Abstract
Ericaceous shrubs often interfere with the growth of black spruce seedlings on regenerating forest sites in Eastern Canada. Mechanical site preparation such as scarification may improve ...this situation, but it is uncertain whether this is solely due to a reduction in direct competition from the shrubs, or also from a sustained improvement in nutritional site quality. We sampled experimental plots in two boreal climate regions (i.e. warmer-drier Abitibi vs. cooler-wetter Côte-Nord) where scarification, performed 18 years earlier, had increased the growth of black spruce relative to non-scarified plots. Trees of scarified plots had closed the canopy more than trees of non-scarified plots in Côte-Nord, but not in Abitibi. Total ground cover of ericaceous shrubs was lower in scarified plots at both sites, the main species being Kalmia angustifolia (i.e. Kalmia) in Abitibi and Rhododendron groenlandicum (i.e. Labrador tea) in Côte-Nord. Scarified plots at both sites had significantly shorter current-year ericaceous rhizomes than non-scarified plots, but the difference between treatments was significantly greater in Côte-Nord than in Abitibi. In Côte-Nord, ericaceous shrubs on scarified plots had a lower specific rhizome mass, higher specific leaf area, lower tannin and higher N concentrations in leaves and litter, and lower N use efficiency than on non-scarified plots. By comparison, scarification in Abitibi affected only one foliar property, namely a reduction in the C:N ratio of Kalmia leaf litter. Forest floor N mineralization rates and black spruce needle N concentrations were higher in scarified than non-scarified plots across both sites. Taken collectively, results suggest that mechanical scarification on ericaceous shrub-dominated cutovers can reduce competitive traits of boreal ericaceous shrubs and improve nutritional site quality, especially in cooler-wetter climates.
Highlights
Mechanical scarification increased growth rates of black spruce saplings.
Scarified plots showed less above- and below-ground growth of ericaceous shrubs.
Foliar and rhizomatous traits suggested higher soil fertility on scarified plots.
Soil N mineralization and spruce needle N were higher on scarified plots.
Positive effects of scarification can be higher in wetter cooler maritime climate.
Aims Natural disturbances leave long-term legacies that vary among landscapes and ecosystem types, and which become integral parts of successional processes at a given location. As humans change land ...use, not only are immediate post-disturbance patterns altered, but the processes of recovery themselves are likely altered by the disturbance. We assessed whether short-term effects on soil and vegetation that distinguish wildfire from forest harvest persist over 60 years after disturbance in boreal black spruce forests, or post-disturbance processes of recovery promote convergence of the two disturbance types. Methods Using semi-variograms and Principal Coordinates of Neighbour Matrices, we formulated precise, a priori spatial hypotheses to discriminate spatial signatures following wildfire and forest harvest both over the short-(16-18 years) and long-term (62-98 years). Results Both over the short- and the long-term, wildfire generated a wide spectrum of responses in soil and vegetation properties at different spatial scales, while logging produced simpler patterns corresponding to the regular linear pattern of harvest trails and to predisturbance ericaceous shrub patches that persist between trails. Conclusions Disturbance by harvest simplified spatial patterns associated with soil and vegetation properties compared to patterns associated with natural disturbance by fire. The observed differences in these patterns between disturbance types persist for over 60 years. Ecological management strategies inspired by natural disturbances should aim to increase the complexity of patterns associated with harvest interventions.
