Pollination biology and seed production of a federally endangered perennial, Echinacea laevigata (asteraceae:heliantheae).
The Piedmont Physiographic Region of the southeastern United States compared with the Coastal Plain Region (Fenneman, 1938), supports relatively few rare plant species (Walker, 1993; Estill and Cruzan, 2001; LeBlond, 2001; Sortie and Weakley, 2001, 2006) and, consequently, has received less attention from conservation botanists. Echinacea laevigata (Boyton and Beadle) Blake (coneflower) is a federal rare species of the Piedmont (Murdock, 1992) and is included in the Center for Plant Conservation's National Collection of Endangered Plants (Urbatsch et al., 2006). Most coneflower populations are small (<50 plants) and over half are declining in numbers (Murdock, 1995).
Managing rare plant populations is facilitated by knowledge of species' reproductive biology (Ratsirarson and Silander, 1996; Kaye, 1999; Timmerman-Erskine and Boyd, 1999; Anderson et al., 2001; Marten and M. Quesada, 2001). Management of small populations, in particular, must consider potential threats to population survival including relatively low seed production and seed germination rates and small seedlings. These reduced reproductive parameters can be caused by sub-optimal plant--pollinator relationships (Sih and Baltus, 1987; Jennersten, 1988; Steffan-Dewenter and Tscharntke, 1999; Suzuki, 2000; Fausto et al., 2001), limiting habitat resources (Horvitz and Schemske, 1988; Lamont et al., 1993; Bosch and Waser, 1999; Morgan, 1999; Cunningham, 2000; Muotoz and Arroyo, 2006; Ne'eman et al., 2006; Farji-Brener and Ghermandi, 2008; Tsaliki and Diekmann, 2009), and genetic mechanisms including local loss of S alleles (Byers and Meagher, 1992; Shore, 1993; Reinartz and Les, 1994) and inbreeding (Menges, 1991, 1995; Young et al., 1996; Fischer and Matthies, 1998; Bruna, 2002; Donaldson et al., 2002; Hooftman et al., 2003; Severns, 2003; Hensen and Oberpreiler, 2005; Kolb, 2005; Ward and Johnson, 2005).
Previous studies of Echinacea laevigata (Edwards and Madsen, 1993; Apsit and Dixon, 2001; Philippi et al., 2001; Alley and Affolter, 2004; Alley et al., 2005; Peters et al., 2009) contribute to the conservation of this species. The study reported here of pollination and reproductive success in both large and small populations will further support coneflower conservation by addressing a primary threat to this rare species, small population size. The goals of the current coneflower study are to (1) describe flowering phenology and diversity of flower visitors in small and large populations and estimate the importance of flower visitor groups as pollinators, (2) compare pollen viability among populations, test for pollen limitation, and compare seed production in outcross, selfed, and autogeitonogamy pollen treatments, (3) determine whether soil cations limit seed production, and (4) compare seed germination rate and seedling size in a small and large population.
SPECIES AND STUDY POPULATIONS
Echinacea laevigata is an herbaceous perennial with short rhizomes that provide limited vegetative reproduction (North Carolina Natural Heritage Program, 2001). Most plants produce a single flowering head on a peduncle that can be up to 40 cm long (Murdock, 1995; Urbatsch et al., 2006). Disc flowers develop in a series of concentric rings, one ring opening daily beginning with the outermost and progressing inward (Wagenius, 2004). The stigma of a disc flower becomes receptive one day after the anthers dehisce. It takes 1012 days for all disc flowers of most individual heads to open. Various bees, beetles and butterflies have been reported visiting coneflower in South Carolina (Edwards and Madsen, 1993), but the importance of these insects as pollinators has, apparently, not been documented.
Echinacea laevigata likely occurred in Piedmont prairies which formerly were common in the southeastern United States but have become nearly extirpated due to fire suppression and anthropogenic habitat conversion (Murdock, 1995; Juras, 1997). In North Carolina, coneflower occurs only in Durham, Granville, Mecklenburg and Rockingham Counties (Buchanan and Finnegan, 2010; Weakley, 2010). This study was conducted during 2004 and 2005 and included a large population in a managed powerline fight-of-way in Granville County, Picture Creek (PC), and five small populations in Granville and Durham Counties, North Carolina (Table 1). Each population was separated from its nearest neighbor population by at least 0.4 km in a matrix of forest habitat and suburban development. The small Knap of Reeds (KOR) and Snow Hill Road (SHR) populations were lost to the study when land managers burned them and the surrounding habitat during spring, 2005. Three other small populations, Freudenberg (FB), Briardale (BD), and Lakeside (LS), were added to the study during 2005. PC included more than 1000 times the number of flowering plants and [greater than or equal to]9 times the daily flowering stem density than the small populations (Table 1).
FLOWERING PHENOLOGY AND FLOWER VISITORS
Flowering phenology.--To assess the likelihood that plant species were competing with E. laevigata for pollinators or facilitating its pollination at PC, we monitored community flowering phenology during 2004. On seven days beginning the third week of May until the fourth week of Jul., approximately weekly, we counted flowering individuals of all non-grass, sedge, or rush species in ten randomly distributed 2 x 10 m transects. We also recorded the blooming period for E. laevigata in each small population. Plant nomenclature followed the PLANTS Database (United States Department of Agriculture, Natural Resources Conservation Service, 2006).
Flower visitors.--Because bees have been reported as common flower visitors to other species of Echinacea (Edwards and Madsen, 1993; Leuszler et al., 1996) and because many bees are generalist flower visitors (Mitchell, 1962), we determined which plant species bees visited at PC during 2004. On seven days beginning the third week of May until the fourth week of Jul., approximately weekly, we observed bee visits during slow walks throughout the length of each 2 x 10 m transect during 900-1100, the peak of bee activity.
