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Tornado and fire effects on tree species composition in a savanna in the Big Thicket National Preserve, southeast Texas, USA
Pores~~;ology Management Forest
Ecology
and Management
91 (1997)
279-289
Tornado and fire effects on tree species composition in a savanna in the Big Thicket National Preserve, southeast Texas, USA Changxiang Liu a,* , Jeff S. Glitzenstein b, Paul A. Harcombe ‘, Robert G. Knox d a Endangered
Resources Branch Texas Parks and Wildlife Department 3000 IH35 South, Suite 100, Austin, TX 78704, b Tall Timbers Research Station, Route I, Box 678, Tallahassee, FL 32312, USA ’ Department of Ecology and Evolutionary Biology, Rice University Houston, TX 77251, USA ’ Biospheric Sciences Branch, NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA Accepted
8 December
USA
1995
Abstract Ordination showed that species composition in a savanna shifted toward mixed pine-hardwood types after the tornado damage in 1983. Of the twenty 250 m2 study plots, one baygall plot remained unchanged in its position in ordination space, as did five mixed pine-hardwood plots and six savanna plots. However, eight savanna plots moved from the savanna space to the mixed pine-hardwood space after the tornado. Prescribed bums in 1986 and 1991 had a modest effect in reversing this trend: only four of the eight plots returned to the savanna ordination space by 1991. The changes in species composition, summarized by movement of plots in ordination space, reflected the conflicting affects of the tornado and fire: the tornado tended to change savanna to mixed pine-hardwood by differentially removing pines and stimulating hardwood growth. Fires tended to reduce hardwood density. However, stand opening and increased fuel loads following the tornado did not result in fires intense enough to dramatically enhance savanna recovery. Keywords:
Prescribed
burning;
Ordination;
Disturbance;
Pinus palustris
1. Introduction Effects of disturbances on vegetation dynamics, species composition, and community structure have attracted much attention from plant ecologists (Bormann and Likens, 1979; Noble and Slatyer, 1980; Pickett, 1980; Oliver, 1981; Shugart, 1984; Pickett and White, 1985). In the southeastern United States, strong winds and fires have been important natural
* Corresponding author. Tel.: (512) 912-7052; 7058; E-mail: [email protected] 0378-l 127/97/$17.00 Copyright PII SO378-1 127(96)03705-X
0 1997 Elsevier
Fax: (512)
Science
912-
B.V.
mill.;
Vegetation
change
processes shaping vegetation patterns (Christensen, 1981, 1988; Glitzenstein and Harcombe, 1988; Basnet et al., 1992, Schwartz, 1994). Strong winds associated with hurricanes, thunderstorms, and tomadoes can result in a great deal of structural damage and high mortality of large trees (Foster, 1988a,b; Brokaw and Walker, 1991; Gresham et al., 1991). Plant species may also have different strategies for responding to damage, e.g. by seeding, sprouting, or regrowing. Some species can regrow even after severe damage and others may die immediately or die later as they become vulnerable to pests and diseases (Pickett and White, 1985). Most pines show limited
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reserved.
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Ecology
sprouting following damage (Glitzenstein and Harcombe, 1988; Boucher, 1990; Boucher et al., 1990; Gresham et al., 1991; Peterson and Picket& 1991). However, fires, mostly low-intensity surface fires in the southeastern United States, are more likely to cause damage to small individuals. Species may also respond differently to fires. They have varying postfire survivorship depending on their resistance and post-fire response strategy. Along the Gulf coast, almost all plant communities are subjected to recurrent disturbances. One unique vegetation association among them is the pine savanna, which has a relatively open canopy and diverse herbaceous ground cover (Marks and Harcombe, 1981; Walker and Peet, 1983). Bridges and Orzell (1989) recognize many savanna communities in the west Gulf Coastal Plain. This association and its diverse herbaceous components are strongly influenced by frequent low-intensity tires (Christensen, 198 1; Marks and Harcombe, 198 1; Walker and Peet, 1983; Ware et al., 1993). Streng and Harcombe (1982) suggest that under some conditions. southeastern savannas can escape control by low-intensity fires and succeed to a more fire resistant vegetation type. With modem fire suppression, this phenomenon is widespread in the southeastern United States (Myers and Peroni, 1983; Martin et al., 1993; Ware et al., 1993). If habitats could escape from fire control naturally, then for savannas to have persisted in the presettlement landscape, there must have been natural mechanisms for converting closed forests back to savannas. One such mechanism might be fires intense enough to kill canopy trees, especially if followed by low-intensity surface fires. Another potential mechanism is stand opening by strong wind (tornadoes, strong hurricanes, or downbursts associated with violent thunderstorms) followed by surface fires. A fairly low-frequency crown disturbance might be adequate to maintain the savanna fraction in the landscape if savannas were normally kept open by frequent low-intensity surface fiies in the past. Disturbance of a well-studied savanna (Streng and Harcombe, 1982) by a tornado (Glitzenstein and Harcombe, 1988), followed by a series of prescribed burns, provided a valuable opportunity to address this possibility. Such a study may have implications for savanna restoration, since, if strong wind fol-
and Management
91 (19971279-289
lowed by fire is effective, the critical features required to achieve the same effect artificialy might be identified. For example, is an abundance of downed woody fuel essential to promote high-intensity surface fires or might the same effect on species composition result from canopy opening and an increase in fine fuel? Study of the interaction of wind disturbance and fire, in as near natural conditions as feasible, can, thus, provide information on effects of disturbance on compositional and structural changes in natural plant communities. 1.1. Study area and methods 1.1.1. Study area
The study area is located in the Big Thicket region of southeastern Texas. The climate of this area is humid subtropical with a long growing season. Rainfall is evenly distributed throughout the year. Average annual temperature is 19.X (30-year average at Port Arthur). Potential evaporation exceeds precipitation during most of the year except in the winter months (National Climatic Data Center data for Beaumont). Temperature and annual rainfall decline gradually from the southeast to the northwest (EarthInfo Inc., 1994). The Big Thicket area is underlain by the Beaumont, Montgomery, Bentley and Willis Pleistocene geologic formations from south to north (Geologic Atlas of Texas, Beaumont sheet, Bureau of Economic Geology, The University of Texas at Austin. 1968). Elevation increases from about 0 to 30m above sea level in the south to 60- 180m above sea level in the north (USGS quadrangle sheets, 7.5 min topographical series, provisional edition, 1984). The region is bounded by the Trinity River to the west and the Neches River to the east. Most of the area drains southeastward into the Neches River via Village Creek. Ultisols are predominant in the region and alfisols and entisols are important in parts of the landscape (Deshotels, 1978). Soil texture ranges from excessively drained sandy soil in the upland and sand ridges, to loamy and clayey soils in swamps and floodplains (Marks and Harcombe, 198 1). Like other areas of the Gulf Coastal Plain, the Big Thicket area is frequented by summer thunderstorms, tropical storms and hurricanes, which were a major source of lightning fires in the presettlement times.
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important role in maintaining this unique vegetation type (Christensen, 1981; Streng and Harcombe, 1982). Limited data collected from the Hickory Creek Unit indicated that fire was more frequent before 1967 than after (Glitzenstein and Harcombe, 1988) with fire intervals ranging from two to seven years from 1928 to 1967. No fire scars were found in sampled trees after that. Fig. 1. Map of study area showing the Hickory Tornado damaged area is shaded. Interval between lines is about 1.52m.
