The efficacy of dispersal: Intuitively, all species that occupy habitats undergoing succession are destined to be replaced in their existing transient habitats and must in time disperse elsewhere to survive. In perennial species dispersal away from the parental site also decreases the annual potential for sibling competition, which oftentimes may be more severe because of similarities in resource use among siblings than is competition with non-sib conspecifics (e.g. Ellstrand and Antonovics 1985; McCall et al. 1989), though numerous examples to the contrary also exist (e.g. Smith 1977; Williams et al. 1983; Kelley 1989; McCall et al. 1989). In species of Solidago diaspore morphology was selected for dispersal effectiveness and dispersal was at least in some respects optimized with the scale of environmental heterogeneity (Werner and Platt 1976; Platt and Weis 1977). Generally, though, the dispersal potential of a species can be increased by altering the pattern of resources allocated to respective diaspore parts through natural selection. In herbaceous wind-dispersed species this alteration can be reflected by reductions in fruit mass, by increasing the drag efficiency of structures such as pappus bristles, by increasing the relative allocation of resources to the dispersal structures (i.e. increasing the pappus bristle mass to fruit mass ratio), by releasing the diaspore at a higher height, or by some combination of these (Harper 1977).

This said, the commonly realized dispersal distances versus potential maximum dispersal distances under the most optimal of conditions are typically disparate values. The distance over which a wind-borne diaspore is dispersed is a function of launch height, rate of descent in still air, and wind velocity, which includes temporal, vertical, and horizontal components (Sheldon and Burrows 1973; Horn et al. 2001). For those species that produce pogonochores (wind-dispersed plumed seeds or fruits) such as oftentimes occurs in the Asteraceae, presumably most, if not all, are viable candidates for long-distance dispersal because variations in wind velocity and direction are greater than is the within-species variation in their rate of descent in still air (Horn et al. 2001).

The pappus as the primary structure affecting dispersal: The question of whether Asteraceae fruits are effectively dispersed over long distances has been debated for the past century (e.g. Small 1918a,b; Carlquist 1966; Sheldon and Burrows 1973; Matlack 1987). The primary structure affecting dispersal success in many genera of the Asteraceae, including the asters, is the pappus (Rowlee; 1893; Small 1918a,b; Carlquist 1967; Sheldon and Burrows 1973). The aster pappus is primitive, setose scabrid or setose denticulate, and represents the fusion of uniseriate rows of cells with the obtuse terminal cell of each row being free and projecting outwards as a lateral cilium for a distance which is less than the diameter of the seta (Small 1918a). The relative efficiency of the pappus as a dispersal structure is easily evaluated by measuring the rate of descent of the propagule through still air. The slower the rate of descent, the greater will be the relative effect of any upward or horizontal air currents on trajectory, and consequently the greater the distance over which dispersal will occur (Sheldon and Burrows 1973).

That the aerodynamics of pogonochores can be adequately modeled by considering the plume as a single long cylinder with a characteristic diameter and angle of attack has been established (Greene and Johnson 1990). However, small differences in the design of the plume may have significant consequences on the aerodynamics of a pogonochore (e.g. Augspurger and Franson 1987; Matlack 1987; Sacchi 1987). In the current study we address dispersal potential among species of commonly co-occurring asters in the genera Doellingeria, Eurybia, Oclemena, and Symphyotrichum. Presumably a high degree of functional convergence exists among not only these genera, but also less closely related, though co-occurring, genera such as Asclepias, Eupatorium, Lilium, Solidago, and Vernonia. During the course of this study we acknowledge that the one-seeded, dry, indehiscent fruit of the Asteraceae, though technically a cypsela, is typically referred to as an achene or seed. Specifically, achenes of the aster species are compared on the basis of mass allocation to respective parts (dispersal structures such as the pappus bristles versus the achene per se) and of their respective rates of descent in still air. Inasmuch as previous studies have hypothesized that selection acts to produce larger seeds in habitats with decreased moisture availability or through successional time (Salisbury 1942; Stebbins 1971; Baker 1972), we question whether the less weedy aster species exhibit a dispersal strategy that mirrors that of the weedier aster species.

