Soil Nutrient Research on the Heber Ranger
District Apache-Sitgreaves National Forest
Hazel peny'
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Abstract
Soil temperatures and vegetative and soil nutrients were
measured during a prescribed fire study in fuelwood harvested pinon-juniper.
Soil temperatures in the top 0-5cm reached 339'~ under slash >25MT/ha.
Vegetative nitrogen and phosphorus decreased as much as 91% due
primarily to volatilization and erosion. Soil nutrient levels increased or
remained unchanged.
Scientists fiom the Rocky Mountain Station are conducting
several studies involving two sites on the Heber Ranger District,
Sitmaves National Forest, Arizona. This discussion emphasizes
the soils aspect of these studies.
The Ancient Tank study is in an area dominated by Juniperus
monosperma (one-seed juniper) with little interspace vegetation
The interspace vegetation is mostly annual fobs with some
Bouteloua gracilis (blue grams). Soils are Trpic Haplustalfs
(Nelson 1992) derived from limestone interbedded with
sandstone. In many areas, the soil is shallow and armored.
In an effort to increase forage, the district used fuelwood
harvesting to remove trees greater than 7.5 cm in basal diameter
from areas averaging 80 ha in size. They wanted to use
commercial hawesters instead of opening the areas up to
personal use cuts because the commercial cutters could be
controlled under contract to complete clean hawests over a set
time period. The problem incurred at the Ancient Tank site was
that most of the sale areas they wanted to cut produced less than
6 cordsh and the cutters felt they could not make money with
the amount of time it would take to get a truck load of wood.
So, the district paid harvesters an average of $40/ha depending
upon the amount of wood they could get off the area (Mortensen
1991). The more wood the cutter could get to sell, the less the
district had to pay.
Harvesting was completed on four areas over a 4 year period
resulting in different age classes of slash When we started the
study, these sites were 1 year, 2 years, 3 years, and 4 y e .
post-harvest. Harvesting resulted in an average of 45MT/ha of
slash which we determined using methodology outlined by
Brown (1974) in the Handbook for Inventorying Downed
Woody Fuel which we modified for use in piilon-juniper.
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Research Soil Scientist, USDA Forest S e ~ c e Rocky
,
Mountain
Forest and Range Experiment Station, Flagstaff, AZ.
The District was l o o m for information on the effects of
burning slash on soil productivity. The Range Conservationist
specifically wanted to know if slash age should be considered
if they did decide to bum. To try and answer the District's
questions as well as some on soil phosphorus and the long-term
effects of fire, we selected four 112-ha blocks in each age class
three were to be burned with one left as a control. Within
each block, we installed three 20 m transects for collection of
soil, litter, ash, and slash samples.
The specific objectives were to measm soil temperatures
associated with a prescribed piilon-juniper slash burn (Perry et
al, in review), and to determine the long-term effects of
prescribed fue on nutrient cyclmg under vaq~ingage classes of
slash and at different soil depths.
Soil temperatures were measured by Steve Sackett and Sally
Haase of the Riverside Forest Fire Laboratory using stainless
steel thermocouples (Sackett and Haase 1992). Thennocouples
were inserted horizontally within the top 10 cm of the soil at
nine random locations in each block under heavy fuel (which
we categorized for this area as greater than 25MT/ha), light he1
(less than 25MT/ha), and in the interspace adjacent to the slash
The thermocouples are attached by cable to data loggers which
are housed in insulated aluminum boxes. Because of time and
equipment restraints, we were only able to instrument two sites.
We chose the one with the lowest slash load at 4 1 M T h which
was the 4-yeardd site and the site with the heaviest fhel, the
2-yeardd site with 51MT/ha
In November 1989, 12 blocks were burned leaving only piles
of white ash and burned stumps. To reach our objective of
determining effects of fire on nutrients, we collected soil at 0-1,
1-2, and 2-5 cm depths along each transect immediately before
and after the burn and again 4 months later in the spring of
1990.
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Soil samples were collected in all four age classes but due
to budget constraints, only the materials from the 2 year and 4
year sites were analyzed. Materials from the 1 year and 3 year
sites are in storage for later analyses. The soil was analyzed for
seven different phosphorus fractions, organic carbon,
exchangeable and total calcium and magnesium as well as total
nitrogen, particle size, and bulk density.
