Shifting Pacific storm tracks as stressors to ecosystems of western North America

Much of the precipitation delivered to western North America arrives during the cool season via midlatitude Pacific storm tracks, which may experience future shifts in response to climate change. Here, we assess the sensitivity of the hydroclimate and ecosystems of western North America to the latitudinal position of cool‐season Pacific storm tracks. We calculated correlations between storm track variability and three hydroclimatic variables: gridded cool‐season standardized precipitation‐evapotranspiration index, April snow water equivalent, and water year streamflow from a network of USGS stream gauges. To assess how historical storm track variability affected ecosystem processes, we derived forest growth estimates from a large network of tree‐ring widths and land surface phenology and wildfire estimates from remote sensing. From 1980 to 2014, cool‐season storm tracks entered western North America between approximately 41°N and 53°N. Cool‐season moisture supply and snowpack responded strongly to storm track position, with positive correlations to storm track latitude in eastern Alaska and northwestern Canada but negative correlations in the northwestern U.S. Ecosystems of the western United States were greener and more productive following winters with south‐shifted storm tracks, while Canadian ecosystems were greener in years when the cool‐season storm track was shifted to the north. On average, larger areas of the northwestern United States were burned by moderate to high severity wildfires when storm tracks were displaced north, and the average burn area per fire also tended to be higher in years with north‐shifted storm tracks. These results suggest that projected shifts of Pacific storm tracks over the 21st century would likely alter hydroclimatic and ecological regimes in western North America, particularly in the northwestern United States, where moisture supply and ecosystem processes are highly sensitive to the position of cool‐season storm tracks.     

Higher climate warming sensitivity of Siberian larch in small than large forest islands in the fragmented Mongolian forest steppe

Forest fragmentation has been found to affect biodiversity and ecosystem functioning in multiple ways. We asked whether forest size and isolation in fragmented woodlands influences the climate warming sensitivity of tree growth in the southern boreal forest of the Mongolian Larix sibirica forest steppe, a naturally fragmented woodland embedded in grassland, which is highly affected by warming, drought, and increasing anthropogenic forest destruction in recent time. We examined the influence of stand size and stand isolation on the growth performance of larch in forests of four different size classes located in a woodland‐dominated forest‐steppe area and small forest patches in a grassland‐dominated area. We found increasing climate sensitivity and decreasing first‐order autocorrelation of annual stemwood increment with decreasing stand size. Stemwood increment increased with previous year's June and August precipitation in the three smallest forest size classes, but not in the largest forests. In the grassland‐dominated area, the tree growth dependence on summer rainfall was highest. Missing ring frequency has strongly increased since the 1970s in small, but not in large forests. In the grassland‐dominated area, the increase was much greater than in the forest‐dominated landscape. Forest regeneration decreased with decreasing stand size and was scarce or absent in the smallest forests. Our results suggest that the larch trees in small and isolated forest patches are far more susceptible to climate warming than in large continuous forests pointing to a grim future for the forests in this strongly warming region of the boreal forest that is also under high land use pressure.     

Age of arboreous Tamarix austromongolica and its growth response to environment in Tongde County of Qinghai,China

Aims The objectives were to identify the age of the arboreous Tamarix austromongolica in the flood plain area of the Qinghai Plateau and clarify the response patterns of T. austromongolica’s growth to the environmental factors. We focused on social issues about whether the T. austromongolica should be protected and how to protect in the reservoir area of a hydropower station. Methods In this study, arboreous T. austromongolica in both reservoir submerged and non-submerged areas were sampled and measured based on the dendrochronology method. The ages were estimated based on the geometrical characteristics of the pith and the identified age of the inner ring. The correlation and response analysis showed the relationship between T. austromongolica’s growth and environmental factors. Important findings We accurately determined the age and historical growth dynamics of the T. austromongolica with large diameter at breast height (DBH). The results showed a special accretion phenomenon in arboreous T. austromongolica, which accelerated the DBH increasing, i.e. no direct relationship existed between the plants’ DBH and ages of the individuals. Radial growth of T. austromongolica, increased rapidly in the 1970s and 1980s and began to stabilize in the late 1980s, and mainly responded to the runoff in July and August of the Yellow River. Increasing runoff would promote the radial growth of T. austromongolica. The growth of the immaturate plant showed significant negative correlation with the wind speed in the growing season. The results will be of theoretical significance to the formation of the special morphology of the T. austromongolica, and will provide scientific practical guidance in designing the protection schemes.     

