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Abstract (from AGU100): Forest conservation and carbon sequestration efforts are on the rise, yet the long‐term stability of these efforts under a changing climate remains unknown. We generate nearly three decades of remotely sensed canopy water content throughout California, which we use to determine patterns of drought stress. Linking these patterns of drought stress with meteorological variables enables us to quantify spatially explicit biophysical drought resistance in terms of magnitude and duration. These maps reveal significant spatial heterogeneity in drought resistance and demonstrate that almost all forests have less resistance to severe, persistent droughts. By identifying the spatial patterning of biophysical...
From 2011 to 2016, California experienced a millennial-intensity drought, generating high levels of tree mortality. Remote sensing has been used to monitor the long-term impacts of drought; however, discriminating dead from live trees in arid and semiarid deciduous woodlands is challenging. The goals of this study were to assess and map the spatial patterns of drought-induced tree mortality in a blue oak (Quercus douglasii) woodland, a highly drought-tolerant species forming savannas along the lower foothills surrounding California's Central Valley. Airborne hyperspectral imagery was used to identify the most important wavelength regions predicting drought-induced blue oak mortality. The best metric to predict canopy...
Categories: Publication; Types: Citation
Abstract (from AGU): The prolonged 2012–2016 California drought has raised many issues including concerns over reduced vegetation health. Drought impacts are complicated by geographical differences in hydroclimatic variability due to a climatic dipole influenced by the Pacific. Analysis of MODIS‐derived Normalized Difference Vegetation Index and self‐calibrated Palmer Drought Severity Index from 2000 to 2018 reveals differences in drought and vegetation responses in Northern versus Southern California (NorCal vs SoCal, see definition in section 2.1). The greatest declines in Normalized Difference Vegetation Index were focused in the SoCal, while NorCal appears not severely affected thus far. It appears that both...
Abstract Urban vegetation is valuable in alleviating local heatwaves. However, drought may decrease vegetation health and limit this cooling effect. Here we use satellite-based Normalized Difference Vegetation Index (NDVI) and Palmer Drought Severity Index (PDSI) to investigate the sensitivity of urban vegetation to drought in Coastal Greater Los Angeles (CGLA) from 2001 to 2020. We applied four statistical models to analyze the relations between 15 socioeconomic variables and the vegetation's sensitivity to drought. We then examined the changes in the cooling effect of the urban vegetation during drought and non-drought periods using remotely sensed land surface temperature (LST) data. The results suggest that...
Categories: Publication; Types: Citation
Abstract (from MDPI): A combination of drought and high temperatures (“global-change-type drought”) is projected to become increasingly common in Mediterranean climate regions. Recently, Southern California has experienced record-breaking high temperatures coupled with significant precipitation deficits, which provides opportunities to investigate the impacts of high temperatures on the drought sensitivity of Mediterranean climate vegetation. Responses of different vegetation types to drought are quantified using the Moderate Resolution Imaging Spectroradiometer (MODIS) data for the period 2000–2017. The contrasting responses of the vegetation types to drought are captured by the correlation and regression coefficients...
Tree loss is increasing rapidly due to drought- and heat-related mortality and intensifying fire activity. Consequently, the fate of many forests depends on the ability of juvenile trees to withstand heightened climate and disturbance anomalies. Extreme climatic events, such as droughts and heatwaves, are increasing in frequency and severity, and trees in mountainous regions must contend with these landscape-level climate episodes. Recent research focuses on how mortality of individual tree species may be driven by drought and heatwaves, but how juvenile mortality under these conditions would vary among species spanning an elevational gradient—given concurrent variation in climate, ecohydrology, and physiology–remains...
Prescribed fire reduces fire hazards by removing dead and live fuels (small trees and shrubs). Reductions in forest density following prescribed fire treatments (often in concert with mechanical treatments) may also lessen competition so that residual trees might be more likely to survive when confronted with additional stressors, such as drought. The current evidence for these effects is mixed and additional study is needed. Previous work found increased tree survivorship in low elevation forests with a recent history of fire during the early years of an intense drought (2012 to 2014) in national parks in the southern Sierra Nevada. We extend these observations through additional years of intense drought and continuing...
