Thomas Elmqvist

Pollination by deceit, floral sex ratios and seed set in dioecious Rubus chamaemorus L

SummaryMale and female flowers of the dioecious perennial herb Rubus chamaemorus L. are similar in general appearance. However, female flowers are somewhat smaller, do not produce any pollen, and contain very small amounts of nectar. Syrphids and bumblebees, which are important pollinators of R. chamaemorus, showed a strong preference for male flowers. Male flowers were also less often rejected by flower visitors than were female flowers, and two different groups of syrphid species stayed longer in male than in female flowers. These observations suggest that female flowers of R. chamaemorus attract pollinators by deceit.Hand-pollination experiments indicated that pollen availability limited seed production of R. chamaemorus in female dominated habitats but not in areas with an equal floral sex ratio. We suggest that the relative importance of factors limiting female reproductive success is not constant, but is influenced by the floral sex ratio of the population. This should apply also to other dioecious species that show variable sex ratios on either a local or regional scale.

The evolution of vivipary in flowering plants

Vivipary in flowering plants is defined as the precocious and continuous growth of the offspring when still attached to the maternal parent. Two main types, true vivipary (involving sexually produced offspring) and pseudovivipary (asexual offspring), may be identified. Vivipary has been described from slightly less than a hundred different species of flowering plants, of which we classify approx. 50% as having true vivipary, with the remaining species being pseudoviviparous. Species with true vivipary tend to inhabit shallow marine habitats while those with pseudovivipary are mostly found in arctic, alpine, or arid habitats. We suggest that all of these habitats are characterized by extraordinarily coarse-grained environments for seedling establishment, albeit with major differences in patch size. In all of these situations the probability of an offspring being dispersed in time or space to a patch better than the parental patch is very low, hence taxa with seed dormancy or seed dispersal mechanisms enjoy no particular advantage. We suggest that future research focus on species that are facultatively pseudoviviparous, as well as on comparisons of ecological and biomechanical aspects of viviparous and non-viviparous mangroves and seagrasses.

Epidemiology of anther-smut disease (Microbotryum violaceum) and numeric regulation of populations ofSilene dioica

SummaryThe interaction between the anther smutMicrobotryum violaceum and its hostSilene dioica was studied during 1985–1990 in 47 populations of different ages, sizes and densities, in an archipelago area in northern Sweden. The sizes of these populations had also been surveyed in the early 1970s. Our results indicate that establishment ofMicrobotryum violaceum is host-size and density dependent. Firstly, young populations ofSilene dioica that became diseased during the study were larger and tended to be more dense than young populations that remained healthy. Secondly, populations diseased in both 1985 and 1990 were found to be larger and tended to be more dense than populations healthy in both years. We were able to document that the pathogen actually failed to establish in two small young populations (diseased plants died shortly after they appeared) and did go extinct in one small old population. Disease incidences within populations did not show large fluctuations between years. The highest increases in disease incidence during the study were found in three relatively young populations that were disease-free at the start of the study. Older populations highly diseased at the start showed less of an increase. Our study indicates thatMicrobotryum violaceum acts as a regulatory factor influencing the rate of increase inSilene dioica populations, once they are sufficiently large to maintain the pathogen. Firstly, seedling density decreased with increased incidence of disease and a seed addition experiment indicated seed-limited recuritment in highly diseased populations. Secondly, those populations that reached very large sizes or densities were either healthy or had very low incidences, indicating the potential for populations that for one reason or another escape an epidemic. However, in the comparison of changes in population size over 16–18 years there was no simple correlation between expansion rate and disease incidence.

Effects of tropical cyclones Ofa and Val on the structure of a Samoan lowland rain forest

In February 1990, tropical cyclone Ofa struck Western Samoa with very strong winds (in excess of 200 km/hr). In December 1991, less than 22 months later, tropical cyclone Val struck the same area with similar intensity. In the moist lowland forest of the Tafua Rain Forest Reserve, Savaii, Western Samoa, we examined the effects of the two cydones on forest structure, tree mortality, and interspecific differences in damage. Average mortality of trees was high after both cyclones (28% and 33%, respectively). In one forest area, subject to a fire after the first storm, mortality was more than 90 percent. The frequency of uprooted trees was 3 1 percent after Ofa, but only 16 percent after Val. Uprooting was significantly more frequent among species lacking buttresses or stilt roots. As a combined effect of the two cyclones, the lowland forest of Tafua suffered a 53 percent tree mortality, with remaining standing trees being severely damaged (topped and with a substantial reduction of main branches). Average tree density dropped from an estimated 476 trees/ha (>5 cm DBH), before Ofa to 225 trees/ha after Val. Existent gap sizes are reflected by the changes in mean canopy cover which decreased from nearly 100 percent before Ofa to 27 percent after Val. Post-cyclone recovery is often observed to be very rapid, but the very large gaps created in the Tafua forest and the simultaneous loss of the whole guild of vertebrate seed dispersers (flying foxes and fruit pigeons) suggests it will be a long time before the upper canopy is once again closed.

