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Ecology |
Department of Biology, The College of Staten Island, City University of New York, Staten Island, New York 10314 USA
Received for publication June 26, 2001. Accepted for publication October 9, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: adaptation • annual dunegrass • coastal ecosystem • Poaceae • salt spray • Staten Island • Triplasis purpurea
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Barbour, 1978
; Barbour, de Jong, and Pavlik, 1985
; Sykes and Wilson, 1988
; Hesp, 1991
; Maun, 1994
; Greipsson and Davy, 1996
). In coastal communities, the distribution of species can sometimes be tied to their tolerance of salt spray (Oosting and Billings, 1942
; Oosting, 1945
; Rozema et al., 1985
; Sykes and Wilson, 1988
; Wilson and Sykes, 1999
). This suggests that adaptation to salt spray within species might be detected within subpopulations that are closest to shore, analogous to the way that intraspecific variation in the tolerance of soil salinity has been documented (Hannon and Bradshaw, 1968
; Rozema et al., 1985
; Wang and Redmann, 1996
; Greipsson, Ahokas, and Vähämiko, 1997
; Hester, Mendelssohn, and McKee, 1996
).
The continued microevolution of improved salt tolerance in coastal subpopulations requires that there be genetic variation in the effects of saltwater spray on life history traits. Although there is limited evidence that there may be a genetic basis to the tolerance of salt sprays (e.g., Humphreys, 1982
), no studies have addressed the adaptive variation that might exist in the response of differentiated subpopulations to salt spray within a coastal species.
In the present study, one objective was to determine whether or not a large coastal population of the annual grass Triplasis purpurea on Staten Island, New York, USA, showed subpopulation differentiation in relation to the effects of salt spray on life history traits. A second related objective was to determine whether or not there was detectable variation among genetically related, full-sib families in their life history responses to airborne salt spray. It was predicted that families from a subpopulation closest to shore would be better adapted to salt sprays and therefore show greater tolerance to the potentially deleterious effects of salt spray previously demonstrated for this species (Cheplick and Demetri, 1999
) compared to families from a subpopulation farther from shore (where salt sprays are minimal). This prediction was partly prompted by previous data that showed (1) salt spray deposition onto T. purpurea plants declines significantly with increasing distance from 30 to 90 m from the shoreline of Staten Island (Cheplick and Demetri, 1999
), and (2) T. purpurea plants closest to shore (30–40 m) typically show the greatest growth and reproduction relative to those farther from shore (80–90 m) (Cheplick and Demetri, 2000
).
It was surmised that T. purpurea could exhibit localized adaptive variation and intrapopulational differentiation in the ability to tolerate salt spray because (1) it has a predominantly self-fertilizing breeding system, maturing most seeds in cleistogamous spikelets on axillary panicles enclosed within leaf sheaths along a culm (Cheplick, 1996a
; Cheplick and Sung, 1998
), and (2) it tends to exhibit very poor seed-dispersal capacity (Cheplick, 1996b, 1998
). In general, these factors should tend to limit the opportunity for gene exchange among subpopulations and promote intrapopulational differentiation (Charlesworth and Charlesworth, 1995
; Linhart and Grant, 1996
; Godt and Hamrick, 1998
; Bohonak, 1999
).
Annual species such as T. purpurea are often abundant in the pioneer zone along coastal beaches (Watkinson and Davy, 1985
; Crawford, 1989
; Hesp, 1991
; Garcia-Mora, Gallego-Fernandez, and Garcia-Novo, 1999
; Cheplick and Demetri, 2000
); however, continual establishment and maintenance of populations from year to year require substantial seed production. Hence, a third objective of the present research was to determine the effects of salt sprays on seed production and germination and also to attempt to discern which of the recorded traits were the critical determinants of seed production.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Duncan and Duncan, 1987
). It is a native annual: in southern New York, seeds germinate in late April to early May, vegetative growth occurs by tiller production and elongation during the summer, and flowering begins in late August. Most seeds are matured on leaf-sheath-enclosed, axillary panicles that bear self-fertilizing, cleistogamous spikelets (Cheplick, 1996a
). Additional chasmogamous spikelets can be produced on terminal panicles that emerge from the tips of large tillers from September through autumnal senescence sometime in late October. The upper culm nodes disarticulate over winter, while the lowermost nodes (with sheath-enclosed seeds) usually remain intact. The following spring, seedlings will often emerge directly from the sheath-enclosed seeds at the lowermost nodes of dried parental culms. Hence, seed dispersal is likely to be very limited for this species (Cheplick, 1996b, 1998
).
