EVENT DENDROGEOMORPHOLOGY OF HURRICANE SANDY TREEFALL, TYLER STATE PARK, PENNSYLVANIA

Field observations following the passage of Hurricane Sandy documented a widespread area of treethrow far beyond its landfall near the Delaware Bay. Whereas most uprooted trees have been removed from residential areas, at Tyler State Park (Bucks County, Pennsylvania) many toppled trees reflect the passage of the storm. At two sites bordering Neshaminy Creek, trunk length measurements (~ minimum original tree height) and breast-height diameters (DBH) of fallen white oak and beech were complemented with dendrochronological sampling. Orientations of twenty large samples (trunk length: 12-37 m; DBH= 0.4-1.0 m) group in a westerly direction (200-300º), with nine clustering within the 270-280º sector. The few trees pointing in easterly directions yielded older ages. Although there are numerous dendrogeomorphological examples of slope creep (rows of gently bent trunks) and rockslides (tilted trees in growth position), no relationship exists between the slope azimuth (mostly N and NE) and orientation of the most recent treefall. This indicates a common external force, consistent with an intense, sustained wind field. The orientation of uprooted trees in Tyler State Park coincides with those in adjacent developments and indicates that most damage was induced by high-speed near-surface easterly winds along the leading edge of Hurricane Sandy. Few trees show effects of earlier fire damage, above-ground breakage (stump in place), and several oriented clusters point to simultaneous falls facilitated by the weight of upwind trees and partially overlapping root masses. Treethrows removed large amounts of soil (root plate diameter up to 4.6 m), with massive crowns likely facilitating the toppling. In the future, coring of surviving trees will be used to evaluate the effect of the storm on ring width, providing a potential geochronological event marker. Our findings show that large oriented treefall remaining in undeveloped and protected sites can provide long-term record of hurricane-induced windthrow. When analyzing abundant fossils and impressions of trees, from Paleozoic to Quaternary successions, in addition to mass-wasting and fluvial transport, intense storms should be considered as possible triggers of preferentially oriented samples.