Importance of canopy position for growth of Celtis laevigata seedlings.
Secondary succession in parts of southern Texas may begin with abandonment of farmland. If farming is stopped, colonization by various annual species occurs quickly, followed by establishment of woody plants like Acacia smallii (Isely), huisache, within five years (Van Auken and Bush, 1985). Acacia smallii dominates the savanna-woodland for the next 25 to 35 years, after which it declines. It is shade intolerant (Bush and Van Auken, 1986a) and grows poorly below its own canopy (Lohstroh and Van Auken, 1987). During this time period, Celtis laevigata (Willd.), Texas sugarberry, becomes established and then dominates the mature community (Bush and Van Auken, 1984). Celtis laevigata is shade tolerant as might be expected for mature community species (Bush and Van Auken, 1986a).
Although C. laevigata is tolerant of low light levels in the greenhouse, we have not identified any reports of its ability to grow in the field or its site requirements for establishment. Late successional species are usually tolerant of low levels of light (Grime, 1965; Loach, 1967, 1970), but may require higher levels of nutrients (Bormann, 1953; Tilman, 1982; Van Auken et al., 1985). Additional factors that could be important in determining plant establishment and dominance are herbivory and ability to compete for resources below ground (Weaver and Clements, 1966; Harper, 1977; Smith, 1980).
This study was designed to determine the importance of canopy position, nutrient addition, herbivory, and root competition on the growth of seedlings of C. laevigata in the field.
Fruits of Celtis Iaevigata were collected in February 1985 from trees located along the San Antonio River, Bexar County, Texas. Seeds were soaked in giberellic acid for 3.5 hours to break dormancy (Kahn, 1968) and then rinsed thoroughly with distilled water. Three seeds were sown in each 15-centimeter diameter by 15-centimeter deep plastic pot containing 1400-gram sieved, Frio clay-loam soil (Taylor et al., 1966) from an early successional site. The soil was a Mollisol, classified as a fine, mixed, thermic, Cumulic Haplustoll, low in carbon (1.29 [+ or -] 0.45 percent, by weight), nitrogen (0.14 [+ or -] 0.03 percent), and phosphorous (8 [+ or -] 2 milligrams per kilogram). Calcium was high (2400 [+ or -] 800 milligrams per kilogram) and magnesium and potassium were at intermediate levels (30 [+ or -] 11 milligrams per kilogram, 11 [+ or -] 5 milligrams per kilogram, respectively) (Bush and Van Auken, 1986b). Rainfall data were taken from the NOAA local climatological data summary (NOAA, 1985).
Seven weeks after sowing, plants were sized according to height and vigor and thinned to one per pot. One-hundred-nineteen plants were used in the experiment. Seven randomly selected plants were kept in the greenhouse and the remaining plants were transplanted to a field site along the San Antonio River. Seven randomly selected plants were placed into each of the 16 treatments. All plants were given 300 milliliters of distilled water every third day for 10 days (equilibration) before initial measurements. Greenhouse plants were harvested for initial measurements by cutting at the cotyledon scars and drying (70[degrees]C) to a constant weight.
The experimental design was [2.sup.4] factorial with canopy position (under an Acacia smallii canopy or in the open), nutrients (added or not added), root competition (roots present or trenching, no woody plant roots), and herbivory (herbivores present or insecticide treatment, no herbivores) as factors. With all combinations, there were 16 treatments. The canopy position was beneath five mature A. smallii trees and the open area was adjacent. Mean light levels were 515 [+ or -] 153 [micro]M*[m.sup.-2]*[sec.sup.-1] under the canopy and 2173 [+ or -] 174 [micro]M*[m.sup.-2]*[sec.sup.-1] in the open. Light levels were measured with a Li-cor[R] LI-188 integrating quantum sensor. Nutrient treatment consisted of 250 milliliters of a complete nutrient solution (Van Auken and Kapley, 1979) once per week starting at zero time for 12 weeks. No nutrient-treatment plants were given 250 milliliters of distilled water at the same time intervals. Root competition was reduced (no competition treatment) by trenching a one square meter perimeter around the plants, to a depth of 20 centimeters. Root competition treatments were untrenched. No herbivory plants were sprayed with malathion (2.95 milliliters per liter) once each week for the duration of the experiment. A four-sided cardboard box prevented the insecticide from being blown onto other test plants. Herbivory treatment plants were sprayed with an equal amount of distilled water at the same time intervals.
