Timing of canopy closure influences carbon translocation and seed production of an understorey herb, Trillium apetalon (Trilliaceae)

BACKGROUND AND AIMS: The light availability on a temperate, deciduous-forest floor varies greatly, reflecting the seasonal leaf dynamics of the canopy trees. The growth and/or reproductive activity of understorey plants should be influenced by the length of the high-irradiance period from snowmelt to canopy closure. The aim of the present study was to clarify how spring-blooming species regulate the translocation of photosynthetic products to current reproduction and storage organs during a growing season in accordance with the changing light conditions. METHODS: Growth pattern, net photosynthetic rate, seed production, and shoot and flower production in the next year of Trillium apetalon were compared between natural and experimentally shaded conditions. Furthermore, translocation of current photosynthetic products within plants was assessed by a labelled carbon-chase experiment. KEY RESULTS: During the high-irradiance period, plants showed high photosynthetic ability, in which current products were initially used for shoot growth, then reserved in the rhizome. Carbon translocation to developing fruit occurred after canopy closure, but this was very small due to low photosynthetic rates under the darker conditions. The shading treatment in the early season advanced the time of carbon translocation to fruit, but reduced seed production in the current year and flower production of the next year. CONCLUSIONS: Carbon translocation to the storage organ had priority over seed production under high-irradiance conditions. A shortened bright period due to early canopy closure effectively restricts carbon assimilation, which greatly reduces subsequent reproductive output owing to low photosynthetic products for fruit development and small carbon storage for future reproduction. As populations of this species are maintained by seedling recruitment, acceleration of canopy closure timing may influence the maintenance and dynamics of populations.     

Tolerance of a perennial herb, <Emphasis Type="Italic">Pimpinella saxifraga</Emphasis>, to simulated flower herbivory and grazing: immediate repair of injury or postponed reproduction?

Perennial, polycarpic herbs can respond to herbivory either by (1) regrowth in the same season in order to compensate for lost reproductive structures or by (2) postponing reproduction until the following growing season. We tested these response patterns with the perennial umbellifer Pimpinella saxifraga by simulating flower herbivory and shoot grazing both in the field and in a common garden experiment. In the field, both simulated flower herbivory and grazing effectively suppressed current reproduction, whereas no statistically significant effects of previous-year treatments on growth or reproduction were found in the following year. In the common garden, in the first year the species fully compensated for simulated flower herbivory in vegetative parameters but seed set was reduced by 26%. After 2 years of flower removal, the plants overcompensated in shoot and root biomass by 47 and 46%, respectively, and compensated fully in reproductive performance. Simulated grazing resulted in 21% lower shoot biomass in the first season, but the root biomass was not affected. In the second season the root biomass increased by 43% as compared to the control plants. However, regrowth following simulated grazing resulted in a significant delay in flowering with the consequence that the seed yield of fertile plants was reduced by 55% as compared to the control plants. These results suggest that in resource-rich garden conditions P. saxifraga may immediately repair injuries caused by flower herbivory, but repairs more extensive shoot injury less successfully. Delayed phenology decreases the benefits of immediate repair. In resource-poor conditions, the benefits of regrowth can be negligible. Accordingly, in our field population, the plants postponed their reproduction until the following year in response to simulated grazing and frequently in response to flower removal. When the plants gain very little from regrowth, the costs of reproduction would select for postponed reproduction in response to injury.     

Delayed timing of breeding as a cost of reproduction

Timing of breeding is a trait with considerable individual variation, often closely linked to fitness because of seasonal declines in reproduction. The drivers of this variation have received much attention, but how reproductive costs may influence the timing of subsequent breeding has been largely unexplored. We examined a population of northern wheatears Oenanthe oenanthe to compare three groups of individuals that differed in their timing of breeding termination and reproductive effort to investigate how these factors may carry over to influence reproductive timing and reproductive output in the following season. Compared to females that bred successfully, females that put in less effort and terminated breeding early due to nest failure tended to arrive and breed earlier in year 2 (mean advancement = 2.2 and 3.3 d respectively). Females that spent potentially more effort and terminated breeding later due to production of a replacement clutch after nest failure, arrived later than other females in year 2. Reproductive output (number of fledglings) in year 2 differed between the three groups as a result of group‐level differences in the timing of breeding in combination with the general seasonal decline in reproductive output. Our study shows that the main cost of reproduction was apparent in the timing of arrival and breeding in this migratory species. Hence, reproductive costs can arise through altered timing of breeding since future reproductive success (including adult survival) is often dependent on the timing of breeding in seasonal systems.     

