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 Southern Forest

Research Note

so-334

Experlment Station March 1987

Ocular and Densimeter Estimates of. Understory

Foliar Cover in Forests of Alabama

Thomas W. Popham and Roger L. Baker

SUMMARY

Foliar cover estimates of woody and herbaceous

understory vegetation were done on twenty l-m* plots

for a variety of forest types in Alabama. The methods of

estimation were ocular, loop-densimeter assisted ocular,

and point frame. The point frame was used as the standard and the other two methods were compared using

chi-square. Some ocular estimates were accurate, but

the number and amount of inaccurate estimates were

sufficient to result in a statistically significant difference

at 01=0.05. A difference existed in the ability of estimators to accurately estimate foliar cover of different vegetation groups. Therefore, ocular and loop-densimeter assisted ocular estimates did not provide consistently

accurate estimates of the proportion of foliar cover of

understory vegetation for forests in Alabama.

INTRODUCTION

The Forest Inventory and Analysis (FIA) unit of the

Southern Forest Experiment Station has the responsibility for surveying and estimating the forest resources of

seven Midsouth States. The Resources Planning Act of

1974 required evaluation of other nontimber resources

such as understory vegetation that might be utilized as

food or cover by wildlife or domestic livestock. One of the

characteristics identified as potentially important in evaluating the understory vegetation was foliar cover. Foliar

cover, as defined by Daubenmire (1968), was considered to be the total amount of ground surface shaded by

all plant parts when plants are standing in their natural

growing condition. Proportion of foliar cover for plant

groups is used in this study rather than percentage.

Pilot studies in Tennessee in 1979 and 1980 had

shown that to attempt species identification was not only

time consuming but also nearly impossible in a broad

area survey. The greatest difficulty came from trying to

identify the immense variety of vegetation that was not

flowering or fruiting. Three plant groups were selected to

measure the growth types present. Although this excluded information as to forage value, the purpose of the

study was to evaluate the methods used, not to provide

forage values. The plant groups selected for sampling

were woody (tree and shrub), grass and grasslike, and

other herbaceous.

The study objective was to evaluate the use of a loopdensimeter to improve the accuracy and precision of

ocular estimates of foliar cover of the three plant groups.

An ocular estimate of foliar cover (subjective) was to be

used rather than an objective measurement technique

such as the point frame because less time was required

to measure each sample plot. The point frame was used

to evaluate the accuracy of the estimates. The loopdensimeter method uses a measuring device, and thus

is not purely a subjective estimate, and it could be easily

transported (Cully 1938, Brown 1954).

METHODS

The FIA unit maintains permanently marked sample

points that systematically grid the entire state of

Alabama. Of a possible 850 plots, 20 were selected at

random for use in comparing methods of estimating foliar cover. Plots selected were sampled between July 14

and September 15, 1982, and were located in 12 counties throughout the State. Plots were 0.5 m by 2 m and

were laid out along randomly selected azimuths at randomly determined distances from the permanent point

markers.

Numbers of *selected plots in each FIA forest type

were: 4 loblolly pine, 2 loblolly &pine-oak, 12 white oakred oak-hickory, and 2 mixed hardwoods.’ After marking

the plot boundaries, two technicians made foliar cover

1 Forest type classification is from the Southern Forest, Experiment

Station Renewable Resources Inventory Work Plan, Alabama 1980-

1981.

Southern Forest Experiment Station/l-10210 U.S. Postal Services Bldg., 701 Loyola Avenue, New Orleans, La. 70113

Forest Service, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Louisiana, Mississippi, E. Oklahoma, Tennessee, E. Texas, Puerto Rico, U.S. Virgin Islands

1
estimates. All vegetation up to 1.52 m in height was

included in the estimates. One technician made ocular

estimates while the other made loop-densimeter estimates. The observations of each technician were

recorded independently to avoid influencing either estimate and to avoid influencing the point frame estimate.

Next, the two technicians jointly made an estimate of

foliar coverage with the point frame. One of the two

technicians was present for the entire study. Observations of the second technician were divided between two

people; the first 12 plots were done by one person and

the next 8 plots by another.

The ocular estimates were done without the aid of any

mechanical device. Previous experience had established that the size of a human fist is approximately

0.01 m2, and a fist was often held directly above or below

the vegetation to assist with the ocular estimate. The

technician could mentally tally the number of O.Ol-m2

areas covered by each understory vegetation plant

group.