The experiment was terminated 12 weeks after initial measurements were taken (1 November 1985). Plants were cut at the cotyledon scars and dry weights were measured. Data were statistically analyzed by an analysis of variance (ANOVA) procedure and means were separated using the Least Significant Difference test. Mortality data was analyzed with a [X.sup.2] test (Steel and Torrie, 1980; SAS Institute, 1982).
Position was the only main factor tested that was significant (ANOVA, F = 64.03, P < 0.0001, Fig. 1). Herbivory, competition, or nutrient additions were not significant (ANOVA, F = 0.00-3.59, P > 0.05). Of the 10 two-way and three-way interactions, only the herbivory-nutrient interaction was significant (ANOVA, F = 4.37, P = 0.0397). Mean aboveground dry weight of Celtis laevigata seedlings grown in the open (grassland, full sunlight) was 0.25 [+ or -] 0.21 grams, whereas mean aboveground dry weight of plants grown below the Acacia canopy (shade) was 0.96 [+ or -] 0.45 grams. Mean aboveground dry weight of the plants harvested at the start of the experiment was 0.14 [+ or -] 0.08 grams. Thus, plants in the open (grassland) increased in dry weight 1.79 times, while those in the shade below the Acacia canopy increased 6.86 times.
[FIGURE 1 OMITTED]
Means of all treatments were separated using the least significant difference test (Table 1). The main differences were in canopy position. Mean values for all plants in treatments in the open were lower than means for plants in treatments below the Acacia canopy. Other differences are slight and should not be considered because other main effects were not significant when tested with ANOVA.
Overall seedling mortality in the field was 36 percent, and 98 percent of these mortalities occurred in the open (grassland, full sunlight), which was highly significant ([X.sup.2] = 36.1, P < 0.005). Rainfall during August 1985 was only 0.45", which was 83 percent below normal, and this was the time when almost all of the Celtis mortalities occurred (Fig. 2). Rainfall during September and October was 28 percent and 36 percent above normal, and only one mortality occurred during this time.
Many abiotic and biotic factors affect the establishment and growth of woody plants (Harper, 1977). However, the present field study identified C. laevigata seedling position relative to an Acacia smallii canopy as the major factor. Growth of C. laevigata seedlings below the Acacia canopy was 3.84 times higher than that of seedlings in open grassland. Reciprocially, mortality of C. laevigata seedlings in the open grassland was 39 times higher compared to those below the Acacia canopy.
Light intensity, an obvious variable influenced by the presence of a woody plant canopy, may be reduced by more than 90 percent by the A. smallii overstory (Bush and Van Auken, 1986a). But, low light levels should reduce, not stimulate, C. laevigata growth. Apparently, the stimulation of C. laevigata seedling growth is mediated by another factor associated with the A. smallii canopy. Soil temperature, soil nutrients, and soil water content all have been shown to change below the canopies of certain woody species (Tiedemann and Klemmedson, 1973a, 1973b, 1986; Bush and Van Auken, 1986b). Higher levels of soil nutrients may be a factor involved in stimulating the growth of C. laevigata. However, we did not find a nutrient stimulation in the present experiment. Soil nutrients, especially nitrogen, have been shown to increase during successional events (Gorham et al., 1979), and total soil nitrogen has been demonstrated to increase during secondary succession in this area (to 2.5 [+ or -] 0.7 g*[kg.sup.-1], Bush and Van Auken, 1986b). In addition, total soil nitrogen is almost twice as high under a 15-year-old Acacia smallii canopy compared to open grassland. Low light levels occur below the canopy, but higher soil nitrogen levels also are found. Added nitrogen, rather than other nutrients, has been shown to stimulate C. laevigata growth in the same Frio soil in greenhouse experiments (Van Auken et al., 1985). Thus, soil nitrogen seems to be a major candidate for causing the increased growth observed.
