抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

The terrestrial carbon sink has been large in recent decades, but its size and location remain uncertain. Using forest inventory data and long-term ecosystem carbon studies, we estimate a total forest sink of 2.4 ± 0.4 petagrams of carbon per year (Pg C year–1) globally for 1990 to 2007. We also estimate a source of 1.3 ± 0.7 Pg C year–1 from tropical land-use change, consisting of a gross tropical deforestation emission of 2.9 ± 0.5 Pg C year–1 partially compensated by a carbon sink in tropical forest regrowth of 1.6 ± 0.5 Pg C year–1. Together, the fluxes comprise a net global forest sink of 1.1 ± 0.8 Pg C year–1, with tropical estimates having the largest uncertainties. Our total forest sink estimate is equivalent in magnitude to the terrestrial sink deduced from fossil fuel emissions and land-use change sources minus ocean and atmospheric sinks.

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A Large and Persistent Carbon Sink in the World’s Forests

Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

/pdf/climate-change-2007-the-physical-science-basis-1x93ttz9jt.pdf

Climate Change 2007: The Physical Science Basis.

https://www.nrs.fs.fed.us/pubs/jrnl/2011/nrs_2011_pan_002-supporting.pdf

Supporting Online Material for A Large and Persistent Carbon Sink in the World's Forests

The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate. About half of the current emissions are being absorbed by the ocean and by land ecosystems, but this absorption is sensitive to climate as well as to atmospheric carbon dioxide concentrations, creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change. Here we present results from a fully coupled, three-dimensional carbon–climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr-1 is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models, resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.

/pdf/acceleration-of-global-warming-due-to-carbon-cycle-feedbacks-3788793i7v.pdf

Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model

Recently developed structural retention harvesting strategies aim to improve habitat and ecological services provided by managed forest stands by better emulating natural disturbances. The potential for elevated mortality of residual trees following such harvests remains a critical concern for forest managers, and may present a barrier to more widespread implementation of the approach. We used a harvest chronosequence combined with dendrochronological techniques and an individual-based neighborhood analysis to examine the rate and time course of residual-tree mortality in the first decade following operational partial "structural retention" harvests in the boreal forest of Ontario, Canada. In the first year after harvest, residual-tree mortality peaked at 12.6 times the preharvest rate. Subsequently, mortality declined rapidly and approached preharvest levels within 10 years. Proximity to skid trails was the most important predictor both of windthrow and standing death, which contributed roughly equally to total postharvest mortality. Local exposure further increased windthrow risk, while crowding enhanced the risk of standing mortality. Ten years after harvest, an average of 10.5% of residual trees had died as a result of elevated postharvest mortality. Predicted cumulative elevated mortality in the first decade after harvest ranged from 2.4% to 37% of residual trees across the observed gradient of skid trail proximity, indicating that postharvest mortality will remain at or below acceptable rates only if skidding impacts are minimized. These results represent an important step toward understanding how elevated mortality may influence stand dynamics and habitat supply following moderate-severity disturbances such as partial harvests, insect outbreaks, and windstorms.

Tree mortality following partial harvests is determined by skidding proximity

Methane flux measurements were carried out at a temperate forest (Haliburton Forest and Wildlife Reserve) in central Ontario (45°17´11´´ N, 78°32´19´´ W) from June to October 2011. Continuous measurements were made by an off-axis integrated cavity output spectrometer that measures methane (CH 4 ) at 10 Hz sampling rates. Fluxes were calculated from the gas measurements in conjunction with wind data collected by a 3-D sonic anemometer using the eddy covariance (EC) method. Observed methane fluxes showed net uptake of CH 4 over the measurement period with an average uptake flux (±standard deviation of the mean) of −2.7 ± 0.13 nmol m −2 s −1 . Methane fluxes showed a seasonal progression with average rates of uptake increasing from June through September and remaining high in October. This pattern was consistent with a decreasing trend in soil moisture content at the monthly timescale. On the diurnal timescale, there was evidence of increased uptake during the day, when the mid-canopy wind speed was at a maximum. These patterns suggest that substrate supply of CH 4 to methanotrophs, and in certain cases hypoxic soil conditions supporting methanogenesis in low-slope areas, drives the observed variability in fluxes. A network of soil static chambers used at the tower site showed reasonable agreement with the seasonal trend and overall magnitude of the eddy covariance flux measurements. This suggests that soil-level microbial processes, and not abiological leaf-level CH 4 production, drive overall CH 4 dynamics in temperate forest ecosystems such as Haliburton Forest.

