The contribution of carbon budget to masting intervals in Veratrum album populations inhabiting different elevations.

PREMISE: Mast flowering/seeding is often more extreme in lower-resource environments, such as alpine compared to lowland habitats. We studied a masting herb that had less extreme masting at higher elevations, and tested if this difference could be explained by higher photosynthetic productivity and/or lower reproductive investment at the higher-elevation sites. METHODS: We examined the relationship between flowering intervals and carbon budget (i.e., the balance between reproductive investment and annual carbon fixation) in a masting herb, Veratrum album subsp. oxysepalum, across five lowland and six alpine populations in northern Japan. We evaluated the previous flowering histories of individual plants based on rhizome morphology and analyzed the masting patterns of individual populations. Total mass of the reproductive organs, as a proxy of reproductive investment, was compared between the lowland and alpine populations. Annual carbon fixation was estimated on the basis of photosynthetic capacity, total leaf area per plant, and seasonal transition of light availability. RESULTS: Interval between high-flowering years was shorter and total reproductive investment was smaller in the alpine than in the lowland populations. Owing to its high photosynthetic capacity and continuous bright conditions, annual carbon fixation per plant was 1.5 times greater in alpine habitat than in lowland habitat. These results suggest that V. album alpine populations have shorter flowering intervals than lowland populations due to faster recovery from energy loss after reproduction. CONCLUSIONS: Our study demonstrated that masting intervals in V. album populations can be explained by habitat-specific carbon budget balances.

Night and day: Contributions of diurnal and nocturnal visitors to pollen dispersal, paternity diversity, and fruit set in an early-blooming shrub, Daphne jezoensis.

PREMISE: Under uncertain pollinator visit conditions, plants often exhibit long flowering periods and generalized pollination systems. Flowering of the gynodioecious shrub Daphne jezoensis occurs in early spring in cool temperate forests. Pollination by nocturnal moths is expected, given the species' tubular-shaped flowers with sweet fragrance and nectar. However, the effectiveness of nocturnal moths under cool conditions is unknown. We evaluated the relative importance of diurnal and nocturnal visitors as pollinators in early spring. METHODS: We investigated flowering duration, flower visitors, and floral scents in a natural population. We experimentally exposed flowers to visitors only during daytime or nighttime using bagging treatments and evaluated the contributions of diurnal and nocturnal insects to fruit set, pollen dispersal distance, and paternity diversity using 16 microsatellite markers. RESULTS: Female flowers lasted ~3 wk, which was ~8 d longer than the flowering period of hermaphrodites. Various insects, including Coleoptera, Diptera, Hymenoptera, and Lepidoptera, visited the flowers during both daytime and nighttime. Flowers emitted volatiles, such as lilac aldehyde isomers and ß-ocimene, which are known to attract moths. Fruit-set rate in the night-open treatment was similar to or higher than that in the day-open treatment. However, pollen dispersal distance in the night-open treatment was shorter than that in the day-open treatment. Paternity diversity was similar in day-open and night-open treatments. CONCLUSIONS: Early-blooming plants ensure pollen receipt and dispersal by having a long flowering period and using both diurnal and nocturnal flower visitors, suggesting the importance of a generalized pollination system under uncertain pollinator visit conditions.

The selective advantage of mast flowering in Veratrum album subsp. oxysepalum: Implications of the predator satiation hypothesis.

PREMISE: Synchronous, highly variable flower or seed production among years within a population (i.e., masting) has been reported in numerous perennial plants. Although masting provides ecological advantages such as enhancing pollination efficiency and/or escape from predator attack, little is known about the degree of these advantages and variations in masting behavior among populations of conspecific plants. METHODS: We determined flowering ramet density and reproductive success (fruit-set success and herbivorous damage) of a perennial herb, Veratrum album subsp. oxysepalum, across six lowland and six alpine populations in northern Japan during 2-3 years. We then analyzed the relationship between floral density and reproductive success to assess the ecological significance of mast flowering. Flowering intervals of individual plants were estimated by counting annual scars on rhizomes. RESULTS: Most populations had mast flowering, but the intervals between flowering for individual plants were shorter in the alpine populations than in the lowland populations. Floral damage by stem borers (dipteran larvae) and seed predation by lepidopteran larvae were intense in the lowland populations. Seed production of individual ramets increased with higher floral density owing to the effective avoidance of floral-stem damage and seed predation. Although stem borers were absent in the alpine habitat, seed predation decreased with higher floral density also in the alpine populations. Pollination success was independent of floral density in both of the alpine and lowland populations. CONCLUSIONS: These results strongly support the predator satiation hypothesis for mast flowering by this species.