To determine how important different groups of insect flower visitors were as coneflower pollinators (Wiggam and Ferguson, 2005), we observed frequencies of visits to coneflower heads and determined quantitative features of pollen loads on individual visitors (Herrera, 1987). During the second--fourth weeks of Jun. of each study year at PC, we counted flower visits and captured visitors in eight randomly distributed 2 X 2 m plots. We counted visits during 41 15-min periods in 2004 and 36 periods in 2005. We limited to [less than or equal to] 8 the number of observation periods per day to minimize undue influence of data from a single day. Fifty-seven observation periods were between 0900 and 1300 and 20 were between 1300 and 1600. Before each observation period in each plot, we counted the blooming coneflower heads so we could express visits on a per head basis. We tested for a pollinator group effect on number of visits per head per 15 min with 1-way ANOVA using SAS PROC GLM (SAS Institute Inc., 2008). We square root transformed data and pooled it over observation plots, days, and years. Differences among pollinator groups were determined with Tukey's studentized range test at an alpha level of 0.05.
Based on observations during the first two weeks of Jun. 2004, the most common flower visitors were Bombus spp. (Apidae; bumblebees), Megachile spp. (Megachilidae, leaf cutter bees), Xylocopa virginica L.(Apidae; carpenter bee), Lygaeus kalmii Stal (Lygaeidae, seed bug), Hesperiidae (skippers), and Nymphalidae (brush-footed butterflies). On 17-18 Jun. 2004 and 8-16 Jun. 2005 we captured visitors in the 2 x 2 m plots. We collected 13 individuals each of Bombus and Megachile and five individuals each of Xylocopa virginica, Lygaeus, Hesperiidae (skippers) and Nymphalidae and determined the distribution of pollen on the body of each at 10-20 X. We assigned a score of 0-4 to each individual based on the location of pollen on its body which reflected the probability of the pollen being deposited on stigmas (Table 3). We analyzed pollen distribution data with the Kruskal-Wallis test for ranked data (Sokal and Rohlf, 1969). Critical [chi square] value was determined in Rohlf and Sokal (1969).
We estimated the number of Echinacea laevigata pollen grains and its proportion in the total pollen load for each captured insect. We placed each insect in 3 ml of detergent solution in a vial and shook the vial until all pollen appeared to be removed from the insect. We removed pollen sacs from the hind legs of Hymenopterans prior to putting them in the detergent solution because this pollen would be unavailable to stigmas. For each individual, we then examined 10 0.05 ml pollen suspension samples at 100 X and counted E. laevigata and non--E. laevigata grains. The mean of the 10 samples per individual was determined, [mean.sub.individual]. For each insect group, we determined the [mean.sub.group], average of the [means.sub.individual], and its confidence interval (Table 3). We tested for a pollinator group effect on number of E. laevigata pollen grains per 0.05 ml sample ([mean.sub.group]) and on proportion of E. laevigata grains per sample with 1-way ANOVA using SAS PROC GLM (SAS Institute Inc., 2008). Data were pooled over observation plots, days, and years. Pollen counts were square root transformed and proportional data were arcsine transformed for these analyses. Pollinator group differences were determined with Tukey's studentized range test at an alpha level of 0.05.
We determined the relative coneflower pollinator importance for each insect visitor group as the sum of the ranks for the four component criteria. We also described the behavior of flower visitors while on coneflower heads, but we did not incorporate this behavior in the linear combination of pollination variables.
Number of insect flower visits and visitor species richness may be reduced in small populations relative to large populations (Sih and Baltus, 1987; Jennersten, 1988; Steffan-Dewenter and Tscharntke, 1999; Suzuki, 2000; Fausto et al., 2001). To determine if this was the case in the large and small coneflower populations, we observed insect flower visitors during the second--fourth weeks of Jun. 2004 and 2005. We counted visits by each insect group during 15 min observation periods. We used the insect counts from PC that were also used to determine pollinator importance. We did not use multiple sampling plots in small populations because they were composed of so few plants. Instead, we counted visits from one vantage point. Number of daily observation periods in the small populations was limited by the small number of flowering heads and their irregular daily occurrence.
For each year, insect visit counts and species richness were square root transformed and then modeled through mixed effects linear model with site as fixed effect and days within site as random effect using SAS PROC MIXED (SAS Institute Inc., 2008). Day within site variance was estimated separately for each site. Differences among sites were determined with Tukey's studentized range test at an alpha level of 0.05.
FACTORS AFFECTING SEED PRODUCTION
Pollen viability.--Low pollen viability has been associated with reduced seed production (Demchik and Day, 1996; Clevenger et al., 2004; Prasad et al., 2006). To estimate pollen viability in the study populations, we determined the % of pollen that stained dark blue in cotton blue-lactophenol, a correlate of pollen viability (Kearns and Inuoye, 1993), during 79 Jun. 2004, for PC, SHR, KOR and during 6-9 Jun. 2005, for BD, FB, and LS. We sampled pollen from eight randomly selected flowering heads at PC and from three randomly selected heads at each of the small populations. We collected anthers from four disk flowers of the day from each head, macerated them together in a drop of stain on a microscope slide and counted presumably viable (dark blue and unshriveled) and non-viable (light blue and/or shriveled) grains at 100 X.
Compatibility.--Seed production by self-incompatible plant species is highly dependent on pollinator activity while that of self-compatible species is largely independent of pollinators. To confirm our suspicion that coneflower is self-incompatible, we compared seed production following three bagging/pollination treatments, bagged heads/no artificial pollination (testing auto-geitonogamy combined with genetic self-compatibility), bagged/ artificial geitonogamy (testing genetic self-compatibility), and unbagged heads (testing combined effects of geitonogamy and cross pollinations). We applied these treatments in PC throughout the second-third weeks of Jun. 2004. Substantial seed production in all three treatments would suggest apomixis.