The loop-densimeter consisted of a steel tape with

one end attached to a slotted wood block (fig. 1). The

other end of the tape was inserted into the slot so that

pushing or pulling the tape through the block formed a

loop of varying area. The tape was scribed to give areas

of 0.01, 0.02, 0.03, 0.04, and 0.05 m2. Using the wood

block as a handle, the estimator held the loop above or

below the vegetation and adjusted the tape to obtain a

direct measure of the area covered. The areas were

totaled to get the proportion of foliar cover.

The point frame is shown in figure 2. Horizontal crossmembers are located at the top and at 38 and 76 cm

from the top (fig. 2). Five holes were drilled at lo-cm

intervals in each crossmember beginning 10 cm from the

legs. Pins 76 cm in length were lowered to the ground or

pushed upward to a 152~cm height to check for pin contact with one of the three vegetation groups as described

in Baker and Thomas (1983).

The point frame was set across the plot at 20 equidistant points along the 2-m plot sides beginning 5 cm from

-

Figure 1 .-Loop-densimetar used in estimating foliar cover.

one end, which resulted in 100 pin readings per plot. The

placement of the point frame introduced some randomness since the legs could seldom be placed at the exact

location of the 20 points along the plot boundary. As

previously stated, the point frame was used as a standard for estimate comparisons (Hutchings and Pase

1963, Baker and Thomas 1983).

Following completion of the data collection for each

plot, results of ocular and loop-densimeter methods of

estimating foliar cover were compared with results from

the point frame method. The unassisted ocular method

was also compared to the loop-densimeter method. A

statistical test based on the chi-square test of a

hypothesized variance described by Freese (1960) was

used to make the comparisons. The test allows the user

to set an acceptable level of accuracy for a trial method

compared to a standard method of measurement. The

test is to determine if the accuracy of the trial method

falls within the set limit with some preselected level of

probability. In this study the probability level used was

0.05.

RESULTS

The range in proportion of foliar cover for woody vegetation was from 0.03 to 0.66 (table 1). For grass and

grasslike vegetation the range was much smaller, and

for coverage by herbaceous vegetation the range was

Figure P.--Point frame used in determining foliar cover.

Thomas W. Popham is mathematical statistician, Southern Plains Area, Agricultural Research Service-USDA, Stillwater, OK; Roger L. Baker is

research soil scientist, Wildlife Habitat and Silviculture Laboratory, Southern Forest Experiment Station, Forest Service-USDA, Nacogdoches, TX.

2
intermediate. These ranges were expected in the treedominated communities observed because FIA plot estimates from Tennessee and southern Alabama had

shown similar values (R. L. Baker unpublished data).

The range of differences and the average difference

between methods of estimation provided information

pertinent to the objective of the study (table 2). The

woody vegetation group, with the higher proportion of

cover values (up to 0.66), had a larger range of differences; however, the average differences were encouragingly low. The average difference was less between

the ocular and loop-densimeter methods than between

either one and the point frame method. The results were

similar for grass and grasslike vegetation except the

range of differences and the average differences were

smaller for these groups than for the woody vegetation.

The largest estimation errors were inconclusive concerning any improvement by using the loop-densimeter,

but large errors were possible with either ocular method

(table 3).

Mean proportion of foliar cover for estimation methods

as compared to the point frame in each vegetation group

were within 1 percent (table 3). For individual plots the

largest estimation error for ocular or loop-densimeter

estimates compared to the point frame occurred for the

first 2 estimates in 7 of 12 comparisons. There was no

evidence that estimates improved with time after the

initial two estimates. Instead, results indicate that variation increased with the amount of foliar cover on plots.

The statistical tests indicate that accuracy within 0.05

was not achieved with either the ocular or loopdensimeter methods for the woody vegetation using the

point frame as the standard (table 4). Accuracy within

0.05 percent probability was obtained with the loopdensimeter and ocular methods for the grass and

grasslike vegetation. However, considering the range of

0.06, this was no real achievement. It also became evident that the point frame was often- less accurate than

the other methods when foliar cover was 2 percent or

less. For herbaceous vegetation only, the estimates from

the loop-densimeter method were closer to point frame

readings than were ocular estimates. Again, the range of

values observed makes this no real achievement. For

the grass and grasslike vegetation, there was no evidence of a difference in accuracy when the ocular was

compared with the loop-densimeter method. Too few

samples were available to test differences between

forest types. Tests between plant groups would not be

valid because of the large differences in variances

(Freese 1960).

