The Spanish and Mexican Baseline of California Tree and Shrubland Distributions Since the Late 18th Century

Historical distributions of 31 tree species, chaparral, and coastal sage scrub described by Spanish land explorers in the late eighteenth and early nineteenth centuries (1769–1806) and in land grant diseños (1784–1846) are reconstructed at 634 localities across central and southern California. This baseline predates most formal botanical surveys by nearly a century, allowing for assessment of vegetation change over the broadest time frame for comparison with pre-historical evidences and future distributions. Spanish accounts are compared with historical sources in the Mexican era (1821–1848), American settlement (1848–1929), and modern range maps of the 1929–1934 Vegetation Type Map (VTM) survey. Among tree species that were recorded in Spanish explorations, the site-specific localities are consistent with VTM maps at the spatial resolution of the land expeditions. In contrast with massive deforestation across eastern North America since European colonization, hardwood and conifer forests in California sustained inconsequential cutting during Hispanic settlement. Spanish accounts and Mexican diseños occasionally provide remarkable detail of fine-scale distributions which have not changed over the past two centuries, including Pinus radiata forest at Cambria and Monterey, the eastern limit of Quercus lobata and Q. agrifolia woodlands with Aesculus californica in the Salinas Valley, as well as isolated stands of Cupressus macrocarpa and C. sargentii. Disjunct occurrences of trees in southern California were recorded at the same places they occur today, including an isolated grove of Q. engelmannii at the Baldwin Park Arboretum, and the Pinus coulteri stand in the mountains above Santa Barbara. The southern margin of mixed conifer forest in the San Bernardino Mountains has remained on the crest of the range since Garcés’ account in 1776. Long-term tree distributions are evaluated with respect to land use, grazing and climate change. We advocate the use of historical records as proxy data for climate change studies

High N, dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought.

Hotter, longer, and more frequent global change-type drought events may profoundly impact terrestrial ecosystems by triggering widespread vegetation mortality. However, severe drought is only one component of global change, and ecological effects of drought may be compounded by other drivers, such as anthropogenic nitrogen (N) deposition and nonnative plant invasion. Elevated N deposition, for example, may reduce drought tolerance through increased plant productivity, thereby contributing to drought-induced mortality. High N availability also often favors invasive, nonnative plant species, and the loss of woody vegetation due to drought may create a window of opportunity for these invaders. We investigated the effects of multiple levels of simulated N deposition on a Mediterranean-type shrubland plant community in southern California from 2011 to 2016, a period coinciding with an extreme, multiyear drought in the region. We hypothesized that N addition would increase native shrub productivity, but that this would increase susceptibility to drought and result in increased shrub loss over time. We also predicted that N addition would favor nonnatives, especially annual grasses, leading to higher biomass and cover of these species. Consistent with these hypotheses, we found that high N availability increased native shrub canopy loss and mortality, likely due to the higher productivity and leaf area and reduced water-use efficiency we observed in shrubs subject to N addition. As native shrub cover declined, we also observed a concomitant increase in cover and biomass of nonnative annuals, particularly under high levels of experimental N deposition. Together, these results suggest that the impacts of extended drought on shrubland ecosystems may be more severe under elevated N deposition, potentially contributing to the widespread loss of native woody species and vegetation-type conversion

Use of ERTS-1 data to assess and monitor change in the Southern California environment

There are no author-identified significant results in this report

Investigation of Cryogenic Current-Voltage Anomalies in SiGe HBTs: Role of Base-Emitter Junction Inhomogeneities

The anomalous current-voltage characteristics of cryogenic SiGe heterojunction bipolar transistors (HBTs) have been a topic of investigation for many years. Proposed explanations include quasiballistic transport of electrons across the base or tunneling from the emitter to the collector, but inconsistencies exist with these hypotheses. Although similar behavior occurs in Schottky junctions and has been attributed to spatial inhomogeneities in the base-emitter junction potential, this explanation has not been considered for SiGe HBTs. Here, we experimentally investigate this hypothesis by characterizing the base-emitter junction ideality factor and built-in potential of a SiGe HBT versus temperature using a cryogenic probe station. The temperature-dependence of the ideality factor and the relation between the built-in potential as measured by capacitance-voltage and current-voltage characteristics are in good qualitative agreement with the predictions of a theory of electrical transport across a junction with a Gaussian distribution of potential barrier heights. These observations support the origin of cryogenic electrical anomalies in SiGe HBTs as arising from lateral inhomogeneities in the base-emitter junction potential. This work helps to identify the physical mechanisms limiting the cryogenic microwave noise performance of SiGe HBTs

