WETLANDS, Vol. 15, No. 4, December 1995, pp. 392-396 © 1995, The Society of Wetland Scientists

EFFECT OF SOIL TEXTURE AND MICROTOPOGRAPHY ON GERMINATION AND SEEDLING GROWTH IN

BOLTONIA DECURRENS (ASTERACEAE), A THREATENED FLOODPLAIN SPECIES

Marian Smith, ~ Terri Brandt, and Jeffrey Stone Southern Illinois Universi~ at Edwardsvitle

Edwardsville, IL 62026

Author for correspondence

Abstract: Boltonia decurrens, a perennial species endemic to the Illinois River Valley, is threatened with extinction. Field observations indicate that B. decurrens is confined to areas with sandy soils formed from alluvial deposits, but there is little understanding of the factors controlling its distribution. Plans for reintro- duction of the species to suitable habitats within itg historical range are hampered by a lack of understanding of the site characteristics necessary for germination and seedling growth. This study examined the effect of soil texture and soil surface features on germination and growth of B. decurrens under greenhouse conditions. We found that both parameters are greater on sand, compared to clay, and on bare soil with either a smooth or lumpy surface, compared to litter-covered surfaces. These conditions are identical to those in which B. decurrens has persisted along the Illinois River and may be imporlant in determining its distribution pat~rn_

Key Words: endemic, floodplain, germination, growth, microtopography, perennial, soil texture, threatened

INTRODUCTION

Boltonia decurrens (Torrey & Gray) Wood, a peren- nial member of the Asteraceae endemic to the Illinois River floodplain (Torrey and Gray 1840), seems to be disappearing from its native habitat (Schwegman and Nyboer 1985), In spite of prolific seed production and the ability to reproduce vegetatively (Schwegman and Nyboer 1985, Smith 1991), the number of naturally occurring populations, which fluctuates annually, con- tinues to decline over time. In 1988, the U.S. Fish and Wildlife Service placed B. decurrens on the national list of threatened species. It is currently listed as en- dangered in Missouri (Wilson 1980) and threatened in Illinois (Herkert 1991).

Despite more than 10 years of censusing populations (U.S. Fish and Wildlife Service 1990) and a number of studies concerning life history and habitat requirements (Smith 1990, Smith 1991, Smith et al. 1993), the re- striction of B. decurrens to the central Illinois River Valley is not understood. The species grows mainly in large sandy areas composed of glacial outwash mate- rials that support a variety of unique species (John Schwegman, pers. comm.). While areas in the lower Illinois River Valley, where B. decurrens does not oc- cur, are also alluvial, soils in the middle reach of tile Illinois River are sandier, containing less clay, than soils

at the southern boundary of the river where it merges with the Mississippi (General Soil Map of Illinois 1982). Members of the B. decurrens Recovery Team suggested that soil texture might be an important factor determining the distribution of the species (U.S. Fish and Wildlife Service 1990). A field study by Smith (1991) also indicated its importance. Because manage- ment c,f endangered species often requires reintroduc- tion or relocation to new areas, knowledge of the char- acteristics of a suitable site is essential.

Germination and seedling establishment are the most vulnerable stages in the life cycle of a plant (Sol- brig 1980), and the environment is extremely heter- ogenous at the scale of the seed (Harper 1977). The soil microtopography into which a seed falls has spe- cial importance and may be the determining factor in the success or failure of a species in a particular habitat (Hamrick and Lee 1987, Fowler 1988, Smith and Ca- pelle 1993). Therefore, to help determine the charac- teristics necessary for the successful establishment of B. decurrens, a study was conducted to examine the effects of soil texture and surface features on germi- nation and seedling growth.

METHODS

In February, 1991, two soil samples, silty clay (6.7% sand, 53.3% silt, 40% clay; hereafter, "c lay")

392

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Figure 1. Germination of Bohonia decurrens at 4-week in- tervals over a 13-week period on gmoolh clay (open circle), smooth sand (filled circle), lumpy clay (open triangle), lumpy sand (closed triangle), litter-covered clay (open box), and litter-covered sand (closed box).

and loamy sand (80.0% sand, 16.7% silt, 3.3% clay; hereafter, " sand" ) (Alvey Laboratory, Belleville, IL), were collected near the Illinois River in Jersey County, IL and f rom a Bol tonia decurrens site in Morgan County, IL. Sixty 10 X 10 X 9 cm pots, filled to within 4 cm of the rim with a commercial potting soil, were separated into two groups, and the surface was covered with a 2-cm layer of either clay or sand. Three surface treatments (smooth, lumpy, or litter) were created by scoring the smooth surface of the soil with a small spatula, or covering the surface with 2 cm of litter (dried stem and leaf material) collected from the B, decurrens site. Ten replicates were created within each of the six soil-surface treatments for a 2 X 3 factorial design containing a total of 60 pots (50 seeds per pot) for a total of 3000 seeds.