Methods
Specimen collection: Mature capitula of the nine aster species, Doellingeria umbellata (Aster umbellatus var. umbellatus; open field), Eurybia macrophylla (A. macrophyllus; woodlot margin), Oclemena nemoralis (A. nemoralis; shoreline), Symphyotrichum lanceolatum (A. lanceolatus subsp. lanceolatus; open field), Symphyotrichum lateriflorum (A. lateriflorus; open field), Symphyotrichum novae-angliae (A. novae-angliae; open field), Symphyotrichum pilosum (A. pilosus; open field), Symphyotrichum puniceum (A. puniceus; open field), and Symphyotrichum urophyllum (A. urophyllus; open field), were harvested from groupings of stems (likely clones) during the fall of 2003. A single species, O. nemoralis, grew on the shore of Toad Lake, Muskoka District, Ontario, Canada, whereas the remaining natural populations were located in native areas on the grounds of the Slippery Rock University of Pennsylvania campus, Slippery Rock, Pennsylvania, USA.

Although seven of the species occurred in the same open field they occupied distinct regions within the habitat as S. puniceum is an obligate wetland species, D. umbellata, S. lanceolatum, and S. novae-angliae are facultative wetland species, S. pilosum and S. urophyllum are upland species, and the status of S. lateriflorum has not been decided. The woodland margin species E. macrophylla is an upland species, whereas the shoreline species O. nemoralis is a facultative wetland species (United States Department of the Army 1987). Each of S. lateriflorum, S. novae-angliae, and S. pilosum is weedy in both Canada and the United States (Anonymous 1990; Mulligan 1992; Chmielewski and Semple 2001a,b, 2003), S. lanceolatum is a weed species in Canada (Mulligan 1992; Chmielewski and Semple 2001a) but not the United States (Anonymous 1990), and none of D. umbellata, E. macrophylla, O. nemoralis, S. puniceum, or S. urophyllum is weedy in Canada or the United States.

Data collection: For each species mature capitula were harvested when natural dispersal was just beginning to occur. At the time of harvest capitula were placed in manila envelopes and returned to the laboratory where they were allowed to air dry. To determine their rate of fall in still air individual achenes were released at the top of a 120 cm length of glass tubing (22 mm internal diameter), and the time taken to fall through 1 m of tubing was measured with a digital stopwatch. The timing of descent did not begin until the achene had traveled 20 cm through the glass tube to allow the achene to first reach terminal velocity (Greene and Johnson 1990). A Cahn C-33 microbalance (+2μg) was used to determine total propagule mass for individual achenes. The length of the pappus bristles (hair) was measured with an ocular micrometer and the number of bristles was counted for each achene. Although Greene and Johnson (1990) measured the mean angle of attack of a hair (qh) by calculating cos-1 of the ratio of pappus radius to hair length, we directly measured the angle of attack of the pappus bristles (the angle from the horizontal to the outermost bristle) with an ocular protractor for each achene. The diameter of the pappus bristles was determined as the average of 25 pappus bristle measurements on a single achene of each species. The pappus bristles were then removed and the achene was re-weighed. The difference between initial achene mass (m) and achene mass less the pappus bristles represented that part of total achene mass allocated to the plume.

Statistical analysis: Formulae used throughout the study and the terms used to define variables relating to seed aerodynamics followed Greene and Johnson (1990). In addition to those variables mentioned above, the following were also utilized during the course of the study: total projected area of the plume (Ap), plume loading (m/Ap), plan area of an imaginary disk of the plumed achene (Ad), solidity (Ap/Ad), drag force (m · g where g is gravitational acceleration and equivalent to 9.81 ms-2), Reynold’s number (where the product of terminal velocity (vf) and mean pappus bristle diameter (dh) are divided by n, the kinematic viscosity of air, which is equal to 1.5X10-5 m2s-1 ), and drag coefficient (CD). Derived quantities were calculated from measured variables, and the theoretical relationship between the square root of plume loading and vf(ρC'D/2g)0.5 was illustrated through the use of Mathematica (Wolfram 2003).

