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• Despite the importance of nonstructural carbohydrates (NSC) for growth and survival in woody plants, we know little about whole-tree NSC storage. The conventional theory suggests that NSC reserves will increase over the growing season and decrease over the dormant season. Here, we compare storage in five temperate tree species to determine the size and seasonal fluctuation of whole-tree total NSC pools as well as the contribution of individual organs. • NSC concentrations in the branches, stemwood, and roots of 24 trees were measured across 12 months. We then scaled up concentrations to the whole-tree and ecosystem levels using allometric equations and forest stand inventory data. • While whole-tree total NSC pools followed the conventional theory, sugar pools peaked in the dormant season and starch pools in the growing season. Seasonal depletion of total NSCs was minimal at the whole-tree level, but substantial at the organ level, particularly in branches. Surprisingly, roots were not the major storage organ as branches stored comparable amounts of starch throughout the year, and root reserves were not used to support springtime growth. • Scaling up NSC concentrations to the ecosystem level, we find that commonly used, process-based ecosystem and land surface models all overpredict NSC storage.

During drought, xylem sap pressures can approach or exceed critical thresholds where gas embolisms form and propagate through the xylem network, leading to systemic hydraulic dysfunction. The vulnerability segmentation hypothesis (VSH) predicts that low-investment organs (e.g. leaf petioles) should be more vulnerable to embolism spread compared to high-investment, perennial organs (e.g. trunks, stems), as a means of mitigating embolism spread and excessive negative pressures in the perennial organs. We tested this hypothesis by measuring air-seeding thresholds using the single-vessel air-injection method and calculating hydraulic safety margins in four northern hardwood tree species of the northeastern United States, in both saplings and canopy height trees, and at five points along the soil–plant–atmosphere continuum. Acer rubrum was the most resistant to air-seeding and generally supported the VSH. However, Fagus grandifolia, Fraxinus americana and Quercus rubra showed little to no variation in air-seeding thresholds across organ types within each species. Leaf-petiole xylem operated at water potentials close to or exceeding their hydraulic safety margins in all species, whereas roots, trunks and stems of A. rubrum, F. grandifolia and Q. rubra operated within their safety margins, even during the third-driest summer in the last 100 yr.

We know surprisingly little about whole‐tree nonstructural carbon (NSC; primarily sugars and starch) budgets. Even less well understood is the mixing between recent photosynthetic assimilates (new NSC) and previously stored reserves. And, NSC turnover times are poorly constrained. We characterized the distribution of NSC in the stemwood, branches, and roots of two temperate trees, and we used the continuous label offered by the radiocarbon (carbon‐14,¹⁴C) bomb spike to estimate the mean age of NSC in different tissues. NSC in branches and the outermost stemwood growth rings had the¹⁴C signature of the current growing season. However, NSC in older aboveground and belowground tissues was enriched in¹⁴C, indicating that it was produced from older assimilates. Radial patterns of¹⁴C in stemwood NSC showed strong mixing of NSC across the youngest growth rings, with limited ‘mixing in’ of younger NSC to older rings. Sugars in the outermost five growth rings, accounting for two‐thirds of the stemwood pool, had a mean age < 1 yr, whereas sugars in older growth rings had a mean age > 5 yr. Our results are thus consistent with a previously‐hypothesized two‐pool (‘fast’ and ‘slow’ cycling NSC) model structure. These pools appear to be physically distinct.

Deciduous angiosperm trees transport xylem sap through trunks and branches in vessels within annual growth rings. Utilizing previous growth rings for sap transport could increase vessel network size and redundancy but may expose new xylem to residual air embolisms in the network. Despite the important role of vessel networks in sap transport and drought resistance, our understanding of cross-ring connections within and between species is limited. We studied cross-ring connections in four temperate deciduous trees using dye staining and X-ray microcomputed tomography (microCT) to detect xylem connectivity across growth rings and quantify their impact on hydraulic conductivity. Acer rubrum and Fraxinus americana had cross-ring connections visible in microCT but only A. rubrum used previous growth rings for axial sap flow. Fagus grandifolia and Quercus rubra, however, did not have cross-ring connections. Accounting for the number of growth rings that function for axial transport improved hydraulic conductivity estimates. These data suggest that the presence of cross-ring connections may help explain aspects of whole-tree xylem sap transport and should be considered for plant hydraulics measurements in these species and others with similar anatomy.