Is increased male flower production a strategy for avoidance of predispersal seed predation in andromonoecious plants?

Floral gender in angiosperms often varies within and among populations. We conducted a field survey to test how predispersal seed predation affects sex allocation in an andromonoecious alpine herb Peucedanum multivittatum. We compared plant size, male and perfect flower production, fruit set, and seed predation rate over three years among nine populations inhabiting diverse snowmelt conditions in alpine meadows. Flowering period of individual populations varied from mid-July to late August reflecting the snowmelt time. Although perfect flower and fruit productions increased with plant size, size dependency of male flower production was less clear. The number of male flowers was larger in the early-flowering populations, while the number of perfect flowers increased in the late-flowering populations. Thus, male-biased sex allocation was common in the early-flowering populations. Fruit-set rates varied among populations and between years, irrespective of flowering period. Fruit-set success of individual plants increased with perfect flower number, but independent of male flower number. Seed predation by lepidopteran larvae was intense in the early-flowering populations, whereas predation damage was absent in the late-flowering populations, reflecting the extent of phenological matching between flowering time of host plants and oviposition period of predator moths. Seed predation rate was independent of male and perfect flower numbers of individual plants. Thus, seed predation is a stochastic event in each population. There was a clear correlation between the proportion of male flowers and the intensity of seed predation among populations. These results suggest that male-biased sex allocation could be a strategy to reduce seed predation damage but maintain the effort as a pollen donor under intensive seed predation.

Ecotypic differentiation of a circumpolar Arctic-alpine species at mid-latitudes: variations in the ploidy level and reproductive system of Vaccinium vitis-idaea.

Although plant species originated from Arctic regions commonly grow in alpine habitats at mid-latitudes, some populations of these species exist also in some specific habitats below the treeline. Local populations at lower elevations may have different origins, ploidy levels, mating systems and/or morphological traits from alpine populations, but comparative studies between alpine and low-elevation populations are scarce. We aimed to reveal the ecological and genetic differentiations between higher and lower populations of Vaccinium vitis-idaea in Hokkaido, northern Japan by comparing 22 populations growing in diverse environments. We analysed the ploidy level of individual populations using flow cytometry. Genetic differentiation among populations, and genetic diversity within populations were calculated using microsatellite markers. Fruit and seed production were recorded under natural conditions, and a pollination experiment was conducted to reveal the variations in mating system across populations. Furthermore, we compared shoot growth and leaf characteristics among populations. Most of the low-elevation populations were tetraploid, whereas all but one of the alpine populations were diploid. Tetraploid populations were clearly differentiated from diploid populations. Some tetraploid populations formed huge clonal patches but genetic diversity was higher in tetraploids than in diploids. Alpine diploids were self-incompatible and produced more seeds per fruit than tetraploid populations. In contrast, tetraploids showed high self-compatibility. Leaf size and foliar production were greater in tetraploid populations. Our results indicate that the genetic compositions of low-elevation tetraploid populations are different from those of alpine diploid populations. Most populations at lower elevations contained unique ecotypes suited to persistence in isolated situations. Local, low-elevation populations of typical alpine species maintain ecologically and genetically specific characteristics and could be valuable in terms of evolutionary and conservation biology. The present study demonstrates the biological importance of small and isolated populations at the edges of species distribution.

Belowground impacts of alpine woody encroachment are determined by plant traits, local climate, and soil conditions.

