ECOLOGICAL CHARACTER OF A SMALL-SCALE LINEAR WIND EVENT
IN AN OLD-GROWTH BOTTOMLAND HARDWOOD STAND IN SOUTH-CENTRAL ARKANSAS
James M. Guldin
Research Forest Ecologist, USDA Forest Service
Federal Building, P.O. Box 1270, Hot Springs, AR 71902
Brian R. Lockhart
Assistant Professor, School of Forest Resources
University of Arkansas at Monticello
P.O. Box 3468, Monticello, AR 71656
Lance Peacock
Arkansas Nature Conservancy
300 Spring Street, Suite 717, Little Rock, AR 72201
ABSTRACT: The Moro Bottoms Research Natural Area, a 45-ha bottomland hardwood
stand in the Moro Creek flowage near Fordyce, Arkansas, is a relict old-growth
stand of oaks, sweetgum, and cypress which the Arkansas Nature Conservancy
acquired from Georgia-Pacific Corporation in 1986. As an element of that
acquisition, a 100% inventory of the stand was obtained on the area. During a
summer evening in 1989, a line of thunderstorms moved across south-central
Arkansas, affecting forest vegetation in a localized manner. These winds
promoted windthrow in a number of forested landscapes including the Moro
Bottoms Research Natural Area. Following the windstorm, a 100% inventory of
all windthrown trees was taken, providing a unique data set to scrutinize the
ecological effects of the windstorm. There were 47 different species of woody
stems on the tract, of which ten were uprooted. Sweetgum and the several
white oak species were proportionally resistant to uprooting, whereas the red
oaks were disproportionally uprooted. A larger proportion of overstory trees
of intermediate diameter (22-30" d.b.h.) were uprooted than of small (12-20"
d.b.h.) or large (32+ d.b.h.) diameters. Analyses focused upon the influence
of species identity, diameter, and azimuth of uprooting, and on the fortuitous
happenstance of a 100% pre-disturbance inventory in characterizing the
ecological pattern of windthrow.
INTRODUCTION
The frequency and magnitude of natural disturbances such as windthrow in
southern bottomland deciduous forests exert an important influence on forest
structure. Catastrophic wind disturbances such as hurricanes and tornados
(Turner, 1935; White, 1979) have long been recognized as a major force in
shaping the structure of forest stands (Curtis, 1943; Oliver and Stephens,
1977; Oliver, 1981). To a lesser extent, the ecological values of winds that
are not catastrophic but merely damaging have received less attention
(Stephens, 1956; Webb, 1989), especially in southern United States forests.
The opportunities for ecological study of such catastrophes is further
limited by the nature of land ownership. In the major and minor river bottoms
that dissect the upper Coastal Plain of the West Gulf region, the richly
productive bottomland hardwood stands are typically owned by forest industry
and non-industrial private landowners. Government ownership in the area has
been the result of recent acquisition. In the event that catastrophic events
occur in these forests, the past actions of landowners have been to salvage
the windthrown trees rapidly. When disturbance affects bottomland forests set
aside as ecological reserves or natural areas, the opportunities to study the
ecological effects of such disturbances increase.
In this paper, we report on the effects of a less-than-catastrophic
disturbance that occurred within one of the rare ecological reserves in the
region -- from a research perspective, a lucky situation. The study objective
was to investigate the effect of a small-scale linear wind associated with
the passing of a line of summer thunderstorms on the the composition,
structure, and direction felled in an old-growth bottomland hardwood stand in
Cleveland County, Arkansas (U.S.A.).
METHODS
The Moro Bottoms Research Natural Area (RNA) encompasses 45 hectares along
Moro Creek in south-central Arkansas. This stand was considered old-growth
based on the large diameters of the overstory trees and ages of selected
trees. Two red oak trees were bored and each was determined to be
approximately 150 years old, while three white oak trees averaged about 250
years old. There was little visual evidence of past harvesting practices and
no evidence of other large disturbances. The Moro Bottoms RNA is one of the
best remaining examples of a typical southern wet-mesic floodplain forest
presently known in Arkansas (unpublished report by the Arkansas Nature
Conservancy; copy on file at the University of Arkansas at Monticello).