On March 2, 1991, all pots were soaked in water

(by submerging pots in water up to the soil surface), and seeds of B. decurrens were sprinkled on the sur- face in each pot. Pots were placed in an unshaded area (700-1200 IzMol quanta m 2 s-L measured at mid-day throughout the study with a LI-185S quantum sensor and a LI-1000 datalogger in instantaneous mode, LI- COR Inc., Lincoln, NE) in the greenhouse at Southern Illinois University at Edwardsville, IL. High/low val- ues for temperature and relative humidity during the study were 38/12°C and 85/42%, respectively. For the duration of the experiment, each pot was given 150 ml water day -I from a hand-held sprinkler to simulate the erosive force of rain. Pots were rotated daily to ameliorate the environmental differences associated with pot position.

Seedlings were counted three times a week, and the approximate date of emergence recorded, Seedling mortality was extremely low (only 2 marked seedlings died during the experiment). Because of the frequency of counting, it is highly unlikely that the mortality of a seedling would be overlooked; therefore, although all data are reported as "germinat ion," they also rep- resent seedling survival.

On June 27, 1991, seedlings were counted for the last time, and because of the threatened status of the plant and a reluctance to measure growth destructively, an estimate of plant size was made by measuring the longest leaf on each seedling. All data were analyzed by ANOVA (a repeated measures ANOVA for seed- ling survival at weekly intervals) (SPSS/PC, Chicago, IL), and means were compared using Student-Neu- mann Keuls (Winer 1971). Data were arcsine-trans- formed prior to analysis as suggested for sets that con- tain percentages outside the 30-70% range (Sokal and Rohlf 1981).

RESULTS

Soil texture and surface treatment significantly af- fected germination, both in terms of timing (Figure 1, Table 1) and total seedling germination after 13 weeks (Figures 2-3, Table 2). At week 13, overall germina- tion was 34% higher in sand than in clay (Figure 2) and higher in sand than in clay on any surface treat- ment (Figure 3a). For both soil textures, germination

Table 1. Summary Repeated Measures ANOVA for germination at 13 weekly intervals.

Source of Variation SS DF MS F P W e e k 5 0 5 4 . . 8 9 1 2 4 2 1 . 2 4 1 1 1 . 4 9 0.00 l Soil × week 231.87 12 19.32 5.11 0.001 Surface X week 333.41 24 13.89 3,68 O.O0l Soil x surtace × week 66.91 24 2.79 0.74 ns Error 2448.31 648 3.76

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Figure 2. Percent germination of Boltonia decurrens after 13 weeks on clay and sand (P<0.001, Pooled DF=58).

on a smooth surface was 17% and 53% greater, re- spectively, compared to a lumpy or litter-covered sur- face (Figure 3b), and the pattern of effect of surface treatment was the same for each soil texture (Figure 3a).

Soil texture and surface treatment significantly af- fected leaf length in plants measured at Week 13. Leaf length was longer in plants from the sand treatment compared to clay (Table 3, Figure 4a) and in the lumpy surface treatment compared to smooth (Table 3, Figure 4b). There were no other differences in leaf length between other combinations of surface treatments (Figure 4b).

DISCUSSION

Our results strongly suggest that soil texture and surface microtopography could affect the distribution pattern of B. decurrens. Germination and seedling growth were significantly greater on sand than on clay at any surface treatment. These results help explain those of a previous field experiment (Smith 1990), in which no germination occurred from 2500 B. decur- rens seeds planted on a site having soils with heavy clay content (53%) adjacent to the Mississippi River at West Alton, MO.

In conLrast to our results, others have reported re- duced germination on sand (Mustart and Cowling 1993) due to low water retention of the soil: however, soil in the present study was watered regularly, reduc- ing the danger of desiccation. Seeds of B. decurrens

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394 WETLANDS, Volume 15, No. 4, 1995

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Figure 3. a. Percent germination of Boltonia decurrens af- ter 13 weeks on smooth (clay and sand), lumpy (clay and sand), and litter-covered (clay and sand) surfaces (P<0.05, Pooled DF= 18 for each pair of values), b. Percent germi- nation after 13 weeks on smooth, lumpy, and litter-covered surfaces (P<0.01, Pooled DF=38).

can germinate in both fall and spring (Smith 1991), but because they germinate only on the surface of standing water or on saturated soil (Schwegman and N y b o c r 1985, U S Fish and Wi ld l i fe Se rv i ce 1990, Smith et al. 1993) under normal field conditions, the effects of reduced soil moisture were not considered relevant to the current study.

