Northern Arizona University
  • Flagstaff, Arizona, United States
Recent publications
Background Most North American temperate forests are plantation or regrowth forests, which are actively managed. These forests are in different stages of their growth cycles and their ability to sequester atmospheric carbon is affected by extreme weather events. In this study, the impact of heat and drought events on carbon sequestration in an age-sequence (80, 45, and 17 years as of 2019) of eastern white pine ( Pinus strobus L.) forests in southern Ontario, Canada was examined using eddy covariance flux measurements from 2003 to 2019. Results Over the 17-year study period, the mean annual values of net ecosystem productivity (NEP) were 180 ± 96, 538 ± 177 and 64 ± 165 g C m –2 yr –1 in the 80-, 45- and 17-year-old stands, respectively, with the highest annual carbon sequestration rate observed in the 45-year-old stand. We found that air temperature (Ta) was the dominant control on NEP in all three different-aged stands and drought, which was a limiting factor for both gross ecosystem productivity (GEP) and ecosystems respiration (RE), had a smaller impact on NEP. However, the simultaneous occurrence of heat and drought events during the early growing seasons or over the consecutive years had a significant negative impact on annual NEP in all three forests. We observed a similar trend of NEP decline in all three stands over three consecutive years that experienced extreme weather events, with 2016 being a hot and dry, 2017 being a dry, and 2018 being a hot year. The youngest stand became a net source of carbon for all three of these years and the oldest stand became a small source of carbon for the first time in 2018 since observations started in 2003. However, in 2019, all three stands reverted to annual net carbon sinks. Conclusions Our study results indicate that the timing, frequency and concurrent or consecutive occurrence of extreme weather events may have significant implications for carbon sequestration in temperate conifer forests in Eastern North America. This study is one of few globally available to provide long-term observational data on carbon exchanges in different-aged temperate plantation forests. It highlights interannual variability in carbon fluxes and enhances our understanding of the responses of these forest ecosystems to extreme weather events. Study results will help in developing climate resilient and sustainable forestry practices to offset atmospheric greenhouse gas emissions and improving simulation of carbon exchange processes in terrestrial ecosystem models.
A (k,r)-coloring of a graph G is a proper k-vertex coloring of G such that the neighbors of each vertex of degree d will receive at least min{d,r} different colors. The r-hued chromatic number, denoted by χr(G), is the smallest integer k for which a graph G has a (k,r)-coloring. This article is intended to survey the recent developments on the studies related to this r-hued colorings. Emphases are on the r-hued colorings of planar graphs, graph families with forbidden minors, and sparse graphs, as well as on the comparison between the r-hued chromatic number and the chromatic number of a graph, and the sensitivity studies of the r-hued chromatic number. It also surveys other related results on r-hued colorings and list r-hued colorings.
Pressures to maximize survey space or mitigate respondent fatigue can lead researchers to employ abbreviated during data collection. This is problematic because short-form measures can suffer from reduced reliability and validity. Thus, we conducted a meta-analysis to determine whether the use of brief measures of the Big Five in business research tends to produce psychometrically sound and criterion-valid results. We compared scale internal consistencies and effect size estimates from our study with meta-analytic estimates for long measures of the Big Five, as established in the literature. Our results indicated that, in general, internal consistency estimates were not substantively different. However, the criterion-related validity comparisons indicated that several point estimates for individual measures did not fall within the credibility intervals obtained from prior meta-analyses. This suggests that although brief measures of the Big Five might appear acceptable for use in business research, caution should be exercised when choosing a brief measure.
Disturbance is one of the fundamental shapers of ecological communities, redistributing resources and resetting successional pathways. Human activities including resources management can influence disturbance regimes and trajectories by actively imposing or suppressing disturbance events or shaping ecosystem recovery via disturbance response. Furthermore, different management objectives may drive different disturbance responses. This suggests that the management jurisdiction to which a land parcel is assigned is likely to influence disturbance management and therefore ecological conditions within that parcel. Here, we combined two exploratory approaches to investigate this linkage. First, we used a systematic literature review to develop a typology of reported disturbance response types and strategies by federal land management agencies in the US. Second, we used Forest Inventory and Analysis (FIA) plot data in five multi-jurisdictional ecosystems containing national parks to investigate the relationship between land ownership and large disturbance occurrence and between disturbance and tree growth rate. We found that agencies vary in the diversity of disturbance response tactics they are reported to employ, and disturbance types vary in the diversity of responses reported in the literature. Disturbance occurrence varied by land ownership type across the FIA dataset, and the direction of tree growth rate was influenced by the interaction between ownership type and disturbance occurrence in two of five examined ecosystems. Although our mixed methods approach was purely exploratory and not mechanistic, our findings suggest that disturbance response is one possible route by which management regimes may influence ecological conditions. Efforts to understand and predict ecological heterogeneity across large landscapes must consider variation in the social system as a potential contributor to such patterns.
