Dust outpaces bedrock in nutrient supply to montane forest ecosystems.

Dust provides ecosystem-sustaining nutrients to landscapes underlain by intensively weathered soils. Here we show that dust may also be crucial in montane forest ecosystems, dominating nutrient budgets despite continuous replacement of depleted soils with fresh bedrock via erosion. Strontium and neodymium isotopes in modern dust show that Asian sources contribute 18-45% of dust deposition across our Sierra Nevada, California study sites. The remaining dust originates regionally from the nearby Central Valley. Measured dust fluxes are greater than or equal to modern erosional outputs from hillslopes to channels, and account for 10-20% of estimated millennial-average inputs of bedrock P. Our results demonstrate that exogenic dust can drive the evolution of nutrient budgets in montane ecosystems, with implications for predicting forest response to changes in climate and land use.

Carbon and water fluxes from ponderosa pine forests disturbed by wildfire and thinning.

Disturbances alter ecosystem carbon dynamics, often by reducing carbon uptake and stocks. We compared the impact of two types of disturbances that represent the most likely future conditions of currently dense ponderosa pine forests of the southwestern United States: (1) high-intensity fire and (2) thinning, designed to reduce fire intensity. High-severity fire had a larger impact on ecosystem carbon uptake and storage than thinning. Total ecosystem carbon was 42% lower at the intensely burned site, 10 years after burning, than at the undisturbed site. Eddy covariance measurements over two years showed that the burned site was a net annual source of carbon to the atmosphere whereas the undisturbed site was a sink. Net primary production (NPP), evapotranspiration (ET), and water use efficiency were lower at the burned site than at the undisturbed site. In contrast, thinning decreased total ecosystem carbon by 18%, and changed the site from a carbon sink to a source in the first posttreatment year. Thinning also decreased ET, reduced the limitation of drought on carbon uptake during summer, and did not change water use efficiency. Both disturbances reduced ecosystem carbon uptake by decreasing gross primary production (55% by burning, 30% by thinning) more than total ecosystem respiration (TER; 33-47% by burning, 18% by thinning), and increased the contribution of soil carbon dioxide efflux to TER. The relationship between TER and temperature was not affected by either disturbance. Efforts to accurately estimate regional carbon budgets should consider impacts on carbon dynamics of both large disturbances, such as high-intensity fire, and the partial disturbance of thinning that is often used to prevent intense burning. Our results show that thinned forests of ponderosa pine in the southwestern United States are a desirable alternative to intensively burned forests to maintain carbon stocks and primary production.

Do high-tannin leaves require more roots?

The well-known deceleration of nitrogen (N) cycling in the soil resulting from addition of large amounts of foliar condensed tannins may require increased fine-root growth in order to meet plant demands for N. We examined correlations between fine-root production, plant genetics, and leaf secondary compounds in Populus angustifolia, P. fremontii, and their hybrids. We measured fine-root (<2 mm) production and leaf chemistry along an experimental genetic gradient where leaf litter tannin concentrations are genetically based and exert strong control on net N mineralization in the soil. Fine-root production was highly correlated with leaf tannins and individual tree genetic composition based upon genetic marker estimates, suggesting potential genetic control of compensatory root growth in response to accumulation of foliar secondary compounds in soils. We suggest, based on previous studies in our system and the current study, that genes for tannin production could link foliar chemistry and root growth, which may provide a powerful setting for external feedbacks between above- and belowground processes.

Ecosystem implications of genetic variation in water-use of a dominant riparian tree.

Genetic variation in dominant species can affect plant and ecosystem functions in natural systems through multiple pathways. Our study focuses on how genetic variation in a dominant riparian tree ( Populus fremontii, P. angustifolia and their natural F(1) and backcross hybrids) affects whole-tree water use, and its potential ecosystem implications. Three major patterns were found. First, in a 12-year-old common garden with trees of known genetic makeup, hybrids had elevated daily integrated leaf-specific transpiration ( E(tl); P=0.013) and average canopy conductance ( G(c); P=0.037), with both E(tl) and G(c) approximately 30% higher in hybrid cross types than parental types. Second, delta(13)C values of leaves from these same trees were significantly more negative in hybrids ( P=0.004), and backcross hybrids had significantly more negative values than all other F(1) hybrid and parental types ( P<0.001). Third, in the wild, a similar pattern was found in leaf delta(13)C values where both hybrid cross types had the lowest values ( P<0.001) and backcross hybrids had lower delta(13)C values than any other tree type ( P<0.001). Our findings have two important implications: (1). the existence of a consistent genetic difference in whole-tree physiology suggests that whole-tree gas and water exchange could be another pathway through which genes could affect ecosystems; and (2). such studies are important because they seek to quantify the genetic variation that exists in basic physiological processes-such knowledge could ultimately place ecosystem studies within a genetic framework.

Potential for assessing long-term dynamics in soil nitrogen availability from variations in delta15N of tree rings.

