Doctoral Dissertations

Date of Award

5-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

Clifford C. Amundsen

Committee Members

Harold R. DeSelm, Lou J. Gross, John C. Rennie

Abstract

An analysis of the temporal dynamics of tree species diversity was made for empirical forests of the States of Arkansas and Mississippi, USA. These two states each had four comprehensive, variable radius plot measurements over a period of three decades. The data came from the Forest Survey, administered by the USDA Forest Service. An important factor contributing to the rigorousness of this study was that the same sample design, involving a minimum of 2,623 plots in Arkansas and 2,806 plots in Mississippi, was implemented in both states over these four survey measurements.

In addition to tree species data, the surveys provided three physiographic categories, three ownership categories, and five stand-size classes. Two measures of tree species diversity were applied to the survey data: (1) tree species richness; and (2) the Mclntosh measure of evenness. The Sorensen quantitative index of similarity was used to contrast tree species changes between survey periods. Data treatment involved applying the two diversity measures and the plot similarity measure to each forested plot in each state for all four surveys. Summary statistics were then derived, along with graphs, to describe the trends of these three metrics through the four survey measurements.

Repeated measures ANOVA tests were conducted to test for significant differences in diversity measures across time and between specific group strata. All tests were conducted at the 0.05 probability level. The tests conducted were of statistical hypotheses. In all cases, the null hypothesis (no difference between respective means) was the subject of statistical testing.

Maps showing spatial distribution were created to show patterns of the three diversity measures across each state. The maps were constructed by aggregating plot-level data to the county level so that each county represented an average value for each diversity metric.

Major findings from this study showed tree species richness levels have declined at the state level for Arkansas and Mississippi. This was confirmed by examination of the empirical evidence and results fi-om the repeated measures ANOVA test (p < 0.0001). Arkansas tree species richness declined by 3.4 percent since 1968, and Mississippi tree species richness declined 10.6 percent fi-om its highest level in 1977.

An additional important revelation was the notable impact plantations were having on tree species richness at the state level. Tests were conducted to discern differences in species richness at the state level by contrasting data sets which included plantations with data sets that excluded plantations. The data showed that the increasing implementation of plantation forestry is having a negative impact on the overall average tree species richness for each state. As the areal extent of plantations continues to increase, their usual monotypic stand structure will continue to pull overall diversity measures of individual states downward. In 1995, plantations contributed to the decrease of Arkansas' tree species richness by 14.1 percent, in contrast to the data set excluding plantations. Mississippi's 1994 tree species richness was reduced by 28.9 percent in a similar comparison.

The absence of harvesting allowed forest land to increase in tree species richness. Plots that had no harvesting activity over the four survey measurements (survivor plots), significantly increased in tree species richness. Arkansas tree species richness increased on these plots by 21.6 percent between 1968 and 1995 while Mississippi increased 43.8 percent from 1967 to 1994.

Tree species richness has declined in these two Midsouth states during four survey measurements covering the mid 1960's to the mid 1990's. The study did not use a causal inference design but did test statistical hypotheses to confirm real differences in the data. Corroborative evidence, supported by empirical evidence in the data, suggests that intense and increasing forestry practices (harvesting and plantation forestry) have attained a level high enough to contribute to the reduction in overall tree species diversity in Arkansas and Mississippi.

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