Tree mortality rates have been increasing globally with mountainous regions experiencing higher temperatures and impacts from the expansion and intensification of pests and invasion by non-native agents. Western North American high-elevation forests exemplify these trends, and they often include one or more species of five-needle white pines (High-5 hereafter). These species share many characteristics critical to defining the structure and function of many subalpine forests. The main threats to High-5 populations include the non-native pathogen Cronartium ribicola, which causes the disease white pine blister rust, climate-driven drought stress, episodic and high mortality from mountain pine beetle (Dendroctonus ponderosae), and wildfires of increasing frequency, size, and intensity. The six High-5 species occurring in western North America (whitebark pine, Pinus albicaulis; limber pine, P. flexilis; southwestern white pine, P. strobiformis; Rocky Mountain bristlecone pine, P. aristata; Great Basin bristlecone pine, P. longaeva; and foxtail pine, P. balfouriana) differ in their health status and threat level. The convergence of threats impacting the rapidly declining species could portend future declines in the species and populations currently less impacted by recent disturbances. Differences in the innate adaptive capacities of the species affect their population trajectories under these novel combinations of stressors. We evaluate the status and outlook for each species and address the following questions: (1) Is the environment changing too fast and the intensity of stressors too great for the species to adapt and recover? (2) Do the species have the heritable traits necessary to sustain fitness under C. ribicola and climatic stresses? (3) Are other mortality factors increasing to the degree that they reduce the populations further and delay or preclude adaptation and population recovery? (4) Can the species escape the stressors through migration? Insights related to these questions provide guidance for forest management to facilitate adaptation and increase the resilience of these species into the future.
Climate change is altering the distribution of woody plants by influencing demographic processes and modifying disturbance regimes. Trailing-edge forests may be particularly vulnerable to these effects because they exist at warm, dry margins of tree distributions. To better understand recent climate-driven changes in trailing-edge forests, we used Landsat time series and 1558 field reference plots to develop annual land cover maps from 1985 to 2020 in two large, biodiverse landscapes in central Arizona, USA. We then combined annual land cover maps with tree ring records and spatial data describing interannual climate, terrain, bark beetle (Curculionidae: Scolytinae) activity, wildfire, and harvest to quantify drivers of forest change. Throughout the two landscapes, forest extent declined by 0.3 % and 0.8 % from 1985 to 2020. However, considerable variation occurred within the study period, with abrupt (ca. 1–2 years) declines in forest extent followed by gradual (ca. 10 years) recovery on each landscape. Pinyon-juniper (Pinus edulis, Pinus monophylla, and/or Juniperus spp.) cover increased from 1985 to ca. 2000 but declined after 2000, a period of extreme drought and regional tree die-off. In contrast, pine-oak (Pinus ponderosa and Quercus spp.) cover increased from 2000 to 2020, primarily due to declines in ponderosa pine and mixed conifer cover over the same period. Wildfire was a key driver of transitions from forest to non-forest cover in our study area, with the occurrence of multiple compounded drought years playing an important role in unburned areas. By driving transitions to alternative forest types or non-forest cover, disturbance and drought will increasingly shape forest dynamics and ecosystem transformations throughout the southwestern US.
Although most scholars characterize the anonymous Middle English Patience as sustained biblical translation or paraphrase, few have examined the work in relation to ongoing concerns about the nature and status of vernacular religious writing in later medieval England. This essay argues that the poem evidences and responds to such concerns through its complex treatment of the commonplace association of vernacular writing with speech. Like a number of other, more explicit vernacular prologues, the opening of Patience uses references to speech to position vernacular writing between the authority of scriptural texts and the immediacy and broad reach of the liturgy and storytelling. The poet further explores the relation between authority and immediacy in his subsequent retelling of the story of Jonah, the disobedient but ultimately effective prophet to the Gentiles. By underscoring qualitative differences between divine and human speech, and between Jonah as penitential witness and inspired prophet, the poet presents vernacular religious writing as foremost hortatory and affective–a means of moving–rather than a medium for theological speculation. Jonah's varied roles as speaker, and his juxtaposition with the prophet Daniel in Cleanness, reveal a poet exploring, rather than narrowly defining, the work and nature of vernacular religious writing.