The seeds from outcrossed spikelets on tillers with terminal panicles have some potential for dispersal by wind, but comprise only about 25% of the seeds matured by these tillers (Cheplick, 1996a
). Furthermore, only 15–35% of the tillers typically have terminal panicles when plants mature in the field (Cheplick, 1996a
). Thus most plants in a population are likely to have originated from the seeds of sheath-enclosed panicles. Unfortunately, it is not possible to know for certain whether or not the seeds used in this study were collected mostly from plants derived from the undispersed seeds of sheath-enclosed panicles (as suspected) or from plants derived from the dispersed seeds of outcrossed panicles.
Along the southeastern facing shore of Staten Island, New York, T. purpurea occurs at locally high densities on sandy beaches at distances 25–90 m from shoreline (Cheplick and Demetri, 2000
). The soils are of the Hooksan series, characterized by thick, sandy sediments (>98% sand) that are nutrient-poor and excessively drained (L. A. Hernandez, Natural Resources Conservation Service, U.S. Deptartment of Agriculture, personal communication). The population studied here occurred at Midland Beach, Staten Island (40°34'4'' N, 74°5'30'' W), a site moderately disturbed by human traffic. The extensive T. purpurea population occurred from 15 to 90 m from the shoreline (i.e., maximum water extent at low tide; Cheplick and Demetri, 2000
). Common co-occurring species included Ammophila breviligulata, Cenchrus tribuloides, Cakile edentula, and Solidago sempervirens. Cheplick and Demetri (1999)
documented that, within this population, the highest levels of salt (175 µg/cm2) were deposited onto T. purpurea individuals growing closest to shore (30–40 m); at distances greater than 70 m, salt deposition was negligible (<20 µg/cm2).
The experiment
On 11 September 1998, 21 mature, seed-producing individuals of T. purpurea were sampled along each of two transects that were 75 m long and parallel to shore. The "near" transect sampled the subpopulation 15 m from shore where salt spray exposure is most intense, while the "far" transect sampled the subpopulation 80 m from shore where salt spray is mostly negligible (Cheplick and Demetri, 1999
). For all individuals, only the upper three nodes of a maternal tiller with sheath-enclosed seeds were collected to control for the seed size variation that occurs along nodes in this species (Cheplick, 1996a
). Seeds were stored within tillers in envelopes at 4°C. Seed families were retained separately for each individual sampled.
Seeds were extracted from tillers in early January 1999 and placed into 9-cm petri dishes containing a 3-mm layer of moist sterile sand for stratification at 4°C. On 12 April 1999, the petri dishes were placed into the greenhouse and exposed to diurnal temperature oscillations of approximately 10°C (maximum of 27°C day, minimum of 17°C night). Seedlings were about 1 cm tall when transplanted into square plastic pots (8 x 8 x 7.4 cm depth) containing a 6 : 2 : 1 mixture of sterile sand, topsoil, and field sand collected from Midland Beach. The soil mix was thoroughly wetted prior to planting of the seedlings.
It was originally desired to plant 12 siblings per each of 13 families per subpopulation; however, due to insufficient numbers of emerging seedlings and some early mortality, final sample sizes were reduced slightly. In the complete experiment, there were 13 families from the near subpopulation and 11 families from the far subpopulation, and both had 8–12 siblings per family.
All pots were completely randomized onto benches in the greenhouse at the College of Staten Island, City University of New York, Staten Island, New York, USA, and watered from the base as needed (every 2–4 d). Seedlings were allowed to establish and grow for 1.5 mo. On the day before saltwater spray treatment was begun, a nondestructive measure of size was recorded. Hereafter referred to as initial size, this measurement consisted of the summed lengths of all tillers from the base to tip, following Cheplick and Demetri (1999)
. Tiller number was also recorded at this time.
For half of the siblings in each family, saltwater spray treatment began on 1 July. Seawater was collected offshore from Midland Beach and stored in a 3.8-L plastic jug at 4°C. This seawater had a salinity of 26.2 ppt at 21.8°C and later measurements showed that salinity did not change with short-term storage. The seawater was applied as a fine mist twice weekly with three sprays (ca. 3 mL) from a bottle held 20 cm from the side of each plant. During each spray application, the soil surface was covered to assure that salt levels within the substrate were not elevated by the treatment. The level of salt deposition that resulted (see RESULTS: Salt deposition onto shoots and seeds) was 213 µg/cm2, just slightly above levels recorded by Cheplick and Demetri (1999)
30–40 m from shore (175 µg/cm2). Control plants were similarly sprayed with distilled water. Salt sprays were continued until 9 August, at which time 50% of the plants had begun to show early signs of senescence. During the experiment, mean (±SD) maximum and minimum greenhouse temperatures on 40 randomly selected days were 27.8 ± 2.1°C and 18.6 ± 1.5°C, respectively.
