Demographic mechanisms in the coexistence of two closely related perennials in a fluctuating environment

The demographic variability and life history differentiation of two closely related shrubs (Atriplex canescens and A. acanthocarpa) were investigated in the Chihuahuan Desert, and the results were interpreted in the context of theories of coexistence in fluctuating environments. Demographic information was recorded during three annual intervals and analyzed employing matrix projection models. A. canescens had lower λ (finite rate of population increase), higher longevity and generation time and slower convergence to a stable population structure than A. acanthocarpa. In favorable years for recruitment (those when, for both species, λ > 1), the λ of A. acanthocarpa was higher than that of A. canescens; in unfavorable years (when λ < 1), the reverse was true. Regardless of conditions (year), A. acanthocarpa had a type 2 survivorship curve (constant rate of mortality with age), while A. canescens had a type 3 survivorship curve (declining mortality with age). Elasticity analyses highlighted the larger influence that fecundity and growth would have in modifying the λ of A. acanthocarpa in comparison to that of A. canescens. In contrast, survival would have a larger influence on the λ of A. canescens. Atriplex acanthocarpa behaved as an opportunistic species that benefitted from sporadic favorable conditions and declined rapidly when conditions deteriorated. In contrast, A. canescens behaved as a tolerant species able to withstand years when conditions were poor, but which could not gain any advantage over A. acanthocarpa when conditions improved. By each having a relative advantage over the other on opposite ends of the contrasting climatic conditions experienced in the Chihuahuan Desert, they are able to coexist. Their contrasting life histories agreed with the theoretical predictions for the operation of the two mechanisms of species coexistence in fluctuating environments: the storage effect and the relative non-linearity of competition. Based on these results, we conclude by speculating on the nature of succession in arid communities.     

Longevity,recruitment and mortality of desert plants in Grand Canyon,Arizona, USA

Abstract. The demography of woody desert plants along the Colorado River in Grand Canyon, Arizona, USA, was analyzed using 355 pairs of replicated photographs taken as long ago as 1872. Longevity, recruitment, and mortality were determined for 38 species characteristic of ungrazed desert scrub. Individual plants that survived 100 yr or more included Acacia greggii, Ambrosia dumosa, Atriplex canescens, A. confertifolia, Echinocactus polycephalus, Ephedra spp., Fouquieria splendens, Larrea tridentata, Lycium andersonii, Opuntia acanthocarpa, O. basilaris, O. erinacea, Pleuraphis rigida, and Yucca angustissima. This is the first evidence of long lifespan for most of these species, particularly the succulents. Most of the long-lived species registered overall increases in population during the past century. Only four species with lifespans ≥ 100 yr had a net loss of individuals between 1889 and the present, and only two decreased between 1923 and the present. It seems likely that climatic fluctuations over the past century are largely responsible for these recruitment and mortality patterns; however, nurse plants, predation refuges and other biotic factors may also play a role.     

Population Status and Reproductive Success of an Endangered Epiphytic Orchid in a Fragmented Landscape

Habitat fragmentation and disturbance are two of the most significant drivers of species extinctions in plant populations. The degree of impact of fragmentation on plant populations depends on the level of specificity of plant–animal interactions, as well as on the availability of suitable sites for seedling recruitment. In this study, we describe the population density and structure, pollen limitation and reproductive success of the endangered tropical orchid Myrmecophila christinae, an epiphytic species with a specialized pollination system. We surveyed a total of 14 populations located in a fragmented landscape. Seedling density was related to habitat disturbance and host plant density; while density of juveniles was related to density of adults. Adult and total individual densities were related to habitat affectation. We also found that fragments <1 ha had significantly fewer seedlings, as well as an over‐representation of large adults. On the other hand, fruit production was higher in fragments >10 ha, and fruit set was significantly lower in highly disturbed fragments. Hand pollination experiments showed that M. christinae was pollen limited in all the studied populations, suggesting that pollen limitation is unrelated to habitat disturbance. Overall, our results suggest that fragmentation has affected key demographic features of M. christinae, including reproduction and recruitment.     