Wearable robotic systems, such as exoskeletons, are designed to assist human motion; however, they are typically only studied during level walking. Before exoskeletons are broadly integrated into unstructured environments, it will be important to evaluate exoskeletons in a broader set of relevant tasks. A balance beam traverse was used to represent a constrained foot placement task for examining balance and stability. Participants (n = 17) completed the task in their own shoes (Pre-Exoskeleton and Post-Exoskeleton trials), and when wearing a lower-limb exoskeleton (Dephy ExoBoot) in both powered and unpowered states. Data were collected via inertial measurement units (on the torso and feet) and analyzed on a pooled level (with data from all participants) and on an individual level (participant-specific confidence intervals). When examining pooled data, it was observed that the exoskeleton had mixed effects on stride stability metrics. When compared to the Post-Exoskeleton shoe control, it was observed that stride duration was increased when wearing the exoskeleton (both powered and unpowered states), while normalized stride length and stride speed were not affected. Despite the changes in stride stability, overall balance (as measured by torso sway) remained unaffected by exoskeleton state. On an individual level, it was observed that not all participants followed these general trends, and within each metric, some increased, some decreased, and some had no change in the Powered Exoskeleton condition when compared to the Post-Exoskeleton Shoe condition: normalized stride length (0% increased, 12% decreased, 88% no change), stride duration (35% increased, 0% decreased, 65% no change), and torso sway (0% increased, 12% decreased, 88% no change). Our findings suggest that the lower-limb exoskeleton evaluated can be used during tasks that require balancing, and we recommend that balancing tasks be included in standards for exoskeleton evaluation.
Climate change affects the Arctic and sub-Arctic regions by exposing previously frozen permafrost to thaw, unlocking soil nutrients, changing hydrological processes, and boosting plant growth. As a result, sub-Arctic tundra is subject to a shrub expansion, called “shrubification”, at the expense of sedge species. Depending on the intrinsic foliar properties of these plant species, changes in foliar mineral element fluxes with shrubification in the context of permafrost degradation may influence topsoil mineral element composition. Despite the potential implications of changes in topsoil mineral element concentrations for the fate of organic carbon, this remains poorly quantified. Here, we investigate vegetation foliar and topsoil mineral element composition (Si, K, Ca, P, Mn, Zn, Cu, Mo, V) across a natural gradient of permafrost degradation at a typical sub-Arctic tundra at Eight Mile Lake (Alaska, USA). Results show that foliar mineral element concentrations are higher (up to 9 times; Si, K, Mo for all species, and for some species Zn) or lower (up to 2 times; Ca, P, Mn, Cu, V for all species, and for some species Zn) in sedge than in shrub species. As a result, a vegetation shift over ∼40 years has resulted in lower topsoil concentrations in Si, K, Zn, and Mo (respectively of 52, 24, 20, and 51%) in highly degraded permafrost sites compared to poorly degraded permafrost sites due to lower foliar fluxes of these elements. For other elements (Ca, P, Mn, Cu, and V), the vegetation shift has not induced a marked change in topsoil concentrations at this current stage of permafrost degradation. A modeled amplified shrubification associated with a further permafrost degradation is expected to increase foliar Ca, P, Mn, Cu, and V fluxes, which will likely change these element concentrations in topsoil. These data can serve as a first estimate to assess the influence of other shifts in vegetation in Arctic and sub-Arctic tundra such as sedge expansion under wetter soil conditions.
Chinese milk vetch (Astragalus sinicus L) is a widely used leguminous green manure in paddy rice culture and has potential to replace or partly replace inorganic nitrogen (N) fertilizer. However, there is little quantitative information regarding replacing inorganic N with milk vetch on CH4 and N2O emissions. This study investigated the relationship between greenhouse gas production and carbon (C) and N release under 0%, 25%, 50%, 75% and 100% substitution of urea-N with milk vetch aboveground litter in a paddy soil. CH4 and N2O emission fluxes were measured using a static chamber method and milk vetch decomposition rates were tested by mesh bag method. After incorporation, C and N release from milk vetch followed a single exponential decay model, with 82.3–89.1% and 98.8–98.9% of the original C and N released during 125 days. CH4 flux was positively correlated with milk vetch substitution ratio, while it was negative correlated with N2O flux. Compared with 0% substitution, 25% substitution decreased global warming potential (GWP) and greenhouse gas intensity (GWP per unit of grain yield) by 12.1% and 10.3%, respectively, while 100% substitution increased GWP and greenhouse gas intensity by 38.4% and 65.2%, respectively. The results showed that soil pH, NH4⁺-N and redox state were significantly correlated with CH4 emission, while C and N remaining in the incorporated milk vetch residue and NH4⁺-N were key factors regulating N2O emission. It could be concluded that replacing 25% of urea-N with milk vetch was most effective in greenhouse gas mitigation while maintaining rice yield.
