SOIL SURVEY OF CATION EXCHANGE CAPACITY (CEC) IN EASTERN HEMLOCK (TSUGA CANADENSIS) AND BLACK BIRCH (BETULA LENTA) FORESTS AT THE MACLEISH FIELD STATION, WEST WHATELY, MA, USA

Eastern hemlock (Tsuga canadensis) forest in western Massachusetts is currently threatened by the invasive pest hemlock woolly adelgid (Adelges tsugae), and black birch (Betula lenta) appears to succeed hemlock following mortality. This study compares the cation exchange capacity (CEC) of soils beneath black birch (Betula lenta) and eastern hemlock (Tsuga canadensis) in 3 paired forest plots at the MacLeish Field Station in West Whately, MA to assess whether soil geochemistry differs beneath each forest type. Soils at the MacLeish Field Station consist of rocky to stony loam, inceptisols that formed on thin glacial till derived from mica schist bedrock of the Waits River Formation. The schist contains bands of quartzite, and impure limestone within the schist increases in abundance to the east.

Soil cores of organic and mineral horizons were collected from 3 hemlock and 3 black birch mature forest patches across the property. At each site, hemlock and black birch plots were selected proximal to one another to minimize differences in topography and underlying soil mineralogy. A higher concentration of impure limestone occurs in schist at Site 3 to the east as compared to bedrock at Sites 1 and 2. Soil cores were collected beneath the canopy of either black birch or hemlock forest patches using a hammer corer (5cm diameter) to a depth of ~17cm, for a total of 20 mineral and 20 organic soil samples for each tree type. Two replicates of each sample were air-dried, and 5.0g were reacted with 0.1M BaCl₂ exchange solution; concentrations of exchangeable cations (exch. Ca, Mg, Na, K, Al, Fe) were detected using an ICP-OES. Exchangeable H was determined by pH measurements of the BaCl₂ solution. Ca and Al were the most abundant exch. cations at all sites, with half of the black birch and nearly a quarter of the hemlock samples reporting >25% of total CEC as exch. Ca. Alternatively, more than half of the hemlock and over a third of the black birch samples reported >50% exch. Al. Mineral soils were observed to contain higher percentages of exch. Al than organic horizons; organic horizons contained a higher percent of exch. Ca. These patterns did not vary between forest types at Sites 1 and 2. However, black birch forest at Site 3 was much higher in exch. Ca and showed very little exch. Al, which could be due to a higher presence of limestone in bedrock at Site 3.