Creek Unit. two contour
Between 1871 and 1991, twenty-seven tropical storms and forty-three hurricanes made landfall along the Texas coast (Bomar, 1995). The observed tornado occurrence in Texas was about 181 per year from 1953 to 1991; the average density of tornadoes in Texas is about 12.95 per lOOOOkm* per year in Texas (Bomar, 1995). Glitzenstein and Harcombe (1988) calculated that the density of tornadoes in south Hardin County was about 5.79 tornadoes per 10 OOOkm* per year from data presented in Trenchard (1977). It was lower than the state average, but much higher than the national average (0.66 per 10 000 km* per year). The Hickory Creek Unit of the Big Thicket National Preserve (Fig. 1) consists of 270 ha of wetland pine savanna and upland forest. It is located in the central part of the Preserve on the Montgomery formation, and is about 40km north of Beaumont, Texas. The closed-canopy stands were derived from former savanna following tire suppression beginning in the early 1930s (Streng and Harcombe, 1982). The savanna overstory is characterized by scattered pines, usually longleaf (Pinus palustris Mill.), and loblolly (Pinus tuedu L.). Note that nomenclature for plant species follows Correll and Johnston (1970) and that for vegetation follows Marks and Harcombe (198 1). Other canopy species include sweetgum (Liquidumbar styraciflua L.), blackgum (Nyssa syluatica Marsh.), southern red oak (Quercus falcatu Michx.), shortleaf pine (Pinus echinatu Mill.), water oak (Quercus nigra L.), and sweet bay (Magnolia uirginiana L.). Common shrub species are wax-myrtle (Myrica cerifera L. and M. heterophylla Raf.) and yaupon (Ilex vomitoria Ait.). Previous studies revealed that fire and poor internal drainage play an
1.2. Field methods
On 10 December 1983, a large tornado touched down in the southwest comer of the Hickory Creek Unit resulting in a damaged area of 3 1.1 ha (Glitzenstein and Harcombe, 1988). To assessthe damage to vegetation and monitor vegetation change afterwards, twenty permanent 250m’ circular plots were established and sampled in 1985 at random distances along three parallel west-east transects traversing the disturbed area. Individuals over 2.0cm DBH (diameter at breast height, about 1.4 m above ground) were tagged, measured and identified within the 250m’ area of each plot. Individuals of 2-5 cm DBH were termed small trees and individuals larger than 5 cm DBH were called large trees. Seedlings were defined as individuals of tree species less than 50cm tall. Saplings were individuals of both tree and shrub species taller than 50cm but less than 2.0cm DBH. Saplings were subdivided into small saplings (taller than 50cm but shorter than 1.4 m) and large saplings (1.4m tall to 2.0cm DBH). Seedlings and saplings were censused along four perpendicular 8.92 m lines radiating outward from plot center. The direction of the first line was determined randomly. Along each line, seedlings were measured within a 50cm wide strip and seedlings within a 20cm wide strip along each line. Downed woody materials were tallied and shrub coverage (intercept lengths of shrubs) was measured along a 10m line on the first and the third lines for seedlings and saplings. The procedure of Brown (1974) was used to survey and calculate weight of woody materials. Fine fuel was collected in a 50cm X 50cm quadrat randomly placed within each plot. Fine fuel samples were sorted into needles, leaves, dead grasses, living materials, and others. Separated materials were ovendried at 70°C and weighed.
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One year after these data were first collected, in the winter of 1986, the tornado-damaged area was subjected to a prescribed fire. Another prescribed fire was conducted five years later in spring 199 1. To assess the effects of these fires, the permanent plots located in 1985 were resampled in the growing season after the 1991 fire. Since there were no direct measurements on 2-4.5 cm DBH individuals in 1983, the density of these individuals was extrapolated from 1985 data in the following manner. If the DBH of an individual in 1985 was less than 3 cm, then the DBH of this individual was assumed less than 2cm in 1983 and to have been excluded; otherwise the individual was treated as having been larger than 2cm DBH in 1983 and the same DBH in 1985 was retained in calculating basal area. This extrapolation might have introduced some bias for estimation of density and basal area in 1983. However, bias was probably not substantial because 1) small trees had little contribution to basal area, and 2) there were not many individuals less than 5cm DBH in these plots before the tornado. Fire intensity was measured by suspending one set of fire-sensitive tablets (Tempil, Big Three Industries, New Jersey, USA) 20cm above ground surface at the center of each plot. I .3. Analyses Data from fifteen of the 20 permanent plots were discussed in a 1988 paper (Glitzenstein and Harcombe, 1988). Five plots were excluded from that paper because these plots were only lightly damaged by the tornado. Data from all twenty plots are included in the current analysis. We sampled the tornado plots as part of a largescale study of fire effects on vegetation in the Big Thicket National Preserve and the nearby Roy E. Larsen Preserve of the Nature Conservancy of Texas. 302 10 X 10m2 fire study plots representing 63 stands were sampled for that study. To best classify vegetation in the tornado study plots, all prebum stands for the fire study as well as 24 stands studied by Marks and Harcombe (1981) in the late 1970s were included in a detrended correspondence analysis (DCA). After assigning vegetation types to tornado plots using ordination results from the large data set, a second DCA ordination was run to track vegetation
Yl f IYY7) 27Y-BY
changes over time using only the fire and tornado plots. Large and small trees were analyzed together for the tornado study plots owing to low numbers of small trees in the 1983 and 1985 data. Post-fire data were treated passively in the ordination so that changes could be referenced to the pre-fire vegetation patterns (see ter Braak, 1987a,b) By 1990-1991 large numbers of small trees had accumulated in some plots. Hence, a third ordination, including only these smaller stems, was run to describe compositional patterns of small trees. In addition to describing overall compositional changes with ordination, we also used paired t-tests to examine changes over time in abundance of individual species. Because of the large numbers of these tests, the type-1 error rate for each test was adjusted using the Dunn-Sidak method (Day and Quinn, 1989).
2. Results 2.1. Changes in tree populations 2.1.1. Impact oj’l983 tornado As a consequence of the tornado, tree (2 2 cm DBH) density and basal area declined substantially between 1983 and 1985 (t = 2.59, P < 0.01; r = 5.40, P < 0.0005; Table 11. Basal area was reduced by 61.1%, though only 22.2% of the trees died between 1983 and 1985. High mortality caused by the tornado was concentrated in large trees (Fig. 2). However, the reduction in pine density (32.6%) was much greater than the loss of hardwoods (19.5%). All three pine species declined substantially as a result of the tornado (Fig. 3). However, effects on hardwoods were variable. Two species, sweegum and blackgum, were relatively unaffected by the tornado. One species which declined greatly was sycamore (Plantanus occidentalis L.). Prior to the tornado, this species was represented by a few large individuals which contributed only 8% of the density, but 24% of pre-tornado basal area. These large individuals were killed by the storm, and no trees of this species occurred in the post-tornado forest. A number of other hardwood species also declined significantly following the tornado. Southern red oak, water oak and dogwood (Comus florida L.1 were
C. Liu et al. /Forest Table 1 Percentage
composition
of density
Species
and Management
and basal area of stems larger
91 (1997)
1983
Total Percentage Total a are stems per hectare
100 984 for density
283
Basal Area 1985
17.5 11.4 2.0 7.3 16.9 2.8 2.4 4.1 3.9 4.9 3.0 23.8
279-289
than 2 cm DBH
Density
Liquidambar styracijlua L. Nyssa syhatica Marsh. Pinus echinata Mill. Pinus palustris Mill. Pinus taeda L. Quercusfalcata Michx. Quercus nigra L. Comusflorida L. Ilex uomitoria Ait. Magnolia cirginiana L. Symplocos tinctoria CL.) L’Her. Others
a Units
Ecology
19.3 16.7 1.0 5.5 16.2 2.1 1.6 3.1 5.0 5.0 1.3 23.2 100 766
and m* ha-’
1990 15.2 8.5 0.1 3.5 10.2 4.3 7.1 10.2 11.1 3.6 0.9 25.2 100 1548
1991
1983
13.8 7.1 0.1 6.0 12.5 3.2 9.4 11.2 11.0 2.5 1.4 21.2 100 1604
1985
1990
1991
6.8 4.4 5.9 21.2 28.1 1.3 1.1 0.7 0.4 0.6 0.5 28.9
16.6 11.4 1.4 16.3 36.6 2.5 1.3 1.1 0.8 1.5 0.4 10.2
11.1 12.8 0.9 18.4 39.2 1.3 1.0 1.3 1.5 1.3 0.1 11.2
10.8 12.7 1.1 19.0 40.5 1.5 1.9 1.8 1.7 1.2 0.1 7.6
100 20.48
100 1.96
100 8.58
100 8.83
for basal area.
