Red oak litter promotes a microarthropod functional group that accelerates its decomposition

The contribution of microarthropod activity to litter decomposition varies widely but can be substantial. Oribatid mites are the most diverse and abundant of the microarthropod groups in forest litter. This experiment was designed to examine the effect of litter type and complexity on the diversity and species composition of oribatid mites, and to test whether alterations in species composition due to litter type affected litter decomposition. In an array of plots on a mixed-hardwood site in the mountains of North Carolina, I exposed microarthropod assemblages to a range of litter types: yellow birch, sugar maple, red oak and two mixed litters. Over several years, the litter types selected oribatid mite assemblages of different species composition. By comparing the decomposition of consecutive cohorts of litter, it was possible to detect differences in decomposition accompanying the shifts in the assemblage. A comparison of the mass loss rates between the two litter cohorts over eighteen months reveals similar trajectories for four litter types. In the oak litter, however, the second cohort disappeared significantly faster than the first. In both years, the litters came from the same trees and were nearly identical in initial carbon and nitrogen contents. Since the response was specific to oak litter, it is unlikely that differences in environmental factors are responsible for the faster mass loss of oak. A significant increase of endophagous oribatid mites, those that burrow into plant material, in the second cohort of oak may account for its accelerated decomposition. The woody petioles and thick leaf-planes of oak leaves provide microhabitats for burrowing mites. Endophage activity can accelerate the litter decomposition both through direct comminution of leaf material and by facilitating microbial growth. Because of their low population growth rates, oribatid populations that are reduced by disturbance are slow to recover and by disrupting these non-resilient populations, disturbance may have long-term repercussions for decomposition.