Floral Density Research Articles

Pollen production of the genus Cupressus

The study of potential pollen emission in the genus Cupressus was carried out in the city of Córdoba (Spain). Pollen production of 227 trees of the represented species of Cupressus in the area (Cupressus sempervirens, C. macrocarpa and C. arizonica) was studied by counting the number of pollen grains per flower, the number of flowers per branch, and the number of branches per m2 of surface area. Total surface area was estimated by considering the shape of the tree as a cone and estimating the average height and radius of each species. Results indicated that there was little variation in the number of pollen grains produced by a flower within each species. Equally, there were few differences in the number of grains per flower among the species, although C. arizonica had slightly fewer than the other species because it possessed fewer stamens per flower. However, there were clear differences in the number of flowers per branch, with C. macrocarpa having the highest floral density, considerably lower in C. arizonica and even lower in C. sempervirens. All of this indicates that the pollen contribution of C. macrocarpa to the atmosphere is far greater than that of C. arizonica and C. sempervirens. The results indicate a different potential emission in each species, and this must be taken into account when determining the quantity of pollen present in the atmosphere. The maximum amount of pollen grains that one individual can spread is provided in this study and would provide useful data in the development of an airborne forecasting system.

Family outcrossing rates and neighborhood floral density in natural populations of swamp milkweed (Asclepias incarnata): potential statistical artifacts

To evaluate how environmental and genetic factors influence mating-system evolution, accurate estimates of outcrossing rates of individual plants (families) are required. Using isozyme markers, we observed wide variation in family outcrossing rates in three natural populations of Asclepias incarnata using three statistical methods: (1) a multilocus maximum-likelihood procedure (tm); (2) a multilocus method-of-moments procedure (ta); and (3) a direct comparison of progeny phenotypes against maternal phenotypes (td). Neighborhood floral-display size was positively correlated with ta in one population, but showed no relationship with any of the other estimates of outcrossing for any population. Monte-Carlo simulations revealed that statistical variation associated with these estimation procedures can be large enough to explain all of the observed variation in outcrossing. We also found that significant, spurious correlations with neighborhood floral display could arise, on average, 7% of the time by chance alone. Our observations suggest that it is difficult to obtain accurate estimates of outcrossing in naturally pollinated plants using the estimation procedures currently available. Moreover, we caution that attempts to interpret observed variation in family outcrossing estimates by observing variation in ecological parameters could be misleading.

New strategies for the chemical thinning of apple (Malus domestica Borkh.) cultivars Queen Cox and Royal Gala

SummaryHigh volume sprays of ammonium thiosulphate (ATS), endothal, sulfcarbamide and pelargonic acid, applied at full bloom, were compared as flower thinners for the apple cultivars Queen Cox and Royal Gala grown on M.9 rootstocks in the south-east of England. Sprays of ATS (10,000 or 15,000 mg l–1) or endothal (500 to 200 mg l–1 reduced initial and final fruit set per 100 floral buds in each of two consecutive years when applied to the same set of trees. Sulfcarbamide reduced fruit set only when applied at concentrations higher than those recommended (4000 mg l–1), and sprays of pelargonic acid had no significant effect on the final set of these two cultivars. The ATS and endothal treatments increased fruit size in the first year of application, but effects in the subsequent year were inconsistent. Sprays in the first year increased floral density in the second year. In both years of treatment, sprays of benzyladenine (BA), applied when fruitlets were approximately 12.mm in diameter, increased the percentage of fruitlets abscinding. As with the ATS and endothal sprays, the effects of the BA spray treatments on fruit size at harvest were variable. Possible reasons for the inconsistencies in fruit size recorded in the thinning trials are discussed.

The influence of relative plant density and floral morphological complexity on the behaviour of bumblebees.

We assessed the combined effects of varying the relative density and the relative floral morphological complexity of plant species on the behaviour of their bumblebee pollinators. Three species of bumblebee (Bombus pascuorum, B. terrestris and B. hortorum) were observed foraging on experimental arrays consisting of pair-wise combinations of four plant species: Borago officinalis, Phacelia tanacetifolia (both with simple flowers), Antirrhinum majus and Linaria vulgaris (both with complex flowers). Plant arrangements consisted of either two simple-flower species, a simple with a complex species or two complex species. The number of plants in each array was constant, while the frequency of each species was manipulated so that it was either rare, equal or common compared with its competitor. Contrary to predictions, rare plants were actually at an advantage in terms of the number of bees attracted per plant. However, rare plants were at a disadvantage in terms of pollen wastage because foragers more often went to a flower of another species after visiting a rare plant. The behaviour of bees on each plant species was further affected by plant floral complexity and the identity of the other species in the array. The three bumblebee species were markedly different in their foraging behaviour and in their responses to varying floral density and complexity. Each species preferred particular flower species. The results are discussed with reference to resource partitioning among bumblebee species.

