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Title: Impacts of long-term precipitation manipulation on hydraulic architecture and xylem anatomy of piñon and juniper in Southwest USA: Xylem acclimation in piñon-juniper woodland
Award ID(s):
1655499 1748133 1440478
Author(s) / Creator(s):
; ; ; ; ; ; ;
Date Published:
Journal Name:
Plant, Cell & Environment
Page Range / eLocation ID:
421 to 435
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    Piñon–juniper (PJ) woodlands are a dominant community type across the Intermountain West, comprising over a million acres and experiencing critical effects from increasing wildfire. Large PJ mortality and regeneration failure after catastrophic wildfire have elevated concerns about the long‐term viability of PJ woodlands. Thinning is increasingly used to safeguard forests from fire and in an attempt to increase climate resilience. We have only a limited understanding of how fire and thinning will affect the structure and function of PJ ecosystems. Here, we examined vegetation structure, microclimate conditions, and PJ regeneration dynamics following ~20 years post‐fire and thinning treatments. We found that burned areas had undergone a state shift that did not show signs of returning to their previous state. This shift was characterized by (1) distinct plant community composition dominated by grasses; (2) a lack of PJ recruitment; (3) a decrease in the sizes of interspaces in between plants; (4) lower abundance of late successional biological soil crusts; (5) lower mean and minimum daily soil moisture values; (6) lower minimum daily vapor pressure deficit; and (7) higher photosynthetically active radiation. Thinning created distinct plant communities and served as an intermediate between intact and burned communities. More intensive thinning decreased PJ recruitment and late successional biocrust cover. Our results indicate that fire has the potential to create drier and more stressful microsite conditions, and that, in the absence of active management following fire, there may be shifts to persistent ecological states dominated by grasses. Additionally, more intensive thinning had a larger impact on community structure and recruitment than less intensive thinning, suggesting that careful consideration of goals could help avoid unintended consequences. While our results indicate the vulnerability of PJ ecosystems to fire, they also highlight management actions that could be adapted to create conditions that promote PJ re‐establishment.

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  2. Extreme temperatures and severe drought events have led to widespread tree mortality worldwide. In semi-arid regions of the Southwest United States, these events pose a significant threat to piñon-juniper (PJ) woodlands. We studied the effects of piñon and juniper mortality on the growth and physiology of existing saplings in PJ woodlands by analyzing water status, photosynthetic activity, and tissue chemistry to gain insights into these impacts. Juniper saplings exhibited improved water status and water use efficiency in response to overstory mortality, whereas piñon saplings did not. Additionally, both piñon and juniper saplings exhibited increased photosynthetic rates, increased photosynthetic capacity, and enhanced growth rates. Our results suggest that saplings of both species responded similarly regardless of whether a mature piñon or juniper died. However, piñon saplings appeared to be more vulnerable to overstory mortality, likely due to the difference in hydraulic strategies between piñon and juniper This study enhances our understanding of the post-mortality recovery process in piñon-juniper ecosystems, providing valuable insights into the contrasting effects of piñon vs. juniper mortality as well as the distinct physiological responses exhibited by piñon and juniper saplings. 
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  3. Climate change and land-use legacies have caused a shift in wildfires and post-fire growing conditions. These changes have strong potential to diminish the resilience of many ecosystems, with cascading effects and feedbacks across taxa. Piñon-juniper (PJ) woodlands are a diverse and widespread forest type in the western US and are home to many obligate and semi-obligate bird species. As such, this system is ideal for understanding wildfire resilience, or lack thereof, in terms of both vegetation and wildlife associations. This study evaluated post-fire vegetation structure and associated avian communities following three wildfires; one that burned one year prior to sampling (recent fire), and two that burned approximately 25 years previously (old fires). Vegetation characteristics and the habitat use of PJ-associated bird species were compared across severely burned patches, unburned refugia, and unburned sites outside of the burn perimeter. We expected wildfire to alter vegetation and bird usage for the first few years post-fire, which we observed in our recent burns. However, even 25-years post-fire, little recovery to PJ woodland had occurred and the associated bird communities had not returned, compared to unburned areas. No piñon regeneration was observed in any burned areas and no juniper regeneration in the recent fire. Piñon seedling densities in unburned sites and refugia averaged 80 ha−1 and 151 ha−1, respectively, while juniper seedling densities were 220 ha−1 in both habitat types. Habitat use for thirteen PJ-associated species were modeled, three of which (Woodhouse’s Scrub Jay, Ash-throated Flycatcher, and Virginia’s Warbler) used all habitats. Four species (American Robin, Gray Vireo, Black-throated Gray Warbler, and Gray Flycatcher) were essentially absent from the old burn habitat, reflecting species-specific need for mature piñon or juniper trees and/or greater canopy cover. Conversely, birds that were present in the old burn habitat (including Virginia’s Warbler, Blue-gray Gnatcatcher, Woodhouse’s Scrub-jay, Ash-throated Flycatcher, and Spotted Towhee) are typically associated with habitat edges, high shrub cover, or cavity nests. Altered vegetation structure and bird habitat use in burned areas 25 years post-fire are evidence for enduring conversion to non-forest vegetation types. However, unburned refugia embedded in burned areas maintain forest attributes and support obligate bird communities, supporting ecological function and biological diversity. 
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  4. 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.A micrometeorological station was used to document the climatic conditions at the study site.  Monitoring the ambient environment in this way allowed us to more easily determine which tree growth responses were driven by changes in the native climate as opposed to those resulting from the rainfall manipulation treatments.  Environmental factors such as temperature, relative humidity, and photosynthetically active radiation (PAR) have a huge impact on the physiological processes that are being explored in this project.  The data collected by the station created a local climatic record which was needed to provide the context in which the treatment effects can be examined and sensor readings can be interpreted. 
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