[FIGURE 2 OMITTED]
Other factors examined, including competition, nutrient supplements, and herbivory, did not cause significant changes in C. laevigata growth. Competition for soil-borne resources, mainly water and nutrients, can play a significant role in establishment and subsequent growth of woody species (Harper, 1977). Trenching effectively eliminates root competition (Ehrenfeld, 1980; Horn, 1985). But, we did not see a difference in C. laevigata growth between trenched and nontrenched plots and concluded that competition between adult and seedling plants was unimportant. Competition with woody plant roots may have been masked by competition with herbaceous plants that were not removed from the study plots (both canopy and open plots). Competition between woody plant seedlings and herbaceous plants can cause significant growth reduction in woody plants (Glendening and Paulsen, 1955; Van Auken and Bush, 1989).
Nutrient additions were expected to have stimulatory effects on the growth of C. laevigata as seen in previous studies (Van Auken et al., 1985). However, additional nitrogen in the canopy soil could have masked that effect. The lack of stimulation of growth of C. laevigata in the open when nutrients were added was probably the result of the early drought and later stimulation of associated herbaceous species by the removal of a nutrient limitation. With the growth limitation removed, the herbaceous species may have used up a nutrient required for C. laevigata growth.
Herbivores can have drastic effects on plant populations and change successional sequences (Harper, 1977; Gilbert, 1985), although not shown in the present study. This study was completed in the autumn and was of limited duration. A study completed over a longer time and including both the spring and autumn growing seasons might show significant growth reduction due to herbivory, if herbivores are present.
Celtis laevigata appears to be a late successional species, requiring high soil nitrogen and capable of growth in canopy shade. Although it can grow in disturbed areas or grasslands, it would do so at reduced rates, depending on the presence of competition and soil nitrogen levels. A more favored site would be in the reduced light and higher soil nitrogen environment below the canopy of A. smallii.
TABLE 1. Mean aboveground dry weight ([+ or -] SD) for Celtis laevigata grown in 16 treatments and initial measurements. Mean values followed by the same letter are not significantly different (ANOVA, LSD P > 0.05). Dry-weight Treatments (grams) Zero time 0.14 [+ or -] 0.08A Open, herbivory, competition, nutrients 0.18 [+ or -] 0.31A Open, herbivory, competition, no nutrients 0.08 [+ or -] 0.03A Open, herbivory, no competition, nutrients 0.23 [+ or -] 0.28AB Open, herbivory, no competition, no nutrients 0.35 [+ or -] 0.36ABC Open, no herbivory, competition, nutrients 0.23 [+ or -] 0.42AB Open, no herbivory, competition, no nutrients 0.17 [+ or -] 0.19A Open, no herbivory, no competition, nutrients 0.40 [+ or -] 0.35ABC Open, no herbivory, no competition, no nutrients 0.28 [+ or -] 0.45AB Under canopy, herbivory, competition, nutrients 1.00 [+ or -] 0.55DE Under canopy, herbivory, competition, no nutrients 0.91 [+ or -] 0.29CDE Under canopy, herbivory, no competition, nutrients 0.85 [+ or -] 0.36CDE Under canopy, herbivory, no competition, no nutrients 0.93 [+ or -] 0.19CDE Under canopy, no herbivory, competition, nutrients 0.88 [+ or -] 0.49CDE Under canopy, no herbivory, competition, no nutrients 0.79 [+ or -] 0.41BCD Under canopy, no herbivory, no competition, nutrients 1.41 [+ or -] 0.93E Under canopy, no herbivory, no competition, no nutrients 0.90 [+ or -] 0.42CDE
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O. W. VAN AUKEN AND R. J. LOHSTROH
Division of Life Sciences, The University of Texas at San Antonio, San Antonio, Texas 78285
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|Author:||Van Auken, O.W.; Lohstroh, R.J.|
|Publication:||The Texas Journal of Science|
|Date:||Feb 1, 1990|
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