• Leaf habit is a major axis of plant diversity that has consequences for carbon balance since the leaf is the primary site of photosynthesis. Nonstructural carbohydrates (NSCs) produced by photosynthesis can be allocated to storage and serve as a resiliency mechanism to future abiotic and biotic stress. However, how leaf habit affects NSC storage in an evolutionary context has not been shown. • Using a comparative physiological framework and an analysis of evolutionary model fitting, we examined if variation in NSC storage is explained by leaf habit. We measured sugar and starch concentrations in 51 oak species (Quercus spp.) growing in a common garden and representing multiple evolutions of three different leaf habits (deciduous, brevideciduous and evergreen). • The best fitting evolutionary models indicated that deciduous oak species are evolving towards higher NSC concentrations than their brevideciduous and evergreen relatives. Notably, this was observed for starch (the primary storage molecule) in the stem (a long-term C storage organ). • Overall, our work provides insight into the evolutionary drivers of NSC storage and suggests that a deciduous strategy may confer an advantage against stress associated with a changing world. Future work should examine additional clades to further corroborate this idea.

Longevity, or organismal life span, is determined largely by the period over which constituent cells can function metabolically. Plants, with modular organization (the ability continually to develop new organs and tissues) differ from animals, with unitary organization (a fixed body plan), and this difference is reflected in their respective life spans, potentially much longer in plants than animals. We draw attention to the observation that palm trees, as a group of monocotyledons without secondary growth comparable to that of lignophytes (plants with secondary growth from a bifacial cambium), retain by means of sustained primary growth living cells in their trunks throughout their organismal life span. Does this make palms the longest-lived trees because they can grow as individuals for several centuries? No conventional lignophyte retains living metabolically active differentiated cell types in its trunk for this length of time, even though the tree as a whole can exist for millennia. Does this contrast also imply that the long-lived cells in a palm trunk have exceptional properties, which allows this seeming immortality? We document the long-life of many tall palm species and their inherent long-lived stem cell properties, comparing such plants to conventional trees. We provide a summary of aspects of cell age and life span in animals and plants. Cell replacement is a feature of animal function, whereas conventional trees rely on active growth centers (meristems) to sustain organismal development. However, the long persistence of living cells in palm trunks is seen not as evidence for unique metabolic processes that sustain longevity, but is a consequence of unique constructional features. This conclusion suggests that the life span of plant cells is not necessarily genetically determined.

PREMISE OF THE STUDY: Despite the strong influence of the frequency and distribution of vessel endings on both hydraulic safety and efficiency, detailed anatomical descriptions or measurements of these structures are generally lacking. METHODS: Here we used high‐resolution x‐ray microcomputed tomography (microCT) to identify and describe xylem vessel endings within Acer rubrum root segments (1.0–2.1 mm diameter, ∼2 mm long). We then compared vessel‐lumen diameter, pit density, vessel element length, and perforation plate angle between non‐ending vessels (those that traverse an entire segment) and those that end within a segment using three‐dimensional image analysis. KEY RESULTS: We found 214 vessel endings, 37 complete vessels, and 385 non‐ending vessels within four A. rubrum root segments. Vessels that ended within the segments tended to have more acute perforation plate angles and had a smaller diameter than those that did not end within the segments. Most vessel diameters tapered within the last few vessel elements, but the perforation plate angle apparently changed over longer distances. Intervessel pit density and vessel element length did not differ between ending and non‐ending vessels. CONCLUSIONS: Vessel endings were surprisingly frequent in A. rubrum roots despite the common perception that root vessels are longer than vessels in other tissues. MicroCT proved to be a useful tool for studying the three‐dimensional arrangement of vessel endings within xylem networks, and these data will be helpful in developing a better understanding of vessel ending microstructure and function.