Initiating Autogenic Restoration on Shallow Semiarid Sites

Our objectives were to evaluate the use of microcatchments in the establishment of Leucaena retusa (little-leaf leadtree) and Atriplex canescens (four-wing saltbush) and their role in the initiation of autogenic landscape restoration processes on a shallow semiarid site. Three six-month-old seedlings of either Leucaena retusa or Atriplex canescens were planted in 1.5-m² microcatchments. An equal number of seedlings was planted in control plots (unmodified soil surface). The water collection effects on shrub survival, standing biomass, and the natural immigration of herbaceous vegetation were determined over 42 months. Planting in microcatchment basins doubled Leucaena seedling survival and resulted in a five-fold increase in standing biomass, compared to the control, during the first growing season. There was a significant increase in soil organic matter in the microcatchment basins within 32 months. At the same time, microcatchments planted with Atriplex canescens seedlings had a ten-fold increase in seedling standing biomass compared to the control. Forty-two months after transplanting, the herbaceous standing crop was significantly greater near Atriplex canescens or in microcatchment basins than in plots with unmodified surface soil. Basins containing Atriplex produced significantly more herbaceous vegetation than basins containing Leucaena, and empty basins produced the least herbaceous vegetation of three basin treatments. These data suggest that landscape-scale procedures that concentrate scarce resources (water, organic matter, nutrients, and propagules), establish keystone species, and ameliorate microenvironmental conditions can initiate autogenic restoration of degraded semiarid ecosystems.     

Pollen source and resource limitation to fruit production in the rare species Eremosparton songoricum (Fabaceae)

Eremosparton songoricum (Litv.) Vass. is a rare, central Asian desert species which shows lower fruit set and seed set (<16%) than most hermaphroditic species. We hypothesized that fruit production was limited by pollen and resources. To evaluate potential fruit abortion due to pollen limitation, supplemental hand‐pollination was undertaken, the mating system was investigated and the foraging behavior of pollinators was recorded. To investigate possibile resource limitation, flowers and young pods were artificially removed and fertilization were manipulated. The results showed that under natural pollination, the number of pollen deposited on the stigma greatly exceed the number of ovules per ovary. Mating system experiments showed that the species is self‐compatible, but depended on pollinators to set seeds. Supplemental outcross pollination increased fruit set significantly. The most frequent effective pollinator Megachile terminate Morawitz, was observed pollinating many flowers of the same individual plant (74.5±1.3%). These results suggested that fruit production is affected by insufficient outcross pollen rather than by pollen quantity. Removal of 2/3 of the flowers and young pods led to significantly higher fruit set, as did addition of fertilizers (N–P–K: 0.025–0.05–0.013 g, N–P–K: 0.05–0.1–0.025 g) showing that reducing resource acceptors and increasing inorganic resources both helps to improve fruit set. We therefore conclude that reproductive success of E. songoricum is limited by both outcross pollen and available nutrients.     

Halo-nitrophilous scrub species and their relationship to the atmospheric concentration of allergenic pollen: case study of the Mediterranean saltbush (Atriplex halimus L., Amaranthaceae)

Halo-nitrophilous scrubs are characterised by their floristic richness in species of the family Amaranthaceae (include Chenopodiaceae) and the Mediterranean saltbush (Atriplex halimus L.) is one of the most characteristic species in the Mediterranean region. Pollen from Amaranthaceae is the main cause of pollinosis at the end of summer and autumn. In this study, the floral phenology of the species Atriplex halimus L., was studied relating it to the atmospheric concentration of Amaranthaceae pollen with the aim to know if it can serve as an indicator of the maximum pollen concentrations. Observations of the male floral phenology of Atriplex halimus were performed over the course of three years in the central Iberian Peninsula (Spain) and the aerobiological pollen data of Amaranthaceae were obtained using a Hirst-type volumetric trap. The results demonstrated that the flowering period of Atriplex halimus closely coincided with the peak pollen levels. Besides, the prevailing movements of air masses in relation to the distribution and abundance of the halo-nitrophilous scrub during the flowering period of Atriplex halimus were studied using a back-trajectory analysis. The results showed that distinct predominant wind patterns led to differences in the quantity of pollen recorded during the pollen season and in the behaviour of the evolution of airborne pollen concentrations.     

