Flowering phenology and reproductive fitness along a mountain slope: maladaptive responses to transplantation to a warmer climate in Campanula thyrsoides

In many biomes, global warming has resulted in advanced and longer growing seasons, which has often led to earlier flowering in plant taxa. Elevational gradients are ideal to study the effects of global warming as they allow transplantation of plants from their original cooler higher elevations down to elevations with a prospective climate. We transplanted plants from ten populations of the European alpine monocarpic herb species Campanula thyrsoides L. to three sites along a steep mountain slope (600, 1,235 and 1,850 m above sea level) in the Swiss Alps and asked whether reproductive phenology adjusts plastically to elevation and if these responses were adaptive, i.e. increased the fitness of plants. We further assessed current genetic differentiation in phenotypic traits and whether any such origin effects were due to adaptation to climatic conditions of origin. Our results showed that transplantation to lower elevations caused strong shifts in phenology, with plants starting growth and flowering earlier than plants placed at higher elevations. However, compared to flower production at high elevation, number of flowers per plant decreased 21 % at mid- and 61 % at low elevation. The shift in phenology thus came with a high cost in fitness, and we suggest that phenology is maladaptive when C. thyrsoides faces temperature conditions deviating from its natural amplitude. We conclude that the frequently reported phenological shift in plant species as a response to global warming may include heavy fitness costs that may hamper species survival.     

Selection on flowering time in Mediterranean high-mountain plants under global warming

Under climate warming, plants will undergo novel selective pressures to adjust reproductive timing. Adjustment between reproductive phenology and environment is expected to be higher in arctic and alpine habitats because the growing season is considerably short. As early- and late-flowering species reproduce under very different environmental conditions, selective pressures on flowering phenology and potential effects of climate change are likely to differ between them. However, there is no agreement on the magnitude of the benefits and costs of early- vs. late-flowering species under a global warming scenario. In spite of its relevance, phenotypic selection on flowering phenology has rarely been explored in alpine plants and never in Mediterranean high mountain species, where selective pressures are very different due to the summer drought imposed over the short growth season. We hypothesized that late-flowering plants in Mediterranean mountains should present stronger selective pressures towards early onset of reproduction than early-flowering species, because less water is available in the soil as growing season progresses. We performed selection analyses on flowering onset and duration in two high mountain species of contrasting phenology. Since phenotypic selection can be highly context-dependent, we studied several populations of each species for 2 years, covering their local altitudinal ranges and their different microhabitats. Surrogates of biotic selective agents, like fruitset for pollinators and flower and fruit loss for flower and seed predators, were included in the analysis. Differences between the early- and the late-flowering species were less than expected. A consistent negative correlational selection of flowering onset and duration was found affecting plant fitness, i.e., plants that bloomed earlier flowered for longer periods improving plant fitness. Nevertheless, the late-flowering species may experience higher risks under climate warming because in extremely warm and dry years the earlier season does not bring about a longer flowering duration due to summer drought.     

A resurrection study reveals limited evolution of phenology in response to recent climate change across the geographic range of the scarlet monkeyflower

Premise of the studyAs global climate change alters drought regimes, rapid evolution of traits that facilitate adaptation to drought can rescue populations in decline. The evolution of phenological advancement can allow plant populations to escape drought, but evolutionary responses in phenology can vary across a species'' range due to differences in drought intensity and standing genetic variation.Methods Mimulus cardinalis, a perennial herb spanning a broad climatic gradient, recently experienced a period of record drought. Here, we used a resurrection study comparing flowering time and stem height at first flower of pre‐drought ancestors and post‐drought descendants from northern‐edge, central, and southern‐edge populations in a common environment to examine the evolution of drought escape across the latitudinal range.Key resultsContrary to the hypothesis of the evolution of advanced phenology in response to recent drought, flowering time did not advance between ancestors and descendants in any population, though storage condition and maternal effects could have impacted these results. Stem height was positively correlated with flowering time, such that plants that flowered earlier were shorter at first flower. This correlation could constrain the evolution of earlier flowering time if selection favors flowering early at a large size.ConclusionsThese findings suggest that rapid evolution of phenology will not rescue these populations from recent climate change. Future work is needed to examine the potential for the evolution of alternative drought strategies and phenotypic plasticity to buffer M. cardinalis populations from changing climate.     

