Shifts in rainfall patterns and increasing temperatures associated with climate change are likely to cause widespread forest decline in regions where droughts are predicted to increase in duration and severity. One primary cause of productivity loss and plant mortality during drought is hydraulic failure. Drought stress creates trapped gas emboli in the water transport system, which reduces the ability of plants to supply water to leaves for photosynthetic gas exchange and can ultimately result in desiccation and mortality. At present we lack a clear picture of how thresholds to hydraulic failure vary across a broad range of species and environments, despite many individual experiments. Here we draw together published and unpublished data on the vulnerability of the transport system to drought-induced embolism for a large number of woody species, with a view to examining the likely consequences of climate change for forest biomes. We show that 70% of 226 forest species from 81 sites worldwide operate with narrow hydraulic safety margins against injurious levels of drought stress and therefore potentially face long-term reductions in productivity and survival if temperature and aridity increase as predicted for many regions across the globe. Safety margins are largely independent of mean annual precipitation, showing that there is global convergence in the vulnerability of forests to drought, with all forest biomes equally vulnerable to hydraulic failure regardless of their current rainfall environment. These findings provide insight into why drought-induced forest decline is occurring not only in arid regions but also in wet forests not normally considered at drought risk

Leaf veins are almost ubiquitous across the range of terrestrial plant diversity, yet their influence on leaf photosynthetic performance remains uncertain. We show here that specific physical attributes of the vascular plumbing network are key limiters of the hydraulic and photosynthetic proficiency of any leaf. Following the logic that leaf veins evolved to bypass inefficient water transport through living mesophyll tissue, we examined the hydraulic pathway beyond the distal ends of the vein system as a possible limiter of water transport in leaves. We tested a mechanistic hypothesis that the length of this final traverse, as water moves from veins across the mesophyll to where it evaporates from the leaf, governs the hydraulic efficiency and photosynthetic carbon assimilation of any leaf. Sampling 43 species across the breadth of plant diversity from mosses to flowering plants, we found that the post-vein traverse as determined by characters such as vein density, leaf thickness, and cell shape, was strongly correlated with the hydraulic conductivity and maximum photosynthetic rate of foliage. The shape of this correlation provided clear support for the a priori hypothesis that vein positioning limits photosynthesis via its influence on leaf hydraulic efficiency.Leaves have sustained virtually all terrestrial ecosystems over the last 400 million years (Kenrick and Crane, 1991;Boyce et al., 2002;Boyce, 2005). Over this period land plants have evolved an enormous diversity of leaf structure; diversity that marks phylogenetic relations among clades and constrains the functional performance and climatic associations of plant species

SummaryThe 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 (r 2 < 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 (r 2 < 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.

The veins that irrigate leaves during photosynthesis are demonstrated to be strikingly more abundant in flowering plants than in any other vascular plant lineage. Angiosperm vein densities average 8 mm of vein per mm 2 of leaf area and can reach 25 mm mm K2 , whereas such high densities are absent from all other plants, living or extinct. Leaves of non-angiosperms have consistently averaged close to 2 mm mm K2 throughout 380 million years of evolution despite a complex history that has involved four or more independent origins of laminate leaves with many veins and dramatic changes in climate and atmospheric composition. We further demonstrate that the high leaf vein densities unique to the angiosperms enable unparalleled transpiration rates, extending previous work indicating a strong correlation between vein density and assimilation rates. Because vein density is directly measurable in fossils, these correlations provide new access to the physiology of extinct plants and how they may have impacted their environments. First, the high assimilation rates currently confined to the angiosperms among living plants are likely to have been unique throughout evolutionary history. Second, the transpiration-driven recycling of water that is important for bolstering precipitation in modern tropical rainforests might have been significantly less in a world before the angiosperms.