For the bagged/no artificial pollination treatment, we bagged 10 heads randomly selected from the central part of the population. We did not manipulate these heads until their contents were harvested in Sep. We randomly selected 10 heads from the central part of the population for the bagged/artificial geitonogamy treatment. When the stigmas of the outer ring of disk flowers in each head were receptive, we began geitonogamy pollinations by gently transferring pollen to stigmas with toothpicks. We reduced the possibility of experimental manipulation effects by minimizing stigma--toothpick tip contact. We repeatedly pollinated each head daily until all of its disk flowers had been pollinated, rebagging the head after each pollination. Immediately after artificial pollinations were completed during the third week of Jun., we randomly selected and bagged 24 heads from among those in the central part of the population that had partly fresh ray flowers, indicating that they had recently completed flowering. These heads were the unbagged treatment. We harvested the 24 heads of the unbagged treatment on 2 Sep. 2004, and the 20 heads of the two bagged treatments on 23 Sep. 2004. We counted plump and non-plump seeds from each head. We also looked for indications of seed herbivory in the harvested heads. To determine if pollination treatment affected the frequency of heads that produced seeds, we performed Fisher's Exact Test using SAS PROC FREQ (SAS Institute Inc., 2008). We also tested for an effect of pollination treatment on numbers of seeds per seed-producing head with a 1-way ANOVA using SAS PROC GLM (SAS Institute Inc., 2008) and untransformed data. Differences among treatments were determined with Tukey's studentized range test at an alpha level of 0.05.
Pollen limitation.--Apart from possible effects of pollen viability and compatibility, pollinator activity may limit seed production by failing to deposit on stigmas that amount of pollen required for maximal seed production. This phenomenon, pollen limitation, has frequently been noted in small populations. To determine if pollen limitation was affecting the coneflower populations, we compared seed production following open-pollination (heads unmanipulated) and cross-pollination (unbagged heads receiving artificial crosspollination) treatments. During the second week of Jun. of both 2004 and 2005, we randomly selected heads which were near anthesis and which appeared equal in size for the two pollination treatments in each population. The number of heads in the small populations was limited by the low number of heads in these populations. In each population, the open-pollination heads remained unmanipulated until they were bagged during the second week of Jul. We artificially cross-pollinated heads throughout the second and third weeks of Jun. of both 2004 and 2005. In PC we cross-pollinated each of 10 heads daily throughout its flowering duration leaving it unbagged between pollinations. For each daily cross-pollination, we used pollen from three plants that were randomly selected from 3-5 m from the maternal plant. We reduced the possibility of experimental manipulation effects by minimizing stigma--toothpick tip contact. In each small population, the number of cross-pollinated heads was <10 because there were so few heads in these populations, SHR (n = 4), KOR (n = 4), BD (n = 8), LS (n = 4), FD (n = 6). Also, the small number of plants flowering daily in these populations limited the number of pollen donors to one on some days and two on other days. We used toothpicks for transferring pollen. During the second week of Jul. of 2004 and 2005, we bagged the cross-pollinated and open-pollinated heads in each population. We harvested these heads during the first week of Sep. of both 2004 and 2005. We determined seed production % for each head by counting plump and shriveled seeds. We had previously determined that plump seeds sliced open and soaked in a 0.1% solution of 2,3,5 triphenyltetrazolium chloride (Kearns and Inouye, 1993) turned pink, indicating viability, while no shriveled seeds turned pink.
For each year and pollination treatment combination, we tested for a population effect on seed production. Also, for each population, we tested for a pollination treatment effect on seed production. We did each of these tests with 1-way ANOVA using SAS PROC GLM. Population and pollination treatment differences were determined with Tukey's studentized range test at an alpha level of 0.05. (SAS Institute Inc., 2008).
Nutrients.--Bosch and Waser (1999) suggested resource limitation as a mechanism of reduced seed production in local populations. A positive correlation between soil nutrient cation amount and seed production would suggest that cations might be limiting seed production. We estimated amounts of [Ca.sup.++], [K.sup.+], and [Mg.sup.++] in the soil for each population. From each population, we collected five soil subsamples from the top 25 mm of the soil profile (plant root zone). Each subsample location was within 1 m ofa coneflower plant and [greater than or equal to]3 m distant from other subsample locations. We combined the five subsamples in a plastic pail, air dried it, pulverized it by hand, and passed it through a 2 mm sieve. The North Carolina Department of Agriculture and Consumer Services, Agronomic Division--Soil Testing Section analyzed the soil samples. They determined the pH of a 1:1 soil:distilled water suspension, the quantities of available calcium, magnesium, and potassium using a Mehlich-3 extraction (Mehlich, 1984), and then calculated cation exchange capacity (CEC) and % base saturation (Mehlich, 1976). We used correlation analysis (Excel Data Analysis Tools, Microsoft Office Corporation, 2003) to assess the relationship between cation amount and seed production following open-pollination (see methods under Flower visitation and pollen limitation below).
We also explored plant size and seed production data for patterns suggesting resource limitation (Griffin and Barrett, 2002). We tested for plant size difference between PC and the combined small populations. The number of disk flowers per cross-pollination head (see methods under Flower visitation and pollen limitation below) was the measure of plant size (Wagenius, 2004). These disk flower numbers were square root transformed for analysis. We pooled data of small populations from both years and PC data from both years. We also tested for plant size difference between pollination treatments (cross- and open-) in each population. For both of the preceding tests, we used a linear model, 1-way ANOVA, with SAS PROC GLM (SAS Institute Inc., 2008). Finally, we assessed the relationship between plant size and seed production of cross-pollination heads for both PC and the small populations. We pooled PC data of 2004 and 2005 and pooled data of all small populations from both years. We used correlation analysis (Excel Data Analysis Tools, Microsoft Office Corporation, 2003) and determined P values from Table Y of Rohlf and Sokal (1969).
