Community and ecosystem responses to experimental ant and forest manipulations

Background/Question/Methods:  Infestation of eastern hemlocks (Tsuga canadensis) by an introduced exotic pest, the hemlock woolly adelgid, has led to extensive mortality of southern hemlock forests. Hemlock mortality due to hemlock woolly adelgid has already reduced ecosystem functions and altered forest hydrology. Terrestrial invertebrates, including forest floor ants, may play important roles in mediating ecosystem processes such as decomposition, and directly influence soil microbial and plant communities and the nutrient processes and pools they regulate. We experimentally manipulated ant presence through ant exclosure and addition experiments established within a large-scale forest canopy manipulation at Coweeta LTER. There are four ant treatments (ant removal using exclosures, ant addition using nest boxes, control, and a disturbance control) with four replicates within each of 3 canopy types (hemlock control, hemlock girdled, and hardwood). We sampled to determine changes in foliar groundcover, soil moisture, soil temperature, potential microbial extracellular enzyme activity, and decomposition rates across the ant and canopy treatments.

Results/Conclusions:   In 2010 and 2011 plants were surveyed to determine species richness, composition, and foliar cover. Plant species richness increased from 2.25 species in hemlock plots to 9.5 species in the hardwood plots, with the most dramatic increases in richness in the ant addition treatments. Foliar cover increased by over 200% between the hemlock control plots and the hardwood control plots across all ant treatments. In 2010 soil samples were collected to examine microbial activity for several key enzymes including betaglucosidase (cellulose degradation), nagase (nitrogen), and phosphotase (phosphorous). Potential nagase activity increased from 40 nmol/h/g in ant free plots to 120 nmol/h/g in ant access plots. In 2009, standard substrate decomposition bags (standard for lignin and cellulose) were deployed for 3, 6, 9, and 15 months. Decomposition rates were higher in the hardwood plots and hemlock girdled plots relative to the hemlock control plots, suggesting an overall acceleration in decomposition rates (due to substrate and climate changes) as hemlocks are lost from eastern forests.