Floral syndrome in Amelanchier nantucketensis (Rosaceae). I. Floral density, bee activity, and characterization of andropetaly

The clonal shrub Amelanchier nantucketensis is andropetalous; a variable number of petals as well as normal anthers both bear germinable pollen. To determine whether floral density associated with andropetaly is related to activity of native bees on flowers, we compared Amelanchier nantucketensis to Amelanchier humilis and Amelanchier stolonifera, which have sterile petals. In samples from four Maine sites, floral density in Amelanchier nantucketensis was 30.3% greater than that for Amelanchier humilis and 39.5% greater than that for Amelanchier stolonifera; in Amelanchier nantucketensis, flowers were more numerous per cubic metre and distance between flowers was shorter. There was a significant correlation across all sites and species between the number of flowers per plant and number of inflorescences visited per bee in 10 min (r = 0.711, p = 0.0001). Statistical t tests separated Amelanchier nantucketensis from both Amelanchier humilis and Amelanchier stolonifera by plant height, nearest stem, nearest flower, and number of inflorescences visited per bee during 10 min. Plant species classification in a canonical discriminant analysis was 83–93% correct with four plant variables, namely distance to nearest flower, distance to nearest inflorescence, plant width, and plant height. Our findings support optimal foraging theory in that bee visits increase with floral density in these Amelanchier species. Key words: Amelanchier, andropetaly, bees, floral density, morphology, Maine.

Floral syndrome in Amelanchier nantucketensis (Rosaceae). II. Bee preference and diversity associated with andropetaly

Pollinator preference has been demonstrated to depend on floral morphology in some plant groups. We tested bee preference for Amelanchier nantucketensis, which has pollen-bearing petals or andropetals, over Amelanchier humilis with its normal petals when these flower together at two Maine sites. In an array experiment with equal floral display for Amelanchier nantucketensis and Amelanchier humilis, almost twice as many bees approached the latter first. Bees responded to some aspect of the larger flowers of Amelanchier humilis when number of inflorescences and height above the ground were similar. We captured 539 bees in 43 species individually on flowers, in malaise traps, and in sweeps; most were common, widely distributed solitary and eusocial bees. Bee species diversity differed slightly between plant species, and guilds differed; 11 bee species were unique to Amelanchier nantucketensis, including 39 individuals of Andrena milwaukiensis, and three species were unique to Amelanchier humilis. For 29 bee species shared by both plant species, abundance on Amelanchier nantucketensis was greater than on Amelanchier humilis for 62%, less for 14%, and the same for 24%. Bees might be attracted to Amelanchier nantucketensis less by andropetaly than by proximity of flowers to the ground, floral density, or unidentified features. Key words: Amelanchier, bees, foraging, morphology, Maine, species diversity.

Fruit set in a deceptive orchid: The effect of flowering phenology, display size, and local floral abundance

We investigated the effect of flowering time, display size, and local floral density on fruit set in Tolumnia variegata, a pollination‐limited orchid that offers no reward to its pollinator(s). During 1990, natural variation in flowering time, display size, and fruit set were monitored in 508 plants at one locality in Puerto Rico. The following season, orchid floral abundance per host tree (Randia aculeata) was manipulated to investigate its effect on fruit set. Four floral abundance treatments were established (700, 500, 300, and 100), each replicated four times. Flowering time was the most important trait affecting fruit set. The proportion of plants setting at least one fruit was significantly high early and late in the season, but low during the flowering peak. Thus, strong disruptive selection differential on flowering phenology was found. Display size had little effect on fruit set. A weak, but significant disruptive selection differential on display size was found. Orchid floral abundance per host tree had a significant effect on fruit set. Early in the season, T. variegata flowers with intermediate number of conspecific flowers exhibited a greater probability of setting fruit than those in host trees with fewer or more flowers. Our results show that flowering phenology may be evolutionarily unstable, possibly a consequence of the deception pollination system. Furthermore, a deception strategy would be relatively unsuccessful in populations where plants are found in either very dense or sparse patches.

Effect of Canopy Gaps and Flower Patch Size on Pollinator Visitation of Impatiens capensis

WALTERS, B. B. AND E. W. STILES (Graduate Program in Ecology and Evolution, Rutgers University, Piscataway, NJ 08855). Effect of canopy gaps and flower patch size on pollinator visitation of Impatiens capensis. Bull. Torrey Bot. Club 123:184-188. 1996.-We measured pollinator visits of Impatiens capensis flowers found in different-sized patches beneath canopy gaps and beneath closed canopy in a mature, old-growth deciduous forest in New Jersey. Honeybees, bumblebees and halictid bees made up the majority of visitors. Visitation rates per flower increased slightly, but not significantly, in relation to flower patch size (number of open flowers) and floral density. By contrast, pollinator visitation rates, floral density and seed production per pod were all significantly higher in patches located beneath forest canopy gaps compared to patches located beneath closed canopy.

