Summary: The evolution of lignified xylem allowed for the efficient transport of water under tension, but also exposed the vascular network to the risk of gas emboli and the spread of gas between xylem conduits, thus impeding sap transport to the leaves. A well‐known hypothesis proposes that the safety of xylem (its ability to resist embolism formation and spread) should trade off against xylem efficiency (its capacity to transport water). We tested this safety–efficiency hypothesis in branch xylem across 335 angiosperm and 89 gymnosperm species. Safety was considered at three levels: the xylem water potentials where 12%, 50% and 88% of maximal conductivity are lost. Although correlations between safety and efficiency were weak (r² < 0.086), no species had high efficiency and high safety, supporting the idea for a safety–efficiency tradeoff. However, many species had low efficiency and low safety. Species with low efficiency and low safety were weakly associated (r² < 0.02 in most cases) with higher wood density, lower leaf‐ to sapwood‐area and shorter stature. There appears to be no persuasive explanation for the considerable number of species with both low efficiency and low safety. These species represent a real challenge for understanding the evolution of xylem.

Summary: Model–data comparisons of plant physiological processes provide an understanding of mechanisms underlying vegetation responses to climate. We simulated the physiology of a piñon pine–juniper woodland (Pinus edulis–Juniperus monosperma) that experienced mortality during a 5 yr precipitation‐reduction experiment, allowing a framework with which to examine our knowledge of drought‐induced tree mortality. We used six models designed for scales ranging from individual plants to a global level, all containing state‐of‐the‐art representations of the internal hydraulic and carbohydrate dynamics of woody plants. Despite the large range of model structures, tuning, and parameterization employed, all simulations predicted hydraulic failure and carbon starvation processes co‐occurring in dying trees of both species, with the time spent with severe hydraulic failure and carbon starvation, rather than absolute thresholds per se, being a better predictor of impending mortality. Model and empirical data suggest that limited carbon and water exchanges at stomatal, phloem, and below‐ground interfaces were associated with mortality of both species. The model–data comparison suggests that the introduction of a mechanistic process into physiology‐based models provides equal or improved predictive power over traditional process‐model or empirical thresholds. Both biophysical and empirical modeling approaches are useful in understanding processes, particularly when the models fail, because they reveal mechanisms that are likely to underlie mortality. We suggest that for some ecosystems, integration of mechanistic pathogen models into current vegetation models, and evaluation against observations, could result in a breakthrough capability to simulate vegetation dynamics. Contents Summary 305 I. Background 305 II. Model–experiment approach 306 III. Simulations of hydraulic failure and carbon starvation 310 IV. On thresholds vs duration of stress as drivers of mortality 311 V. Interdependence of hydraulic failure and carbon starvation 314 VI. Next‐generation, traditional, and empirical models 316 VII. A path forward 317 VIII. Conclusions 318 Acknowledgements 318 References 318

Many factors such as poverty, ineffective institutions and environmental regulations may prevent developing countries from managing how natural resources are extracted to meet a strong market demand. Extraction for some resources has reached such proportions that evidence is measurable from space. We present recent evidence of the global demand for a single commodity and the ecosystem destruction resulting from commodity extraction, recorded by satellites for one of the most biodiverse areas of the world. We find that since 2003, recent mining deforestation in Madre de Dios, Peru is increasing nonlinearly alongside a constant annual rate of increase in international gold price (∼18%/yr). We detect that the new pattern of mining deforestation (1915 ha/year, 2006–2009) is outpacing that of nearby settlement deforestation. We show that gold price is linked with exponential increases in Peruvian national mercury imports over time (R² = 0.93, p = 0.04, 2003–2009). Given the past rates of increase we predict that mercury imports may more than double for 2011 (∼500 t/year). Virtually all of Peru's mercury imports are used in artisanal gold mining. Much of the mining increase is unregulated/artisanal in nature, lacking environmental impact analysis or miner education. As a result, large quantities of mercury are being released into the atmosphere, sediments and waterways. Other developing countries endowed with gold deposits are likely experiencing similar environmental destruction in response to recent record high gold prices. The increasing availability of satellite imagery ought to evoke further studies linking economic variables with land use and cover changes on the ground.

The goal of this research project was to determine the water transport behaviour of earlywood versus latewood in the trunk of 21‐year‐old Douglas‐fir [Pseudostuga menziesii (Mirb.) Franco] trees. Specific conductivity (k_s) and the vulnerability of xylem to embolism were measured on a single growth ring and in a subset of earlywood and latewood samples within the same ring. Earlywood/latewood ratio, trunk water potential (Ψ) and relative water content (RWC) were used to predict differences in conductivities and vulnerability to embolism. Earlywood has about 11 times the k_s of latewood, and up to 90% of the total flow occurred through the earlywood. Earlywood’s vulnerability to embolism followed the same trend as that of the whole wood, with 50% loss of conductivity at –2.2 MPa (P₅₀). Latewood was more vulnerable to embolism than earlywood at high Ψ, but as Ψ decreased, the latewood showed very little further embolism, with a P₅₀ <–5.0 MPa. The lowest trunk Ψ estimated in the field was about –1.4 MPa, indicating that latewood and earlywood in the field experienced about 42% and 16% loss of k_s, respectively. The higher vulnerability to embolism in latewood than in earlywood at field Ψ was associated with higher water storage capacity (21.8% RWC MPa^–1 versus 4.1% RWC MPa^–1, latewood and earlywood, respectively). The shape of the vulnerability curve suggests that air seeding through latewood may occur directly through pores in the margo and seal off at lower pressure than earlywood pores.