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Ecology |
Département de biologie and Centre d'études nordiques, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4
Received for publication July 17, 2001. Accepted for publication October 2, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: beach abrasion • Caryophyllaceae • coastal dunes • Honckenya peploides • Leymus mollis • plant succession • Poaceae • sand burial • zonation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Indeed, sandy substrates have poor water-holding capacity that leads to rapid percolation of precipitation water (Salisbury, 1952
), with significant nutrient leaching (Kellman and Roulet, 1990
). Strong winds and relatively fine particle size lead to high substrate mobility, with local accumulation and erosion (Olson, 1958
; Hesp, 1989
). Furthermore, winds carry saltwater droplets (salt spray) that accumulate on plant tissues and also contribute to substrate salinity. Tides may increase substrate salinity and, in some rare events (e.g., major storms), cause upper beach erosion (Maun, 1984
; Lee and Ignaciuk, 1985
). All these conditions are generally unfavorable for the various life stages of plants and particularly for seedlings (Maze and Whalley, 1992
; Maun, 1996
).
However, sand movement, salt spray, soil salinity, and pH all typically decrease, while soil organic matter content and macronutrient concentration increase from the upper beach to the stabilized dunes (Salisbury, 1952
; Ranwell, 1972
; Hundt, 1985
; Imbert and Houle, 2000
); in other words, abiotic conditions become less restrictive to plant growth. Significant changes in these conditions occur even over relatively short distances, for instance from the upper beach to the foredune (approximately 15 m; Houle, 1997b
). Plant zonation on coastal dunes may thus reflect individual species tolerance to such environmental conditions (Oosting and Billings, 1942
; Barbour and de Jong, 1977
; Wilson and Sykes, 1999
).
On coastal dunes in subarctic Québec, Honckenya peploides (L.) Ehrh. (Caryophyllaceae) colonizes the upper beach where it forms small mounds called embryo dunes. Leymus mollis Trin. (Poaceae) is mostly restricted to the foredune; however, a few individuals successfully establish on the upper beach, particularly on embryo dunes (Houle, 1997b
). The aim of this study was to evaluate H. peploides and L. mollis seedling tolerance to sand burial, salt spray, soil salinity, and nutrient and water availability in greenhouse experiments. We hypothesized that the spatial segregation of the two species on subarctic coastal dunes was associated with differential tolerance of their seedlings to abiotic stresses (Houle, 1997a, b
).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Experiments
Greenhouse experiments were conducted at the Centre d'études nordiques research station at Whapmagoostui-Kuujjuaraapik (Great Whale; 55°17' N, 77°45' W) on the east coast of Hudson Bay, in northern Québec, Canada. The effects of sand burial, soil salinity, salt spray, and nutrient and water availability on the growth and survival of Leymus and Honckenya seedlings were determined. During the experiments, day length was maintained at a minimum of 14 h while temperature fluctuated around 20°C. Leymus and Honckenya seedlings were collected on a coastal dune system, approximately 3 km north of the research station in mid-June of 1997 (Gagné, 1998
). One week after transplantation, seedlings were submitted to a specific treatment or treatment combination (see below) for a period of 7 wk. At the end of the experiments, seedlings were carefully washed and their biomass was sorted into below- and aboveground parts. Tissues were dried at 70°C for 36 h and weighed.
Sand burial experiment
Small holes (0.5 cm diameter) were made at the bottom of each one of 15 plastic containers (0.02 m3). These were draped with a geotextile membrane and filled with dune sand. In each container, ten 5.2 cm diameter plastic cylinders were vertically driven into the sand and filled with dune sand. Initially, 10 cm of each cylinder emerged from the sand surface. Each cylinder contained one seedling, i.e., there were five cylinders with one Leymus seedling and five cylinders with one Honckenya seedling per container. Levels of sand burial were randomly assigned to each seedling within a container (0, 0.5, 1.0, 1.5, and 2.0 cm/wk; randomized block design). Once per week, cylinders were pulled out from the sand on a length corresponding to a given level of burial and then filled with dune sand. Weekly, 20 mL of a 4-g/L nutrient solution (Plant Prod 20-20-20; Plant Products, Brampton, Ontario, Canada) were added to each cylinder. Seedlings were watered daily, except on the days of nutrient addition.
