Effects of forest canopy density and epixylic vegetation on nutrient concentrations in decaying logs of a subalpine fir forest

Background: Deadwood and the associated epixylic vegetation influence nutrient cycles in forest ecosystems. Open canopies strongly regulate deadwood decomposition and disrupt epixylic vegetation on logs. However, it is un-clear how the forest canopy density and epixylic vegetation growth affect the nutrient concentrations in deadwood.Methods: We measured the concentrations of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sodium (Na), magnesium (Mg), and manganese (Mn) in experimentally exposed decaying logs placed in gaps, at the edge of gaps, and under the closed canopy during a four-year decomposition experiment in a Subalpine Faxon fir forest (Abies fargesii var. faxoniana) on the eastern Qinghai-Tibetan Plateau, China. To assess the effect of the epixylic vegetation, we experimentally removed it from half of the logs used in the study.Results: Under open canopy conditions in the gap and at the edge, the concentrations for most of the nutrients in the bark and the highly decayed wood were lower than under the closed canopy. The effect of the epixylic treatment on nutrient concentrations for all but K and Na in barks varied with the decay classes. Significantly lower concentrations of N, P, Ca, and Mn following the removal of epixylic vegetation were observed in the wood of decay class IV. Epixylic vegetation significantly increased most nutrient concentrations for decaying barks and wood under open canopy conditions. In contrast, epixylic vegetation had no or minimal effects under the closed canopy.Conclusions: Forest canopy density and epixylic vegetation significantly alter the nutrient concentrations in decaying logs. Open canopies likely accelerate the rate of nutrient cycling between the epixylic vegetation and decaying logs in subalpine forests.