Differences in leaf litter decomposition rates between mesophytic and upland forest species in restored upland oak forests

Background/Question/Methods

Decomposition of leaf litter is a critical process in the cycling of nutrients through forest ecosystems. In the interior southern U.S., open oak woodlands were once common, and were adapted to and maintained by fire. However, over a century of fire suppression has allowed the growth of fire sensitive and shade tolerant (mesophytic) plant species. Continued fire suppression increases the prevalence of these mesophytic species, closes the canopy and alters environmental conditions, potentially changing the activity of decomposer microorganisms. Leaves of mesophytic species also differ physically and chemically from oaks, changing overall litter characteristics. Thus, increased mesophytic leaf litter likely alters decomposition rates and influences associated microbial decomposer activity.

A litterbag approach was used to investigate differences in leaf litter decomposition rates between upland and mesophytic tree species, along with how decomposition rates differed between restored upland oak forests and unrestored (mesophytic) forests. Leaves came from two species characteristic of upland oak forests (Quercus stellata, Quercus falcata), two mesophytic species (Liquidambar styraciflua, Ulmus alata), and two intermediate species (Quercus alba, Carya tomentosa). Litterbags were placed in restored and unrestored forests and collected at eight week intervals to determine mass loss and decomposer microbial enzyme activity throughout the year.

Results/Conclusions

Across restoration treatments, leaf litter decomposition varied primarily by leaf species, with the mesophytic/intermediate species L. styraciflua, U. alata, and C. tomentosa decomposing at faster rates than the oaks (Q. stellata, Q. falcata, and Q. alba). Enzyme activity was associated with the stage of decomposition. Phosphatase, β-glucosidase, cellobiohydrolase, and β-N-acetylglucosaminidase activity increased as decomposition began, and also correlated with mass loss, then decreased as more labile organic compounds were degraded. In addition, these enzymes were correlated with each other. Activity of phenol oxidase and peroxidase was initially low, but increased as more recalcitrant materials such as lignin were degraded.

Slower decomposition of litter from upland oak tree species may be beneficial for the upland oak ecosystem, as it would remain for longer periods and serve as a potential fuel for the fires that are necessary to maintain this ecosystem. This research furthers our knowledge of leaf litter characteristics and how they contribute to maintaining these threatened upland oak forests. Understanding these characteristics adds a level of knowledge that will aid land managers in restoring and preserving the upland oak ecosystem.