For each of the variables for the respective species mean values of total achene mass, percent of total achene mass allocated to the plume, number of pappus bristles in a plume, pappus bristle length, angle of attack of the pappus bristles, terminal velocity, total projected area of the plume, plume loading, plan area of an imaginary disk of the plumed achene, solidity, drag force, Reynold’s number, and drag coefficient were compared with an analysis of variance (PROC GLM) (SAS Institute Inc. 1997). A posteriori comparisons among group means for the respective species were conducted with the Student-Newman-Keuls multiple range test (PROC GLM, SNK option) (SAS Institute Inc. 1997).

Multiple regression with terminal velocity as the dependent variable and the number of pappus bristles in a plume, pappus bristle length, angle of attack of the pappus bristles and achene mass (less the pappus) as the predictor variables, was initiated with PROC REG (SAS Institute Inc. 1997) for each species. The STB option was included as part of the MODEL statement. Because the beta coefficients resulting from this option are measured in standard deviations as opposed to the original units they may be directly compared to one another thereby indicating the relative strength of each of the predictors.

Because measures of the coefficient of variation eliminate variation in the data resulting as a consequence of the magnitude of the data, comparisons within species were made between pairs of achene characteristics (total achene mass, percent of the total achene mass allocated to the plume, number of pappus bristles in a plume, pappus bristle length, and angle of attack of the pappus bristles) per se, as well as with terminal velocity and the total projected area of the plume using the variance ratio test for coefficients of variation (Zar 1984). Because the achene characteristics have a genetic basis and are formed over a relatively short period of time we hypothesized that they would be less variable than total achene mass within a species.

Results
Descriptive statistics for total achene mass, percent of total achene mass allocated to the plume, number of pappus bristles in a plume, pappus bristle length, angle of attack of the pappus bristles, terminal velocity, total projected area of the plume, plume loading, plan area of an imaginary disk of the plumed achene, solidity, drag force, Reynold’s number, and drag coefficient are summarized for each species (Table 1). Similarities between and among species are oftentimes associated with the species’ weed status, though for some characters (angle of attack of the pappus bristles, terminal velocity, projected area of the plume, plan area of an imaginary disk of the plumed seed, and drag coefficient) the generic associations predominate.

Table 1. Summary of: (A) total achene mass (mg) [F=713.47; df=8, 891; P<0.0001]; (B) percent of total achene mass allocated to the plume [F=235.08; df=8, 891; P<0.0001]; (C) number of bristle hairs in a plume [F=231.85; df=8, 891; P<0.0001]; (D) bristle hair length (mm) [F=686.43; df=8, 891; P<0.0001]; (E) angle of attack of the pappus bristles (degrees) [F=58.58; df=8, 891; P<0.0001]; (F) terminal velocity (m·s-1) [F=149.30; df=8, 891; P<0.0001]; (G) projected area of the plume (mm2) [F=610.78; df=8, 891; P<0.0001]; (H) plume loading (mg·mm-2) [F=4.67; df=8, 891; P<0.0001]; (I) plan area of an imaginary disk of the plumed seed (mm2) [F=328.05; df=8, 891; P<0.0001]; (J) solidity [F=15.72; df=8, 891; P<0.0001]; (K) drag force (mg·m·s-2) [F=713.74; df=8, 891; P<0.0001]; (L) Reynold’s number [F=268.23; df=8, 891; P<0.0001]; (M) drag coefficient [F=204.36; df=8, 891; P<0.0001]. Results of interspecific comparisons for variables A-M are presented above between square brackets. Mean values followed by the same letter are not significantly different following a posteriori comparisons with the Student–Newman Keuls multiple range test.

