U.S. Department of Agriculture

Forest Service Research Note SE-363 March 1981

IMPACT OF PRESCRIBED FIRE ON UNDERSTORY AND

FOREST FLOOR NUTRIENTS

by Walter A. Hough’

ABSTRACT.-The impact of low-intensity prescribed fires on slash pine/saw-palmetto/gallberry

understory and forest floor nutrients was estimated from measurements before and after burning. Highly significant correlations existed between weight loss of these fuel components and the weight loss of

several elements. Energy loss was also highly correlated with forest floor and understory weight loss. The

data also indicate that annual or biennial burns do not reduce the macronutrients in the soil surface layer.

but, in fact, increase them; and that even though a significant amount of N (1 IO kg/ ha) may be lost by

burning a Syear fuel accumulation there still is 160 kg/ ha of N in the ash and residue following the fire.

as well as about 700 kg/ ha in the upper 10 cm of the soil profile.

Keywords: Serenoa repens, Ilex glabra, Pinus elliortii, Pinus palustris, pine/ saw-palmetto/gallberry.

macronutrients, micronutrients.

Nutrients are constantly being added to and released from undisturbed forest stands. When trees

are harvested and removed or when fire is used to

reduce hazardous fuel buildups, the nutrient status

of the stand is changed.

The data reported here are taken from a comprehensive investigation of the use of prescribed fire in

the South and the effects of such fires on air quality

(Southern Forest Fire Laboratory 1976). The

information presented is for the aboveground plant

parts of the saw-palmetto (Serenoa repens (Bartr.)

Small)/gallberry (Zlex glabra (L.) Gray) understory vegetative complex and the associated forest

floor material found under natural slash (Pinus

elliottii Engehn.)/ longleaf pine (P. palustris Mill.)

stands. These components of the forest stand are

drastically altered by the low-intensity fires used in

prescribed burning operations. Dry-weight

measurements and element analysis were made

before and after burning to determine the quantity

of nutrients in the various understory and forest

floor components.

‘Hough was located at the Southern Forest Fire Laboratory,

Macon, Ga., when this work was done, and is now Assistant

Station Director, Research Planning and Application, Asheville, NC.

Southeastern Forest Experiment Station - Asheuille, North Carolina

METHODS

Vegetative materials used in this study were

plants and forest floor layers collected from eight

forest stands in Georgia and Florida, with understories dominated by the saw-palmetto/ gallberry

plant association. Prior to the burns applied in these

tests, the time since last disturbance of the understory and forest floor varied from 1 to 8 years. Past

disturbance in all stands was from prescribed fire.

Prescribed backfires were applied to the forest

stands, and fuels were sampled before each burn

and immediately after the fire was out. Vegetative

samples were collected by species and size class

(foliage; stems < 0.64 cm, 0.64 to 2.5 cm, 2.5 to 7.6

cm, and > 7.6 cm) from I-m’plots. Forest floor

materials were separated by size class only on these

plots. Samples were ovendried at 8YC and

weighed, and all samples of the same type material

from a single stand were combined and run through

a hammer mill with 2cm screen to obtain a coarsely ground, well-mixed composite sample. After

milling, the material was stored in paper bags until

analyzed.
Subsamples of this material were analyzed for

element composition at a cooperating laboratory.2

The coarsely ground material was ovendried at

8O”C, ground to pass a 20-mesh screen, and a

portion was ashed at 500°C for4 h. Ash digestion in

a standard 5-percent Li-HCl solution preceded

analysis with a Spark-Emission direct-reading

spectrograph of the following: P, K, Ca, Mg, Mn,

Fe, B, Cu, Zn, Al, MO, Sr, Ba, and Na. Total S inanother portion was measured following dry ashing of

plant tissue with MgO, using a Leco Automatic

Sulfur Analyzera Total N in another portion of the

dried and ground material was measured by a semimicro Kjeldahl method with an Autoanalyzer.

