Linking ecosystem function and hydrologic regime to inform restoration of a forested peatland

Drainage is a globally common disturbance in forested peatlands that impacts peat soils, forest communities, and associated ecosystem functions, calling for informed hydrologic restoration strategies. The Great Dismal Swamp (GDS), located in Virginia and North Carolina, U.S.A., has been altered since colonial times, particularly by extensive ditch networks installed to lower water levels and facilitate timber harvests. Consequently, peat decomposition rates have accelerated, and red maple has become the dominant tree species, reducing the historical mosaic of bald cypress, Atlantic white-cedar, and pocosin stands. Recent repair and installation of water control structures aim to control drainage and, in doing so, enhance forest community composition and preserve peat depths. To help inform these actions, we established five transects of 15 plots each (75 plots total) along a hydrologic gradient where we measured continuous water levels and ecosystem attributes, including peat depths, microtopography, and forest composition and structure. We found significant differences among transects, with wetter sites having thicker peat, lower red maple importance, greater tree density, and higher overall stand richness. Plot-level analyses comported with these trends, clearly grouping plots by transects (via nonmetric multidimensional scaling) and resulting in significant correlations between specific hydrologic metrics and ecosystem attributes. Our findings highlight hydrologic controls on soil carbon storage, forest structure, and maple dominance, with implications for large-scale hydrologic restoration at GDS and in other degraded forested peatlands more broadly.

Cortical electrical stimulation alters erythrocyte perfusion pattern in the cerebral capillary network of the rat

The effect of direct cortical electrical stimulation on the pattern of erythrocyte perfusion in the capillary network of the rat cerebral cortex was studied by fluorescence intravital video-microscopy. The movement of fluorescently labeled red blood cells (FRBCs) in individual capillaries 50-70 microm subsurface in the dorsal somatosensory cortex was visualized using a closed cranial window. Cortical stimulation electrodes were placed on opposite sides of the window. FRBC velocity (mm/s) and supply rate (cells/s) were measured in 51 capillaries from six rats before and during electrical stimulation of increasing intensities (15-s trains of 3-Hz, 3-ms, 0.5-5.0-mA, square pulses). FRBC velocity, supply rate, and the instantaneous capillary erythrocyte content (lineal cell density, LCD, cells/mm) increased with the stimulation current and reached maxima of 110, 160 and 33% above control, respectively. Capillaries with low resting velocity showed a greater response than those with high resting velocity. The fraction of capillaries in which FRBC velocity increased was not constant, but increased with the stimulation current, as did the magnitude of the velocity change in these capillaries. A few capillaries showed a negative FRBC velocity response at stimulations <4 mA. These results suggest that a robust rise in the fraction of responding (engaged) capillaries and a smaller rise in the capillary LCD contribute to neuronal activation-induced cortical hyperemia. Thus, capillary engagement and erythrocyte recruitment appear to represent important components of the cortical functional hyperemic response. These results provide insight into some of the specific hemodynamic changes associated with functional hyperemia occurring at the capillary level.

Toluidine blue O and methylene blue as endothelial redox probes in the intact lung

There is increasing evidence that the redox activities of the pulmonary endothelial surface may have important implications for the function of both lungs and blood. Because of the inherent complexity of intact organs, it can be difficult to study these activities in situ. Given the availability of appropriate indicator probes, the multiple-indicator dilution (MID) method is one approach for dealing with some aspects of this complexity. Therefore, the objectives of the present study were to 1) evaluate the potential utility of two thiazine redox indicators, methylene blue (MB) and toluidine blue O (TBO), as MID electron acceptor probes for in situ pulmonary endothelium and 2) develop a mathematical model of the pulmonary disposition of these indicators as a tool for quantifying their reduction on passage through the lungs. Experiments were carried out using isolated rabbit lungs perfused with physiological salt solution with or without plasma albumin over a range of flow rates. A large fraction of the injected TBO disappeared from the perfusate on passage through the lungs. The reduction of its oxidized, strongly polar, relatively hydrophilic blue form to its colorless, highly lipophilic reduced form was revealed by the presence of the reduced form in the venous effluent when plasma albumin was included in the perfusate. MB was also lost from the perfusate, but the fraction was considerably smaller than for TBO. A distributed-in-space-and-time model was developed to estimate the reduction rate parameter, which was approximately 29 and 1.0 ml/s for TBO and MB, respectively, and almost flow rate independent for both indicators. The results suggest the utility particularly of TBO as an electron acceptor probe for MID studies of in situ pulmonary endothelium and of the model for quantitative evaluation of the data.

Postnatal remodeling of the leptomeningeal vascular network as assessed by intravital fluorescence video-microscopy in the rat

An intriguing characteristic of the ontogenic development of the cerebral vasculature is the rapid differentiation of the neonatal leptomeningeal vascular plexus into the mature, adult network form. The physiological and cellular mechanisms of this cerebrovascular remodeling process are unclear. The objective of this work was to determine and correlate changes in vascular density, network pattern and flow velocity in leptomeningeal microvessels of the rat during postnatal development in vivo. To this end, microvascular diameter, segment length, and vascular density of reconstructed leptomeningeal networks were measured from video-recordings of the microcirculation visualized through a cranial window in 0-15-day-old Sprague-Dawley rats. The velocity of erythrocytes in the microvessels was measured by frame to frame tracking of fluorescently labeled red blood cells. We found that surface vascular density (total vessel length per area), node density and segment density (object per area) decreased significantly by the second week after birth. Anastomosing vascular polygons, characteristic to newborn networks, became less numerous and larger in diameter during the postnatal 2-week period, indicating progressive rarefaction of the networks. Vessel diameter and red cell velocity showed transient increases at 1.5 weeks. The velocity/diameter ratio (V/D), an index of wall shear rate, increased by the age of 1.5 weeks and remained unchanged afterwards. There was a negative correlation between V/D and diameter at 1 week; this relationship was reversed to a positive correlation at 2 weeks. We conclude that postnatal remodeling of the leptomeningeal vascular network is associated with rarefaction and an adaptation of vessel caliber to wall shear rate. These changes may contribute to arterio-venous differentiation and redistribution of blood flow from the superficial to the intracortical vasculature in the developing brain.

An angiographic method for in vivo study of arteries of the circle of Willis in small animals

An X-ray imaging technique designed to allow sequential diameter measurements of the cerebral vessels in intact, anesthetized small animals under relatively physiological conditions is described. The ferret and the rabbit were chosen as potentially useful animal models for studying the cerebrovascular system because of the advantageous anatomic characteristics of these relatively small species. A commercially available and relatively inexpensive X-ray imaging system with a small focal spot provides good spatial resolution. An external carotid perfusion loop allows for 1) the introduction of low-osmolality contrast medium without changing perfusion pressure or flow and 2) measurement of internal carotid and circle of Willis pressures at the same time that the vessel images are obtained. In the present study, detection of small changes in the diameters of the small vessels is facilitated by an algorithm utilizing the X-ray absorption by the entire vessel cross section. This avoids some of the problems of edge detection for small cylindrical vessels wherein the contrast is less than optimal and diminishes as the vessel perimeter is approached.