each reduced in density by more than 40%. Water oak and dogwood also experienced large decreases in basal area (55% and 40%, respectively). However, these three species made up only a small portion of the tree population (Table 1). Before the tornado, the ranking of overstory species by basal area was loblolly > longleaf > sweetgum > shortleaf > blackgum. Loblolly pine remained as the dominant species (in basal area) after the tornado. The ranks of the other species changed as follows: sweetgum > longleaf > blackgum > southern red oak. This changed order reflected differing amounts of mortality caused by the tornado. 2.1.2. 1985-1990 (post-tornado trends) Damaged trees (including broken, pinned, top dead, and uprooted trees and trees with major crown damage) continued to die between 1985 and 1990 (Fig. 21, and damaged trees alive in 1985 had significantly lower survivorship than trees without major damage (chi-squared test, P < 0.001): about 60% of the 144 damaged trees alive in 1985 died between 1985 and 1990 compared to only 34% of the 273 trees without major damage that died in the same period. Continued high mortality of damaged trees in the half decade following the tornado may well have
0
IO
8 i
20
30
40
50
60
30
40
50
60
30
40
50
60
died
during
19851990
-1
0
IO
20
0
10
20
DBH (cm)
Fig. 2. Size distribution 1983-1985, 1985-1990,
of trees that and 1990-1991.
the periods
284
C. Liu et al. /Forest
--------
Ecology
and Management
91 (1997)
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-----F==-= ‘\
\---
-----._______------_-----
63
85
86
87
88
89
90
91
83
85
86
87
68
89
90
91
33
85
86
87
88
89
90
91
83
85
86
87
88
89
90
91
year
year
Fig. 3. Changes in basal area (top panels) and density (bottom panels) of trees (larger than 2 cm DBH) of major species. The values for years from 1986 to 1989 are interpolated from the data of 1985 and 1990 to show the trend of change over time. Note that the y axes of the two top panels are on different scales.
been a consequence, at least in part, of increased susceptibility of these trees to the 1986 fire. Densities of hardwoods increased dramatically following the tornado (Fig. 4). By 1990, over 50% of the trees were less than 1Ocm DBH and tree density was already significantly higher than before the tor-
nado (t = 2.91; P < 0.005). Yaupon, dogwood, water oak, and southern red oak more than tripled in density, although they only made up a small portion of the total basal area (Table 1). In contrast to the other hardwoods, blackgum and sweetgum did not experience large increases in density. However, ow-
8 -1
1983
1985
8
1990
0
10
20
30
4.0
50
0
60
DBH (cm)
Fig. 4. Size distribution
of trees alive (2 2cm
10
20
30
40
DBH (cm) DBH)
in 1983, 1985, 1990, and 1991.
Xl
60
C. Liu et al. /Forest
Ecology
and Management
ing to continued low rates of mortality, these species maintained high levels of basal area and continued as codominants with Ioblolly pine. By 1991, densities of longleaf and loblolly pines also exceeded pretornado levels though the density of shortleaf pine was still far below the 1983 level.
of seedling,
sapling,
Species
and small tree populations Seedlings 1985 a
Shrub/small tree Myrica cerifera L./ M. Heterophylla Raf. Magnolia oirginiana L. Cornusflorida L. Symplocos tinctoria (L.) L’Her. Persea borbonia CL.1 Spreng. Ilex opaca Ait. Sassafras albidum (Nutt.) Nees. Ilex Llomitoria Ait. Tree Pinus taeda L. Pinus palustris Mill. Pinus echinata Mill. Liquidambar styraciflua L. Nyssa sylvatica Marsh. Quercus falcata Michx. Quercus nigra L. others Total * from
Glitzenstein
and Harcombe
1991
9807
na
3176 1121 560 1588 187 373 2054 6351 187 187 1494 1027 93 280 2709
(1988);
per hectare)
Small saplings 1990
31194
(stems
Large
saplings
1985 a
1990
1991
1985 a
na
2559
696
157
71 16 na na 5 na na
55 21 na na 4 na na
1065 280 579 467 93 280 3867
205 36 244 75 14 23 319
59 14 0 17 23 1 3 6
27 10 0 16 36 2 6 4
37 0 0 915 93 37 187 953
114 25 0 155 11 16 27 280
na
na
b estimated
11412 from
285
pine and water oak increased significantly ( P < .Ol). Also, longleaf pines began to emerge from the grass stage in large numbers (Table 2). These young pines probably became established when the tornado opened up the overstory. Establishment of pines, especially longleaf pine, was found only in open places where other species did not form a dense thicket. In summary, two species, shortleaf pine and sycamore, were negatively affected by the tornado and failed to recover. Two dominant species, blackgum and sweetgum remained relatively unchanged throughout the entire period. All other major tree species declined following the tornado and then increased afterwards. There was little indication that fires in 1986 and 1991 had slowed post-tornado recovery of any species. Loblolly pine was the most abundant species in number of seedlings in 1985. However, there were virtually no pine saplings (Table 2). In contrast, several hardwood species had already developed substantial sapling and seedling populations by sprouting or seeding. By 1991, seedling numbers for
2.1.3. 1991 jire The fire was cool and patchy. Only six of the twenty plots burned completely, six burned partially, and the remainder did not bum at all. Measured fire temperatures were all less than 253°C compared to a temperature range of 153-500°C in sandhill and upland pine forests measured in other parts of the Big Thicket National Preserve. Most of the 110 trees that died (i.e. were topkilled) during the 1991 fire were less than 5.0 cm DBH (Fig. 4). Despite the loss of some individuals, overall density was not significantly affected by this low-intensity fire (t = 0.21 and P = 0.84 for small trees). Contrary to the expected declines, some species actually increased in basal area and density after the fire. For example, basal areas of longleaf Table 2 Composition
91 (19973 279-289
2241
1985 data.
Small trees
1990
1991
0
48
36
4
4
0
0
148 23 150 34 0 0 95
336 374 93 93 37 56 990
71 52 48 37 7 16 457
14 55 46 20 0 11 432
28 10 8 6 4 4 16
36 10 10 12 4 4 28
40 152 14 128 20 12 160
26 172 22 104 14 16 164
52 21 0 120 2 14 21 84
0 0 0 336 19 0 131 75
18 7 0 102 4 11 16 318
21 9 0 137 0 2 34 241
30 12 0 42 30 0 6 68
40 14 0 54 46 2 6 120
64 16 0 174 44 56 106 182
82 58 0 162 36 36 136 156
921
2540
1211
1058
1983 b
268
1985
390
1990
1168
1991
1184
286
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Ecology
and Management
91 (1997)
279-289
all species combined had declined to about 10% of the 1985 level. Saplings and small trees of pines began to appear in 1990. By 1991, they made up 5-25% of the sapling and small tree populations in the study plots. However, the fire had little overall effect on the sapling population (t = 1.10; P =: 0.285). The most common shrub species were yaupon and wax-myrtle. Shrub coverage was significantly higher in 1991 than in 1990 (t = 2.78; P = 0.013). This is largely a result of rapid resprouting of shrubs after the fire.
w E N’8 5, 5: a
2.2. Ordination analysis of vegetation change 3
50
100
150 AXIS 1
200
250
3ocl
Fig. 5. Vegetation types of twenty tornado study plots (pre-tornado) on an ordination diagram of all fire stands from the Big Thicket based on tree density. Tornado plots were circled: there were one baygall plot, five mid-slope plots, and 14 savanna plots in 1983.
The ordination based on tree density showed that the vegetation in the tornado-damaged area was not uniformly savanna prior to the tornado (Fig. 5). Of the twenty plots, fourteen were classified as w&land pine savanna (savanna>, five were mid-slope mixed
a
b
Savanna ‘.., ‘..,
.i.,
Savanna c
Y----k /
“I,. /’
100
c
150
200
250
d
Fig. 6. Movement of Hickory Creek tornado plots on an ordination diagram from 1983 to 1991 based on tree density: a. plots remained in their types (one baygall plot, three mid-slope plots, and six savanna plots); b. plots changed from mid-slope to savanna then back to mid-slope (n = 2); c. plots changed from savanna to mid-slope then back to savanna (n = 4); d. plots changed from savanna to mid-slope and remained mid-slope (n = 4).
