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31.
Robert M. Scheller Alec M. Kretchun E. Louise Loudermilk Matthew D. Hurteau Peter J. Weisberg Carl Skinner 《Ecosystems》2018,21(4):643-656
Climate-driven increases in wildfires, drought conditions, and insect outbreaks are critical threats to forest carbon stores. In particular, bark beetles are important disturbance agents although their long-term interactions with future climate change are poorly understood. Droughts and the associated moisture deficit contribute to the onset of bark beetle outbreaks although outbreak extent and severity is dependent upon the density of host trees, wildfire, and forest management. Our objective was to estimate the effects of climate change and bark beetle outbreaks on ecosystem carbon dynamics over the next century in a western US forest. Specifically, we hypothesized that (a) bark beetle outbreaks under climate change would reduce net ecosystem carbon balance (NECB) and increase uncertainty and (b) these effects could be ameliorated by fuels management. We also examined the specific tree species dynamics—competition and release—that determined NECB response to bark beetle outbreaks. Our study area was the Lake Tahoe Basin (LTB), CA and NV, USA, an area of diverse forest types encompassing steep elevation and climatic gradients and representative of mixed-conifer forests throughout the western United States. We simulated climate change, bark beetles, wildfire, and fuels management using a landscape-scale stochastic model of disturbance and succession. We simulated the period 2010–2100 using downscaled climate projections. Recurring droughts generated conditions conducive to large-scale outbreaks; the resulting large and sustained outbreaks significantly increased the probability of LTB forests becoming C sources over decadal time scales, with slower-than-anticipated landscape-scale recovery. Tree species composition was substantially altered with a reduction in functional redundancy and productivity. Results indicate heightened uncertainty due to the synergistic influences of climate change and interacting disturbances. Our results further indicate that current fuel management practices will not be effective at reducing landscape-scale outbreak mortality. Our results provide critical insights into the interaction of drivers (bark beetles, wildfire, fuel management) that increase the risk of C loss and shifting community composition if bark beetle outbreaks become more frequent. 相似文献
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Nia J. Bryant Robert C. Piper Lois S. Weisman Tom H. Stevens 《The Journal of cell biology》1998,142(3):651-663
A large number of trafficking steps occur between the last compartment of the Golgi apparatus (TGN) and the vacuole of the yeast Saccharomyces cerevisiae. To date, two intracellular routes from the TGN to the vacuole have been identified. Carboxypeptidase Y (CPY) travels through a prevacuolar/endosomal compartment (PVC), and subsequently on to the vacuole, while alkaline phosphatase (ALP) bypasses this compartment to reach the same organelle. Proteins resident to the TGN achieve their localization despite a continuous flux of traffic by continually being retrieved from the distal PVC by virtue of an aromatic amino acid–containing sorting motif. In this study we report that a hybrid protein based on ALP and containing this retrieval motif reaches the PVC not by following the CPY sorting pathway, but instead by signal-dependent retrograde transport from the vacuole, an organelle previously thought of as a terminal compartment. In addition, we show that a mutation in VAC7, a gene previously identified as being required for vacuolar inheritance, blocks this trafficking step. Finally we show that Vti1p, a v-SNARE required for the delivery of both CPY and ALP to the vacuole, uses retrograde transport out of the vacuole as part of its normal cellular itinerary. 相似文献
36.
The heart is one of the least regenerative organs in the body, and highly vulnerable to the increasing incidence of cardiovascular diseases in an aging world population. Cell-based approaches aimed at cardiac repair have recently caused great public excitement. But clinical trials of patients’ own skeletal myoblasts or bone marrow cells for transplantation have been disappointing. Human embryonic stem cells (hESCs) form bona fide cardiomyocytes in vitro which are readily generated in mass culture and are being tested in animal models of heart damage. The early results, while encouraging, underscore that much remains to be done. This review focuses on the many challenges that remain before hESCs-mediated repair of the human heart becomes a reality. 相似文献
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Christelle Hély Kelly K. Caylor Peter Dowty Samuel Alleaume Robert J. Swap Hank H. Shugart Christopher O. Justice 《Ecosystems》2007,10(7):1116-1132
Abstract
We present a regional fuel load model (1 km2 spatial resolution) applied in the southern African savanna region. The model is based on a patch-scale production efficiency
model (PEM) scaled up to the regional level using empirical relationships between patch-scale behavior and multi-source remote
sensing data (spatio-temporal variability of vegetation and climatic variables). The model requires the spatial distribution
of woody vegetation cover, which is used to determine separate respiration rates for tree and grass. Net primary production,
grass and tree leaf death, and herbivory are also taken into account in this mechanistic modeling approach. The fuel load
model has been calibrated and validated from independent measurements taken from savanna vegetation in Africa southward from
the equator. A sensitivity analysis on the effect of climate variables (incoming radiation, air temperature, and precipitation)
has been conducted to demonstrate the strong role that water availability has in determining productivity and subsequent fuel
load over the southern African region. The model performance has been tested in four different areas representative of a regional
increasing rainfall gradient—Etosha National Park, Namibia, Mongu and Kasama, Zambia, as well as in Kruger National Park,
South Africa. Within each area, we analyze model output from three different magnitudes of canopy coverage (<5, 30, and 50%).
We find that fuel load ranges predicted by the model are globally in agreement with field measurements for the same year.
High rainfall sustains green herbaceous production late in the dry season and delays tree leaf litter production. Effect of
water on production varies across the rainfall gradient with delayed start of green material production in more arid regions. 相似文献
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Robert A. Krebs 《Journal of Insect Behavior》1991,4(2):221-233
Variation in copulation duration of Drosophila mojavensisstrains was influenced by both sexes. Males maintained predominant control, as copulation duration of pairs from different strains was more similar to that of the strain from which the male was derived, but female origin also contributed significantly to the duration of copulation. Variation among strains was controlled by genes acting additively in both sexes. The size of both males and females also affected copulation duration. Small males copulated longer on average than large males, while males paired with large females copulated longer than those paired with small females. The importance of copulation duration to fitness was tested by correlation analyses with male size, female size, female remating latency, and number of eggs laid prior to female remating. Longer copulations stimulated earlier oviposition, possibly by increasing accessory gland secretions that are passed by males during copulation. 相似文献