Among-population variation in costs of reproduction in the long-lived orchid Gymnadenia conopsea: an experimental study

A cost of reproduction in terms of reduced future performance underlies all life-history models, yet costs have been difficult to detect in short-term experiments with long-lived plants. The likelihood of detecting costs should depend on the range of variation in reproductive effort that can be induced, and also on the shape of the cost function across this range, which should be affected by resource availability. Here, we experimentally examined the effects of both reduced and increased fruit production in two populations of the long-lived orchid Gymnadenia conopsea located at sites that differ in length of the growing season. Plants that were prevented from fruiting produced more flowers in the population with a longer growing season, had higher survival in the other population, and grew larger compared to control plants in both populations. Fruit production was pollen-limited in both populations, and increased reproductive investment after supplemental hand-pollination was associated with reduced fecundity the following year. The results demonstrate that the shape of the cost function varies among fitness components, and that costs can be differentially expressed in different populations. They are consistent with the hypothesis that differences in temporal overlap between allocation to reproduction and other functions will induce among-population variation in reproductive costs.     

Size-dependent sex allocation in a monocarpic perennial herb, <Emphasis Type="Italic">Cardiocrinum cordatum</Emphasis> (Liliaceae)

The size-dependent sex allocation model predicts that the relative resource allocation to female function often increases with plant size in animal-pollinated plants. If size effects on reproductive success vary depending on the environmental conditions, however, the size dependency may differ among populations. We tried to detect site-specific variation in size-dependent sex allocation of a monocarpic hermaphrodite with reference to light availability. Multiple flowers and fruits were sampled from the individuals of Cardiocrinum cordatum, a monocarpic understory herb, and pollen, ovule and seed production were measured with reference to the plant size in two populations. Furthermore, frequency and foraging behavior of pollinator visitation was observed. Ovule production per flower increased with plant size in both populations, while pollen production per flower increased with size only in the population under sparse canopy. Therefore, proportional allocation to male function decreased with plant size in the population under closed canopy, but did not change in the population under sparse canopy. Pollinators usually visited only one flower per plant, indicating the negligible geitonogamous pollination in this species. Although seed production under closed canopy was lower than that under sparse canopy, seed-set rate per flower and seed mass per fruit were independent of plant size in either of the populations. Size-dependent sex allocation in this species was site-specific, suggesting that not only resource storage before reproduction (i.e., plant size) but also resource availability of environment throughout the reproductive process (i.e., light availability) affect reproductive performance in this species.     

Additive and non-additive effects of simulated leaf and inflorescence damage on survival, growth and reproduction of the perennial herb Arabidopsis lyrata

Herbivores may damage both leaves and reproductive structures, and although such combined damage may affect plant fitness non-additively, this has received little attention. We conducted a 2-year field experiment with a factorial design to examine the effects of simulated leaf (0, 12.5, 25, or 50% of leaf area removed) and inflorescence damage (0 vs. 50% of inflorescences removed) on survival, growth and reproduction in the perennial herb Arabidopsis lyrata. Leaf and inflorescence damage negatively and independently reduced flower, fruit and seed production in the year of damage; leaf damage also reduced rosette size by the end of the first season and flower production in the second year. Leaf damage alone reduced the proportion of flowers forming a fruit and fruit production per plant the second year, but when combined with inflorescence damage no such effect was observed (significant leaf × inflorescence damage interaction). Damage to leaves (sources) caused a greater reduction in future reproduction than did simultaneous damage to leaves and inflorescences (sinks). This demonstrates that a full understanding of the effects of herbivore damage on plant fitness requires that consequences of damage to vegetative and reproductive structures are evaluated over more than 1 year and that non-additive effects are considered.     

Frost damage and winter nitrogen uptake by the grass Poa pratensis L.: consequences for vegetative versus reproductive growth

Frost damage can decrease nitrogen uptake by grasses over winter, and it can also decrease biomass production over the following growing season. However, it is not clear to what extent reduced nitrogen uptake over winter decreases grass production, or whether is it merely a symptom of root damage. We examined the growth response of the grass Poa pratensis L. (Kentucky bluegrass) to variation in the timing of freezing and nitrogen availability over winter in London, Ontario, Canada. All tillers were transplanted into untreated soil in early spring, and at peak seed maturation, root, shoot, and reproductive biomass were measured. There was an interaction between freezing and increased winter nitrogen availability, whereby nitrogen addition increased tiller biomass under ambient temperatures, but decreased tiller biomass in combination with a late winter freeze. The nitrogen response of ambient temperature tillers occurred primarily via increased seed production, whereas for frozen tillers seed production was generally absent. Our results support the hypothesis that nitrogen uptake over winter can increase growing season productivity in P. pratensis, but also demonstrate that increased nitrogen availability increases tiller vulnerability to frost. These results have important implications for grass responses to the alteration of soil freezing dynamics with climate change.     

