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

   https://doi.org/10.6073/pasta/4c7b3696c3101375fbde251622748738  

   Pockman, William; McDowell, Nathan (January 2016, Environmental Data Initiative)

   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.  Water potential measurements were used to monitor the water stress of the two target species across the four treatment regimes.  Sampling for water potentials occurred twice daily.  One set of samples was collected hours before dawn and another set was collected at mid-day.  The predawn readings provided the “least-stressed†tree water content values as they were collected after the trees had returned to equilibrium over the evening and had yet to start transpiring.  The mid-day values, collected after tree-level respiration had been occurring for hours and when the daily temperatures were highest, represented the opposite “most-stressed†scenario. To gauge the effect of the irrigation treatment on the water content of the trees, we sampled water potentials just before and just after irrigation events.    

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2. Differences in the Hydration State of Riparian Pecan Trees Between Rural and Urban Settings

   https://doi.org/10.21423/twj.v15i1.7174  

   Snook, Michael; Matheny, Ashley; Restrepo_Acevedo, Ana Maria; Ulatowski, Maria (February 2024, Texas Water Journal)

   Urbanization causes changes in near-surface meteorology and rainfall-runoff relationships that threaten to place hydraulic stress on vegetation. The goal of this study was to investigate the differences in riparian zone tree hydration state, as indicated by leaf water potential, between an urban and a rural stream site, and to understand how the trees respond differently to precipitation events. At the rural stream site, the streambed was dry due to persistent drought conditions, whereas the urban stream site had established flow due to urban water inputs. The trees at the urban site were found to suffer less hydraulic stress than the trees at the rural site, as indicated by predawn leaf water potential measurements. Additionally, trees at the rural site were found to regulate stomatal openness to reduce transpiration on the day before rain, but not after, due to the presence of near-surface moisture introduced by the rain event. Trees at the urban site did not have to regulate stomatal openness before or after the rain, as the established flow in the stream provided consistent water access. These findings support the viability of protecting and preserving riparian ecosystems in urban settings. 

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3. Ecosystem-scale rainfall manipulation in a Pinon-Juniper woodland: Tree Sapwood and Leaf Area Data (2011)

   https://doi.org/10.6073/pasta/f4fbe653cd0a2b7bf645675fdc2294fa  

   Pockman, William; McDowell, Nathan (January 2015, Environmental Data Initiative)

   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. 

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

   https://doi.org/10.6073/pasta/74fa1f04faa8d8d9a8c9e8e5acf7a56e  

   Pockman, William; McDowell, Nathan (January 2016, Environmental Data Initiative)

   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. 

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5. Drought supersedes warming in determining volatile and tissue defenses of piñon pine (Pinus edulis)

   https://doi.org/10.1088/1748-9326/ab1493  

   Trowbridge, Amy M.; Stoy, Paul C.; Adams, Henry D.; Law, Darin J.; Breshears, David D.; Helmig, Detlev; Monson, Russell K. (June 2019, Environmental Research Letters)

   Abstract Trees are suffering mortality across the globe as a result of drought, warming, and biotic attacks. The combined effects of warming and drought onin situtree chemical defenses against herbivory have not been studied to date. To address this, we transplanted mature piñon pine trees—a well-studied species that has undergone extensive drought and herbivore-related mortality—within their native woodland habitat and also to a hotter-drier habitat and measured monoterpene emissions and concentrations across the growing season. We hypothesized that greater needle temperatures in the hotter-drier site would increase monoterpene emission rates and consequently lower needle monoterpene concentrations, and that this temperature effect would dominate the seasonal pattern of monoterpene concentrations regardless of drought. In support of our hypothesis, needle monoterpene concentrations were lower across all seasons in trees transplanted to the hotter-drier site. Contrary to our hypothesis, basal emission rates (emission rates normalized to 30 °C and a radiative flux of 1000μmol m⁻²s⁻¹) did not differ between sites. This is because an increase in emissions at the hotter-drier site from a 1.5 °C average temperature increase was offset by decreased emissions from greater plant water stress. High emission rates were frequently observed during June, which were not related to plant physiological or environmental factors but did not occur below pre-dawn leaf water potentials of −2 MPa, the approximate zero carbon assimilation point in piñon pine. Emission rates were also not under environmental or plant physiological control when pre-dawn leaf water potential was less than −2 MPa. Our results suggest that drought may override the effects of temperature on monoterpene emissions and tissue concentrations, and that the influence of drought may occur through metabolic processes sensitive to the overall needle carbon balance. 

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