The future capacity of forest ecosystems to sequester atmospheric carbon is likely to be influenced by CO₂-mediated shifts in nutrient cycling through changes in litter chemistry, and by interactions ...with pollutants like O₃. We evaluated the independent and interactive effects of elevated CO₂ (560 μl l⁻¹) and O₃ (55 nl l l⁻¹) on leaf litter decomposition in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) at the Aspen free air CO₂ enrichment (FACE) site (Wisconsin, USA). Fumigation treatments consisted of replicated ambient, +CO₂, +O₃, and +CO₂ + O₃ FACE rings. We followed mass loss and litter chemistry over 23 months, using reciprocally transplanted litterbags to separate substrate quality from environment effects. Aspen decayed more slowly than birch across all treatment conditions, and changes in decomposition dynamics of both species were driven by shifts in substrate quality rather than by fumigation environment. Aspen litter produced under elevated CO₂ decayed more slowly than litter produced under ambient CO₂, and this effect was exacerbated by elevated O₃. Similarly, birch litter produced under elevated CO₂ also decayed more slowly than litter produced under ambient CO₂. In contrast to results for aspen, however, elevated O₃ accelerated birch decay under ambient CO₂, but decelerated decay under enriched CO₂. Changes in decomposition rates (k-values) were due to CO₂- and O₃-mediated shifts in litter quality, particularly levels of carbohydrates, nitrogen, and tannins. These results suggest that in early-successional forests of the future, elevated concentrations of CO₂ will likely reduce leaf litter decomposition, although the magnitude of effect will vary among species and in response to interactions with tropospheric O₃.
Genome-wide association studies (GWAS) have not consistently detected replicable genetic risk factors for ischemic stroke, potentially due to etiological heterogeneity of this trait. We performed ...GWAS of ischemic stroke and a major ischemic stroke subtype (large artery atherosclerosis, LAA) using 1,162 ischemic stroke cases (including 421 LAA cases) and 1,244 population controls from Australia. Evidence for a genetic influence on ischemic stroke risk was detected, but this influence was higher and more significant for the LAA subtype. We identified a new LAA susceptibility locus on chromosome 6p21.1 (rs556621: odds ratio (OR)=1.62, P=3.9×10(-8)) and replicated this association in 1,715 LAA cases and 52,695 population controls from 10 independent population cohorts (meta-analysis replication OR=1.15, P=3.9×10(-4); discovery and replication combined OR=1.21, P=4.7×10(-8)). This study identifies a genetic risk locus for LAA and shows how analyzing etiological subtypes may better identify genetic risk alleles for ischemic stroke.
The two major disturbance types of boreal black spruce forest in north— central Quebec, Canada — natural disturbance by wildfire and anthropogenic disturbance by harvest -may affect processes of ...recovery differently and leave distinct postdisturbance soil and vegetation spatial patterns. We tested whether 1) spatial patterns of physico-chemical soil organic layer properties, black spruce diameter and density, and understory ericaceous shrub cover, differ between these two principal disturbance types; 2) operations associated with forest harvest result in distinct, regular spatial patterns of these same variables related to presence of machine trails; and 3) ericaceous shrub presence is a potential factor contributing to the legacy of spatial patterns after harvest. We explored these patterns on black spruce-feathermoss forest stands, including fire-origin stands (18 and 98 years) and stands originating from harvest (16 and 62 years) in central Quebec, Canada. We used two spatial analysis methods, spectral analysis and principal component analysis in the frequency domain, to characterize and relate spatial patterns of these soil and vegetation variables, measured along 50-m transects on each site. Spatial patterns of distribution of soil and vegetation variables were different on the burned and the harvested forest sites. Wildfire gave rise to spatial patterns in soil and vegetation variables at multiple scales, reflecting the complexity generated by variable burn intensity. Patterns following forest harvest were mainly related to the regular structure defined by trails created by logging operations. In contrast to burned sites, ericaceous shrub patterns on harvested sites were strongly associated with spatial arrangements of spruce diameter and density, promoting absence of canopy closure and persistence of trails. Moreover, different spatial signatures did not converge in the long-term (62-98 years) between the two disturbance types. The divergence in spatial structure between natural and anthropogenic disturbances has implications for ecosystem structure and function in the longer term.
