Throughfall-Reduction Drying Effects on Forest Floor Plant Tissue Biomass (Oi+Oe soil horizons) and Moisture Content in Four Lowland Panamanian Forests

Objectives: Climatic drying is predicted for many tropical forests, yet effects on soil properties across moisture and soil gradients within tropical forests remain poorly characterized, hampering predictions of forest-climate feedbacks. We hypothesized that drying would suppress soil CO2 fluxes (i.e., respiration) in already-drier tropical forests by further reductions in soil moisture, but increases CO2 fluxes in wetter tropical forests by alleviating anaerobiosis and soil saturation. We measured soil CO2 fluxes, soil moisture, soil temperature, and forest floor biomass during wet-dry cycles (2015 – 2022) in four Panamanian forests that vary in rainfall and soil fertility. We also surveyed all tree species and identified to species in 2018 and 2019.Results: We found that soil moisture peaked in the wet season and declined in the dry season. Measured soil CO2 fluxes declined in the dry season and peaked in the early wet season ahead of peak soil moisture, resulting in a lower soil moisture optimum for respiration than previously modeled. Forest floor biomass peaked in the dry season, in contrast. Chronic throughfall exclusion also suppressed soil moisture across the four forests to 20cm depths, and also initially suppressed soil CO2 fluxes across forests. There was sustained suppression of soil CO2 fluxes after four years in the wettest forest only (-28 ± 4% during the dry season), but elevated soil CO2 fluxes in a fertile forest after four years (+75 ± 28% during the late wet season). The unexpected negative drying effect in the wettest, most infertile forest could have resulted from reduced vertical flushing of nutrients into soils, as the drying effect increased with time. Including hydro-nutrient interactions in ecosystem models could improve predictions of tropical forest-climate feedbacks (results presented in Cusack et al. 2023). Datasets included: Datasets included here include .csv and .xls files for forest floor biomass (equivalent to the Oi+Oe soil horizons), and a subset include forest floor biomass moisture content (weight/weight) in the study plots. There is also a .kml file that includes coordinates for all 32 plots included in the study of four forests (n = 4 throughfall reduction and n = 4 control plots per site). No special software is needed to open these files.

研究目标：诸多热带森林被预测将经历气候干旱化，但热带森林内沿水分与土壤梯度的土壤性状响应仍未得到充分表征，这阻碍了森林-气候反馈机制的预测精度。我们提出如下假说：在已经较为干旱的热带森林中，进一步的水分减少会抑制土壤二氧化碳通量（soil CO₂ fluxes）；而在更为湿润的热带森林中，干旱化可通过缓解厌氧环境（anaerobiosis）与土壤积水状况，提升土壤二氧化碳通量。我们于2015年至2022年间，在4个降雨与土壤肥力存在差异的巴拿马森林中，开展了干湿循环下的土壤二氧化碳通量、土壤含水量、土壤温度以及地表凋落物生物量的野外监测。此外，我们于2018年与2019年完成了所有乔木物种的调查与物种鉴定工作。

研究结果：我们观测到，土壤含水量在雨季达到峰值，旱季则出现下降；土壤二氧化碳通量在旱季降低，且早于土壤含水量峰值在湿季初期达到峰值，这表明土壤呼吸（respiration）的最适土壤含水量低于此前模型的预测值。与之形成对比的是，地表凋落物生物量在旱季达到峰值。长期穿透雨排除（throughfall exclusion）处理同样使4个森林20厘米深度的土壤含水量出现下降，且最初也抑制了所有森林的土壤二氧化碳通量。仅在最湿润的森林中，四年后仍存在持续的土壤二氧化碳通量抑制（旱季期间较对照组降低28%±4%）；而在一个高肥力森林中，四年后土壤二氧化碳通量出现上升（湿季后期较对照组升高75%±28%）。在最湿润且肥力最低的森林中观测到的意外干旱负效应，可能源于养分向土壤垂直淋溶的减少——这一抑制效应随时间推移逐渐增强。在生态系统模型中纳入水-养分交互作用，可提升热带森林-气候反馈的预测精度（相关研究结果详见Cusack等人2023年的工作）。

数据集说明：本次公开的数据集包含各研究样地的地表凋落物生物量（对应Oi+Oe土壤发生层（soil horizons））的.csv与.xls格式文件，部分数据集还包含了研究样地内地表凋落物的含水量（重量/重量比）。此外还包含一个KML（.kml）格式文件，记录了4个森林研究中全部32个样地的坐标信息——每个样地设置4个穿透雨减少处理样方与4个对照样方（n = 4 throughfall reduction and n = 4 control plots per site）。无需特殊软件即可打开上述文件。