Numerous researchers have used the isotopic signatures of C, H, and O in tree rings to provide a long-term record of changes in the physiological status, climate, or water-source use of trees. The frequently limiting element N is also found in tree rings, and variation in its isotopic signature may provide insight into long-term changes in soil N availability of a site. However, research has suggested that N is readily translocated among tree ring of different years; such infidelity between the isotopic compositions of the N taken up from the soil and the N contained in the ring of that growth year would obscure the long-term N isotopic record. We used a 15-year 15N-tracer study to assess the degree of N translocation among tree rings in ponderosa pine (Pinus ponderosa) trees growing in a young, mixed-conifer plantation. We also measured delta13C and delta15N values in unlabeled trees to assess the degree of their covariance in wood tissue, and to explore the potential for a biological linkage between them. We found that the maximum delta15N values in rings from the labeled trees occurred in the ring formed one-year after the 15N was applied to the roots. The delta15N value of rings from labeled trees declined exponentially and bidirectionally from this maximum peak, toward younger and older rings. The unlabeled trees showed considerable interannual variation in the delta15N values of their rings (up to 3 and 5 per thousand), but these values correlated poorly between trees over time and differed by as much as 6 per thousand. Removal of extractives from the wood reduced their delta15N value, but the change was fairly small and consistent among unlabeled trees. The delta13C and delta15N values of tree rings were correlated over time in only one of the unlabeled trees. Across all trees, both delta13C values of tree rings and annual stem wood production were well correlated with annual precipitation, suggesting that soil water balance is an important environmental factor controlling both net C gain and transpirational water loss at this site. Our results suggest that interannual translocation of N among tree rings is substantial, but may be predictable enough to remove this source of variation from the tree-ring record, potentially allowing the assessment of long-term changes in soil N availability of a site.

Physiological condition and water source use of Sonoran Desert riparian trees at the Bill Williams River, Arizona, USA.

We investigated the environmental water sources used in mid-summer by three Sonoran Desert phreatophytic riparian tree species, Salix gooddingii, Populus fremontii, and the exotic Tamarix spp., at sites that differed in water table depth. Salix gooddingii was most sensitive to water table decline, as evidenced by lower predawn water potentials. Although P. fremontii was less sensitive to water table decline than S. gooddingii, its leaf gas exchange was the most responsive to atmospheric water stress imposed by high leaf-to-air vapor pressure deficit. Tamarix spp. was least sensitive to water table decline and showed no reduction of predawn water potential over the measured range of depth to groundwater. Comparison between D/H of xylem and sampled environmental water sources suggest that S. gooddingii and P. fremontii used groundwater at most sites with no change in water source as depth to groundwater varied. In contrast, xylem D/H of Tamarix spp. was depleted in deuterium compared to groundwater at most sites, suggesting use of water from an unsampled source, or discrimination against deuterium during water uptake. This study highlights the difficulty in sampling all water sources in large-scale studies of riparian ecosystems with complex subsurface hydrogeology.

Leaf gas exchange characteristics differ among Sonoran Desert riparian tree species.

We investigated leaf gas exchange responses to leaf temperature, leaf-to-air vapor pressure deficit (VPD), and predawn and midday shoot water potential (psipd and psimd, respectively) of two native Sonoran Desert riparian tree species, Fremont cottonwood (Populus fremontii S. Wats.) and Goodding willow (Salix gooddingii Ball), and one exotic riparian tree species, saltcedar (Tamarix chinensis Lour. and related species). Measurements were made at two sites over 2 years that differed climatically. Because multiple linear regression models explained less than 29% of the variation in stomatal conductance (gs) and less than 48% of the variation in net photosynthetic rate (Pn) of all species, we used boundary-line analysis to compare gas exchange responses among species. Gas exchange rates were high in all species. The hyperbolic relationship between Pn and gs suggested that initial reductions in gs at high gs did not inhibit Pn. Reductions in gs of cottonwood and willow occurred at psimd values at or below previously reported xylem cavitation thresholds (-1.6 and -1.4 MPa, respectively), indicating tight stomatal regulation of water loss and a narrow cavitation safety margin. In contrast, reductions in gs of saltcedar occurred at psimd values well above the cavitation threshold (-7.0 MPa), but at much lower psimd values than in cottonwood and willow, suggesting a wider cavitation safety margin and less tight regulation of water loss in saltcedar. High VPD had a smaller effect on leaf gas exchange in willow than in cottonwood. In contrast, willow had a less negative psipd threshold for stomatal closure than cottonwood. Compared with cottonwood and willow, leaf gas exchange of saltcedar was more tolerant of high VPD and low psipd. These physiological characteristics of saltcedar explain its widespread success as an invader of riparian ecosystems containing native Fremont cottonwood and Goodding willow in the Sonoran Desert.

Restoration of myoelectrical propagation across a jejunal transection using microsurgical anastomosis.

