Doctoral Dissertations

Date of Award

6-1987

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Botany

Major Professor

Edward E. C. Clebsch

Committee Members

Hazel R. Delcourt, Hal R. DeSelm, Gary F. McCracken

Abstract

Northern white cedar, (Thuja occidentalis L.), occupies a main range surrounding the Great Lakes region of North America. In the southern portion of its range, this species occurs in disjunct populations both in the Ridge and Valley Province and in surrounding areas of the Southern Appalachians. Previous researchers have proposed that these populations are relict from a once-continuous distribution during the last glacial maximum (20,000 years before present). In the southern part of its range, northern white cedar often occurs on calcareous substrates and is unusual among boreal disjunct species in the south since it occurs at relatively low elevations where these substrates are more common.

Other taxa with more northerly ranges occur within stands of northern white cedar at some sites in its southern disjunct range. The presence of persistent seeps and northerly, protected aspects are necessary site factors for sustaining populations of other disjunct taxa in addition to northern white cedar.

The genetic composition of stands as revealed by allozyme analysis indicates the conservation of genetic variation by the use of an asexual breeding system in some sites where conditions are not conducive to reproduction by sexual means. In sites where there is a history of severe disturbance the populations are almost entirely monogenic even though there is often evidence of abundant sexual reproduction. In other sites without evidence of recent disturbance and where conditions are suitable for sexual reproduction, the genetic composition of stands is more variable than in any other sites examined in the main or disjunct ranges of this species. I propose that genetic diversity within these stands results from the use of alternate breeding systems over a long period of time, possibly extending back before the last glacial maximum in some populations east of the Appalachians.

The diameter class distributions within populations of northern white cedar reflect the disturbance history of these stands and the reproductive strategies of northern white cedar. Population increases and decreases during periods of climatic fluctuations and anthropogenic disturbance are consistent with the present genetic composition of these stands. There is evidence that in its southern disjucnt range, northern white cedar potentially is valuable as a dendroclimatic indicator of past weather conditions in this region.

The results of this study indicate the value of these southern disjunct populations of northern white cedar not only as a model for the investigation of the population dynamics of a glacial relict species, but also as a conservatory of genetic and species diversity.

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