Global climate and land use change are causing woody plant encroachment in arctic, alpine, and arid/semi-arid ecosystems around the world, yet our understanding of the belowground impacts of this phenomenon is limited. We conducted a globally distributed field study of 13 alpine sites across four continents undergoing woody plant encroachment and sampled soils from both woody encroached and nearby herbaceous plant community types. We found that woody plant encroachment influenced soil microbial richness and community composition across sites based on multiple factors including woody plant traits, site level climate, and abiotic soil conditions. In particular, root symbiont type was a key determinant of belowground effects, as Nitrogen-fixing woody plants had higher soil fungal richness, while Ecto/Ericoid mycorrhizal species had higher soil bacterial richness and symbiont types had distinct soil microbial community composition. Woody plant leaf traits indirectly influenced soil microbes through their impact on soil abiotic conditions, primarily soil pH and C:N ratios. Finally, site-level climate affected the overall magnitude and direction of woody plant influence, as soil fungal and bacterial richness were either higher or lower in woody encroached versus herbaceous soils depending on mean annual temperature and precipitation. All together, these results document global impacts of woody plant encroachment on soil microbial communities, but highlight that multiple biotic and abiotic pathways must be considered to scale up globally from site- and species-level patterns. Considering both the aboveground and belowground effects of woody encroachment will be critical to predict future changes in alpine ecosystem structure and function and subsequent feedbacks to the global climate system.

When spring ephemerals fail to meet pollinators: mechanism of phenological mismatch and its impact on plant reproduction.

The flowering phenology of early-blooming plants is largely determined by snowmelt timing in high-latitude and high-altitude ecosystems. When the synchrony of flowering and pollinator emergence is disturbed by climate change, seed production may be restricted due to insufficient pollination success. We revealed the mechanism of phenological mismatch between a spring ephemeral ( Corydalis ambigua) and its pollinator (overwintered bumblebees), and its impact on plant reproduction, based on 19 years of monitoring and a snow removal experiment in a cool-temperate forest in northern Japan. Early snowmelt increased the risk of phenological mismatch under natural conditions. Seed production was limited by pollination success over the 3 years of the pollination experiment and decreased when flowering occurred prior to bee emergence. Similar trends were detected on modification of flowering phenology through snow removal. Following snowmelt, the length of the pre-flowering period strongly depended on the ambient surface temperature, ranging from 4 days (at greater than 7°C) to 26 days (at 2.5°C). Flowering onset was explained with an accumulated surface degree-day model. Bumblebees emerged when soil temperature reached 6°C, which was predictable by an accumulated soil degree-day model, although foraging activity after emergence might depend on air temperature. These results indicate that phenological mismatch tends to occur when snow melts early but subsequent soil warming progresses slowly. Thus, modification of the snowmelt regime could be a major driver disturbing spring phenology in northern ecosystems.

Theory of coevolution of cytoplasmic male-sterility, nuclear restorer and selfing.

Gynodioecy is a sexual polymorphism in angiosperms, where hermaphroditic and female individuals coexist. This is often caused by a cytoplasmic genetic element (CGE) that destroys male functions, which is called cytoplasmic male-sterility (CMS). On the other hand, nuclear genes tend to evolve the ability to restore male function. The coevolutionary process of CMS and the restoration has been studied theoretically. Recently, a theoretical study suggested that these coevolutionary dynamics could be influenced by the rate of selfing within populations, although it assumed that the selfing rate of a population was a fixed parameter. Accordingly, we theoretically study the coevolution of three traits in this paper: CMS, nuclear restorer and selfing rate, in which we hypothesize that selfing evolution can suppress CMS evolution under some conditions. The analysis indicates three significant properties of the system; (1) CMS-restorer evolution can result in bistability under a given selfing rate, (2) the coevolution of three traits can realize intermediate levels of selfing, and (3) the evolution of high levels of selfing is conditionally associated with no CMS and/or no restoration, which may support our hypothesis.

Ecotypic divergences of the alpine herb Potentilla matsumurae adapted to fellfield-snowbed habitats across a series of mountain sky islands.