Data collection
Life span was recorded as the number of days from planting until complete senescence, when green tissues were no longer evident. Seeds on terminal panicles were collected as they matured to minimize loss due to disarticulation. Most seeds (>90%) were matured on axillary panicles within leaf sheaths at each node and some plants (and many tillers) did not produce emergent panicles, a situation typical of this species in field populations (Cheplick, 1996a
). Total number of seeds is therefore the sum of seeds produced on both terminal and axillary panicles.
Also recorded at the time of death were the number of tillers and the dry mass of roots and shoots after drying at 60°C to constant mass. Because seeds were later assessed for germinability and viability (see below), air-dried mass after 1 mo (rather than oven-dried mass) was obtained for all seeds produced per plant. Derived variables were mean seed mass (mass of all seeds/total number of seeds) and reproductive effort (mass of all seeds/vegetative mass).
Determination of salt deposition onto shoots and seeds
To determine the quantity of salt deposited onto shoots by the weekly sprays, 13 plants not used in the experiment were sprayed three times on 11 August and 13 August. Immediately after drying of the second spraying, shoot area was then determined with a leaf area meter (LI-COR LI-3100; LI-COR, Lincoln, Nebraska, USA). Each plant was immersed and agitated for 1 min in 150 mL of distilled water to remove external salts from the shoot (Cheplick and Demetri, 1999
). A conductivity meter (YSI-Model 30, YSI, Yellow Springs, Ohio, USA) was used to measure conductivity of the rinse. From the standard curve developed by Cheplick and Demetri (1999)
, the quantity of salt deposited could be expressed per unit of shoot surface area.
To determine whether or not significant quantities of salt had been deposited onto seeds prior to the germination trial, seeds from 11 salt-sprayed and 7 control plants were collected. Each seed family was agitated for 20 sec in 150 mL distilled water and the conductivity of the rinse was recorded. Total seed mass was determined so that the quantity of surficial salt could be expressed per unit mass of seeds.
Germination trial
All sheath-enclosed seeds from the experimental plants were stored dry in paper envelopes at 4°C. From 16 June to 18 July 2000, seeds were stratified at 4°C in petri dishes on water-moistened filter paper (9 cm in diameter). Seeds from 3–5 plants per family per treatment were subjected to a germination trial to explore potential effects of salt sprays of maternal plants on germinability and viability in the two subpopulations.
Seed dishes were placed into an incubator set on a diurnal cycle of 12 h light at 25°C and 12 h dark at 15°C (Cheplick and Sung, 1998
). Germination was recorded as radicle emergence, using a dissection microscope, once a week until no further germination occurred (34 d). Despite treatment of seeds with a 10% chlorine bleach rinse, some surficial mold developed. All moldy seeds were counted and discarded; they are not included in the calculation of final germination percentage.
After the germination trial, a subsample of 20 ungerminated, nonmoldy seeds per family were tested for viability with a 0.5% solution of tetrazolium chloride, following Cheplick and Sung (1998)
. Seeds were soaked in the solution for 2 h on a 40°C hot plate and examined under a dissection microscope. Pink-stained embryos were scored as viable.
Data analyses
A mixed-model, partially nested analysis of covariance (ANCOVA) was used to analyze all variables in the experiment. The covariate was initial size: initial number of tillers for the analysis of final number of tillers or summed tiller lengths for the analysis of all other variables. The components of the complete ANCOVA model and corresponding F tests are shown in Table 1. Distance from shore each subpopulation occurred and treatment were fixed effects; family nested within distance was a random effect. The Statistical Analysis System, version 6.09 (SAS, 1990
) was used for all ANCOVAs. To improve homogeneity of variances and normality, final number of tillers was square-root transformed, number of seeds and mean seed mass were log10 transformed, and reproductive effort was arcsine, square-root transformed (Underwood, 1997
). Only means adjusted for the covariate (i.e., least squares means) are reported in the tables and figures.