How plant allometry influences bud phenology and fruit yield in two Vaccinium species

Background and AimsUnderstanding how plant allometry, plant architecture and phenology contribute to fruit production can identify those plant traits that maximize fruit yield. In this study, we compared these variables and fruit yield for two shrub species, Vaccinium angustifolium and Vaccinium myrtilloides, to test the hypothesis that phenology is linked to the plants’ allometric traits, which are predictors of fruit production.MethodsWe measured leaf and flower phenology and the above-ground biomass of both Vaccinium species in a commercial wild lowbush blueberry field (Quebec, Canada) over a 2-year crop cycle; 1 year of pruning followed by 1 year of harvest. Leaf and flower phenology were measured, and the allometric traits of shoots and buds were monitored over the crop cycle. We hand-collected the fruits of each plant to determine fruit attributes and biomass.Key ResultsDuring the harvesting year, the leafing and flowering of V. angustifolium occurred earlier than that of V. myrtilloides. This difference was related to the allometric characteristics of the buds due to differences in carbon partitioning by the plants during the pruning year. Through structural equation modelling, we identified that the earlier leafing in V. angustifolium was related to a lower leaf bud number, while earlier flowering was linked to a lower number of flowers per bud. Despite differences in reproductive allometric traits, vegetative biomass still determined reproductive biomass in a log–log scale model.ConclusionsGrowing buds are competing sinks for non-structural carbohydrates. Their differences in both number and characteristics (e.g. number of flowers per bud) influence levels of fruit production and explain some of the phenological differences observed between the two Vaccinium species. For similar above-ground biomass, both Vaccinium species had similar reproductive outputs in terms of fruit biomass, despite differences in reproductive traits such as fruit size and number.     

Evidence for genetic differentiation in timing of maturation among nine‐spined stickleback populations

Timing of maturation is an important life‐history trait that is likely to be subjected to strong natural selection. Although population differences in timing of maturation have been frequently reported in studies of wild animal populations, little is known about the genetic basis of this differentiation. Here, we investigated population and sex differences in timing of maturation within and between two nine‐spined stickleback (Pungitius pungitius) populations in a laboratory breeding experiment. We found that fish from the high‐predation marine population matured earlier than fish from the low‐predation pond population and males matured earlier than females. Timing of maturation in both reciprocal hybrid crosses between the two populations was similar to that in the marine population, suggesting that early timing of maturation is a dominant trait, whereas delayed timing of maturation in the pond is a recessive trait. Thus, the observed population divergence is suggestive of strong natural selection against early maturation in the piscine‐predator‐free pond population.     

Vegetative and reproductive phenological traits of Rhizophora mucronata Lamk. and Sonneratia alba Sm.

Mangrove phenology is important in understanding the past, present and future response of mangrove species to impacts of climate change. Our study is the first long term direct observation of the phenology of Rhizophora mucronata and Sonneratia alba in Kenya. Objective of the study was to determine, interpret and document the timing of the various phenoevents and phenophases, and to establish relationships between phenology and the climatic variables. Phenological traits were investigated in six monospecific mangrove stands in Gazi Bay, south of Mombasa, for 2 years. Leaf emergence, leaf fall, flower bud, flower and fruit initiation data were recorded every fortnight in 54 shoots of 9 trees at each site. Continuous leaf emergence and loss characterized by multimodal peaks was observed for the two species. Leaf emergence and leaf fall peaked in the wet months and was reduced in the dry months. There was a relationship between the leaf emergence and drop with the reproductive phenology in the two species. Mean leaf longevity for R. mucronata and S. alba was 12.8 ± 1.2 and 4.9 ± 0.5 months respectively. The reproductive cycle took approximately 16–20 months in R. mucronata and 4–5 months in S. alba. Bud initiation in R. mucronata was seasonal and occurred in October and September. Buds were observed for 8–11 months developing slowly on the shoots. Shifts were observed in the timing of flower initiation, and the flowering period lasted in total for 4–5 months. The time period from flower appearance to the developmental start of immature propagules lasted about 1 month at reforested sites and up to 5 months in stands of natural vegetation. In R. mucronata initiation of immature propagules was not seasonal and varied among the sites. Fruiting in S. alba was short and seasonal without overlap, and shifts were observed in the timing of flowering, flowering and fruiting peaks. Abscission of reproductive parts started in July and in June, respectively, in the years 2005 and 2006. Fruiting was observed starting in August in both years, and fruiting peak was reached in October in 2005 and one month earlier in 2006. In R. mucronata vegetative and reproductive phenophases significantly correlated with climatic variables, whereas in S. alba only leaf emergence and leaf fall correlated with temperature. The flowering plasticity in the reproductive phenology of the two species indicates possible sensitivity to certain climatic and environmental triggers. Our results also indicate that R. mucronata trees have a distinctly higher investment in the reproductive cycle than S. alba.     