Overcompensation in response to simulated herbivory in the perennial herb <Emphasis Type="Italic">Sedum maximum</Emphasis>

A positive effect of herbivory on plant reproduction (overcompensation) has been documented mostly in monocarpic plants. Iteroparous perennials can be used to test whether enhanced reproduction in 1 year has negative future consequences as predicted by optimal allocation models. This study was intended to verify this prediction in the iteroparous herb Sedum maximum, applying mechanically simulated herbivory. I monitored 132 labelled S. maximum individuals during 2 years of study. They were randomly assigned to two groups: clipped and control. Infructescence dry mass, total seed dry mass, seed size, germination rate and an increase of root dry mass during the season were assessed in the experimental plants. Since only roots can survive to the next season, root dry mass was considered a reliable measure of allocation to future performance. Clipped plants showed increased fruit and seed dry mass versus the controls, with no other aspect of reproduction affected. Apical bud removal also had a positive effect on increase of root dry mass. The results indicate true overcompensation in response to simulated herbivory with no future costs of increased reproduction. Moreover, increased plant reproduction as a result of herbivory is likely to persist in the following years: clipping increased not only seed production but also root growth. This response is inconsistent with the results of optimal allocation models and the discrepancy is probably due to violation of the resource limitation assumption. Plants adapted to tolerate herbivory seem not to reproduce at the maximum rate when undamaged, but rather withhold resources to be allocated to reproduction after herbivory.     

Reproductive trade-offs in genetically distinct clones ofVaccinium macrocarpon,the American cranberry

Patterns of resource allocation to growth, current reproduction, and potential future reproduction were quantified in six genetically distinct cultivars ofVaccinium macrocarpon. For all cultivars (genotypes), vegetative size is positively correlated with some measures of current reproduction (fruit and flower number) but negatively correlated with others (seed number per fruit, seed weight per fruit). Vegetative growth in the current year is significantly related to the production of reproductive terminal buds, a measure of the potential for reproduction in the following year. Stems with low levels of current reproduction — lower flower number, fruit number, and seed weight — were more likely to form reproductive terminal buds than stems with higher levels of current reproduction. Individual genotypes differed significantly for vegetative size, fruit number, fruit weight, seed number, and seed weight, as well as for the frequency of fruiting stems and reproductive terminal buds produced. Genotypes were segregated in principal component space, indicating overall differences between them in allocation to the suite of variables measured. These results indicate the possibility of fitness differences among cultivars due to genetically determined allocation strategy, which has implications for fitness differences among genotypes within natural populations.     

Reduction of generation time in Eucalyptus globulus

The plant growth retardant paclobutrazol was applied by foliar spray to seedlings of Eucalyptus globulus (Labill.) to determine its effects on reproductive activity. It was shown to induce flowering in vegetatively juvenile seedlings and increase reproductive activity in vegetatively adult material. The former case represented a 50% (three year) reduction in generation time for commercial seedlings. Paclobutrazol treatment did not appear to have a negative effect on any aspect of reproductive development, germination of new seed, or early seedling growth, while having a distinct positive effect on reproductive output. This suggests that paclobutrazol may be an effective tool enabling tree breeders to reduce generation time and increase reproductive output in E. globulus. Foliar spraying during autumn was shown to be an effective means and timing of paclobutrazol application, with results evident in the following flowering season. Considerable persistence of paclobutrazol effects was noted in the second season after application. Pot size and growth temperatures also play roles in determining the extent of new reproductive activity each season in E. globulus seedlings.     

