Unmanned Aerial Vehicle − Based Rangeland Monitoring: Examining a Century of Vegetation Changes

Rangelands comprise a large component of the terrestrial land surface and provide critical ecosystem services, but they are degrading rapidly. Long-term rangeland monitoring with detailed, nonsubjective, quantitative observations can be expensive and difficult to maintain over time. Unmanned aerial vehicles (UAVs) provide an alternative means to gather unbiased and consistent datasets with similar details to field-based monitoring data. Comparing summer 2017 UAV images with long-term plot measurements, we demonstrate that rangeland vegetation cover changes can be accurately quantified and estimate an increase in total absolute shrub/subshrub cover from 34% in 1935 to > 80% in 2017 in central Arizona. We recommend UAV image-based rangeland monitoring for land managers interested in a few specific and dominant species, such as the foundation species, indicator species, or invasive species that require targeted monitoring. Land managers can identify and continuously monitor trends in rangeland condition, health, and degradation related to specific land use policies and management strategies.     

Geospatial Assessment of Grazing Regime Shifts and Sociopolitical Changes in a Mongolian Rangeland

Drastic changes have occurred in Mongolia’s grazing land management over the last two decades, but their effects on rangelands are ambiguous. Temporal trends in Mongolia’s rangeland condition have not been well documented relative to the effects of long-term management changes. This study examined changes in grazing land use and rangeland biomass associated with the transition from the socialist collective to the current management systems in the Tsahiriin tal area of northern Mongolia. Grazing lands in Tsahiriin tal that were formerly managed by the socialist collective are now used by numerous nomadic households with their privately owned herds, although the lands remain publicly owned. Grazing pressure has more than tripled and herd distribution has changed from a few spatially clustered large herds of sheep to numerous smaller herds of multiple species. Landsat image–derived normalized-difference vegetation index estimates suggest that rangeland biomass significantly decreased (P < 0.001) from the collective to the postcollective periods. The observed decrease was significantly correlated with changes in the grazing management system and increased stocking density (P < 0.001), even when potential climate-induced changes were considered. Furthermore, field- and Satellite Pour l’Observation de la Terre imagery–based rangeland assessments in 2007 and 2008 indicate that current rangeland biomass is low. Spatial pattern analyses show that the low biomass is uniform throughout the study site. The observed decrease in rangeland biomass might be further accelerated if current grazing land use continues with no formal rangeland management institution or organized, well-structured efforts by the local herding households.     

Energetics of large fullerenes: balls, tubes, and capsules

First-principles calculations were performed to compare the energies of 29 different fullerene structures, with mass number from 60 to 240, and of eight nonhelical graphite tubes of different radii. A quantity called the planarity, which indicates the completeness of the pi-bonding, is the single most important parameter determining the energetics of these structures. Empirical equations were constructed for the energies of nonhelical tubes and for those fullerene structures that may be described as balls or capsules. For a given mass number, bail-shaped fullerenes are energetically favored over capsular (tube-like) fullerenes.     

Characterizing Western Juniper Expansion via a Fusion of Landsat 5 Thematic Mapper and Lidar Data

Juniper encroachment into shrub steppe and grassland systems is one of the most prominent changes occurring in rangelands of western North America. Most studies on juniper change are conducted over small areas, although encroachment is occurring across large regions. Development of image-based methods to assess juniper encroachment over large areas would facilitate rapid monitoring and identification of priority areas for juniper management. In this study, we fused Landsat 5 Thematic Mapper and Light Detection and Ranging (lidar)–based juniper classifications to evaluate juniper expansion patterns in the Reynolds Creek Experimental Watershed of southwestern Idaho. Lidar applications for characterizing juniper encroachment attributes at finer scales were also explored. The fusion-based juniper classification model performed well (83% overall accuracy). A comparison of the resulting juniper presence/absence map to a 1965 vegetation cover map indicated 85% juniper expansion, which was consistent with tree-ring data. Comparisons of current and previous canopy-cover estimates also indicated an increase in juniper density within the historically mapped juniper distribution. Percent canopy cover of juniper varied significantly with land-cover types highlighting areas where intensive juniper management might be prioritized.     

Time-domain measurements of nanomagnet dynamics driven by spin-transfer torques

We present time-resolved measurements of gigahertz-scale magnetic dynamics caused by torque from a spin-polarized current. By working in the time domain, we determined the motion of the magnetic moment throughout the process of spin-transfer-driven switching, and we measured turn-on times of steady-state precessional modes. Time-resolved studies of magnetic relaxation allow for the direct measurement of magnetic damping in a nanomagnet and prove that this damping can be controlled electrically using spin-polarized currents.     

