Tropical Tree Species Identity and Diameter at Breast Height in a Throughfall-Reduction Drying Experiment 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. 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 tree species identity and diameter at breast height (dbh) 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：学界预测诸多热带森林将面临气候干旱化趋势，但目前针对热带森林内沿湿度梯度与土壤梯度变化的土壤特性响应机制仍缺乏充分刻画，这一现状阻碍了森林-气候反馈预测精度的提升。本研究提出假说：气候干旱化将通过进一步降低土壤湿度，抑制已处于较干旱状态的热带森林的土壤二氧化碳通量（soil CO2 fluxes，即土壤呼吸）；而在更为湿润的热带森林中，干旱化可通过缓解土壤厌氧状态与积水状况，提升土壤二氧化碳通量。研究于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，测定了干湿循环过程中的土壤二氧化碳通量、土壤湿度、土壤温度以及地表枯落物生物量。此外，研究团队还在2018年与2019年对所有乔木树种开展调查并完成物种鉴定。

研究结果：结果显示，土壤湿度在湿季达到峰值，干季时出现下降；测定得到的土壤二氧化碳通量在干季降低，且在土壤湿度达到峰值前的湿季早期就已达到峰值，这表明土壤呼吸的最优土壤湿度低于此前模型的预测结果。长期穿透雨排除处理（throughfall exclusion）同样使4个森林20厘米深度处的土壤湿度出现下降，且最初会抑制所有森林的土壤二氧化碳通量。仅在最湿润的森林中，经过4年处理后土壤二氧化碳通量仍持续受到抑制（干季较对照组降低28%±4%）；而在一处高肥力森林中，经过4年处理后土壤二氧化碳通量反而出现上升（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出现了出乎意料的负向干旱效应，这可能源于养分向土壤的垂直淋溶作用减弱，且该干旱抑制效应随处理时间延长而加剧。在生态系统模型（ecosystem models）中纳入水-养分交互作用（hydro-nutrient interactions），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年发表的成果）。

包含数据集：本数据集包含研究样地内乔木物种组成与胸径（diameter at breast height, DBH）的.csv和.xls格式文件；此外还有包含4个森林研究中全部32个样地坐标的.kml文件（每个样地设置4个穿透雨减少处理样地与4个对照样地，即n=4）。打开此类文件无需使用特殊软件。