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279-289
2’ Baygall Sandhill
:
Fig. 7. Movement DBH).
of the Hickory
Creek tornado
plots between
LOwerslOpe
1990 and 1991 on an ordination
hardwood-pine type (mid-slope), and one was wetland baygall shrub thicket (baygall). In the following eight years, mid-slope and baygall plots generally remained within their original community types in the ordination space (Fig. 6a). An exception was two mid-slope plots which became savanna in 1985 but had changed back to mid-slope by 1990 (Fig. 6b). This temporary change was probably due to reduced tree density in these two plots after the tornado rather than a true compositional change. All tornado plots showed movement toward mid-slope after the tornado, but changes after 1985 were not unidirectional toward any particular type. Six of the 14 original savanna plots remained as savanna during the entire period. Type changes occurred in the other eight savanna plots (Fig. 6c and d). One of these became a mid-slope plot between 1990 and 1991. Seven plots moved from savanna to mid-slope between 1983 and 1985; three of these changed back to savanna after 1985, while four stayed as mid-slope (Fig. 6~). However, three of these four plots, which stayed as mid-slope, showed some movement towards savanna after the 1991 fire. Patchiness of the 1991 fire helps to explain the ordination results discussed above: nine of the ten savanna plots burned completely or partially during this fire; only three of the nine mid-slope plots
diagram
based on small tree density
(2-5 cm
burned; the single baygall plot did not bum. This bum pattern reflects the moisture gradient from savanna to baygall. Savanna plots were more open and relatively drier than mid-slope and baygall plots. The ordination based only on small tree density (2-5cm DBH) indicated that eight of the twenty plots (four savanna plots and four mid-slope plots) moved toward savanna between 1990 and 1991 (Fig. 7), two mid-slope plots moved toward upper-slope, and the others changed little. As discussed above, this same trend was also apparent in the ordination of all trees, though not so obviously (Fig. 6). Movement toward savanna demonstrates some recovery of this community type, at least in the small trees. In part, this was due to fire-induced hardwood mortality. However, growth of pines (longleaf and loblolly) to larger size classes also contributed to a shift towards pine-dominated vegetation.
3. Discussion Tornado-related mortality was limited mostly to large trees. Many small trees managed to survive this severe canopy disturbance. In this sense, the tornado was an intermediate disturbance rather than a catastrophic event. As in other forests (Abrams and
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Nowacki, 19921, the effect of intermediate disturbance in the Hickory Creek Unit was to accelerate succession towards more shade-tolerant but less fire-tolerant overstory species. Surface fires are more frequently occurring and less severe disturbances than tornadoes. In the southeastern United States, natural and prescribed fires are generally of low intensity and restricted to the ground surface. Such fires largely affect small individuals. Generally, pines are more resistant to low-intensity fires than are other species of trees (Garren, 1943, Williamson and Black, 1981). Fire top-kills hardwoods and shrubs and enhances regeneration of pines (Wahlenburg, 1946). In this sense, fire acts to reverse successional trends stimulated by tornadoes and hurricanes. Pine savanna is a fire-dependent type (Komarek, 1974; Christensen, 198 1; Marks and Harcombe, 1981). Streng and Harcombe (1982) predicted that this community should succeed toward closed-canopy forest in the absence of fire and that flammability would decline as this trend continued. The open canopy and increase in fuel created by the 1983 tornado presented an opportunity to more effectively use prescribed burning for savanna restoration (Glitzenstein and Harcombe, 1988). However, the predicted increase in fire intensity was not observed in either the 1986 or 199 1 fires, probably because both these bums were conducted under restricted burning conditions. Furthermore, dead logs burned only in small areas and might not have contributed greatly to overall fire intensity over a large area. Thus, succession to hardwoods went largely unchecked in many plots. In plots with low densities of trees, pines regenerated well. The large increase in longleaf pine between 1990 and 1991 indicates that this important savanna species was indeed recovering from the tornado. Undoubtedly, this was partially due to prescribed fire. However, not all the plots have recovered to their pre-tornado locations in ordination space. This suggests that the combination of the tornado and the current prescribed burning program has not been sufficient to fully restore the longleaf that once dominated this savanna community. A more aggressive prescribed burning program may be necessary to reverse the overall trend towards hardwood dominance initiated by the 1983 tornado. A final conclusion to be drawn from this analysis is that wind
91 (19971279-289
damage is unlikely to play a critical role in converting vegetation from a fire-resistant to a fire-dependent state. Infrequent high-intensity fires are the more likely possibility.
Acknowledgements
We are grateful to Gary Cox and Linda C. Kaiser for their assistance in field work and to the Big Thicket National Preserve for conducting prescribed fires. We thank Dr. David D. Diamond of Texas Parks and Wildlife Department and two anonymous reviewers for reviewing the manuscript. Financial support was provided by a National Park Service grant SWRO7029-O-0006.
References Abrams, M.D. and Nowacki, G.J., 1992. Historical variation in fire, oak recruitment and post-logging acceierated succession in central Pennsylvania. Bull. Torrey Bot. Club, 119: 19-28. Basnet, K., Likens, G.E., Scatena, F.N. and Lugo, A.E., 1992. Hurricane Hugo: damage to a tropical rain forest. J. Trap. Ecol., 8: 41-55. Bomar, G.W., 1995. Texas Weather, 2nd edition. University of Texas Press, Austin, Texas, 275 pp. Bormann, F.H. and Likens, G.E., 1979. Catastrophic disturbance and the steady state in northern hardwood forests. Am. Sci., 67: 660-690. Boucher, D.H., 1990. Growing back after hurricanes. Bioscience, 40: 163-166. Boucher, D.H., Vandermeer, J.H., Yih, K. and Zamora, N., 1990 Contrasting hurricane damage in tropical rain forest and pine forests. Ecology, 71: 2022-2024. Bridges, E.L. and Orxell, S.L., 1989. Longleaf pine communities of the West Gulf Coastal Plain. Nat. Areas J., 9: 246-263. Brokaw, N.V.L. and Walker, L.R., 1991. Summary of the effects of Caribbean hurricanes on vegetation. Biotropica, 23: 442447. Brown, J.K., 1974. Handbook for Inventorying Downed Woody Material. USDA For. Serv. Gen. Tech. Rep. IN?‘-16, Intermountain Forest and Range Experiment Station, Ogden, Utah. 24 PP. Christensen, N.L., 1981. Fire regimes in southeastern ecosystems. In: H.L. Mooney (editor), F%e regimes and ecosystem properties. USDA For. Serv. Gen. Tech. Rep. QO-26, pp. 112- 136. Christensen, N.L., 1988. Vegetation of the southeastern Coastal Plain. In: M.G. Barbour and W.D. Billinga (Editors), North America Terrestrial Vegetation. Cambridge University Press. Cambridge, UK, pp. 317-364.
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Correll, D.S. and Johnston, M.C., 1970. Manual of the Vascular Plants of Texas. Texas Research Council, Austin, Texas, 1881 PP. Day, R.W. and Quinn, G.P., 1989. Comparisons of treatments after an analysis of variance in ecology. Ecol. Monogr., 59: 433-463. Deshotels, J.D., 1978. Soil Survey of Big Thicket National Preserve, Texas, USA. USDI National Park Service/USDA Soil Conservation Service/Texas Agricultural Experimental Station, College Station, Texas. EarthInfo, Inc., 1994. CD* National Climatic Data Center Summary of the Day. Boulder, Colorado. Foster, D.R., 1988a. Disturbance history, community organization and vegetation dynamics of the old-growth pisgah forest, south-western New Hampshire, USA. J. Ecol., 76: 105-134. Foster, D.R., 1988b. Species and stand response to catastrophic wind in central New England, USA. J. Ecol., 76: 135-151. Garren, K.H., 1943. Effects of fire on vegetation of the sontheastem United States. Bot. Rev., 9: 617-654. Glitzenstein, J.S. and Harcombe, P.A., 1988. Effects of the December 1983 tornado on forest vegetation of the Big Thicket, Southeast Texas, USA. For. Ecol. Manage., 25: 269-290. Gresham, C.A., Williams, T.M. and Lipscomb, D.J., 1991. Hurricane Hugo wind damage to southeastern U.S. coastal forest tree species. Biotropica, 23: 420-426. Komarek, E.V., 1974. Effects of tire on temperate forests and related ecosystems: southeastern United States. In: T.T. Kozlowski and C.E. Ahlgren (Editors), Fire and Ecosystems. Academic Press, New York, pp. 251-277. Marks, P.L. and Harcombe, P.A., 1981. Forest vegetation of the Big Thicket, southeast Texas. Ecol. Monogr., 51: 287-305. Martin. W.H.. Boyce, S.G. and Echtemacht, AC., 1993. Biodiversity of the Southeastern United States-Upland Terrestrial Communities. John Wiley and Sons Inc., New York, 373 pp. Myers, R.L. and Peroni, P.A., 1983. Approaches to determining aboriginal fire use and its impact on vegetation. Bull. Ecol. Sot. Am., 64: 217-218. Noble, I.R. and Slatyer, R.O., 1980. The use of vital attributes to predict successional changes in plant communities subject to recurrent disturbances. Vegetatio, 43: 5-21.
