United States

Department of Precommercial Thinning of

Water mpelo Stands on the

Mobile-Tensaw River Delta:

Third-Year Results

Agkculture

Forest Service

Southern

Research Station

Research Paper

SW-1 7

J.C.G. Goelz and J.S. Meadows
The Authors:

J.C.G. Goelz, Principal Forest Biometrician, and J.S. Meadows,

Principal Silviculturist, Southern Research Station, U.S. Department

of Agriculture, Forest Service, Stoneville, MS 38776.

March 1999

Southern Research Station

P.O. Box 2680

Asheville, NC 28802
Precommercial Thinning of Water Tupelo

Stands on the Mobile-Tensaw River Delta:

Third-Year Results

J.C.G. Goelz and J.S. Meadows

Abstract

Three Cyear-old stands were selected for precommercial thinning.

Treatment consisted of two components: (I) thinning water tupelo (Nyssa

aqua&a L.) stump sprouts and dense patches of water tupelo seedlings

and (2) cutting all stems of Carolina ash (Fmxinus caroliniana Mill.) and

black willow (S&x n&a Marsh.) (cleaning). This approach provided a 2

by 2 factorial with two replications in each stand. Thinning increased

average diameter growth of water tupelo stump sprouts, with the greatest

increase in one stand that had the highest density of stumps and apparently

the highest productivity; cleaning did not increase diameter growth.

Thinning decreased stand basal area growth; cleaning did not affect basal

area growth. Mortality of water tupelo stump sprouts was negligible.

Cleaning may increase the number of water tupelo seedlings, although the

evidence from this study is not compelling. Precommercial thinning will

probably provide a favorable response in stands with high density of

stumps and high productivity.

Keywords: Cleaning, coppice, diameier growth, mortality, precommercial

thinning, water tupelo, wetlands.

Introduction

Water tupelo (Nyssu aquatica L.) stands tend to be more

dense than other bottomland hardwood stands (Goelz 1995,

Putnam and others 1960). Young, postclearcut stands are

also dense, with many sprouts arising from each stump. Most

of these sprouts will die before they are harvested, thus their

growth is lost to mortality. Carolina ash (Fruxinus

cumliniana Mill.) or black willow (Sulix nigru Marsh.),

often abundant in these young stands, are not merchantable

on these sites.

Mature water tupelo stands in the Mobile-Tensaw River

Delta are often harvested with helicopter systems, a method

too expensive to use for thinning. Ground conditions often

preclude using rubber-tired or tracked equipment for

thinning. As commercial thinning may not be practical in

these stands, interest has arisen in conducting precommercial

thinning to achieve a merchantable size (currently, 3-inch

[in.] top diameter) at an earlier age, reducing rotation length

as well as minimizing unsalvageable losses of growth to

mortality.

Previous studies have indicated that thinning is not

particularly beneficial for water tupelo stands. Kennedy

(1983) found that thinning mature water tupelo stands in the

Atchafalaya Basin of Louisiana did not increase diameter

growth. In McGarity’s (1979) study of 60-year-old muck

swamp forests, thinning increased growth of individual trees,

but control plots had the greatest volume growth.

Although mortality of individual sprouts is expected as each

stump undergoes self-thinning, previous studies have

indicated that mortality is high for entire stumps of water

tupelo. DeBell(197 1) reported high survival for water

tupelo stump sprouts 1 year after harvesting; however, at age

4,45 percent of the stumps with sprouts had died. Kennedy

(1982) reported that because mortality of young water tupelo

sprout clumps was high, tupelo coppice would not comprise

an important component of the mature stand. After 6 years,

only 9 percent of the stumps cut in May and 18 percent of

the stumps cut in November had live sprouts.

The high density of stems in the 3- to 4-year-old stands in

the Mobile-Tensaw River Delta could decrease rapidly if

patterns of mortality are similar to those found in the

Atchafalaya Basin. This study is intended to determine (1)

whether diameter and height growth rates of water tupelo

stump sprouts respond to thinning; (2) whether stump sprout

thinning will improve survival of the remaining sprouts; and

(3) whether cutting all Carolina ash and black willow

(cleaning) will improve growth and survival of tupelo stump

sprouts and seedlings.

Initial results from this study (Goelz and others 1993)

indicate that thinning increases annual diameter growth by

27 to 37 percent over unthinned plots, although cleaning

does not improve growth. The treatments had negligible

influence on the number of water tupelo seedlings, although

flooding that extended into June during the first growing

season following treatment probably caused a 30-percent

reduction in seedling numbers. Unlike previous
observations, mortality of tupelo stump sprouts was

negligible.

Methods

Three locations were selected on Scott Paper Company land

on the delta of the Mobile and Tensaw Rivers in Alabama.