Soil salinity and salt spray experiment
In this experiment, two factors were studied: soil salinity (with five levels: 0, 2.5, 5, 10, and 20 g sea salt/L) and salt spray (with two levels: 0 and 10 g sea salt/L) in a full factorial design, with 15 replicates per treatment combination, per species. Because it was almost impossible to spray seedlings individually, a split-plot design was used (whole plot: salt spray; subplot: soil salinity; Montgomery, 1984
). Seedlings were transplanted individually into 110-cm3 pots filled with dune sand. Weekly, 10 mL of a 4 g/L nutrient solution (Plant Prod 20-20-20) were provided to each seedling. The day after nutrient addition, 20 mL of a 0, 2.5, 5, 10, or 20 g sea salt/L solution were added to the soil, and 10 mL of a 0- or 10-g sea salt/L solution were sprayed onto the seedlings. Three days later, seedlings were sprayed again. Seedlings were watered daily with tap water, except on the days nutrients were added or the saline solution and/or salt spray were applied.
Nutrient stress experiment
Seedlings of Leymus and Honckenya were individually transplanted into 110-cm3 pots filled with dune sand. There were 15 replicates per species (in a randomized block design). The effect of nutrient availability on seedling growth was evaluated by adding one of four concentrations of a nutrient solution: 0, 1, 2, or 4 g/L of a complete fertilizer (Plant Prod 20-20-20). Weekly, 10 mL of the nutrient solution were provided to each seedling (thus 0, 2, 4, or 8 mg of N per week for level 0, 1, 2, or 4 g/L, respectively). Seedlings were watered on every other day with tap water.
Drought experiment
For this experiment, seedlings were individually transplanted into 400-cm3 pots. Pots were draped with a geotextile membrane and filled with dune sand. The four levels of water availability (each with 15 replicates, in a randomized block design) were 0, 1, 3, and 5 d without watering for each of three consecutive periods of 14 d. At day 1 and 8, 25 mL of a 4-g/L complete nutrient solution (Plant Prod 20-20-20) were added to each pot. Seedlings assigned to 1, 3, and 5 d of drought were not watered at day(s) 2; 2, 3, and 4; and 2, 3, 4, 5, and 6, respectively. The day after the drought treatment (respectively, days 3, 5, and 7), pots were placed in water for about 10 min to allow saturation. On the other days, pots were watered from above with tap water.
Statistical analyses
Total biomass and root/shoot ratio were analyzed with one- (sand burial, nutrient addition, and drought experiments) or two-factor (soil salinity and salt spray experiment) analyses of variance (ANOVA). When significant differences among means were detected (P 
0.05), multiple comparisons were performed using LSD tests (Fisher's least significant difference; Sokal and Rohlf, 1995
). Root : shoot ratios were transformed to obtain homogeneous variances before the analyses.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Substrate salinity and salt spray experiment
Leymus seedling survival remained high under all treatment combinations (0-g salt spray level: 93, 100, 93, 100, and 87%; 10-g salt spray level: 100, 93, 93, 100, and 93% for 0, 2.5, 5, 10, and 20 mg sea salt/L, respectively). Salt spray exacerbated the growth reduction caused by high soil salinity, except at the very highest soil salinity level (significant salt spray x substrate salinity interaction, Fig. 2). Leymus seedlings of the intermediate substrate salinity level (i.e., 5.0 g sea salt/L) had the lowest root : shoot ratio (Fig. 2). Salt spray had no significant effect on this variable.
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Nutrient stress experiment
Seedling survival rates were lower for Honckenya (73, 73, 73, and 60% for 0, 1, 2, and 4 g/L, respectively) than for Leymus (100, 93, 100, and 100% for 0, 1, 2, and 4 g/L, respectively). Both Leymus and Honckenya seedlings were quite sensitive to nutrient availability (Fig. 3). Total biomass increased about four times for Honckenya and two times for Leymus as the concentration of the nutrient solution increased from 0 to 4 g/L. Meanwhile the root : shoot ratio decreased for Honckenya, but did not change significantly for Leymus.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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showed that soil physicochemistry, sand accretion/erosion, and local climatic conditions all contribute to the harshness of the upper beach environment on coastal dunes in subarctic Québec. For instance, between June 1996 and August 1997, he estimated that approximately 12 cm of sand accumulated on embryo dunes. He also demonstrated that the substrate was saline and contained very little water (1–2% moisture in the top 10 cm). Furthermore, he showed that the substrate was very poor in nutrients, with an average concentration of 0.007% N, 0.945 ppm P, and 14.34 ppm K. Such extreme, limiting conditions are typical of other coastal dune systems (e.g., Ranwell, 1972
; Hundt, 1985
; Hesp, 1991
).
