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EFFECTS OF COOL SEASON PRESCRIBED FIRE ON UNDERSTORY VEGETATION IN A MIXED PINE HARDWOOD FOREST OF EAST TEXAS.

Abstract.--Response of understory woody and herbaceous vegetation to late winter tire was evaluated in an upland Pinus taeda L. (loblolly pine) mixed hardwood forest within Mission Tejas State Park in east Texas. Understory woody plants, which regenerated following the first tire were hardwood species less than 3 m tall. Pine regeneration was scarce. After a second late winter fire, all understory woody stems decreased in number. After two growing seasons following the second fire, more stems were present than pre-treatment. Regeneration was primarily hardwoods sprouting from perennial rootstocks. Also, woody vine and herbaceous cover increased during the two growing season post-tire period. Little change in composition and numbers of species occurred in the control plots during the study period. Analyses revealed a decrease in hardwood stems and a significant decrease in pine. Low pine regeneration may have been due to drought, poor seed crop and hardwood competition during the study. Results suggest that i nfrequent cool season fire favors hardwood regeneration in upland pine/hardwood communities.

Resumen.--El resultado de la existencia de vegetacion herbacea y de especies de arboles del ultimo incendio de invierno fue evaluado en un bosque de tierras elevadas poblado por una variedad de pinos (Pinus taeda L.) y por especies latifoliadas dentro de 'Mission Texas State Park' al este de Texas. Las especies de arbustos, las cuales se regeneraron despues del primer incendio, fueron especies de madera menos de 3m de altura. La regeneracion de pinos fue escasa. Despues de un segundo incendio de invierno, todos los tallos de arbustos (tallos de especies latifoliadas) decrecieron en numero. Despues de dos estaciones de crecimiento a continuacion del segundo incendio, mas tallos estuvieron presentes que en el pre-tratamiento. La regeneracion fue primordialmente de brotes de especies latitoliadas de raices existentes y perennes. Tambien, los troncos de vid (tallos de vid) y cubiertas de herbaceos incrementaron durante las dos estaciones de crecimiento despues del periodo del incendio. Hubo un poco de cambio en la composicion y el numero de especies en las parcelas de control durante el periodo de estudio. Los analisis revelaron una significante disminucon en tallos de especies latifoliadas con una disminucion en Los pinos. La baja regeneracion de pinos se pudo haber debido a la sequfa, pocas semillas de cultivo y la competencia de las especies latifoliadas durante el estudio. Los resultados sugieren que la infrecuencia de las estaciones de incendio favorece la regeneracion de arboles (o especies latifoliadas) en comunidades mixtas de pinos y especies latifoliadas.

As ecosystems evolve, certain plant species have acquired structural and physiological traits which enable them to survive fire. Species in fire adapted plant communities may become dependent on periodic fire for survival (Reeves 1970; 1973). Mutch (1970) hypothesized that certain plant communities evolve in repeatedly burned areas to become more flammable to maintain themselves. Pinus sp. communities in the western gulf coastal plain of the US are one such example. Southern pines are pioneer species which are typically shade intolerant and require large scale disturbance such as fire for their perpetuation (Buckner 1989). Fire plays an important role in arresting hardwood invasion that competes with light, nutrient and moisture relations (Edwards 1987; Nixon & Ward 1986). However, mixed pine/hardwood stands in the South is a forest type that develops naturally, as a result of successional forces, usually in the absence of disturbance such as fire (Cooper 1989). The pine component may be lost if fire is excl uded. Upland oak communities also suffer from fire exclusion, often becoming mixed hardwood stands with diminished oak populations and decreased oak regeneration (Van Lear & Watt 1993; Brose et al. 1999).

Komarek (1965) suggested that natural fuel accumulations in the understory would allow summer fires caused by lightning to burn sufficiently to maintain pine/grassland communities against encroachment of hardwoods. Fire resistant adaptations in the assemblage of many southern plant species include deep fibrous roots, below ground or soil surface apical meristems, and underground vegetative reproductive organs which account for overall resistance to fire injury (Owesnby 1991). Woody species such as pines have thick bark, which serves as protection for xylem and phloem tissues near the tree base, and reproductive terminal buds which may be protected in a needle rosette and resistant to heat damage (Wade & Johansen 1989; Reeves & Corbin 1985). Root sprouting following top removal in hardwoods is a fire resistant adaptation. Quercus sp. (oaks) are prolific sprouters following fire and may have an advantage where fire is periodic in upland communities (Van Lear & Watt 1993). Grano (1970) studied the effect of con trolled burning on understory hardwoods in a P. taeda L./P. echinata Mill. (loblolly/shortleaf pine) forest and showed that following a single winter burn most of the hardwood stems were killed, but shortly thereafter sprouted abundantly.