A Range-Expanding Shrub Species Alters Plant Phenological Response to Experimental Warming

Shifts in plant species phenology (the timing of life-history events such as flowering) have been observed worldwide in concert with rising global temperatures. While most species display earlier phenology with warming, there is large variation among, and even within, species in phenological sensitivity to rising temperatures. Other indirect effects of climate change, such as shifting species composition and altered species interactions, may also be contributing to shifting plant phenology. Here, we describe how experimental warming and the presence of a range-expanding species, sagebrush (Artemisia rothrockii), interact to influence the flowering phenology (day of first and peak flowering) and production (number of flowers) of an alpine cushion plant, Trifolium andersonii, in California’s White Mountains. Both first flowering and peak flowering were strongly accelerated by warming, but not when sagebrush was present. Warming significantly increased flower production of T. andersonii, but less so in the presence of sagebrush. A shading treatment delayed phenology and lowered flower production, suggesting that shading may be the mechanism by which sagebrush presence delayed flowering of the understory species. This study demonstrates that species interactions can modify phenological responses to climate change, and suggests that indirect effects of rising temperatures arising from shifting species ranges and altered species interactions may even exceed the direct effects of rising temperatures on phenology.     

REPRODUCTIVE PHENOLOGIES IN LOBELIA INFLATA (LOBELIACEAE) AND THEIR ENVIRONMENTAL CONTROL

Flowering and fruiting phenologies of individual plants and flowers of Lobelia inflata, a North American summer annual, were studied in the field and greenhouse to determine whether onset of flowering and fruit maturation were correlated, and the degree to which these reproductive phenologies were influenced by the environment. Within each of two field populations, larger plants flowered earlier and produced more flowers than smaller plants. Onset of flowering was positively correlated with onset of fruit maturation but not perfectly so. Two factors decreased the intensity of this correlation. First, at the flower level, the earlier a flower bloomed, the longer the resulting fruit took to develop. Second, fruit development times varied significantly among individual plants. In the greenhouse, individuals watered more frequently attained greater size and flowered earlier than individuals watered less frequently. Nutrient additions did not affect plant size or onset of flowering. These results indicate that for the summer annual Lobelia inflata, reproductive phenologies are phenotypically correlated, and that timing of reproduction is resource and size dependent, as it is for other monocarpic plant species.     

Warm springs reduce parasitism of the cereal leaf beetle through phenological mismatch

Variation in weather among years may affect biological control of insect pests by influencing how well matched in phenology specialist parasitoids are with their pest hosts. A 10‐year study in western North America (Utah) revealed greater change with warm versus cool springs in the life cycle timing of the cereal leaf beetle (CLB), Oulema melanopus (L.), than of its principal enemy, the parasitoid wasp Tetrastichus julis (Walker). The beetle laid eggs, and larval populations developed in crop fields earlier on a calendar‐day basis, but nonetheless after more degree‐days had accumulated, in warmer than in cooler springs. The phenology of parasitism by wasps, in contrast, varied little among springs in relation to accumulated degree‐days. Consequently, in warmer springs, larval phenology of the CLB was delayed relative to adult parasitoid activity, and parasitism was reduced. Presently, a significant degree of biological control of the CLB results from parasitism by T. julis. By promoting phenological mismatch between host and parasitoid, however, a warming climate could weaken this biological control of the insect pest.     

