Close to the edge: effects of repeated severe drought on stem hydraulics and non-structural carbohydrates in European beech saplings

Severe drought events threaten tree water transport system, productivity and survival. Woody angiosperms generally die when embolism-induced loss of hydraulic conductance (PLC) surpasses 80-90% under intense water shortage. However, the recovery capability and possible long-term carry-over effects of repeated drought events could dictate the fate of species' population under climate change scenarios. Potted saplings of European beech (Fagus sylvatica L.) were subjected to two drought cycles in two consecutive growing seasons, aiming to induce minimum leaf water potentials (Ψmd) of about -4 MPa, corresponding to hydraulic thresholds for survival of this species. In the first cycle, a well-irrigated (C) and a drought-stressed group (S) were formed, and, in the following summer, each group was divided in a well-irrigated and a drought-stressed one (four groups in total). The impact of the multiple drought events was assessed by measuring wood anatomical traits, biomass production, water relations, stem hydraulics and non-structural carbohydrate (NSC) content. We also investigated possible connections between stem hydraulics and carbon dynamics during the second drought event and following re-irrigation. S plants had lower Ψmd and maximum specific hydraulic conductivity (Ks) than C plants in the following growing season. Additionally, aboveground biomass production and leaf number were lower compared to C trees, resulting in lower water consumption. However, PLC was similar between groups, probably due to the production of new functional xylem in spring. The second drought event induced 85% PLC and promoted conversion of starch-to soluble sugars. Nevertheless, 1 week after re-irrigation, no embolism repair was observed and soluble sugars were reconverted to starch. The previous drought cycle did not influence the hydraulic performance during the second drought, and after re-irrigation S plants had 40% higher wood NSC content. Our data suggest that beech cannot recover from high embolism levels but multiple droughts might enhance stem NSC availability.

Vestured pits and scalariform perforation plate morphology modify the relationships between angiosperm vessel diameter, climate and maximum plant height

Shared ancestry among species and correlation between vessel diameter and plant height can obscure the mechanisms linking vessel diameter to current climate distributions of angiosperms. Because wood is complex, various traits may interact to influence vessel function. Specifically, pit vesturing (lignified cell wall protuberances associated with bordered pits) and perforation plate morphology could alter the relationships between vessel diameter, climate and plant height. Using phylogenetically informed analyses, we tested for associations between vessel diameter, climate and maximum plant height across angiosperm species with different pit vesturing (presence/absence) and perforation plate morphology (simple/scalariform and quantitative variation). We show significantly larger changes in vessel diameter and maximum plant height across climates for species with vestures and simple perforation plates, compared to nonvestured species and those with scalariform plates. We also found a significantly greater increase in height for a given increase in vessel diameter with lower percentage of scalariform plates. Our study provides novel insights into the evolution of angiosperm xylem by showing that vessel pit vesturing and perforation plate morphologies can modify relationships among xylem vessels, climate and height. Our findings highlight the complexity of xylem adaptations to climate, substantiating an integrative view of xylem function in the study of wood evolution.

Hydraulic architecture and vulnerability to drought-induced embolism in southern boreal tree species of Inner Asia

The branch xylem of six important Inner Asian southern boreal forest trees was studied for wood-anatomical and hydraulic traits in order to infer the species' drought tolerance from embolism resistance, potential hydraulic conductivity, mean conduit diameters and conduit density. The only studied angiosperm tree, Betula pendula Roth, was much more sensitive to cavitation than all five conifers (evergreen or summer-green), even when using 88% loss of conductivity (P88) in birch, but 50% (P50) in the conifers as critical thresholds. This suggests that pioneer birch forests, which have widely replaced the conifer climax forests after anthropogenic disturbance (e.g., logging, man-made fire), are more vulnerable to climate warming-induced drought than the original conifer forests. In contrast to expectation, the generally more drought-exposed light taiga species (Larix sibirica Ledeb., Pinus sylvestris L.) did not have consistently lower P50 and P88 values than the dark taiga conifers, suggesting that other drought survival traits are equally important. Among the dark-taiga species, only Pinus sibirica Du Tour, but not Abies sibirica Ledeb. and Picea obovata Ledeb., had relatively high P50 values indicating higher vulnerability. In the light-taiga forest, P. sylvestris revealed lower embolism resistance than L. sibirica. In the face of rapid climate warming and drying in Inner Asia, the drought survival strategies of southern boreal tree species deserve further intensive study, which should include other drought survival traits.