PERFORMANCE OF SEEDS AND SEEDLINGS FROM A LARGE AND A SMALL POPULATION
A direct relationship between population size and seed and seedling performance has been been described for several plant taxa (Menges, 1991, 1995; Fischer and Matthies, 1998; Bruna, 2002; Donaldson et al., 2002; Hooftman et al., 2003; Kolb, 2005). We looked for differences in coneflower seed performance and seedling size between the large PC population and the small SHR population. During the first week of Sep. 2004, we harvested randomly selected open-pollinated heads from PC and available heads from SHR and selected the plump seeds from each head. We pooled the plump seeds of each population and moist-stratified them (4 C for 50 d). On 4 Mar. 2005, we planted them at PC in 10 4 X 2 m field blocks at a depth of 0.1-0.2 mm. To minimize the chances of coneflower seedlings emerging from a soil seed bank, we located the blocks approximately 100 m from the edge of the coneflower population. Prior to planting, we cleared vegetation from the blocks with herbicide and clipping. Within each block, we planted 20 PC seeds in one 0.5 X 0.5 m plot, 20 SHR seeds in another 0.5 x 0.5 m plot and left the third plot unplanted as a control. We recorded numbers of new seedlings (seed germinations) and seedling deaths in each plot weekly from 11 Mar. to 29 Jul. On 19 Aug. we counted the leaves on each surviving seedling.
We tested for a population effect on field seed germination and seedling survivorship with a randomized block ANOVA using SAS PROC ANOVA (SAS Institute Inc., 2008). Data for these analyses were arc sine transformed. We tested for a population effect on seedling leaf number with a mixed model ANOVA using SAS PROC MIXED (SAS Institute Inc., 2008). Leaf numbers were square root transformed for this analysis. Means separation for these analyses was determined with Tukey's studentized range test at an alpha level of 0.05.
[FIGURE 1 OMITTED]
On 5 Mar. 2005, we planted 10 open-pollination seeds from PC at a depth of 0.1-0.2 mm in each of 20 greenhouse pots containing 4 : 1 loam : sand substrate. We planted 10 seeds from SHR in 20 other pots containing the same substrate. Plants were maintained on a greenhouse bench, lightly watered to maintain a moist substrate surface, and rotated weekly to avoid position effects. Greenhouse temperatures ranged from 23 C to 29 C. We recorded numbers of germinations and seedling deaths for each pot weekly until no more germination occurred for two weeks in a row. Then all seedlings except the largest in each pot were removed. We harvested the seedlings on 22-24 Aug. 2005, dried them at 43 C to constant weight and recorded their dry weight.
We tested for a population effect on greenhouse seed germination, seedling survivorship and seedling dry weight with 1-way ANOVA using SAS PROC GLM. For these analyses, germination and survivorship data were arc sine transformed; dry weight data was untransformed. Means separation for these analyses was determined with Tukey's studentized range test at an alpha level of 0.05.
FLOWERING PHENOLOGY AND FLOWER VISITORS
Flowering phenology.--Coneflower was first seen flowering at PC on 25 May 2004, and by 3 Jun. it was the most abundant blooming species in the community (Fig. 1). It remained most abundant throughout Jun. At its blooming peak, coneflower accounted for 68% of the flowering individuals in the community. Blephila ciliata (Linnaeus) Bentham, Penstemon australis Small, and Rosa caroliniana Linnaeus bloomed along with coneflower during late May-early Jun. and Houstonia longifolia Gaertner did so during early Jun.--early Jul. In 2004, Echinacea laevigata flowered at SHR and KOR from 4 to 26 Jun. and 4 to 28 Jun., respectively. In 2005, flowering occurred at BD, LS, and FB from 3 to 23 Jun., 5 to 30 Jun., and 6 to 27 Jun., respectively. Blephila ciliata, P. australis, R. caroliniana, and H. longifolia did not occur with any of the small coneflower populations.
Flower Visitors.--We saw bees visiting coneflower in the PC study transects during 25 May-6 Jul. Bees visited Blephila ciliata, Penstemon australis, and Rosa caroliniana until 3 Jun., but not later. We did not see bees visit Houstonia longifolia. Bees visited one plant of Cirsium caroliniana (Walter) Fernald and B.G. Schubert located outside of the study transects on 3 Jun., but not later. Bees began visiting Centrosema virginiana (Linnaeus) Bentham and Liatris squarrosa (Linnaeus) Michaux in Jul. after coneflower had completed flowering.
The order of coneflower pollinator importance was Bombus > Megachile = Xylocopa = Hesperiidae > Lygaeus = Nymphalidae. Herrera (1987) also found that butterflies were relatively unimportant pollinators compared to various bees. Bumblebees ranked higher than all other insect groups for number of visits to coneflower (Table 3). They accounted for 73% of all visits during 2004 and 93% during 2005. Additionally, individual bumblebee visitors carried nearly as many Echinacea laevigata pollen grains as carpenter bees (Table 3). There were no differences among pollinator groups for proportion of coneflower pollen in pollen load or pollen location on insect body. All bee and seed bug visitors crawled over flower heads and contacted receptive stigmas with their mouth parts, ventral thorax, abdomen, and legs. Skippers and butterflies remained stationary while they probed disk flowers with their proboscides. Other insects which visited coneflower occasionally included Hymenopterans Apis meUifera L. (Apidae), several Halictidae species, and Ammophila sp. Dahlbom (Sphecidae); Coleopterans, Brachyloptura vegans Olivier and Typocerus zebra Olivier (Cerambycidae); Lepidopterans, Eusarca confusaria Hubner (Geometridae) and Harrisina sp. (Zygaenidae); and Dipterans, Eristalis transverses Wiedemann, Milesia virgineinsis Drury, and Toxomerus sp. (Syrphidae).