Software bots have been facilitating several development activities in Open Source Software (OSS) projects, including code review. However, these bots may bring unexpected impacts to group dynamics, as frequently occurs with new technology adoption. Understanding and anticipating such effects is important for planning and management. To analyze these effects, we investigate how several activity indicators change after the adoption of a code review bot. We employed a regression discontinuity design on 1,194 software projects from GitHub. We also interviewed 12 practitioners, including open-source maintainers and contributors. Our results indicate that the adoption of code review bots increases the number of monthly merged pull requests, decreases monthly non-merged pull requests, and decreases communication among developers. From the developers’ perspective, these effects are explained by the transparency and confidence the bot comments introduce, in addition to the changes in the discussion focused on pull requests. Practitioners and maintainers may leverage our results to understand, or even predict, bot effects on their projects.
Ecosystem primary productivity is a key ecological process influencing many ecosystem services, including carbon storage. Thus, clarifying how primary productivity in terrestrial ecosystems responds to climatic variability can reveal key mechanisms that will drive future changes in the global carbon budget. Satellite products of canopy greenness are widely used as proxies for vegetation productivity to evaluate how ecosystems respond to climate variability. However, to what degree inter-annual variations in productivity are consistent with greenness and how this relationship varies spatially remains unclear. Here we investigated the strength of the coupling between inter-annual variations in leaf area index (LAI, a measure of greenness) and ecosystem gross primary productivity (GPP) derived from eddy covariance towers, i.e., the r² of the LAI-GPP relationship. Overall, inter-annual GPP and LAI were highly coupled (i.e., high r²) in arid grasslands, but were fully decoupled in mesic evergreen broadleaf forests, indicating that this relationship varies strongly along aridity gradients. A possible mechanism of the spatial variation in the LAI-GPP relationship is that the tradeoff between ecosystem structure (LAI) and physiology (photosynthesis per unit leaf area) becomes stronger in more humid climates. Land models overestimated the r² of LAI-GPP correlation for most ecosystem types and failed to capture the spatial pattern along aridity gradients. We conclude that relying on greenness products for evaluating inter-annual changes in vegetation productivity may bias assessments, especially in tropical rainforest ecosystems. Our findings may also reconcile observed disparities between responses in greenness and GPP during drought in Amazon forests.
Wildfires play a vital role in ecosystems and are one of the most significant natural processes that shape the northern Sierra Nevada and southern Cascade Mountain Ranges in northeastern California. At the same time, across the American West, a history of fire exclusion, timber salvage and harvest, and human-assisted reforestation has caused widespread changes in forest vegetation patterns over the past century. Federal agencies such as the National Park Service (NPS) and US Forest Service (USFS) have different missions and approaches to these land management activities, creating a mosaic of ecological conditions across managed landscapes both before and after wildfires. We studied vegetation recovery following a major fire that crossed jurisdictional boundaries to explore how management history (fire exclusion and timber harvest and salvage) and post-fire reforestation efforts may influence vegetation regeneration. Using both field-based data collection and remote sensing Normalized Difference Vegetation Index (NDVI) analyses, we assessed differences in vegetation communities found in the NPS and USFS portions of the footprint of the 2012 Reading Fire in northeastern California. We found that a legacy of timber harvest combined with fire exclusion was associated with denser shrub regeneration and minimal sapling recovery. In this context, areas that were reforested post-fire returned a relatively diverse and abundant suite of mixed-conifer tree species to the landscape. In areas with a history of fire exclusion that regenerated without human interference after the fire, saplings did establish on their own but were predominantly white fir (Abies concolor), a shade-tolerant species that is known to increase wildfire severity.
The unexpected detection of ∼16 wt.% monoclinic tridymite, a high-temperature silica polymorph, within an otherwise lacustrine mudstone in Gale crater, Mars raises significant questions about its formation and the extent of magmatic evolution on that planet. The rock sample, analyzed by the X-ray diffractometer onboard the Curiosity rover, also contained feldspar, cristobalite, and opaline silica (±Si-glass). Monoclinic tridymite is extremely rare on Earth, and has only been discovered in silicic volcanic environments, high-temperature impact settings, and extraterrestrial rocks. We review the most common formation pathways of natural tridymite and run thermodynamical models to investigate possible formation mechanisms. We consider the broader context of the sample to propose a formation and transport mechanism based on: (1) the mineralogical assemblage of the mudstone and rocks in the vicinity, (2) the composition of the mudstone layer, and (3) the overall geological context. Based on the large amount of tridymite, the high SiO2 and low Al2O3 concentration of the mudstone, and the low temperature context within distal lacustrine mudstone, we propose that an explosive eruption released Si-rich ashes, which were deposited into Gale crater's watershed as a tridymite-rich ashfall along with cristobalite, feldspar, Ti-oxide, and Si-rich glass, when Gale was still a lake (Hesperian). The dissolution of Si-rich glass and mineral sorting during transport would have concentrated tridymite, caused opaline silica precipitation, and relatively lowered the Al2O3 concentration. This scenario implies that explosive volcanism on Mars occurred during the Hesperian and might not be restricted to basaltic eruptions, revealing the complexity of Mars magmatism.
