214 ing through the colluviums, causing saturation and the loss of interparticle cohesion. In the Otter Creek watershed, colluvium is formed by weathering and erosion of the Fort Payne Forma- tion (siltstone, shale, and chert with limestone lenses). The col- luvium is composed of silt- to clay-sized silicates with fragments as large as boulders. During May 1–2, 2010, the rainfall records were broken with more than 43 cm of rainfall in the Otter Creek watershed (National Weather Service 2010). The rock and soil slides formed during May 1–2, 2010 in Radnor Lake State Natu- ral Area are hydrogeomorphically similar to those described for the treeless developments studied by Miller and Wiethe (1975), except the slides occurred in dense forests (Loeb et al. 2010). Field Measurements During June 2010, field measurements were taken in all of the Radnor Lake State Natural Area landslides caused by the May 1–2, 2010 rainfall that felled and killed trees and saplings. Based upon the presence or absence of large rocks embedded in and bedrock exposed under the root plates, the landslides were di- vided into two groups: soil slides (no rock evident) or rockslides (rock visible). Diameter at breast height (DBH; 1.37 m height) was measured for the trees (DBH ≥ 10 cm) and saplings (10 cm > DBH > 2.5 cm) that fell following the rainfall of May 1–2, 2010 and died as indicated by separation of the root mass from the ground and the leaves were brown. Percent slope was mea- sured at each tree and sapling fall. Species were classified as hav- ing a surface root system, including red maple (Acer rubrum), sugar maple (Acer saccharum), common hackberry (Celtis oc- cidentalis), sugarberry (Celtis laevigata), and willow (Salix Loeb and King: Landslides and the Urban Forest spp.), versus tap root or heart root systems following Bernatzky (1978), Dirr (2009), and Wray (1997). There are several mod- els which define the root plate (Coder 2010); however, none fit the situation of trees lost to landslides. The method used to mea- sure the root plate diameter and root plate depth used by Gas- son and Cutler (1990) in studying uprooted trees following an extraordinary gale is also appropriate for landslides. Root plate diameter was measured parallel to the scarp face to the extrem- ity of the longest root exposed. Root plate depth was measured from the soil surface at the center of the stem to the endpoint of the longest exposed root aligned with the center of the stem. Statistical Analysis The data analyzed were tree and sapling stem counts by species; basal area; root plate diameter and root plate depth; and percent slope. Table 1 contains the rockslide and soil slide data for stems of trees and saplings by species, and results from the 2009 sur- vey of Radnor Lake State Natural Area (Loeb et al. 2010). The survey used 194 plots with 8 m radius located in transects across the Natural Area including all the areas containing the rockslides and soil slides. The sapling and trees totals by species from the Natural Area survey were transformed with division by 22 and rounding to the integer to enable determination of significant differences from the soil slide and rockslide results by species and tree versus sapling using the chi square test. Basal area was calculated for all multiple stem and single stem trees to relate tree size to root plate diameter and root plate depth. For a few saplings and small trees embedded in the colluvium and root mass of much larger trees, root plate depth and diameter could Table 1. Trees and saplings in soil slides, rockslides, and the Radnor Lake State Natural Area survey (Loeb et al. 2010). Natural Area survey results were transformed from the original totals with division by 22 and rounding to the integer value. An asterisk (*) indicates less than 11 saplings or trees were present in the Natural Area survey. Species (common) Acer negundo (boxelder) Acer rubrum (red maple) Acer saccharum (sugar maple) Ailanthus altissima (tree of heaven) Asimina trifolia (pawpaw) Carya glabra (pignut hickory) Carya ovata (shagbark hickory) Carya tomentosa (mockernut hickory) Celtis occidentalis (common hackberry) Celtis laevigata (sugarberry) Fraxinus americana (white ash) Juglans nigra (black walnut) Liriodendron tulipifera (tulip tree) Prunus avium (sweet cherry) Prunus serotina (black cherry) Quercus alba (white oak) Quercus muehlenbergii (chinkapin oak) Quercus prinus (chestnut oak) Quercus rubra (northern red oak) Quercus velutina (black oak) Robinia psuedoacacia (black locust) Sassafras albidum (sassafras) Tilia americana (American basswood) Ulmus rubra (slippery elm) Additional speciesz Total z 1 20 0 0 1 1 1 1 0 3 0 0 1 0 0 0 0 0 0 1 0 1 0 0 32 Soil Slides Saplings 1 Trees 2 1 20 1 0 0 5 1 18 5 15 2 1 4 5 1 2 2 5 0 5 4 1 4 2 106 0 9 0 1 0 0 0 2 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 14 Rockslides Saplings 0 Trees 0 0 32 5 0 3 6 3 26 3 13 2 1 3 1 1 2 4 6 1 1 5 3 2 1 124 Natural Area Survey Saplings 2 * 47 10 11 2 * * 2 * * 0 * 0 0 * 0 * * * * * 2 3 7 86 Trees 1 * 20 1 3 1 * * 4 * 3 1 * 0 * * 2 1 1 1 1 1 1 3 7 52 Additional species for rockslides: Salix spp. (willow); soil slides: Aesculus hippocastanum (horsechestnut); and Natural Area survey: Acer saccharinum (silver maple), Aesculus glabra (Ohio buckeye), Carya cordiformis (bitternut hickory), Cornus florida (flowering dogwood), Crataegus spp. (hawthorn), Diospyros virginiana (common per- simmon), Fagus grandifolia (American beech), Gleditsia triacanthos (honeylocust), Juniperus virginiana (eastern redcedar), Morus alba (white mulberry), Ostrya virginiana (hophornbeam), Platanus occidentalis (American sycamore), Pyrus malus (European crabapple), Quercus bicolor (swamp white oak), and Ulmus americana (American elm). ©2011 International Society of Arboriculture
September 2011
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