PS 32-24
Forest community structure, but not ecosystem processes, differs 25 years after eastern hemlock removal in an "accidental experiment"
The spread of the exotic hemlock woolly adelgid (Adelges tsugae) throughout eastern North America threatens the survival of eastern hemlock (Tsuga canadensis) forests and has prompted pre-emptive and salvage logging. Because eastern hemlock is a foundation tree species, its loss and subsequent replacement by deciduous tree species is expected to greatly influence ecosystem structure and function. In this study, we took advantage of a 25-year-old “accidental experiment” that involved hemlock removal by logging in the late 1980s at Smith College’s MacLeish Field Station in western Massachusetts to investigate how microclimate, ecosystem processes, and forest floor animal communities might change in the decades following eastern hemlock loss. We compared measures of ecosystem structure (microarthropod communities and density, basal area, soil organic horizon thickness, leaf litter input) and function (soil nitrogen cycling, soil carbon-to-nitrogen ratio C:N), as well as microclimate (temperature, relative humidity, light levels as photosynthetically active radiation, soil moisture) in mature hemlock forest plots and adjacent young black birch (Betula lenta) forest plots that developed following selective patch-logging of hemlock.
Results/Conclusions
We found evidence of significant differences in community and ecosystem structure between mature hemlock and young black birch forests. Specifically, greater densities of microarthropods occurred in the forest floors of hemlock plots (p < 0.05), possibly due to significantly thicker organic horizons and greater leaf litter inputs (18% more). Nitrogen mineralization rates did not differ substantially between hemlock and black birch forest plots during the first growing season investigated, but these rates increased (due to ammonia production) in the hemlock soils during the subsequent two growing seasons when several hemlocks became infested by eastern hemlock scale (Fiorinia externa) and hemlock woolly adelgid.
Our results indicate substantial changes in forest floor structure and microarthropod populations after a shift from hemlock to deciduous tree dominance; however, longer-term changes in a key ecosystem process, nitrogen cycling, were less evident. Disturbance and mortality of hemlock trees appear to cause short-lived increases in nitrogen mineralization rates, but more permanent shifts to the higher steady-state nitrogen mineralization rates expected under deciduous trees might require several decades. Other similar “accidental experiments” with hemlock removal likely exist and should be taken advantage of before intact reference stands are lost to hemlock woolly adelgid or pre-emptive salvage logging.