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Understanding how plants survive drought and cold is increasingly important as plants worldwide experience dieback with drought in moist places and grow taller with warming in cold ones. Crucial in plant climate adaptation are the diameters of water-transporting conduits. Sampling 537 species across climate zones dominated by angiosperms, we find that plant size is unambiguously the main driver of conduit diameter variation. And because taller plants have wider conduits, and wider conduits within species are more vulnerable to conduction-blocking embolisms, taller conspecifics should be more vulnerable than shorter ones, a prediction we confirm with a plantation experiment. As a result, maximum plant size should be short under drought and cold, which cause embolism, or increase if these pressures relax. That conduit diameter and embolism vulnerability are inseparably related to plant size helps explain why factors that interact with conduit diameter, such as drought or warming, are altering plant heights worldwide.

While plant height is the main driver of variation in mean vessel diameter at the stem base (VD) across angiosperms, climate, specifically temperature, does play an explanatory role, with vessels being wider with warmer temperature for plants of the same height. Using a comparative approach sampling 537 species of angiosperms across 19 communities, we rejected selection favouring freezing‐induced embolism resistance as being able to account for wider vessels for a given height in warmer climates. Instead, we give reason to suspect that higher vapour pressure deficit (VPD) accounts for the positive scaling of height‐standardized VD (and potential xylem conductance) with temperature. Selection likely favours conductive systems that are able to meet the higher transpirational demand of warmer climates, which have higher VPD, resulting in wider vessels for a given height. At the same time, wider vessels are likely more vulnerable to dysfunction. With future climates likely to experience ever greater extremes of VPD, future forests could be increasingly vulnerable. In angiosperms, individuals from warmer climates have wider vessels than similar‐sized plants in colder climates. This trend is consistent with scaling of vessel diameter with vapour pressure deficit (VPD). Because VPD will be higher in future climates, and because wider vessels are potentially more vulnerable to embolism, future forests could be increasingly vulnerable. [ABSTRACT FROM AUTHOR]

All students of xylem structure–function relations need to be familiar with the work of Sherwin Carlquist. He studies xylem through the lens of the comparative method, which uses the appearance of similar anatomical features under similar conditions of natural selection to infer function. "Function" in biology implies adaptation; maximally supported adaptation inferences require experimental and comparative xylem scientists to work with one another. Engaging with comparative inferences of xylem function will, more likely sooner rather than later, bring one to the work of Sherwin Carlquist. To mark his 90th birthday, I highlight just a few examples of his extraordinarily perceptive and general comparative insights. One is "Carlquist's Law", the pervasive tendency for vessels to be solitary when background cells are conductive. I cover his pioneering of "ecological" wood anatomy, viewing xylem variation as reflecting the effects of selection across climate and habit variation. Another is the embolism vulnerability–conduit diameter relationship, one of the most widely invoked structure–function relationships in xylem biology. I discuss the inferential richness within the notion of Carlquistian paedomorphosis, including detailed functional inferences regarding ray cell orientation. My final example comes from his very recent work offering the first satisfactory hypothesis accounting for the geographical and histological distribution of scalariform perforation plates as an adaptation, including "Carlquist's Ratchet", why scalariform plates are adaptive but do not re‐evolve once lost. This extraordinarily rich production over six decades is filled with comparative inferences that should keep students of xylem function busy testing for decades to come. [ABSTRACT FROM AUTHOR]

Projected climatic warming has direct implications for future disturbance regimes, particularly fire-dominated ecosystems at high latitudes, where climate warming is expected to be most dramatic. It is important to ascertain the potential range of climate change impacts on terrestrial ecosystems, which is relevant to making projections of the response of the Earth system and to decisions by policymakers and land managers. Computer simulation models that explicitly model climate–fire relationships represent an important research tool for understanding and projecting future relationships. Retrospective model analyses of ecological models are important for evaluating how to effectively couple ecological models of fire dynamics with climate system models. This paper uses a transient landscape-level model of vegetation dynamics, Alaskan Frame-based Ecosystem Code (ALFRESCO), to evaluate the influence of different driving datasets of climate on simulation results. Our analysis included the use of climate data based on first-order weather station observations from the Climate Research Unit (CRU), a statistical reanalysis from the NCEP–NCAR reanalysis project (NCEP), and the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Model simulations of annual area burned for Alaska and western Canada were compared to historical fire activity (1950–2000). ALFRESCO was only able to generate reasonable simulation results when driven by the CRU climate data. Simulations driven by the NCEP and MM5 climate data produced almost no annual area burned because of substantially colder and wetter growing seasons (May–September) in comparison with the CRU climate data. The results of this study identify the importance of conducting retrospective analyses prior to coupling ecological models of fire dynamics with climate system models. The authors’ suggestion is to develop coupling methodologies that involve the use of anomalies from future climate model simulations to alter the climate data of more trusted historical climate datasets. [ABSTRACT FROM AUTHOR]

This numerical study examines the impact of urban growth and release of aerosols, moisture, and heat on precipitation for Fairbanks, Alaska, a remote city at high latitude. The remote location allows atmospheric changes to be attributed to the city and permits examination of their regional-scale impact. The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) with three nested, interacting domains was run over Alaska. An analysis of variance separates the variance in precipitation ascribable to one urban causal factor from variance ascribable to other urban factors. Statistical analysis using an unreplicated 24-factorial design detected significant precipitation modification by individual urban effects, plus interactions among effects. Release of aerosols, release of moisture, and concurrent urban growth and aerosol release statistically significantly affect downwind precipitation with 95% confidence. The significant response to urban aerosol release results from directly caused shifts in cloud microphysical processes. For the other simulations, altered buoyancy indirectly initiates shifts in cloud microphysical paths. Additional moisture from urban sources contributes to increased downwind precipitation. Locations of maximum increase in precipitation vary for the various urban effects because of different mechanisms involved. All urban effects investigated slightly influenced accumulated precipitation predicted for the innermost domain, but not for the outer domains; prediction efficiency depends on grid increment size because processes are resolved on the finer grid that do not appear on the coarse grid. Changes in precipitation can have both positive and negative ecological and societal impacts; identifying which urban effects might cause precipitation changes can help in cost-effective planning to ameliorate negative consequences. [ABSTRACT FROM AUTHOR]