PREMISE: Divergent selection due to environmental heterogeneity can lead to local adaptation. However, the ecological and evolutionary processes of local adaptation that occurs across multiple regions are often unknown. Our previous studies reported on the ecotypic divergence within a local area of variation of Potentilla matsumurae, an alpine herb adapted to the fellfield-snowbed environment. Here we investigated large-scale geographic patterns of ecotypic differentiation in this species to infer local adaptation and selective forces across multiple regions. METHODS: We compiled information on the overall distributions of fellfield and snowbed habitats on the mountains in Japan across the distribution of the species. Next, we conducted common garden experiments to test the adaptive divergence of the fellfield-snowbed plants derived from multiple regions. Finally, we evaluated phylogeographic structures based on cpDNA and allozyme variations and inferred the evolutionary history of ecotype differentiation. RESULTS: The mosaic distribution of the fellfield-snowbed ecotypes across isolated mountaintops constitutes indirect evidence for habitat-specific natural selection. The significant difference in survivorship between the ecotypes observed in a controlled snow environment provides more substantial evidence of local selection. Phylogeographic structures support the hypothesis that ecotypic divergence events from fellfield to snowbed populations occurred independently in at least two distinct regions. CONCLUSIONS: Ecotypic divergence of P. matsumurae has occurred across a series of mountain sky islands. Local selection in snowy environments is a driving force that maintains the divergent ecotypes across multiple mountain regions and can contribute to the diversification of plants in heavy-snow regions.

Association between community assemblage of flower colours and pollinator fauna: a comparison between Japanese and New Zealand alpine plant communities.

BACKGROUND AND AIMS: Flower colour plays a major role in the attraction and decision-making of pollinators. Different functional groups of pollinators tend to prefer different flower colours, and therefor may lead to different flower colour compositions among different communities depending on the visual system of the dominant pollinators. However, few studies have investigated the linkage between pollinator fauna and flower colour composition in natural communities, a theme we explored in the present study. METHODS: Flower spectral reflectance of 106 Japanese and 96 New Zealand alpine plants in the wavelength range 300-700 nm were measured. The composition of pollinator fauna in the communities and the types of pollinators for each plant species were also investigated. KEY RESULTS: Based on bee and fly colour vision models, as well as a principal components analysis, considering phylogenetic non-independence between plant species, flower colours appeared to vary according to pollinator type rather than geographical region. Consequently, flower colour composition differed between the regions, reflecting the bee/fly mixed pollinator fauna of Japan and the fly-dominant pollinator fauna of New Zealand. According to the bee colour vision model, the majority of the colours of hymenopteran-pollinated flowers appeared to be discriminated by bees. In contrast, many of the colours of dipteran-pollinated flowers would not be discriminated by bees and flies. CONCLUSION: The results suggest that the differences in flower colour composition between Japanese and New Zealand alpine communities are due to differences in the pollinator fauna in these communities rather than differences in abiotic factors between the geographical regions and the phylogenetic origin of the communities.

Variations in ramet performance and the dynamics of an alpine evergreen herb, Gentiana nipponica, in different snowmelt conditions.

PREMISE OF THE STUDY: Variation in demographic parameters reflects the life-history strategies of plants in response to specific environments. We aimed to investigate the intraspecific variation in life-history traits of a clonal alpine herb, Gentiana nipponica, in various snowmelt conditions. METHODS: Individual ramets within genets accumulate leaves for 7-9 yr without shedding, and die after reproduction. We tested the physiological function of accumulated leaves for reproduction and monitored the ramet demography in early, intermediate, and late snowmelt populations over 3 yr. Then, we simulated ramet dynamics using the demographic parameters. KEY RESULTS: Old leaves had a carbon storage function, and the initiation of reproduction depended on the amount of ramet leaves. Growth and reproductive performance were highest in the population with an intermediate snowmelt period. The early snowmelt population showed short persistence periods due to restricted growth and high mortality of the ramets. The late snowmelt populations showed slow growth, but high survival rate of the ramets, in which the ramet size at reproduction was smallest and fruit formation was often suppressed by the short growing period. CONCLUSIONS: Limiting factors dictating the distribution of G. nipponica differed between the early and late snowmelt habitats. High mortality and restricted growth, because of the harsh environment, determine the distribution limit toward earlier snowmelt locations. By contrast, late snowmelt strongly limited fecundity because of the short period for fruit maturation. The difference in snowmelt time provides a clear gradient of selective forces that may promote local adaptation among neighboring populations.

