Potential role of soil moisture deficit in the distribution ...

Potential role of soil moisture deficit in the distribution of Cakile edentula Department of Botany, University of Maryland, College Park, MD, U.S.A. 20742 AND

LARRYW. DOUGLASS

Can. J. Bot. Downloaded from www.nrcresearchpress.com by Depository Services Program on 06/06/13 For personal use only.

Department of Animal Sciences, University of Maryland, College Park, MD, U.S.A. 20742 Received December 9, 1985 TYNDALL, R. W., A. H. TERAMURA, and L. W. DOUGLASS. 1986. Potential role of soil moisture deficit in the distribution of Cakile edentula. Can. J. Bot. 64: 2789 -2791. Soil water potential, leaf water potential, and stomatal conductance of Cakile edentula (Bigelow) Hooker were compared between beach and foredune habitats on Cumtuck Bank, North Carolina. All three variables were significantly lower on the foredune than on the beach. Low soil water potential on the foredune may contribute to low survival and growth inhibition by lowering leaf water potential and stomatal conductance. TYNDALL, R. W., A. H. TERAMURA et L. W. DOUGLASS. 1986. Potential role of soil moisture deficit in the distribution of Cakile edentula. Can. J. Bot. 64: 2789 -2791. Le potentiel hydrique du sol, le potentiel hydrique foliaire et la conductance stomatique de Cakile edentula (Bigelow) Hooker ont CtC comparCs entre deux habitats: la grkve et l'avant-dune 21 Cumtuck Bank, en Caroline du Nord. Les trois variables Ctaient significativement plus basses pour l'avant-dune que pour la gkve. Le potentiel hydrique du sol plus bas pour l'avant-dune peut contribuer 2I une survie plus faible et 2I l'inhibition de croissance en causant l'abaissement du potentiel hydrique foliaire et de la conductance stomatique. [Traduit par la revue]

Introduction Cakile edentula (Bigelow) Hooker is the dominant species on beaches of the mid-Atlantic (Tyndall 1985). On adjacent stabilized foredunes. however. it is not even a subdominant as relative frequency is less than 3 % and aboveground biomass is less than 0.3 g . m-2 (Hanis et al. 1983; Levy 1976). The relatively few plants that do occur on the foredune usually become established in open areas created during autumnal and winter storms (Tyndall 1985). In Canada, Keddy (1982) documented significantly lower survival and reproductive output of foredune plants as compared with those on the beach. Similarly, Payne and Maun (1984) found that survivorship and fruit production were highest on the lakeward end of the beach gradient along Lake Huron and decreased landward. Factors believed to contribute to these differences include nitrogen deficiency and competition (Keddy 1982), insect damage and disease (Payne and Maun 1984), and low soil water potential (McLeod and Murphy 1977; Payne and Maun 1984). The purpose of this study was to test the hypothesis that soil moisture deficit contributes to population differences of C. edentula between beach and foredune habitats along stabilized sections of the North Carolina Outer Banks. Selection of this factor was based upon preliminary reconnaissances of two obvious disparities between these habitats. First, beach soil seldom dries to a depth exceeding 150 mm year-round and remains very moist immediately below. In contrast, foredune soil progressively dries from the surface to a depth exceeding 200 mm usually by midJune to late June. Second, the beach habitat is typified by an open community, while foredunes are artificially stabilized with dense stands of perennial grasses. The latter characteristic suggested that soil moisture in the foredune could be depleted early in the growing season as a result of high evapotranspirative demand, while that in the beach does not become limiting. In this study, soil water 'Present address: 9933 Campus Way South, Largo, MD, U.S.A. 20772. Printed in Canada 1 Imprim6 au Canada

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FIG. 1. Total soil water potential (mean SEM) of the beach and foredune until senescence of C. edentula on the foredune. Means compared by ANOVA have the same SEM. Significant differences at the P I0.05 level are indicated by asterisks. The upper 200 mrn of soil on the foredune was completely dry on June 24th.

potential ($,) was compared between the beach and foredune, and leaf water potential ($,) and stomatal conductance to water vapor (g,) were used to detect habitat differences in plant water status.

