Leaf litter composition and understory herb effects on temporal decomposition dynamics in a northern hardwood forest

Background/Question/Methods

In temperate deciduous forests, overall ecosystem C balance is largely driven by the annual production and turnover of leaf litter, and litter turnover is partially controlled by the microbial production of extracellular enzymes (EE).  Recent evidence suggests that temporal patterns of EE activity in litter and soil differ from the patterns expected based on litter decomposition models.  Additionally, the presence of understory herbs represents highly labile litter that may “prime” C mineralization.  We used sequential litterbag harvesting to address the interactive effects of litter species composition and understory herb presence on temporal decomposition dynamics in a mature hardwood forest.  Mesh litterbags were filled with recently senesced beech, maple, or a 50:50 beech and maple mixture. Twelve replicates of each litter type were placed at 6 locations (n=72 litterbags per litter treatment) and replicate litterbags were collected monthly for 12 months.  To test the influence of understory vegetation on decomposition dynamics, we placed an additional 144 litterbags (n=24 for each litter type in each understory type) at the same locations but in areas with high understory herb density and with no understory herbs.  Replicate litterbags were collected from each location after 6, 8, 10 and 18 months.

Results/Conclusions

Litter type significantly effected decomposition in both experiments; maple had the smallest mass remaining (54 ±4 %) after 1 year followed by mixed litter (61 ±3 %) and beech (69 ±5 %).  In the 12-month study, EE activity generally followed this trend with significantly higher levels in maple followed by mixed litter and beech.  Some hydrolytic enzymes had relatively constant rates across the study period while others (chitinase and cellulose) peaked at ~8 months.  Oxidative enzymes remained low in the early months before peaking at 11 months.  When we examined understory effects, we found that EE did not typically differ between understory/no understory areas, but typically differed between litter types.  EE spiked at ~8 months and tended to be greatest in the mixed and maple litter and lowest in the beech litter.  Mass loss was strongly correlated with hydrolytic enzymes in the early stages of decomposition and oxidative enzymes exhibited stronger correlations after 10 months. Overall, we found little evidence that the forest understory “primes” litter decomposition; any herbaceous litter pulse-effect might be rapid.  These results confirm expected temporal relationships between decomposition and litter lability, and further suggest tighter than expected coupling between EE production and decomposition stage.