In natural fire adapted communities, fire exclusion results in the buildup of hazardous fuels, and poses the threat of wildfire which may cause irreparable damage to both the plant and animal community as a whole (VanLear & Waldrop 1989). Therefore it may be desirable to manipulate vegetation through the use of prescribed fire to arrest successional trends for the benefit of a variety of resources such as timber, grazing, recreation or the restoration of natural areas (Lewis & Harshbarger 1976).

A prescribed fire can be anthropogenic or occur naturally under certain circumstances in wildland areas. The Society of American Foresters defines prescribed burning as the "controlled application of fire to wildland fuels in either their modified or natural state, under specified environmental conditions which allow the fire to be confined to a predetermined area and at a time to produce the fire line intensity and a rate of spread required to attain planned resource management objectives" (McPherson et al. 1990). Traditionally a prescribed fire was one intentionally ignited by humans for specific management objectives. Currently, naturally occurring fires resulting from ignition by lightning in a designated area, usually a wilderness, are also called prescribed natural fire.

In the fall of 1991, a prescribed burning experiment was initiated at Mission Tejas State Historical Park through a cooperative arrangement between Texas Parks and Wildlife Department and the Arthur Temple College of Forestry at Stephen F. Austin State University. The objectives of the study were to (1) provide a detailed description of both the woody and herbaceous vegetation before and after treatment by fire, (2) determine changes of the number of woody stems before and after treatment by fire and (3) evaluate fire effects on hardwood and pine regeneration.

STUDY SITE

The study site lies within Mission Tejas State Historical Park, Houston County, which is approximately 69 km west of Nacogdoches, Texas. Percent slope ranged from 30% to nearly level ground. Plots were all located within the Trawick soil series (kaolinitic, thermic Mollic Hapludalf) which consist of deep, well drained, moderately to slowly permeable soils that formed in thick marine sediments.

The park's forested areas consisted of a canopy of P. taeda with an emerging midstory of mixed hardwood. The pine overstory basal area ranged from 18.4 to 27.5 [m.sup.2] throughout the study area. The forest was representative of the prominent P. taeda L./P. echinata cover type that is widespread throughout the southern US [SAF cover type 80 (Mann 1980)]. The understory vegetation was dense, consisting of a variety of pine seedlings, hardwoods, vines and several species of grasses and forbs.

METHODS

In 1991, an area within the park was designated for burning treatment, while adjacent areas served as a control. Twenty (6.36 [m.sup.2]) circular vegetation plots were randomly selected and permanently established. Ten plots were designated for treatment and ten served as a control. Burned and unburned sections were equally representative of the vegetation types in the study area.

Late winter burns (early March) in 1992 and 1993 were conducted according to a fire plan setting general parameters (after Wade & Lunsford 1989) of damp soil, 30% to 55% relative humidity, temperature less than 60 degrees Fahrenheit, and light but directionally steady wind. Prescribed fires were accomplished using strip-heading fire technique (Wade & Lunsford 1989) generally, with flanking fires used on steep slopes. A deep layer of leaf litter (mainly pine needles) was the dominant fuel. Pine cones and small to medium sized downed woody material also were interspersed. Fire lines were hand-constructed to separate burn and control plots, and to protect historic structures.

A woody stem survey was conducted before the initial fire of 1992. Post-fire inventories were performed in July of 1992, 1993 and 1994. The 1994 growing season data included woody vegetation, as well as a descriptive inventory of the vine and herbaceous components. A total of four sampling surveys occurred.

Following the second prescribed fire, subplots were nested within each 6.36 [m.sup.2] plot for the purpose of sampling woody vine and herbaceous species. Two (0.2 [m.sup.2]) woody vine plots were arranged along the east and west boundaries of the larger plot, and four (0.1 [m.sup.2]) plots were established in points of all cardinal directions and permanently marked.