Effects of climate and demography on reproductive phenology of a harvested marine fish population

Shifts in phenology are a well‐documented ecological response to changes in climate, which may or may not be adaptive for a species depending on the climate sensitivity of other ecosystem processes. Furthermore, phenology may be affected by factors in addition to climate, which may accentuate or dampen climate‐driven phenological responses. In this study, we investigate how climate and population demographic structure jointly affect spawning phenology of a fish species of major commercial importance: walleye pollock (Gadus chalcogrammus). We use 32 years of data from ichthyoplankton surveys to reconstruct timing of pollock reproduction in the Gulf of Alaska and find that the mean date of spawning has varied by over 3 weeks throughout the last >3 decades. Climate clearly drives variation in spawn timing, with warmer temperatures leading to an earlier and more protracted spawning period, consistent with expectations of advanced spring phenology under warming. However, the effects of temperature were nonlinear, such that additional warming above a threshold value had no additional effect on phenology. Population demographics were equally as important as temperature: An older and more age‐diverse spawning stock tended to spawn earlier and over a longer duration than a younger stock. Our models suggest that demographic shifts associated with sustainable harvest rates could shift the mean spawning date 7 days later and shorten the spawning season by 9 days relative to an unfished population, independent of thermal conditions. Projections under climate change suggest that spawn timing will become more stable for walleye pollock in the future, but it is unknown what the consequences of this stabilization will be for the synchrony of first‐feeding larvae with production of zooplankton prey in spring. With ongoing warming in the world’s oceans, knowledge of the mechanisms underlying reproductive phenology can improve our ability to monitor and manage species under changing climate conditions.     

Quantitative genetics of body size and timing of maturation in two nine-spined stickleback (Pungitius pungitius) populations

Due to its influence on body size, timing of maturation is an important life-history trait in ectotherms with indeterminate growth. Comparison of patterns of growth and maturation within and between two populations (giant vs. normal sized) of nine-spined sticklebacks (Pungitius pungitius) in a breeding experiment revealed that the difference in mean adult body size between the populations is caused by differences in timing of maturation, and not by differential growth rates. The fish in small-sized population matured earlier than those from large-sized population, and maturation was accompanied by a reduction in growth rate in the small-sized population. Males matured earlier and at smaller size than females, and the fish that were immature at the end of the experiment were larger than those that had already matured. Throughout the experimental period, body size in both populations was heritable (h² = 0.10–0.64), as was the timing of maturation in the small-sized population (h² = 0.13–0.16). There was a significant positive genetic correlation between body size and timing of maturation at 140 DAH, but not earlier (at 80 or 110 DAH). Comparison of observed body size divergence between the populations revealed that Q _ST exceeded F _ST at older ages, indicating adaptive basis for the observed divergence. Hence, the results suggest that the body size differences within and between populations reflect heritable genetic differences in the timing of maturation, and that the observed body size divergence is adaptive.     

Response to joint selection on germination and flowering phenology depends on the direction of selection

Flowering and germination time are components of phenology, a complex phenotype that incorporates a number of traits. In natural populations, selection is likely to occur on multiple components of phenology at once. However, we have little knowledge of how joint selection on several phenological traits influences evolutionary response. We conducted one generation of artificial selection for all combinations of early and late germination and flowering on replicated lines within two independent base populations in the herb Campanula americana. We then measured response to selection and realized heritability for each trait. Response to selection and heritability were greater for flowering time than germination time, indicating greater evolutionary potential of this trait. Selection for earlier phenology, both flowering and germination, did not depend on the direction of selection on the other trait, whereas response to selection to delay germination and flowering was greater when selection on the other trait was in the opposite direction (e.g., early germination and late flowering), indicating a negative genetic correlation between the traits. Therefore, the extent to which correlations shaped response to selection depended on the direction of selection. Furthermore, the genetic correlation between timing of germination and flowering varies across the trait distributions. The negative correlation between germination and flowering time found when selecting for delayed phenology follows theoretical predictions of constraint for traits that jointly determine life history schedule. In contrast, the lack of constraint found when selecting for an accelerated phenology suggests a reduction of the covariance due to strong selection favoring earlier flowering and a shorter life cycle. This genetic architecture, in turn, will facilitate further evolution of the early phenology often favored in warm climates.     