Categories: Publication; Types: Citation
Abstract (from ScienceDirect): Drought is expected to become an increasingly important stressor on forests globally, and understanding the physiological mechanisms driving tree drought response is essential for developing effective mitigation and conservation measures for these ecosystems. In 2014, during California’s 2012–2016 “hotter” drought in which higher temperatures exacerbated the effects of low water availability, many giant sequoia trees in the Sierra Nevada mountains exhibited foliage dieback at levels previously unreported. We hypothesized that this apparent drought-induced foliage dieback was associated with spatial patterns of site water balance and consequently tree water status and physiology. As...
Reproduction is a key component of ecological resilience in forest ecosystems, so understanding how seed production is influenced by extreme drought is key to understanding forest recovery trajectories. If trees respond to mortality-inducing drought by preferentially allocating resources for reproduction, the recovery of the stand to pre-drought conditions may be enhanced accordingly. We used a 20-year annual seed capture data set to investigate whether seed production by three tree genera commonly found in the Sierra Nevada (Abies, Pinus, and Calocedrus) was correlated with variation in local weather, which included an extreme drought spanning multiple years. We tested whether average seed production differed during...
Categories: Publication; Types: Citation
Abstract (from Ecological Society of America): Tree mortality is an important outcome of many forest fires. Extensive tree injuries from fire may lead directly to mortality, but environmental and biological stressors may also contribute to tree death. However, there is little evidence showing how the combined effects of two common stressors, drought and competition, influence post‐fire mortality. Geographically broad observations of three common western coniferous trees subjected to prescribed fire showed the likelihood of post‐fire mortality was related to intermediate‐term (10 yr) pre‐fire average radial growth, an important component of tree vigor. Path analysis showed that indices of competition and drought...
Drought-associated woody-plant mortality has been increasing in most regions with multi-decadal records and is projected to increase in the future, impacting terrestrial climate forcing, biodiversity and resource availability. The mechanisms underlying such mortality, however, are debated, owing to complex interactions between the drivers and the processes. In this Review, we synthesize knowledge of drought-related tree mortality under a warming and drying atmosphere with rising atmospheric CO2. Drought-associated mortality results from water and carbon depletion and declines in their fluxes relative to demand by living tissues. These pools and fluxes are interdependent and underlay plant defences against biotic...
Categories: Publication; Types: Citation
Between 2012 and 2016, California suffered one of the most severe droughts on record. During this period Sequoiadendron giganteum (giant sequoias) in the Sequoia and Kings Canyon National Parks (SEKI), California, USA experienced canopy water content (CWC) loss, unprecedented foliage senescence, and, in a few cases, death. We present an assessment of the vulnerability of giant sequoia populations to droughts that is currently lacking and needed for management. We used a temporal trend of remotely sensed CWC obtained between 2015 and 2017, and recently georeferenced giant sequoia crowns to quantify the vulnerability of 7,408 individuals in 10 groves in the northern portion of SEKI. CWC is sensitive to changes in...
Categories: Publication; Types: Citation
Drought has impacted the Colorado River basin for the past 20 years and is predicted to continue. In response, decisions about how much water should be stored in large reservoirs and how much water can be consumptively used will be necessary. These decisions have the potential to limit riverine ecosystem management options through the effect water-supply decisions have on reservoir elevations. We used projected hydrology and river temperatures to compare the outcome of combinations of water storage scenarios and consumptive use limits on metrics associated with ecosystem management of the Colorado River in Grand Canyon. Ecosystem management metrics included the ability to implement designer flows, temperature suitability...