Methods The study site was located on Cumtuck Bank, North Carolina, about 9 km north of Kitty Hawk (36O10'54.6" N, 75'45'5.2" W). During seedling, adult vegetative, and reproductive stages of growth, g, and $L were measured for C. edentula plants on the beach and foredune. Whenever possible, measurements were made over 2 consecutive days in a randomized complete block with replication experimental design. This design permits a doubling of replication without confounding the results with environmental differences between days. On each sampling day, two healthy, uppermost, fully expanded leaves (samples) were measured from each of three plants

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T i m e of day ( h l FIG. 2. Diurnal conductance and total leaf water potential (mean SEM) of C. edentula on the beach and foredune during various stages of growth. Means compared by ANOVA have the same SEM. Significant differences at the P 5 0.05 level are indicated by asterisks. Foredune plants were dead by July 15 and thus are not represented in the two diagrams on the right. (replicates). A 50-m section of beach and foredune was subdivided into three subsections for randomly selecting one individual per subsection. Because the scoured seaward slope of the foredune was too steep for sampling, all randomly selected plants on the foredune occurred on the broad (2-6 m), relatively flat top. On the beach, the entire vegetative zone was sampled. In addition to ecophysiological measurements, phenological and morphological conditions were noted during each sampling event. All g, measurements were made with a LI-700 transient porometer (LI-COR, Inc., Logan, UT). During the first sampling event $, was estimated using a 5-14 press (Campbell Scientific Inc. Logan, UT). For the second sampling episode, $, was estimated with leaf-cutter psychrometers (No. 76-1 lC, and No. 82-22 microvoltmeter; J. R. D. Merrill Specialty Equipment, Inc., Logan, UT). The latter also was used to calibrate the 5-14 press. For $, comparisons between habitats, five samples (one per 10-m subsection) were collected from the upper 200 mm in each plant sampling area. Most of the root system of C. edentula was restricted to this layer on the foredune, and taproots seldom exceeded 300 mm. Soil moisture sampling occurred concurrently with g, and $, measurements, until all Cakile plants died on the foredune. When present, the dry surface layer was poured carefully out of the aluminum sampling cylinder (25 mm diameter) so that percent dry weight of the moist portion could be determined. The moist layer was transferred to tightly sealed plastic bags and weighed within 3 h. In a preliminary experiment, percent dry weight was not affected by 12 h of storage. In the laboratory, the original percent dry weight was restored in sealed beakers using deionized water, and $, was measured with PCT-55-15 soil psychrometers and a HR-33T dewpoint microvoltmeter (Wescor, Inc., Logan, UT). Analysis of variance (Proc GLM, Anonymous, 1982) was used to compare habitat means by time of sampling, unless variances were heterogeneous (Proc Means for variances). In the latter case, t-tests were used for normal data (Proc Univariate), and the Mann- Whitney U-test for data which remained nonnormal after log transformation (Sokal and Rohlf 1981). Unless otherwise stated, the significance level is P I0.05.

Results Soil water potential was significantly lower on the foredune than on the beach, except on May 30 immediately following a rainstorm (Fig. 1). As observed during preliminary reconnaissances, the soil on the foredune became completely dry to a depth of at least 200 mm by late June, while soil water potential on the beach remained above -0.3 MPa. Similarly, both $L and g, of C. edentula seedlings on the foredune were significantly lower than those on the beach (Fig. 2). Most foredune plants were beginning to senesce in mid-June, and all were dead before the July 15 g, measurements. In contrast, beach plants did not begin fruiting until late June, and senescence did not begin until early August. Foredune plants seldom branched and had longer internodes, smaller leaves, and much greater insect damage than beach plants. In addition, foredune root systems were less extensive and taproots were much smaller than those of beach plants. Discussion In the beach habitat, moisture was not limiting during seedling establishment and adult vegetative growth since $, remained high (greater than -0.3 MPa; $, at field capacity was -0.2 MPa). High $, was reflected in high $L; midday values were about -0.4 and -0.8 MPa for seedlings and adults, respectively. Similar findings were reported by DeJong (1979) in California: (i) at a depth of 300 mm, $, was greater than - 1.0 MPa during spring and seldom was less than - 1.5 MPa year-round, and (ii) plant water potentials at midday for taprooted species ranged from about -0.4 to - 1.1 MPa during spring and were rarely less than - 1.8 MPa year-round. In contrast with the beach environment, soil moisture deficit appears to be an important limiting factor in the foredune habitat. As the upper 200 rnrn of soil was drying, both g, and llhL