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Oliver, C.D., 1981. Forest development in North America following major disturbances. For. Ecol. Manage., 3: 153-168. Peterson, C.J. and Picket& S.T.A., 1991. Treefall and resprouting following catastrophic windthrow in an old-growth hemlockhardwoods forest. For. Ecol. Manage., 42: 205-217. Pickett, S.T.A. and White, P.S., 1985. The Ecology of Natural Disturbance and Patch Dynamics. Academic Press, Orlando, FL, 472 pp. Pickett, S.T.A., 1980. Non-equilibrium coexistence of plants. Bull. Torrey Bot. Club, 107: 238-248. Schwartz, M.W., 1994. Natural distribution and abundance of forest species and communities in northern Florida. Ecology, 75: 687-705. Shugart, H.H., 1984. A theory of forest dynamics. SpringerVerlag, New York, 278 pp. Streng, D.R. and Harcombe, P.A., 1982. Why don’t east Texas savannas grow up to forest? Am. Midl. Nat., 108: 278-294. ter Braak, C.J.F., 1987a. The analysis of vegetation-environment relationships by canonical correspondence analysis. Vegetatio, 69: 69-77. ter Braak, C.J.F., 1987b. CANOCO-A FORTRAN Program for Canonical Community Ordination. TN0 Institute of Applied Computer Science, Wageningen, The Netherlands, 95 pp. Trenchard, M.H., 1977. Baseline climatological data for the Big Thicket National Preserve. Report to the National Park Service, Big Thicket National Preserve, Beaumont, Texas. Wahlenburg, W.G., 1946. Longleaf Pine: Its Use, Ecology, Regeneration, Protection, Growth, and Management. Charles Latbrop Pack Forestry Foundation, Washington, DC, 429 pp. Walker, J. and Peet, R.K., 1983. Composition and species diversity of pine-wiregrass savannas of the Green Swamp, North Carolina. Vegetatio, 55: 163-179. Ware, S., Frost, C. and Doerr, P.D., 1993. Southern mixed hardwood forest: the former longleaf pine forest. In: William H. Martin, Stephen G. Boyce and Arthur C. Echtemacht (Editors), Biodiversity of the Southeastern United States. John Wiley and Sons Inc., New York, pp. 447-493. Williamson, B.G. and Black, E.M., 1981. High temperature of forest tires under pines as a selective advantage over oaks. Nature, 293: 643-644.
Ecology
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Tornado and fire effects on tree species composition in a savanna in the Big Thicket National Preserve, southeast Texas, USA Changxiang Liu a,* , Jeff S. Glitzenstein b, Paul A. Harcombe ‘, Robert G. Knox d a Endangered
Resources Branch Texas Parks and Wildlife Department 3000 IH35 South, Suite 100, Austin, TX 78704, b Tall Timbers Research Station, Route I, Box 678, Tallahassee, FL 32312, USA ’ Department of Ecology and Evolutionary Biology, Rice University Houston, TX 77251, USA ’ Biospheric Sciences Branch, NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA Accepted
8 December
USA
1995
Abstract Ordination showed that species composition in a savanna shifted toward mixed pine-hardwood types after the tornado damage in 1983. Of the twenty 250 m2 study plots, one baygall plot remained unchanged in its position in ordination space, as did five mixed pine-hardwood plots and six savanna plots. However, eight savanna plots moved from the savanna space to the mixed pine-hardwood space after the tornado. Prescribed bums in 1986 and 1991 had a modest effect in reversing this trend: only four of the eight plots returned to the savanna ordination space by 1991. The changes in species composition, summarized by movement of plots in ordination space, reflected the conflicting affects of the tornado and fire: the tornado tended to change savanna to mixed pine-hardwood by differentially removing pines and stimulating hardwood growth. Fires tended to reduce hardwood density. However, stand opening and increased fuel loads following the tornado did not result in fires intense enough to dramatically enhance savanna recovery. Keywords:
Prescribed
burning;
Ordination;
Disturbance;
Pinus palustris
1. Introduction Effects of disturbances on vegetation dynamics, species composition, and community structure have attracted much attention from plant ecologists (Bormann and Likens, 1979; Noble and Slatyer, 1980; Pickett, 1980; Oliver, 1981; Shugart, 1984; Pickett and White, 1985). In the southeastern United States, strong winds and fires have been important natural
* Corresponding author. Tel.: (512) 912-7052; 7058; E-mail: [email protected] 0378-l 127/97/$17.00 Copyright PII SO378-1 127(96)03705-X
0 1997 Elsevier
Fax: (512)
Science
912-
B.V.
mill.;
Vegetation
change
processes shaping vegetation patterns (Christensen, 1981, 1988; Glitzenstein and Harcombe, 1988; Basnet et al., 1992, Schwartz, 1994). Strong winds associated with hurricanes, thunderstorms, and tomadoes can result in a great deal of structural damage and high mortality of large trees (Foster, 1988a,b; Brokaw and Walker, 1991; Gresham et al., 1991). Plant species may also have different strategies for responding to damage, e.g. by seeding, sprouting, or regrowing. Some species can regrow even after severe damage and others may die immediately or die later as they become vulnerable to pests and diseases (Pickett and White, 1985). Most pines show limited
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reserved.
