Water relations and hydraulic architecture of two Patagonian steppe shrubs: Effect of slope orientation and microclimate

On a local scale, topography influences microclimate, vegetation structure and the morpho-physiological attributes of plants. We studied the effects of microclimatic differences between NE- and SW-facing slopes on the water relations and hydraulic properties of two dominant shrubs of the Patagonian steppe in Argentina (Retanilla patagonica and Colliguaja integerrima). The NE-facing slope had higher irradiance and air saturation deficits and lower soil water availability and wind speed than the SW-facing slope. Predawn and midday Ψ_L and osmotic potentials were significantly lower in shrubs on the NE-facing slope. Osmotic adjustment and more elastic cell walls helped the plants to cope with a more xeric environment on NE-facing slope. Higher water deficits on NE-facing slope were partially compensated by a higher leaf and stem water storage. While stem hydraulic efficiency did not vary between slopes, leaf hydraulic conductance was between 40% and 300% higher on the NE-facing slope. Changes observed in leaf size and in SLA were consistent with responses to mechanical forces of wind (smaller and scleromorphic leaves on SW-facing slope). Morpho-physiological adjustments observed at a short spatial scale allow maintenance of midday Ψ_L above the turgor loss point and demonstrate that leaves are more responsive to microclimatic selective pressures than stems.     

Three‐dimensional gravity‐current flow within a subaqueous bend: Spatial evolution and force balance variations

The nature of three‐dimensional flow in submarine channel bends is poorly understood, largely due to the absence of detailed data from natural channels. Herein, data from density‐driven flows in a large reservoir on the Huanghe (Yellow) River are presented showing the spatio‐temporal variation of flow around a subaqueous bend. The data demonstrate for the first time that reversed helical flow, relative to that found in river channel bends, can occur from the centrifugal forcing of flow, even when the Coriolis force acts in the opposite direction. The data also suggest that reversed helical flow fields in submarine channels may be more frequent than currently estimated, notably for bends where Coriolis and centrifugal forces combine in the same direction. In addition, this study provides the first field evidence suggesting that sinuous submarine channels can exhibit an asymmetry in helical flow orientation between left and right‐turning bends, which will have major implications for the morphodynamics of submarine channels, their resultant patterns of sedimentation and, ultimately, the distribution of depositional units across submarine fan systems.     

Stochastic Distance Based Geological Boundary Modeling with Curvilinear Features

Obtaining accurate geological boundaries and assessing the uncertainty in these limits are critical for effective ore resource and reserve estimation. The uncertainty in the extent of an ore body can be the largest source of uncertainty in ore resource estimation when drilling is sparse. These limits are traditionally interpreted deterministically and it can be difficult to quantify uncertainty in the boundary and its impact on ore tonnage. The proposed methodology is to consider stochastic modeling of the ore boundary with a distance function recoding of the available data. This technique is modified to incorporate non-stationarities in the form of a locally varying anisotropy field used in kriging and sequential Gaussian simulation. Implementing locally varying anisotropy kriging retains the geologically realistic features of a deterministic model while allowing for a stochastic assessment of uncertainty. A case study of a gold deposit in Northern Canada is used to demonstrate the methodology. The proposed technique generates realistic, curvilinear geological boundary models and allows for an assessment of the uncertainty in the model.     

Time–space continuity of daily maps of fractional snow cover and albedo from MODIS

Using reflectance values from the seven MODIS “land” bands with 250 or 500 m resolution, along with a corresponding cloud product, we estimate the fraction of each 500 m pixel that snow covers, along with the albedo of that snow. The daily products have data gaps and errors because of cloud cover and sensor viewing geometry. Rather than make users interpolate and filter these patchy daily maps without completely understanding the retrieval algorithm and instrument properties, we use the daily time series to improve the estimate of the measured snow properties for a particular day. We use a combination of noise filtering, snow/cloud discrimination, and interpolation and smoothing to produce our best estimate of the daily snow cover and albedo. We consider two modes: one is the “predictive” mode, whereby we estimate the snow-covered area and albedo on that day using only the data up to that day; the other is the “retrospective” mode, whereby we reconstruct the history of the snow properties for a previous period.     