During 2004, insects visited heads in PC five and 16 times as often as in KOR and SHR, respectively ([F.sub.2,17] = 11.93, P < 0.001, Table 4). However, during 2005, insects visited heads in FB more often than in PC ([F.sub.3,21] = 27.22, P < 0.001, Table 4). We often saw Bombus pollinators emerging from the ground, apparently from a nest, at the edge of the FB population. Insects visited heads in PC three and four times as often as in LS and BD, respectively. During both study years, insect visitor species richness was higher in PC than in each small population ([F.sub.2,17] = 18.51, P < 0.0001 and [F.sub.3.21] = 9.05, P = 0.001).
FACTORS AFFECTING SEED PRODUCTION
Pollen viability.--Pollen % viability for PC was 94.6 [+ or -] 1.19 (n = 8); SHR, 98.3 [+ or -] 0.58 (n = 3); KOR, 93.2 [+ or -] 1.53 (n = 3); BD, 98.7 [+ or -] 1.15 (n = 3); LS, 97.7 [+ or -] 0.58 (n = 3) and FB, 99.3 [+ or -] 0.58 (n = 3). These data suggest that low pollen viability did not limit coneflower seed production.
Compatibility.--All 24 of the unbagged, open-pollinated heads produced seeds, but only 20% (2/10) of the heads receiving either self pollen or no pollen produced seeds. The probability of seed production depended on pollination treatment (P < 0.001). Comparing those heads that did produce seeds, % production was greater ([F.sub.2.25] = 13.1, P < 0.001) for open-pollinated heads, 42.0 [+ or -] 15.7 (n = 24), than those receiving self pollen, 2.0 [+ or -] 0.19 (n = 2), or no pollen, 4.3 [+ or -] 0.31 (n = 2). We saw no evidence of seed herbivory. We concluded that coneflower is nearly exclusively outcrossing but occasionally produces a few seeds via geitonogamy. These results did not indicate apomictic seed production. This compatibility system is like that previously reported for other Echinacea species (McGregor, 1968; Parrish and Bazzaz, 1979; Leuszler et al., 1996; Wagenius, 2004, 2006).
Pollen limitation.--Seed production patterns differed between years. During 2004, there was no difference for either open-pollination or cross-pollination between PC and the small populations ([F.sub.2,24] = 1.90, P = 0.16 and [F.sub.1,10] = 1.36, P = 0.27, Table 5). However, open-pollination seed production in PC exceeded that in all of the small populations during 2005 ([F.sub.3,35] = 17.69, P < 0.001). Seed set was lower in BD than in LS and FB. Cross-pollination seed set also was lowest in BD ([F.sub.3,16] = 3.39, P = 0.04, Table 5). We concluded that seed production during 2005 was limited in the small populations relative to PC and this limitation was strongest in BD.
In PC during 2005, the relatively low cross-pollination seed production was unexpected given relatively high open-pollination seed set in this population (Table 5). Since we barely touched stigmas when we cross-pollinated flowers, it seems unlikely that stigma damage caused this result. However, the cross-pollinated heads had only 67% the number of disk flowers as open-pollinated heads. It is possible that these smaller plants were less reproductively fit than the larger open-pollinated plants. Because of this equivocal result, we do not make inferences based on comparisons of cross-pollination seed production between PC and LS or FD. However, we feel justified in concluding that cross-pollination seed production was higher in PC than in BD.
In the small populations of both 2004 and 2005, cross-pollination failed to increase seed production relative to open-pollination (Table 5). This indicated that pollinators were realizing the seed producing potential of these populations; seed production was not pollen-limited. The seed production limitation in small populations relative to PC during 2005 was caused by a mechanism(s) other than pollen limitation.
Nutrients.--Each cation, [Ca.sup.++], [K.sup.+], and [Mg.sup.++], was present in appreciable amounts in soils supporting both the large and small coneflower populations (Table 2). Cation exchange capacities of all these soils were near the maximum for North Carolina soils and base saturations were high (Hardy et al., 2003). These high base conditions were comparable with those previously reported for soils supporting Echinacea laevigata (Dayton, 1966). There was not a relationship between total soil cation amount and open-pollination seed production ([r.sub.5] = 0.612, P = 0.14; Table 2).
There was no difference in plant size between PC and the pooled small populations ([F.sub.1,30] = 0.402, P = 0.53). Plant size in open- and cross-pollination treatments differed in only PC during 2005 ([F.sub.l,16] = 5.195, P = 0.04). In this population, cross-pollinated heads had only 67% the number of disk flowers, suggesting that they could be less reproductively fit than the open-pollination heads. There was no relationship between plant size and seed production in the cross-pollination treatment in PC ([r.sub.16] = 0.195, P = 0.44) or in the pooled small populations ([r.sub.12] = 0.342, P = 0.23). These results suggested that, with the possible exception of PC during 2005, resources that regulate plant size did not limit seed production in the study populations.
PERFORMANCE OF SEEDS AND SEEDLINGS FROM A LARGE AND A SMALL POPULATION
Field seed germination for SHR and PC did not differ ([F.sub.1,9] = 4.61, P = 0.06, Table 6), but seedling survivorship was greater for SHR ([F.sub.1,9] = 5.11, P = 0.05). On average, each fieldraised PC seedling had 25% more leaves than each SHR seedling ([F.sub.1,19] = 8.50, P = 0.02). SHR and PC greenhouse germination ([F.sub.1,38] = 0.88, P = 0.35) and survivorship ([F.sub.1,38]= 3.04, P = 0.09) did not differ. Individual Picture Creek seedlings weighed 52% more than those from SHR ([F.sub.1,36] = 22.89, P < 0.001).