Isotropic plasma-thermal atomic layer etching of superconducting TiN films using sequential exposures of molecular oxygen and SF6/_6/H2_2 plasma

Microwave loss in superconducting titanium nitride (TiN) films is attributed to two-level systems in various interfaces arising in part from oxidation and microfabrication-induced damage. Atomic layer etching (ALE) is an emerging subtractive fabrication method which is capable of etching with Angstrom-scale etch depth control and potentially less damage. However, while ALE processes for TiN have been reported, they either employ HF vapor, incurring practical complications; or the etch rate lacks the desired control. Further, the superconducting characteristics of the etched films have not been characterized. Here, we report an isotropic plasma-thermal TiN ALE process consisting of sequential exposures to molecular oxygen and an SF$_6$/H$_2$ plasma. For certain ratios of SF$_6$:H$_2$ flow rates, we observe selective etching of TiO$_2$ over TiN, enabling self-limiting etching within a cycle. Etch rates were measured to vary from 1.1 \r{A}/cycle at 150 $^\circ$C to 3.2 \r{A}/cycle at 350 $^\circ$C using ex-situ ellipsometry. We demonstrate that the superconducting critical temperature of the etched film does not decrease beyond that expected from the decrease in film thickness, highlighting the low-damage nature of the process. These findings have relevance for applications of TiN in microwave kinetic inductance detectors and superconducting qubits.Comment: 17 pages, 7 figure

Nonlinear Photoelasticity to Explicate Acoustic Dephasing Dynamics

Detection and controlling of acoustic (AC) phonon phase have been strenuous tasks although such capability is crucial for further manipulating thermal properties. Here, we present a versatile formalism for tracing AC nanowaves with arbitrary strain compositions by incorporating the nonlinear photoelasticity (PE) into ultrafast acoustics where broad AC spectrum encompassing thermally important THz frequency range should be collected far beyond Brillouin frequency. The initial AC phase upon displacive carrier generation could be inherently varied depending on the bipolar AC compositions by implementing externally biased piezoelectric diodes. The importance of adopting nonlinear PE is then manifested from the transient phase shift either abrupt at the point of diffuse surface scattering or gradual during phonon-phonon or phonon-electron scattering events based on which the ratio of nonlinear to linear PE coefficient is experimentally extracted as a function of the detection probe energy, reaching 0.98 slightly below the bandgap. As the probing energy is rather set away from the bandgap, AC phase is completely invariant with any scattering events, exhibiting the conventional trend at Brillouin frequency in linear regime. Under potent influence of nonlinear PE, the AC dephasing time during the propagation are quantified as a function of AC wavepacket size and further correlated with intrinsic and extrinsic AC scattering mechanisms in electron reservoir

An efficient algorithm to calculate intrinsic thermoelectric parameters based on Landauer approach

The Landauer approach provides a conceptually simple way to calculate the intrinsic thermoelectric (TE) parameters of materials from the ballistic to the diffusive transport regime. This method relies on the calculation of the number of propagating modes and the scattering rate for each mode. The modes are calculated from the energy dispersion (E(k)) of the materials which require heavy computation and often supply energy relation on sparse momentum (k) grids. Here an efficient method to calculate the distribution of modes (DOM) from a given E(k) relationship is presented. The main features of this algorithm are, (i) its ability to work on sparse dispersion data, and (ii) creation of an energy grid for the DOM that is almost independent of the dispersion data therefore allowing for efficient and fast calculation of TE parameters. The inclusion of scattering effects is also straight forward. The effect of k-grid sparsity on the compute time for DOM and on the sensitivity of the calculated TE results are provided. The algorithm calculates the TE parameters within 5% accuracy when the K-grid sparsity is increased up to 60% for all the dimensions (3D, 2D and 1D). The time taken for the DOM calculation is strongly influenced by the transverse K density (K perpendicular to transport direction) but is almost independent of the transport K density (along the transport direction). The DOM and TE results from the algorithm are bench-marked with, (i) analytical calculations for parabolic bands, and (ii) realistic electronic and phonon results for $Bi_{2}Te_{3}$.Comment: 16 Figures, 3 Tables, submitted to Journal of Computational electronic

Using machine learning to advance synthesis and use of conservation and environmental evidence

This is the final version. Available from Wiley via the DOI in this record. National Institute for Health ResearchScience for Nature and People Partnershi