Effect of fire on phytolith coloration

Dark‐colored phytoliths are often found preserved in paleosols and archaeological sediments. Some practitioners believe these darkened phytoliths provide evidence of fire histories, while others suggest alternative reasons for their occurrence. This study examines the effect of fire on phytolith appearance and discusses the extent to which color may be used as proxy evidence for fire. The results of this study demonstrate that under oxidative conditions of openair fire, the color of phytoliths can be altered, although dark‐colored phytoliths also occur naturally in some unburned plant species. Despite some overlap observed between burned and unburned color in phytoliths, clear differences are apparent in the way this color appears optically. In particular, transparent and opalescent qualities were found to occur in nature as opposed to a dull opaque appearance of charred phytoliths. Although fire‐induced color change is probably limited to a portion of the phytolith assemblage, phytolith color remains a tool that can be confidently used to indicate the presence of fire in various sedimentary contexts. © 2006 Wiley Periodicals, Inc.     

Separation of pedogenic and lithogenic components of magnetic susceptibility in the Chinese loess/palaeosol sequence as determined by the CBD procedure and a mixing analysis

Magnetic susceptibility variations in the Chinese loess/palaeosol sequences have been used extensively for palaeoclimatic interpretations. The magnetic signal of these sequences must be divided into lithogenic and pedogenic components because the palaeoclimatic record is primarily reflected in the pedogenic component. In this paper we compare two methods for separating the pedogenic and lithogenic components of the magnetic susceptibility signal: the citrate-bicarbonate-dithionite (CBD) extraction procedure, and a mixing analysis. Both methods yield good estimates of the pedogenic component, especially for the palaeosols. The CBD procedure underestimates the lithogenic component and overestimates the pedogenic component. The magnitude of this effect is moderately high in loess layers but almost negligible in palaeosols. The mixing model overestimates the lithogenic component and underestimates the pedogenic component. Both methods can be adjusted to yield better estimates of both components. The lithogenic susceptibility, as determined by either method, suggests that palaeoclimatic interpretations based only on total susceptibility will be in error and that a single estimate of the average lithogenic susceptibility is not an accurate basis for adjusting the total susceptibility. A long-term decline in lithogenic susceptibility with depth in the section suggests more intense or prolonged periods of weathering associated with the formation of the older palaeosols.
The CBD procedure provides the most comprehensive information on the magnitude of the components and magnetic mineralogy of loess and palaeosols. However, the mixing analysis provides a sensitive, rapid, and easily applied alternative to the CBD procedure. A combination of the two approaches provides the most powerful and perhaps the most accurate way of separating the magnetic susceptibility components.     

Spatial and Temporal Variability in Estuary Habitat Use by American Alligators

Estuarine habitat occupied by Alligator mississippiensis, a primarily freshwater species, is spatially and temporally heterogeneous largely due to a salinity gradient that fluctuates. Using long-term night light survey data, we examined seasonal patterns in alligators’ habitat use by size classes in midstream and downstream estuary zones of Shark River, Everglades National Park, in southern Florida. We observed predominantly large-sized alligators (total length?≥?1.75 m); observations of alligators in the small size classes (0.5 m?≤?total length?<?1.25 m) were rare especially in the higher-salinity downstream zone. The density of alligators in the downstream zone was lower than that of the midstream zone during the dry season when salinity increases due to reduced precipitation. Conversely, the density of the large size alligators was higher in the downstream zone than in the midstream zone during the wet season, likely because of reduced salinity. We also found a significant declining trend over time in the number of alligators in the dry season, which coincides with the reported decline in alligator relative density in southern Florida freshwater wetlands. Our results indicated high adaptability of alligators to the fluctuating habitat conditions. Use of estuaries by alligators is likely driven in part by physiology and possibly by reproductive cycle, and our results supported their opportunistic use of estuary habitat and ontogenetic niche shifts.