Fast blood-flow simulation for large arterial trees containing thousands of vessels

Blood flow modelling has previously been successfully carried out in arterial trees to study pulse wave propagation using nonlinear or linear flow solvers. However, the number of vessels used in the simulations seldom grows over a few hundred. The aim of this work is to present a computationally efficient solver coupled with highly detailed arterial trees containing thousands of vessels. The core of the solver is based on a modified transmission line method, which exploits the analogy between electrical current in finite-length conductors and blood flow in vessels. The viscoelastic behaviour of the arterial-wall is taken into account using a complex elastic modulus. The flow is solved vessel by vessel in the frequency domain and the calculated output pressure is then used as an input boundary condition for daughter vessels. The computational results yield pulsatile blood pressure and flow rate for every segment in the tree. This solver is coupled with large arterial trees generated from a three-dimensional constrained constructive optimisation algorithm. The tree contains thousands of blood vessels with radii spanning ~1 mm in the root artery to ~30 μm in leaf vessels. The computation takes seconds to complete for a vasculature of 2048 vessels and less than 2 min for a vasculature of 4096 vessels on a desktop computer.     

Use of Bayesian analysis with individual‐based modeling to project outcomes of coral restoration

Various approaches to coral restoration have been developed to help increase rate of reef recovery from perturbations, among the most common of which is coral transplantation. Success is often evaluated based on short‐term observations that capture only the initial phase of space colonization by coral transplants. Here, an individual‐based model is developed to quantify uncertainty in future trajectories in experimental plots given past observations. Empirical data were used to estimate probabilistic growth, survival, and fission rates of Acropora pulchra and A. intermedia (order Scleractinia) in a sandy reef flat (Bolinao, Philippines). Simulations were initialized with different densities (25 or 50 transplants per species per 16 m²) to forecast possible coral cover trajectories over a 5‐year period. Given current conditions, there is risk of local extinction which is higher in low‐density plots for both species, and higher for A. intermedia compared to A. pulchra regardless of density. While total coral cover is projected to increase, species composition in the future is more likely to be highly uneven. The model was used to quantify effect on recovery rate of protection from pulse anthropogenic disturbances, given different initial transplantation densities. When monitoring data are limited in time, stochastic models may be used to assess whether the restoration trajectory is heading toward the desired state and at what rate, and foresee system response to various adaptive interventions.     

Long‐term climate and competition explain forest mortality patterns under extreme drought

Rising temperatures are amplifying drought‐induced stress and mortality in forests globally. It remains uncertain, however, whether tree mortality across drought‐stricken landscapes will be concentrated in particular climatic and competitive environments. We investigated the effects of long‐term average climate [i.e. 35‐year mean annual climatic water deficit (CWD)] and competition (i.e. tree basal area) on tree mortality patterns, using extensive aerial mortality surveys conducted throughout the forests of California during a 4‐year statewide extreme drought lasting from 2012 to 2015. During this period, tree mortality increased by an order of magnitude, typically from tens to hundreds of dead trees per km², rising dramatically during the fourth year of drought. Mortality rates increased independently with average CWD and with basal area, and they increased disproportionately in areas that were both dry and dense. These results can assist forest managers and policy‐makers in identifying the most drought‐vulnerable forests across broad geographic areas.     

Effects of host abundance on larch budmoth outbreaks in the European Alps

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