Samples of litter and slash were collected immediately before
the fire and ash was collect following the burn. We attempted
to collect ash the following spring but there had been so much
movement and mixing of soil and ash by wind that we were
unable to collect a clean ash sample. Determinations made on
these samples were oven d ~ yweight, total phosphorus, calcium,
magnesium, and nitrogen.
Figure 1 illustrates the maximum temperatures we recorded
under heavy slash in the top 5 cm of soil. The highest
temperature recorded at the 2 year site was 95 OC and the mean
was 70 O C . On the 4 year site, the highest temperature recorded
was 339 O C and the mean of all nine locations was 203 O C .
Figure 2 illustrates longer temperature durations on the
4-yeardd site where temperatures remained over 100 OC for
more than 14 hours and return to near ambient at approximately
35 hours after ignition. The measured temperatures never
exceeded 100 O C on the 2 year site and returned to near ambient
within 10 hours. I say 'near' ambient because the fire causes a
darkening of the surface and removal of shade producing slash
so the ambient temperatures on these sites will be higher
following the fh.
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Ambient Temp.
Highest temp. recorded out
of 9 measurement points
Mean of highest temps.
recorded at all 9 pomts
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1
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50
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0
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I 2 Year Site
4 Year Site
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HOURS A F E R IGNITION
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Figure 2.
Soil temperature durations in the 0-5 cm soil depth
on a piiion-juniper slash burn in central AS.
-203
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b...-.;.....i
2 Year Site
have accumulated heavier materials nearer the soil surface due
primarily to weathering and gravity. This accumulation of
surface fuel resulted in higher, longer duration soil temperatures.
Figure 3 also illustrates the loss of vegetative material on a
weight basis. On the 2 year site, there was an 86% decrease in
vegetative material h m slash and litter to ash and a 52%
decrease on the 4 year site. These losses are attributed primarily
to wind during and immediately following the fire which carried
large amounts of ash off site. As mentioned earlier, four months
after the fire wind had moved enough soil and ash to make
collection of ash samples impossible. Malchus Baker, a Research
Hydrologist at the Rocky Mountain Station in Flagstaff9 is
currently studying wind erosion on burned and crushed sites on
the Heber District.
----- - .-1-339
C
150
and compaction Secondly, slash on the 4 year site appeared to
, ,
4 Year Site
Figure 1.
Soil temperatures associated with a piiion-juniper
slash burn in central Arizona.
Slash weights reported for these areas were 5 1MTh on the
2 year site and 41MT/ha on the 4 year site. Why then do we
see this difference in soil temperatures? It could be the result to
two thmgs. First, the litter layer on the 4 year site (Figure 2)
was heavier and denser with an average weight of 29MTh and
depth of 0.5 c m Whereas, the 2 year site had 18MT/ha litter at
an average depth of 1.8 cm. The extra 2 years slash was on the
ground on the 4 year site allowed for greater litter accumulation
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2 Year
Slash
DLitter
4 Year
KlAsh
Figure 3.
Vegetative material weights on a piiionjuniper slash
burn in central Arizona.
Nutrient determinations (Table 1) illustrate a decrease in
nitrogen and phosphorus from that contained in the slash and
litter to what remained in the ash. This decrease is due primarily
to volatilization and erosional losses. On the 2 year site, we had
a 91% decrease in both phosphorus and nitrogen On the 4 year
site, which had somewhat less erosional ash loss, we found an
88% decrease in phosphorus and 72% decrease in nitrogen
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Table 1.
Changes in vegetative nitrogen and phosphorus
(kglha) following a prescribed pinonjuniper slash burn in
central Arizona.
2 Year Site
4 Year Site
Nitrogen
Phosphorus
Nitrogen
Phosphorus
Pre-Burn
(Litter+Slash)
290.5
34.8
312.2
31.1
Post-Burn
(Ash)
25.8
3.0
87.7
10.1
% Decrease
91.1
91.4
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71.9
67.5
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One year following the fm, there was no vegetation in the
bumed area and 3 112 years following the bum there were only
a few annual fohs in the burned areas with grass cover right
up to the edge of the bum.