PATTERNS OF FRUIT PRODUCTION IN A NEOTROPICAL ORCHID: POLLINATOR VS. RESOURCE LIMITATION

Patterns of growth, and of flower and fruit production, were monitored over two years in a population of the orchid Aspasia principissa Reichb. f. in central Panama. Observations and experimental manipulations were used to determine the relative importance of pollinator and resource limitation on fruit production. Within a season, fruit production was limited by pollinator availability. Fruit set for hand-pollinated flowers was over six times greater than that for naturally pollinated flowers. However, in plants that produced more than one fruit, fruit size declined in subsequent fruits, indicating that resources could limit seed production within a season. Plants producing fruits in 1986, on average, produced smaller shoots and inflorescences in 1987 relative to plants that flowered but produced no fruit in 1986. Thus, plants are likely to be resource limited over their lifetimes. Most individuals of reproductive size (82.5%) did not produce fruit over a three-year period. The reproductive dominance of a few individuals in this population of Aspasia principissa may have important implications for understanding the population structure of the species and the high species diversity of orchids.     

Temporal variation in the female components of reproductive success over the extended flowering season of a Mediterranean perennial herb

We examined the reproductive success of the perennial herb Lobularia maritima during its extended flowering and fruiting season. The within- and between-year variability of the female components of reproductive success (from flower, fruit and seed production to seed survival, seed germination and seedling establishment) were analysed during four flowering seasons. All the components of reproductive success studied showed a significant within- and between-year variation. September was the period of the year with the maximum values of flower and fruit production, and the highest germination and establishment rates. Nevertheless, seed losses due to both predispersal and postdispersal seed predation during this period were also the highest, seriously reducing seed output in this period. On the other hand, in those periods in which seed production was low, i.e. January and May, the percentage of seeds lost to seed predators was the lowest. Reproductive success in each period of the flowering season was estimated using a simple demographic model, in which the information concerning all the components already calculated was integrated. The two variables used to estimate reproductive success in each period, i.e. the number of new individuals produced per plant and the probability of a seed becoming an adult plant, showed relatively small differences over the year. These results suggest a counter-balance of the different components of reproductive success in this species, with favourable and unfavourable periods for the different components being compensated during its extended flowering season.     

Timing of reproductive and vegetative development in Saxifraga oppositifolia in an alpine and a subnival climate

Morphogenesis of floral structures, dynamics of reproductive development from floral initiation until fruit maturation, and leaf turnover in vegetative short-stem shoots of Saxifraga oppositifolia were studied in three consecutive years at an alpine site (2300 m) and at an early- and late-thawing subnival site (2650 m) in the Austrian Alps. Marked differences in the timing and progression of reproductive and vegetative development occurred: individuals of the alpine population required a four-month growing season to complete reproductive development and initiate new flower buds, whereas later thawing individuals from the subnival sites attained the same structural and functional state within only two and a half months. Reproductive and vegetative development were not strictly correlated because timing of flowering, seed development, and shoot growth depended mainly on the date of snowmelt, whereas the initiation of flower primordia was evidently controlled by photoperiod. Floral induction occurred during June and July, from which a critical day length for primary floral induction of about 15 h could be inferred. Preformed flower buds overwinter in a pre-meiotic state and meiosis starts immediately after snowmelt in spring. Vegetative short-stem shoots performed a full leaf turnover within a growing season: 16 (+/-0.8 SE) new leaves per shoot developed in alpine and early-thawing subnival individuals and 12 (+/-1.2 SE) leaves in late-thawing subnival individuals. New leaf primordia emerged continuously from snowmelt until late autumn, even when plants were temporarily covered with snow. Differences in the developmental dynamics between the alpine and subnival population were independent of site temperatures, and are probably the result of ecotypic adaptation to differences in growing season length.     

Effects of seed release timing on plant life-history and seed production in a population of a desert annual,shape Blepharis sindica (Acanthaceae)

Growth, phenology, survivorship, and seed production were observed in a population of a desert annual, Blepharis sindica, with reference to the variation in the timing of seedling emergence. The population consisted of several cohorts induced by rain-cued seed release within a growing season. The fate of 100 individuals of six cohorts was monitored throughout the growing season. Earlier-established cohorts had significantly larger plant sizes and higher reproductive outputs than later cohorts. The time and duration of each phenological stage varied among the cohorts, and they were also influenced by plant size. Mortalities at the seedling stage, vegetative stage, and reproductive stage increased with the delay of seed release. Seed release was concentrated in the early growing season. Fecundity was highest in the earliest cohort and decreased monotonically in later cohorts. The results suggested that even in temporally varying environments, the superiority of early emergent plants was evident. The seed release patterns in temporally fluctuating desert environments are discussed as a compromise between 'diversified bet-hedging' and an optimal timing for maximizing the reproductive success in a growing season.     