Seedling mortality in a number of species has also

Table 2. Summary two-way ANOVA of the effect of soil texture and surface treatment on total germination after 13 weeks.

12

Source of Variation SS DF MS F P

Soil 201.47 1 201.67 9.99 0.01 Surface 379,20 2 189.60 9.40 0.001 Soil x surface 16.53 2 8.27 0.41 ns Error t 089.20 54 20.17

been reported to be affected by soil moisture, due to the failure of an emerging root to become anchored in the soil (Harper et M. 1961, Hamrick and Lee 1987, Smith and Capelle 1992). However, if soil moisture is not limiting, sand offers a more porous surface for root establishment and growth (Brady 1974), factors which are particularly important when seedlings and emerg- ing radicles are extremely small, as they are in B. de- currens (Smith et al. 1993).

The importance of surface microtopography for ger- mination and seedling establishment has been dem- onstrated for a number of plants (Harper et al. 1965), with optimal surface texture varying among species ( t o m e s and Elberse 1976). Smith and Capelle (1992) reported that Cichorium intybus L. germinated best on soil with higher heterogeneity, but Hamrick and Lee (1987) found that Carduus nutans L. had higher ger- mination, survival, and biomass production on smooth soil surfaces.

In the present study, germination response to micro- topography was divided into three distinct groups: sand (smooth and lumpy), clay (smooth and lumpy), and litter (sand and clay). Germination on both soil textures resulted in similar curves in the smooth and lumpy treatments up to Week 5, with litter treatments lagging; however, beyond Week 5, sand clearly be- came the optimal soil texture. Because seedlings of B. decurrens are small and require high light for growth (Smith et al. 1993), it is important for germination to occur rapidly before competing vegetation overtops the young plants. The slow germination demonstrated on litter-covered soil, whether sand or clay, corre- sponds to field observations (Schwegman and Nyboer 1985, US Fish and Wildlife Service 1990) that, in ar- eas where litter and standing vegetation are not

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Figure 4. a. Leaf length of Boltonia decurrens seedlings after 13 weeks on clay and sand (P<0.001, Pooled DF-58). b. Leaf length after 13 weeks on smooth, lumpy, and litter- covered surfaces (P<0.001, Pooled DF=38).

cleared, B. decurrens is rapidly excluded from the community. Suppression of germination by litter has been reported for a variety of other wetland species (Middleton et al. 1991), and the removal of litter re-

Table 3. Summary 2-way ANOVA for effect of soil texture and surface treatment on leaf length at Week 13.

Source of Variation SS DF MS F P Surface treatment 32.40 2 16.20 9.79 0.001 Soil 108.00 1 108.00 65.26 0.001 Surface treatment x soil 8.63 2 4.32 2.60 ns Error 89.38 54 1.66

396 WETLANDS, Volume 15, No. 4, 1995

suited in a significant increase in the number of species and the number of individuals of a species recruited from the seed bank in an experimental marsh complex (van der Valk 1986).

Soil texture seemed to have a greater effect on ger- mination than surface treatment, as both smooth and lumpy surfaces on sand showed higher percentages than on any clay treatment (14% and 12% for smooth and lumpy sand, compared to 10% and 8% for smooth and lumpy clay). This may be because even smooth sandy surfaces have more topographical relief than clay, due to the larger size of the soil particles (Brady 1974, Hillel 1982, Mustart and Cowling 1993).

Leaf length was significantly longer in Week 13 seedlings grown on sand, regardless of surface fea- tures. Leaf area/leaf and leaf number/seedling were previously shown to be positively correlated with leaf length (R2=0.90 and R2=0.84, respectively; Smith 1990); therefore, seedlings with longer leaves have greater photosynthetic surface area. This may be crit- ical for a species requiring high light such as B. de- currens (Smith et al. 1993), either in the fall when it is germinating beneath litter or in the spring when competition from other seedlings is greater.

Our results clearly indicate that germination is better on sand, compared to clay, and on bare soil (either smooth or lumpy) compared to littered-covered sur- faces. These conditions are identical to those in which B. decurrens has persisted along the Illinois River. The largest populations grow on alluvial sand deposits, subject to recurrent flooding that smooths the surface of the soil and clears it of living and dead vegetation to create safe sites for invasion. Future plans for re- introduction of the species must consider soil texture and the frequency of flooding, which prepares the mi- crotopography of the surface for the establishment of seedlings of Boltonia decurrens.