Flowering phenology and flower visitors.--The most important coneflower pollinators we saw were species of bumblebees that have been reported to visit more than 75 plant genera (Mitchell, 1962). Additionally, Bombus foragers often visit several plant species during individual foraging flights (Mosquin, 1971; Kunin, 1993); they are generalist flower visitors. In the current study, few plant species that attracted bumblebees flowered at the same time as coneflower. However, at other locations, co-flowering species may compete with coneflower for Bombus pollinators (Mosquin, 1971; Gross and Werner, 1983; Campbell, 1985; Jennersten and Nilsson, 1993; Kunin, 1993; Ramsey, 1995) or facilitate its pollination (Feldman et al., 2004; Ghazoul, 2006; Lopezaraiza-Mikel et al., 2007; La'zaro et al., 2009). Wagenius (pers. comm.) indicated that co-flowering Amorpha canescens Pursh competed with Echinacea angustifolia De Candolle for Bombus pollinators rather than facilitating its pollination. Accordingly, the importance of generalist bumblebees as coneflower pollinators may differ from that reported in the current study (Mitchell et al., 2009), depending on community flowering phenology. It is important that species' abundances and flowering phenologies are included in descriptions of rare species' pollination biology and seed production.
Factors affecting seed production.--During 2005, seed production was limited in LS, FD, and more so in BD. Since there was no indication that soil cations, other resources that regulate plant size, pollen viability, or pollen limitation reduced seed production, we speculate that a genetic mechanism caused this limitation. Peters et al. (2009) determined from genetic data that inbreeding was not elevated in small coneflower populations relative to larger populations but did consider loss of S alleles via genetic drift a serious threat to small population viability. Accordingly, we suggest that a paucity of S alleles was the most likely reason for the reduced seed set. Future studies of seed set following artificial crosspollinations between neighboring coneflower populations and determining frequencies of inter-population pollinator flights would help interpret patterns of seed production among populations.
Although a direct relationship between levels of pollinator service and seed production has been described by recent workers (Yates and Ladd, 2005; Aguilar et al., 2006; Campbell and Husband, 2007), neither the current study or Wagenius and Lyon (2010) found a direct relationship. We showed that coneflower seed set in KOR and SHR during 2004 equaled that in PC even though flower visits were 5 and 16 times more frequent, respectively, in PC. Wagenius and Lyon (2010) reported abundant solitary Halictidae bee visits to but reduced seed production in small, isolated populations of Echinacea angustifolia. A consideration of pollinator behavior and size can contribute to explaining the different seed set--pollinator service trends in these two studies. Halictid bees typically remain within local flower patches (Ginsberg, 1985) and respond to landscape features at a fine ([less than or equal to] 250 m) spatial scale (Steffan-Dewenter et al., 2002). This behavior tends to promote pollination among plants that may be closely related or share common S alleles. Alternatively, Bombus intersperses long inter-flower flights with short flights (Ginsberg, 1985; Schulke and Waser, 2001; Kraus et al., 2009) and, in general, responds to conditions at a coarser landscape scale (Steffan-Dewenter et al., 2002). Steffan-Dewenter and Tscharntke (1999) showed that Bombus pollinator abundance decreased less than solitary bees at plant isolation distances up to 1000 m. Behavior of bumblebees, more than halictid bees, tends to promote pollination among plants that have different S alleles. Additionally, since bumblebees are larger than halictids, they can accumulate larger pollen loads from more pollen donor plants. These large pollen loads accumulated from several donor plants may mean that few bumblebee visits are required for maximal seed production. In combination, these results suggest that halictid bees would be less effective than Bombus in cross-pollinating isolated plants and may partly explain the different results noted by Wagenius and Lyon (2010) and the current study. In summary, both pollinator foraging behavior and effectiveness in response to plant community composition and blooming phenologies must be considered in interpretations of population size and isolation on seed production.
Performance of seeds and seedlings from a large and a small population.--SHR seedlings were smaller than PC seedlings. Inbreeding has previously been suggested as a likely cause for relatively small seedlings in small populations (Menges, 1991, 1995; Young et al., 1996; Fischer and Matthies, 1998; Bruna, 2002; Donaldson et al., 2002; Hooftman et al., 2003; Severns, 2003; Hensen and Oberpreiler, 2005; Kolb, 2005; Ward and Johnson, 2005). However, inbreeding was previously shown not to be elevated in SHR or other small coneflower populations (Peters et al., 2009). Alternatively, chronically open habitat can select for relatively small seedlings (Crawley and Nachapong, 1985; McGinley et al., 1987; Tripathi and Khan, 1990). The amount of litter and living vegetation has been controlled at SHR by annum mowing. Since the mid-1980s, PC has been mowed only occasionally (Walker, 2009) so dense annual stands of grasses and forbs have produced a substantial litter layer. Accordingly, we suggest adaptation to local site conditions as a more likely explanation than inbreeding for the relatively small SHR seedlings. Future studies are needed of possible demographic consequences of variable coneflower seedling sizes in habitats that differ in amount of vegetative and litter cover.
Conservation recommendations.--Based on results of our study, we make the following management suggestions: (1) Conservation workers should maintain Bombus habitat near coneflower populations (Sipes and Tepedino, 1995; Tepedino et al., 1997). Habitat should include nest sites (Alford, 1975) and spring-blooming species which provide queens with pollen and nectar as they establish nests early in the season and help habituate Bombus foragers to coneflower habitat (Waser and Real, 1979; Tepedino et al., 1997). (2) If coneflower seed set is low, workers could reduce the flower abundance of those co-flowering species in the community which attract bumblebees (Lopezaraiza-Mikel et al., 2007). Coneflower seed production in response to flower reduction should be monitored because it could be reduced instead of increased. (3) If outbreeding depression is not indicated among neighboring coneflower populations (Waser and Price, 1989; Tallmon et al., 2004; Seltmann et al., 2009), workers should manage habitat separating these populations to encourage inter-population pollinator flights (Cane, 2001), thereby reducing the probability of local S allele loss.