Geographic mosaics of phenology,host preference,adult size and microhabitat choice predict butterfly resilience to climate warming

The climate‐sensitive butterfly Euphydryas editha exhibited interpopulation variation in both phenology and egg placement, exposing individuals to diverse thermal environments. We measured ‘eggspace’ temperatures adjacent to natural egg clutches in populations distributed across a range of latitudes (36°8′–44°6′) and altitudes (213–3171 m). Eggs laid > 50 cm above the ground averaged 3.1°C cooler than ambient air at 1 m height, while eggs at < 1 cm height averaged 15.5°C hotter than ambient, ranging up to 47°C. Because of differences in egg height, eggs at 3171 m elevation and 20.6°C ambient air experienced mean eggspace temperatures 7°C hotter than those at 213 m elevation and ambient 33.3°C. Experimental eggs survived for one hour at 45°C but were killed by 48°C. Eggs laid low, by positively geotactic butterflies, risked thermal stress. However, at populations where eggs were laid lowest, higher oviposition would have incurred incidental predation from grazers. Interpopulation variation in phenology influenced thermal environment and buffered exposure to thermal stress. At sites with hotter July temperatures, the single annual flight/oviposition period was advanced such that eggs were laid on earlier dates, with cooler ambient temperatures. The insects possessed two mechanisms for advancing egg phenology; they could advance timing of larval diapause‐breaking and/or shorten the life cycle by becoming smaller adults. Mean weight of newly‐eclosed females varied among populations from 92 to 285 mg, suggesting that variable adult size did influence phenology. Possible options for in situ mitigation of thermal stress include further advancing phenology and raising egg height. We argue that these options exist, as evidenced by current variation in these traits and by failure of E. editha to conform to restrictive biogeographic constraints, such as the expectation that populations at equatorial and poleward range limits be confined to higher and lower elevations, respectively. This optimistic example shows how complex local adaptation can generate resilience to climate warming.     

Asymmetric effects of cooler and warmer winters on beech phenology last beyond spring

In temperate trees, the timings of plant growth onset and cessation affect biogeochemical cycles, water, and energy balance. Currently, phenological studies largely focus on specific phenophases and on their responses to warming. How differently spring phenology responds to the warming and cooling, and affects the subsequent phases, has not been yet investigated in trees. Here, we exposed saplings of Fagus sylvatica L. to warmer and cooler climate during the winter 2013–2014 by conducting a reciprocal transplant experiment between two elevations (1,340 vs. 371 m a.s.l., ca. 6°C difference) in the Swiss Jura mountains. To test the legacy effects of earlier or later budburst on the budset timing, saplings were moved back to their original elevation shortly after the occurrence of budburst in spring 2014. One degree decrease in air temperature in winter/spring resulted in a delay of 10.9 days in budburst dates, whereas one degree of warming advanced the date by 8.8 days. Interestingly, we also found an asymmetric effect of the warmer winter vs. cooler winter on the budset timing in late summer. Budset of saplings that experienced a cooler winter was delayed by 31 days compared to the control, whereas it was delayed by only 10 days in saplings that experienced a warmer winter. Budburst timing in 2015 was not significantly impacted by the artificial advance or delay of the budburst timing in 2014, indicating that the legacy effects of the different phenophases might be reset during each winter. Adapting phenological models to the whole annual phenological cycle, and considering the different response to cooling and warming, would improve predictions of tree phenology under future climate warming conditions.     

