Phase imaging and the lever-sample tilt angle in dynamic atomic force microscopy

The phase shift in amplitude-controlled dynamic atomic force microscopy (AFM) is shown to depend on the cantilever-sample tilt angle. For a silicon sample and tip the phase shift changes by nearly 15º for a change in tilt angle of 15º. This contribution to the phase results from the oscillating tip\u27s motion parallel to the surface, which contributes to the overall energy dissipation. It occurs even when the measurements are carried out in the attractive regime. An off-axis dynamic AFM model incorporating van der Waals attraction and a thin viscous damping layer near the surface successfully describes the observed phase shifts. This effect must be considered to interpret phase images quantitatively. © 2004 American Institute of Physics

Challenges facing gap-based silviculture and possible solutions for mesic northern forests in North America

Gap-based silvicultural systems were developed under the assumption that richness, and diversity of tree species and other biota positively respond to variation in size of harvest-created canopy gaps. However, varying gap size alone often does not meet diversity objectives and broader goals to address contemporary forest conditions. Recent research highlights the need to consider site factors and history, natural disturbance models, within-gap structure and recruitment requirements in addition to light resources for desired tree diversity. This synthesis brings together silvicultural developments and ecological literature on gap-based management, highlighting interactions with other factors such as microsite conditions, non-tree vegetation and more. We pose a revised concept for managers and researchers to use in prescriptions and studies focused on integrated overstory and understory manipulations that increase structural complexity within and around canopy openings

Acute ischaemic stroke associated with SARS-CoV-2 infection in North America

BACKGROUND: To analyse the clinical characteristics of COVID-19 with acute ischaemic stroke (AIS) and identify factors predicting functional outcome. METHODS: Multicentre retrospective cohort study of COVID-19 patients with AIS who presented to 30 stroke centres in the USA and Canada between 14 March and 30 August 2020. The primary endpoint was poor functional outcome, defined as a modified Rankin Scale (mRS) of 5 or 6 at discharge. Secondary endpoints include favourable outcome (mRS ≤2) and mortality at discharge, ordinal mRS (shift analysis), symptomatic intracranial haemorrhage (sICH) and occurrence of in-hospital complications. RESULTS: A total of 216 COVID-19 patients with AIS were included. 68.1% (147/216) were older than 60 years, while 31.9% (69/216) were younger. Median [IQR] National Institutes of Health Stroke Scale (NIHSS) at presentation was 12.5 (15.8), and 44.2% (87/197) presented with large vessel occlusion (LVO). Approximately 51.3% (98/191) of the patients had poor outcomes with an observed mortality rate of 39.1% (81/207). Age \u3e60 years (aOR: 5.11, 95% CI 2.08 to 12.56, p\u3c0.001), diabetes mellitus (aOR: 2.66, 95% CI 1.16 to 6.09, p=0.021), higher NIHSS at admission (aOR: 1.08, 95% CI 1.02 to 1.14, p=0.006), LVO (aOR: 2.45, 95% CI 1.04 to 5.78, p=0.042), and higher NLR level (aOR: 1.06, 95% CI 1.01 to 1.11, p=0.028) were significantly associated with poor functional outcome. CONCLUSION: There is relationship between COVID-19-associated AIS and severe disability or death. We identified several factors which predict worse outcomes, and these outcomes were more frequent compared to global averages. We found that elevated neutrophil-to-lymphocyte ratio, rather than D-Dimer, predicted both morbidity and mortality

Productivity Relationships and Developmental Dynamics Over 26 Years Within Mixed Aspen-White Spruce Stands in Minnesota

The productivity and dynamics of mixed aspen (Populus tremuloides)-white spruce (Picea glauca) forests were examined over a 26-year period across a series of replicated long-term silvicultural trials within northern Minnesota. Long-term growth and biomass production rates were compared between pure aspen, pure white spruce, and mixed aspen-white spruce stands established through planting and/or natural regeneration following clear-cut harvesting in 1982. Interestingly, there were no differences in spruce annual mortality rates between pure and mixed stands, with spruce averaging 1.2 ± 0.26 and 1.4 ± 0.20 % mortality in mixed and pure stands, respectively. In contrast, spruce in pure stands had significantly greater height growth rates than those found in mixtures. Aspen height growth and mortality rates were unaffected by spruce, as there were no differences between aspen grown in pure and mixed stands. Spruce height growth rates were much lower than aspen within mixed stands, resulting in rapid stratification into two-storied stands that contained a lower canopy stratum of white spruce at 4.9 ± 0.3 m and an upper canopy stratum of aspen at 13.8 ± 0.5 m after 26 years of development. Comparisons of productivity rates between pure and mixed stands indicated that there were no significant differences in biomass production rates between pure aspen and mixed aspen-white spruce stands (7.84 ± 0.08 and 6.61 ± 0.97 Mg/ha/yr), whereas pure white spruce stands had much lower levels of productivity (0.98 ± 0.05 Mg/ha/yr). Notably, the highest level of productivity observed across the study was within a stratified, mixed aspen white spruce stand (8.71 Mg/ha/yr); suggesting the potential for higher levels of production within these species mixtures relative to pure populations. In addition, productivity levels associated with spruce within mixtures was largely additive to aspen production, highlighting the potential for increasing productivity in pure aspen systems through spruce underplanting

The Influence of Cutting Cycle and Stocking Level on the Structure and Quality of Managed Old-Growth Northern Hardwoods

The effects of 57 years of uneven-aged management on the structure and quality of old-growth northern hardwood forests were examined within a long-term Forest Service study in the Dukes Experimental Forest, Michigan. Treatments within the study included several differing stocking levels (30, 50, 70, and 90 ft2/acre) and cutting cycles (5, 10, 15, and 20 years) replicated across three blocks. Tree growth and quality, as well as stand structural attributes were examined within each treatment and compared to an unmanaged Research Natural Area (RNA). Comparisons of tree grade between treatments indicated that cutting cycle length and stocking level had no effect on tree quality. In contrast, average annual diameter growth was greatest in lower stocking treatments, and regression analyses of tree growth versus residual stocking level indicated that optimum levels of growth were achieved between residual basal areas of 50 and 70 ft2/acre. Comparisons of stand structure between treatment areas and the unmanaged RNA suggested that volumes of CWD in the high residual basal area treatments most closely approximated those found within unmanaged old-growth. Treatments with high residual basal areas and longer cutting cycles had significantly greater snag basal area and volume than other treatments and were closer in this respect to the RNA than other treatments. Analysis of diameter distributions over time indicated that cutting cycle had little effect on distribution shape over the course of this study. In contrast, treatments with lower residual basal areas developed from increasing q to concave or negative exponential distributions over time, while the 90 ft2/acre treatment displayed an increasing q distribution throughout the experiment. The findings of this study suggest that manipulation of stocking levels within northern hardwood stands is of greater importance than cutting cycle in controlling stand growth, as well as influencing the development of structural complexity