Wood to Soil 0-10 cm data and Wood to Soil 10-20 cm data to detect the imprint of decaying logs (30-80 cm diameter) from two hurricane cohorts (Hugo, 1989, and Georges, 1998)

Many trees fell during Hurricanes Hugo (1989) and Georges (1998) in Puerto Rico. A debris removal experiment suggested that coarse woody hurricane debris slowed canopy recovery by fueling microbial nitrogen immobilization. We analyzed C, N, microbial biomass C and root length in paired soil samples taken under versus 20-50 cm away from large trunks of two species felled by Hugo and Georges three times during wet and dry seasons during the two years after Georges. Data on soil P and other nutrients have not yet been analyzed. Soil microbial biomass, C and N were higher under than near logs of both age cohorts. Frass from wood boring beetles may induce the early effects. Root length was greater under logs at 0-10 cm depth during the dry season, and away from logs in the wet season, but varied independently of microbial biomass. Thus decaying wood can provide resources exploited by tree roots. Percent soil C and N were significantly higher under than near logs in both the 0-10 and 10-20 cm samples. Microbial biomass C varied significantly among seasons at 0-10 cm depth but differences between positions (under vs away) were only suggestive. Surface soil on the upslope side of the logs had significantly more N and microbial biomass, likely from accumulation of leaf litter above the logs on steep slopes. This study shows that C and N accumulate significantly more in soil under than near decaying logs, even in logs that had only decayed for 7 months, and thus contributes to soil heterogeneity. Tree roots track and exploit resource and nutrient hotspots as they change locations between seasons, so the soil heterogeneity in soil fertility is important for forest productivity.
Soil phosphorus (P) availability is most often the most limiting nutrient in wet tropical forests. Total soil P was measured by complete digestion in samples from the upper 10 cm; Olsen extractable P (available) was also measured. Total soil P concentrations were significantly greater under than away from decomposing logs of both species and both hurricane cohorts. Although Guarea guidonia had higher wood and leaf litter P concentrations than Dacryodes excelsa, the effect of decomposing logs of D. excelsa on total soil P was proportionately greater because the lower background P content of soil on ridges where D. excelsa is a co-dominant compared to the lower slopes and valleys as well as limed soils in former coffee plantations where G. guidonia is abundant. This disparity together with lower forest productivity after removal of debris from hurricane Hugo (MRCE experiment) and simulated hurricane debris (Canopy Trimming Experiment) indicate that wood removal from ridge sites is likely to reduce future forest productivity. Differences in soil P content upslope minus downslope beneath logs indicated that accumulation of leaf litter on the upslope side was nearly as important as P from decaying wood.
Support for this work was provided by grants BSR-8811902, DEB-9411973, DEB-9705814 , DEB-0080538, DEB-0218039 , DEB-0620910 , DEB-1239764, DEB-1546686, and DEB-1831952 from the National Science Foundation to the University of Puerto Rico as part of the Luquillo Long-Term Ecological Research Program. Additional support provided by the University of Puerto Rico and the International Institute of Tropical Forestry, USDA Forest Service.