The native water tupelo stands on these locations wem

clearcut in 198687. Four seasons of growth occurred before

this study began; each clearcut was 40 to 60 acres (a~).

Treatments were applied in the autumn of 1990. All stems

over 2 in. in diameter at breast height were felled when the

stands were clearcut. The locations are good for water tupelo

and are densely stocked to water tupelo, Carolina ash,

baldcypress (Taxodium distichum [L.] Rich.), and black

willow with other species present. Before treatment, the

locations had 5,000 to 10,000 stems per acre with 300 to

3,500 water tupelo seedlings per acre. Sprout-origin stems

were larger and appeared to dominate the locations, although

seedlings were abundant in patches where sprouts were

absent.

A 2- by 2-factorial design with two randomly selected

replications at each location was used. The first factor

consists of two treatments--thinning or not thinning water

tupelo. Thinning comprises two components:

1. All water tupelo stumps were thinned to one or two best

sprouts where “best” was defined as the largest well-formed

sprout that originated low on the stump. When two wellspaced sprouts of good form and low origin were present,

both were left.

2. Water tupelo seedlings were thinned with a machete

wherever they occurred in a dense patch (more than 20 per

100 square feet [ft?] in patches of 100 fi? or larger). The

tallest water tupelo seedlings were left at a density of

approximately one per 36 ft2 (or a nominal 6- by 6-ft

spacing).

The second factor consists of two treatments-cleaning or

not cleaning the Carolina ash and black willow. All ash and

willow were cut as close to the ground as possible with a

chainsaw. These factors provide four treatment

combinations: (1) no thinning, no cleaning (control); (2) no

thinning, cleaning; (3) thinning, no cleaning; and (4)

thinning and cleaning.

Eight 0.786-ac treatment plots were placed in each location.

As the soils near the riverbank are much different from the

rest of the stand, all treatment plots were at least 12 chains

from the river. Three potential treatment plots were

discarded as they represented intersections of pull-boat

channels and were poorly stocked. Pull-boat channels are

remnants from previous harvests when logs on “pull-boats”

were winched through the harvested area to the riverbank.

The eight plots were otherwise adjacent to each other on a

square grid two plots wide by four plots long. When

potential treatment plots were discarded, a new plot was

placed adjacent to the other plots in the opposite direction

from the riverbank.

The measurement plot (0.304 ac) was in the center of each

treatment plot. Small stems were counted by species and

origin (sprout or seedling). Stems >2.5 in. in diameter at

breast height were permanently numbered and the following

measurements were taken-diameter at breast height,

species, origin, number of sprouts on stump, and crown class

(Kraft’s tree class) (Daniel and others 1979). Total height

was measured on 25 randomly selected trees per plot, unless

~25 stems were present 22.5 in. in diameter at breast height.

These trees were remeasured 1 and 3 years after the study

began.

Analyses were by analysis of variance or covariance

analysis. The design is a 2 by 2 factorial with two levels for

thinning, two levels for cleaning, and three blocks (referred

to as locations) with two replications per block. Effect of

locations was considered fixed. For individual tree

characteristics, such as diameter and height growth,

measurement plot averages were used. For height growth,

only data were used from water tupelo trees that were

dominant or codominant at both measurements. Significance

tests were conducted at an a level of 0.05.

Results and Discussion

Average Diameter Growth of Water ‘Ibpelo

Plot average diameter growth from 1990-93 is presented in

figure 1. The initial average plot diameter was not a

significant covariate and was not included. Thinning and

location and the thinning by location interaction were

significant. Cleaning and the thinning by cleaning interaction
2.0

1.8-

1.6-

t

. Thinned

O Unthinned

Location

Figure l--Plot-average diameter growth for thinned and unthinned plots for three locations. Means

represent averages across cleaning treatments. Error bars represent the mean plus or minus twice the

pooled standard error.

terms were not significant, thus each point in figure 1

represents averages of plots that are cleaned and not cleaned.

Locations 1 and 3 were similar in regard to diameter growth

on thinned and unthinned plots. Location 2 had greatest

diameter growth and response to thinning. The greater

magnitude of response to thinning may have occurred

because this location had the greatest density of water tupelo

stumps (Goelz and others 1993). making initial competition

greater. As diameter growth in unthinned plots for location 2

was greater than for the other two locations, productivity of

this location may be higher, and thinning response is

generally greatest for sites of highest productivity.