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). In a path diagram, a one-headed arrow indicates a causal effect of one variable on another, while a two-headed arrow represents a correlation between two variables. The SAS procedure PROC CALIS was used to generate path coefficients, which are standardized partial regression coefficients. Because path analysis makes the same assumptions as other parametric techniques, initial size, life span, vegetative mass, and number of seeds were log10 transformed and final number of tillers was square-root transformed prior to analysis. Separate path analyses were performed for the salt spray and control groups.
For seed germination and viability data, row by column tests of independence using the G statistic were performed to ascertain whether or not percentage germination or viability depended on salt spray (Sokal and Rohlf, 1981
). All families within each of the two subpopulations were combined for analysis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Only two traits were affected by salt sprays. The final number of tillers was significantly reduced by saltwater spray, whereas RE was significantly higher in that treatment (Table 3). The latter effect was likely due to the fact that mean shoot mass was slightly lower and mean number of seeds was slightly greater for salt-sprayed plants relative to control plants (Table 3).
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; Schlichting and Pigliucci, 1998
). For some families in each subpopulation (e.g., F8 and N14), there was a reduction in life span with salt spray, but in others (e.g., F13 and N2), plants survived longer with salt spray (Fig. 1). Altogether only 5 of 13 families (38%) from the near subpopulation showed a reduction in life span with salt spray, and 4 of 11 families (36%) from the far subpopulation showed a similar reduction. Life span is important to fitness in T. purpurea as it is significantly correlated with seed production (Fig. 2), albeit via an indirect effect on tillering and biomass production (see below). The slope of the line depicting this relationship for salt-sprayed plants (1.58) was about three times greater than that of control plants (0.54) (Fig. 2).
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Germination trial
Although there was tremendous variation among families in the total number of seeds that germinated, percentage germination (expressed as a fraction of the total number of nonmoldy seeds) was relatively low overall (Table 4). For families from the subpopulation nearest to shore, germination was significantly reduced by salt spray (31% vs. 36% in the control). In contrast, for families from the subpopulation farthest from shore, the situation was reversed: germination was greater for the salt-sprayed plants (30% vs. 25% in the control).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Epperson, 1990
; Williams, 1994
; Linhart and Grant, 1996
). Often such differentiation is attributed to agents of natural selection such as toxic soils, competitors, or other environmental factors that vary in intensity along a spatial axis (Linhart and Grant, 1996
). Despite the predominantly self-fertilizing breeding system and poor seed dispersal capacity of Triplasis purpurea (both factors that severely restrict gene flow) (Cheplick, 1996a, b
) and the continuous exposure of plants close to shore to salt deposition (Cheplick and Demetri, 1999
), we did not detect any evidence of subpopulation differentiation and local adaptation to salt sprays within this coastal population.
For other plant species where adaptation to local conditions was not detected, it has often been argued that phenotypic plasticity has evolved in response to fine-scale, unpredictable environmental heterogeneity, rather than genetic adaptation of population subgroups (Hickman, 1975
; Cartica and Quinn, 1982
; Rapson and Wilson, 1988
; Rice and Mack, 1991
; Galloway and Fenster, 2000
). Genetically based plasticity in relation to the effects of salt spray was certainly detectable for some traits in T. purpurea (Figs. 3 and 4). However, similar to Cartica and Quinn's (1982)
study of the coastal perennial Solidago sempervirens, the differences in size and fecundity detected between T. purpurea in the field at varying distances from shore (Cheplick and Demetri, 2000
) are probably environmentally induced. Given that salt spray deposition levels can vary greatly within a population, depending not only on proximity to the shore, but also on wind intensity and direction, topography, and the timing of rainfall episodes (Boyce, 1954
; Edwards and Claxton, 1964
; Barbour, 1978
; Cheplick and Demetri, 1999
), plasticity of growth and reproduction may be a viable buffer against the selective elimination of suboptimal genotypes (Sultan, 1987
; Rice and Mack, 1991
).