Standing geographic variation in eclosion time and the genomics of host race formation in Rhagoletis pomonella fruit flies

Taxa harboring high levels of standing variation may be more likely to adapt to rapid environmental shifts and experience ecological speciation. Here, we characterize geographic and host‐related differentiation for 10,241 single nucleotide polymorphisms in Rhagoletis pomonella fruit flies to infer whether standing genetic variation in adult eclosion time in the ancestral hawthorn (Crataegus spp.)‐infesting host race, as opposed to new mutations, contributed substantially to its recent shift to earlier fruiting apple (Malus domestica). Allele frequency differences associated with early vs. late eclosion time within each host race were significantly related to geographic genetic variation and host race differentiation across four sites, arrayed from north to south along a 430‐km transect, where the host races co‐occur in sympatry in the Midwest United States. Host fruiting phenology is clinal, with both apple and hawthorn trees fruiting earlier in the North and later in the South. Thus, we expected alleles associated with earlier eclosion to be at higher frequencies in northern populations. This pattern was observed in the hawthorn race across all four populations; however, allele frequency patterns in the apple race were more complex. Despite the generally earlier eclosion timing of apple flies and corresponding apple fruiting phenology, alleles on chromosomes 2 and 3 associated with earlier emergence were paradoxically at lower frequency in the apple than hawthorn host race across all four sympatric sites. However, loci on chromosome 1 did show higher frequencies of early eclosion‐associated alleles in the apple than hawthorn host race at the two southern sites, potentially accounting for their earlier eclosion phenotype. Thus, although extensive clinal genetic variation in the ancestral hawthorn race exists and contributed to the host shift to apple, further study is needed to resolve details of how this standing variation was selected to generate earlier eclosing apple fly populations in the North.     

Relationships between Population Size, Genetic Diversity and Fitness Components in the Rare Plant Dictamnus albus in Central Germany

An increasing number of studies support the hypothesis that smaller populations face a higher risk of extinction, and declining population sizes are therefore one of the focal points in plant conservation. In small populations, loss of genetic diversity is often related to reduced reproductive fitness. For the rare Dictamnus albus in Central Germany, an earlier study had already confirmed a significant correlation between population size and genetic diversity. In order to assess whether these variables correlate with fitness components, plant height; flower, fruit and seed production; and germination were studied in a total of 11 populations of different size. In the seven populations that were sampled over two consecutive years, differences among populations and among years were tested using a Two-Way ANOVA. Co-linearity among variables was assessed using principal component analysis (PCA), followed by calculating correlations between ordination axes and both population size and genetic diversity. Plant height and flower number were uncorrelated to the other variables and, together with germination, did not show any correlation to either population size or genetic diversity. However, both size and genetic diversity of populations correlated significantly with other PCA axes that reflected reproductive components such as fruit number, seed number, seed fruit ratio, and seed mass. Our results support the idea that reproduction is hampered in small populations and raise concerns over the loss of genetic diversity in D. albus.     

Flower and Fruit Characteristics in Small and Isolated Populations of a Fleshy-Fruited Shrub

Abstract: During a warm period after the last glaciation the fleshy-fruited shrub, Prunus mahaleb, extended its area of distribution from the Mediterranean basin towards central Europe. Under a subsequent cooler climate, and probably increased competition from trees, the species became restricted to steep, south-facing cliffs and scree slopes with a particularly sunny and warm microclimate. Thus, the small relict populations in central Europe have been isolated for about 5000 - 6000 years. To investigate whether or not reproductive traits of P. mahaleb are affected by isolation and population size, several flower and fruit characteristics were sampled in ten populations in Switzerland. A Canonical Correlation Analysis (CCA) based on these reproductive characteristics differentiated the populations. The first CCA axis was determined by flower size, pollination frequency, pulp and seed mass. This ran parallel to increasing precipitation and decreasing altitude. The second axis correlated with the number of fruits per raceme and seed viability, and ran parallel to increasing sunshine. None of the six flower and fruit variables correlated with population size or with the degree of isolation. However, flower size, pollination frequency, pulp and seed mass showed a negative correlation with altitude. The results suggest that, in populations of P. mahaleb in central Europe, climatic conditions explain more of the observed variation in flower and fruit characteristics than population size or degree of isolation.     

