Date of Award

6-2009

Document Type

Union College Only

Degree Name

Bachelor of Arts

Department

Environmental Science and Policy

First Advisor

John Garver

Language

English

Keywords

tree, arbology, hemlock, precipitation, slope

Abstract

Tilted Tsuga canadensis (Eastern Hemlock) were cored on the Bowman Creek slump (N 42.80306, W -74.25895) and the tree-ring analysis allows reconstruction of periods of slope instability. During ground movement, trees tilted due to the stress of movement, and conifers respond by growing eccentrically with larger rings with reaction wood on the downslope side of the tree, and smaller rings on the upslope side. The difference on ring width can be ascribed to tree tilting. The slump is complex and it has three “steps” that likely correspond to where slip surfaces intersect the surface, and the lowest part slipped about 0.5 to 1.0 m in July 2008. The following chronology of tree tilting and ring asymmetry that is likely related to ground movement is recognized: 1) Recent movement is well recorded across the slump and some trees are currently the most eccentric rings that have grown in the last 175 yr. Tree A6 grew thick rings of reaction wood three years prior to failure in 2008, which suggests ground deformation preceded slip. All trees on the upper part of the slump (C) have wildly eccentric rings (2005-P), which suggest slip is imminent if the same pattern follows (to A6), and this overall recent activity is likely related to the fact that the area is experiencing enhanced precipitation. 2) All tilted trees across the slump show eccentric growth in annual rings beginning in 1970, with a recovery phase from this event lasting until 1990. Very high regional precipitation in the 1970’s is likely related to this period of landsliding and slope instability. The data from this interval suggest that after the movement of the slump was initiated at the bottom (A), the motion of flow slowly propagated upslope until the upper portions of the slump, (B) and then (C) had motion that followed successively over a period of 6-10 yr. 3) 1942-1946 appears to be a single, short-lived event that is also recognized in the Plotterkill Preserve. 4) 1873-1880 may have been when a slip occurred that caused a canopy gap recorded in rapid release from suppression growth from c. 1880 to 1910. 5) 1828-1833 may have been a time of instability on the slope as indicated by only a single tree (B4), which shows eccentric growth and reaction wood. This research is crucial for regional and local planners, because the main result suggest that we have entered a period of enhanced slope instability. It is important, to map and understand urban settings that might be affected by potential slope failures during this period of enhanced precipitation.

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