Differences between technician estimators were

tested. Ocular estimates from the technician who was

present for the entire study (estimator A) were the same

Table 1 .-Range of foliar cover values* for each method of estimating

cover and each vegetation group

Vegetation Estimation

arouo method

Foliar cover

values

Minimum Maximum

W&Y Ocular 0.03 0.65

Loop-densimeter 0.03 0.65

Point frame 0.03 0.66

Grass and grasslike Ocular 0.00 0.06

Loop-densimeter 0.00 0.06

Point frame 0.00 0.07

Herbaceous Ocular 0.00 0.25

Loop-densimeter 0.00 0.18

Point frame 0.00 0.23

*As a proportion of total cover for that group.

Table 2.-Range of difference and average difference in foliar cover* from a comparison of estimation methods in each vegetation group

Vegetation

Estimation

methods

compared

Differences in

foliar cover

Minimum Maximum Average

W&Y Ocular vs. densimeter -0.08

Ocular vs. point frame -0.08

Densimeter vs. point frame -0.17

Grass and grasslike Ocular vs. densimeter

Ocular vs. point frame

Densimeter vs. point frame

-0.02

-0.04

-0.05

Herbaceous Ocular vs. densimeter

Ocular vs. point frame

Densimeter vs. point frame

*As a proportion of total cover for that group.

-0.05 0.07 0.0005

-0.15 0.03 -0.0080

-0.10 0.01 -0.0085

0.21

0.11

0.05

0.01

0.01

0.01

0.0055

-0.0120

-0.0175

-0.0005

-0.0055

-0.0050

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Table S-Percentage error for each vegetation group and estimation method as compared to the point

frame method of measuring foliar cover

Vegetation Estimation

croup method

woody Ocular

Densimeter

Grass and grasslike Ocular

Densimeter

Herbaceous Ocular 0.03 5 0 6 5

Densimeter 0.03 5 0 6 7

Proportion

0.01

0.01

Largest error of estimation

Positive Negative

------ Percent _-___-_

100 7 3

4 5 6 4

5 0 7 5

5 0 7 1

*Mean of 20 plots.

Table 4.-Chi-square test of accuracy indicating the probability of obtaining a difference between

estimation methods in each vegetatioi, group

Vegetation

croup

Woody

Estimation Chi-square probability*

methods

compared 0.01 0.02 0.05 0.10

Ocular vs. densimeter l 1 l . Ocular vs. point frame l * *

Densimeter vs. point frame l

l *

Grass and grasslike Ocular vs. densimeter

Ocular vs. point frame * t

Densimeter vs. point frame l

l

Herbaceous Ocular vs. densimeter l *

Ocular vs. point frame l l *

Densimeter vs. point frame *

*

‘Asterisk indicates that methods for obtaining cover were not equal.

*Probability is less than or equal to 0.05 of getting a chi-square value greater than observed for

difference in-proportion of 0.01, 6.02, 0.05, 0.16

as point frame readings for all three vegetation types.

Estimator A had considerable field experience. However, when the densimeter was used to assist in ocular

estimates made by estimator A, estimates were comparable only for grass and grasslike and herbaceous

vegetation, not for woody vegetation. Estimator B (the

technician estimating values for the first 12 plots) had

considerably less field experience, but by using the densimeter, estimates of foliar cover were comparable to

point frame readings for all three vegetation types. Ocular estimates were comparable to point frame readings

only for grass and grasslike vegetation.

CONCLUSION

We concluded from this evidence that the use of a

loop-densimeter in these forest types did not improve

foliar cover estimates over the ocular method already in

use and may hinder an experienced estimator. The loopdensimeter may be useful in training field personnel to

estimate foliar cover, but an experienced, trained technician can produce usable ocular estimates. The results

did indicate the ocular estimates of understory vegetation should not be expected to be within 0.05 percent

probability of the point frame estimate in similar communities. It is recommended that regular checks be made

with a point frame to provide field personnel a reference

standard for ocular estimates.

LITERATURE CITED

Baker, Ft. L.; Thomas, C. E. A point frame for circular

plots in southern forest-ranges. Journal of Range

Management. 36:(1):121-123. 1983.

Brown, D. Methods of surveying and measuring vegetation. Commonwealth Bureau of Pastures and Field

Crops Bulletin 42. Reading, England; Bradley & Son,

Ltd., The Crown Press; 1954. 223 p.

Culley, M. Densimeter, an instrument for measuring the

density of ground cover. Ecology. 19(4):588-590.

1938.

Daubenmire, Ft. Plant communities, a textbook of synecology. New York; Harper & Row; 1968. 300 p.

Fresse, F. Testing accuracy. Forest Science. 6(2):139-

145. 1960.

Hutchings, S. S.; Pase, C. P. Measurement of plant

cover-basal, crown, leaf area. p. 22-30 In: Range

Research Methods. US. Department of Agriculture,

Forest Service Misc. Publ. No. 940. 1963: 22-30.