TYNDALL ET AL.

were significantly lower on the foredune as compared with the beach. Furthermore, plant senescence coincided with complete drying of this layer. The effect of low 4,on the foredune is probably contributory and confounded with other variables. Greater insect damage to foredune plants than to beach plants may contribute to population differences. Competition for light may also be important as indicated by longer internodes on foredune plants. If low N is a contributory factor, then its effect may be masked by that of A,or C. edentula does not respond to N deficiency in the same manner reported for its west coast counterpart, C. maritima. When the latter was N stressed (Holton 1980, cited in Barbour et al. 1985), g, was significantly higher for stressed than for unstressed plants. However, as pointed out by Barbour et al. (1985), very little is known about N as well as P and K in North American dunes. The importance of soil moisture deficit throughout the geographic range of C. edenhtla is probably a function of plant density. Dunes without plant cover have soil moisture levels similar to the beach, but the surface is very mobile (Tyndall 1985). Those with moderate densities are characterized by microsites with soil moisture deficits or excessive sand burial and erosion (van der Valk 1974). In dunes with very dense cover, such as the foredune in this study, sand movement is minimal (less than 20 mm-year-', Tyndall 1985) and soil moisture deficit is maximal. In summary, soil moisture deficit is probably an important limiting factor in the distribution of C. edentula. Low IC., on the foredune probably contributes to seedling mortality and growth inhibition by reducing $L and g,. Soil water potential is not an important limiting factor in the beach habitat.

Acknowledgements This project was made possible by Curt Mason, Chief of the Duck Field Research Facility, U.S. Army Corps of Engineers. Appreciation is extended to G. Bichner, W. A. Birkemeier, D. S. Fanning, B. R. Flamm, I. R. Forseth, B. Grogg, C. Judge, H. Klein, M. Leffler, J. Lydon, H. C. Miller,

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I. Miller, J. Monks, R. Townsend, and S. C. Wheeler for assistance. This study was funded partially by the Office of the Dean for Graduate Studies and Research, University of Maryland. ANONYMOUS. 1982. SAS user's guide (82.3 VMICMS). SAS Institute Inc., Cary, NC.

BARBOUR, M. G . , T. M. DEJONG, and B. M. PAVLIK. 1985. Marine beach and dune plant communities. In Physiological ecology of North American plant communities. Edited by B. F. Chabot and H. A. Mooney. Chapman and Hall, London. DEJONG,T. M. 1979. Water and salinity relations of Californian beach species. J. Ecol. 67: 647-663. HARRIS, R. L., G . F. LEVY,and J. E. PERRY. 1983. Reevaluation of vegetational characteristics at the CERC Field Research Facility, Duck, North Carolina (MR 834). Department of the Army, Coastal Engineering Research Center, Fort Belvoir, VA. HOLTON, B. 1980. Some aspects of the nitrogen cycle in a northern California coastal dune-beach ecosystem, with emphasis on Cakile maritima. Ph.D. dissertation, University of California, Davis. KEDDY, P. A. 1982. Population ecology on an environmental gradient: Cakile edentula on a sand dune. Oecologia, 52: 348-355. LEVY,G . F. 1976. Vegetative study at the Duck Field Research Facility, Duck, North Carolina (MR 76-6). Department of the Army, Coastal Engineering Research Center, Fort Belvoir, VA. MCLEOD, K. W., and P. G . MURPHY. 1977. Establishment of Ptelea trifoliata on Lake Michigan sand dunes. Am. Midl. Nat. 97: 350-362. PAYNE, A. M., and M. A. MAUN.1984. Reproduction and survivorship of Cakile edentula var. lacustris along the Lake Huron shoreline. Am. Midl. Nat. 111: 86-95. SOKAL, R. R., and F. J. ROHLF.1981. Biometry: the principles and practice of statistics in biological research. W. H. Freeman and Co., San Franscisco. TYNDALL, R. W. 1985. Role of seed burial, salt spray, and soil moisture deficit in plant distribution on the North Carolina Outer Banks. Ph.D. thesis, University of Maryland, College Park. VANDERVALK,A. G . 1974. Environmental factors controlling the distribution of forbs on coastal foredunes in Cape Hatteras National Seashore. Can. J. Bot. 52: 1057-1073.