Average Height Growth of Dominant and

Codominant Water lbpelo

Treatments did not significantly affect plot average height

growth of dominant and codominant water tupelo trees from

1990-93, although location effect was significant. Average

height growth for locations 1, 2, and 3 was 5.8.8.2, and 6.0

ft, respectively, which further substantiates the argument that

location 2 had inherently greater productivity. The reason for

the higher productivity is unknown. One possibility is that

the stand harvested at location 2 was probably younger than

those at locations 1 and 3, and the smaller stumps have more

vigorous sprouts. Evidence for this possibility is personal

3
6 0

50

40

30

20

1 0

b w Thinned

I 0 Unthinned

2

Location

Figure 24asal area growth for thinned and unthinned plots for three locations. Means represent

averages across cleaning treatments. Error bars represent the mean plus or minus twice the pooled

standard error.

observation of the size of the stumps. Soils and flooding

regimes may also affect productivity. Although the effects

were not significant at a of 0.05, thinning and cleaning,

alone or in combination, had a negative effect on height

growth.

Basal Area Growth

Thinning and locations significantly affected plot basal area

growth (fig. 2). Thinning decreased basal area growth on all

locations. The cleaning and all interaction terms were not

significant. Although the reduction of basal area growth

appears considerable, much of the basal area growth on the

unthinned plots may be lost to future mortality when sprouts

within stumps undergo natural thinning, while the fewer

stems on the thinned plots may survive until final rotation.

Change in Quadratic Mean Diameter

Change in quadratic mean diameter is determined by growth

of initially measured trees and the number and size of

ingrowth trees. Thinning, cleaning, locations, and thinning X

location interaction terms significantly affect change in

quadratic mean diameter (fig. 3). The thinning by cleaning

interaction term was not significant. Thinning increases

growth in quadratic mean diameter, and the effect of

thinning is greatest for location 2. Cleaning decreased

growth in quadratic mean diameter; cleaning decreased

average diameter growth and basal area growth, but the

effect was not significant for those variables. Thus, we

discount the effect here; this result may be a type one error.

Quadratic mean diameter can be considered analogous to

stand maturity. A stand with a greater quadratic mean

diameter is, therefore, closer to rotation age. For locations 1

and 3, thinning had a modest effect on quadratic mean

diameter. However, the effect of thinning was much greater

for location 2. The increase in quadratic mean diameter in 3

years for the thinned plots on location 2 may be similar to

the increase unthinned plots at the same location will attain

in 6 years. Thus, the thinned plots may be maturing almost

twice as fast as the unthinned plots. If this difference is

4
Location

Figure Change in quadratic mean diameter (DQ) for four treatments and three locations. Error bars

remesent the mean plus or minus twice the pooled standard error. Control at location 1 is hidden by

thinned and cleaned.

maintained for several years, the reduction in rotation age

may make precommercial thinning economically viable on

sites similar to location 2.

Mortality

Only 8 out of more than 1,000 trees permanently numbered

after treatment have died from 1990-93. The mortality of

seven trees was attributed to animal damage rather than

density-dependent mortality or disintegration of the

supporting stump. This very low mortality contrasts studies

of water tupelo coppice in other areas where few stumps

maintained living sprouts (DeBell 1971, Kennedy 1982).

The Mobile-Tensaw River Delta has not been greatly

affected by major hydrological alteration, although the areas

studied by DeBell(l97 1) and Kennedy (1982) have been.

Thus, stump mortality may remain inconsequential in this

relatively undisturbed wetland. In the near future, densitydependent natural thinning of the stump sprouts will occur.

As more sprouts die, treatments will likely have an effect on

sprout mortality as treatments did affect diameter growth and

thus vigor.

Change in Numbers of Water Tupelo Seedlings

Equation 1 represents a conceptual form to predict number

of water tupelo seedlings.

N1993 = A + B(N,990) (1)

where

N = the number of water tupelo seedlings in 1993 or 1990,

and

A and B = linear functions of dummy variables representing

treatments and locations that were significant.

Thus, B can be considered survival while A represents new

regeneration. However, because B was not constrained to be

between 0 and 1, it is not entirely survival. If the number of

5
new seedlings in 1993 was related to the number of existing

seedlings in 1990, then this component would be subsumed

into B. As we consider that component to be small relative

to survival, B can often be considered to be survival. A

negative value for B would probably indicate that the

number of tupelo seedlings is wildly fluctuating from year to

year. A value for B > 1 would indicate that survival is

probably high, and that the number of new seedlings was

positively related to the initial number of seedlings. The

final equations for A and B follow:

A = 536(TC) -595(L,TC)489(L2TC)

B = (0.984 + 0.327(C))

(2)

Conclusions

These third-year results indicate that cleaning has little or no

beneficial effects on the water tupelo stands selected for this

study. The response to thinning seems to depend on stand

and site characteristics. The response was greatest for

location 2, which seemed to have the greatest site quality

and which had few large gaps between sprout clumps and,

thus, a high density of stumps. However, the magnitude of

the effects might change as the stands develop and mortality

of the sprouts become more common as natural thinning

occurs within stumps.