Restricted female function of hermaphrodites in a gynodioecious shrub, Daphne jezoensis (Thymelaeaceae).

Gynodioecy is the coexistence of hermaphrodites and females in a population. It is supposed to be an intermediate stage in the evolutionary pathway from hermaphroditism to dioecy in angiosperm. Hermaphrodites gain fitness through both seed and pollen production whereas females gain fitness only through seed production. As females spread in a gynodioecious population, sexual selection prompts hermaphrodites to invest in male function and male-biased hermaphrodites prevail. In the gynodioecious shrub Daphne jezoensis (Thymelaeaceae), female frequency is stably around 50% in most populations, and fruit-set rate of hermaphrodites is commonly low. Therefore, D. jezoensis is likely at a later stage in the evolutionary pathway. Female function of hermaphrodites (fruit-set rate, selfing rate, seed size, and germination rate) was assessed in three populations under natural conditions. In order to evaluate the potential seed fertility and inbreeding depression by selfing in hermaphrodites, hand pollination treatments were also performed. Over a 2-year period under natural conditions, 18-29% of hermaphrodites and 69-81% of females set fruit. Across all three populations, the mean fruit-set rate ranged 9.5-49.2% in females and only 3.9-10.2% in hermaphrodites. Even with artificial outcross-pollination, 59-91% of hermaphrodites failed to set any fruit. When self-pollination was performed in hermaphrodites, both of fruit-set and germination rates were decreased, indicating early-acting inbreeding depression. In addition, more than half of the hermaphrodite seeds were produced by selfing under natural pollination, but pollinator service was still required. Totally, hermaphrodites performed poorly as seed producers because of the intrinsically-low fruiting ability and a combination of autogamous selfing and strong inbreeding depression, indicating the absence of reproductive assurance. These results indicate that the mating system of D. jezoensis is functionally close to dioecy.

A linkage between flowering phenology and fruit-set success of alpine plant communities with reference to the seasonality and pollination effectiveness of bees and flies.

To clarify the linkage between flowering phenology and pollination success in alpine plant communities, we quantified the seasonality of flower visitors, the temporal transition of floral resources, and the variation in pollination success of alpine plants in northern Japan. Bumble bees, syrphid flies, and non-syrphid flies were the predominant flower visitors. Foraging activity of bumble bees increased toward the late flowering period reflecting the life cycle of colony development. The activity of syrphid flies was sensitive to ambient temperature, while that of non-syrphid flies remained high throughout the season. Flower production of bee-pollinated plants fluctuated significantly between years with a bimodal pattern peaking in the early and late periods, while flower production of fly-pollinated plants was less variable between years. Fruit-set success of bee-pollinated plants increased considerably from the early to the late flowering period, while the trend for fly-pollinated plants was less marked. Three times more visits of dipteran insects are necessary for fly-pollinated plants to achieve fruiting success comparable to bee-pollinated plants. Bumble bees are potentially excellent pollinators, but the visitation frequency is low early in the season. Lower pollination ability of dipteran insects may be compensated for by abundant flower visits. The relationships between flowering phenology and fruit-set success of alpine plant communities highly depend on the type of pollinators.

Elevation alters ecosystem properties across temperate treelines globally.

Temperature is a primary driver of the distribution of biodiversity as well as of ecosystem boundaries. Declining temperature with increasing elevation in montane systems has long been recognized as a major factor shaping plant community biodiversity, metabolic processes, and ecosystem dynamics. Elevational gradients, as thermoclines, also enable prediction of long-term ecological responses to climate warming. One of the most striking manifestations of increasing elevation is the abrupt transitions from forest to treeless alpine tundra. However, whether there are globally consistent above- and belowground responses to these transitions remains an open question. To disentangle the direct and indirect effects of temperature on ecosystem properties, here we evaluate replicate treeline ecotones in seven temperate regions of the world. We find that declining temperatures with increasing elevation did not affect tree leaf nutrient concentrations, but did reduce ground-layer community-weighted plant nitrogen, leading to the strong stoichiometric convergence of ground-layer plant community nitrogen to phosphorus ratios across all regions. Further, elevation-driven changes in plant nutrients were associated with changes in soil organic matter content and quality (carbon to nitrogen ratios) and microbial properties. Combined, our identification of direct and indirect temperature controls over plant communities and soil properties in seven contrasting regions suggests that future warming may disrupt the functional properties of montane ecosystems, particularly where plant community reorganization outpaces treeline advance.