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sprouting following damage (Glitzenstein and Harcombe, 1988; Boucher, 1990; Boucher et al., 1990; Gresham et al., 1991; Peterson and Picket& 1991). However, fires, mostly low-intensity surface fires in the southeastern United States, are more likely to cause damage to small individuals. Species may also respond differently to fires. They have varying postfire survivorship depending on their resistance and post-fire response strategy. Along the Gulf coast, almost all plant communities are subjected to recurrent disturbances. One unique vegetation association among them is the pine savanna, which has a relatively open canopy and diverse herbaceous ground cover (Marks and Harcombe, 1981; Walker and Peet, 1983). Bridges and Orzell (1989) recognize many savanna communities in the west Gulf Coastal Plain. This association and its diverse herbaceous components are strongly influenced by frequent low-intensity tires (Christensen, 198 1; Marks and Harcombe, 198 1; Walker and Peet, 1983; Ware et al., 1993). Streng and Harcombe (1982) suggest that under some conditions. southeastern savannas can escape control by low-intensity fires and succeed to a more fire resistant vegetation type. With modem fire suppression, this phenomenon is widespread in the southeastern United States (Myers and Peroni, 1983; Martin et al., 1993; Ware et al., 1993). If habitats could escape from fire control naturally, then for savannas to have persisted in the presettlement landscape, there must have been natural mechanisms for converting closed forests back to savannas. One such mechanism might be fires intense enough to kill canopy trees, especially if followed by low-intensity surface fires. Another potential mechanism is stand opening by strong wind (tornadoes, strong hurricanes, or downbursts associated with violent thunderstorms) followed by surface fires. A fairly low-frequency crown disturbance might be adequate to maintain the savanna fraction in the landscape if savannas were normally kept open by frequent low-intensity surface fiies in the past. Disturbance of a well-studied savanna (Streng and Harcombe, 1982) by a tornado (Glitzenstein and Harcombe, 1988), followed by a series of prescribed burns, provided a valuable opportunity to address this possibility. Such a study may have implications for savanna restoration, since, if strong wind fol-
and Management
91 (19971279-289
lowed by fire is effective, the critical features required to achieve the same effect artificialy might be identified. For example, is an abundance of downed woody fuel essential to promote high-intensity surface fires or might the same effect on species composition result from canopy opening and an increase in fine fuel? Study of the interaction of wind disturbance and fire, in as near natural conditions as feasible, can, thus, provide information on effects of disturbance on compositional and structural changes in natural plant communities. 1.1. Study area and methods 1.1.1. Study area
The study area is located in the Big Thicket region of southeastern Texas. The climate of this area is humid subtropical with a long growing season. Rainfall is evenly distributed throughout the year. Average annual temperature is 19.X (30-year average at Port Arthur). Potential evaporation exceeds precipitation during most of the year except in the winter months (National Climatic Data Center data for Beaumont). Temperature and annual rainfall decline gradually from the southeast to the northwest (EarthInfo Inc., 1994). The Big Thicket area is underlain by the Beaumont, Montgomery, Bentley and Willis Pleistocene geologic formations from south to north (Geologic Atlas of Texas, Beaumont sheet, Bureau of Economic Geology, The University of Texas at Austin. 1968). Elevation increases from about 0 to 30m above sea level in the south to 60- 180m above sea level in the north (USGS quadrangle sheets, 7.5 min topographical series, provisional edition, 1984). The region is bounded by the Trinity River to the west and the Neches River to the east. Most of the area drains southeastward into the Neches River via Village Creek. Ultisols are predominant in the region and alfisols and entisols are important in parts of the landscape (Deshotels, 1978). Soil texture ranges from excessively drained sandy soil in the upland and sand ridges, to loamy and clayey soils in swamps and floodplains (Marks and Harcombe, 198 1). Like other areas of the Gulf Coastal Plain, the Big Thicket area is frequented by summer thunderstorms, tropical storms and hurricanes, which were a major source of lightning fires in the presettlement times.
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important role in maintaining this unique vegetation type (Christensen, 1981; Streng and Harcombe, 1982). Limited data collected from the Hickory Creek Unit indicated that fire was more frequent before 1967 than after (Glitzenstein and Harcombe, 1988) with fire intervals ranging from two to seven years from 1928 to 1967. No fire scars were found in sampled trees after that. Fig. 1. Map of study area showing the Hickory Tornado damaged area is shaded. Interval between lines is about 1.52m.
Creek Unit. two contour
Between 1871 and 1991, twenty-seven tropical storms and forty-three hurricanes made landfall along the Texas coast (Bomar, 1995). The observed tornado occurrence in Texas was about 181 per year from 1953 to 1991; the average density of tornadoes in Texas is about 12.95 per lOOOOkm* per year in Texas (Bomar, 1995). Glitzenstein and Harcombe (1988) calculated that the density of tornadoes in south Hardin County was about 5.79 tornadoes per 10 OOOkm* per year from data presented in Trenchard (1977). It was lower than the state average, but much higher than the national average (0.66 per 10 000 km* per year). The Hickory Creek Unit of the Big Thicket National Preserve (Fig. 1) consists of 270 ha of wetland pine savanna and upland forest. It is located in the central part of the Preserve on the Montgomery formation, and is about 40km north of Beaumont, Texas. The closed-canopy stands were derived from former savanna following tire suppression beginning in the early 1930s (Streng and Harcombe, 1982). The savanna overstory is characterized by scattered pines, usually longleaf (Pinus palustris Mill.), and loblolly (Pinus tuedu L.). Note that nomenclature for plant species follows Correll and Johnston (1970) and that for vegetation follows Marks and Harcombe (198 1). Other canopy species include sweetgum (Liquidumbar styraciflua L.), blackgum (Nyssa syluatica Marsh.), southern red oak (Quercus falcatu Michx.), shortleaf pine (Pinus echinatu Mill.), water oak (Quercus nigra L.), and sweet bay (Magnolia uirginiana L.). Common shrub species are wax-myrtle (Myrica cerifera L. and M. heterophylla Raf.) and yaupon (Ilex vomitoria Ait.). Previous studies revealed that fire and poor internal drainage play an
1.2. Field methods
On 10 December 1983, a large tornado touched down in the southwest comer of the Hickory Creek Unit resulting in a damaged area of 3 1.1 ha (Glitzenstein and Harcombe, 1988). To assessthe damage to vegetation and monitor vegetation change afterwards, twenty permanent 250m’ circular plots were established and sampled in 1985 at random distances along three parallel west-east transects traversing the disturbed area. Individuals over 2.0cm DBH (diameter at breast height, about 1.4 m above ground) were tagged, measured and identified within the 250m’ area of each plot. Individuals of 2-5 cm DBH were termed small trees and individuals larger than 5 cm DBH were called large trees. Seedlings were defined as individuals of tree species less than 50cm tall. Saplings were individuals of both tree and shrub species taller than 50cm but less than 2.0cm DBH. Saplings were subdivided into small saplings (taller than 50cm but shorter than 1.4 m) and large saplings (1.4m tall to 2.0cm DBH). Seedlings and saplings were censused along four perpendicular 8.92 m lines radiating outward from plot center. The direction of the first line was determined randomly. Along each line, seedlings were measured within a 50cm wide strip and seedlings within a 20cm wide strip along each line. Downed woody materials were tallied and shrub coverage (intercept lengths of shrubs) was measured along a 10m line on the first and the third lines for seedlings and saplings. The procedure of Brown (1974) was used to survey and calculate weight of woody materials. Fine fuel was collected in a 50cm X 50cm quadrat randomly placed within each plot. Fine fuel samples were sorted into needles, leaves, dead grasses, living materials, and others. Separated materials were ovendried at 70°C and weighed.
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One year after these data were first collected, in the winter of 1986, the tornado-damaged area was subjected to a prescribed fire. Another prescribed fire was conducted five years later in spring 199 1. To assess the effects of these fires, the permanent plots located in 1985 were resampled in the growing season after the 1991 fire. Since there were no direct measurements on 2-4.5 cm DBH individuals in 1983, the density of these individuals was extrapolated from 1985 data in the following manner. If the DBH of an individual in 1985 was less than 3 cm, then the DBH of this individual was assumed less than 2cm in 1983 and to have been excluded; otherwise the individual was treated as having been larger than 2cm DBH in 1983 and the same DBH in 1985 was retained in calculating basal area. This extrapolation might have introduced some bias for estimation of density and basal area in 1983. However, bias was probably not substantial because 1) small trees had little contribution to basal area, and 2) there were not many individuals less than 5cm DBH in these plots before the tornado. Fire intensity was measured by suspending one set of fire-sensitive tablets (Tempil, Big Three Industries, New Jersey, USA) 20cm above ground surface at the center of each plot. I .3. Analyses Data from fifteen of the 20 permanent plots were discussed in a 1988 paper (Glitzenstein and Harcombe, 1988). Five plots were excluded from that paper because these plots were only lightly damaged by the tornado. Data from all twenty plots are included in the current analysis. We sampled the tornado plots as part of a largescale study of fire effects on vegetation in the Big Thicket National Preserve and the nearby Roy E. Larsen Preserve of the Nature Conservancy of Texas. 302 10 X 10m2 fire study plots representing 63 stands were sampled for that study. To best classify vegetation in the tornado study plots, all prebum stands for the fire study as well as 24 stands studied by Marks and Harcombe (1981) in the late 1970s were included in a detrended correspondence analysis (DCA). After assigning vegetation types to tornado plots using ordination results from the large data set, a second DCA ordination was run to track vegetation
Yl f IYY7) 27Y-BY
changes over time using only the fire and tornado plots. Large and small trees were analyzed together for the tornado study plots owing to low numbers of small trees in the 1983 and 1985 data. Post-fire data were treated passively in the ordination so that changes could be referenced to the pre-fire vegetation patterns (see ter Braak, 1987a,b) By 1990-1991 large numbers of small trees had accumulated in some plots. Hence, a third ordination, including only these smaller stems, was run to describe compositional patterns of small trees. In addition to describing overall compositional changes with ordination, we also used paired t-tests to examine changes over time in abundance of individual species. Because of the large numbers of these tests, the type-1 error rate for each test was adjusted using the Dunn-Sidak method (Day and Quinn, 1989).
