A reevaluation of the use of rhizome scars to age plants of Trillium erectum (Melanthiaceae)1

PREMISE OF STUDY

Herbaceous perennials are important long-lived plants in North American forests. Trillium has been used as a model organism to examine the effects of ecological processes on age structure in herbaceous forest perennials. Here, the methods of aging Trillium rhizomes are critically examined. •

METHODS

Rhizomes of seedlings, single-bract plants, three-bract nonflowering plants, and flowering plants of Trillium erectum were examined. The patterns of cataphyll and scape scars on rhizomes were examined with respect to demographic category. •

KEY RESULTS

Trillium rhizomes produce two cataphyll scars per year on single-bract, three-bract nonflowering, and flowering plants. Scape scars were only evident on rhizomes of three-leaved nonflowering and flowering plants. The percentage of intact rhizomes ranged from 10-67% for three-bract nonflowering plants, and 0-51% for flowering plants. Rhizomes in all demographic categories had evidence of recessing tissues from the oldest portion of the rhizome indicating that accurate age estimates are not possible on many plants. •

CONCLUSIONS

Accepted methods of aging Trillium rhizomes have significant drawbacks. The primary problem is that rhizomes rot from the oldest portions in all demographic categories. A second problem is that plants producing multiple scape scars in a given year could mistakenly be counted as multiple years. Finally, confusing terminology and misrepresentations in Trillium literature suggests that many previous studies did not correctly determine age. Given the challenges of aging Trillium, we suggest that future studies use rhizome aging to study early demographic stages only and the ecological processes that influence their growth.

The evolution of self-compatible and self-incompatible populations in a hermaphroditic perennial, Trillium camschatcense (Melanthiaceae)

The evolution of selfing from outcrossing ancestors is known to have occurred repeatedly in angiosperms. Theoretical studies have argued that the transition from outcrossing to selfing is accomplished more easily than the reverse case, and phylogenetic analyses involving self-compatible (SC) and self-incompatible (SI) species has basically supported this assumption. The evolutionary direction of self-compatibility and self-incompatibility was examined in Trillium camschatcense, which contains geographically widespread SC populations, and restricted SI populations. Ecological surveys have revealed that the SC populations were suitable for outcrossing, and selfing in these populations did not confer any fitness advantage. Since reproductive fitness indicates the possibility of an evolutionary shift from self-compatibility to self-incompatibility, the phylogenetic relationships of SI and SC populations of T. camschatcense were investigated based on cpDNA variations and nuclear DNA microsatellite polymorphisms. Although phylogenetic analyses did not provide credible evidence to determine evolutionary direction, the SI populations turned out to be monophyletic with extremely low genetic differentiation. Based on these results, we proposed two possible scenarios for the evolutionary backgrounds of SI and SC populations in T. camschatcense. The plausibility of each scenario was evaluated based on the reproductive and geographical features of the mating systems.

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.

Antagonistic effects of seed dispersal and herbivory on plant migration

The two factors that determine plant migration rates - seed dispersal and population growth - are generally treated independently, despite the fact that many animals simultaneously enhance plant migration rate via seed dispersal, and decrease it via negative effects of herbivory on population growth. Using extensive empirical data, we modelled the antagonistic effects of seed dispersal and herbivory by white-tailed deer on potential migration rates of Trillium grandiflorum, a forest herb in eastern North America. This novel antagonistic interaction is illustrated by maximum migration rates occurring at intermediate, but low herbivory (< 15%). Assuming herbivory < 20% and favourable conditions for population growth during post-glacial migration, seed dispersal by deer can explain rates of migration achieved in the past, in contrast to previous models of forest herb migration. However, relatively unfavourable conditions for population growth and increasingly intense herbivory by deer may compromise plant migration in the face of present and future climate change.

Ecology: Linking Species Diversity and Genetic Diversity

Although there is a great deal of interest in the biological diversity of species and of genes, it is only recently that researchers have begun to investigate the processes that exert parallel influences on these different levels of diversity.

Floral density, pollen limitation, and reproductive success in Trillium grandiflorum

Decreases in floral density can disrupt mutualistic interactions between plants and their pollinators, and decrease reproductive success. I addressed the relationship between floral density and plant reproductive success using two experimental approaches: a pollen supplementation experiment in 12 populations of Trillium grandiflorum that naturally varied in floral density, and a transplant experiment in which floral density was manipulated in plots at four experimental sites. In the pollen supplementation experiments, the degree of pollen limitation, in terms of fruit set and seed set, decreased with floral density. Further, in the experimental sites, plant reproductive success increased asymptotically with floral density. These results demonstrate the value of simultaneously conducting experiments in both experimental sites and natural populations to understand how population density influences plant reproductive success. Factors that reduce the density of this perennial herb, such as habitat fragmentation and herbivory by white-tailed deer ( Odocoileus virginianus), should be expected to limit its reproduction.