Key messagePathogen-induced defoliation resulted in a reduction in transpiration, an upregulation of photosynthesis in the early growing season, and no change in NSC reserves across stem, root, and foliar tissues.The defoliation of eastern white pine (Pinus strobus L.) by native fungi associated with white pine needle damage (WPND) can substantially reduce foliar area for much of the growing season in the northeastern United States. Chronic defoliations in the region are known to have slowed growth rates in symptomatic stands, but the physiological impacts of WPND as it relates to tree water use and carbon assimilation are largely unresolved. We investigated how the severity of WPND defoliation influences transpiration throughout the course of a growing season. We also assessed leaf-level gas exchange between defoliation severity classes and needle age over time. Finally, we compared concentrations of non-structural carbohydrates (NSC) between defoliation severity classes in five different tissue types over time. We found that trees experiencing a high-severity defoliation had 20% lower sap flux density compared to low-severity individuals. We found that rates of photosynthesis were significantly influenced by the needle age class and time of year, while instantaneous water use efficiency was higher across all needle age classes late in the growing season. Our findings suggest that the residual current-year foliage of high-severity defoliated trees compensated for the loss of mature second- and third-year foliage in the early portion of the growing season. This study found that soluble sugars and starch varied significantly over time and by tissue type, but defoliation severity had little effect on NSC concentrations. Together with reduced basal area increment in high-severity trees relative to low-severity trees, this indicates that WPND-affected trees are prioritizing NSC storage over secondary growth.

The leaf axis of members of the order Cycadales ('cycads') has long been recognized by its configuration of independent vascular bundles that, in transverse section, resemble the Greek letter omega (hence the 'omega pattern'). This provides a useful diagnostic character for the order, especially when applied to paleobotany. The function of this pattern has never been elucidated. Here we provide a three-dimensional analysis and explain the pattern in terms of the hydraulic architecture of the pinnately compound cycad leaf. The genus Cycas was used as a simple model, because each leaflet is supplied by a single vascular bundle. Sequential sectioning was conducted throughout the leaf axis and photographed with a digital camera. Photographs were registered and converted to a cinematic format, which provided an objective method of analysis. The omega pattern in the petiole can be sub-divided into three vascular components, an abaxial 'circle', a central 'column' and two adaxial 'wings', the last being the only direct source of vascular supply to the leaflets. Each leaflet is supplied by a vascular bundle that has divided or migrated directly from the closest wing bundle. There is neither multiplication nor anastomoses of vascular bundles in the other two components. Thus, as one proceeds from base to apex along the leaf axis, the number of vascular bundles in circle and column components is reduced distally by their uniform migration throughout all components. Consequently, the distal leaflets are irrigated by the more abaxial bundles, guaranteeing uniform water supply along the length of the axis. The omega pattern exemplifies one of the many solutions plants have achieved in supplying distal appendages of an axis with a uniform water supply. Our method presents a model that can be applied to other genera of cycads with more complex vascular organization.

Premise of research. The extant genera of cycads (order Cycadales) can be readily distinguished by the anatomy of their leaflets. In particular, the genus Dioon possesses a unique cellulosic fiber type in the leaflet mesophyll, not found in any other genus. We examine living material of all 15 species of Dioon to confirm preliminary observations and provide details of wall structure in comparison with fibers in the leaf axis. Methodology. Unembedded sample material was sectioned on a sliding microtome transversely and longitudinally, and standard histochemical tests were utilized to identify cell types. SEM was utilized to analyze fiber anatomy. Maceration of samples provided cell types for comparison. Pivotal results. All species of Dioon have the same configuration of cell types in the leaflets, the most conspicuous element being elongated fibers with multilamellate cellulosic cell walls. Such fibers do not appear in the leaf axis. This fiber type is seemingly unique among all cycad genera. Conclusions. Dioon leaflets possess a fiber type of distinctive wall structure, unique among the extant cycads. This finding clarifies an earlier report in which such fibers were described as “gelatinous fibers.”