Bottom-up effects of nutrient availability on flower production, pollinator visitation, and seed output in a high-Andean shrub

Soil nutrient availability directly enhances vegetative growth, flowering, and fruiting in alpine ecosystems. However, the impacts of nutrient addition on pollinator visitation, which could affect seed output indirectly, are unknown. In a nutrient addition experiment, we tested the hypothesis that seed output in the insect-pollinated, self-incompatible shrub, Chuquiraga oppositifolia (Asteraceae) of the Andes of central Chile, is enhanced by soil nitrogen (N) availability. We aimed to monitor total shrub floral display, size of flower heads (capitula), pollinator visitation patterns, and seed output during three growing seasons on control and N addition shrubs. N addition did not augment floral display, size of capitula, pollinator visitation, or seed output during the first growing season. Seed mass and viability were 25–40% lower in fertilised shrubs. During the second growing season only 33% of the N addition shrubs flowered compared to 71% of controls, and a significant (50%) enhancement in vegetative growth occurred in fertilised shrubs. During the third growing season, floral display in N addition shrubs was more than double that of controls, received more than twice the number of insect pollinator visits, and seed output was three- to four-fold higher compared to controls. A significant (50%) enhancement in vegetative growth again occurred in N addition shrubs. Results of this study strongly suggest that soil N availability produces strong positive bottom-up effects on the reproductive output of the alpine shrub C. oppositifolia. Despite taking considerably longer to be manifest in comparison to the previously reported top-down indirect negative effects of lizard predators in the same study system, our results suggest that both bottom-up and top-down forces are important in controlling the reproductive output of an alpine shrub.     

Snow cover and extreme winter warming events control flower abundance of some,but not all species in high arctic Svalbard

The High Arctic winter is expected to be altered through ongoing and future climate change. Winter precipitation and snow depth are projected to increase and melt out dates change accordingly. Also, snow cover and depth will play an important role in protecting plant canopy from increasingly more frequent extreme winter warming events. Flower production of many Arctic plants is dependent on melt out timing, since season length determines resource availability for flower preformation. We erected snow fences to increase snow depth and shorten growing season, and counted flowers of six species over 5 years, during which we experienced two extreme winter warming events. Most species were resistant to snow cover increase, but two species reduced flower abundance due to shortened growing seasons. Cassiope tetragona responded strongly with fewer flowers in deep snow regimes during years without extreme events, while Stellaria crassipes responded partly. Snow pack thickness determined whether winter warming events had an effect on flower abundance of some species. Warming events clearly reduced flower abundance in shallow but not in deep snow regimes of Cassiope tetragona, but only marginally for Dryas octopetala. However, the affected species were resilient and individuals did not experience any long term effects. In the case of short or cold summers, a subset of species suffered reduced reproductive success, which may affect future plant composition through possible cascading competition effects. Extreme winter warming events were shown to expose the canopy to cold winter air. The following summer most of the overwintering flower buds could not produce flowers. Thus reproductive success is reduced if this occurs in subsequent years. We conclude that snow depth influences flower abundance by altering season length and by protecting or exposing flower buds to cold winter air, but most species studied are resistant to changes.     

Reproduction as a bottleneck to treeline advance across the circumarctic forest tundra ecotone

The fundamental niche of many species is shifting with climate change, especially in sub‐arctic ecosystems with pronounced recent warming. Ongoing warming in sub‐arctic regions should lessen environmental constraints on tree growth and reproduction, leading to increased success of trees colonising tundra. Nevertheless, variable responses of treeline ecotones have been documented in association with warming temperatures. One explanation for time lags between increasingly favourable environmental conditions and treeline ecotone movement is reproductive limitations caused by low seed availability. Our objective was to assess the reproductive constraints of the dominant tree species at the treeline ecotone in the circumpolar north. We sampled reproductive structures of trees (cones and catkins) and stand attributes across circumarctic treeline ecotones. We used generalized linear mixed models to estimate the sensitivity of seed production and the availability of viable seed to regional climate, stand structure, and species‐specific characteristics. Both seed production and viability of available seed were strongly driven by specific, sequential seasonal climatic conditions, but in different ways. Seed production was greatest when growing seasons with more growing degree days coincided with years with high precipitation. Two consecutive years with more growing degree days and low precipitation resulted in low seed production. Seasonal climate effects on the viability of available seed depended on the physical characteristics of the reproductive structures. Large‐coned and ‐seeded species take more time to develop mature embryos and were therefore more sensitive to increases in growing degree days in the year of flowering and embryo development. Our findings suggest that both moisture stress and abbreviated growing seasons can have a notable negative influence on the production and viability of available seed at treeline. Our synthesis revealed that constraints on predispersal reproduction within the treeline ecotone might create a considerable time lag for range expansion of tree populations into tundra ecosystems.     