Analysis of C-H-O, total ash, Si, and energy

content was carried out on separate subsamples of

the coarsely ground materiaL4 Following ovendrying at 85’C, the sample was ground to pass a IOOmesh screen. C-H-O measurements were made with

a Perkin-Eimer Elemental Analyzer. Heat values of,

the ground materials were determined in a Parr

Bomb Calorimeter. Si content was measured by

acid digestion of ashed material.

Estimates were made of the weight per unit area

of fuel components before and after the burns.

Chemical analysis of understory and forest floor

material was used to estimate the total weight per

unit area of each element in the stand prior to and

following the burn.

The data presented are represenative of “total”

amounts of elements found in both organic and

inorganic forms in plant material. ‘The values

reported give no indication of how much material is

“available” for plant growth.

RESULTS

The reduction of hazardous fuel buildup was the

principal reason for the type of prescribed burning

used in this study. In the stands burned, total understory and forest floor fuel leduction ranged from 40

to 74 percent. Consumption of the understory

varied from 55 to 88 percent, while forest floor

reduction ranged from 25 to 75 percent. The actual

fuel reduction on a dry-weight basis depends on the

weather-fuel moisture conditions and is also highly

dependent on the amount of fuel that has accumulated on the site. For example, a prescribed burn in a

stand where fuel has accumulated for only 1 or 2

years could result in a high percentage of fuel reduction (75 percent) but may actually consume only

‘Soil Testing and Piant Analysis Laboratory, University of

Georgia, Athens, Georgia.

‘Use of trade names does not constitute an endorsement by

the U.S. Department of Agriculture or the Forest Service to the

exclusion of other products that may be suitable.

4Engineering Experiment Station. Georgia Institute of

Technology, Atlanta, Georgia.

6,900 kg/ ha. However, a burn in a stand where fuel .

has accumulated for 5 years could give a lower percentage reduction (53 percent) but actually consume

more fuel--17,300 kg/ ha. A procedure for estimating fuel consumption on prescribed burns in the

South has been proposed by Hough (1978).

Measurements made during application of prescribed backfires showed that all burns were of low

intensity for all fire behavior factors. Flame length

varied from 0.15 to 1.2 m, forward rate of spread

ranged from 0.15 to 0.8 m per minute, and Byram’s

(1959) fireline intensity varied from about 50 to 275

kWjm. Only the understory and forest floor were

directly involved in the fire; overstory foliage was

not scorched and on most sample points the H layer

was not burned.

Results of nutrient-loss calculations based on

chemical and biomass data derived from samples

collected before and after each prescribed burn were

variable, but do indicate some general trends. The

data also show tlhat many more samples must be collected ii’ information about micronutrients is to be

meaningful. !_lnderstory and forest floor fuels show

high variability, thus requiring intensive sampling

to estimate weight of material before and after fire.

Inaccuracies in the chemical analysis of elements

occurring in very low concentrations in unburned

samples may have also introduced errors.

Another source of error was the failure to collect

all of the ash left on the surface after the fire. Because of the relatively large-size sample plots (1 m2),

the ash, char, and unburned material were collected by hand. A small fraction of this material was

observed to drift off the plot and escape collection.

Because there was no way to estimate how much ash

was lost on each plot, no attempt was made to adjust the field data.

There appeared to be a high degree of correlation between weight loss of the forest floor (FF) plus

understory (US) and weight loss of several elements

(table 1). Energy loss was also highly correlated with

FF + US weight loss. Regression analysis was used

to provide the following equations:

C = 136.49 + 0.4730 (X), r2 = 0.99

N = -10.55 +- 0.0071 (X), r2 = 0.96

P = -0.44 + 0.00031 (X), r’ = 0.86

Mg = -0.36 + 0.00045 (X), r2 = 0.89

S = -1 .I9 + 0.00067 (X), r* = 0.98

where element weight loss (kg/ ha) is a function of

US & FF weight loss, X (kg! ha).