The effects of nutrient addition on floral characters and pollination in two subalpine plants, <Emphasis Type="Italic">Ipomopsis aggregata</Emphasis> and <Emphasis Type="Italic">Linum lewisii</Emphasis>

The availability of soil and pollination resources are main determinants of fitness in many flowering plants, but the degree to which each is limiting and how they interact to affect plant fitness is unknown for many species. We performed resource (water and nutrients) and pollination (open and supplemental) treatments on two species of flowering plants, Ipomopsis aggregata and Linum lewisii, that differed in life-history, and we measured how resource addition affected floral characters, pollination, and reproduction (both male and female function). We separated the direct effects of resources versus indirect effects on female function via changes in pollination using a factorial experiment and path analysis. Resource addition affected I. aggregata and L. lewisii differently. Ipomopsis aggregata, a monocarp, responded to fertilization in the year of treatment application, increasing flower production, bloom duration, corolla width, nectar production, aboveground biomass, and pollen receipt relative to control plants. Fertilization also increased total seed production per plant, and hand-pollination increased seeds per fruit in I. aggregata, indicating some degree of pollen limitation of seed production. In contrast, fertilization had no effect on growth or reproductive output in the year of treatment on L. lewisii, a perennial, except that fertilization lengthened bloom duration. However, delayed effects of fertilization were seen in the year following treatment, with fertilized plants having greater aboveground biomass, seeds per fruit, and seeds per plant than control plants. In both species, there were no effects of resource addition on male function, and the direct effects of fertilization on female function were relatively stronger than the indirect effects via changes in pollination. Although we studied only two plant species, our results suggest that life-history traits may play an important role in determining the reproductive responses of plants to soil nutrient and pollen additions.     

Influence of reproductive traits on pollination success in two <Emphasis Type="Italic">Daphne</Emphasis> species (Thymelaeaceae)

Taxonomically related species can differ in a number of reproductive traits, which may translate into a differential mating system and pollination success. Here we compare two hermaphroditic insect-pollinated Daphne species (D. rodriguezii and D. gnidium) which differ in distribution (island endemic vs. mediterranean) and floral traits (long- vs. short-tube corolla). We investigated their mating system and pollen limitation by means of hand-pollination experiments and quantified the diversity and abundance of flower visitors by direct observations. Plant size and five reproductive traits (flower production, proportion of viable anthers, pollen production, flower tube length and tepal area) were studied to assess how they contribute to reproductive success, measured as proportion of pollen grains germinated per stigma and fruit set. Selfing was very low and pollen limitation existed in both species, though was higher in D. rodriguezii probably due to the scarcity of flower visitors. The low fruit set in both species suggests that most of the pollen grains found on stigmas are self-pollen. Pollinators appeared to favour some floral traits (specifically, flower tube length or tepal area) in both species, although flower crop in D. rodriguezii was the only reproductive trait influencing fruit set. In both species, the highest variability in reproductive traits and pollination success was within individuals. Our findings suggest that despite both species showed similar mating system, dependency on outcrossing pollen and selection of floral traits, pollen limitation was higher in D. rodriguezii, probably as a higher proportion of self-pollen arrives to its stigmas.     

Experimental evidence for stronger cacao yield limitation by pollination than by plant resources

Both pollination and resource limitation may cause low fruit:flower ratios in plants, but pollen and resource limitation have never been contrasted in commercially important crop species. Here we experimentally investigated the relative effect of pollen limitation and resource limitation in Theobroma cacao. In Central Sulawesi, Indonesia, we applied different relative levels of hand pollination (10%, 40%, 70% and 100% of available flowers up to 2 m height) to mature cacao trees in two separate experiments encompassing (1) different light (shade roofs) and nitrogen (fertilizer application) treatments, and (2) water availability (throughfall displacement) treatments. None of the resource availability treatments had a significant effect, while number of mature pods and yield increased non-linearly with pollination intensity up to 200% of current yield levels. The largest benefits were reached by increasing pollination from 10% to 40%, with non-significant increases beyond that level. Despite an increase of fruit abortion with pollination intensity, T. cacao yield is determined, at least on the short term, by the number of flowers pollinated. This suggests pollination deficit in crops can be very large and that a better knowledge of pollen and resource limitation to devise adequate pollinator management strategies may be critical for increasing production.     

No evidence for simultaneous pollen and resource limitation in Aciphylla squarrosa: A long‐lived,masting herb

Abstract For successful reproduction animal pollinated plants must provide resources for both pollinator attraction and offspring production, and theory suggests that resources and pollen delivery limit reproduction simultaneously. We conducted a series of experiments involving supplemental pollination, flower removal, fertilizer addition and foliage removal to investigate the interaction of resources and pollen on fruit‐set of Aciphylla squarrosa, a long‐lived, dioecious, masting herb in Wellington, New Zealand. Reducing floral display decreased open‐pollinated fruit‐set, suggesting that display size is a reflection of an optimal investment between attraction and fecundity. In combination with supplemental pollination, resource reduction and fertilization addition did not alter fruit‐set, suggesting that changes in resource availability did not limit reproduction in the current year. In addition, supplemental pollination of non‐manipulated treatments did not increase fruit‐set, demonstrating that plants were not naturally pollen limited. While we found that simultaneous pollen and resource limitation did not occur within a season, this is possibly mitigated by life history patterns including mast flowering and a storage taproot. Multiple year studies are required to further examine simultaneous resource and pollen limitation.     