All woody species were identified, their stems counted and heights measured. All live woody vegetation at three meters in height or less was inventoried. In addition, for the vine and herbaceous plots, percent coverage of plants present on the forest floor was estimated. The percent coverage for the vine and herbaceous groups were placed in 0-33%, 34-66% or 67-100% classes. Nomenclature follows Nixon (1985) and Correll & Johnston (1970).

The relative frequency (frequency of a species/frequency of all species x 100), density (number of individuals of a species/total number of individuals x 100), and importance values (relative frequency + relative density) for each species of woody vegetation were determined for all plots (Kent & Coker 1992). A repeated measures design was utilized and an analysis of variance (ANOVA) was performed to test the effects of prescribed burning on the number of woody stems in the understory (SAS, 1990). Two sets of ANOVA's were performed ([alpha] = 0.05). Stems for both pine and hardwood stems were tallied and pooled within each treatment type (i.e., control, burn). These stems were separated on their regenerative characteristics. Pine regeneration was primarily from seed, while all hardwood regeneration was from both seed and resprouting from rootstocks. For burn plots in particular, hardwood regeneration was mostly resprouts. For the purpose of the analysis, treatment and the year sampling occurred were the main variables of interest. The response variables were number of pine and hardwood stems in the understory. A Duncan's multiple range test was applied to separate the means (Duncan 1955).

RESULTS

Effect of fire on species composition.--Changes in species composition for woody seedlings and shrubs were analyzed by making year to year comparisons in burn and control plots. Following the initial prescribed fire conducted late winter of 1992, the treatment plots had a substantial change relative to control. Prior to burning, P. taeda made up 45% of the importance value, but essentially disappeared after the first burn. In addition, Cercis canadensis L. (eastern redbud) and Ilex vomitoria Ait. (yaupon holly) increased in importance value, while Quercus falcata Michx. (southern red oak) and Viburnum rifidulum Raf. (rusty blackhaw) greatly decreased (Table 1). The post-fire inventory showed an overall increase in the density of woody species by nearly 65%. The control plots following treatment had a 58% increase in the number of species, with a dominant presence of P. taeda (Table 1).

After the second late winter burn conducted in 1993, understory vegetation was surveyed and compared with post-burn 1992 results. For the treatment plots following the second fire, composition of the understory vegetation changed significantly. While Q. falcata, I. vomitoria, and C. canadensis comprised nearly 68% of the importance value in 1992, post-fire analysis revealed that Sassafras albidum Nutt. Nees. (sassafras), Rhus coppalina L. (sumac) and V. rufidulum increased, representing 66% of the importance value. Total number of species in the treatment plots from 1992 to 1993 decreased by 15% (Table 1). A comparison of the post-fire inventories for the control plots from 1992 to 1993 revealed that while P. taeda remained dominant both years, Q. falcata representing 22.5% of the importance value in 1992, was overshadowed by I. vomitoria and Liquidambar styraciflua L. (sweetgum) in 1993 (Table 1).

A 1993 and 1994 growing season comparison for all plots was conducted. Results for the burned plots in 1993 showed that S. albidum and R. coppalina represented a majority of the importance value. However, by 1994, the dominant species present were L. styraciflua and Calli carpa americana L. (American beautyberry) making up 45% of the importance value. Total number of species observed from 1993 to 1994 increased by 25%, but the control plots showed little change. P. taeda continued to be dominant; whereas, L. styraciflua which held 10% of the importance value in 1993, was exceeded by Q. falcata. Total number of species observed in the control plots from 1993 to 1994 showed an increase of only seven percent (Table 1).

A two year post-burn survey of woody vines for the treatment and control plots were similar. Toxicodendron radicans (L.) Kuntze (Poison ivy) and Parthenocissus quinquefolia L. (Virginia creeper) were the dominant vines. Percent coverage by vines and herbs increased substantially in burn plots when compared to control (Fig. 1). Percent cover for herbaceous plants increased significantly in the second growing season following fire as expected from previous research (Kush et al. 1999; Waldrop et al. 1987; Reeves & Halls 1973), and responses were greater for the burn plots than control (Fig 1).

Effect of treatment on height classes and number of woody stems. -- Total number of stems 1 m or less increased from 33% to 87% in the treatment plots, but stems in the 1-2 m height class decreased two fold. Control plots consisted mostly of stems 1 m or less. There was a decline in the number of stems for all plots following the second burn. Stems 1 m or less in the treatment plots decreased (88%) compared to initial burn results. In addition, stems 1-2 m decreased, while a 17.5% increase was observed for the 2-3 m height class. For control plots, stems 1 m or less decreased nearly nine fold from 1992 to 1993. Stems in the 1-2 m class declined as well, while the 2-3 m class increased by 35%.

