Response of soil invertebrates to forest canopy inputs along a productivity gradient

Previous studies have suggested that herbivory in forest canopies can influence forest floor processes such as nutrient cycling and decomposition. We studied the response of litter decomposition to a moisture/productivity gradient with manipulations of the effects of canopy herbivory at the Coweeta Hydrologic Laboratory, North Carolina. Litterbags containing Quercus rubra L. and Acer rubrum L. litter were placed at three elevations along the gradient and sampled monthly for two years. Microarthropods, nematodes, and litter mass loss responses to the productivity gradient were measured. The relative abundance of Collembola and three suborders of mites (Oribatida, Mesostigmata and Prostigmata) was compared across the gradient. Mass loss was greater at the middle and high elevation sites in both years and was correlated with increased numbers of oribatid mites per gram of litter. The abundance of all the above microarthropods (of which oribatics were the most common) was also greater on the middle and high elevation sites and greater on two- year -old litter than on one- year- old litter. Nematode densities were also greater on the older litter. The herbivore inputs study, simulating the effects of canopy herbivory, included frass additions, throughfall additions, greenfall exclusion, total litter exclusion, and controls, Experimental additions of frass to plots on the low and middle elevation sites led to an increase in Collembola abundance in litterbags from those plots. Plots with frass and artificial throughfall additions also showed increased numbers of fungal feeding and bacterial feeding nematodes in some months. Numbers of oribatid and prostimatid mites were reduced in litter exclusion plots. Results from these studies suggest not only significant influences of elevation on litter decomposition and soil fauna abundance but direct links between canopy herbivory and responses in population densities of forest floor biota.