Alternatively, the usual salt spray levels found along the shore of Staten Island may not be significant enough to function as a potent agent of natural selection. This may be especially true for species like T. purpurea because the cuticular surface of many grass leaves is not completely wettable, resulting in the beading of droplets on the leaf surface and reduced entrance of harmful chlorides (Boyce, 1954
). For 29 species from sand dunes in New Zealand, most of the grasses were not adversely affected by experimental salt sprays (Sykes and Wilson, 1988
). For T. purpurea in the present study, only the number of tillers was significantly reduced by salt spray (Table 3), although shoot mass was also reduced slightly. The levels of salt deposited onto the shoots during 1 wk (213 µg/cm2) was in between the low (133 µg/cm2) and high (327 µg/cm2) weekly salt spray treatments of Cheplick and Demetri (1999)
. In their study, only the highest salt spray level significantly reduced T. purpurea shoot mass and seed production; the lower salt spray regime mostly only reduced tiller numbers, similar to this study. Levels of salt deposition at 30–40 m from shore were between 160 and 175 µg/cm2 at Midland Beach, Staten Island, on one sample date following a week of no rain (Cheplick and Demetri, 1999
). Although salt deposition may be greater than this during some periods (e.g., periods of high wind and wave activity, longer periods of no rainfall), it is possible that the reproductive fitness of T. purpurea is not substantially impacted by airborne salt under most circumstances, minimizing the opportunity for selective discrimination among genotypes. The lack of a significant family (i.e., genetic) effect on seed production (Table 2) supports this proposition. The establishment of substantial numbers of seedlings in close proximity to shore and their vigorous growth and reproduction there (Cheplick and Demetri, 2000
), suggest that T. purpurea is moderately resistant to low levels of salt deposition and can persist locally on coastal beaches. This contention also supports previous research showing that the distribution of T. purpurea seedlings on a coastal beach in North Carolina, USA, was not limited by salt sprays (Tyndall et al., 1987
).
Although seed production was unaffected by salt spray, percentage germination was reduced by salt spray for the subpopulation nearest to shore, contrary to expectations if plants there were better adapted to the local environment (Table 4). The far subpopulation, in contrast, showed improved germination following salt spray and greater viability of ungerminated seeds. This is the first report of a potential maternal effect mediated by airborne salt in a coastal species. Previously, it was demonstrated that the soil nutrient levels under maternal T. purpurea plants had a pronounced influence on the germination of their seeds (Cheplick and Sung, 1998
). It is presently unknown why salt spray reduced germination only in one subpopulation and whether or not such a maternal effect has any impact on seedling establishment or selection in the field. The possible mechanism for salt-mediated maternal effects is also elusive, although elevated salt levels on seed surfaces were not detected; this is not surprising given that all seeds used in the germination trial had matured on leaf sheath-enclosed panicles. Mean seed mass also did not differ between salt-sprayed and control plants (Table 3).
Family effects and evolutionary consequences
Although there was no evidence of differentiation among the subpopulations for any life history trait, there was a highly significant family component (within each subpopulation) to the phenotypic variation detected in the experiment (Table 2). This indicates that there are pronounced differences among these inbred family groups and the opportunity for genetic "kinship" substructuring within this population (Heywood, 1991
). In species like T. purpurea that are predominantly self-fertilizing and of limited dispersal capacity (Cheplick, 1996a, b
) among-family variation is commonly observed (Schemske, 1984
; Cheplick and Quinn, 1986, 1988
; Heywood, 1991
; Williams, 1994
; Linhart and Grant, 1996
).
Local substructuring within a population due to variation among genetically related groups (i.e., families) has implications for the understanding of multilevel selection (Stevens, Goodnight, and Kalisz, 1995
) and metapopulation theory (Husband and Barrett, 1996
). Neighborhood models of group selection (Wilson, 1987
) may be appropriate for T. purpurea because the restriction of dispersal caused by seed retention along nodes within the leaf sheaths of maternal parents when they senesce in autumn often results in small groups of siblings that emerge from these seeds the following spring (Cheplick, 1996b, 1998
). When genetically variable traits like life span and vegetative mass impact fitness (Figs. 2 and 4), some sibling groups may be selectively favored relative to others (Wilson, 1987
; Cheplick, 1993
; Kelly, 1996, 1997
). Thus, when dispersal is limited and selfing is common (Stevens, Goodnight, and Kalisz, 1995
), selection may need to be considered as a process that operates on a "nested hierarchy of units" (Wilson and Dugatkin, 1997
). In short, population genetic structure appears to be a typical feature of coastal plants and important to their long-term dynamics (Clark, 1990
).
Because restricted dispersal is relatively common in plants, the resulting spatial patchiness can increase the stability and persistence of the metapopulation (Husband and Barrett, 1996
; Thrall, Burdon, and Murray, 2000
). For the annual T. purpurea, mostly existing in fragmented populations along the shore of Staten Island, repeated episodes of colonization and extinction are expected as new sibling groups establish in recently disturbed habitat patches and gradually decline as long-lived perennials develop (e.g., Ammophila breviligulata; Cheplick and Demetri, 2000
). Closer attention to the role variation among sibling groups plays in the long-term dynamics of the metapopulation is needed to elucidate the putative significance of multilevel selection in microevolution.