 

SPECIES

A

B

C

D

E

F

G

H

I

J

K

L

M

Symphyotrichum lanceolatum

0.127fg

21.0b

37c

4.0d

48ab

0.36g

2.18d

0.08c

2.34e

1.29ab

1.24fg

0.54f

0.60d

Symphyotrichum lateriflorum

0.109g

17.3d

28d

2.7h

51a

0.53d

0.84f

0.32ab

1.04g

1.16b

1.07g

0.62e

0.43d

Symphyotrichum novae-angliae

0.318d

16.0d

37c

4.5c

49ab

0.50d

3.92c

0.09c

2.92d

1.44a

3.12d

1.16b

5.69c

Symphyotrichum pilosum

0.143f

19.1c

27d

3.3g

48ab

0.38fg

1.07ef

0.16bc

1.65f

0.79c

1.40f

0.44g

0.36d

Composite – weedy species

0.174

18.3

32

3.6

49

0.44

2.00

0.16

1.99

1.17

1.71

0.69

1.77

Doellingeria umbellate

0.775a

6.3f

47b

3.6f

21e

0.80a

3.83c

0.24abc

3.39c

1.16b

7.60a

1.34a

32.34a

Eurybia macrophylla

0.642b

17.2d

40c

5.9b

26d

0.45e

5.11b

0.14bc

8.67a

0.63c

6.29b

0.75d

11.22b

Oclemena nemoralis

0.396c

32.8a

74a

6.2a

34c

0.42f

8.39a

0.05c

8.19b

1.08b

3.88c

0.62e

9.50b

Symphyotrichum puniceum

0.380c

5.3f

27d

3.8e

45b

0.75b

1.41e

0.37a

2.40e

0.75c

3.73c

1.00c

4.94c

Symphyotrichum urophylla

0.235e

12.2e

37c

3.3g

46ab

0.57c

1.47e

0.18bc

1.73f

1.07b

2.30e

0.67e

1.94d

Composite – nonweedy species

0.485

14.7

45

4.6

34

0.60

4.04

0.20

4.88

0.94

4.76

0.88

11.98

That the theoretical relationship between the square root of plume loading [(m/Ap)0.5] and vf(ρC'D/2g)0.5 is defined by a line with a slope of 1.0 (see Greene and Johnson 1990) demonstrates that the drag coefficient of a screen adequately describes the aerodynamics of these plumed seeds (Fig. 1). Individual values in these figures that deviate from the predicted slope are typically a consequence of the angle of attack of the pappus bristles (i.e. large θ) as opposed to atypically large or small values for achene mass, the number of pappus bristles, or the length of the pappus bristles.Based on the rejection of the F-value for each of the multiple regressions (Table 2), the independent variables, number of pappus bristles, pappus bristle length, angle of attack of the pappus bristles, and achene weight (less the weight of the pappus bristles) reliably predicts the dependent variable, terminal velocity.

The respective coefficients of variation, which indicate how well the model fits the data, range from 11.3 to 24.9 for the weedy species and 14.9 to 19.6 for the non-weedy species (Table 2). Using pappus bristle length for Symphyotrichum lanceolatum as the example, the associated beta coefficient may be interpreted as follows: a one standard deviation increase in pappus bristle length corresponds to a 0.46 decrease in terminal velocity (Table 2). Independent variables that are not significant (the coefficients are not significantly different from 0) are bolded (Table 2). Generalizing, the number of pappus bristles has no effect on terminal velocity among the weedy species though does have a significant decreasing effect in the three non-weedy obligate/facultative wetland species, Doellingeria umbellata, Oclemena nemoralis, and S. puniceum. The length of the pappus bristles either has a substantial effect on decreasing terminal velocity in the weedy species, or none at all. The same is true, but to a lesser degree in the non-weedy species. The angle of attack of the pappus bristles has a significant positive effect on terminal velocity among the weedy species and also on the three non-weedy obligate/facultative wetland species, but not the remaining non-weedy species. Achene weight (less the pappus bristles) has a significant positive effect on terminal velocity across all species, regardless of whether they are weedy or not.

