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.

1989年雨果飓风（Hurricane Hugo）与1998年乔治飓风（Hurricane Georges）席卷波多黎各期间，大量林木倒伏。一项残体移除实验表明，飓风产生的粗木质残体（coarse woody debris）通过促进微生物固持氮元素，延缓了林冠恢复进程。我们在乔治飓风过后的两年内，于干湿季分别采集了三次成对土壤样本：样本分别取自雨果与乔治飓风倒伏的两个树种的大伐倒木下方，以及伐倒木20-50cm以外区域，并对样本中的碳（C）、氮（N）、微生物生物量碳以及根长进行了分析。目前尚未对土壤磷（P）及其他养分的相关数据开展分析。两类腐解时长组的伐倒木下方，土壤微生物生物量、碳与氮含量均高于伐倒木附近区域。蛀木甲虫产生的虫粪或许是引发这一早期效应的诱因。在旱季，0-10cm土层中伐倒木下方的根长更长；而在湿季，根长则在伐倒木以外区域更长，且根长变化与微生物生物量无显著关联。由此可见，腐解木材可为林木根系提供可利用的养分资源。在0-10cm与10-20cm土层的样本中，伐倒木下方的土壤碳、氮占比均显著高于伐倒木附近区域。在0-10cm土层中，微生物生物量碳含量随季节呈现显著差异，但伐倒木下方与外侧区域的差异仅具有统计学趋势。伐倒木上坡侧的表层土壤氮含量与微生物生物量显著更高，这大概率源于陡坡上伐倒木上方累积的枯落物输入。本研究表明，即便腐解仅7个月的伐倒木，其下方土壤的碳、氮积累量也显著高于附近区域，由此提升了土壤异质性。林木根系会随季节变化追踪并利用养分热点区域，因此土壤肥力异质性对森林生产力至关重要。
在湿润热带森林中，土壤有效磷（P）通常是最主要的限制性养分。研究人员通过全消解法测定了0-10cm表层土壤的全磷含量，同时也测定了奥尔森浸提有效磷（Olsen extractable P）。两类倒伏树种以及两类飓风残体的腐解木下方，土壤全磷浓度均显著高于外侧区域。尽管圭亚那木果楝（Guarea guidonia）的木材与枯落物磷浓度高于翼子榄（Dacryodes excelsa），但翼子榄腐解木对土壤全磷的提升效应比例更高。究其原因，翼子榄作为共优势种的山脊土壤本底磷含量，低于下坡、河谷区域，以及圭亚那木果楝广泛分布的前咖啡种植园施石灰土壤。这一差异，结合飓风残体移除实验（MRCE实验）与模拟飓风残体的林冠修剪实验（Canopy Trimming Experiment）中观测到的森林生产力下降现象，表明移除山脊区域的腐木可能会降低未来的森林生产力。伐倒木上下坡侧的土壤磷含量差值表明，上坡侧枯落物累积对土壤磷的贡献几乎与腐解木材释放的磷相当。
本研究得到美国国家科学基金会（National Science Foundation）向波多黎各大学提供的多项拨款资助，拨款编号包括BSR-8811902、DEB-9411973、DEB-9705814、DEB-0080538、DEB-0218039、DEB-0620910、DEB-1239764、DEB-1546686以及DEB-1831952，相关资助属于卢奎洛长期生态研究计划（Luquillo Long-Term Ecological Research Program）的一部分。此外，本研究还得到波多黎各大学以及美国农业部林务局热带森林国际研究所（International Institute of Tropical Forestry, USDA Forest Service）的支持。