Acknowledgments.--We thank Dr. Cavell Brownie and Wade Wall for initial statistical assistance; Andy Walker, Angela Richardson, Julie Roszco, Jennifer Petite, and Caitlin Elam for their help with data collection; Dave Stephan for insect identification; Jenny Xiang for use of her lab facilities; Donna Wright for providing greenhouse space; and the NCSU Phytotron for research space.
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SUBMITTED 13 DECEMBER 2010
ACCEPTED 26 JANUARY 2012
J. M. STUCKY (1)
Department of Plant Biology, Box 7612, North Carolina State University, Raleigh 2769-5
L. E. GADD
North Carolina Natural Heritage Program, 1601 MSC Raleigh 27699
Department of Statistics, Box 8203, North Carolina State University, Raleigh 27695
(1) Corresponding author: Telephone: (919) 515-2227; e-mail: email@example.com
TABLE 1.--Experimental populations of Echinacea laevigata General location; Population (county, coordinates) Habitat Picture Creek Granville, 36.1[degrees]N, 60 m wide powerline (PC) 78.7[degrees]W right-of-way Snow Hill Rd. Durham, 36.1[degrees]N, roadside adjacent to (SHR) 78.9[degrees]W oak forest Knap of Reeds Granville, 36.1[degrees]N, 10 m wide abandoned (KOR) 78.8[degrees]W powerline right of-way adjacent to oak forest Briardale (BD) Durham, 36.1[degrees]N, roadside adjacent to 78.9[degrees]W oak forest Lakeside (LS) Durham, 36.1[degrees]N roadside adjacent to 78.9[degrees]W oak forest Freudenberg Durham, 36.1[degrees]N edge of oak forest (FB) 78.9[degrees]W Distance to Daily no. nearest E. flowering laevigata No. plants that stems/[m.sup.2], population flowered during mean [+ or -] Population (km) growing season [SD.sup.1] Picture Creek 6.2 [approximately 7.2 [+ or -] 2.02 (PC) equal to] 50,000 Snow Hill Rd. 0.8 23 0.04 [+ or -] 0.220 (SHR) Knap of Reeds 6.2 31 0.08 [+ or -] 0.052 (KOR) Briardale (BD) 0.4 15 0.10 [+ or -] 0.076 Lakeside (LS) 0.4 17 0.09 [+ or -] 0.057 Freudenberg 0.8 48 0.78 [+ or -] 0.564 (FB) (1) Stems/[m.sup.2] determined as number of flowering plants divided by rectangular dimensions of area occupied for SHR and KOR during 2004 and BD, LS and FB during 2005; determined for PC as number of flowering plants in 2 X 2 m pollinator observation plots during 2004 and 2005 TABLE 2.--Nutrient status of soils and open-pollination seed production for experimental populations Base saturation Total (% of CEC concentration of CEC (1) occupied base cations (meq/100 by base (meq/100 Population pH [cm.sup.3]) cations) (2) [cm.sup.3]) PC 6.3 25.2 94 23.7 SHR 6.5 20.5 96 19.7 KOR 6.1 21.4 93 19.9 BD 6.1 18.6 89 16.6 LS 5.9 17.5 91 15.9 FB 6.5 23.2 95 22.0 Open-pollination seed production Population (%) (3) PC 42, 56 (4) SHR 52 KOR 50 BD 22 LS 38 FB 42 (1) Cation exchange capacity (2) Base cations include [Ca.sup.++] , [K.sup.+], and [Mg.sup.++] (3) From Table 5 (4) Seed production for 2004 and 2005 TABLE 3.--Pollinator importance of Echinacea laevigata flower visitors Location 2 of pollen loads on body; mean rank (3), range Pollinator group n (1) Rank Bombus spp. (8) 13 3.8 (a), 3-4 3.5 Xylocopa virginica (L.) 5 2.7 (a), 2-4 3.5 Lygaeus kalmii Stal 5 3.3 (a), 3-4 3.5 Megachile spp (9) 13 3.1 (a), 0-4 3.5 Hesperiidae (10) 5 1.2 (a), 0-4 3.5 Nymphalidae (11) 5 1.0 (a), 0-2 3.5 No. of E. laevigata pollen grains per 0.05 ml sample; [mean.sub.group] (4,5) 95% CL Pollinator group Rank Bombus spp. (8) 39.3 (ab), 4.0-110 4.25 Xylocopa virginica (L.) 97.4 (a), 22.3-222.3 5.5 Lygaeus kalmii Stal 12.9 (b), 5.2-24.8 3 Megachile spp (9) 13.4 (b), 6.9-21.9 3 Hesperiidae (10) 1.5 (b), 0-6.4 3 Nymphalidae (11) 2.8 (a), 0-11.4 3 Proportion of E. laevigata pollen grains per 0.05 ml sample; [mean.sub.group] (4,6), 95% CL Pollinator group Rank Bombus spp. (8) 76.3 (a), 55.8-91.9 3.5 Xylocopa virginica (L.) 86.6 (a), 77.8-93.2 3.5 Lygaeus kalmii Stal 95.3 (a), 72.4-100 3.5 Megachile spp (9) 73.7 (a), 61.4-84.4 3.5 Hesperiidae (10) 51.4 (a), 16.3-99.8 3.5 Nymphalidae (11) 60.6 (a), 0-100 3.5 Visits/head/15 min; mean 4,7, 95% CL Pollinator group Rank Sum of ranks Bombus spp. (8) 2.3 (a), 1.7-2.6 6 17.3 Xylocopa virginica (L.) 0.011 (c), >0.01-0.01 2 14.5 Lygaeus kalmii Stal >0.01 (c), >0.01-0.01 2 12 Megachile spp (9) 0.1 (b), 0.07-0.15 4.5 14.5 Hesperiidae (10) 0.1 (b), 0.05-0.13 4.5 14.5 Nymphalidae (11) >0.01 (c), 0->0.01 2 12 (1) Number of insects evaluated for the first three criteria (2) Location scores: 4--all pollen on body parts most likely to contact stigmas; 3 most pollen on body parts most likely to contact stigmas but some on dorsal surface, wings, or in pollen sacs; 2--all pollen on body parts not likely to contact stigmas; 1 < 10 pollen grains on the body; 0 no pollen evident (3) Kruskal-Wallis test, [H.sub.