Latitudinal variation in seed weight and flower number in Prunella vulgaris

Studies of seed-weight variation across altitudinal and latitudinal gradients have led to conflicting hypotheses regarding the selective value of this traint in relation to the length of the growing season. Growing-season length may also influence the evolution of seed number, and population differentiation in seed weight may be constrained by a negative genetic correlation between seed weight and seed number within populations. We examined variation in seed weight and an estimate of seed number (flower number) and the covariance of these traits among populations of Prunella vulgaris at five latitudes between northern Michigan and South Carolina. We measured seed weight and flower number in native habitats and in a common environment to determine the extent to which patterns observed in the field reflect genetic differentiation. We observed no genetically based variation in seed weight across the latitudinal gradient, although genetic variation among populations within a latitude was observed. In contrast to the lack of variation in seed weight, flower number increased clinally from northern Michigan to Tennessee in a common environment. Population mean flowering date in a common environment was successively later from north to south. Later-flowering individuals appear to achieve a larger size before flowering and consequently possess more resources for seed production. This difference may account for the greater flower production of late-flowering, southern populations. Independence of population mean seed weight and flower number across the latitudinal gradient suggests that population differentiation in seed weight has not been constrained by a trade-off between seed size and number within populations.     

Selection on floral display in insect-pollinated Primula farinosa: effects of vegetation height and litter accumulation

Grazing reduces litter thickness and vegetation height and may thereby indirectly affect reproductive success and selection on floral characters in plants with prostrate growth. Reductions in litter thickness and vegetation height should influence both the resource status of plants with leaves positioned close to the ground and the significance of inflorescence height for interactions with pollinators and seed predators. We experimentally examined how simulated grazing of surrounding vegetation affected pollen limitation, fruit predation and fecundity of short-scaped and long-scaped Primula farinosa, which differ markedly in floral display and therefore in expected attractiveness to pollinators. Litter removal and pruning of surrounding vegetation increased fruit and seed production per plant in the year of the treatment and the probability of flowering in the following year. Pollen limitation of fruit and seed production was stronger in the short-scaped morph than in the long-scaped morph, but was not significantly affected by litter removal and simulated grazing of surrounding vegetation. Supplemental hand-pollination reduced seed size in the year of the treatment and flowering probability in the second year, and these effects did not differ among scape morphs or grazing treatments. The results suggest that grazing indirectly favours seed production in P. farinosa, mainly because it increases the resource status of plants that escape damage. Contrary to expectation, there was no strong evidence that litter accumulation and tall vegetation increase the severity of pollen limitation or reduce the relative performance of the short-scaped morph.     

Allocation to reproduction following experimental defoliation in Platanthera bifolia (Orchidaceae)

The roles of herbivory and pollination success in plant reproduction have frequently been examined, but interactions between these two factors have gained much less attention. In three field experiments, we examined whether artificial defoliation affects allocation to attractiveness to pollinators, pollen production, female reproductive success and subsequent growth in Platanthera bifolia L. (Rich.). We also recorded the effects of inflorescence size on these variables. We studied the effects of defoliation on reproductive success of individual flowers in three sections of inflorescence. Defoliation and inflorescence size did not have any negative effects on the proportion of opened flowers, spur length, nectar production or the weight of pollinia. However, we found that hand-pollination increased relative seed production and defoliation decreased seed set in most cases. Interactions between hand-pollination and defoliation were non-significant indicating that defoliation did not affect female reproductive success indirectly via decreased pollinator attraction. Plants with a large inflorescence produced relatively more seeds than plants with a small inflorescence only after hand-pollination. The negative effect of defoliation on relative capsule production was most clearly seen in the upper sections of the inflorescence. In addition to within season effects of leaf removal, defoliated P. bifolia plants may also have decreased lifetime fitness as a result of lower seed set within a season and because of a lower number of reproductive events due to decreased plant size (leaf area) following defoliation. Our study thus shows that defoliation by herbivores may crucially affect reproductive success of P. bifolia.