The 1994 inventory indicated resurgence in the numbers of hardwood stems for all plots. Total number of stems present in the burned plots doubled compared to 1993 data. Stems less than 1 m made up 52% of the total number of stems present. Control plots had a uniform distribution of stems for all height classes.

Analysis of the number of pine stems revealed statistically differences in the year sampling occurred (P=0.0051), and treatment effect on number of stems present, P=0.0305 (Table 2). Repeated fire substantially reduced pine stem numbers compared with both the pre-burn survey and corresponding control plots. Preliminary inventories of pine stems taken in 1992 differed significantly from post-fire 1992, 1993 and 1994 (Table 3). Pre-burn counts in 1992 had the highest number of understory pine stems observed (35) for all plots sampled.

Sampling year (P= 0.0001) and treatment effect (P=0.0002) were significant for hardwood stems (Table 2). Interaction between the variables "sampling year" and "treatment" was found to be significant (P= 0.0010). Duncan's multiple range test for the interaction effect was performed (Table 4). It was found that 1992 post-burn data for treatment plots had the highest (146) number of stems, and differed significantly from the 1992 post-burn data for control. There was no significant difference between burn and control plots for the 1992 pre-burn, and 1993 inventories. The lowest number of hardwood stems found in the understory were found in the control plots during the 1993 and 1994 vegetation surveys (Table 4).

DISCUSSION

Effect of fire on composition.--Throughout the experiment, no single species consistently dominated the treatment or control plots. The burn plots had the greatest change over time. The prescribed fire executed in the late winter of 1992 had a significant effect on the burn plots. The burn eradicated nearly all pine seedlings, and hardwood stems became prominent. Young pine seedlings lack the thermal properties necessary to survive direct exposure to fire (Haywood et al. 1998; Wade & Johansen 1989; Westveld 1949). This is best illustrated with comparisons of importance values for species between the treatment and control plots following the initial fire in 1992 (Table 1).

The overwhelming presence of understory hardwood stems in the treatment plots is evidence that hardwood species are favored by late winter burning. By one growing season later, numbers of hardwood stems increased more than four times the number of stems prior to the initial fire. Greater than 90% of the stem counts of hardwoods were derived as re-sprouts from perennial rootstocks.

Late winter prescribed burns are effective in top-killing hardwoods up to three inches in ground diameter (Langdon 1981). However, there appears to be little impact on the rootstocks, and resprouting of top killed hardwoods will generally occur (Boyer 1993; Van Lear & Waldrop 1989). The mix of hardwoods following the fire relative to pre-burn data, suggests variable tolerance and re-sprouting vigor in certain species (Boyer 1993). Studies suggest that following fire, rapid re-sprouting responses of understory hardwoods and shrubs are probably necessary to compete with fast growing perennial grasses and forbs for transitory nutrients and space (Olsen 1992).

Treatment plots had a contrast of woody vine and herbaceous vegetation occurrences relative to control (Fig. 1). One can easily see that fire favors herbaceous and vine species. There were significantly more herbaceous species in the burned versus control plots. Rhizomatous species whose reproductive organs are below ground or protected from fire, can withstand the disturbance of periodic prescribed fire (Owensby 1991). Growth following fire as well as the opening of the stand to sunlight are obvious components in the facilitation and establishment of understory vegetation. However, for germination, light seeded species disseminated by wind often favor exposed mineral soil surfaces created by fire (Smith 1986).

The results of the herbaceous data are congruent with findings from a similar study performed in a southern Piedmont loblolly stand, where season of burn response to herbaceous plants were examined. Herbaceous plants were more abundant in burned versus unburned plots regardless of season of burn (Cushwa et al. 1966).

Besides relative importance values for herbaceous vegetation observed, a comparison of percent cover (Fig. 1) illustrates the encouragement of understory growth following prescribed fire. A combination of increased shading as well as prominent litter layer in the control plots are perhaps the more notable factors in the discouragement of herbaceous vegetation establishment.