Due to the rarity and unique character of this old-growth bottomland
hardwood stand, the Arkansas Nature Conservancy, a land conservation
organization, purchased this stand from Georgia-Pacific Corporation in 1985.
The area was then acquired by the Arkansas Natural Heritage Commission for
continued preservation. As part of the acquisition process, a consulting
forester with extensive experience in bottomland hardwood management was hired
to conduct a 100% inventory of all trees greater than approximately 25 cm
d.b.h. This consulting forester noted that this stand was unique, and to the
best of his knowledge, "the only old-growth stand with substantial volumes of
red and figured redgum remaining in the mid-south." Gum and redgum refers to
sweetgum (Liquidambar styraciflua L.).
One summer evening in July, 1989, a line of thunderstorms moved across
south-central Arkansas. These storms were accompanied by unusually high winds
and caused windthrow in many areas, among them the Moro Bottoms RNA. The
storm uprooted 207 trees, comprising ten different species. Since this area
was designated a research natural area, no salvage operations were conducted
and the windthrown trees were left to decompose naturally. In August, 1989,
a 100% inventory of the windthrown trees was conducted by species, diameter
(d.b.h. for trees >25 cm), and azimuth direction of each fallen tree.
Information from this post-disturbance inventory was then compared to the
pre-disturbance inventory to determine post-disturbance stand composition.
Two sources of error were possible in comparing pre- and post-disturbance
stands. First, since three years had passed since the initial inventory,
trees may have increased in diameter enough to have moved into larger diameter
classes. A second source of error involved measurement error. Trees measured
during the post-disturbance inventory were in contact with the soil surface,
in some cases with the bole half-buried in soil. Therefore, calipers were
used to measure tree diameters instead of measuring tapes, as was done in the
first inventory. Based on analyses of diameter distributions, we felt that
these sources of error were small, especially in light of the value of the
100% pre- and post-disturbance inventories. Data from both inventories were
summarized into importance values for each species representing the summation
of relative density and relative dominance (i.e., basal area; Spurr and
Barnes, 1980).
RESULTS
Species Group Designations
A total of 47 woody species were tallied during the pre-disturbance
inventory. Overstory species were placed into the following species or
species groups (scientific names follow Duncan and Duncan, 1988): (1)
sweetgum; (2) white oaks, including white oak (Q. abla L.), overcup oak
(Q. lyrata Walt.), and swamp chestnut oak (Q. michauxii Nutt.); (3) cherrybark
oak (Q. pagoda Raf.); (4) other red oaks including water oak (Q. nigra L.) and
willow oak (Q. phellos L.); (5) hickory species including pignut hickory
(Carya glabra (Mill.) Sweet), shagbard hickory (C. ovata Mill.), and mockernut
hickory (C. tomentosa (Poir.) Nutt.); (6) baldcypress (Taxodium distichum L.);
(7) two ash species, green ash (Fraxinus pennsylvanica Marsh.) and white ash
(F. americana L.); (8) other hardwoods including red maple (Acer rubrun L.),
common persimmon (Diospyros virginiana L.), American beech (Fagus grandifolia
Ehrh.), American holly (Ilex opaca Ait.), blackgum (Nyssa sylvatica Marsh.),
and winged elm (Ulmus alata Michx.); and (9) loblolly pine (Pinus taeda L.).
Species groups were designated by the consulting forester during the pre-
disturbance inventory; therefore, the same classification was used during the
post-disturbance inventory.
Pre-disturbance Conditions
A total of 3,456 trees >25 cm d.b.h. were tallied during the
pre-disturbance inventory. Based on the pre-disturbance diameter
distribution, the 40-cm diameter class contained the greatest number of trees
with 454 stems ( Figure 1 ). The largest diameter tree was a 135-cm sweetgum.
Beginning with the 40-cm diameter class, the diameter distribution conformed
to the classic reverse-J curve. This distribution may represent an uneven-
aged stand, although this may or may not be case since only five trees were
actually aged.