Trendlines for these equations are shown in

figure 1 for comparison. The energy loss equation

was:

Energy loss (kJ/ ha) = 7217.6 + 19.41 (X), rz =

0.99. There was a weak but significant correlation
‘I

between FF + US weight loss and weight loss of K,

Ca, and Mn. The following equations resulted from

regression analysis:

K = 4.25 + 0.00054 (X), r2 = 0.45

Ca = 8.70 + 0.00107 (X), r2 = 0.51

Mn = 0.277 + 0.000028 (X), r2 = 0.47

Although only limited data are presented on the

effects of prescribed burning on nutrient content,

results are within the variation that can be found in

the literature. For example, N loss has been

reported to vary from 20 to 900 kg/ ha for various

fuel and fire situations (Allen 1964; Christensen

1977; Grier 1975; Harwood and Jackson 1975;

Wells and others 1979). Loss of P ranged from 0.1 to

10 kg/ ha, Ca losses ranged from 4 to 100 kg/ ha, and

K loss from 4 to 282 kg/ ha.

Allen (1964) and Allen and others (1969) investigated heather burning in northern England and its

effect on the distribution of mineral nutrients. They

found over half the C, N, and S in the heather was

driven off with the smoke. They found that any loss

from burning the plant complex can be restored by

minerals contained in precipitation.

Most citations give data from one burn or perhaps different intensities of burn but in the same fuel

conditions (Kodama and VanLear 1980). None present data over a range of fuel weights and, therefore. there are not enough data in the literature to

test the regression equations presented in this paper.

For low-intensity prescribed burns used in slash

pine/ saw-palmetto/ gallberry stands, these regression equations can be used to estimate the loss of N.

P, Mg, S, C, and energy if the amount of fuel consumed is measured or estimated (Hough 1978).

Also, for the low-intensity fires studied here, the

equations developed for Ca, K, and Mn are

probably representative of similar-type burns.

Fire releases nutrients to the atmosphere, the remaining forest floor material, and the mineral soil.

The litter and vegetative components of a slash pine

stand constitute a large nutrient sink if undisturbed. It appears, therefore, that prescribed burning at 4- to 5-year intervals not only reduces the fire

hazard caused by fuel buildup but also rapidly releases a portion of the nutrient capital that has been

made temporarily unavailable because of the slow

decomposition rate of biomass and litter. Even

though 100 kg/ha of N in the understory and litter

materials might be lost in a prescribed burn of a pine

stand with a 5-year fuel accumulation, there still remains about 160 kg/ ha in the residue and ash, 116

kg/ ha in the slash pine overstory, as well as about

700 kg/ha in the upper 10 cm of the soil profile. If

the contributions from precipitation and dry fallout are added, plus new growth of vegetative

material that may fix N on the site (Wells and others

1979), it is not surprising that N does not decrease

following one burn or even repeated burning (Wells

1971). In fact, a more detailed analysis of the soil

chemical properties on the Baker County, Florida,

plots shows that prescribed burning does not

deplete nutrient levels but is beneficial to nutrient

cycling.’

& 90

2

a0

.J i‘0

6 0 1

5 0

4 0 E

3 0

2 0

FL

S

IO Mg

--P

0

L_.

4,OooO 6,000 12,000 16,ooO 20,000 24,000

LOSS OF UNDERSTORY t LITTER (kg/ ha)

Figure 1 .-Relationship between understory plus litter dryweight loss and loss of various elements following a prescribed fire.

CONCLUSIONS

Although these data are from a limited number

(n = 10) of prescribed burns, there are several relevant features that should be noted. The C fraction is

highly volatile and weight loss is positively and

significantly correlated with fuel weight loss.

Energy loss is also directly correlated with loss of

understory and forest floor weight. Loss of N, P,

Mg, and S shows a highly significant correlation

with fuel weight loss. K, Ca, and Mn weight loss is

not as strongly correlated. Several of the micronutrients showed increases following these fires

(Cu, Al, and MO), while others showed medium to

large losses (Fe, Sr, B, Ba, and Zn). This indicates

that sampling procedures must be intensified and

improved if reliable results are expected for micronutrients.

5William H. McKee, Jr., Soil Scientist, Southeastern Forest

Experiment Station, Charleston, South Carolina, personal communication.
Table I.-Average fuel weight and nutrient loss, nutrient remaining in the ash and unburned

residue following low-intensity prescribed fires, and nutrient in the 0- to IO-cm soil layer

Item

.Prescribed burn location (county)

Baker, Clinch, Pierce, Ware.

