Composition and tree-size distributions of the Snyder-Middleswarth old-growth forest, Snyder County, Pennsylvania

Castanea, Mar 2003 by Zawadzkas, Peter P, Abrahamson, Warren G

METHODS

Study Site

Our study was conducted within the Snyder-Middleswarth Natural Area, part of the Bald Eagle State Forest, 8 km northwest of Troxelville, Pennsylvania (40[degrees]48'N, 77[degrees]17'W). The site has a temperate climate with warm, humid summers and only moderately cold, humid winters. The region's highest average monthly temperature of 23[degrees]C occurs in July and the lowest of -3[degrees]C occurs in January. Mean annual precipitation for the region is 103 cm and is evenly distributed throughout the year (30-yr means, NOAA data for Williamsport, Pennsylvania).

The old-growth forest is located in a narrow and steep ravine between two ridges that run east to west; Buck Mountain lies to the north and Thick Mountain to the south. The ravine, created by Swift Run, has well-developed north-facing and south-facing slopes as well as a bottomland. Elevations in the sampled area ranged from 451 m to 548 m, with slopes varying in steepness from 1-68%. The predominant soils are extremely stony and sandy well-drained loams that have weathered from sandstone and shale and have low to moderate available water capacity (Eckenrode 1985). The soils of the bottomland, slopes, and Buck Mountain ridge top are mapped as Hazelton (mesic Typic Dystrochrept) or Clymer (mesic Typic Hapuldult) soils while the Thick Mountain ridge top is covered by lingers (mesic Typic Hapludult) soils.

Thanks originally to the inaccessibility of this ravine and in 1965 to its preservation as a National Natural Landmark, the approximately 135-ha portion of this forest included in our study has never been logged. The extent of direct human impact to the area is a footpath that runs along the northern bank of Swift Run and loops back along the north-facing slope. Hunting of deer and other game is allowed in the adjoining state forestlands.

There have been many natural disturbances within this old-growth forest during the past three decades (Abrahamson, pers. obs.). Windstorms, especially those associated with snow or ice events, have toppled a number of the larger hemlock and yellow birch throughout the stand. Furthermore, the crowns of slope and ridge top trees frequently show evidence of wind and/or ice damage. Gypsy moth outbreaks have occurred periodically within central Pennsylvania since the mid-1970s and likely have markedly impacted the oak canopies within the study area during one or two growing seasons.

Sampling

Five independent topographic transects of 16 nested quadrats each were used to sample within the old-growth forest. Each transect of quadrats ran parallel to Swift Run, in order to follow the elevation contour. One transect was located on the bottomland, one was located approximately halfway up each ridge, and one was located along the top of each ridge. Each nested 10 x 10 m quadrat was separated by 20 m from the adjacent quadrats within the same transect. A nested 5 x 5 m quadrat shared its northeast corner with that of the 10 x 10 m quadrat. There were 80 such nested quadrats sampled within the old-growth forest. All woody stems >6 cm diameter at breast height (dbh) were identified and measured in the 10 x 10 m quadrats; all woody stems

Tree ages were not determined as no coring was performed; coring studies were beyond the scope of this initial study of this old-growth forest. We recognize the limitations of using size distributions when inferring past disturbances (Lorimer 1985), however, we feel that in the absence of tree-age data, size-distribution data provide at least some insight into the successional status of species.

All non-vegetation measurements were taken from the northeast corner of each nested quadrat. Quadrat coordinates were recorded using a Magellan(R) GPS 4000 XL and quadrat elevation was determined from a digital topographic map (DeLorme 1999) by inserting the GPS coordinates for each quadrat. Forest overstory density (i.e., canopy cover) was quantified with a Robert E. Lemmon spherical densiometer model-C. We measured canopy cover in each of the four cardinal directions and averaged these values to produce a single canopy-cover measure for each quadrat. Aspect, which was determined by compass, and slope, which was measured with a Suunto(R) clinometer, were also recorded for each quadrat. A soil sample was taken with a soil auger at the northeast corner of each nested quadrat to a depth of 10 cm. Forty-three of the 80 nested quadrats were covered by rocky "talus" where only organic soil materials could be sampled. Consequently, we lack particle-size data for these nested quadrats.