Sweetgum had the greatest density among species or species groups with 30%,
or 1036 of the trees >25 cm d.b.h. Sweetgum also had the greatest dominance
at 33% of total basal area giving this species an importance value of 63
(Table 1). White oaks had the second greatest importance among species or
species groups at 33 composed of a relative density of 14% and a relative
dominance of 19%. Other red oak species had the third greatest importance at
26. Hickory species had the fourth greatest importance at 22. This value was
composed of a relative density of 13% but a relative dominance of only 9%,
indicating that the mean stand diameter of the hickories was considerabley
less than that of sweetgum and the oaks. Cherrybark oak had the fifth
greatest importance at 21. Finally, importance values for each of the
remaining species or species groups was >16.
Post-disturbance Conditions
The severe winds came from the northwest; therefore, the greatest number of
trees fell southeast (100-180 degrees; Figure 2 ). But some trees fell in a
different direction than the way the wind was blowing. This may have been due
to localized vortices of wind developing on a occasional basis, or may have
been due to physical deflection of trees by associates that fell on them. The
post-disturbance inventory did not attempt to measure these 'domino' effects.
A total of 207 trees, or 6.0% of the total number of trees >25 cm d.b.h.,
blew over during the wind event. Based on the post-disturbance diameter
distribution, trees from the 35-120 cm diameter classes fell during the wind
event ( Figure 1 ). Most windthrow occurred among the 55-100 cm diameter
classes. The largest tree felled was a 110 cm white oak, although few trees
larger than 105 cm were lost.
Table 1. Pre-disturbance density, relative density, basal area, relative
basal area, and importance values by species or species groups
for the Moro Bottoms RNA. (Density = stems/ha; relative =%
basal area = sq.m./ha)
Species or Density Relative Basal Relative Importance
species group density area basal area value
------------- ------- -------- ------- ---------- ----------
Sweetgum 1,036 29.98 280.78 33.36 63
White oaks 488 14.12 155.90 18.52 33
Cherrybark oak 369 10.68 85.80 10.19 21
Other red oaks 456 13.19 106.45 12.65 26
Hickories 466 13.48 72.95 8.67 22
Baldcypress 252 7.29 61.29 7.28 15
Ashes 48 1.39 8.04 0.95 2
Other hardwoods 238 6.89 44.11 5.24 12
Loblolly pine 103 2.98 26.43 3.14 6
Total 3,456 100.00 846.75 100.00 200
In terms of post-disturbance importance, sweetgum gained in importance from
63 to 67 out of a possible 200 (Table 2). White oaks also had a slight
increase in importance from 32.6 to 32.9. These increases in importance for
both sweetgum and white oaks, even though each lost trees during the wind
event, was due to the far greater number of red oaks lost during the storm,
both cherrybark oak and other red oaks. Cherrybark oak decreased in
importance by 3 (from 21 to 18) while the other red oaks group also decreased
by 3 (from 26 to 23). Hickory species also had a slight decrease in
importance, from 22 to 21, while the remaining species or species groups had
slight increases in importance.
Based on post-disturbance diameter distributions by species, only 2% of the
sweetgum trees were felled, even though sweetgum composed 30% of the trees.
Trees were felled from the 40-105 cm diameter classes with greatest losses
(4 trees) occurring in the 40 and 65 diameter classes ( Figure
3 ). A total of
22 white oak trees were felled from the 40-110 cm diameter classes with
greatest losses occurring in the 65-85 cm diameter classes ( Figure
4 ).
Fifty-four cherrybark oak trees were felled from the 35-105 cm diameter
classes. Greatest losses (at least 4 trees) occurred from the 45-95 cm
diameter classes ( Figure 5 ). In several places the post-disturbance diameter
dropped below zero. This was due to the previously mentioned sources of error
between the two inventories.
