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 km , 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.

Purpose Over the past two decades, consequential life cycle assessment (CLCA) has emerged as a modeling approach for capturing environmental impacts of product systems beyond physical relationships accounted for in attributional LCA (ALCA). Put simply, CLCA represents the convergence of LCA and economic modeling approaches. Method In this study, a systematic literature review of CLCA is performed. Results While initial efforts to integrate the two modeling methods relied on simple partial equilibrium (PE) modeling and a heuristic approach to determining affected technologies, more recent techniques incorporate sophisticated economic models for this purpose. In the last 3 years, Multi-Market, Multi-Regional PE Models and Computable General Equilibrium models have been used. Moreover, the incorporation of other economic notions into CLCA, such as rebound effects and experience curves, has been the focus of later research. Since economic modeling can play a prominent role in national policy-making and strategic/corporate environmental planning, developing the capacity to operate LCA concurrent to, or integrated with, these models is of growing importance. Conclusions This paper outlines the historical development of such efforts in CLCA, discusses key methodological advancements, and characterizes previous literature on the topic. Based on this review, we provide an outlook for further research in CLCA.

*: These authors contributed equally 24 25One-sentence summary: 26The gas phase of mesophyll conductance is impacted by the 3D traits tortuosity, path 27 lengthening and airspace connectivity, in addition to porosity. 2 JME, GTR, and CRB conceived the study and developed the methods, with 31 contributions from MEG and AJM; JME and GTR acquired and analyzed the data; ABR 32 performed the phylogenetic analyses; JME and GTR wrote the manuscript; ABR, MEG, 33 AJM, and CRB complemented the writing. 3 ABSTRACT 42The leaf intercellular airspace (IAS) is generally considered to have high conductance to 43 CO 2 diffusion relative to the liquid phase. While previous studies accounted for leaf-level 44 variation in porosity and mesophyll thickness, they omitted 3D IAS traits that potentially 45 influence IAS conductance (g IAS ). Here we re-evaluated the standard equation for g IAS 46 by incorporating tortuosity, lateral path lengthening, and IAS connectivity. We measured 47 and spatially mapped these geometric IAS traits for 19 Bromeliaceae species with CAM 48 or C3 photosynthetic pathways using X-ray microCT imaging and a novel computational 49 approach. We found substantial variation in porosity (0.04-0.73 m 3 m -3 ), tortuosity 50 -2 s -1 bar -1 ) plants due to a coordinated decline in these IAS traits. Our re-evaluated 54 equation also generally predicted lower g IAS values than the former one. Moreover, we 55 observed high spatial heterogeneity in these IAS geometric traits throughout the 56 mesophyll, especially within CAM leaves. Our data show that IAS traits that better 57 capture the 3D complexity of leaves strongly influence g IAS and that the impact of the 58 IAS on mesophyll conductance should be carefully considered with respect to leaf 59 anatomy. We provide a simple function to estimate tortuosity and lateral path 60 lengthening in the absence of access to imaging tools such as X-ray microCT or other 61 novel 3D image-processing techniques. INTRODUCTION 63By volume, as little as 3% and up to 73% (this study) of 64 the inside of a leaf is composed of air. Such a wide range of values results from the 65 multiple roles that mesophyll cells play in leaf function, the degree of reticulation of the 66 embedded vein network, and cell size and shape, all reflecting the various adaptations 67 plants have made in colonizing nearly every terrestrial habitat on Earth. From an 68 evolutionary perspective, the transition from oceans to land exposed plant tissues to air, 69 which dramatically lowered the resistance for CO 2 diffusion to chloroplasts by ~10,000-70fold. Evolutionary development of the leaf intercellular airspace (IAS) is therefore 71 considered a key innovation to profit from that lowered diffusion resistance (Ligrone et 72 al., 2012). Yet, terrestrial inhabitation also exposed leaves to the risk of desiccation. 73Plants presumably navigated this trade-off by developing a complex spatial cellular 74 arrangement in order to produce a more or less tortuous 3D IAS network that rapidly 75 delivered CO 2 t...

Maintaining high rates of photosynthesis in leaves requires efficient movement of CO 2 from the atmosphere to the mesophyll cells inside the leaf where CO 2 is converted into sugar. CO 2 diffusion inside the leaf depends directly on the structure of the mesophyll cells and their surrounding airspace, which have been difficult to characterize because of their inherently three-dimensional organization. Yet faster CO 2 diffusion inside the leaf was probably critical in elevating rates of photosynthesis that occurred among angiosperm lineages. Here we characterize the three-dimensional surface area of the leaf mesophyll across vascular plants. We show that genome size determines the sizes and packing densities of cells in all leaf tissues and that smaller cells enable more mesophyll surface area to be packed into the leaf volume, facilitating higher CO 2 diffusion. Measurements and modelling revealed that the spongy mesophyll layer better facilitates gaseous phase diffusion while the palisade mesophyll layer better facilitates liquid-phase diffusion. Our results demonstrate that genome downsizing among the angiosperms was critical to restructuring the entire pathway of CO 2 diffusion into and through the leaf, maintaining high rates of CO 2 supply to the leaf mesophyll despite declining atmospheric CO 2 levels during the Cretaceous.

Summary The mesophyll surface area exposed to intercellular air space per leaf area (Sm) is closely associated with CO2 diffusion and photosynthetic rates. Sm is typically estimated from two‐dimensional (2D) leaf sections and corrected for the three‐dimensional (3D) geometry of mesophyll cells, leading to potential differences between the estimated and actual cell surface area. Here, we examined how 2D methods used for estimating Sm compare with 3D values obtained from high‐resolution X‐ray microcomputed tomography (microCT) for 23 plant species, with broad phylogenetic and anatomical coverage. Relative to 3D, uncorrected 2D Sm estimates were, on average, 15–30% lower. Two of the four 2D Sm methods typically fell within 10% of 3D values. For most species, only a few 2D slices were needed to accurately estimate Sm within 10% of the whole leaf sample median. However, leaves with reticulate vein networks required more sections because of a more heterogeneous vein coverage across slices. These results provide the first comparison of the accuracy of 2D methods in estimating the complex 3D geometry of internal leaf surfaces. Because microCT is not readily available, we provide guidance for using standard light microscopy techniques, as well as recommending standardization of reporting Sm values.