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

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.  The focus of this study was to determine the effects of rainfall manipulation on our two target tree species.  Therefore, the analysis of the water relations of these trees was an essential component of the project.  Sap-flow within each individual target tree was monitored through the use of Granier probes.  These monitoring efforts provided a window on processes such as transpiration and the night-time re-filling of the xylem tissue.  Drought tolerance and adaptation strategies were also explored by comparing differences in sap-flow rates across treatment types and between species.

气候模型（climate models）预测，全球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。我们搭建了一套大型原位实验系统，用以探究降水变化对生态系统的影响。该实验在占地1600平方米（40米×40米）的样地中开展，共设置4种处理方式，在由矮松（Pinus edulis）与单籽杜松（Juniper monosperma）构成的天然生态系统中，每种处理设置3个重复样地。由于这两个物种拥有发达的根系，因此需开展大规模的人工调控方能有效改变其土壤水分有效性。
具体处理方案如下：1）灌溉组：额外补充水分；2）干旱组：仅接收自然降雨量的55%；3）覆盖对照组：接收自然降水，但可抵消实验装置本身带来的非处理干扰；4）自然对照组：完全接收自然降水。
相较于自然对照组、覆盖对照组与灌溉组，我们的干旱处理装置有效降低了土壤水势（soil water potential）与容积含水量（volumetric water content）。在生长季，干旱组与覆盖对照组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℃，同时在塑料遮罩装置的上下方均观测到短期的日间最高气温骤升现象。
干旱与灌溉处理对树木的黎明前水势（predawn water potential）、木质部液流（xylem sap-flow）与净光合速率（net photosynthesis）均产生了显著影响：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了显著下降。额外灌溉则使植株的水势与木质部液流速率较其他处理组均出现显著提升。
本实验设计可有效实现生态系统尺度下的植物水分胁迫调控，可覆盖多种干旱情境，并可模拟与历史干旱事件烈度相当的长期干旱——过往此类干旱事件曾导致这两个物种出现大范围死亡。本研究的核心目标是明确降水调控对这两种目标树种的影响，因此对这些树木水分关系的分析是本项目的核心环节。我们通过Granier探针（Granier probes）监测每株目标树木的木质部液流，该监测手段为蒸腾作用与夜间木质部组织补水等过程提供了观测窗口。此外，我们还通过对比不同处理组间以及两个物种间的液流速率差异，探究了物种的干旱耐受与适应策略。