Optical and structural properties of ZnO films deposited on GaAs by pulsed laser deposition

doi:10.1063/1.373643ZnO films were synthesized on GaAs substrates at different growth conditions by pulse laser deposition. High-purity (99.999%) oxygen was used as the ambient gas. The pressure of the ambient oxygen gas for ZnO film growth was varied from 20 to 50 mTorr, and the growth temperature from 300 to 450 °C. ZnO films showed very strong bound exciton peaks located between 3.37 and 3.35 eV. The full width at half maximum of the bound exciton peak is less than 5 meV. These results indicate ZnO films on GaAs substrates can be used for optical devices such as light-emitting diodes. The other significant properties of textured ZnO films on GaAs substrates are described.This work was supported in part by University of Missouri Research Board under Grant No. RB95-061, the U.S. Army Research Office ~Research Triangle Park, N.C.! under Grant No. DAAH04-94-G-0305, and the Office of Naval Research-Electronics Division, Contract No. N00014-99-1-0288

Impact of intra- versus inter-annual snow depth variation on water relations and photosynthesis for two Great Basin Desert shrubs

Snowfall provides the majority of soil water in certain ecosystems of North America. We tested the hypothesis that snow depth variation affects soil water content, which in turn drives water potential (Ψ) and photosynthesis, over 10 years for two widespread shrubs of the western USA. Stem Ψ (Ψ stem) and photosynthetic gas exchange [stomatal conductance to water vapor (g s), and CO2 assimilation (A)] were measured in mid-June each year from 2004 to 2013 for Artemisia tridentata var. vaseyana (Asteraceae) and Purshia tridentata (Rosaceae). Snow fences were used to create increased or decreased snow depth plots. Snow depth on +snow plots was about twice that of ambient plots in most years, and 20 % lower on -snow plots, consistent with several down-scaled climate model projections. Maximal soil water content at 40- and 100-cm depths was correlated with February snow depth. For both species, multivariate ANOVA (MANOVA) showed that Ψ stem, g s, and A were significantly affected by intra-annual variation in snow depth. Within years, MANOVA showed that only A was significantly affected by spatial snow depth treatments for A. tridentata, and Ψ stem was significantly affected by snow depth for P. tridentata. Results show that stem water relations and photosynthetic gas exchange for these two cold desert shrub species in mid-June were more affected by inter-annual variation in snow depth by comparison to within-year spatial variation in snow depth. The results highlight the potential importance of changes in inter-annual variation in snowfall for future shrub photosynthesis in the western Great Basin Desert

Climate control of terrestrial carbon exchange across biomes and continents

Peer reviewe

Nutrient Acquisition from Soils with Patchy Nutrient Distributions: Importance of Patch Size, Degree of Variability and Root Uptake Kinetics

Effectiveness of nitrate (NO3-) and phosphate (P) uptake in a patchy nutrient environment by tussocks of Agropyron desertorum (crested wheatgrass) was assessed with simulations using existing nutrient-uptake models parameterized with field data. Simulations conducted with Monte Carlo techniques were used to expose roots to different levels of nutrient variability with numbers of patches set at 10 large or 1000 small patches over the rooting zone of a single tussock. With constant uptake kinetics, simulated whole-plant uptake of both NO3- and P by established or growing roots was highest in soils with the nutrients distributed uniformly but declined with increasing soil nutrient variability for both nutrients. The decline was greater for NO3- than for P and was greater for both ions in soils with nutrients distributed in the 10 large patches. The reduction in predicted nutrient uptake with increasing variability was more pronounced with higher mean concentrations of P, while mean concentration had little effect on the pattern of NO3- uptake. However, allowing increased uptake kinetics with increasing nutrient concentration, as measured earlier for Agropyron desertorum, indicated that changes in kinetics could offset the lower whole-plant uptake rates in soils with patchy nutrient distributions. The effects of soil moisture and root distribution on whole-plant uptake of NO3- and P were also simulated. With constant kinetics, nutrient uptake in drier soil with uniformly distributed soil moisture was less affected by patchy nutrient distributions than was nutrient uptake in moister soil. Benefits associated with enhanced uptake kinetics with patchy nu- trients distributions were reduced in drier soils for NO3-, but were similar for P. Variability in soil moisture had little effect on patterns of P and N03- uptake. Differences in distri- butions of distances between fine roots had little influence on the patterns of NO3- and P uptake