Table 2 lists soil nutrient changes for the top 5 cm of soil.
We found increases in total nitrogen, avadable phosphorus, and
exchangeable calcium and magnesium We found no decreases
in soil nutrients. It is well documented that there is a release of
nutrients following fire due to oxidation of organic materials
once held in the vegetative materials (Isaac and Hopkins 1937,
Smith 1970, Lewis 1974, Stark 1977, DeBano and C o d 1978,
Stdmck et al. 1982, Wright and Bailey 1982, Giovanni et al.
1988). In this study, however, it's obvious when considering that
there is little or no vegetation in the burned area that something
else is happening in the soil. Could it be a destruction of soil
flora and fauna (Hayward and Tissot 1936, Ahlgren and Ahlgren
1965, Wright and Bailey 1982)? Very probable. An alteration
of nutrient ratios? Probable. Release of some unknown
compounds from the resinous wood slash? Maybe. Hydrophobic
condition of the soil? A test for hydrophobicity in this instance
indicated no problems but hydrophobic soils have been known
to occur following piiion-juniper frres. Could it be a lack of seed
source to recolonize the burned areas? Possible in some of the
12 burned blocks, but not in all. Could it be a combination of
conditions, some of which we can only guess at and some that
have never o c c m d to us? This is the most likely scenario of
all.
Answers to some of these questions were what we were
hoping for with a long-term study until one fateful day 2 y e .
ago when there was a rniscornrnunication among some folks
working out on the Ancient Tank Site. The District decided not
to burn the remaining hectares of slash, based partly on our
fmdings. They have a roller chopper that they pull behind a
tractor to break up the slash and get it down into the soil surface.
It can then decompose faster and provide an effective soil cover.
Table 2. - Changes in soil nutrient levels following prescribed
piiion-juniper slash burning in central Arizona.
Total nitrogen
2 Year Site
4 Year Site
No change
35% increase
Total phosphorus
No change
No change
Total calcium
No change
No change
Total magnesium
No change
No change
Available phosphorus
83% increase
93% increase
Exchangeable calcium
129% increase
170% increase
Exchangeable
magnesium
No change
45% increase
Organic carbon
No change
No change
pH
No change
No change
The chopper &o produces trenches that provide microclimate
conducive to seedling establishment and appear to inmase
infiltration, although that hasn't been measured. The chopped
areas are much more aesthetically pleasing than untreated or
bumed slash, and areas that were chopped only 1 year ago
appear to have greater forage production and species diversity.
So, what was the problem on the burned sites? The tractor
operator had been told to chop everythmg in the fuelwood
harvested areas. "Everyhng?" he asks. "Yes, everythug." By
now you've probably guessed that even though there was no
slash to chop on the burned blocks and the permanent transect
markers were sticking up all over, he ran the chopper through
the blocks churning up the soil on the transects and flattening
the markers.
Well, thugs happen, and we gained some good information
but we didn't accomplish any of our long-term goals. That brings
us to the second study we're conducting on the Heber Ranger
District.
The Mud Tank area has approximately 60% Pinus edulis
(piilon pine) and 40% juniper species, primarily one-seed juniper
but with some Juniperus deppeana (alligator bark juniper) on
better sites. Soils are derived from limestone parent material
interbedded with sandstone. The study is made up of 33 4-ha
blocks over which four silvicultural treatments are being applied
by commercial fuelwooding. In this case, there is enough
fuelwood that the cutters pay the district to cut.
The silvicultud treatments are single-tree selection where
tree densities are reduced approximately 50% while maintaining
diameter distribution, overstory removal where everything
greater than 18 cm basal diameter is cut, type conversion where
all trees are cut, and no cutting. Soil nutrient work is being done
in the overstory removal, type conversion, and no cut treatments.
There are 10 4-ha blocks in each treatment and half of these
will have prescribed slash burns.
Gerald Gottfried, Research Forester with the Rocky Mountain
Station in Flagstaff, is studying effects of all four silviculttd
treatments and prescribed fm on tree regeneration and growth.