Table 2. Post-disturbance density, relative density, basal area, relative
basal area, and importance values by species or species groups
for the Moro Bottoms RNA. (Density = stems/ha; relative =%
basal area = sq.m./ha)
Species or Density Relative Basal Relative Importance
species group density area basal area value
------------- ------- -------- ------- ---------- ----------
Sweetgum 1,014 31.21 270.99 35.56 67
White oaks 466 14.34 144.49 18.96 33
Cherrybark oak 315 9.70 65.01 8.53 18
Other red oaks 390 12.00 80.07 10.51 23
Hickories 424 13.05 61.98 8.13 21
Baldcypress 251 7.73 61.05 8.01 16
Ashes 48 1.48 8.04 1.05 3
Other hardwoods 238 7.33 44.11 5.78 13
Loblolly pine 103 3.17 26.43 3.47 7
Total 3,249 100.01 762.17 100.00 201
In the other red oak species group, 66 trees or 14.5% of the original
number of other red oaks were lost during the wind event. Losses occurred
from the 35-110 cm diameter classes with greatest losses (at least 4 trees)
occurring in the 50-90 cm diameter classes, similar to cherrybark oak ( Figure
6).
In addition to the losses among cherrybark oak and the other red oaks, 42
hickory trees were lost during the severe wind. Losses occurred from the
35-100 cm diameter classes with greatest losses (at least 6 trees) occurring
in the 40, 55, 60, and 65 cm diameter classes ( Figure
7 ). These values were
similar to cherrybark oak and the other red oaks.
As for the remaining species and species groups, few trees were felled
during the severe wind. Only one 55-cm baldcypress was felled ( Figure 8 ). No ash trees or other hardwood trees
>25 cm d.b.h. were found blown down or toppled by blown over trees ( Figure 9 s 9 & Figure
10, respectively). Finally, no
loblolly pine trees were found on the ground ( Figure
11
).
DISCUSSION
One might ask why more trees were lost in the intermediate diameter classes
compared to the smaller and larger diameter classes. Cherrybark oak is
generally considered to be the most desirable tree species in southern United
States bottomland hardwoods (Burns and Honkala, 1990). It has excellent
growth and form, making it a highly desirable timber species. This
desirability results from the sturdy nature of the lumber produced from red
oak in general, and particularly from cherrybark oak. But this sturdiness may
have ecological ramifications. We hypothesize that cherrybark oak is not as
flexible a species when subjected to severe wind. That is, because the bole
is stiffer, the crown of cherrybark oak buffets about less than that of
sweetgum or the white oaks. Instead, the crowns of cherrybark oak resist wind
by depending more on a sturdy bole. The disadvantage of this strategy is
that, in cases of severe wind, cherrybark oaks of intermediate diameter may
not be as resistant to windthrow. Since this species apparently does not sway
as much as other species, an increased incidence of blowdown may occur.
In addition to there being fewer larger trees, lower windthrow of
cherrybark oaks >80 cm may be due to a higher ratio of diameter to crown
surface area -- that is, a given crown area supported by a much thicker bole.
A cursory examination of the crowns of cherrybark oak suggests little
difference in crown size between intermediate and codominant trees. The
larger stem diameter of the codominant trees may make them more resistant to
blowdown than smaller associates that have similar crown size. These larger
trees may also have had more expansive root systems and thus greater anchorage
in the soil, but this is speculative. Similar arguments used to explain the
loss of trees in the intermediate diameter classes in cherrybark oak may help
explain losses in the other red oak and hickory species groups.
A possible ecological advantage of this sturdiness is, that during near
calm to mild wind events, cherrybark oak crowns are better able to compete
against other species during episodes of crown abrasion. This has important
stand development consequences in mixed-species hardwood stands (see Oliver,
1976; Clatterbuck & Hodges, 1986; and Kittredge, 1988).
CONCLUSIONS
An important point relevant to the objectives of this meeting involves the
fortuitous happenstance of a 100% pre-disturbance inventory on a relatively
large stand. Given the increasing number of natural areas being set aside, it
is imperative that as complete an inventory as possible be made on these
unique areas. While greater detail can be gathered from small permanent
plots, additional information can be gained by supplementing permanent plot
data with a 100% inventory of at least the overstory trees. Although several
errors were present in comparing pre- and post- disturbance inventories,
primarily the result of diameter growth of trees during the three years
between inventories along with measurement error, these errors were deemed
relatively small in comparison to the results obtained from these complete
inventories.
Finally, results showing a species-specific and a size-specific pattern to
windthrow (i.e., cherrybark oak, other red oaks, and hickories of intermediate
diameters being more likely to fall during severe wind events) would probably
not have been possible with sampling techniques.
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