Extremely high proportions of male flowers and geographic variation in floral ratios within male figs of Ficus tikoua despite pollinators displaying active pollen collection

Most plants are pollinated passively, but active pollination has evolved among insects that depend on ovule fertilization for larval development. Anther‐to‐ovule ratios (A/O ratios, a coarse indicator of pollen‐to‐ovule ratios) are strong indicators of pollination mode in fig trees and are consistent within most species. However, unusually high values and high variation of A/O ratios (0.096–10.0) were detected among male plants from 41 natural populations of Ficus tikoua in China. Higher proportions of male (staminate) flowers were associated with a change in their distribution within the figs, from circum‐ostiolar to scattered. Plants bearing figs with ostiolar or scattered male flowers were geographically separated, with scattered male flowers found mainly on the Yungui Plateau in the southwest of our sample area. The A/O ratios of most F. tikoua figs were indicative of passive pollination, but its Ceratosolen fig wasp pollinator actively loads pollen into its pollen pockets. Additional pollen was also carried on their body surface and pollinators emerging from scattered‐flower figs had more surface pollen. Large amounts of pollen grains on the insects' body surface are usually indicative of a passive pollinator. This is the first recorded case of an actively pollinated Ficus species producing large amounts of pollen. Overall high A/O ratios, particularly in some populations, in combination with actively pollinating pollinators, may reflect a response by the plant to insufficient quantities of pollen transported in the wasps’ pollen pockets, together with geographic variation in this pollen limitation. This suggests an unstable scenario that could lead to eventual loss of wasp active pollination behavior.     

Reproductive biology of Pachira aquatica Aubl. (Malvaceae: Bombacoideae): a tropical tree pollinated by bats,sphingid moths and honey bees

We investigated the reproductive biology, including the floral biology, pollination biology, breeding system and reproductive success, of Pachira aquatica, a native and dominant tropical tree of fresh water wetlands, throughout the coastal plain of the Gulf of Mexico. The flowers present nocturnal anthesis, copious nectar production and sugar concentration (range 18–23%) suitable for nocturnal visitors such as bats and sphingid moths. The main nocturnal visitors were bats and sphingid moths while bees were the main diurnal visitors. There were no differences in legitimate visitation rates among bats, moths and honey bees. Bats and honey bees fed mainly on pollen while moths fed on nectar, suggesting resource partitioning. Eight species of bats carried pollen but Leptonycteris yerbabuenae is probably the most effective pollinator due to its higher pollen loads. The sphingid moths Manduca rustica, Cocytius duponchel and Eumorpha satellitia were recorded visiting flowers. Hand pollination experiments indicated a predominant outcrossing breeding system. Open pollination experiments resulted in a null fruit set, indicating pollen limitation; however, mean reproductive success, according to a seasonal census, was 17 ± 3%; these contrasting results could be explained by the seasonal availability of pollinators. We conclude that P. aquatica is an outcrossing species with a pollination system originally specialized for bats and sphingid moths, which could be driven to a multimodal pollination system due to the introduction of honey bees to tropical America.     

Breeding system and bird pollination of Camellia pubipetala,a narrowly endemic plant from karst regions of south China

Camellia pubipetala is an endemic and endangered species with small and isolated populations occurring only in karst regions in Guangxi of south China. To understand the reproductive biology of C. pubipetala and its possible influences upon its endangered status, its breeding system and pollination ecology were studied in the Longhushan (LHS) and Longzhao (LZ) populations of this species. The flowering duration of the C. pubipetala populations spanned from late January to early April and anthesis of a single flower usually lasted 5–7 days. This species is homogamous, and the pollen and stigma are viable throughout anthesis. Each bagged flower could secrete 141.5 μL of nectar at a sugar concentration of 25.0% during anthesis. The observed high pollen/ovule ratio, and the results of hand-pollination experiments indicated that this species obligately outcrosses. Open pollination resulted in a significantly decreased fruit set (6.7%) and seed set (38.9%) compared to supplementary pollination treatment (23.3% and 64.7%, respectively), which is indicative of a pollen limitation in the process of pollination. The primary pollinator of C. pubipetala is the sunbird Aethopyga christinae and its visiting frequency is quite low, whereas the honeybee Apis cerana is only an occasional pollinator in wild populations. Low reproductive rates in C. pubipetala were found to be a consequence of few species of pollinators and their low visiting frequency. Pollen limitation may be a crucial factor that contributes to the endangered nature of this species. Artificial pollination and the release of pollinators are effective ways to increase the fruit and seed yield of this species.     