2. Results 2.1. Changes in tree populations 2.1.1. Impact oj’l983 tornado As a consequence of the tornado, tree (2 2 cm DBH) density and basal area declined substantially between 1983 and 1985 (t = 2.59, P < 0.01; r = 5.40, P < 0.0005; Table 11. Basal area was reduced by 61.1%, though only 22.2% of the trees died between 1983 and 1985. High mortality caused by the tornado was concentrated in large trees (Fig. 2). However, the reduction in pine density (32.6%) was much greater than the loss of hardwoods (19.5%). All three pine species declined substantially as a result of the tornado (Fig. 3). However, effects on hardwoods were variable. Two species, sweegum and blackgum, were relatively unaffected by the tornado. One species which declined greatly was sycamore (Plantanus occidentalis L.). Prior to the tornado, this species was represented by a few large individuals which contributed only 8% of the density, but 24% of pre-tornado basal area. These large individuals were killed by the storm, and no trees of this species occurred in the post-tornado forest. A number of other hardwood species also declined significantly following the tornado. Southern red oak, water oak and dogwood (Comus florida L.1 were
C. Liu et al. /Forest Table 1 Percentage
composition
of density
Species
and Management
and basal area of stems larger
91 (1997)
1983
Total Percentage Total a are stems per hectare
100 984 for density
283
Basal Area 1985
17.5 11.4 2.0 7.3 16.9 2.8 2.4 4.1 3.9 4.9 3.0 23.8
279-289
than 2 cm DBH
Density
Liquidambar styracijlua L. Nyssa syhatica Marsh. Pinus echinata Mill. Pinus palustris Mill. Pinus taeda L. Quercusfalcata Michx. Quercus nigra L. Comusflorida L. Ilex uomitoria Ait. Magnolia cirginiana L. Symplocos tinctoria CL.) L’Her. Others
a Units
Ecology
19.3 16.7 1.0 5.5 16.2 2.1 1.6 3.1 5.0 5.0 1.3 23.2 100 766
and m* ha-’
1990 15.2 8.5 0.1 3.5 10.2 4.3 7.1 10.2 11.1 3.6 0.9 25.2 100 1548
1991
1983
13.8 7.1 0.1 6.0 12.5 3.2 9.4 11.2 11.0 2.5 1.4 21.2 100 1604
1985
1990
1991
6.8 4.4 5.9 21.2 28.1 1.3 1.1 0.7 0.4 0.6 0.5 28.9
16.6 11.4 1.4 16.3 36.6 2.5 1.3 1.1 0.8 1.5 0.4 10.2
11.1 12.8 0.9 18.4 39.2 1.3 1.0 1.3 1.5 1.3 0.1 11.2
10.8 12.7 1.1 19.0 40.5 1.5 1.9 1.8 1.7 1.2 0.1 7.6
100 20.48
100 1.96
100 8.58
100 8.83
for basal area.
each reduced in density by more than 40%. Water oak and dogwood also experienced large decreases in basal area (55% and 40%, respectively). However, these three species made up only a small portion of the tree population (Table 1). Before the tornado, the ranking of overstory species by basal area was loblolly > longleaf > sweetgum > shortleaf > blackgum. Loblolly pine remained as the dominant species (in basal area) after the tornado. The ranks of the other species changed as follows: sweetgum > longleaf > blackgum > southern red oak. This changed order reflected differing amounts of mortality caused by the tornado. 2.1.2. 1985-1990 (post-tornado trends) Damaged trees (including broken, pinned, top dead, and uprooted trees and trees with major crown damage) continued to die between 1985 and 1990 (Fig. 21, and damaged trees alive in 1985 had significantly lower survivorship than trees without major damage (chi-squared test, P < 0.001): about 60% of the 144 damaged trees alive in 1985 died between 1985 and 1990 compared to only 34% of the 273 trees without major damage that died in the same period. Continued high mortality of damaged trees in the half decade following the tornado may well have
0
IO
8 i
20
30
40
50
60
30
40
50
60
30
40
50
60
died
during
19851990
-1
0
IO
20
0
10
20
DBH (cm)
Fig. 2. Size distribution 1983-1985, 1985-1990,
of trees that and 1990-1991.
the periods
284
C. Liu et al. /Forest
--------
Ecology
and Management
91 (1997)
279-289
-----F==-= ‘\
\---
-----._______------_-----
63
85
86
87
88
89
90
91
83
85
86
87
68
89
90
91
33
85
86
87
88
89
90
91
83
85
86
87
88
89
90
91
year
year
Fig. 3. Changes in basal area (top panels) and density (bottom panels) of trees (larger than 2 cm DBH) of major species. The values for years from 1986 to 1989 are interpolated from the data of 1985 and 1990 to show the trend of change over time. Note that the y axes of the two top panels are on different scales.
been a consequence, at least in part, of increased susceptibility of these trees to the 1986 fire. Densities of hardwoods increased dramatically following the tornado (Fig. 4). By 1990, over 50% of the trees were less than 1Ocm DBH and tree density was already significantly higher than before the tor-
nado (t = 2.91; P < 0.005). Yaupon, dogwood, water oak, and southern red oak more than tripled in density, although they only made up a small portion of the total basal area (Table 1). In contrast to the other hardwoods, blackgum and sweetgum did not experience large increases in density. However, ow-
8 -1
1983
1985
8
1990
0
10
20
30
4.0
50
0
60
DBH (cm)
Fig. 4. Size distribution
of trees alive (2 2cm
10
20
30
40
DBH (cm) DBH)
in 1983, 1985, 1990, and 1991.
Xl
60
C. Liu et al. /Forest
Ecology
and Management
ing to continued low rates of mortality, these species maintained high levels of basal area and continued as codominants with Ioblolly pine. By 1991, densities of longleaf and loblolly pines also exceeded pretornado levels though the density of shortleaf pine was still far below the 1983 level.
of seedling,
sapling,
Species
and small tree populations Seedlings 1985 a
Shrub/small tree Myrica cerifera L./ M. Heterophylla Raf. Magnolia oirginiana L. Cornusflorida L. Symplocos tinctoria (L.) L’Her. Persea borbonia CL.1 Spreng. Ilex opaca Ait. Sassafras albidum (Nutt.) Nees. Ilex Llomitoria Ait. Tree Pinus taeda L. Pinus palustris Mill. Pinus echinata Mill. Liquidambar styraciflua L. Nyssa sylvatica Marsh. Quercus falcata Michx. Quercus nigra L. others Total * from
Glitzenstein
and Harcombe
1991
9807
na
3176 1121 560 1588 187 373 2054 6351 187 187 1494 1027 93 280 2709
(1988);
per hectare)
Small saplings 1990
31194
(stems
Large
saplings
1985 a
1990
1991
1985 a
na
2559
696
157
71 16 na na 5 na na
55 21 na na 4 na na
1065 280 579 467 93 280 3867
205 36 244 75 14 23 319
59 14 0 17 23 1 3 6
27 10 0 16 36 2 6 4
37 0 0 915 93 37 187 953
114 25 0 155 11 16 27 280
na
na
b estimated
11412 from
285
pine and water oak increased significantly ( P < .Ol). Also, longleaf pines began to emerge from the grass stage in large numbers (Table 2). These young pines probably became established when the tornado opened up the overstory. Establishment of pines, especially longleaf pine, was found only in open places where other species did not form a dense thicket. In summary, two species, shortleaf pine and sycamore, were negatively affected by the tornado and failed to recover. Two dominant species, blackgum and sweetgum remained relatively unchanged throughout the entire period. All other major tree species declined following the tornado and then increased afterwards. There was little indication that fires in 1986 and 1991 had slowed post-tornado recovery of any species. Loblolly pine was the most abundant species in number of seedlings in 1985. However, there were virtually no pine saplings (Table 2). In contrast, several hardwood species had already developed substantial sapling and seedling populations by sprouting or seeding. By 1991, seedling numbers for
2.1.3. 1991 jire The fire was cool and patchy. Only six of the twenty plots burned completely, six burned partially, and the remainder did not bum at all. Measured fire temperatures were all less than 253°C compared to a temperature range of 153-500°C in sandhill and upland pine forests measured in other parts of the Big Thicket National Preserve. Most of the 110 trees that died (i.e. were topkilled) during the 1991 fire were less than 5.0 cm DBH (Fig. 4). Despite the loss of some individuals, overall density was not significantly affected by this low-intensity fire (t = 0.21 and P = 0.84 for small trees). Contrary to the expected declines, some species actually increased in basal area and density after the fire. For example, basal areas of longleaf Table 2 Composition
91 (19973 279-289
2241
1985 data.