Table 2. Summary of respective F-values and probabilities associated with the multiple regressions, coefficients of variation (CV), and beta coefficients for each of the species following multiple regression of terminal velocity with the predictor characters number of pappus bristles (Number), pappus bristle length (Length), angle of attack of the pappus bristles (Degrees), and achene weight (less the weight of the pappus bristles) (Weight). Respective coefficients that are not significantly different (alpha=0.01) from 0 are bolded.

 

Species	F	Pr > F	CV	Number	Length	Degrees	Weight
Symphyotrichum lanceolatum	19.42	<0.0001	18.7	-0.01	-0.46	0.40	0.47
Symphyotrichum lateriflorum	10.75	<0.0001	24.9	-0.05	-0.14	0.46	0.31
Symphyotrichum novae-angliae	9.35	<0.0001	11.3	-0.09	-0.10	0.45	0.60
Symphyotrichum pilosum	20.59	<0.0001	16.8	-0.03	-0.68	0.39	0.39
Composite - weedy species	77.61	<0.0001	21.6	0.04	-0.70	0.37	0.75
Doellingeria umbellata	21.63	<0.0001	17.5	-0.42	-0.22	0.23	0.57
Eurybia macrophylla	3.79	0.0066	17.0	-0.14	-0.23	0.19	0.31
Oclemena nemoralis	20.78	<0.0001	14.9	-0.27	-0.01	0.53	0.44
Symphyotrichum puniceum	23.37	<0.0001	19.6	-0.54	-0.13	0.31	0.51
Symphyotrichum urophylla	5.20	0.0008	18.1	-0.14	-0.32	0.19	0.37
Composite - non-weedy species	155.90	<0.0001	23.5	-0.14	-0.45	0.26	0.53

Coefficients of variation for total achene mass, percent of the total achene mass allocated to the plume, number of pappus bristles, pappus bristle length, angle of attack of the pappus bristles, terminal velocity, total projected area of the plume, plume loading, plan area of an imaginary disk of the plumed seed, solidity, drag force, Reynold’s number, and drag coefficient are summarized for each of the species (Table 3).

 

 

Figure 1. Relationship between (m/Ap)0.5 and vf(ρC'D/2g)0.5 for each of the aster species. The depicted line represents a slope of 1.0. Table 3. Summary of coefficients of variation: (A) total achene mass; (B) percent of total achene mass allocated to the plume; (C) number of bristle hairs in a plume; (D) bristle hair length; (E) angle of attack of the pappus bristles; (F) terminal velocity; (G) projected area of the plume; (H) plume loading; (I) plan area of an imaginary disk of the plumed seed; (J) solidity; (K) drag force; (L) Reynold’s number; (M) drag coefficient.

 

SPECIES	A	B	C	D	E	F	G	H	I	J	K	L	M
Symphyotrichum lanceolatum	21.4	28.1	23.8	10.5	31.7	24.7	45.1	127.3	53.9	95.3	21.4	24.7	70.9
Symphyotrichum lateriflorum	23.9	47.0	28.9	14.2	30.7	29.4	51.0	458.4	67.8	89.6	23.9	29.4	87.4
Symphyotrichum novae-angliae	25.6	23.7	28.5	9.9	24.4	13.1	45.1	42.9	48.6	34.8	25.6	13.1	72.3
Symphyotrichum pilosum	21.5	39.4	19.8	10.4	25.0	22.5	35.0	73.1	47.7	83.7	21.5	22.5	52.6
Composite – weedy species	55.3	36.7	29.7	21.8	28.2	28.8	80.4	450.5	65.0	79.8	55.3	45.3	173.5
Doellingeria umbellate	21.4	43.7	26.3	7.6	81.1	23.7	33.2	79.8	28.0	29.0	21.4	23.7	59.8
Eurybia macrophylla	19.0	25.8	26.1	11.8	43.5	17.9	29.1	44.0	29.5	35.5	19.1	17.9	47.2
Oclemena nemoralis	18.3	21.3	17.4	7.5	38.4	20.0	27.3	48.7	32.1	27.7	18.2	20.0	47.3
Symphyotrichum puniceum	24.1	36.9	31.4	14.5	38.8	27.0	46.9	92.0	60.2	52.3	24.1	27.0	70.9
Symphyotrichum urophylla	24.8	26.7	18.7	10.6	31.7	19.6	39.2	44.4	53.1	59.3	24.8	20.0	71.4
Composite – nonweedy species	46.0	73.6	42.1	29.0	51.8	35.2	72.9	106.8	71.6	48.0	46.0	38.6	118.3