He is also working with Bill Kruse, Research Range Scientist
with the Rocky Mountain Station in Flagstaff, on
understoIy/overstory production Bill Kruse is also monitoring
small mammal populations and is worlung with me on nutrient
cychg in the soiltblue grama system throughout the growing
season.
Specific soil nutrient objectives are to measure the soil
temperatures associated with these Werent levels of fbelwood
harvest and to once again attempt to determine long-term effects
of fuelwood harvest and fire on nutrient cycling. The nutrient
work has been expanded over what we were doing on the
Ancient Tank study. Samples of soil and blue grama have been
collected once every 2 weeks throughout the growing season
for the past 3 summers. This sampling will continue for at least
5 years post-treatment. Soil, litter, and slash will be collected
within a couple of days before burning and soil and ash will be
collected as soon after burning as it is cool enough to work in
the area and annually for at least 5 years post-treatment. We're
currently getting our downed woody fuel measurements and will
be measuring concentrations of nitrogen, phosphorus, calcium,
magnesium, sodium, potassium, manganese, copper, and zinc in
the slash, litter, and ash. The soils work has also been expanded
to include ammonia and nitrate; exchangeable phosphorus,
calcium, and magnesium; total nitrogen, phosphorus, calcium,
magnesium, sodium, potassium, manganese, copper, and zinc;
organic carbon; pH; and bulk density.
Because fuelwood harvest could not be completed in one
year, ten of the blocks were harvested last year and will be
burned in Fall 1993. The other twenty will be completed in May
1993 and will be burned in Fall 1994.
LITERATURE CITED
Ahlgren, Isabel F. and Clifford E. Ahlgren 1%5. Effects of
prescribed burning on soil microorganisms in a Minnesota
jack pine forest. Ecology. 46(3): 304-310.
Brown, James K. 1974. Handbook for inventorying downed
woody material. USDA Forest Service. General Technical
Report INT-16. Ogden, UT. 24pp.
DeBano, L.F. and C.E. Conrad. 1978. The effect of fire on
nutrients in a chapanal ecosystem. Ecology. 59(3):489-497.
Giovannini, G., S. Lucchesi, and M. Giacktti. 1988. Effect
of heating on some physical and chemical m e t e r s related
to soil aggregation and erodibility. Soil Science.
146(4):244-261.
Heyward, F m k and A.N. Tissot. 1936. Some changes in the
soil fauna associated with forest f m in the longleaf pine
region Ecolopy. 17(4): 659466.
Isaac, Leo A. and Howard G. Hopkins. 1937. The forest soil of
the douglas fir region, and changes wrought upon it by
logging and slash burning. Ecology. 18:264-279.
Lewis, Jr., William M. 1974. Effects of fire on nutrient
movement in a South Carolina pine forest. Ecology.
55; 1120-1 127.Mortensen, Bruce. 199 1. Personal
communication USDA Forest Service. Apache-Sitpaves
N.F. Heber Ranger District. Overgaard, AZ.
Nelson, Chris. 1992. Personal communication USDA Forest
Service. Apache-Sitgreaves N.F. Springerville, AZ.
Peny, Hazel M., Sally M. Haase, and Stephen S. Sackett. In
Review. Soil temperatures associated with a pinyon-juniper
slash burn in central Arizona. USDA Forest Service. Rocky
Mountain Forest and Range Experiment Station FlagsW,
AZ.
Sackett, Stephen S. and Sally M. Haase. 1992. Measuring soil
and tree temperatures during prescribed fires with
thermocouple probes. USDA Forest Service. Pacific
Southwest Research Station. General Technical Report
PSW-GTR-131. Berkeley, CA. 15pp.
Smith, D.W. 1970. Concentrations of soil nutrients before and
after fire. Canadian Journal of Soil Science. 50:17-29.
Stark, Nellie M. 1977. Fire and nutrient cychg in a douglasfirAarch forest. Ecology. 58:16-30.
Stednick, John D., Larry N. Tripp, and Robert J. McDonald.
1982. Slash burning effects on soil and water chemistry in
southeastern Alaska. Journal of Soil and Water Consemation
37(2): 126-128.
Wright, Henry A. and Arthur W. Bailey. 1982. Fire Ecology.
United States and Southern Canada. John Wiley & Sons. New
York. 501 pp.