Effects of plant size and weather on the flowering phenology of the organ pipe cactus (Stenocereus thurberi)

Background and Aims

Flowering phenology is a critical life-history trait that influences reproductive success. It has been shown that genetic, climatic and other factors such as plant size affect the timing of flowering and its duration. The spatial and temporal variation in the reproductive phenology of the columnar cactus Stenocereus thurberi and its association with plant size and environmental cues was studied.

Methods

Flowering was monitored during 3 years in three populations of S. thurberi along a latitudinal gradient. Plant size was related to phenological parameters. The actual and past weather were used for each site and year to investigate the environmental correlates of flowering.

Key Results

There was significant variation in the timing of flowering within and among populations. Flowering lasted 4 months in the southern population and only 2 months in the northern population. A single flowering peak was evident in each population, but ocurred at different times. Large plants produced more flowers, and bloomed earlier and for a longer period than small plants. Population synchrony increased as the mean duration of flowering per individual decreased. The onset of flowering is primarily related to the variance in winter minimum temperatures and the duration to the autumn–winter mean maximum temperature, whereas spring mean maximum temperature is best correlated with synchrony.

Conclusions

Plant size affects individual plant fecundity as well as flowering time. Thus the population structure strongly affects flowering phenology. Indications of clinal variation in the timing of flowering and reproductive effort suggest selection pressures related to the arrival of migrating pollinators, climate and resource economy in a desert environment. These pressures are likely to be relaxed in populations where individual plants can attain large sizes.Key words: Flowering phenology, optimal timing, plant size, Sonoran Desert, Stenocereus thurberi, temperature     

Floral phenology and compatibility of sawgrass, Cladium jamaicense (Cyperaceae)

Sawgrass, Cladium jamaicense, is the dominant macrophyte in the Florida Everglades. We examined sawgrass flowering phenology and compatibility reactions in ex situ and in situ populations over 2 yr. Sawgrass flowers in May in southern Florida. Flower maturation was relatively synchronous within an inflorescence. Along the entire inflorescence, functionally male flowers emerged initially, followed by stigmas, then anthers of hermaphroditic flowers. Flowers of each sex expanded over 2 d with less than 1 d in between, totaling 6- 7 d for an inflorescence to complete flowering. Hand pollinations showed that sawgrass was self-compatible and not pollen-limited, because open pollinations produced fruit set similar to self- and cross-pollinations. Fruit set was low in autogamy and manipulation treatments. Manipulation treatments were used to study the effect of exposure to airborne pollen during hand pollinations. This treatment thus provides a useful technique for studies on the in situ compatibility of wind-pollinated graminoids. Sawgrass was able to self-fertilize, but the timing of flower maturation on an inflorescence promoted outcrossing. Actual outcrossing rates in sawgrass thus depend on clonal architecture and the timing of floral maturation on other inflorescences within a clone rather than on inflorescences of other genets in a population.     

Fecundity limits in Frangula alnus (Rhamnaceae) relict populations at the species’ southern range margin

The geographic range of many temperate plant species is constrained by climate, but it remains little known how climate affects population performance at low-latitude range margins. This study investigated the reproduction of the Eurasian tree Frangula alnus in relict populations near its southwestern range limit in southern Spain. The aim was to identify the principal stages and causes of ovule loss experienced by these marginal populations. More than 6,800 flowers were monitored over 2 years, insect observations and different experiments were carried out to assess levels of pollen and resource limitation, as well as the influence of flowering phenology on seed production. Most ovule losses occurred during flower anthesis and were due to strong cross-pollen limitation. Fruit set was affected by tree size, light regime and flowering phenology, probably through their effects on pollinator behaviour. Fruit set was almost zero throughout the first half of the flowering season. Then it increased paralleling changes in pollinator abundance but was soon overridden by increasing ovule desiccation due to summer drought. Successful seed production was mostly confined to a brief period near the end of the flowering season. Adverse weather during this period in the second study year resulted in a threefold bud and flower mortality and a 50% decrease of fruit production. Spring rainfalls in southwestern Spain have diminished more than 30% through past decades leading to an earlier onset of summer drought. This trend and its adverse effects on seed production may contribute to explaining the recent decline of F. alnus at its southwestern range limit.Electronic Supplementary Material Supplementary material is available in the online version of this article at