Small trees
1990
1991
0
48
36
4
4
0
0
148 23 150 34 0 0 95
336 374 93 93 37 56 990
71 52 48 37 7 16 457
14 55 46 20 0 11 432
28 10 8 6 4 4 16
36 10 10 12 4 4 28
40 152 14 128 20 12 160
26 172 22 104 14 16 164
52 21 0 120 2 14 21 84
0 0 0 336 19 0 131 75
18 7 0 102 4 11 16 318
21 9 0 137 0 2 34 241
30 12 0 42 30 0 6 68
40 14 0 54 46 2 6 120
64 16 0 174 44 56 106 182
82 58 0 162 36 36 136 156
921
2540
1211
1058
1983 b
268
1985
390
1990
1168
1991
1184
286
C. Liu et al./
Forest
Ecology
and Management
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all species combined had declined to about 10% of the 1985 level. Saplings and small trees of pines began to appear in 1990. By 1991, they made up 5-25% of the sapling and small tree populations in the study plots. However, the fire had little overall effect on the sapling population (t = 1.10; P =: 0.285). The most common shrub species were yaupon and wax-myrtle. Shrub coverage was significantly higher in 1991 than in 1990 (t = 2.78; P = 0.013). This is largely a result of rapid resprouting of shrubs after the fire.
w E N’8 5, 5: a
2.2. Ordination analysis of vegetation change 3
50
100
150 AXIS 1
200
250
3ocl
Fig. 5. Vegetation types of twenty tornado study plots (pre-tornado) on an ordination diagram of all fire stands from the Big Thicket based on tree density. Tornado plots were circled: there were one baygall plot, five mid-slope plots, and 14 savanna plots in 1983.
The ordination based on tree density showed that the vegetation in the tornado-damaged area was not uniformly savanna prior to the tornado (Fig. 5). Of the twenty plots, fourteen were classified as w&land pine savanna (savanna>, five were mid-slope mixed
a
b
Savanna ‘.., ‘..,
.i.,
Savanna c
Y----k /
“I,. /’
100
c
150
200
250
d
Fig. 6. Movement of Hickory Creek tornado plots on an ordination diagram from 1983 to 1991 based on tree density: a. plots remained in their types (one baygall plot, three mid-slope plots, and six savanna plots); b. plots changed from mid-slope to savanna then back to mid-slope (n = 2); c. plots changed from savanna to mid-slope then back to savanna (n = 4); d. plots changed from savanna to mid-slope and remained mid-slope (n = 4).
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2’ Baygall Sandhill
:
Fig. 7. Movement DBH).
of the Hickory
Creek tornado
plots between
LOwerslOpe
1990 and 1991 on an ordination
hardwood-pine type (mid-slope), and one was wetland baygall shrub thicket (baygall). In the following eight years, mid-slope and baygall plots generally remained within their original community types in the ordination space (Fig. 6a). An exception was two mid-slope plots which became savanna in 1985 but had changed back to mid-slope by 1990 (Fig. 6b). This temporary change was probably due to reduced tree density in these two plots after the tornado rather than a true compositional change. All tornado plots showed movement toward mid-slope after the tornado, but changes after 1985 were not unidirectional toward any particular type. Six of the 14 original savanna plots remained as savanna during the entire period. Type changes occurred in the other eight savanna plots (Fig. 6c and d). One of these became a mid-slope plot between 1990 and 1991. Seven plots moved from savanna to mid-slope between 1983 and 1985; three of these changed back to savanna after 1985, while four stayed as mid-slope (Fig. 6~). However, three of these four plots, which stayed as mid-slope, showed some movement towards savanna after the 1991 fire. Patchiness of the 1991 fire helps to explain the ordination results discussed above: nine of the ten savanna plots burned completely or partially during this fire; only three of the nine mid-slope plots
diagram
based on small tree density
(2-5 cm
burned; the single baygall plot did not bum. This bum pattern reflects the moisture gradient from savanna to baygall. Savanna plots were more open and relatively drier than mid-slope and baygall plots. The ordination based only on small tree density (2-5cm DBH) indicated that eight of the twenty plots (four savanna plots and four mid-slope plots) moved toward savanna between 1990 and 1991 (Fig. 7), two mid-slope plots moved toward upper-slope, and the others changed little. As discussed above, this same trend was also apparent in the ordination of all trees, though not so obviously (Fig. 6). Movement toward savanna demonstrates some recovery of this community type, at least in the small trees. In part, this was due to fire-induced hardwood mortality. However, growth of pines (longleaf and loblolly) to larger size classes also contributed to a shift towards pine-dominated vegetation.
3. Discussion Tornado-related mortality was limited mostly to large trees. Many small trees managed to survive this severe canopy disturbance. In this sense, the tornado was an intermediate disturbance rather than a catastrophic event. As in other forests (Abrams and
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Nowacki, 19921, the effect of intermediate disturbance in the Hickory Creek Unit was to accelerate succession towards more shade-tolerant but less fire-tolerant overstory species. Surface fires are more frequently occurring and less severe disturbances than tornadoes. In the southeastern United States, natural and prescribed fires are generally of low intensity and restricted to the ground surface. Such fires largely affect small individuals. Generally, pines are more resistant to low-intensity fires than are other species of trees (Garren, 1943, Williamson and Black, 1981). Fire top-kills hardwoods and shrubs and enhances regeneration of pines (Wahlenburg, 1946). In this sense, fire acts to reverse successional trends stimulated by tornadoes and hurricanes. Pine savanna is a fire-dependent type (Komarek, 1974; Christensen, 198 1; Marks and Harcombe, 1981). Streng and Harcombe (1982) predicted that this community should succeed toward closed-canopy forest in the absence of fire and that flammability would decline as this trend continued. The open canopy and increase in fuel created by the 1983 tornado presented an opportunity to more effectively use prescribed burning for savanna restoration (Glitzenstein and Harcombe, 1988). However, the predicted increase in fire intensity was not observed in either the 1986 or 199 1 fires, probably because both these bums were conducted under restricted burning conditions. Furthermore, dead logs burned only in small areas and might not have contributed greatly to overall fire intensity over a large area. Thus, succession to hardwoods went largely unchecked in many plots. In plots with low densities of trees, pines regenerated well. The large increase in longleaf pine between 1990 and 1991 indicates that this important savanna species was indeed recovering from the tornado. Undoubtedly, this was partially due to prescribed fire. However, not all the plots have recovered to their pre-tornado locations in ordination space. This suggests that the combination of the tornado and the current prescribed burning program has not been sufficient to fully restore the longleaf that once dominated this savanna community. A more aggressive prescribed burning program may be necessary to reverse the overall trend towards hardwood dominance initiated by the 1983 tornado. A final conclusion to be drawn from this analysis is that wind
91 (19971279-289
damage is unlikely to play a critical role in converting vegetation from a fire-resistant to a fire-dependent state. Infrequent high-intensity fires are the more likely possibility.
Acknowledgements
We are grateful to Gary Cox and Linda C. Kaiser for their assistance in field work and to the Big Thicket National Preserve for conducting prescribed fires. We thank Dr. David D. Diamond of Texas Parks and Wildlife Department and two anonymous reviewers for reviewing the manuscript. Financial support was provided by a National Park Service grant SWRO7029-O-0006.
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