Discussion

Efficient dissemination of wind-dispersed propagules is not only dependent upon launch height and the effectiveness of the pappus in keeping the propagule airborne (i.e. rate of fall in still air of the propagule), but also the environmental conditions (i.e. humidity, horizontal wind velocity, strength and frequency of updrafts) in effect at the time of their release from the inflorescence. As such, these three principal factors will dictate the horizontal distance over which an airborne propagule will travel.

 

The consequences of launch height: Though potentially variable both within and among the studied species, launch height ranged between 0.4 m for Oclemena nemoralis to 1.5 m for Symphyotrichum puniceum. As a factor affecting dispersal distance per se, all else being equal, launch height is generally of greater concern if obstructions are nearby compared to when they are not. Except for Eurybia macrophylla, a woodland margin species, and O. nemoralis, a shoreline species, the remaining aster species grew in close association with shoots of such genera as Asclepias, Eupatorium, Lilium, Solidago, and Vernonia, shoots that were approximately comparable in height to those of the aster species, and thus potential barriers to the flight of airborne aster achenes. For these achenes to disperse any distance horizontally would necessitate that they be subjected to updrafts upon release to clear the local vegetation first. As a woodland margin species, achenes of E. macrophylla would not initially encounter obstacles provided that dispersal was away from, as opposed to into, the wood lot. The dispersal of O. nemoralis achenes would be unobstructed for a considerable distance if they were blown offshore, but concurrently would have a strong likelihood of landing in water. Achenes dispersed inland would in this instance have to contend with the under-story vegetation of a boreal forest. Only dispersal parallel to the shoreline could be expected to provide suitable germination sites.

 

Factors affecting the rate of descent: The rate of descent of aster achenes in still air is dependent upon the relationship that exists between load (achene mass) and drag potential of the pappus bristles (the parachute effect). Although these two morphological factors pre-determine dispersal, potential they are temporally separated both generationally and developmentally. The pappus bristles represent a modified calyx and are essentially mature at the time of pollination represent a modified calyx (Cronquist 1981), are genetically maternal, and as such reflect maternal character evolution. Phenotypically the pappus bristles are a consequence of the interaction between the genotype and the environment to which it was exposed during floral development prior to pollination. This said, reproductive characters, such as pappus bristles, that are formed over a comparatively short period of time tend to be less plastic to changes in the environment than are vegetative characters (Semple and Chmielewski 1987). Regardless, the number of pappus bristles, their length, and their angle of attack is for all intents and purposes defined before the load is established. Chmielewski and Ruit (2002) postulated that the key role of the pappus in Doellingeria umbellata and S. novae-angliae was to facilitate dispersal as opposed to encourage germination. Further, achene load did not differ between ray and disk achenes of D. umbellata thereby suggesting that dispersability would be comparable between them (Chmielewski 1999).
 

Within a species the length of the pappus bristles was the least variable of the pappus bristle characters measured and plume loading was the most variable. Specifically, coefficients of variation across characters were at least twice those for pappus bristle length, and for angle of attack of the pappus bristles the coefficient of variation was up to approximately twenty times that of pappus bristle length in the weedy species but less so in the non-weedy species. The coefficient of variation for solidity in the weedy species was also considerably larger than for the non-weedy species, though S. novae-angliae was an exception. The generally larger values for the coefficient of variation for the composite species groups in contrast to the species per se indicate considerable variation does exist among the species within a specific ecological group (weedy versus non-weedy). Because achenes were not pre-screened visually for a complete complement of pappus bristles, but rather were randomly selected based on their plumpness and thus presumed viability (see Chmielewski 1991), variation in this character was greater than if pre-screening had occurred because in some cases nearly all the pappus bristles had been lost naturally prior to dispersal.
 