We have applied "whole-genome shotgun sequencing" to microbial populations collected en masse on tangential flow and impact filters from seawater samples collected from the Sargasso Sea near Bermuda. ...A total of 1.045 billion base pairs of nonredundant sequence was generated, annotated, and analyzed to elucidate the gene content, diversity, and relative abundance of the organisms within these environmental samples. These data are estimated to derive from at least 1800 genomic species based on sequence relatedness, including 148 previously unknown bacterial phylotypes. We have identified over 1.2 million previously unknown genes represented in these samples, including more than 782 new rhodopsin-like photoreceptors. Variation in species present and stoichiometry suggests substantial oceanic microbial diversity.
To investigate the risk of radiation-induced optic neuropathy according to total radiotherapy dose and fraction size, based on both retrospective and prospectively collected data.
Between October ...1964 and May 1989, 215 optic nerves in 131 patients received fractionated external-beam irradiation during the treatment of primary extracranial head and neck tumors. All patients had a minimum of 3 years of ophthalmologic follow-up (range, 3 to 21 years). The clinical end point was visual acuity of 20/100 or worse as a result of optic nerve injury.
Anterior ischemic optic neuropathy developed in five nerves (at mean and median times of 32 and 30 months, respectively, and a range of 2-4 years). Retrobulbar optic neuropathy developed in 12 nerves (at mean and median times of 47 and 28 months, respectively, and a range of 1-14 years). No injuries were observed in 106 optic nerves that received a total dose of < 59 Gy. Among nerves that received doses of > or = 60 Gy, the dose per fraction was more important than the total dose in producing optic neuropathy. The 15-year actuarial risk of optic neuropathy after doses of > or = 60 Gy was 11% when treatment was administered in fraction sizes of < 1.9 Gy, compared with 47% when given in fraction sizes of > or = 1.9 Gy. The data also suggest an increased risk of optic nerve injury with increasing age.
As there is no effective treatment of radiation-induced optic neuropathy, efforts should be directed at its prevention by minimizing the total dose, paying attention to the dose per fraction to the nerve, and using reduced-field techniques where appropriate to limit the volume of tissues that receive high-dose irradiation.
Elevated CO
2 resulted in changes in chemical foliar composition that are likely to impact herbivory and decomposition
Atmospheric chemical composition affects foliar chemical composition, which in ...turn influences the dynamics of both herbivory and decomposition in ecosystems. We assessed the independent and interactive effects of CO
2 and O
3 fumigation on foliar chemistry of quaking aspen (
Populus tremuloides) and paper birch (
Betula papyrifera) at a Free-Air CO
2 Enrichment (FACE) facility in northern Wisconsin. Leaf samples were collected at five time periods during a single growing season, and analyzed for nitrogen, starch and condensed tannin concentrations, nitrogen resorption efficiencies (NREs), and C:N ratios. Enriched CO
2 reduced foliar nitrogen concentrations in aspen and birch; O
3 only marginally reduced nitrogen concentrations. NREs were unaffected by pollution treatment in aspen, declined with O
3 exposure in birch, and this decline was ameliorated by enriched CO
2. C:N ratios of abscised leaves increased in response to enriched CO
2 in both tree species. O
3 did not significantly alter C:N ratios in aspen, although values tended to be higher in +CO
2+O
3 leaves. For birch, O
3 decreased C:N ratios under ambient CO
2 and increased C:N ratios under elevated CO
2. Thus, under the combined pollutants, the C:N ratios of both aspen and birch leaves were elevated above the averaged responses to the individual and independent trace gas treatments. Starch concentrations were largely unresponsive to CO
2 and O
3 treatments in aspen, but increased in response to elevated CO
2 in birch. Levels of condensed tannins were negligibly affected by CO
2 and O
3 treatments in aspen, but increased in response to enriched CO
2 in birch. Results from this work suggest that changes in foliar chemical composition elicited by enriched CO
2 are likely to impact herbivory and decomposition, whereas the effects of O
3 are likely to be minor, except in cases where they influence plant response to CO
2.