Foraging responses of bumble bees to rewardless floral patches: importance of within-plant variance in nectar presentation.

Spatiotemporal variation in nectar distribution is a key factor affecting pollinator movements between flowers and plants within a population. Pollinators having systematic searching ability can flexibly respond to the reward condition of floral patches, and they tend to revisit rewarding patches. However, foraging behaviour may be influenced by the nectar distribution within populations. To evaluate the effects of unrewarding experiences and plant distribution, we compared bumble bee foraging behaviours between naturally rewarding and artificially rewardless (by nectary removal) patches in two aconite populations with different plant densities. Visitation frequency to the patches, number of successive flower visits within inflorescences, and successive inflorescence visits within patches were recorded. Nectar production and standing crop were also measured. Bumble bees increased the movements between neighbouring inflorescences instead of leaving the patches when they faced rewardless flowers. A large variance in nectar production existed among flowers within plants. This might explain the observed bumble bee behaviour, because they could be rewarded by moving to the adjacent inflorescences even after a rewardless experience. Our results imply that a highly variable nectar reward in a population might mask the disadvantage of completely rewardless individuals.

Short-Term Responses in Maximum Quantum Yield of PSII (Fv/Fm) to ex situ Temperature Treatment of Populations of Bryophytes Originating from Different Sites in Hokkaido, Northern Japan.

There is limited knowledge available on the thermal acclimation processes for bryophytes, especially when considering variation between populations or sites. This study investigated whether short-term ex situ thermal acclimation of different populations showed patterns of site dependency and whether the maximum quantum yield of PSII (Fv/Fm) could be used as an indicator of adaptation or temperature stress in two bryophyte species: Pleurozium schreberi (Willd. ex Brid.) Mitt. and Racomitrium lanuginosum (Hedw.) Brid. We sought to test the hypothesis that differences in the ability to acclimate to short-term temperature treatment would be revealed as differences in photosystem II maximum yield (Fv/Fm). Thermal treatments were applied to samples from 12 and 11 populations during 12 or 13 days in growth chambers and comprised: (1) 10/5 °C; (2) 20/10 °C; (3) 25/15 °C; (4) 30/20 °C (12 hours day/night temperature). In Pleurozium schreberi, there were no significant site-dependent differences before or after the experiment, while site dependencies were clearly shown in Racomitrium lanuginosum throughout the study. Fv/Fm in Pleurozium schreberi decreased at the highest and lowest temperature treatments, which can be interpreted as a stress response, but no similar trends were shown by Racomitrium lanuginosum.

Size-dependent sex allocation and reproductive investment in a gynodioecious shrub.

In sexually dimorphic plants, resource allocation to reproduction often differs between sex morphs. In gynodioecious species, i.e. coexisting hermaphrodite and female plants within a population, females often produce more fruits than hermaphrodites. Since fruit production is costlier than flower production, hermaphrodites and females may regulate flower and fruit production differently in response to resource availability. To clarify the gender-specific strategies of reproductive allocation, we assessed sexual dimorphism in reproductive traits, size-dependent resource allocation, morphological traits, and photosynthetic capacity in a natural population of a gynodioecious shrub, Daphne jezoensis Hermaphrodites had larger flowers and increased flower number with plant size at a rate greater than females, but showed consistently smaller fruit production. Although females did not increase flower production as much as hermaphrodites did as their size increased, they produced 3.7 times more fruits than did hermaphrodites. Despite a large sexual difference in fruiting ability based on hand-pollination, total resource investment in reproduction (the sum of flower and fruit mass) was similar between sex morphs across plant sizes, and there was little sexual difference in the cost of reproduction, i.e. the negative effect of current reproduction on future reproductive effort, in the natural population. In addition, there were no sexual differences in the resource allocation to vegetative organs (leaf and root mass) and photosynthetic capacity (light response photosynthetic rates). Under natural conditions, pollen limitation strongly restricted the fruit production of females, resulting in similar cost of reproduction between hermaphrodites and females.

