Summer Monsoon Rains Affect Litter Decomposition Dynamics Beneath Tree Canopies in a Piñon-Juniper Woodland.

Piñon (Pinus edulus) - Juniper (Juniperus monosperma) (PJ) woodlands cover 17 million hectares in the western US.  However, these numbers are currently shrinking, due to multiple, prolonged, drought induced die-offs among piñon trees.  Further piñon mortality is likely affecting the rates at which litter deposition contributes to soil organic matter (SOM) pool, and soil carbon storage.  To assess the combined impacts of piñon mortality on these processes, we deployed foliage litter bags under tree canopies, at two different field sites in New Mexico.  One site included the widespread presence of girdled piñons, while the other site did not.  Litter bags were deployed in February 2013, and harvested in March, June, and August.  Time 0, and duff samples were also collected.  We analyzed activity rates of alanine aminopeptidase, alkaline phosphatase, β-D-glucosidase, β-D-xylosidase, and β-N-acetyl glucosaminidase.  The litter results showed that the mass loss rates within from the litter bag samples were negligible until August, when the mean OM fraction was 0.85g OM/ g litter which is beginning to show progress towards the mean OM fraction from the background litter, which was measured at 0.75g.  Our EEA analyses showed distinct patterns when comparing EEA profiles across the different sampling periods.  Activity rates from the initial, March, and June samples were all highly similar, while the August litter samples showed activity profiles nearly identical to the duff samples.   Additionally, activity rates among the cell wall degrading enzymes, β-D-glucosidase, β-D-xylosidase, and β-N-acetyl glucosaminidase, under junipers were all ≥ 26% higher at girdled, than beneath the junipers, at control.  These results illustrate how mortality within one dominant tree species, i.e., piñons, can significantly affect the litter decomposition dynamics across a given field site.  Further, the decomposition dynamics of this freshly deposited litter varies significantly across the growing season, in association with increased soil moisture due to summer rain events.