Pollen limitation,fruit abortion,and autonomous selfing in three populations of the perennial herb Ruellia nudiflora

Multiple factors determine plant reproductive success and their influence may vary spatially. This study addresses several factors influencing female reproductive success in three populations of Ruellia nudiflora, specifically we: (i) determine if fruit set is pollen‐limited and if pollinator visitation rates are related to this condition; (ii) estimate fruit set via autonomous self‐pollination (AS) and relate it to the magnitude of herkogamy; and (iii) evaluate if fruit abortion is a post‐pollination mechanism that determines the magnitude of pollen limitation. At each site we marked 35 plants, grouped as: unmanipulated control (C) plants subjected to open pollination, plants manually cross‐pollinated (MP), and plants excluded from pollinators and only able to self‐pollinate autonomously (AS). Fruit set was greater for MP relative to C plants providing evidence for pollen limitation, while a tendency was observed for lower fruit abortion of MP relative to C plants suggesting that fruit set is influenced not only by pollen delivery per se, but also by subsequent abortion. In addition, although pollinator visits varied significantly among populations, the magnitude of pollen limitation did not, suggesting that pollinator activity was not relevant in determining pollen limitation. Finally, fruit set tended to decrease with the degree of herkogamy for AS plants, but this result was inconclusive. These findings have contributed to identify which factors influence reproductive success in populations of R. nudiflora, with potentially relevant implications for population genetic structure and mating system evolution of this species.     

Effects of salinity on growth,membrane permeability and root hydraulic conductivity in three saltbush species

The species of the genus Atriplex have been introduced in West Asia and North Africa to determine their adaptability for use as fodder species. These halophytes are well adapted to extreme environmental conditions and may possess interesting properties for soil rehabilitation. The effect of NaCl stress on growth, water relation and mineral nutrition were investigated in three xero-halophyte species of Atriplex used for rehabilitation of arid steppe in Algeria. Atriplex halimus, Atriplex canescens and Atriplex nummularia, were cultivated in hydroponic conditions and treated with increasing doses of NaCl (0–300 mM). All species showed positive plant growth for low and moderate levels of salinity. A. halimus had higher dry weight production than A. nummularia and A. canescens in high salinity concentration. Increasing concentration of salinity induced decrease in chlorophyll content (Chl a and b) and root hydraulic conductivity (L₀) in all species, especially in A. canescens. All three species showed marked increase in electrolyte leakage across the salinity gradient. In addition all species were able to accumulate a large quantity of sodium (Na), chloride (Cl) and proline and to maintain higher relative water content, which was probably associated with a greater capacity for osmotic adjustment, whereas potassium (K) and calcium (Ca) decreased with increase salinity. The data suggest that salt tolerance strategies in all Atriplex species could involve a delicate balance among ion accumulation, osmotic adjustment, production of osmotica and maintenance of relative water content and growth.     

Erosion of a pollination mutualism along an environmental gradient in a south Andean treelet,Embothrium coccineum (Proteaceae)

The limit of a species’ distribution can be determined biotically if an environmental gradient causes the loss of critical mutualists such as pollinators. We assessed this hypothesis for Embothrium coccineum, a self‐incompatible red‐flowered treelet growing along a strong west‐east precipitation gradient from rainforest to forest‐steppe ecotone in the rain shadow of the southern Andes in northwestern Patagonia. For 16 populations along this gradient, we quantified composition of the pollinator assemblage, pollination efficiency and limitation, and reproductive output. The treelet has a generalized pollination system, but the hummingbird Sephanoides sephaniodes was the most effective pollinator. The relative importance of this hummingbird as a flower visitor within populations influenced pollen transfer and fruit set more strongly than local precipitation. As hummingbirds and other pollinators, including passerine birds and nemestrinid flies, were replaced by bees towards the dry eastern range limit, pollen limitation increased and reproduction eventually failed. These results support the hypothesis that pollinators can act as important biotic filters influencing plant distribution, and warn against predictions of geographical range shifts based solely on climatic variables.     