Reproducible measurement of single-molecule conductivity

A reliable method has been developed for making through-bond electrical contacts to molecules. Current-voltage curves are quantized as integer multiples of one fundamental curve, an observation used to identify single-molecule contacts. The resistance of a single octanedithiol molecule was 900 +/- 50 megohms, based on measurements on more than 1000 single molecules. In contrast, nonbonded contacts to octanethiol monolayers were at least four orders of magnitude more resistive, less reproducible, and had a different voltage dependence, demonstrating that the measurement of intrinsic molecular properties requires chemically bonded contacts.     

Comparing local vs. global visible and near-infrared (VisNIR) diffuse reflectance spectroscopy (DRS) calibrations for the prediction of soil clay, organic C and inorganic C

Local, field-scale, VisNIR-DRS soil calibrations generally yield the most accurate predictions but require a substantial number of local calibration samples at every application site. Global to regional calibrations are more economically efficient, but don't provide sufficient accuracy for many applications. In this study, we quantified the value of augmenting a large global spectral library with relatively few local calibration samples for VisNIR-DRS predictions of soil clay content (clay), organic carbon content (SOC), and inorganic carbon content (IC). VisNIR models were constructed with boosted regression trees employing global, local + global, and local spectral data, using local samples from two low-relief, sedimentary bedrock controlled, semiarid grassland sites, and one granitic, montane, subalpine forest site, in Montana, USA. The local + global calibration yielded the most accurate SOC predictions for all three sites [Standard Error of Prediction (SEP) = 3.8, 6.7, and 26.2 g kg^− 1]. This was similarly true for clay (SEP = 95.3 and 102.5 g kg^− 1) and IC (SEP = 5.5 and 6.0 g kg^− 1) predictions at the two semiarid grassland sites. A purely local calibration produced the best validation results for soil clay content at the subalpine forest site (SEP = 49.2 g kg^− 1), which also had the largest number of local calibration samples (N = 210). Using only samples from calcareous soils in the global spectral library combined with local samples produced the best SOC and IC results at the more arid of the two semiarid sites. Global samples alone never achieved more accurate predictions than the best local + global calibrations. For the temperate soils used in this study, the augmentation of a large global spectral library with relatively few local samples generally improved the prediction of soil clay, SOC, and IC relative to global or local samples alone.     

Spatial patterns of Douglas-fir and aspen forest expansion

Spatial patterns, rates, and density of encroaching forests into adjacent grasslands have important implications for long-term land use management and resource planning. This study examines the effects of Douglas-fir (Pseudotsuga menzeisii) and aspen (Populus tremuloides) regeneration mechanisms on sucker and seedling spatial patterns, distance from adult trees, and density in encroaching forests. A total of 8,924 aspen suckers and 1,244 Douglas-fir seedlings were counted and mapped in 2,920 quadrats (5 m × 5 m) in 106 plots along a lower forest-grassland ecotone in the Centennial Valley, MT, USA. Sucker and seedling spatial patterns were analyzed using Morisita’s I index. Average sucker and seedling density per quadrat and distance from adult trees were estimated for each plot and compared between aspen-dominated plots and Douglas-fir-dominated plots using ANOVA tests. Aspen suckers were established in a clustered spatial pattern at a significantly higher density and a significantly shorter distance from the adult trees. In contrast, Douglas-fir seedlings were established in varying spatial patterns at a significantly lower density and a significantly greater distance from the adult trees. Forest encroachment into the adjacent grassland in the Centennial Valley is occurring in contrasting patterns and at varying rates and densities due to the difference in aspen and Douglas-fir regeneration mechanisms.     

LiDAR-Based Classification of Sagebrush Community Types

Sagebrush (Artemisia spp.) communities constitute the largest temperate semidesert in North America and provide important rangelands for livestock and habitat for wildlife. Remote sensing methods might provide an efficient method to monitor sagebrush communities. This study used airborne LiDAR and field data to measure vegetation heights in five different community types at the Reynolds Creek Experimental Watershed, southwestern Idaho: herbaceous-dominated, low sagebrush (Artemisia arbuscula) –dominated, big sagebrush (Artemisia tridentata spp.) –dominated, bitterbrush (Purshia tridentata) -dominated, and other vegetation community types. The objectives were 1) to quantify the correlation between field-measured and airborne LiDAR-derived shrub heights, and 2) to determine if airborne LiDAR-derived mean vegetation heights can be used to classify the five community types. The dominant vegetation type and vegetation heights were measured in 3 × 3 m field plots. The LiDAR point cloud data were converted into a raster format to generate a maximum vegetation height map in 3-m raster cells. The regression relationship between field-based and airborne LiDAR-derived shrub heights was significant (R²=0.77; P value < 0.001). An analysis of variance test with all pairwise post hoc comparisons indicated that LiDAR-derived vegetation heights were significantly different among all vegetation community types (all P values < 0.01), except for herbaceous-dominated communities compared to low sagebrush-dominated communities. Although LiDAR measurements consistently underestimated vegetation heights in all community types, shrub heights at some locations were overestimated due to adjacent taller vegetation. We recommend for future studies a smaller rasterized pixel size that is consistent with the target vegetation canopy diameter.