Moderate-scale mapping methods of aspen stand types : a case study for Cedar Mountain in southern Utah

Quaking aspen (Populus tremuloides Michx.) are the most widely distributed tree species across North America, but its dominance is declining in many areas of the western United States, with certain areas experiencing rapid mortality events over the past decade. The loss of aspen from western landscapes will continue to profoundly impact biological, commercial, and aesthetic resources associated with aspen. However, many options are available for its restoration. Advances in remote sensing technologies offer cost-effective means to produce spatial and quantitative information on the distribution and severity of declining aspen at many scales. This report describes the development and application of transferable remote sensing and geographic information system methodologies to accurately classify aspen condition within areas of delineated aspen woodland cover. These methodologies were applied on Cedar Mountain in southern Utah within the Colorado Plateau to map three aspen stand conditions (healthy, damaged, and seral) successfully. Using moderate-scale imagery (2008 Landsat TM data), digital elevation model derivatives, high-resolution National Agriculture Imagery Program imagery, and a decision tree modeling approach, a spatially explicit 2008 landscape assessment of Cedar Mountain aspen was produced with an overall accuracy of 81.3% (Kappa [к] or KHAT accuracy measure = 0.69, n = 445). Of the total area mapped as aspen within the 12,139-ha study area, healthy aspen was the most abundant with 49% (5960 ha), followed by damaged with 35% (4210 ha), and seral with an estimated 16% (1968 ha) coverage. Aspen classification maps, derived from remotely sensed digital imagery and ancillary datasets, can offer objective management information to land managers to utilize when planning, implementing, and evaluating aspen restoration activities

Effects of photoinhibition on whole-plant carbon gain assessed with a photosynthesis model

Werner C, Ryel RJ, Correia O, Beyschlag W. Effects of photoinhibition on whole-plant carbon gain assessed with a photosynthesis model. Plant, Cell & Environment. 2001;24(1):27-40

The Western Aspen Alliance: Promoting Sustainable Aspen Ecosystems in Western North America

A consortium of researchers and managers has formed the Western Aspen Alliance (WAA) to coordinate and facilitate advances in aspen ecology in Western North America. Our prime goal is to disseminate state-of-the-science aspen information to interested managers, scientists, the public, and other users. Recent events, some potentially related to climate change, have spurred interest in aspen ecology. For example, reports of sudden aspen mortality, large-scale seedling establishment following wildfires, and documentation of trophic interactions between wolves, elk, and aspen, have all significantly modified our understanding of aspen ecosystems. We realize, regionally, that an aging aspen cohort will need to successfully regenerate to maintain sustainable populations. Wildlife pressure on aspen regeneration is an issue of concern for managers throughout western North America. The WAA will incorporate these lessons into an ongoing resource bank for managers throughout the region. From a science perspective, we wish to engender a cross-disciplinary network of researchers willing to take on pertinent aspen topics. For example, there is a current need to assess the extent of Sudden Aspen Decline (SAD), seral/stable aspen stands, and historic aspen coverage. Basic research on aspen physiology, disturbance ecology, water yield, genetics, herbivory, and biodiversity/trophic interactions issues are also desired. The social/aesthetic value of aspen is an another up-and-coming research area in need of work. We will form working groups for these issues and pursue additional aspen topics that arise. A central role of the WAA will be to sponsor field visits, workshops, conferences, and collaboration between researchers. We have recently initiated a bibliographic database of aspen research and management topics for use by WAA members. Though sponsored by Utah State University’s College of Natural Resources and the USDA Forest Service, Rocky Mountain Research Station, we are currently working with numerous state and federal agencies, NGO’s, and universities. We welcome your input and participation