Sand burial experiment
In the present study, we showed that experimental burial significantly reduced Leymus seedling biomass. Several studies have reported similar responses for other dune plants (Sykes and Wilson, 1990
; Zhang and Maun, 1990
; Martinez and Moreno-Casasola, 1996
). Sand burial appears to negatively affect seedling growth and, eventually, survival by depriving of light a significant portion of the photosynthetically active plant surface. Yet, in some species, sand burial stimulates growth (Lee and Ignaciuk, 1985
; Sykes and Wilson, 1990
), for instance, by improving conditions around the roots, such as water availability (Zhang and Maun, 1992
). In our experiment, Leymus seedling growth was not stimulated by sand burial. However, abundant daily watering probably obscured any such ameliorating effect on water status from sand burial. In the natural habitat, where soil moisture is typically low, growth stimulation may indeed occur after sand burial events (Gagné, 1998
). Unlike other species (Sykes and Wilson, 1990
; Martinez and Moreno-Casasola, 1996
; Brown, 1997
), Leymus root : shoot ratio was relatively unresponsive to sand burial.
Because few Honckenya seedlings survived through our burial experiment, we cannot conclude definitively about the effect of sand burial on this species. Nevertheless, with null survival at burial rates of 1.5 and 2.0 cm/wk, it appears that Honckenya seedlings are much less tolerant than Leymus seedlings to sand burial.
Substrate salinity and salt spray experiment
Both Leymus and Honckenya seedling biomass decreased with increases in soil salinity, particularly in combination with simulated salt spray. Similar results have been presented for Atriplex laciniata (Lee and Ignaciuk, 1985
), Scaevola sericea (Alpha, Drake, and Goldstein, 1996
), Leymus arenarius (Greipsson and Davy, 1996
), and Atriplex patula (Ungar, 1996
). Growth stimulation at low saline conditions was not observed, which is contrary to the results of Lee and Ignaciuk (1985)
for several strandline species (Atriplex glabriuscula, Cakile maritima, and Salsola kali). Although Honckenya growth reduction in response to salinity was comparable to that of Leymus, seedling survival rates were quite different: only a few Honckenya seedlings survived to the highest salinity treatment.
Nutrient stress experiment
Leymus and Honckenya seedlings were very tolerant of low nutrient availability; however, even low to moderate nutrient additions increased biomass (Hawke and Maun, 1988
; Martinez and Rincon, 1993
; Valverde, Pisanty, and Rincon, 1997
). The root : shoot ratio decreased with increases in nutrient availability for Honckenya, but not for Leymus. Valverde, Pisanty, and Rincon (1997)
similarly observed reductions in the root/shoot ratio of three out of six dune species. The poor nutrient status of the dune habitat, thus, likely limits Leymus and Honckenya seedling growth, but not their survival.
Drought stress experiment
The low soil moisture typical of coastal dunes is considered to be an important seedling mortality factor (McLeod and Murphy, 1983
; Maun, 1985, 1994
). For instance, with weekly watering, 92% of Ammophila breviligulata seedlings survived in the field until the end of the growth season while under natural conditions none survived (Maun and Krajnyk, 1989
). In our greenhouse experiments, Leymus and Honckenya survived successive periods of 5 d of drought. However, even after 5 d without watering, mean soil moisture did not decrease below 2.5% (data not shown), whereas it averaged 1.9% and 1.1% in 1996 and 1997 in the field (Gagné, 1998
). Yet, Leymus and Honckenya seedling growth was reduced by low water availability.
Greenhouse experiments integrated with field studies
Greenhouse experiments allowed us to isolate and verify the effects of diverse abiotic factors on Leymus and Honckenya seedling growth and survival. However, in the natural habitat, environmental factors are likely to interact, and it is difficult to evaluate their full consequences only from controlled experiments. For instance, nutrient addition increased Leymus ramet biomass in a greenhouse experiment, but had no effect in the natural habitat (Houle, 1997a
). Two-year seedling censuses combined with abiotic variable measurements in the natural habitat (Gagné, 1998
) put the present results in perspective.
