Ecosystem-Scale Rainfall Manipulation in a Piñon-Juniper Forest at the Sevilleta National Wildlife Refuge, New Mexico: Soil Temperature Data (2006-2013)

Climate models predict that water limited regions around the world will become drier and warmer in the near future, including southwestern North America. We developed a large-scale experimental system that allows testing of the ecosystem impacts of precipitation changes. Four treatments were applied to 1600 m2 plots (40 m × 40 m), each with three replicates in a piñon pine (Pinus edulis) and juniper (Juniper monosperma) ecosystem. These species have extensive root systems, requiring large-scale manipulation to effectively alter soil water availability.  Treatments consisted of: 1) irrigation plots that receive supplemental water additions, 2) drought plots that receive 55% of ambient rainfall, 3) cover-control plots that receive ambient precipitation, but allow determination of treatment infrastructure artifacts, and 4) ambient control plots. Our drought structures effectively reduced soil water potential and volumetric water content compared to the ambient, cover-control, and water addition plots. Drought and cover control plots experienced an average increase in maximum soil and air temperature at ground level of 1-4° C during the growing season compared to ambient plots, and concurrent short-term diurnal increases in maximum air temperature were also observed directly above and below plastic structures. Our drought and irrigation treatments significantly influenced tree predawn water potential, sap-flow, and net photosynthesis, with drought treatment trees exhibiting significant decreases in physiological function compared to ambient and irrigated trees. Supplemental irrigation resulted in a significant increase in both plant water potential and xylem sap-flow compared to trees in the other treatments. This experimental design effectively allows manipulation of plant water stress at the ecosystem scale, permits a wide range of drought conditions, and provides prolonged drought conditions comparable to historical droughts in the past – drought events for which wide-spread mortality in both these species was observed.  Soil temperature impacts both the abiotic and biotic processes at our site. The rate of evaporation, soil water content, VPD, and many other environmental factors are directly or indirectly affected by the temperature of the system. By monitoring the soil temperature at our site, we were able to determine its influence on the target trees and their associated physiological functions. Differences in soil temperature between plots can be impacted by the drought and cover-control structures used in our rainfall-manipulation treatments. Therefore, measuring soil temperatures in all three cover types and all four treatment regimes also allowed us to tease-out the temperature differences that were an artifact of the treatment structures as opposed to the actual treatments.Â

气候模型预测，全球范围内的水分受限区域在未来将愈发干旱炎热，北美西南部亦在此列。本研究构建了一套大型实验系统，用于探究降水变化对生态系统的影响。
实验在皮尼翁松（Pinus edulis）与单籽杜松（Juniper monosperma）组成的生态系统中开展，共设置1600平方米（40米×40米）的样地，并为每种处理设置3个重复。这两种植物拥有发达的根系，因此需要通过大尺度的操控手段才能有效改变土壤水分有效性。
实验共设置4种处理：1）灌溉组样地：施加额外补水；2）干旱组样地：仅接收55%的自然降雨量；3）覆盖对照组样地：接收自然降雨量，用于评估实验设施本身带来的干扰效应；4）自然对照组样地：完全接收自然降雨。
相较于自然对照组、覆盖对照组与灌溉组样地，本研究的干旱处理装置可有效降低土壤水势与体积含水量。生长季内，干旱组与覆盖对照组样地的地表最高土壤温度与空气温度较自然对照组平均升高1~4℃；同时在塑料装置上下方区域，也观测到最高空气温度出现短期的日间峰值升高。
干旱与灌溉处理均对树木黎明前水势、树干液流与净光合速率产生显著影响：相较于自然对照组与灌溉组树木，干旱组树木的生理功能出现显著下降。与其余处理组的树木相比，补水灌溉可显著提升植株水势与木质部液流速率。
本实验设计可在生态系统尺度上有效操控植物水分胁迫，能够模拟多种干旱情境，同时可实现与历史干旱事件相当的长期干旱处理——此类历史干旱曾导致这两种植物出现大范围死亡。
本实验样地的土壤温度会影响非生物与生物过程。蒸发速率、土壤含水量、水汽压亏缺（Vapor Pressure Deficit, VPD）等诸多环境因子均直接或间接受系统温度调控。通过对样地土壤温度的监测，本研究明确了其对目标树种及其相关生理过程的调控作用。
降雨操控处理中使用的干旱装置与覆盖对照装置，会对样地间的土壤温度差异产生影响。因此，对3种覆盖类型与4种处理组的土壤温度进行监测，可帮助我们厘清：究竟是实验装置本身带来的温度差异，还是实际处理本身导致了温度变化。