T = a dummy variable that is 1 when the plot has been

thinned, and 0 when not thinned;

C = a dummy variable that is 1 when the plot has been

cleaned, and 0 when not cleaned;

L1 = a dummy variable for location 1; and

L2 = a dummy variabte for location 2.

The adjusted R* was 0.95 and the standard error of the

estimate is 12 1 trees per plot. “A’ corrects for an

anomaly-a plot on location 3 that had been thinned and

cleaned had a very large number of seedlings. Thus the

importance of A may be discounted.

The equation for B indicates that for thinned and control

plots the number of seedlings in 1993 ahnost equaled the

number of seedlings in 1990 (“survival” = 0.984). For plots

that had been cleaned, the “survival” term is >l, probably

indicating that the causes of high initial seedling numbers

also caused higher numbers of new regeneration. Thus

“survival” for all seedlings was less than the 0.984

parameter value would indicate. Although the equation

indicates that cleaning increases the number of tupelo

seedlings, a long, late flood in 199 1 may have affected the

reliability of these. results. The flood reduced seedling

numbers by approximately 30 percent between 1990 and

199 1, and the increase of tupelo seedlings from 199 1 to

1993 represent a recovery from these losses. Because these

numbers fluctuate drastically, random chance may cause a

perceived treatment effect. For these reasons, making any

inferences about the treatment effect on tupelo seedling

numbers is unwarranted.

Literature Cited

Daniel, T.W.; Helms, J.A.; Baker, ES. 1979. Principles of silviculture, 2d

ed. New York: McGraw-Hill Book Co. 500 p.

DeBeU, D.S. 197 1. Stump sprouting after harvest cutting in swamp tupelo.

Res. Pap. SE-83. Asheville, NC: U.S. Department of Agriculture, Forest

Service, Southeastern Forest Experiment Station. 6 p.

Goelx, J.C.G. 1995. Stocking guides for water tupelo and baldcypress.

Southern Journal of Applied Forestry. 19: 105-108.

Go&s, J.C.G.; Meadows, IS.; Willingham, P.W. 1993. Precommercial

thinning of water tupelo stands on the Mobile-Tensaw River Delta. In:

Brissette, John C., ed. Proceedings of the seventh biennial southern

silvicuhural research conference; 1992 November 17- 19; Mobile, AL.

Gen. Tech. Rep. SO-93. New Orleans: U.S. Department of Agriculture,

Forest Service, Southern Forest Experiment Station: 105-108.

Kennedy, HE, Jr. 1982. Growth and survival of water tupelo coppice

regeneration after six growing seasons. Southern Journal of Applied

Forestry. 6: 133-135.

Kennedy, H.E., Jr. 1983. Water tupelo in the Atchafalaya basin does not

benefit from thinning. Res. Note SO-298. New Orleans: U.S.

Department of Agriculture, Forest Service, Southern Forest Experiment

Station. 3 p.

McCarity, RW. 1979. Ten-year results of thinning and clear-cutting in a

muck swamp timber type. Southern Journal of Applied Forestry.

3: 6467.

Putnam, J.A.; Fumivai, G.M.; M&night, J.S. 1960. Management and

inventory of southern hardwoods. Agric. Handb. I8 I. Washington, DC:

U.S. Department of Agriculture. 102 p.


Goelz, J.C.G.; Meadows, J.S. 1999. Precommerical thinning of water tupelo stands on the

Mobile-Tensaw River Delta: third-year tesults. Res. Pap. SRS-17. Asheville, NC: U.S.

Department of Agriculture, Forest Service, Southern Research Station. 6 p.

Three 4-year-old stands were selected for precommercial thinning. Treatment consisted of two

components: (I) thinning water tupelo (Nyssa aquatica L.) stump sprouts and dense patches of

water tupelo seedlings and (2) cutting all stems of Carolina ash (Fruxinus caroliniana Mill.)

and black willow (Salix nigra Marsh.) (cleaning). This approach provided a 2 by 2 factorial

with two replications in each stand. Thinning increased average diameter growth of water

tupelo stump sprouts, with the greatest increase in one stand that had the highest density of

stumps and apparently the highest productivity; cleaning did not increase diameter growth.

Thinning decreased stand basal area growth; cleaning did not affect basal area growth.

Mortality of water tupelo stump sprouts was negligible. Cleaning may increase the number of

water tupelo seedlings, although the evidence from this study is not compelling.

Precommercial thinning will probably provide a favorable response in stands with high density

of stumps and high productivity.

Keywords: Cleaning, coppice, diameter growth, mortality, precommercial thinning, water

tupelo, wetlands.
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