Effect of treatment on height classes and number of woody sterns.--The first fire caused prolific resprouting of hardwoods and a higher abundance of stems 1 m or less, resulting from the top killing of these species in the taller classes. Hofstetter (1974), in a study of prescribed fire in the southern Florida pinelands, found that most perennial shrubs and trees produced more stems than were present prior to fire; and, the predominance of those stems were found in the smaller height classes. In this study however, the second fire reduced the proportion of stems shorter than 2 m, leaving a relatively higher proportion of stems in the 2-3 m height class. Taller stems are generally larger in diameter, and are better able to survive fire due to thicker bark.

Quercus falcata was the most aggressive resprouter from the first fire, but was top killed after the second burn. However, L. styraciflua retained overall dominance, surpassing 1992 pre-burn results of stem numbers for all height classes. Some research suggests that in the absence of other prolific root sprouters, oaks would gradually dominate the advance regeneration pool (Van Lear & Watt 1993; Waldrop et al. 1987). Brose et al. (1999) found that upland oaks would dominate a stand treated with an intense spring fire. Other combinations of season and fire intensity produced mixed hardwood stands with a highly variable oak presence.

Following the second late winter burn, there was a remarkable decline in the number of species in the understory. It is reasonable to assume that a second burn would contribute to the loss of pine and hardwood components, since one growing season would fall short in the amount of time needed for recovery to pre-burn levels. In the burn plots where the pine component was already significantly reduced, number of stems decreased by nearly five fold when compared to pre-burn numbers. The decline in such numbers may be attributed to a number of factors. It is possible that a poor seed year coupled with a winter fire, which would consume regeneration, could contribute to these low numbers. Season of burn may play a role in the regeneration of P. taeda and P. echinata. Brender & Cooper (1968) recommended prescribed burning the growing season immediately preceding an expected seed crop as the ideal time for enhancing the regeneration of loblolly. A drought occurred in the growing season immediately preceding the sec ond burn, and may have impeded growth on all plots in the study area. In 1993, the average rainfall in Houston County was less than 2.54 cm in July and August (N.C.D.C. 1993). Evidence of mortality in the control plots was exemplified by decreases in both pine and hardwood stems. Newly germinated pine seedlings suffer from any extended dry period and probably dried up before sampling was conducted in July 1993. Silker (1957), in a study of the effect of prescribed burning on the understory vegetation of a P. taedal P. echinata forest of east Texas, found an appreciable reduction in the amount of hardwood regeneration when drought occurred during the growing season following fire occurrence.

Two growing seasons following the second prescribed burn, hardwood regeneration had surpassed pre-burn levels for all height classes involved. These results suggest that growing seasons with adequate precipitation, seed source and rootstocks are essential for the renewal of the pine/hardwood understory on burn plots.

The results for L.styraciflua and Quercus sp. suggest that prescribed fire does not affect all hardwoods similarly. Results from this study suggest that frequent late winter fire will eradicate small pine seedlings, top kill hardwoods less than 5 cm in diameter, and increase the proportion of resprouting hardwood stems during the following growing season. Numerous studies have shown that summer burns are more effective in reducing hardwood stems in the understory than winter burns (Waldrop et al. 1987; Langdon 1981; Lottie et al. 1960); and Hodgkins (1958) found that annual winter burning for five years had little or no effect in diminishing sprouting ability of repeatedly top killed hardwoods. Expected percent top kill by summer fire is greater than 80% for 2.5 cm dbh class and approximately 60% for the 5 cm class (Wade & Lunsford 1989; Grano 1970).

CONCLUSIONS

A single late winter fire produced numerous stems of hardwood regeneration, and simultaneously eliminated small pine seedlings in the understory. Hardwood regeneration was primarily from underground perennial rootstocks, and tended to occupy the lower height class. A second late winter fire the following year was effective in reducing numbers of hardwood stems in all height classes to three meters. Drought conditions during peak growing months probably contributed to the overall decline of numbers of understory stems. Pine seedlings remained dominant in the control plots, although subsequent pine regeneration was insignificant. A litter layer in the control may have mitigated drought conditions, thus enabling pine seedlings to survive.

Two year growing season data following fire indicated that prolific hardwood regeneration in the understory surpassed the pre-burn inventories conducted before fire. Quercus falcata and L. styraciflua appear to be the most aggressive re-sprouters of the hardwood component. Results suggest that an additional growing season with adequate precipitation would be favorable for hardwoods dominance in the understory. Pine regeneration in the burn plots was insignificant.