► We examine soil and ericad properties 15years after careful logging and scarification. ► These properties differed on trails and protection strips after careful logging. ► Ericad-induced changes in ...soil properties contributed to perpetuate the trail pattern. ► Scarification had a positive effect on ericad reduction but altered C and N availability.
The practice of careful logging has been mandated in northeastern Canada’s coniferous and mixedwood forests as a means of protecting natural regeneration while minimizing soil disturbance by machinery. This practice imposes evenly spaced, parallel trails for circulation of harvesting machinery, while most of the understory and soil is left intact in wide protection strips between the trails. The objective of this study was to examine soil physico-chemical and microbial properties, and ericaceous shrub performance (aboveground biomass) following careful logging and with or without subsequent scarification. By sampling 32 cutover forest sites (formerly black spruce-dominated) in the Abitibi-Temiscamingue region of boreal northwestern Quebec, we evaluated how (a) the presence of alternating bands of protection strips and skid trails (on non-scarified sites), and (b) a scarification treatment consisting of mixing the soil organic layer with the surface mineral soil, influenced soil and vegetation properties over time (4–15years after careful logging). On non-scarified sites, careful logging showed a differential compaction pattern consisting of bands of disturbed zones or trails alternating with protection zones. Both soil properties and ericaceous shrub biomass estimates tracked these alternating compacted and non-compacted areas on the cutover. Soil N (Nt), soil percentage microbial C to total C (Cmic/Ct), and Kalmia angustifolia (Kalmia) and Vaccinium spp. (Vaccinium) aboveground biomass differed on strips and trails 8 and 15years after harvest. The observed differences in ericaceous shrub biomass between strips and trails, and correlations between biomass and soil variables, suggest potential shrub interference with soil processes. These observations suggest that ericaceous species may promote differences in soil properties and plant communities that result in a long-term spatial signature perpetuating the trail pattern. Scarification reduced Kalmia and Vaccinium aboveground biomass and had a negative effect on Cmic/Nmic and Cmic/Ct, 15years after harvest. Forest understory plants, such as ericaceous shrubs, may play a key role in driving belowground properties, forest regeneration, and long-term succession on these sites. Understanding the ecology and spatial distribution of these species may thus have important implications for ecosystem structure and function as well as timber production.
Purpose: Results of percutaneous transluminal angioplasty (PTA) in selected cases have been reported to be equal or superior to those of arterial bypass graft surgery, with a lower morbidity and ...mortality. We performed PTA of stenotic or occlusive lesions in patients with limb-threatening ischemia, hoping to improve our overall success and decrease morbidity in this group of patients. The results of PTA in the limb-salvage setting was evaluated.
Methods: From 1992 to 1995, 307 PTAs were performed in 257 patients. One hundred sixty-one (63%) patients had diabetes mellitus, and 32 (12%) patients had renal failure. All patients were evaluated by means of pulse volume recordings and ankle brachial indices at 1 and 6 weeks after PTA and at 3 month intervals thereafter. Seventeen patients (9%) were lost to follow-up. The continued success or failure of PTA was defined by means of noninvasive vascular laboratory criteria, patency by means of pulse examination, the need for subsequent bypass grafting across the index lesion, and limb salvage.
Results: The 1-year patency rates for external iliac PTAs (56%) were significantly lower (
P < .05) than those for common iliac PTAs (87%). Infrainguinal PTAs at the femoral, popliteal, and tibial level had 1-year patency rates of less than 15%.
Conclusion: Common iliac artery PTA is justified in most cases in which it is feasible. However, when PTAs are performed below the inguinal ligament, the results are markedly worse. One-year patency rates of PTA in this group of patients with threatened limbs are inferior to the patency rates of arterial bypass grafts, even when these bypasses are performed with a prosthetic material. PTA should not be considered as a primary treatment modality for patients with infrainguinal arterial occlusive disease who also have limb-threatening ischemia, except in unusual circumstances. (J Vasc Surg 1998;28:1066-71.)
PDF
