Woody encroachment of grasslands: Near-surface thermal implications assessed through the lens of an astronomical event.

Temperature has long been understood as a fundamental condition that influences ecological patterns and processes. Heterogeneity in landscapes that is structured by ultimate (climate) and proximate (vegetation, topography, disturbance events, and land use) forces serve to shape thermal patterns across multiple spatio-temporal scales. Thermal landscapes of grasslands are likely shifting as woody encroachment fragments these ecosystems and studies quantifying thermal fragmentation in grassland systems resulting from woody encroachment are lacking. We utilized the August 21st, 2017, solar eclipse to mimic a rapid sunrise/sunset event across a landscape characterized as a grassland to experimentally manipulate levels of solar radiation in the system. We then quantified changes in near-surface temperatures resulting from changes in solar radiation levels during the eclipse. Temperatures were monitored across three grassland pastures in central Oklahoma that were characterized by different densities (low, medium, and high) of Juniperus virginiana to understand the impact of woody encroachment on diurnal temperature patterns and thermal heterogeneity in a grassland's thermal landscape. The largest temperature range across sites that occurred during the eclipse was in the mixed grass vegetation. Similarly, the largest change in thermal heterogeneity occurred in the grassland with the lowest amount of woody encroachment. Thermal heterogeneity was lowest in the highly encroached grassland, which also experienced the lowest overall change in thermal heterogeneity during the eclipse. Time series models suggested that solar radiation was the most influential factor in predicting changes in thermal heterogeneity as opposed to ambient temperature alone. These results suggest that highly encroached grasslands may experience lower diurnal variability of temperatures at the cost of a decrease in the overall thermal heterogeneity of that landscape. It appears that fine-scale spatio-temporal thermal variation is largely driven by solar radiation, which can be influenced by vegetation heterogeneity inherent within a landscape.

Invasive woody plants as foci of tick-borne pathogens: eastern redcedar in the southern Great Plains.

Habitat preference and usage by disease vectors are directly correlated with landscapes often undergoing anthropogenic environmental change. A predominant type of land use change occurring in the United States is the expansion of native and non-native woody plant species in grasslands, but little is known regarding the impact of this expansion on regional vector-borne disease transmission. In this study, we focused on the impact of expanding eastern redcedar (Juniperus virginiana; ERC) and tested two hypotheses involving relationships between habitat preferences of adult tick species in rural habitats in central Oklahoma. Using CO2 traps, we collected ticks from two densities of ERC and grassland and screened adult ticks for the presence of pathogen DNA. We found support for our first hypothesis with significantly more Amblyomma americanum (Linnaeus) and Dermacentor variabilis (Say) collected in ERC habitats than in grassland. Our second hypothesis was also supported, as Ehrlichia- and Rickettsia-infected A. americanum were significantly more likely to be collected from ERC habitats than grassland. As the first evidence that links woody plant encroachment with important tick-borne pathogens in the continental United States, these results have important ramifications involving human and companion animal risk for encountering pathogen-infected ticks in the southern Great Plains.

Dyakonov-Tamm wave guided by a twist defect in a structurally chiral material.

The boundary-value problem for a Dyakonov-Tamm wave guided by a twist defect in a structurally chiral material and propagating along the bisector of the twist defect was formulated. The resulting dispersion equation was numerically solved. Detailed analysis showed that either two or three different Dyakonov-Tamm waves can propagate, depending on the value of the twist angle. These waves have different phase speeds and degrees of localization to the twist-defect interface.

Energy flux in a surface-plasmon-polariton wave bound to the planar interface of a metal and a structurally chiral material.

The spatial profile of the energy flux in a surface-plasmon-polariton (SPP) wave bound to the planar interface of a metal and a structurally chiral material (SCM) shows strong dependencies on the pitch of the SCM. More than one SPP-wave mode at the same frequency (or free-space wavelength) being possible for the chosen interface, the energy-flux profiles of different modes are different. Although the energy flux on the metal side of the interface decays exponentially with distance from the interface, the profile on the SCM side exhibits oscillations as a function of distance from the interface and decay rates, which are highly variable. Because of the periodic nature of the SCM, the Floquet-Lyapunov theorem may be invoked to explain this behavior.

Surface waves at a biaxial bicrystalline interface.

Localized to the planar interface of two identical dielectric biaxial crystals with optic ray axes parallel to the interface but with a relative twist about an axis perpendicular to the interface, electromagnetic wave propagation along the bisectrix of the two crystallographic orientations is possible with either real-valued or complex-valued transverse decay constants as the twist angle varies from 0 degrees to an upper limit (< or =90 degrees) that is dependent on the angle between the two optic ray axes.

Surface waves with simple exponential transverse decay at a biaxial bicrystalline interface.

The dispersion equation for surface waves--with simple transverse exponential decay at the interface of identical biaxial crystals with a relative twist about the axis normal to the interface and propagating along a bisector of the angle between the crystallographic configurations on either side of the interface--has several solutions of which only one is physical. The selected type of surface wave is possible only for a restricted range of the twist angle, which depends on the ratio of the maximum and the minimum of the principal refractive indexes and the angle between the optic ray axes.