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Southeastern Geographer Vol. XXXI, No. 1, May 1991, pp. 1-14 WINDFALL DISTURBANCE IN A PIEDMONT UPLANDS FOREST* Michael E. Lewis INTRODUCTION. Natural disturbances (fire, wind, volcanic eruption, mass movement, flooding) are an important element of change in the vegetation patterns of many areas. (J) Studies of windfall disturbance in the southeastern United States have generally examined old growth stands among the cove hardwoods or conifers of the Appalachian Highlands . (2) Two scales of disturbance have been emphasized: small treefall gaps ( TABLE 1 GAPMAKER PARAMETERS, BIRKHEAD UPLANDS NATURAL AREA Species No. of Gapmakers Gapmaker dbh (cm) Gapmaker Type (%) edian range Uproot Bole Snap-off Branch Breakage Uproot Index1 Heart Rot/ Disease (%) Scarlet Oak43 Quercus coccĂ­nea Oak snags1 1 Quercus spp. Black Oak10 Quercus velutina Southern Red Oak8 Quercus falcata White Oak7 Quercus alba Tulip Poplar2 Hriodendron tulipifera Shortleaf Pine2 Pinus echinata Red Maple1 Acer rubrum Mockernut Hickory1 Carya tomentosa Chestnut Oak1 Quercus prinus AU Species86 51 53 49 47 54 37 31 40 26 26 44 34-78 39-58 36-52 37-69 36-58 32-42 23-39 23-78 53 64 60 75 71 100 100 100 100 60 42 27 30 25 29 100 35 5 9 10 0.12 0.27 0.11 0.20 0.20 0.35 0.17 0.16 0.70 0.18 42 100 40 25 14 0 0 0 0 0 292 in O C H X m > H SO Z O W O O I X W SS 1 Uproot index = mean depth of uproot hole/mean diameter of uprooted mass 2 Does not include oak snags Vol. XXXI, No. 1 rainwater draining off the rocky ridges flowed across midslopes where infiltration quickly saturated the thin mineral soils. Infiltration into the shallow soils also produced throughflow downslope into the wet ravines . The saturated mineral soils, in combination with a deep regolith, weakened the anchorage of canopy trees, particularly those with spreading crowns and shallow roots. As a result, uprooting of sound, healthy trees was common; fully 60% of the gapmakers were uprooted, while only 29% (exclusive of dead snags) showed evidence of heart rot or serious disease. The smallest uproot index was recorded for scarlet oak and black oak (0.12 and 0.11). The uproot holes for those species were only 20 to 50 cm deep, and root masses, including soil and loose regolith, were often more than 4 m across. White oaks and southern red oaks similar in size to the scarlet and black oaks generally created deeper holes with smaller root mass diameters; both had uproot indexes of 0.2. The single chestnut oak among the gapmakers had the largest uproot index (0.70); it had several large, deep roots pulled from a fractured outcrop of weathered rock. Bole snap-offs were second in importance to uprooting as a gapmaking mechanism. The number of snapped boles was greatest for scarlet oak (18 individuals), but snapped boles also occurred in black oak (3), southern red oak (2), and white oak (2). Heart rot and other forms of senescence had weakened many of the snapped trees, but only 3 oak snags were among the 30 bole-snapped trees. These results differ from Lorimer's study of wind disturbance in a cove hardwood forest of the southern Appalachian Mountains. (JO) Windfall effects in that study were concentrated among old, senescent trees regardless of species; 21 of the 28 gapmakers (75%) had snapped off at the bole because of heart rot associated with age and disease. The cove forest of Lorimer's study differed from the Birkhead Uplands in species composition (poplar-beech-basswood-hemlock) and physical setting (gentle cove slopes formed on a poorly fractured bedrock of graywacke and quartzite). In contrast, the oak-dominated Birkhead Uplands are situated on steeper slopes underlain by a deeply weathered regolith of fractured gabbro and basalt. This geomorphic setting contributed to the vulnerability of shallow-rooted but otherwise sound trees of the scarlet oak-black oak-southern red oak group while favoring the survival of white oak and chestnut oak. Comparison of the pre-disturbance canopy composition with the species composition of the gapmakers showed that windthrow is clearly TABLE...

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