Catastrophic windthrow in the southern Appalachians: characteristics of pits and mounds and initial vegetation responses

Abstract

We characterized pit and mound (PM) topography resulting from catastrophic wind in the Coweeta Basin, and located 48 PMs across a variety of forest types. Our measurements included pit length, width, and depth; and mound height, thickness, and width. Species of fallen trees were identified, and DBH (diameter at breast height, 1.37 m) was measured for biomass determination. We identified five distinct microsites at each PM: mound face, mound top, pit bottom, pit-wall, and intact forest floor. On each microsite, we measured photosynthetically active radiation (PAR), soil temperature, and soil moisture, and took soil samples from four microsites (intact forest floor, pit wall, pit bottom, mound top) to determine carbon and nitrogen concentrations. Treefall direction was marginally non-random. Three PM dimensions were significantly related to fallen tree biomass: mound width; mound height; and pit width. Other relationships failed because (1) rooting depth of the fallen tree was not necessarily proportional to tree size; and (2) trees that fell striking other trees often slid back into the pit, altering its dimensions. PAR was highest at mound top (250 μmol m −2 s −1) and lowest in pit bottom (70 μmol m −2 s −1). Mean soil temperature varied ≈3°C across microsites, and soil moisture ranged from 24% on the mound top and mound face to 34% in the pit bottom. Nitrogen and carbon concentrations were significantly higher on the forest floor ([N] = 0.23%; [C] = 4.73%) than on the other three microsites ([N] = 0.08–0.10%; [C] = 1.4–2.2%). Over time, soil nutrition and microsite instability, due to erosion and settling, may be the most influential factors determining rates of vegetative establishment in PMs. We characterized initial vegetative recovery in 27 blowdown sites. Trees were placed in one of two damage classes: direct wind damage (direct); and damage due to the fall of another tree (indirect). Basal and/or bole sprouting, and live or dead crowns were noted. Blowdown areas ranged from 181 to 4043 m 2 and averaged 1175 m 2. Mean diameter of indirectly damaged trees was 50% of the mean for trees directly damaged, but both had similar minimum diameters. Overall, the biomass of indirectly damaged trees accounted for <10% of total biomass but 33% of the total number of stems. Of the indirectly damaged trees, 38% were topped, 82% exhibited basal or bole sprouting, and 21% had live crowns. By contrast, of the directly damaged trees, only 5% were topped, <50% were sprouting, and only 11% had live crowns.