The influence of epiphytic lichens on the nutrient cycling of an Oak Woodland

We examined the importance of the epiphytic lichen, Ramalina menziesii, in a blue oak (Quereus douglasii) woodland of California, using a factorial experiment in which lichens were removed from tree canopies and the soil surface for a 3-yr period. Each blue oak contained an average of 3.8 kg of lichen biomass, totaling 590 kg/ha for the entire woodland; oak leaf biomass was 958 kg/ha. Removal of lichens affected the interception of rainfall by the forest canopy and the deposition of water and nutrients in throughfall collected beneath the canopy. Trees with lichens had higher deposition of total N, organic N, Ca, Mg, Na, and Cl, and a lower deposition of SO₄ under their canopy; deposition of NO₃, NH₄, K, and total P was not affected. Accounting for the uptake of nutrients by lichens, we calculate that canopy lichens enhance the receipt of N and P from the atmosphere by 2.85 and 0.15 kg-ha^-1·yr^-1, respectively. The bulk deposition (wetfall + sedimentation) of N and P for the same period was 0.88 and 0.06 kg·ha^-1·yr^-1, respectively. Because retranslocation of N occurred prior to leaf abscission, the percentage of N in leaf litter was approximately half that in lichen litter. Lichen litter significantly slowed the decomposition of oak leaves on the forest floor. Oak leaf and lichen litter decomposition was slow, with low N release; whereas P release was much more rapid. The removal of lichens had no effect on leaf biomass or nutrient content, acorn productivity, or tree-ring increment of the oak trees, probably because soil pools of N and P were large relative to the magnitude of the flux changes caused by the manipulations. In fact, no differences in soil mineralization rates among the treatments were detectable. Overall, the manipulations significantly affected annual intersystem capture of nutrients, but the magnitude of these changes was insufficient to affect intrasystem fluxes or forest productivity.