Timing of Tree Density Increases, Influence of Climate Change, and a Land Use Proxy for Tree Density Increases in the Eastern United States

Brice B Hanberry

doi:10.3390/land10111121

Abstract

Long-term observations inform relationships among changes in vegetation, climate, and land use. For the eastern United States, I compared the timing of tree change, comprised of density and diversity increases, with the timing of climate change, as measured by change point detection of the Palmer Modified Drought Index (PMDI) that accounts for water balance, in two prairie ecological provinces, four grassland landscapes, and four forest landscapes. Historical evidence supplied documentation of tree density increases between approximately 1860 and 1890 in the two prairie provinces of grasslands bordering eastern forests. Additionally, because timing of tree increases paralleled when land area reached ≥25% agricultural use, I categorized grassland and forest landscapes that increased to ≥25% agricultural area during 1860, 1880, 1900, and 1920. One change point detection method identified no significant PMDI change points during the 1800s. The other method found the southern prairie province, bordering eastern forests, had change points of 1855 and 1865 during an interval of relative dryness. Only two of four grassland landscapes, and one of four forest landscapes had change points, which occurred during relative dryness or were continuous with historical variation. Inconsistent changes in moisture availability did not provide correlations with comprehensive tree increases, but land use change corresponded with tree changes based on timing, magnitude and direction of change, and mechanism. The agricultural threshold may provide the critical missing component that allows progression in analysis of land use change effects on vegetation.

Highlights

Historical records impart the necessary context to identify general timing and magnitude of vegetation changes, and to separate land use and climate influences on vegetation changes
In the eastern United States, precipitation has generally increased during the 1900s, since the 1970s, which can result in confounding climate change with wholesale land use changes since Euro-American settlement [1,2,3]
Climate changes that increase or decrease moisture and continuity of herbaceous fuels may affect fire frequency. This approach to determine timing of vegetation change in tree density and diversity accounted for both climate and land use change

Summary

Introduction

Historical records impart the necessary context to identify general timing and magnitude of vegetation changes, and to separate land use and climate influences on vegetation changes. The degree to which climate and land use contribute to changes in vegetation is not established. Examination of current trends in tree density or diversity during recent decades, without consideration of comprehensive forest transitions during past centuries, will not discern whether trends are new, or long-standing transitions due to changing baseline conditions. Depending on the timing of when tree change is first documented to occur, examination of the corresponding climate may be decades or centuries too late. In the eastern United States, precipitation has generally increased during the 1900s, since the 1970s, which can result in confounding climate change with wholesale land use changes since Euro-American settlement [1,2,3]

Methods

Results

Discussion

Conclusion