Foliar Herbivory Affects Floral Characters and Plant Attractiveness to Pollinators: Implications for Male and Female Plant Fitness

Foliar Herbivory Affects Floral Characters and Plant Attractiveness to Pollinators: Implications for Male and Female Plant Fitness

Pollinator Foraging Strategies in Mixed Floral Arrays: Density Effects and Floral Constancy

The profitability of any foraging strategy may be in part dependent on the number of foragers adopting it, and so it can be viewed mathematically as a “game.” We explore here the implications of a game-theoretic model of pollinators foraging on a mixed array of two flower species, with individual pollinators either specializing on one or the other, or else acting as generalists. We further allow for the possibility of specialist advantages in foraging efficiency. The predicted mixture of pollinator behavioral strategies depends on the overall floral density and the relative densities of the two flower species. Where floral resources are scarce, pollinators should behave as generalists whereas when resources are superabundant, specialization on the single most profitable flower type (all else being equal, the commonest one) is favored. At intermediate floral densities, a mix of floral specializations (floral constancy) is favored, so long as the rarer flower's density does not fall below some critical minimum level. Where the rarer flower is too rare, a mixture of generalists and common-species specialists is favored. Rare flowers are at a reproductive disadvantage in all cases, but their relative success is highest where their pollinators are flower constant.

Mating system variation in hybridizing irises: Effects of phenology and floral densities on family outcrossing rates

The frequency of outcrossing in two hybridizing species of Iris was estimated for populations and for individual fruits. Effects of floral phenology and the local densities of flowers on outcrossing rates were examined and the potential for hybrid seed formation under different pollen environments was assessed. The populations examined differed with respect to the spatial distribution of plants and the level of genetic structure; the I. fulva population consisted of a number of low density patches and appeared to have some genetic differentiation whereas the I. hexagona population consisted of a single high-density experimental plot of randomly distributed genotypes. Population outcrossing rate estimates were relatively high (0.67–0.90) for both species. The distribution of family outcrossing rates tended to be bimodal for both species with individual fruits either having the majority of seeds fertilized by outcrossed donors or being almost entirely selfed. The frequency of outcrossed fruits increased with the number of flowers open at other plants and decreased when more flowers were open on the same stem in I. hexagona. In I. fulva the opposite trends were apparent; outcrossing decreased when more flowers were open on other stems and increased when more flowers were open on the same stem. The unexpected responses of outcrossing frequency in I. fulva may have been a consequence of higher levels of vegetative reproduction and genetic structure and the behaviour of pollen vectors. Differences in pollen prepotency and the higher selfing rates observed at low floral densities in I. hexagona may have contributed to the observed patterns of hybrid seed formation. The analyses of family outcrossing rates provide important information on factors responsible for mating system variation and evolution.

Correlates of male mating success in two species of Anthidium bees (Hymenoptera : Megachilidae)

Field observations were made on the mating behavior of two congeneric species of solitary bees, Anthidium porterae in an arid grassland and A. palliventre in a coastal sand dune habitat. Males of both species exhibited resource defence polygyny and defended hostplants to gain access to females foraging for nectar and pollen. The mating frequencies of marked and measured resident (territorial) males were monitored during periods of continuous observation, following which measurements of territory size and floral resources were obtained. Mating success of A. palliventre males was strongly influenced by territory characteristics: Males that defended small areas with a few rich hostplant patches mated more often than males that held larger territories containing many hostplant patches of low floral density. Large males generally held high-quality sites and thus had a mating advantage over smaller individuals. In A. porterae, on the other hand, male mating success was unrelated to any measure of territory quality. Copulation frequency and male size were positively correlated, however, apparently due to the increased ability of large males to seize and hold females for mating. The two species also differed in the incidence of non-territorial, “sneaky” males. While absent in A. palliventre, sneaky males accounted for 12% of all mating observed in A. porterae. Males of A. porterae that displayed sneaky tactics mated, on average, as often as resident males.