The production of woody vines and herbaceous species seems to respond to prescribed fire. Increase in percent cover by both vines and herbaceous plants from 1993 to 1994 shows the importance of an additional growing season, as well as adequate rainfall for recovery following fire.

Prescribed fire is often used to control hardwoods in forest stands. Results indicate that infrequent cool season fires in mature upland pine/hardwood stands may actually favor hardwoods instead of pine. Integrating prescribed fire into forest management requires thoughtful consideration of long term goals along with understanding how periodicity and seasonality of fire affect a variety of species assemblages.

ACKNOWLEDGMENTS

We wish to thank Dr. Mike Legg, and Dr. Herschel Reeves at the Arthur Temple College of Forestry, Stephen F. Austin State University. In addition, this project would not have been possible without the financial support from Texas Parks and Wildlife. Alonso Escuelante assisted in the translation of the abstract into Spanish.

LITERATURE CITED

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Table 1. Importance values * for the
understory woody pine and hardwood
species showing before and after fire
effects on composition.
Species 1992 1993 1994
Treatment plots Before After
Sassafras albidum (Nutt.) Nees. 27.46
Rhus coppalina L. 25.40 15.35
Ilex vomitoria Ait. 16.22
Liquidambar styraciflua L. 25.04 22.25
Pinus taeda L. 66.80
Quercus falcata Michx. 33.81 26.24 17.74
Viburnum rufidulum Raf. 16.99 19.65 16.87
Callicarpa americana L. 16.86 14.85 19.52 19.56
Cercis canadensis L. 15.04 17.66 19.42
Others 50.50 99.99 90.23 106.55
Total 200.00 200.00 200.00 200.00
Hex vomitoria Ait. 9.09 12.46 11.10
Liquidambar styraciflua L. 8.87 12.11
Pinus taeda L. 61.78 50.92 50.84 52.36
Quercus falcata Michx. 25.20 28.38 13.83
Viburnum rufidulum Raf. 27.69 17.76 19.86 19.61
Callicarpa americana L. 31.50 20.23 21.49 20.90
Others 44.96 73.62 83.24 82.20
Total 200.00 200.00 200.00 200.00
(*)Importance values are the sum of
relative density and frequency.
Table 2. Repeated Measures ANOVA for the effects
of prescribed burning on pine and hardwood stems.
Variable df MS f P
Pine stems
Sample year 3 4453.750 4.76 0.0051
Treatment 1 4620.800 4.94 0.0305
Sample year X treat 3 1003.900 1.07 0.3685
Plot (treat) 18 1960.619 2.09 0.0190
Hardwood stems
Sample year 3 24250.912 17.55 0.0001
Treatment 1 22545.612 16.31 0.0002
Sample year X treat 3 8655.745 6.26 0.0010
Plot (treat) 18 2331.390 1.69 0.0713
Table 3. Average number ot pine
stems per plot by year and
treatement
Survey Year Number plots Mean Pine Stems 1
 per treatment Burn
1992, pre-burn survey 10 35.la
1992, post-burn 10 5.2b *
1993, post burn 10 l.lb *
1993, post-burn 10 1.1b *
1994 10 1.6b *
Survey Year Mean Pine Stems
 Control
1992, pre-burn survey 33.3a
1992, post-burn 47.4a
1993, post burn 16.lb
1993, post-burn 16.1b
1994 18.5b
(1)Within column means followed
by the same letter do not vary
significantly ([alpha] = 0.05)1
(*)Burn means differ significantly
with corresponding control mean ([alpha] = 0.05).
Table 4.
Average number of
hardwood stems per plot
by year and treatment.
Survey Year Number plots Mean Hardwood Mean Hardwood
 per Treatment Stems 1 Burn Stems Control
1992, pre-burn survey 10 36.5bc 41.la
1992, post-burn 10 145.7a * 58,5a
1993, post-burn 10 25.0c 18.4a
1994 10 68.6b * 23.5a
(1)Within column means followed by the same
letter do not vary significantly ([alpha] = 0.05).
(*)Burn means differ significantly with
corresponding control mean ([alpha] = 0.05).


[Graph omitted]
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Article Details
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Author:Clendenin, Michelle; Ross, William G.
Publication:The Texas Journal of Science
Article Type:Statistical Data Included
Geographic Code:1U7TX
Date:Feb 1, 2001
Words:5818
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