Year of Award

2011

Document Type

Thesis

Degree Type

Master of Interdisciplinary Studies (MIS)

Degree Name

Interdisciplinary Studies

Department or School/College

Interdisciplinary Studies Program

Committee Chair

Paul Alaback

Commitee Members

Steve Siebert, Rustem Medora

Keywords

demography, herbaceous understory, life-history characteristics, long-lived perennial, northern Rockies, population structure, spring ephemeral, Trillium

Abstract

Trillium ovatum is a long-lived, spring ephemeral that grows in moist forest habitats of the western United States. Extensive investigations of Trillium, including T. ovatum, have been accomplished, yet no prior studies of T. ovatum have been conducted at the eastern edge of its distribution in Montana, where populations experience increased aridity and highly variable climatic conditions as compared to other regions where it occurs. This study examines life-history characteristics and demography of T. ovatum in three representative populations that were sampled over 9 growing seasons to determine: 1) life history stages, stage class structure, and yearly transitions among stage classes; 2) age and size of plants and relationships among age, size, and stage class; 3) minimum age of reproduction and fecundity; 4) forest structure and site characteristics. Life-history stages of T. ovatum include: cotyledon; one-leaf vegetative; three-leaf vegetative (juvenile); three-leaf reproductive (flowering); and three-leaf nonflowering regressive stages. Juvenile plants comprise the majority of each population. Flowering plants represent the only means of reproduction, and plants are slow to reach sexual maturity, which takes a minimum of 18 years. Age and number of seeds produced are positively correlated with individual plant size. Yearly fecundity measures of fruit set, number of seeds produced per plant, and seeds per square meter are highly variable. Trillium exhibit stage class regressions, with an average of 27% of reproductive adults periodically retrogressing to a nonflowering stage. Dormancy was observed in all adult stage classes. Mortality is generally low (<1.5%), and yearly tallies of new plants showed that more plants were ‘recruited’ into the population than died, but no clear changes in population density were documented during the period of this study. Trillium ovatum face many of the same threats as other forest understory species—logging, overharvest for horticultural/medicinal use, competition from nonnative species, and overgrazing. Low fecundity and recruitment may limit the ability of these populations to recover from stress or mortality events, and make them sensitive to climatic changes and increases in fire disturbances that are likely to occur in this region. Continued long-term monitoring will be invaluable for understanding responses of long-lived forest understory species to the complex interplay of these environmental stresses.

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