Categories: Publication; Types: Citation
Wetland managers in the Central Valley of California, a dynamic hydrological landscape, require information regarding the amount and location of existing wetland habitat to make decisions on how to best use water resources to support multiple wildlife objectives, particularly during drought. Scientists from the U.S. Geological Survey Western Ecological Research Center (WERC), Point Blue Conservation Science (Point Blue), and the U.S. Fish and Wildlife Service (USFWS) partnered to learn how wetland and flooded agricultural habitats used by waterfowl and shorebirds change during the non-breeding season (July–April) particularly during drought. During extreme drought conditions, the ability to provide sufficient water...
Categories: Publication; Types: Citation
Tree die-off, driven by extreme drought and exacerbated by a warming climate, is occurring rapidly across every wooded continent—threatening carbon sinks and other ecosystem services provided by forests and woodlands. Forecasting the spatial patterns of tree die-off in response to drought is a priority for the management and conservation of forested ecosystems under projected future hotter and drier climates. Several thresholds derived from drought-metrics have been proposed to predict mortality of Pinus edulis, a model tree species in many studies of drought-induced tree die-off. To improve future capacity to forecast tree mortality, we used a severe drought as a natural experiment. We compared the ability of existing...
Categories: Publication; Types: Citation
Identifying the weather thresholds that can transform plant communities is key to assessing the vulnerability of ecosystems to drought and climate shifts, and thus enabling adaptive management to mitigate their impacts on land resources. We asked whether and how drought contributes to decline of big sagebrush, a widespread shrub of the western US that is critical for wildlife such as the imperiled sage grouse yet is poorly adapted to fire. Our objective was to quantitatively define “ecological drought” – water deficits that result in impacts to ecosystems - based on a precise set of weather and soil moisture conditions that are associated with failure of sagebrush stands to recover and sites converted into low-diversity...
Categories: Publication; Types: Citation
Abstract (from SpringerLink): This paper presents the first record of fire in Pacific coast salt marshes; the 1993 Green Meadows Fire and the 2013 Camarillo Springs Fire burned an area of Salicornia-dominated salt marsh at Point Mugu, CA. These fires inspire concern about resiliency of ecosystems not adapted to fire, already threatened by sea-level rise (SLR), and under stress from extreme drought. We monitored vegetation percent cover, diversity, and soil organic carbon (SOC) in burned and unburned areas of the salt marsh following the 2013 Camarillo Springs Fire and used remotely sensed Normalized Vegetation Difference Index (NDVI) analysis to verify the in situ data. Two years following the fire, vegetation percent...
Abstract (from AMS100): Between water years 2012 and 2017, the Truckee–Carson river system in the western United States experienced both historic-low and record-high Sierra Nevada snowpack, anomalously warm temperatures, and winter and spring flooding. As part of an ongoing collaborative modeling research program in the river system, researchers conduct annual interviews with key local water managers to characterize local climate adaptation strategies and implementation barriers, and identify science information needs to prioritize ongoing research activities. This article presents new findings from a third wave of interviews conducted with the same water managers following the historic 2017 wet year. Comparison...
We aimed to improve the scientific capacity to estimate climate extremes, evaluate their effects on natural resources, and enhance a platform for derivation of and access to customized climate information for the full extent of the Southwest. Extreme climate can have substantial effects on species, ecological and evolutionary processes, and the health of visitors to public lands. Researchers generally can specify the climate-extreme metrics, and the extents and resolutions of those metrics, most relevant to their scientific objectives and the practical applications of their work. However, such application-specific data rarely are available. We screened global climate models (GCMs) on the basis of their realism...
This study builds on a collaboration with a water resource management community of practice in the Upper Colorado River Basin to develop scenarios of future drought and assess impacts on water supply reliability. Water managers are concerned with the impacts of warming on water year streamflow, but uncertainties in projections of climate make the application of these projections to planning a challenge. Instead, water managers considered a plausible scenario for future drought to be historical droughts to which warming is added. We used a simple statistical model of water year streamflow with temperatures increased by 1 °C to 4 °C, and then examined reductions in flow and runoff efficiency (RE) with each degree...
Categories: Publication; Types: Citation