/pdf/methane-fluxes-measured-by-eddy-covariance-and-static-447aj64jpj.pdf

Methane fluxes measured by eddy covariance and static chamber techniques at a temperate forest in central Ontario, Canada

SUMMARY (1) The growth and fate of individual plants in a natural self-thinning stand of Impatiens paNida located along a disturbed forest edge in southeastern Pennsylvania were followed from canopy closure to the onset of flowering (eight weeks). The primary study population consisted of 161 plants measured at the beginning and end of this period. Survivorship, lodging and herbivory of these individuals were monitored weekly, and flowering status was recorded at the end of the study. A second set of twenty representative but easily accessible plants at the same location were measured repeatedly the following year to determine the shape of the growth curve of individual plants during this same time interval. (2) Mortality in the primary population was 25%. Lodging was an important cause of death. Smaller plants were more likely to lodge and to suffer herbivore damage than were larger plants. Lodging, independent of plant size, accounted for 16% of mortality explained by a path analysis model; the effects of herbivory on mortality (independent of plant size) were not significant, however. Death caused by lodging in a natural self. . thinning population supports the hypothesis that biomechanical constraints may have substantial effects on self-thinning relationships and patterns of size structure change in herbaceous plant populations. (3) A decrease in above-ground biomass (negative growth) was found to occur in 21 % of the surviving plants in the main study population. Although models of plant competition usually assume that plants with zero or negative growth die, 88% of the surviving plants displaying negative growth flowered by the end of the study. Growth curves for the twenty repeatedly measured plants were approximately linear over the course of the study period. (4) The survival of plants with negative growth affects the function relating biomass (B) to density (D) and the function relating size inequality (G) to mean plant mass (m) over the course of self-thinning. Had these plants died during the study period, the observed log Blog D slope (- 1.73) would have been very close to the mean thinning slope observed across many species (-0.85). Because many non-growing and shrinking plants survived, size inequality did not decrease during self-thinning. We suggest that the ability of plants to survive sustained suppression may have a major and predictable effect on both the selfthinning trajectory and on changes in size variability over the course of stand development. Our results emphasize the importance of field studies aimed at understanding the behaviour of suppressed plants.

https://www.jstor.org/stable/pdfplus/2260767.pdf

Growth, death and size distribution change in an impatiens pallida population

We used dendroecological techniques to analyze the temporal pattern in diameter growth following selection harvests in stands dominated by Acer saccharum Marsh. in central Ontario and examined differences in growth responses related to tree size, damage, and orientation relative to canopy gaps. While dendroecological studies have commonly assumed that trees show immediate growth responses to gap creation (i.e., within 12 years), we found that the growth enhancement in A. saccharum was gradual and did not reach a peak until 35 years following gap creation. Trees of intermediate size showed the largest proportional growth increases after gap creation, with the largest responses observed in trees on the north side of gaps. Trees with visible damage to the crown or bole had significantly lower preharvest basal area increments than trees with little or no damage, but showed greater proportional growth responses to gap creation. Both the long observed time delay in tree growth response to canopy opening and t...

The time course of diameter increment responses to selection harvests in Acer saccharum

https://tspace.library.utoronto.ca/bitstream/1807/108180/4/Removing_bias_from_2017_TSpace.pdf

Sean C. Thomas

Papers

Tree mortality following partial harvests is determined by skidding proximity

Methane fluxes measured by eddy covariance and static chamber techniques at a temperate forest in central Ontario, Canada

Growth, death and size distribution change in an impatiens pallida population

The time course of diameter increment responses to selection harvests in Acer saccharum

Removing bias from LiDAR-based estimates of canopy height: Accounting for the effects of pulse density and footprint size