Masters Theses

Date of Award

5-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Botany

Major Professor

Edward E. C. Clebsch

Committee Members

Gary L. Walker, Mitchell B. Cruzan, Sally P. Horn

Abstract

In the southern portion of its disjunct range, northern white cedar (Thuja occidentalis L.) is a glacial relict that typically occurs on north-facing, limestone cliffs along deeply entrenched stream and river valleys. Cliff Thuja populations occur across a mosaic of physiographic provinces that comprise a heterogeneous landscape of variable topographic, lithologic, and climatic conditions. Habitat heterogeneity is observed within cliff sites, where Thuja stems occur on sheer cliff faces and less frequently along thin corridors cliff top and/or cliff bottom habitat. Several associate species, including additional relict ones, are also distributed among these cliff habitats I sampled and described the cliff environment for eight Thuja sites from April to October of 1994 using 10 m wide horizontal and vertical belt transects Leafy twig tissue was collected at each site from all individual stems occurring within transect boundaries Eleven allozyme loci were used to describe the genetic structure within and among cliff populations.

I found significant environmental differences in soil pH and nutrient concentrations within and among cliff Thuja sites. Between site variation was attributed to changes in underlying lithology among physiographic provinces Environmental heterogeneity within sites supported two distinct microhabitats - the cliff top and cliff face across all sites examined. A combination of autochthonous and allochthonous nutrient inputs via physical and chemical weathering processes, surface transport of soils and detritus in solution, and the subsurface percolation of water through mineral-rich bedrock may all influence the observed edaphic differences between microhabitats.

Patterns of genetic structure among cliff populations indicate that genetic drift following population fragmentation and differential rates of northward migration among physiographic provinces throughout the present interglacial period may have had a pronounced effect on the distribution of genetic variation. Moreover, these data suggest that the effects of genetic drift were probably less severe in physiographic provinces containing a greater abundance of suitable habitat and/or natural north-south oriented corridors of migration, and more severe in regions containing less suitable environmental conditions. A significant partitioning of genetic information within populations suggests that stems occurring on the cliff face contain greater genetic variation than those on the cliff top. Sampling design and spatial heterogeneity between stems from different microhabitats may account for the observed differences It is also possible that this pattern illustrates the effects of intraspecific competition on the establishment of cliff top seedlings. The presence allelic combinations in cliff face stems that could have been derived from cliff top parents suggests that the cliff face may provide environmental conditions, or safe sites, necessary for seedling establishment. This conclusion is supported by the observation of many more seedlings on cliff faces and increased shading on cliff tops. An alternative explanation is that Thuja populations may have been restricted exclusively to cliff faces during the warmest part of the Hypsithermal Interval, where they maintained genetic variation by the asexual production of modules Cliff face restricted populations may then have resumed sexual reproduction in less hostile environmental conditions and have since expanded to the cliff top. If this explanation is correct, one would expect the former scenario of competitive exclusion to be a relatively recent population dynamic

It is likely that events of natural history since the last glacial maxima, life- history characteristics (especially the use of alternative reproductive systems), intrapopulation interactions, and environmental heterogeneity have all acted to produce the present genetic structure exhibited by cliff populations of northern white cedar in its southern disjunct range.

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