Functional significance of variation in bryophyte canopy structure.

In most bryophytes, the thickness of boundary layers (i.e., unstirred layers) that surrounds plant surfaces governs rates of water loss. Architectural features of canopies that influence boundary layer thickness affect the water balance of bryophytes. Using field samples (9.3 cm diameter cushions) from 12 populations (11 species) of mosses and liverworts, we evaluated the relationship between canopy structure and boundary layer properties. Canopy structure was characterized using a contact surface probe to measure canopy depth along perpendicular transects at spatial scales ranging from 0.8 to 30 mm on 186 points per sample. Semivariance in depth measurements at different spatial scales was used to estimate three architectural properties: surface roughness (L(r)), the scale of roughness elements (S(r)), and fine-scale surface texture, the latter characterized by the fractal dimension (D) of the canopy profile. Boundary layer properties were assessed by evaporation of ethanol from samples in a wind-tunnel at wind speeds from 0.6 to 4.2 m/s and applied to characterize mass transfer using principles of dynamic similarity (i.e., using dimensionless representations of conductance and flow). In addition, particle image velocimetry (PIV) was used to visualize and quantify flow over two species. All cushions exhibited the characteristics of turbulent as opposed to laminar boundary layers, and conductance increased with surface roughness. Bryophyte canopies with higher L(r) had greater conductances at all wind speeds. Particle image velocimetry analysis verified that roughness elements interacted with flow and caused turbulent eddies to enter canopies, enhancing evaporation. All three morphological features were significantly associated with evaporation. When L(r), S(r), and D were incorporated with a flow parameter into a conductance model using multiple linear regression, the model accounted for 91% of the variation in mass transfer.

Variation in carbon isotope discrimination within and among Sphagnum species in a temperate wetland.

Field samples of bryophytes are highly variable in carbon isotope discrimination values (Δ, a measure of 13CO2 uptake relative to 12CO2), but it is unknown what affects Δ under field conditions, or how variation in Δ relates to bryophyte performance. This study employed field and greenhouse common garden studies to evaluate the influence of microsite, seasonal, and genetic variation on Δ in peatmosses. Three species of Sphagnum that occupy hollow (S. recurvum), carpet (S. palustre), and hummock (S. tenerum) habitats were sampled for relative growth rates (RGR), C:N ratio, and Δ throughout a growing season. Values of Δ ranged from 19.0 to 27.1‰. This variation was unrelated to species (P=0.61). However, Δ varied seasonally (P<0.001), with lower discrimination in the spring (mean 22.5‰), followed by summer (23.8‰) and winter (24.7‰). There was also significant microsite variation (P=0.015) which disappeared when plants were grown in a common garden. In both spring and summer, microsite variation in Δ was inversely related to RGR (P<0.001), but unrelated to C:N ratios (P>0.08). These results suggest that environmental, not genetic, variation at microsites affects Δ in non-vascular plants. However, environmental control of Δ is unlike that in vascular plants where water limitation lowers chloroplastic demand and increases resistance to carbon uptake. In non-vascular plants, water limitation lowers chloroplastic demand and decreases resistance to carbon uptake. These processes have additive effects and generate high spatial and seasonal variability in Δ.

Cryptosporidiosis: an unrecognized cause of diarrhea in elderly hospitalized patients.

Human infection with Cryptosporidium species has been increasingly noted in the past decade. We conducted a broad-based longitudinal review in a community setting and found that a Cryptosporidium species was detected in one-third of the specimens screened over a 5-year period. Thirty-six patients were identified, comprising three distinct clinical groups: persons with human immunodeficiency virus (HIV) infection (18 patients); young, otherwise healthy persons (5 patients); and, surprisingly, chronically ill elderly persons (13 patients). In six (46%) of the 13 elderly patients, both Cryptosporidium and Clostridium difficile toxin was identified, suggesting that Cryptosporidium may be a copathogen in some instances of nosocomial diarrhea. Acquisition in an institutional setting was suspected for nine (69%) of the elderly and three (17%) of the HIV-infected patients. Elderly patients with chronic illnesses constitute a newly recognized category of persons at risk for cryptosporidial infection. In this group cryptosporidiosis may be far more common than previously recognized, may be acquired institutionally, and can mimic and occur with Clostridium difficile-associated diarrhea.

Clinical characteristics, management strategies, and cost implications of a statewide outbreak of enterovirus meningitis.

A statewide outbreak of enterovirus meningitis occurred in Rhode Island in the summer of 1991. A comprehensive chart review was conducted for determining the demographic and clinical characteristics of the epidemic and for assessing the interinstitutional variation in treatment strategies. Four hundred eight cases were reported, and enteroviruses were recovered in 61 (68%) of 90 cases in which viral isolation was attempted. Of six isolates that were serotyped, all were echovirus 30. Analysis of cerebrospinal fluid revealed that an increased total white blood cell count and an increased protein concentration were associated with increasing patient age; however, the percentage of polymorphonuclear leukocytes in the cerebrospinal fluid decreased with increasing patient age and longer durations of illness. Marked variations in treatment strategies between institutions were noted in the use of computed tomography of the head, the administration of empirical antimicrobial agents, and the duration of hospitalization. Considerable health resources could have been saved by rapid detection of the virus, dissemination of information about the outbreak, and a conservative approach to clinical management.