Autonomous self‐pollination in the nectarless orchid Pogonia minor

The pollination biology of the nectarless orchid Pogonia minor was investigated in central Japan. The investigation revealed that the solitary flowers failed to attract pollinators, while high rates of fruit set were observed in the natural population. Comparable levels of fruit set were obtained in bagged, artificial self‐pollinated and artificial cross‐pollinated plants, indicating that the species is not pollinator‐limited for fruit set under natural conditions. Autonomous self‐pollination in P. minor resulted from a reduced rostellum, which allowed contact between the pollinia and the stigma. Self‐pollination is thought to be an adaptive response that provides reproductive assurance under conditions of pollinator limitation. Since pollen limitation is generally known to be frequent among deceptive orchids, strong pollen limitation is probably a driving force in the autonomous self‐pollination mechanism in the nectarless orchid P. minor.     

Reproductive Consequences of Clonal Growth in Stenocereus eruca, a Rare Clonal Cactus of the Sonoran Desert

Stenocereus eruca is a prostrated, self-incompatible cactus endemic to the Sonoran Desert that regenerates primarily through clonal propagation. Clonal growth is expected to affect mate availability by influencing the number and spatial distribution of mating types. In this paper we examine the role of clonal growth on female fecundity through a series of pollination experiments in a population of S. eruca. We set up a pollen supplementation experiment using five distance treatments with pollen collected at 1, 10, 100, 1000 and 25000 m from receptor flowers during the years 2001 and 2002 and evaluated genetic sifmilarities between pairs of receptor-donor ramets through RAPD markers. Our data on fruit set, number of seeds/fruit, germination and overall fecundity revealed that S. eruca show a significant reduction in female fecundity when pollination occurs between ramets located at short distances (1 and 10 m), while genetic data showed high levels of similarity at those distances. The reduction in female fecundity is apparently a consequence of geitonogamy and inbreeding depression. Our data suggest that clonal growth and geitonogamy are likely to be partially responsible for the low levels of sexual reproduction and seedling recruitment observed in populations of S. eruca. Co-ordinating editor: H. Kudoh     

Wind pollination and insect pollination of two temperate willow species,Salix miyabeana and Salix sachalinensis

Pollination systems of Salix miyabeana and Salix sachalinensis were studied at a riverside in northern Japan in order to measure the balance of wind pollination and insect pollination. In 1996, 19 clones of each species were selected, and seed-set success by a net-bagging (to exclude insect visitation) and an artificial pollination (to remove pollen limitation) were compared to by natural pollination. For S. miyabeana, the same experiment was repeated in two populations in 1997. Proportion of seed set through wind pollination dominated in both species in this study. Pollen limitation was common under natural conditions, and S. sachalinensis relied more on insect pollination for seed production than S. miyabeana. Meteorological factors such as precipitation and hours of sunshine during the flowering season influenced the potential reproductive activity of the willow between years. In the wet and cloudy spring of 1996, clones which obtained high seed set depended more on insect pollination for both species, whereas in the dry and sunny spring of 1997, such clones depended more on wind pollination for S. miyabeana. Because the efficiency of wind pollination seemed to be more sensitive to fluctuating weather conditions than insect pollination, insect pollination was considered to play an assurance role for seed production in these willows.     

Pollen Flow,Fecundity, and the Adaptive Significance of Heterostyly in Palicourea padifolia (Rubiaceae)1

Palicourea padifolia is a distlous shrub visited primarily by hummingbirds and bees in mid-elevation rain forests in Costa Rica. At Las Cruces Biological Station, the population of P. padifolia is composed of equal numbers of pin and thrum plants and morphs are randomly distributed. Like the majority of distylous species, P. padifolia exhibits a self and intramorph-incompatible mating system. The analysis of stigmatic pollen loads obtained from emasculated flowers shows that pollen flow between floral morphs is disassortative, which supports the Darwinian hypothesis concerning the adaptive significance of heterostyly. Pins experience greater disassortative pollination than thrums. Pins also set more seed than thrums, suggesting that the higher frequency of compatible pollen deposited on pin stigmas results in greater fecundity; alternatively, greater seed set in pins could be due to the differential allocation of resources to male and female function between morphs. Overall, seed set for the Las Cruces popularion of P. padifolia is much lower than the potential maximum. Factors which could contribute to seed set limitation include the frequency and/or spatial pattern of pollinator visits, pollen availability, and resource availability.