Restricted pollination on Oceanic Islands : Pollination of Ceiba pentandra by flying foxes in Samoa

The very low diversity of animal pollinators on remote oceanic islands can act as a potent biotic filter to potential plant immigrants. To assess the relative difficulties faced by colonizing plant species, we studied the pollination ecology of kapok, Ceiba pentandra (Bombacaceae), a widespread aboriginal introduction to Pacific islands. The Samoan form of this tree is self-fertile, but requires pollination for fruit set. In continental areas C. pentandra attracts a diverse assemblage of pollinators, but in Samoa it is pollinated only by Pteropus tonganus (Megachiroptera: Pteropodidae)an example of a plant exclusively dependent on one bat species for pollination. The pollination system is dystrophic, however, since foraging flying foxes may destroy up to 50 percent of C. pentandra flowers and developing fruits. Though resource defense is regarded as rare in nectarivorous bats, P. tonganus vigorously defends small feeding territories on the rich, but readily depleted, C. pentandra nectar resource. This intraspecies conflict probably shapes patterns of pollen transfer within and among trees, and enhances cross-pollination. Our study highlights the importance of conserving pollinators in remote oceanic islands with depauperate pollinator faunas.

Differences in response to defoliation between males and females of Silene dioica

SummaryThe response by male and female plants to herbivory was studied by experimental defoliation of the dioecious perennial herb Silene dioica in a green-house. Male and female plants were defoliated prior to and during the early flowering phase at two intensities (50% and 100% of leaf-area removed) in two consecutive years. Defoliation resulted in a decrease in the number of flowers initiated in both sexes, while a larger delay of peak flowering and a higher mortality was observed in males compared to females. In female plants, severe defoliation resulted in a reduction in seed number per capsule and in seed size compared to control. Females showed a negative correlation between the production of flowers in the first and second season in all treatments, while flowering in males the first season was not correlated with flowering in the second season. Females also showed a lower frequency of flowering than males during the two seasons studied. However, during the flowering period, males allocated significantly more biomass to flowers than did females. This outcome supports the idea that females may have a higher total reproductive expenditure than males, but males have a higher reproductive effort during flowering. Male rosette leaves were significantly preferred by the generalist herbivore Arianta arbustorum in experiments. This preference was most pronounced in trials with leaves from fertilized plants compared to nonfertilized plants. A greater storage of resources in aboveground leaves during winter by males compared to females may explain the higher preference for male leaves and the higher male mortality following early defoliation. Furthermore, males are smaller than females and may have a lower ability than females to replace lost resources needed for reproduction when defoliated early in the season.

Sexual dimorphism and between-year variation in flowering, fruit set and pollinator behaviour in a boreal willow.

In Salix myrsinifolia-phylicifolia, males and females produced approximately the same number of catkins per individual, but males produced more flowers per catkin. Females had a higher concentration of sugars in the nectar than males, but the standing crop of nectar did not differ between sexes. Females had a hexoserich nectar, males a sucrose-dominated nectar. Bumblebees discriminated between male and female willows, but their preference changed during the day and differed between bumblebees with different tongue length. The relative abundance of different species of bumblebees varied considerably between years. Hand-pollination in 2 populations significantly increased both fruit and seed set per catkin. Production of flowers in female plants showed a large between-year variation. Both average fruit set and seed set per fruit were <50% during 1981-1985. Fruit set showed greater variation than did seed set per fruit. The early time of flowering and the strongly female biased sex ratios commonly observed in populations of this willow (60-80% females), increase the risk of pollen limitation. The production of many more flowers than fruist may represent a bet-hedging strategy to match the uncertainty in the conditions for fruit maturation, although it may also be of value as it potentially allows the plant to selectively mature fruits of a high genetic quality. -from Authors (Less)

Tropical Rain Forest Recovery from Cyclone Damage and Fire in Samoa1

In 1990 and 1991, Samoa was struck by two cyclones, Ofa and Val. In the Tafua Rain Forest Preserve on the island of Savai’i, one part of the forest also burned after the first cyclone. Here we report on patterns of regeneration and changes in tree species composition in the Tafua lowland rain forest after five years of recovery from cyclone and fire disturbance. In the unburned area, tree canopy cover increased from 27 percent after the last cyclone to 58 percent, and in the burned area from below 12 to 49 percent. Nine of the ten most common tree species decreased in relative abundance in the entire forest after the last cyclone. One fast growing pioneer species, Macaranga harveyana, now makes up 42 percent of the total number of trees (.5 cm DBH) in the unburned area and 86 percent in the burned area. Large interspecific differences occur in size distribution and there are at least four distinguishable regeneration patterns, which may be related to shade tolerance. Mean number of species per plot was generally higher in the unburned area than in the burned area, while the Shannon evenness index was higher in the unburned than in the burned area only for trees above 1 cm DBH. Species with fruits known to be fed upon by birds and/or bats generally made up a larger proportion of all trees in the burned than in the unburned area. In contrast to other studies of postcyclone regeneration, in which recovery is often rapid due to resprouting of trees, recovery in the Tafua forest was a slow process with regeneration more dependent on vertebrate seed dispersal than on resprouting.