The architecture of the capitulum in the Asteraceae will in part dictate the number of florets possible, and the degree of achene set within a capitulum will in part dictate the final angle of attack of the pappus bristles as well as achene mass. The greater the floret number and the greater the seed set within a capitulum, the greater the angle of attack of the pappus bristles due to crowding. Presumably the pappus bristles of unfertilized achenes would provide little, if any, resistance to the pappus bristles of adjacent maturing achenes, whereas this would not be true of maturing achenes. This presents an evolutionary paradox in that the more successful the pollination event, the smaller the potential for dispersal due to an increased terminal velocity. Alternatively, the less successful the pollination event is, the less likely an achene would be dispersed from the parent plant because of the increased terminal velocity as a consequence of increased achene mass relative to pappus mass, assuming that the plant does not alter the absolute amount of resources allocated to the achenes (Werner and Platt 1976). Although variation in each of the pappus bristle characters exists, and in some instances is pronounced, it would appear that these are not primarily responsible for the variation that exists in terminal velocity.

 

Terminal velocity as a factor affecting dispersal distance: Terminal velocity among species included in this study ranged between 0.36 to 0.80 m s-1. These values are comparable to those reported for aster species such as O. acuminata (0.41 m s-1 as Aster acuminatus) and S. prenanthoides (0.66 m s-1 as A. prenanthoides) (Matlack 1987), other plumed, potentially co-occurring Asteraceae such as Cirsium vulgare (0.36 m s-1), Euthamia graminifolia (0.12-0.39 m s-1), Solidago altissima (0.28 m s-1), S. canadensis (0.17-0.29 m s-1), S. gigantea (0.24 m s-1), S. missouriensis (0.16-0.31 m s-1), S. speciosa (0.38-0.41 m s-1) and Tragopogon porrifolius (0.30-0.36 m s-1), or other plumed, potentially co-occurring, species such as Apocynum cannabinum  (0.15 m s-1) or Asclepias syriaca L. (0.27 m s-1) (Werner and Platt 1976; Morse and Schmitt 1985; Matlack 1987).
 

It is important to realize that achenes of plants with statistically different terminal velocities could, under similar conditions, exhibit identical horizontal dispersal distance. As an example, achenes of O. nemoralis (terminal velocity of 0.42 m s-1) and D. umbellata (terminal velocity of 0.80 ms-1) would each take approximately 1 s to descend to the ground from their typical release height in still air. This result is easily explained by the fact that D. umbellata is typically twice as tall as is O. nemoralis. Because the distance over which an achene is dispersed is intimately tied to both terminal velocity and release height, terminal velocity alone provides little comparative information on the potential distance achenes of different plants within a species or between/among species will be dispersed. As a generalization, it would appear that the weedier aster species, as a consequence of both increased plant height and comparatively lower values for terminal velocity would remain airborne longer than would the achenes of the non-weedy aster species.
 

The values of terminal velocity for plumed seeds in general would also suggest that, irrespective of species, genus, family or weediness, a high degree of functional convergence is exhibited. Specifically, the pappus bristles provide sufficient drag to allow seeds to remain aloft to be dispersed a comparatively short distance locally (<5 m) under normal conditions. Long distance dispersal is fortuitous and is a consequence of conditions occurring at the time of dispersal, and therefore, not a function of design. Few achenes actually do escape beyond the shadow of the parent plant (Levin and Kerster 1974), though this is not unique among the asters.