Florida” Georgiab Georgia’ Georgiad

_--____________________kglha_____________-______

Understory and litter loss

N loss

N in residue

N in soil (O-10 cm)

P loss

P in residue

P in soil (O-10 cm)

K loss

K in residue

K in soil (O-10 cm)

Ca loss

Ca in residue

Ca in soil (O-10 cm)

Mg loss

Mg in residue

Mg in soil (O-10 cm)

s loss

S in residue

S in soil (O-10 cm)

Mn loss

Mn in residue

Mn in soil (O-10 cm)

6,510 7,397 18,691

39 43 112

28 22 161

1,099 746 732

1.5 2.5 4.6

4.2 2.8 15.7

76.5 51.4 41.0

9.0 9.0 10.6

5.7 5.2 34.9

53.4 38.2 33.1

12.9 25.3 23.0

21.7 15.1 70.0

289.5 140.5 85.0

1.8 4.6 7.7

6.5 3.4 16.5

70.3 41.2 32.8

3.4 3.8 10.8

3.3 2.2 15.0

30.1 15.3 20.5

0.4 0.7 0.6

0.7 0.5 2.1

7.2 6.8 6.8

25.380

194

109

__

8.4

10.6

__

24.5

23.9

_ _

49.5

57.7

__

12.4

14.2

__

17.4

10.8

_ _

1.3

1.8

“Plots unburned for 1 or 2 years but stands had been burned repeatedly on an annual (n = 1) and biennial (n = 2) basis over the last 14

years.

bPlots (n = 2) unburned for 1 year with possibly two burns applied during life of stand.

‘Plots (n = 4) unburned for 5 years with possibly three burns applied during life of stand.

“Plot (n = 1) unburned for 8 years with possibly three burns applied during life of stand.

LITERATURE CITED

Allen, S. E.

1964. Chemical aspects of heather burning. J. Appl. Ecol.

11347-367.

Allen, S. E., C. C. Evans, and H. M. Grimshaw.

1969. The distribution of mineral nutrients in soil after

heather burning. Oikos 20: 16-25.

Byram, G. M.

1959. Combustion of forest fuels. In Forest fire: Control

and use. p. 61-89. K. P. Davis. McGraw-Hill Book

Co., New York.

Christensen, N. L.

1977. Fire and soil-plant nutrient relations in a pinewiregrass Savannah on the coastal plain of North

Carolina. Oecologia 3 1:27-44.

Grier, C. C.

1975. Wildfire effects on nutrient distribution and leaching in a coniferous ecosystem. Can. J. For. Res.

5599~605.

Harwood, C. E., and W. D. Jackson.

1975. Atmospheric losses of four plant nutrients during

a forest fire. Aust. For. 38(2):92-99.

Hough, W. A.

1978. Estimating fuel weight consumed by prescribed

fires in the South. USDA For. Serv., Res. Pap.

SE-187, 12 p. Southeast. For. Exp. Stn., Asheville,

N.C.

Kodama, H. E., and D. H. Van Lear.

1980. Prescribed burning and nutrient cycling relationships in young loblolly pine plantations. South.

J. Appl. For. 4(3): 118-121.

Southern Forest Fire Laboratory Staff.

1976. Southern forestry smoke management guidebook.

USDA For. Serv., Gen. Tech, Rep. SE-IO, 14 p.

Southeast. For. Exp. Stn., Asheville, N.C.

Wells, C. G.

1971. Effects of prescribed burning on soil chemical

properties and nutrient availability. p. 86-99.

Proc. Prescribed Burning Symp. [Charleston, SC..

April 1971.1

Wells, C. F., R. E. Campbell, L. F. DeBano, C. E. Lewis,

R. L. Fredriksen, E. C. Franklin, R. C. Froelich, and P. H.

Dunn.

1979. Effects of fire on soil: a state-of-knowledge review.

USDA For. Serv., Gen. Tech. Rep. WO-7,34 p.

Washington, D.C.