The aim of this study was to determine whether microsurgical anastomosis can restore propagation of jejunal pacesetter potentials (PPs) across a site of canine jejunal transection and preserve motility and transit in bowel distal to the transection. A complete jejunal transection with exact microsurgical anastomosis was performed in five dogs, while five dogs with intact jejunum and five dogs with complete transection and end-to-end conventional macrosurgical anastomosis were used as controls. Long-term recording electrodes and intraluminal, open-tipped pressure catheters were implanted in all dogs. The mean frequency of PPs decreased distal to the transection in both groups of transected dogs. However, aborad propagation of PPs across the anastomosis occurred episodically by 3 months in each dog that had a microsurgical anastomosis, but never occurred in any dog with a conventional macroanastomosis. Moreover, the motility and transit in bowel distal to the transection were unaltered in the dogs with a microsurgical anastomosis, whereas they decreased in the dogs with a macroanastomosis. The conclusion was that microsurgical anastomosis of transected canine jejunum restored episodic propagation of PPs across the anastomosis, and preserved motility and maintained transit in bowel distal to the anastomosis. The conventional macroanastomosis did none of these.

Hydraulic lift: a potentially important ecosystem process.

Hydraulic lift is the process by which some deep-rooted plants take in water from lower soil layers and exude that water into upper, drier soil layers. Hydraulic lift is beneficial to the plant transporting the water, and may be an important water source for neighboring plants. Recent evidence shows that hydraulically lifted water can promote greater plant growth, and could have important implications for net primary productivity, as well as ecosystem nutrient cycling and water balance.

Boxelder water sources and physiology at perennial and ephemeral stream sites in Arizona.

To assess the influence of stream water on leaf gas exchange and water potential in different sized boxelder trees (Acer negundo L.), we compared these characteristics in trees growing beside a perennial stream and a nearby ephemeral stream in a montane-riparian forest in northern Arizona. Patterns of tree water use were quantified by stable isotope analysis (delta(18)O). Physiological characteristics were similar for large and small trees. Similarity between sites in predawn and daytime water potentials and xylem delta(18)O indicated that stream water was not a physiologically important water source. Seasonal and site variations in light-saturated net photosynthetic rate were significantly related to leaf-to-air vapor pressure deficit (r = -0.691) and foliar nitrogen concentration (r = 0.388). Although deep water was the dominant water source, surface soil water was utilized following precipitation, especially by small trees. We conclude that net carbon gain and severity of water stress are only weakly coupled to stream water availability.

Competition for nitrogen between plants and soil microorganisms.

Experiments suggest that plants and soil microorganisms are both limited by inorganic nitrogen, even on relatively fertile sites. Consequently, plants and soil microorganisms may compete for nitrogen. While past research has focused on competition for inorganic nitrogen, recent studies have found that plants/mycorrhizae in a wide range of ecosystems can use organic nitrogen. A new view of competitive interactions between plants and soil microorganisms is necessary in ecosystem where plant uptake of organic nitrogen is observed.

Use of a pulse oximeter for determination of systolic blood pressure in a helicopter air ambulance.

INTRODUCTION: Traditional methods of determining blood pressure may be unreliable (auscultation or palpation) or unavailable (direct arterial cannulation) in the air medical environment. The authors investigated the combination of a pulse oximeter with a standard sphygmomanometer (blood pressure) cuff as an alternative method. METHOD: The pulse oximeter is applied to a finger on the same upper extremity on which a standard blood pressure cuff had been applied. A baseline blood pressure was obtained by palpation or an automated blood pressure device. One minute later, the systolic blood pressure (SBP) was determined by inflating the blood pressure cuff until the pulsatile display on the pulse oximeter was obliterated. This was taken as the systolic blood pressure. RESULTS: Complete data were obtained on 116 patients, with 223 data pairs. The SBP as obtained by the baseline method was strongly correlated with the SBP obtained by the pulse oximeter display obliteration method (r = 0.90, p < 0.001). CONCLUSION: The obliteration of the wave form display on a pulse oximeter is an accurate, convenient, inexpensive and readily available alternative method of determining SBP.

In situ estimates of annual net nitrogen mineralization and nitrification in a subarctic watershed.

Annual estimates of surface soil nitrogen transformations were determined using an in situ method in four different subarctic vegetation types within a watershed in southwestern Alaska. The net nitrogen mineralization estimates were 22.5, 0.5, 4.7, and 2.7 kg-N ha-1 yr-1 for the alder, dry tundra, moist tundra, and white spruce sites, respectively. Only the soil from the alder site showed net nitrification (about 10 kg-N ha-1 yr-1). Annual inogranic nitrogen flux from the overlying organic layer to the mineral soil was almost seven times greater than net N production in the surface mineral soil in the alder site, indicating that the alder forest floor is potentially a substantial source for plant-available N. Rates of mobilization of N from the surface organic layers of the other sites were similar to net N production rates in surface mineral soils. In situ rates of N transformations showed a similar trend among sites as did laboratory estimates conducted in a previous study, suggesting a strong substrate control of N transformations in these soils.