How do sink and source activities influence the reproduction and vegetative growth of spring ephemeral herbs under different light conditions?

Spring ephemeral herbs inhabiting deciduous forests commonly complete reproduction and vegetative growth before canopy closure in early summer. Effects of shading by early canopy closure on reproductive output and vegetative growth, however, may vary depending on the seasonal allocation patterns of photosynthetic products between current reproduction and storage for future growth in each species. To clarify the effects of sink-source balance on seed production and bulb growth in a spring ephemeral herb, Gagea lutea, we performed a bract removal treatment (source reduction) and a floral-bud removal treatment (sink reduction) under canopy and open conditions. Leaf carbon fixations did not differ between the forest and open sites and among treatments. Bract carbon fixations were also similar between sites but tended to decrease when floral buds were removed. Seed production was higher under open condition but decreased by the bract-removal treatment under both light conditions. In contrast, bulb growth was independent of light conditions and the bract-removal treatment but increased greatly by the bud-removal treatment. Therefore, leaves and bracts acted as specialized source organs for vegetative and reproductive functions, respectively, but photosynthetic products by bracts were flexibly used for bulb growth when plants failed to set fruits. Extension of bright period was advantageous for seed production (i.e., source limited) but not for vegetative growth (i.e., sink limited) in this species.

Early onset of spring increases the phenological mismatch between plants and pollinators.

Climate warming accelerates the timing of flowering and insect pollinator emergence, especially in spring. If these phenological shifts progress independently between species, features of plant-pollinator mutualisms may be modified. However, evidence of phenological mismatch in pollination systems is limited. We investigated the phenologies of a spring ephemeral, Corydalis ambigua, and its pollinators (bumble bees), and seed-set success over 10-14 years in three populations. Although both flowering onset and first detection of overwintered queen bees in the C. ambigua populations were closely related to snowmelt time and/or spring temperature, flowering tended to be ahead of first pollinator detection when spring came early, resulting in lower seed production owing to low pollination service. Relationships between flowering onset time, phenological mismatch, and seed-set success strongly suggest that phenological mismatch is a major limiting factor for reproduction of spring ephemerals. This report demonstrates the mechanism of phenological mismatch and its ecological impact on plant-pollinator interactions based on long-term monitoring. Frequent occurrence of mismatch can decrease seed production and may affect the population dynamics of spring ephemerals.

Demand-driven resource investment in annual seed production by a perennial angiosperm precludes resource limitation.

The limits on annual seed production have long been characterized as restriction by either pollination success or resource provision to seed development. This expected dichotomy between pollen and resource limitation is based on the assumption that reproductive resources are fixed, which is reasonable for semelparous species. In contrast, iteroparity can ease the constraints on reproductive output per breeding season, if resources can be either mobilized from past storage or borrowed against future performance. For perennial plants, these options allow enhanced reproductive investment in response to unusually good pollination, so that annual seed production may not be pollen or resource limited. We assessed demand-governed reproductive investment by manipulating both resource supply capacity (partial defoliation) and resource demand (pollination quality: fully self-pollination, fully cross-pollination, or combinations of partial self- and cross-pollination within the inflorescence) for a forest herb, Stenanthium occidentale, which is subject to strong pre-dispersal inbreeding depression. Insensitivity to partial defoliation indicated that reproductive output was not source regulated. Instead, demand by developing seeds governs resource distribution, as demonstrated by elevated photosynthate translocation to fruits on fully cross-pollinated plants and the ability of completely defoliated plants to produce seeds. Such contingent resource allocation eliminates a simple dichotomy between pollen receipt and resource availability as limits on annual seed production. Instead, such flexible reproductive investment allows iteroparous perennials to participate maximally in current reproduction (as determined by ovule production) following superior pollination, or to conserve resources for future reproduction following poor pollination.