Variable effects of salt spray
A significant family by treatment interaction was detected for life span and mean seed mass (Table 2). This indicates genetic variability in plasticity exists within this T. purpurea population. Life span was reduced by salt spray in some families (Fig. 1) and, because a long life span is associated with greater seed production (Fig. 2), such families may be selected against in the areas closest to shore. However, the life span of many other families was clearly unaffected and sometimes increased by salt spray, although there was widespread variation in this life history trait for families in subpopulations both near and far from shore (note spread of points in Fig. 1).
Mean seed mass was similarly erratic in terms of the response of the families to salt spray (Fig. 3). Families that showed a reduction in mean seed mass under salt spray may be at a selective disadvantage relative to other families. Larger seed mass in T. purpurea results in the production of more vigorous seedlings (Cheplick and Sung, 1998
) that are more competitive (Cheplick and Wickstrom, 1999
) and better able to emerge after burial in sand (Cheplick and Grandstaff, 1997
).
The determinants of fitness
In T. purpurea, any factor that impacts vegetative mass is likely to directly influence seed production, a crude measure of annual fitness (Fig. 4). The number of tillers does not have any effect on seed production beyond its direct relation to vegetative mass. For the indeterminate annual plant, length of the growing season can profoundly determine seed crop size (Harper, 1977
). Opportunistic reproduction on newly formed tillers late in the season can increase seed output in T. purpurea (Cheplick, 1996a
). However, even in the relatively homogeneous greenhouse environment, it was discovered that life span could vary greatly among families (Fig. 1). Hence, the potential exists for microevolutionary changes in life span within this population via its indirect relation to maternal fitness, especially in areas where exposure to airborne salt is common.
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FOOTNOTES |
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2 Author for reprint requests (gpcsi{at}cunyvm.cuny.edu
)
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LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Barbour M. G. T. M. De Jong B. M. Pavlik 1985 Marine beach and dune plant communities. In B. F. Chabot and H. A. Mooney [eds.], Physiological ecology of North American plant communities, 296–322. Chapman and Hall, New York, New York, USA
Bohonak A. J. 1999 Dispersal, gene flow, and population structure. Quarterly Review of Biology 74: 21-45[CrossRef][Medline]
Boyce S. G. 1954 The salt spray community. Ecological Monographs 24: 29-67[CrossRef][ISI]
Cartica R. J. J. A. Quinn 1982 Resource allocation and fecundity of populations of Solidago sempervirens along a coastal dune gradient. Bulletin of the Torrey Botanical Club 109: 299-305[CrossRef][ISI]
Charlesworth D. B. Charlesworth 1995 Quantitative genetics in plants: the effect of the breeding system on genetic variability. Evolution 49: 911-920[CrossRef][ISI]
Cheplick G. P. 1993 Reproductive systems and sibling competition in plants. Plant Species Biology 8: 131-139
Cheplick G. P. 1996a Cleistogamy and seed heteromorphism in Triplasis purpurea (Poaceae). Bulletin of the Torrey Botanical Club 123: 25-33[CrossRef][ISI]
Cheplick G. P. 1996b Do seed germination patterns in cleistogamous annual grasses reduce the risk of sibling competition?. Journal of Ecology 84: 247-255[CrossRef]
Cheplick G. P. 1998 Seed dispersal and seedling establishment in grass populations. In G. P. Cheplick [ed.], Population biology of grasses, 84–105. Cambridge University Press, Cambridge, UK
Cheplick G. P. H. Demetri 1999 Impact of saltwater spray and sand deposition on the coastal annual Triplasis purpurea (Poaceae). American Journal of Botany 86: 703-710
Cheplick G. P. H. Demetri 2000 Population biology of the annual grass Triplasis purpurea in relation to distance from shore on Staten Island, New York. Journal of Coastal Conservation 5: 145-154
