SOIL BIOGEOCHEMISTRY OF FOREST SUCCESSION FOLLOWING LOGGING OF EASTERN HEMLOCK TREES AT THE MACLEISH FIELD STATION, WEST WHATELY, MASSACHUSETTS, U.S.A

Spread of the hemlock woolly adelgid (Adelges tsugae; HWA) in New England’s eastern hemlock (Tsuga canadensis) forests has caused forest succession by deciduous hardwood species, particularly black birch (Betula lenta). Because tree species influence soil nutrient cycling, succession of hemlock by black birch could change forest soil chemistry. In attempt to simulate the effects of this succession on nutrient cycling, we compared soil geochemistry from plots of mature hemlock and juvenile black birch that regrew after logging of hemlock 20 years ago. For 9 incubation periods from May 2011 to October 2012, we measured net soil nitrogen mineralization rates in the mature hemlock and juvenile black birch stands. For reference, net nitrogen mineralization rates for a mature black birch plot were measured over 4 incubation periods from May 2012 to October 2012. In addition soil pH, exchangeable acidity and exchangeable base cations were measured.

In the juvenile black birch (BB) organic soil horizon, low net N mineralization rates for 2011-2012 equal rates observed in hemlock (HEM) soil during 2011. BB results are dissimilar to the results from mature black birch (MBB) soils, which have high net N mineralization and nitrification rates. These results suggest that hemlock soil exhibits strong control over N mineralization, even after hemlock trees are gone. Surprisingly, HEM soils experienced an increase in N mineralization between 2011 and 2012, without a change in nitrification rate. Increased net N mineralization might be explained by recent spread of the HWA infestation in this forest.

As expected, MBB has higher base saturation (66%) and lower exchangeable Al³⁺ (2.7 meq/100g soil) than HEM soil (45% and 5.5 meq/100g soil). However, BB has lower base saturation (29%) and higher exchangeable Al³⁺ (7.5 meq/100g soil) than both HEM and MBB. These results suggest that clear cutting lowers input of base cations to soil via throughfall, which increases exchangeable acidity of the soil later succeeded by BB. Throughfall beneath the currently thin BB canopy has not resupplied base cations to the exchangeable cation pool, in part due to higher Al³⁺ that inhibits exchange.