Modeling nutrient accumulation and release in downed woody debris

Background/Question/Methods

In forest inventories, downed woody debris (DWD) is often categorized into discrete decay classes along the continuum of decomposition. As wood progresses through the stages of decay, it plays a unique role in forest carbon and nutrient cycling by acting as a sink for N, P and Ca via translocation from soil and simultaneously a source of C and K. In managed forests, additions of harvest slash contribute a large pulse of new material to the forest floor at discrete intervals significantly altering the carbon and nutrient storage of the total DWD pool. Nutrient import into this wood can outpace the loss of mass during decay and this is reflected on the stand scale as an accumulation of nutrients in DWD in excess of initial conditions. To model the magnitude and timing of DWD nutrient accumulation and release following a harvest event we employed a novel approach combining a pre and post-harvest DWD inventory and destructive sampling of logs with respiration and leachate monitoring. 

Results/Conclusions

During decay, C content in logs was reduced by 63% on a mass basis along a chronosequence from decay class 1 to 5, while N, P, Ca and Mg increased by 479%, 247%, 179% and 16% respectively, corroborating prior reports of nutrient concentration increase as a result of fungal colonization. At the stand scale, matrix model projections indicated a peak accumulation of N and P in DWD eight years following harvest slash input and a rapid loss of potassium in the same time interval. Preliminary results indicate that a small proportion (<5%) of total DWD carbon is exported as water-borne DOC, peaking during the intermediate stages of decay (decay class 3), while respiration was highest in early decay (class 2) wood. Leaching of C and N from DWD was highly variable within and among decay classes, but consistently exceeded litter exudates suggesting that DWD is a point-source of labile C and N. The export of N in leachate increased with advanced decay, but diminished in the final stage of decay (class 5). The results present a new approach to evaluating the impact of harvest slash removal and highlight the unique role of DWD in accumulating nutrients during the course of decay.