Cheplick G. P. K. Grandstaff 1997 Effects of sand burial on purple sandgrass (Triplasis purpurea): the significance of seed heteromorphism. Plant Ecology 133: 79-89[CrossRef][ISI]
Cheplick G. P. J. A. Quinn 1986 Self-fertilization in Amphicarpum purshii: its influence on fitness and variation of progeny from aerial panicles. American Midland Naturalist 116: 394-402[CrossRef][ISI]
Cheplick G. P. J. A. Quinn 1988 Quantitative variation of life history traits in amphicarpic peanutgrass (Amphicarpum purshii) and its evolutionary significance. American Journal of Botany 75: 123-131[CrossRef][ISI]
Cheplick G. P. L. Sung 1998 Effects of maternal nutrient environment and maturation position on seed heteromorphism, germination, and seedling growth in Triplasis purpurea (Poaceae). International Journal of Plant Sciences 159: 338-350[CrossRef]
Cheplick G. P. V. M. Wickstrom 1999 Assessing the potential for competition on a coastal beach and the significance of variable seed mass in Triplasis purpurea. Journal of the Torrey Botanical Society 126: 296-306[CrossRef][ISI]
Clark J. S. 1990 Population and evolutionary implications of being a coastal plant: long-term evidence from the North Atlantic Coasts. Reviews in Aquatic Sciences 2: 509-533[ISI]
Crawford R. M. M. 1989 Studies in plant survival: ecological case histories of plant adaptation to adversity. Blackwell Scientific Publishers, Oxford, UK
Duncan W. H. M. B. Duncan 1987 The Smithsonian guide to seaside plants of the Gulf and Atlantic coasts from Louisiana to Massachusetts, exclusive of lower peninsular Florida. Smithsonian Institution Press, Washington, D.C., USA
Edwards R. S. S. M. Claxton 1964 The distribution of air-borne salt of marine origin in the Aberystwyth area. Journal of Applied Ecology 1: 253-263
Epperson B. K. 1990 Spatial patterns of genetic variation within plant populations. In A. H. D. Brown, M. T. Clegg, A. L. Kahler, and B. S. Weir [eds.], Plant population genetics, breeding, and genetic resources, 229–253. Sinauer Associates, Sunderland, Massachusetts, USA
Galloway L. F. C. B. Fenster 2000 Population differentiation in an annual legume: local adaptation. Evolution 54: 1173-1181[CrossRef][ISI][Medline]
Garcia-Mora M. R. J. B. Gallego-Fernandez F. Garcia-Novo 1999 Plant functional types in coastal foredunes in relation to environmental stress and disturbance. Journal of Vegetation Science 10: 27-34
Godt M. J. J. L. Hamrick 1998 Allozyme diversity in the grasses. In G. P. Cheplick [ed.], Population biology of grasses, 11–29. Cambridge University Press, Cambridge, UK
Greipsson S. H. Ahokas S. Vähämiko 1997 A rapid adaptation to low salinity of inland-colonizing populations of the littoral grass Leymus arenarius. International Journal of Plant Sciences 158: 73-78[CrossRef]
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
Hannon N. A. D. Bradshaw 1968 Evolution of salt tolerance in two coexisting species of grass. Nature 220: 1342-1343
Harper J. L. 1977 Population biology of plants. Academic Press, New York, New York, USA
Hesp P. A. 1991 Ecological processes and plant adaptations on coastal dunes. Journal of Arid Environments 21: 165-191[ISI]
Hester M. W. I. A. Mendelssohn K. L. McKee 1996 Intraspecific variation in salt tolerance and morphology in the coastal grass Spartina patens (Poaceae). American Journal of Botany 83: 1521-1527[CrossRef][ISI]
Heywood J. S. 1991 Spatial analysis of genetic variation in plant populations. Annual Review of Ecology and Systematics 22: 335-355[CrossRef][ISI]
Hickman J. C. 1975 Environmental unpredictability and plastic energy allocation strategies in the annual Polygonum cascadense (Polygonaceae). Journal of Ecology 63: 689-701[CrossRef]
Hitchcock A. S. 1950 Manual of the grasses of the United States, 2nd ed. (revised by A. Chase). United States Department of Agriculture Miscellaneous Publication 200. Dover Publications, Washington, D.C., USA
Humphreys M. O. 1982 The genetic basis of tolerance to salt spray in populations of Festuca rubra L. New Phytologist 91: 287-296[CrossRef][ISI]
