Mass loss dynamics during decomposition of leaf litter along the Ecological Staircase

Background/Question/Methods

We used a leaf litter decomposition study to test whether plants on nutrient-rich or nutrient-poor sites reinforce high or low nutrient availability along the Ecological Staircase, a series of marine terraces located on the coast of northern California. While the terraces formed on the same parent material and share similar modern day climates, they differ substantially in soil age (from 100,000 ybp to >1,000,000 ybp) with younger sites exhibiting more nutrient-rich conditions than older sites.  Concomitant with the shift in soil nutrient status along this chronosequence is a shift in the ecosystems occupying the terraces; the younger two terraces (1 and 2) support full-sized Pinus muricata (Bishop pine), Pseudotsuga menziesii (Douglas-fir), and Sequoia sempervirens (redwood) forests, while the older three terraces (3-5) support substantially smaller trees of the same species, dubbed “pygmy” forests. Our study used senescent P. muricata litter collected from terraces 1, 2, and 5 and included an in situ experiment, reciprocal experiments with a common litter source (terrace 2) and a common site of decomposition (terrace 2), and a litter sterilization experiment. Decomposing litter was collected six times over the course of two years.

Results/Conclusions

Here we focus on results from initial analyses of mass loss data after one year of decomposition. Litter source (P < 0.0001), site of decomposition (P = 0.0004), and microbial colonization (P < 0.0001) significantly affected mass loss. Within the in situ experiment (i.e., litter decomposed on the collection site), litter lost the most mass at the youngest terrace and the least mass at the oldest terrace, with 58.47 %, 67.78%, and 76.62% of initial mass remaining across the three terraces. Litter from the intermediate-aged terrace in the common litter experiment also lost a decreasing proportion of mass as terrace age increased (61.82%, 67.78%, and 76.38% of initial mass remained across terraces 1, 2, and 5, respectively). Litter decomposed in the common site (terrace 2) experiment followed the same pattern; mass loss decreased with litter collected from the youngest to the oldest terrace, with 62.75%, 67.78%, and 69.48% of initial mass remaining  across terraces 1, 2, and 5, respectively. Sterilization decreased mass loss in all experiments, suggesting an important role of litter microbial communities in facilitating mass loss. Taken together, these results suggest that litter differences may serve as a more proximate control over mass loss dynamics than site differences.