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Herbivory is a fundamental ecological force in the evolution of plant physiological, morphological, and chemical attributes. In this study, we explored how browsing pressure by local deer populations affected leaf form and function in two California native tree species, Quercus agrifolia (coast live oak) and Umbellularia californica (California bay laurel). Specifically, we investigated how leaf and stem vascular attributes differed between browsed and non-browsed zones of each species. Browsing significantly altered traits such as leaf to phloem ratios and leaf area, but we observed few meaningful differences in leaf and stem anatomy between browsed and non-browsed material. We discuss these results in the context of leaf and stem adaptations to herbivory and water use efficiency and explore future research considerations for investigating leaf and stem vascular trait development with herbivore presence.

The wide size range of conifer tracheids and angiosperm vessels has important consequences for function. In both conduit types, bigger is better for conducting efficiency. The gain in efficiency with size is maximized by the control of conduit shape, which balances end-wall and lumen resistances. Although vessels are an order of magnitude longer than tracheids of the same diameter, they are not necessarily more efficient because they lack the low end-wall resistance of tracheids with torus-margo pits. Instead, vessels gain conducting efficiency over tracheids by achieving wider maximum diameters. End-walls contributed 56-64% to total xylem resistance in both conduit types, indicating that length limits conducting efficiency. Tracheid dimensions may be more limited by unicellularity and the need to supply strength to homoxylous wood than by the need to protect against cavitation. In contrast, the greater size of the multicellular vessel is facilitated by fibers that strengthen heteroxylous wood. Vessel dimensions may be most limited by the need to restrict intervessel pitting and cavitation by air-seeding. Stressful habitats that promote narrow vessels should favor coexistence of conifers and angiosperms. The evolution of vessels in angiosperm wood may have required early angiosperms to survive a phase of mechanic and hydraulic instability.

Maceration technique allows isolating and studying the structure of plant cells, providing essential insights into plant physiology and responses to environmental factors. Despite its importance, the methods sections of publications often lack sufficient detail to properly apply maceration technique, preventing broader applications beyond industrial and wood identification studies. Here we describe maceration protocols (P), as guidelines, to allow successful cell separation, in woody and herbaceous plants, for further studying the structure of vascular tissues (xylem and phloem), using Franklin’s solution instead of the traditional, more toxic Jeffrey’s. We included sample preparation, equipment and chemicals, recommended stains and means of slide preparation that ensure clear observation and imaging, and identified potential pitfalls and safety practices. P1 provides a simple xylem and phloem maceration technique, with non-permanent slides preparation, suitable for observation under light, fluorescence or confocal microscopy. P2 and P3 are suitable for producing permanent slides of macerated xylem cells. P2 has been successfully used for macerating xylem of angiosperms and gymnosperms from at least 34 families and 25 orders, with densities ranging widely. P3 uses minimal substances and time for dehydration and staining. These flexible protocols may contribute to unlocking the potential of maceration technique to advance research in ecology and evolution.

Angiosperm radiation in the Cretaceous is thought to have profoundly diminished the success of the conifers, the other major woody plant group present at the time. However, today the conifers persist and often thrive despite their supposed inferiority in vegetative and reproductive function. By exploring this apparent conflict for global tree dominance, we seek here to reveal patterns that explain not only how the allegedly inferior conifers persist among angiosperms but also why some conifer groups became extinct in the Cretaceous. We find that despite the profound contrast between the dominant conifer families in the Southern and Northern Hemispheres, all conifers can be characterized by a common set of functional attributes that allow them to exist in an important group of niches, from high latitudes to the equator. In these environments, conifers are often highly efficient at outcompeting, outliving, or outsurviving angiosperms. Hence, we conclude that conifer success cannot be dismissed as being uniquely associated with habitats that are unfavorable for angiosperms.

The ferns comprise one of the most ancient tracheophytic plant lineages, and occupy habitats ranging from tundra to deserts and the equatorial tropics. Like their nearest relatives the conifers, modern ferns possess tracheid-based xylem but the structure-function relationships of fern xylem are poorly understood. Here, we sampled the fronds (megaphylls) of 16 species across the fern phylogeny, and examined the relationships among hydraulic transport, drought-induced cavitation resistance, the xylem anatomy of the stipe, and the gas-exchange response of the pinnae. For comparison, the results are presented alongside a similar suite of conifer data. Fern xylem is as resistant to cavitation as conifer xylem, but exhibits none of the hydraulic or structural trade-offs associated with resistance to cavitation. On a conduit diameter basis, fern xylem can exhibit greater hydraulic efficiency than conifer and angiosperm xylem. In ferns, wide and long tracheids compensate in part for the lack of secondary xylem and allow ferns to exhibit transport rates on a par with those of conifers. We suspect that it is the arrangement of the primary xylem, in addition to the intrinsic traits of the conduits themselves, that may help explain the broad range of cavitation resistance in ferns.

Ice formation in the xylem sap produces air bubbles that under negative xylem pressures may expand and cause embolism in the xylem conduits. We used the centrifuge method to evaluate the relationship between freeze-thaw embolism and conduit diameter across a range of xylem pressures (P[subscript x]) in the conifers Pinus contorta and Juniperus scopulorum. Vulnerability curves showing loss of conductivity (embolism) with P[subscript x] down to -8 MPa were generated with versus without superimposing a freeze-thaw treatment. In both species, the freeze-thaw plus water-stress treatment caused more embolism than water stress alone. We estimated the critical conduit diameter (D[subscript f]) above which a tracheid will embolize due to freezing and thawing and found that it decreased from 35 [micro]m at a P[subscript x] of -0.5 MPa to 6 [micro]m at -8 MPa. Further analysis showed that the proportionality between diameter of the air bubble nucleating the cavitation and the diameter of the conduit (kL) declined with increasingly negative P[subscript x]. This suggests that the bubbles causing cavitation are smaller in proportion to tracheid diameter in narrow tracheids than in wider ones. A possible reason for this is that the rate of dissolving increases with bubble pressure, which is inversely proportional to bubble diameter (La Place's law). Hence, smaller bubbles shrink faster than bigger ones. Last, we used the empirical relationship between P[subscript x] and D[subscript f] to model the freeze-thaw response in conifer species.

