thermometers at each sampling site
The depth of ash layer and organic matter were measured at each sampling site
Visual assessments were made of percent canopy cover and surrounding vegetation descriptions
Altitude and coordinates were recorded for each sample location using the “My Altitudes” application
Each soil sample was weighed when wet, then dried in an oven at 104.4℃ for 8 hours. The dried samples were then weighed again to calculate the percentage of soil moisture

Measuring soil temperature and taking soil sample at one of the sites

Gathering soil sample at the last site

Team 1 at Bear Paw Reserve, California

Team 1 at the lab analyzing soil samples

raw data

Soil temperature with notation about other factors

Soil weight before and after being dried with soil moisture percentage

results & discussion

The relationship between the change in elevation along the 90 m transect and the change in soil moisture

Q1: How does soil moisture change in response to a slope on a burnt site?

Soil moisture significantly decreases as the elevation increases.
p = 0.0579 > 0.05 shows that there is not enough evidence to support the claim that higher elevation will have lower soil moisture and vice versa.
r² = 0.9884 indicates that 98.84% of the variance of the soil moisture being studied is explained by the variance of the elevation.
It is is considered an almost best fit line, which shows that there is a small amount of other factor affects on the data (e.g. vegetation, canopy cover, ash layer, sampling size, etc.).

The relationship between the ash thickness and soil moisture

Q2: How does ash layer thickness affect soil moisture?

5 out of 12 sample sites had an ash layer.
There was a slight trend showing that as the thickness of the ash layer increased, the percentage of soil moisture in the soil decreases, however, this trend was not significant
There is no strong correlation between ash thickness and soil moisture as r2 value is too small.
There is not enough data to support hypothesis because percentage of soil moisture varied at 0 cm ash layer.

Q3: Is there a thermal gradient response to slope?

The First Figure demonstrates average soil temperature at 5 cm depth with respects to elevation. It shows a moderate effect size with an upward trend line which means the elevation increased, the temperature of soil increases, however, this trend was not significant (p = 0.259)
The Second Figure demonstrates average soil temperature at 10 cm depth with respects to elevation. It shows a weak or low effect size with a downward trend line which means the elevation increased, the temperature of soil decreases, however, this trend was not significant (p = 0.095)
The Last Figure demonstrates the difference between the average temperatures at 10 cm and 5 cm depth. Standard error bars are shown. It shows a slight trend of the difference between the soil temperatures at 10 cm depth and 5 cm depth, however, this trend was not significant since sample sizes were small.
r² values of soil temperature at 5 cm depth and 10 cm depth indicate that at least some variability in the data cannot be accounted for by the model
This shows that there is not enough evidence to support the claim that the soil temperature decreases with increase of altitude
The variation may mainly come from small sampling size and sampling time (early morning at the bottom, and noon at the top) as well as other factors (e.g. vegetation, canopy cover, ash layer, etc.)

comparison between California and alaska research

The Bear Paw Reserve is located in the San Bernardino Mountains of California, the El Dorado fire covered the area back in the fall of 2020. The Caribou-Poker Creek Research Watershed in Interior Alaska was decimated by the summer wildfire in 2004. Since the fire, the Bear Paw Reserve has been closed to the general public, the CPCRW is a restricted access area for research purposes only.

In both ecosystems we studied the thermal gradience, soil temperature, and soil moisture. At CPCRW the thermal gradients calculated were between the organic and mineral layers, whereas the thermal gradients at Bear Paw were calculated using the 5 cm and 10 cm soil temperature. At Bear Paw the slope of the site analyzed was 19 degrees, for CPCRW it was 4.7 degrees.

Location of Bear Paw Reserve in the Interior of California. Diagram of field site with white dots indicating where each sample was collected and where on the transect

Location of the Caribou Poker Creek Research Watershed in the Interior of Alaska. This map shows the locations and elevations of the selected sites on the 120 meter vertical transect along an East facing slope. This area corresponds to a back burnt region of the boreal forest in discontinuous permafrost

At the top of the slope at Bear Paw Reserve, the soil moisture content was 1.53% and at the bottom was 3.76%, which resulted in a difference of 2.23% and an average for the transect of 2.56%. For the soil temperature at 5cm, there was a difference of 7.1℃ with the top recorded at 33.7℃ and the bottom 26.6℃. The average temperature came to be 28.2℃. The area has been damaged with burnt trees, ashes covering the ground, and less canopy cover. It has almost been a year since the fire began and the damages are still present, but vegetation is slowly growing back.

From the results of the data from Caribou-Poker Creek Research Watershed there was a 15% moisture content difference when comparing the top to the bottom of the vertical transect. The soil moisture content of the top of the slope was 113% and the bottom was 98% the average for the transect was 138%. For the soil temperature there was a 0.6℃ temperature difference with the top recording 6.8℃ and the bottom temperature recorded as 6.2℃. The average temperature for the transect was 6.16℃. For the area after the 2004 fire the regrowth includes birch, black spruce, grass mounds, berries, etc. Water tracks have formed from water runoff in the topography due to the changing vegetation density and gradients after the fire.

FURTHER DIRECTIONS

Assessing the color of the ash in each site would be beneficial in future studies because it would provide insight on the albedo and how it connects to soil temperature and soil moisture.
Increasing the length of the vertical transact and the amount of soil samples taken at each site would bring more accuracy to the findings.

ACKNOWLEDGEMENTS

We thank Dr. Katie Spellman, Dr. Javier Fochesatto, Dr. Elena Sparrow from UAF, Abby Ramirez and Jeff Cutkomp from SAC MESA, and Crystal Castillo from Cal Poly San Luis Obispo for research support and training. We would also like to thank The Wildlands Conservancy for making their Bear Paw Reserve location available. Funding for this research experience is supported from the MESA program and Santa Ana College and a cooperative agreement with NASA under Award No. NNX16AC52A.

citations

Sustainability and Climate, Office of. “Drought and Tree Mortality in the Pacific Southwest Region.” Forest Service, US Department of Agriculture, July 2017, www.fs.usda.gov/ccrc/sites/default/files/documents/files/r5-droughtfactsheet.pdf.

USDA Forest Service, Fire and Aviation Management. “El Dorado Fire.” InciWeb the Incident Information System, San Bernardino National Forest , 2020, inciweb.nwcg.gov/incident/7148/.

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