Individual Variation in Nectar Production and Its Effect on Fitness in Kalmia Latifolia

During summer 1986, we assessed the effect of individual variation in per flower nectar production rate on one component of plant reproductive success (percent fruit set) mediated through differential pollinator attractiveness. Thirty—two individual mountain laurels, Kalmia latifolia, were monitored for average 24—h nectar production in individual flowers, average per flower pollinator visitation rate, average floral density, and percent fruit set. Significant positive correlations occurred between (1) average 24—h nectar production by individual flowers and average visitation rate per flower, and (2) average visitation rate per flower and percent fruit set. Pollinators appeared to be differentially attracted to individual shrubs that produce larger quantities of nectar per flower, and the increase visitation rate promoted a higher percentage fruit set because K. latifolia is pollination limited. However, there was no correlation between an individual shrub's 24—h nectar production in 1986 and the same individual's 24—h nectar production in 1985. Thus, while pollinator behavior appears to influence this component of plant reproductive success in Kalmia, any selective advantage to individuals may vary between flowering seasons.

The inheritance of floral characters in white clover (Trifolium repens)

SummaryA diallel cross was carried out among seven varieties of white clover to investigate the inheritance of some of those characters which affect potential seed yield. Peduncle length and number of seeds/floret have low narrow‐sense heritabilities, while peduncle length displays the added complications of heterozygosity and non‐allelic interactions. Consequently, improvements in these characters will be difficult. Seed number m‐2 is a composite character which apparently exhibits heterozygosity. Higher seed yields will therefore be obtained by selection for its component characters, particularly floret number and floral density, both of which are apparently inherited in a comparatively straightforward manner. These results are discussed in the context of improving the potential seed yield of white clover.

Interactions for Pollination among Coflowering Shrubs

To examine competition for pollination in a natural community, I measured pollinator sharing, pollinator dependence, and limitation of fruit set for four colflowering shrub species in The Great Swamp, Rhode Island. These species included all potential competitors for pollination and have had the long, extensive associations deemed necessary for competitive coevolution. The four shrubs shared the major pollinators (bees), and three of the four species depended upon pollinators for fruit set; but pollination limitation due to competition was detected in only one species. Despite the lack of competition, the shrubs interacted negatively and stronly for bumble bees, which were major pollinators. The species differentially attracted bumble bees, and their relative attractive (competitive) abilities were predicted by an index based on average daily sucrose production per flower x local floral density as: Ilex opaca > Kalmia latifolia > Gaylussacia frondosa > Kalmia angustifolia. Bumble bees rarely visited the less attractive species during flowering overlap in 3 of 4 yr. Competitive avoidance was achieved, not by specialization and divergence as is commonly hypothesized, but by having traits that confer tolerance of rare pollinator visits (e.g., long—lived flowers or self—pollination) or by having diverse pollinator assemblages (small bees and bumble bees). K. latifolia was the only shrub specialized for pollination (by bumble bees), and this specialization increased, rather than decreased, the risk of pollination limitation because K. latifolia was a poorer competitor for bumble bees than Ilex. Pollination limitation was prevented or reduced by the extended longevity of unpolinated flowers. Flowers could remain viable > 2 wk until Ilex ceased flowering and bumble bees switched to K. latifolia K. angustifolia, the poorest competitor, was visited by many species of small bees and could also self—pollinate, so fruit set was maintained even in the absence of bumble bees, Gaylussacia lacked these traits, and it was pollination limited: flowers are short—lived and cannot self—pollinate. For this species small bees were important in maintaining fruit set when bumble bees switched to more attractive species. These traits for competitive avoidance are advantageous when pollination is uncertain for any reason, such as unfavorable weather for pollinator foraging. As a result, their presence cannot be unambiguously attributed to past competition, although they currently are important in reducing competition among these shrubs. Such traits as generalization for resource use of tolerance of sparse resources may be as important as specialization or greater competitive ability for the coexistence of species in competitive communities.

Mating patterns in self-compatible distylous populations of Amsinckia (Boraginaceae)

Progeny tests of naturally pollinated pin- and thrum-form plants of two species of Amsinckia were conducted to determine whether net selfing, random mating, or net disassortative mating occurred in natural populations. The locus determining flower form was used as a marker gene. Amsinckia spectabilis experienced close to random mating in a dense population, but in a small, diffuse population a high rate of selfing was indicated. Pin plants of Amsinckia vernicosa var. furcata experienced 44.3% net disassortative mating, and thrum plants in this species experienced complete (100%) disassortative mating.Compared with A. spectabilis, stigmas and anthers are about twice as far apart in A. vernicosa var. furcata; yet the pollinators are essentially similar, suggesting that greater separation of stigmas and anthers greatly improves the efficiency of distyly at promoting disassortative pollination. The two populations of A. spectabilis studied indicate that autogamy and (or) geitonogamy is much higher in diffuse populations. The results support previous studies that disassortative pollination is greater in thrum-form flowers than in pin flowers. Distyly without self-incompatibility can be an effective outbreeding system but is much more sensitive to variations in floral structure, population density, and pollinator behavior than are self-incompatible breeding systems.