Husband B. C. S. C. H. Barrett 1996 A metapopulation perspective in plant population biology. Journal of Ecology 84: 461-469[CrossRef][ISI]
Kelly J. K. 1996 Kin selection in the annual plant Impatiens capensis. American Naturalist 147: 899-918[CrossRef][ISI]
Kelly J. K. 1997 Fitness variation across a subdivided population of the annual plant Impatiens capensis. Evolution 51: 1100-1111[CrossRef][ISI]
Linhart Y. B. M. C. Grant 1996 Evolutionary significance of local genetic differentiation in plants. Annual Review of Ecology and Systematics 27: 237-277[CrossRef][ISI]
Maun M. A. 1994 Adaptations enhancing survival and establishment of seedlings on coastal dune systems. Vegetatio 111: 59-70[ISI]
Miller R. E. N. L. Fowler 1993 Variation in reaction norms among populations of the grass Bouteloua rigidiseta. Evolution 47: 1446-1455[CrossRef][ISI]
Oosting H. J. 1945 Tolerance to salt spray of plants of coastal dunes. Ecology 26: 85-89[CrossRef][ISI]
Oosting H. J. W. D. Billings 1942 Factors effecting vegetational zonation on coastal dunes. Ecology 23: 131-142[CrossRef][ISI]
Rapson G. L. J. B. Wilson 1988 Non-adaptation in Agrostis capillaris L. (Poaceae). Functional Ecology 2: 479-490
Rice K. J. R. N. Mack 1991 Ecological genetics of Bromus tectorum. III. The demography of reciprocally sown populations. Oecologia 88: 91-101[CrossRef][ISI]
Rozema J. P. Bijwaard G. Prast R. Bruekman 1985 Ecophysiological adaptations of coastal halophytes from foredunes and salt marshes. Vegetatio 62: 499-521[CrossRef][ISI]
SAS. 1990 Statistical Analysis System, Version 6.09. SAS Institute, Cary, North Carolina, USA
Schemske D. W. 1984 Population structure and local selection in Impatiens pallida (Balsaminaceae), a selfing annual. Evolution 38: 817-832[CrossRef][ISI]
Schlichting C. D. M. Pigliucci 1998 Phenotypic evolution: a reaction norm perspective. Sinauer Associates, Sunderland, Massachusetts, USA
Shipley B. 2000 Cause and correlation in biology: a user's guide to path analysis, structural equations and causal inference. Cambridge University Press, Cambridge, UK
Sokal R. R. F. J. Rohlf 1981 Biometry, 2nd ed. W. H. Freeman, San Francisco, California, USA
Stevens L. C. J. Goodnight S. Kalisz 1995 Multilevel selection in natural populations of Impatiens capensis. American Naturalist 145: 513-526[CrossRef][ISI]
Sultan S. E. 1987 Evolutionary implications of phenotypic plasticity in plants. Evolutionary Biology 21: 127-178[ISI]
Sykes M. T. J. B. Wilson 1988 An experimental investigation into the response of some New Zealand sand dune species to salt spray. Annals of Botany 62: 159-166
Thrall P. H. J. J. Burdon B. R. Murray 2000 The metapopulation paradigm: a fragmented view of conservation biology. In A. G. Young and G. M. Clarke [eds.], Genetics, demography and viability of fragmented populations, 75–95. Cambridge University Press, Cambridge, UK
Tyndall R. W. A. H. Teramura C. L. Mulchi L. W. Douglas 1987 Effects of salt spray upon seedling survival, biomass, and distribution on Currituck Bank, North Carolina. Castanea 52: 77-86[ISI]
Underwood A. J. 1997 Experiments in ecology: their logical design and interpretation using analysis of variance. Cambridge University Press, Cambridge, UK
Wang X. Y. R. E. Redmann 1996 Adaptation to salinity in Hordeum jubatum L. populations studied using reciprocal transplants. Vegetatio 123: 65-71[CrossRef][ISI]
Watkinson A. R. A. J. Davy 1985 Population biology of salt marsh and sand dune annuals. Vegetatio 62: 487-497[CrossRef][ISI]
Williams C. F. 1994 Genetic consequences of seed dispersal in three sympatric forest herbs. II. Microspatial genetic structure within populations. Evolution 48: 1959-1972[CrossRef][ISI]
Wilson D. S. L. A. Dugatkin 1997 Group selection and assortative interactions. American Naturalist 149: 336-351[CrossRef][ISI]
Wilson J. B. 1987 Group selection in plant populations. Theoretical and Applied Genetics 74: 493-502[CrossRef][ISI]
Wilson J. B. M. T. Sykes 1999 Is zonation on coastal sand dunes determined primarily by sand burial or by salt spray? A test in New Zealand dunes. Ecology Letters 2: 233-235[CrossRef][ISI]
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