ACKNOWLEDGMENTS

The authors thattk Tom Keevin, USACE, John Schwegman, Illinois Department of Natural Resources, and Martin Stoecker for their help in planning and carrying out this study. This work was supported by funds from the U.S. Army Corps of Engineers, Illinois Department of Natural Resources and U.S. Fish and Wildlife Service.

LITERATURE CITED

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Fowle~; N. L. 1988. What is a safe site?: neighbor, litter, germina- tion date, and patch effects. Ecology 69:947 961.

General Soil Map of Illinois. 1982. Agricultural Experiment Sta- tion, College of Agriculture, University of Illinois at Urbana- Champaign, Urbana, IL, USA.

Hamrick, J. L. and J. M. Lee. 1987. Effect o f soil surface topog- raphy and litter cover on the germination, survival and growth of musk thistle (Carduus nutans). American Journal o f Botany 74: 451-457.

Harper, L. 1977. Population Biology o f Plants. Academic Press, New York, NY, USA.

Harper, L., I. N. Clatworthy, I. H. McNaughton, and G. R. Sagar. 1961. The evolution and ecology of closely related species living in the same area. Evolution 15:209-227.

Harper, L., J. T. Williams, and G. R. Sugar. 1965. The behaviour of seeds in soil. I. The heterogeneity of soil surfaces and its role in determining the establ ishment of plants from seed. Journal of Ecology 53:273-286.

Herkert, J. R. (ed.). 1991. Endangered and Threatened Species of Illinois: Status and Distribution. 1:13. Illinois Endangered Species Protection Board, Springfield, IL, USA.

Hillel, D. 1982. Introduction to Soil Physics. Academic Press, New York, NY, USA.

Middleton, B. A., A. G. Van der Valk, D. H. Mason, R. L. Williams and C. B. Davis. 1991. Vegetation dynamics and seed banks of a monsoonal wetland overgrown with Paspalum distichum L. in northern India. Aquatic Botany 40:239-259.

Mustart, P. J. and R. M. Cowling. 1993. Effects of soil and seed characteristics on seed germination and their possible roles in de- termining field emergence patterns of four Agulhas Plain (South Africa) Proteaceae. Canadian Journal of Botany 71:1363-1368.

Oomes, J. M. and W. T. Elberse. 1976. Germination o f six grassland herbs in microsites with different water contents. Journal of Ecol- ogy 64:745-755.

Schwegman, J. E. and R. W. Nyboer. 1985. The taxonomic and population status of Boltonia decurrens (Torrey & Gray) Wood. Castanea 50:112-115.

Smith, M. 1990. Basic life history characteristics o f Boltonia de- currens (Decurrent false aster). Report to the U.S. Army Corps of Engineers. St. Louis, MO, USA.

Smith, M. 1991. Life history o f the Decurrent false aster. Report to Illinois Department o f Natural Resources, Springfield, IL, USA.

Smith, M. and J. Capelle. 1992. Effects o f soil surface microtopog- raphy and litter cover on germination, growth and biomass pro- duction of chicory (Cichorium ino'bus L.). American Midland Naturalist 128:246-253.

Smith, M., Y. Wu, asid O. Green. 1993. Effect of light and water- stress on photosynthesis and biomass production in Boltonia de- currens (Asteraceae), a threatened species. American Journal of Botany 80: 859-864.

Sokal, R. R. and E J. Rohlf. 1981. Biometry, 2 ~ Edition. W. H. Freeman and Company, New York, NY, USA.

Solbrig, O. T. 1980. Demography and natural selection, p. 1-20. In O.T. Solbrig (ed.) Demography and Evolution in Plant Popula- tions. Botanical Monographs 15. Blackwell Scientific, Oxford. UK.

Torrey, J. and A. Gray. 1840. The Flora of North America. 1:!88. Wiley and Putnam, New York, NY, USA,

U. S. Fish and Wildlife Service. 1990. Decurrent False Aster Re- covery Plan. U. S. Fish and Wildlife Service, Twin Cities, MN, USA.

van dcr Valk, A, G. 1986. The impact o f litter and annual plants on recruitment from the seed-bank of a lacustrine wetland. Aquat- ic Botany 24:1.3-26.

Wilson, J .H . 1980. Rare and Endangered Species of Missouri. Mis- souri Department o f Conservation. Jefferson City, MO, USA.

Winer, B. J. 1971. Statistical Principles in Experimental Design (2 "~ ed_'J. McGraw-Hill . NewYork. NY. USA.

Manuscript received 10 July 1995; revision received 18 September 1995: accepted 19 September 1995.