Water transport in conifers occurs through single-celled tracheids that are connected to one another via intertracheid pit membranes. These membranes have two components: the porous margo, which allows water to pass through the membrane, and the impermeable torus, which functions to isolate gas-filled tracheids. During drought, tracheids can become air filled and thus hydraulically dysfunctional, a result of air entering through the pit membrane and nucleating cavitation in the water column. What are the hydraulic tradeoffs associated with cavitation resistance at the pit level, and how do they vary within the structural components of the intertracheid pit? To address these questions, we examined pit structure in 15 species of Cupressaceae exhibiting a broad range of cavitation resistances. Across species, cavitation resistance was most closely correlated to the ratio of the torus to pit aperture diameter but did not vary systematically with margo porosity. Furthermore, our data indicate that constraints on pit hydraulic efficiency are shared: the pit aperture limits pit conductivity in more drought-resistant taxa, while increased margo resistance is more likely to control pit conductivity in species that are more vulnerable to cavitation. These results are coupled with additional data concerning pit membrane structure and function and are discussed in the context of the evolutionary biogeography of the Cupressaceae.

The understory of the redwood forests of California's coast harbors perennial ferns, including Polystichum munitum and Dryopteris arguta. Unusual for ferns, these species are adapted to the characteristic Mediterranean‐type dry season, but the mechanisms of tolerance have not been studied. The water relations of P. munitum and D. arguta were surveyed for over a year, including measures of water potential (Ψ), stomatal conductance (gₛ) and frond stipe hydraulic conductivity (K). A dehydration and re‐watering experiment on potted P. munitum plants corroborated the field data. The seasonal Ψ varied from 0 to below −3 MPa in both species, with gₛ and K generally tracking Ψ; the loss of K rarely exceeded 80%. Quantile regression analysis showed that, at the 0.1 quantile, 50% of K was lost at −2.58 and −3.84 MPa in P. munitum and D. arguta, respectively. The hydraulic recovery of re‐watered plants was attributed to capillarity. The seasonal water relations of P. munitum and D. arguta are variable, but consistent with laboratory‐based estimates of drought tolerance. Hydraulic and Ψ recovery following rain allows perennial ferns to survive severe drought, but prolonged water deficit, coupled with insect damage, may hamper frond survival. The legacy effects of drought on reproductive capacity and community dynamics are unknown.

The Cupressaceae clade has the broadest diversity in habitat and morphology of any conifer family. This clade is characterized by highly divergent physiological strategies, with deciduous swamp-adapted genera-like Taxodium at one extreme, and evergreen desert genera-like Cupressus at the other. The size disparity within the Cupressaceae is equally impressive, with members ranging from 5-m-tall juniper shrubs to 100-m-tall redwood trees. Phylogenetic studies demonstrate that despite this variation, these taxa all share a single common ancestor; by extension, they also share a common ancestral habitat. Here, we use a common-garden approach to compare xylem and leaf-level physiology in this family. We then apply comparative phylogenetic methods to infer how Cenozoic climatic change shaped the morphological and physiological differences between modern-day members of the Cupressaceae. Our data show that drought-resistant crown clades (the Cupressoid and Callitroid clades) most likely evolved from drought-intolerant Mesozoic ancestors, and that this pattern is consistent with proposed shifts in post-Eocene paleoclimates. We also provide evidence that within the Cupressaceae, the evolution of drought-resistant xylem is coupled to increased carbon investment in xylem tissue, reduced xylem transport efficiency, and at the leaf level, reduced photosynthetic capacity. Phylogenetically based analyses suggest that the ancestors of the Cupressaceae were dependent upon moist habitats, and that drought-resistant physiology developed along with increasing habitat aridity from the Oligocene onward. We conclude that the modern biogeography of the Cupressaceae conifers was shaped in large part by their capacity to adapt to drought.

Desiccation-tolerant (DT) plants can dry past – 100 MPa and subsequently recover function upon rehydration. Vascular DT plants face the unique challenges of desiccating and rehydrating complex tissues without causing structural damage. However, these dynamics have not been studied in intact DT plants. We used high resolution micro-computed tomography (microCT), light microscopy, and fluorescence microscopy to characterize the dynamics of tissue desiccation and rehydration in petioles (stipes) of intact DT ferns. During desiccation, xylem conduits in stipes embolized before cellular dehydration of living tissues within the vascular cylinder. During resurrection, the chlorenchyma and phloem within the stipe vascular cylinder rehydrated before xylem refilling. We identified unique stipe traits that may facilitate desiccation and resurrection of the vascular system, including xylem conduits containing pectin (which may confer flexibility and wettability); chloroplasts within the vascular cylinder; and an endodermal layer impregnated with hydrophobic substances that impede apoplastic leakage while facilitating the upward flow of water within the vascular cylinder. Resurrection ferns are a novel system for studying extreme dehydration recovery and embolism repair in the petioles of intact plants. The unique anatomical traits identified here may contribute to the spatial and temporal dynamics of water movement observed during desiccation and resurrection.