Honckenya seedling mortality rates reached close to 100% both in 1996 and 1997 on the upper beach of a coastal dune system at Kuujjuaraapik-Whapmagoostui, where our greenhouse experiments were performed (Gagné, 1998
). Important sand burial events appeared to be responsible for that mortality. Van der Valk (1974)
and Maun (1984)
also observed high mortality rates in dune plants attributable to major sand accumulation. For Leymus, seedling mortality rates were 17% and 33% in 1996 and 1997, respectively, in the natural habitat (Gagné, 1998
). In 1997, some Leymus seedlings from the embryo dunes survived up to approximately 7 cm of sand accumulation during the growth season. The fact that Leymus seedlings survived better than Honckenya seedlings at the upper edge of the beach contradicts our initial hypothesis. Indeed, as the upper edge of the beach is occupied almost exclusively by mature Honckenya (only a few Leymus individuals are present; Houle, 1997b
), we expected seedling survival to be higher for Honckenya than for Leymus at the upper edge of the beach.
If Leymus seeds are available on the upper beach (Gagné, 1998
) and seedlings survive relatively well there, how can mature individuals be rare in this habitat? We suggest that factors such as massive abrasion events may be responsible for the quasi-absence of Leymus on the upper beach. Coastal systems are subject to periodic massive erosion via violent storm waves and, potentially, ice thrust (Maun, 1984
; Harris and Davy, 1986
; Costa, Seeliger, and Cordazzo, 1991
). Although no massive erosion occurred between September 1996 and June 1997 at the upper edge of the beach studied by Gagné (1998)
, between September 1995 and June 1996, a massive beach erosion occurred (J.-M. Gagné and G. Houle, personal observations). In June 1996, Honckenya stems and rhizomes were exposed. The embryo dunes of 1995 were now flattened-out surfaces. Such erosion (by wind, wave, or ice) is capable of uprooting plants, especially seedlings, and causing their death. Yet rhizomes can be fragmented and disperse and establish closer to the shoreline or even colonize new substrate (Maun, 1984
; Harris and Davy, 1986
).
Erosion seems the most probable factor explaining the rarity of Leymus on the upper beach. Maun (1984)
observed that the extension of Ammophila breviligulata populations toward the beach was limited by erosion. Honckenya subvertical rhizomes appear to confer a greater tolerance to erosion than Leymus subhorizontal and vertical rhizomes. At the end of the growth season, the succulent leaves of Honckenya senesce and stems, if buried, become rhizomes that bear adventitious roots and numerous buds. Clones are thus deeply rooted in the sand (Marie-Victorin, 1995
), reducing the risk of root desiccation after erosion events and allowing rapid recovery (Maun, 1994
). Erosion exposes adventitious roots and buds on Leymus rhizomes which then become susceptible to dessication.
The upper beach thus represents a habitat strongly influenced by frequent disturbances. At lower latitudes, this habitat is mostly occupied by annuals such as Cakile edentula (Doing, 1985
). Short life cycles in combination with fruit buoyancy allow annuals to better survive periodic erosions (Ignaciuk and Lee, 1980
). At higher latitudes, annuals are absent from the upper beach (Imbert and Houle, 2000
). Although Leymus appears more tolerant to salt spray, soil salinity, drought, and sand burial, low tolerance to disturbance in the form of erosion may restrict its presence on the upper beach of high-latitude coastal dunes.
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FOOTNOTES |
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2 Author for reprint request (gilles.houle{at}bio.ulaval.ca
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Barbour M. G. T. M. de Jong 1977 Response of west coast beach taxa to salt spray, seawater inundation, and soil salinity. Bulletin of the Torrey Botanical Club 104: 29-34[CrossRef][ISI]
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Doing H. 1985 Coastal fore-dune zonation and succession in various parts of the world. Vegetatio 61: 65-75
Gagné J.-M. 1998 Écologie comparée de la colonisation des dunes côtières de Poste-de-la-Baleine, Québec subarctique, par Honckenya peploides and Elymus mollis. Master's thesis, Université Laval, Québec, Canada
Greipsson S. A. J. Davy 1996 Sand accretion and salinity as constraints on the establishment of Leymus arenarius for land reclamation in Iceland. Annals of Botany 78: 611-618
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