(5)] = 7.620, P > 0.05, no differences among mean ranks (4) Means in a column with the same superscript are not different according to ANOVA with Tukey's multiple pairwise mean comparison; [alpha] = 0.05 (5) ANOVA used square root transformed data for [mean.sub.group] [F.sub.5,35] = 6.23, P < 0.001 (6) ANOVA used arcsine transformed data for [mean.sub.group], [F.sub.5,35] = 1.11, P = 0.37 (7) ANOVA used square root transformed data, n = 76 observation periods, [F.sub.5,449] = 246.29, P < 0.001 (8) Includes B. bimaculatus Cresson, B. citrinus Smith, B. griseocollis Degeer, B. impatiens Cresson, B. pennsylvanicus Degeer (9) Includes M. brevis Say, M.mendica Cresson, M. sculpturalis Smith, M. texana Cresson, and M. xylocopoides Smith (10) Includes Atrytone logan Edwards, Atrytonopsis hianna Scudder, Epargyreus clarus Cramer, Polites sp. probably themistocles Latreille and Thorybes bathyllus J.E. Smith (11) Includes Euptoieta claudia Cramer, Speyeria cybek F., and Vanessa virgineinsis Drury TABLE 4.--Insect visitation in Echinacea laevigata populations No. visits/ Visitor species # 15 min. head/15 min. richness/15 min. Population intervals (n) (SE) (1) (SE) 2004 Picture Creek 42 1.6 (0.28) (a) 2.8 (0.21) (a) Knap of Reeds 7 0.3 (0.20) (a) 0.8 (0.41) (b) Snow Hill Road 5 0.1 (0.24) (a) 0.6 (0.45) (b) 2005 Picture Creek 36 4.8 (0.19) (a) 2.5 (0.17) (a) Briardale Road 12 1.2 (0.32) (c) 1.5 (0.26) (b) Lakeside Drive 6 1.6 (0.76) (c) 1.0 (0.38) (b) Freudenberg 6 8.7 (1.54) (a) 1.0 (0.38) (b) (1) Tukey's means groupings, a-value for Tukey's <0.05 (2) Means with the same superscript in a column within individual years are not different using Tukey's studentized test TABLE 5.--Seed production in open-and supplemental-pollination treatments in each population % viable seeds No. head (1,2); Pollination surviving per mean Year Population treatment heads [+ or -] SD 2004 PC Open 24 42 [+ or -] 15 (a) Cross 10 47 [+ or -] 18 (A) SHR Open 6 52 [+ or -] 21 (a) Cross 23 62 [+ or -] 5 (A) KOR Open 15 50 [+ or -] 14 (a) Cross 3 - 2005 PC Open 10 56 [+ or -] 13 (a) Cross 83 35 [+ or -] 9 (A) BD Open 15 22 [+ or -] 13 (c) Cross 43 27 [+ or -] 14 (B) LS Open 8 38 [+ or -] 12 (b) Cross 23 45 [+ or -] 8 (A) FD Open 10 42 [+ or -] 5 (b) Cross 6 49 [+ or -] 13 (A) ANOVA; Pollination treatment effect within Year Population population 2004 PC [F.sub.1,32] = 0.79, P = 0.38 SHR [F.sub.1,16] = 0.41, P = 0.54 KOR - 2005 PC [F.sub.1,16] = 14.42, P = 0.002 BD [F.sub.1,17] = 0.43, P = 0.52 LS [F.sub.1,4] = 0.52, P = 0.51 FD [F.sub.1,14] = 2.81, P = 0.12 (1) Populations with the same lower case superscript within a year do not differ for open-pollination seed set (2) Populations with the same upper case superscript within a year do not differ for supplemental pollination seed set (3) Longhorn beetle bored into peduncles of some heads in this treatment. Seed production in these heads not included in statistical analyses TABLE 6.--Seed germination, seedling survivorship, and seedling size in the field and greenhouse Experimental % Seed germination; environment Population mean [+ or -] SD Field PC 40.5 [+ or -] 12.12 (a) SHR 54.5 [+ or -] 17.56 (a) Greenhouse PC 65.5 [+ or -] 28.57 (a) SHR 55.5 [+ or -] 23.95 (a) % Seedling Experimental survivorship; environment Population mean [+ or -] SD Field PC 36.0 [+ or -] 12.87 (b) SHR 49.5 [+ or -] 16.91 (a) Greenhouse PC 97.4 [+ or -] 11.23 (a) SHR 98.8 [+ or -] 3.80 (a) Experimental Seedling size (2); environment Population mean [+ or -] SD Field PC 5.1 [+ or -] 1.56 (a) SHR 4.1 [+ or -] 1.23 (b) Greenhouse PC 22.0 [+ or -] 4.31 (a) SHR 14.4 [+ or -] 5.40 (b) (1) Means with the same superscript in a column within an individual experimental environment are not different (2) Number of leaves per field seedling; dry weight (g) of individual greenhouse seedlings
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|Author:||Stucky, J.M.; Gadd, L.E.; Arellano, C.|
|Publication:||The American Midland Naturalist|
|Date:||Jul 1, 2012|
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