Thermal infrared spectra taken by the Emirates Mars Infrared Spectrometer (EMIRS) on‐board the Emirates Mars Mission (EMM) spacecraft are well suited for the retrieval of surface temperatures, the atmospheric temperature profile from the surface to ∼40 km, and the column abundance of dust aerosols, water ice clouds, and water vapor. A constrained linear inversion retrieval routine that includes multiple scattering has been developed and optimized for this purpose. Here, we present a brief overview of the retrieval algorithm and first atmospheric science results from observations taken by EMIRS over the first Earth year of EMM Science Phase operations. These retrievals show extensive water ice clouds, typical for the aphelion season of these observations, and the expected north polar summer maximum and subsequent equatorward transport of water vapor is well documented. An unusually strong and early regional dust storm and its associated thermal response were also observed.
Co-production of knowledge (through project design or research) is viewed as an effective approach to solving environmental problems, which may also increase community adaptive capacity in the face of climate change. However, the reality is that little is known about long-term impacts of co-production on researchers, communities, and outputs. We qualitatively analyzed case studies to understand co-production processes and related adaptive capacity outcomes. These 13 case studies were developed to identify impacts of the United States Department of Agriculture National Institute of Food and Agriculture water (2001-2013) and climate (2010-2015) portfolios, which funded projects focused on research, education, and extension related to climate and water issues on working lands. Case study data included interviews, survey responses, and analysis of reports and publications related to a single project. We found that projects which were responsive to specific needs and assets of stakeholders had strong connections to adaptive capacity outcomes, but that these projects did not necessarily entail highly interactive practices of co-production of knowledge (e.g., stakeholder-driven research with continuous interactions between academic and non-academic partners). Our research provides evidence to suggest that, in some contexts, engagement approaches that are less time-and resource-intensive for stakeholders may be as effective at building adaptive capacity as highly interactive co-production efforts.
Looking back on the 20th century, “language revolution” is not an overstatement. Professor Tsu presents a captivating account of the technology of reading and writing in China that over the years built upon the historical tasks of unification around a national language and the standardization of its writing system. The story, however, begins in Chapter 1 with the Hundred Days Reform of 1898. Credit goes to fugitive scholar Wang Zhao whose early understanding of the language-literacy interface presaged the sweeping changes of the modern period.
Background Previous studies suggested associations between the oral microbiome and lung cancer, but studies were predominantly cross-sectional and underpowered. Methods Using a case-cohort design, 1,306 incident lung cancer cases were identified in the Agricultural Health Study, NIH-AARP Diet and Health Study, and Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Referent subcohorts were randomly selected by strata of age, sex, and smoking history. DNA was extracted from oral wash specimens using the DSP DNA Virus Pathogen kit, the 16S rRNA gene V4 region was amplified and sequenced, and bioinformatics were conducted using QIIME 2. Hazard ratios (HR) and 95% confidence intervals (CI) were calculated using weighted Cox proportional hazards models. Results Higher alpha diversity was associated with lower lung cancer risk (Shannon index HR 0.90; 95% CI: 0.84–0.96). Specific principal component vectors of the microbial communities were also significantly associated with lung cancer risk. After multiple testing adjustment, greater relative abundance of three genera and presence of one genus were associated with greater lung cancer risk, while presence of three genera were associated with lower risk. For example, every standard deviation increase in Streptococcus abundance was associated with 1.14 times the risk of lung cancer (95% CI: 1.06–1.22). Associations were strongest among squamous cell carcinoma cases and former smokers. Conclusions Multiple oral microbial measures were prospectively associated with lung cancer risk in three US cohort studies with associations varying by smoking history and histologic subtype. The oral microbiome may offer new opportunities for lung cancer prevention.
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7,040 members
Ophelia Wang
  • School of Earth and Sustainability
Alan Lew
  • Department of Geography, Planning, and Recreation
Marco Aurelio Gerosa
  • School of Informatics, Computing, and Cyber Systems
Nancy Collins Johnson
  • Environmental & Biological Sciences
Flagstaff, Arizona, United States