Architectural constraints versus evolutionary pressures: For all intents and purposes, propagule load is essentially initiated once fertilization occurs, though final load is yet to be determined. This in part could explain the large differences observed in the respective coefficient of variation values. Following fertilization both the embryo and endosperm represent unique genetic combinations and the success of each during the next critical stages of development will dictate propagule load, though load is dependent not only on the genetic makeup of the embryo and endosperm but also the ability of the maternal plant to provide the necessary resources for development to occur considering current environmental conditions. Further, variation in total propagule mass either within or among species reflects a balance among the architectural constraints imposed by the floral structures, the evolutionary pressures associated with dispersal, persistence, predator avoidance, germinability, and seedling competition (Harper 1977; Jolls and Werner 1989), and the environmental conditions to which the developing propagule is exposed.

Although the pappus bristles of aster species are the morphological agents facilitating dispersal, it would appear that achene mass is more often than not the single most important actual achene character dictating terminal velocity and thus the potential horizontal distance over which a seed may be dispersed, though for the weedy species angle of attack of the pappus bristles may be equally important. Achene mass is indeed quite variable among aster species, with at least a 10-fold difference being evident (e.g.. Delisle 1938; Wetmore and Delisle 1939; Peterson and Bazzaz 1978; Havercamp and Whitney 1983; Pitelka et al. 1983; Chmielewski and Ruit 2002). Additionally, with the exception of S. urophylla, a non-weedy species, the weedy species S. lateriflorum, S. lanceolatum, S. novae-angliae, and S. pilosum produce the lightest achenes in the aster species studied. Past studies have suggested that because achene mass in some species is not correlated with total germination that the evolutionary significance of achene mass is associated with pre-germination phenomena such as dispersal or post-germination phenomena such as seedling survival or vigor (Chmielewski 1991; Prinzie and Chmielewski 1994). Achene weight neither affected the likelihood of ray or disk achenes of D. umbellata from germinating (Chmielewski 1999). Similarly, the probablity of ray or disk achenes of Eurybia divaricata (as A. divaricatus) germinating was not affected by achene weight (Chmielewski and Huff 1995). However, germination in Oclemena acuminata (Pitelka et al. 1983) S. lanceolatum (Chmielewski 1991) and S. pilosum var. pilosum (Prinzie and Chmielewski 1994) did demonstrate an increased likelihood of germination with increased achene weight. Regardless, larger seeds would be harder to disperse, and minimally require stronger winds than would comparatively smaller seeds (Willson and Traveset 2002) to achieve the same degree of dispersal.
 

The results of our study strongly suggest that intraspecific variation in achene mass affects dispersability of propagules such that larger achenes have a comparatively higher terminal velocity and concurrent shorter horizontal distance traveled for a specific set of wind conditions. Because the increased weight of propagules is known to enhance initial root development and thereby increase the probability of seedling establishment (Stebbins 1971, Baker 1972), the tradeoff that exists between dispersability and the starting resource capital of seedlings is evolutionarily sound (Morse & Schmitt 1985). In the less weedy aster species selection should favor the production of comparatively heavier, more vigorous propagules with a decreased likelihood of reaching more distant habitats as opposed to safe sites within the same location (Werner and Platt 1976) if for no other reason than that larger seeds would require stronger winds (Willson and Traveset 2000). In contrast, the weedier species produce achenes with a comparatively greater potential for distance dispersal and this potential is realized because of the relationship between both plant height and terminal velocity. Inasmuch as Chmielewski and Ruit (2002) concluded that the key role of the pappus in D. umbellata and S. novae-angliae is to facilitate dispersal as opposed to encourage germination, the results of the current study would suggest that we can dismiss dispersal as being an important pre-germination phenomenon in non-weedy asters and concentrate future attention on post germination phenomena such as seedling survival or vigor.

Temporal separation of pappus structure and achene mass determination means that two of the three primary factors affecting dispersal (height being the other) vary independently. Thus, combining these characters to produce the final propagule phenotype is random and results in essentially all pappus structure/achene mass combinations being produced. As a result it is difficult to envision how the forces of natural selection could possibly affect the final propagule phenotype by acting upon plume structure and achene mass in a coordinated fashion.

 

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