Similarities and differences in the balances between leaf, xylem and phloem structures in Fraxinus ornus along an environmental gradient

The plant carbon balance depends on the coordination between photosynthesis and the long-distance transport of water and sugars. How plants modify the allocation to the different structures affecting this coordination under different environmental conditions has been poorly investigated. In this study, we evaluated the effect of soil water availability on the allocation to leaf, xylem and phloem structures in Fraxinus ornus L. We selected small individuals of F. ornus (height ~2 m) from sites contrasting in soil water availability (wet vs dry). We measured how the leaf (LM) and stem + branch biomass (SBM) are cumulated along the stem. Moreover, we assessed the axial variation in xylem (XA) and phloem tissue area (PA), and in lumen area of xylem vessels (CAxy) and phloem sieve elements (CAph). We found a higher ratio of LM:SBM in the trees growing under drier conditions. The long-distance transport tissues of xylem and phloem followed axial patterns with scaling exponents (b) independent of site conditions. PA scaled isometrically with XA (b ~ 1). While CAxy was only marginally higher at the wet sites, CAph was significantly higher at the drier sites. Our results showed that under reduced soil water availability, F. ornus trees allocate relatively more to the leaf biomass and produce more conductive phloem, which is likely to compensate for the drought-related hydraulic limitations to the leaf gas exchanges and the phloem sap viscosity.

Do tall tree species have higher relative stiffness than shorter species?

PREMISE OF THE STUDY

In 1757 Leonhard Euler demonstrated that to avoid bending tall columns needed to be stiffer but not stronger than shorter columns of equal diameter and material density. Many researchers have concluded that trees have a fixed stiffness to basic density ratio, and therefore, trees adjust for increasing height by adding mass to adjust stem form. But the wood science literature points to considerable variance in stiffness with respect to green wood density.

METHODS

Using the vast global repository of green wood mechanical properties, we compared relative stiffness and relative strength between taller and shorter species. For North American trees, we examined stem moisture distribution.

KEY RESULTS

For all regions of the world, taller species on average possessed greater stiffness, but not strength, than shorter species of equal basic specific gravity. We looked for a possible universal mechanism that might allow taller tree species to adjust stiffness without affecting xylem specific gravity and concluded that the evidence points to a decrease in cellulose microfibril angle in structural cell walls combined with possible increases in holocellulose percentage. The evidence is strongest for conifers. We also showed that tall conifers have the ability to adjust the distribution of xylem moisture to maximize conduction while minimizing column load.

CONCLUSIONS

Our research reveals that taller trees have developed internal stem adjustments to minimize diameter increase while attaining ever-greater heights, thus enabling these taller species to reduce energy expended on biomass accumulation while gaining greater access to solar radiation.

The Physiological Mechanisms Behind the Earlywood-To-Latewood Transition: A Process-Based Modeling Approach

In extratropical ecosystems, the growth of trees is cyclic, producing tree rings composed of large-lumen and thin-walled cells (earlywood) alternating with narrow-lumen and thick-walled cells (latewood). So far, the physiology behind wood formation processes and the associated kinetics has rarely been considered to explain this pattern. We developed a process-based mechanistic model that simulates the development of conifer tracheids, explicitly considering the processes of cell enlargement and the deposition and lignification of cell walls. The model assumes that (1) wall deposition gradually slows down cell enlargement and (2) the deposition of cellulose and lignin is regulated by the availability of soluble sugars. The model reliably reproduces the anatomical traits and kinetics of the tracheids of four conifer species. At the beginning of the growing season, low sugar availability in the cambium results in slow wall deposition that allows for a longer enlargement time; thus, large cells with thin walls (i.e., earlywood) are produced. In late summer and early autumn, high sugar availability produces narrower cells having thick cell walls (i.e., latewood). This modeling framework provides a mechanistic link between plant ecophysiology and wood phenology and significantly contributes to understanding the role of sugar availability during xylogenesis.

Feeling stretched or compressed? The multiple mechanosensitive responses of wood formation to bending

Background and Aims

Trees constantly experience wind, perceive resulting mechanical cues, and modify their growth and development accordingly. Previous studies have demonstrated that multiple bending treatments trigger ovalization of the stem and the formation of flexure wood in gymnosperms, but ovalization and flexure wood have rarely been studied in angiosperms, and none of the experiments conducted so far has used multidirectional bending treatments at controlled intensities. Assuming that bending involves tensile and compressive strain, we hypothesized that different local strains may generate specific growth and wood differentiation responses.

Methods

Basal parts of young poplar stems were subjected to multiple transient controlled unidirectional bending treatments during 8 weeks, which enabled a distinction to be made between the wood formed under tensile or compressive flexural strains. This set-up enabled a local analysis of poplar stem responses to multiple stem bending treatments at growth, anatomical, biochemical and molecular levels.

Key Results

In response to multiple unidirectional bending treatments, poplar stems developed significant cross-sectional ovalization. At the tissue level, some aspects of wood differentiation were similarly modulated in the compressed and stretched zones (vessel frequency and diameter of fibres without a G-layer), whereas other anatomical traits (vessel diameter, G-layer formation, diameter of fibres with a G-layer and microfibril angle) and the expression of fasciclin-encoding genes were differentially modulated in the two zones.

Conclusions

This work leads us to propose new terminologies to distinguish the 'flexure wood' produced in response to multiple bidirectional bending treatments from wood produced under transient tensile strain (tensile flexure wood; TFW) or under transient compressive strain (compressive flexure wood; CFW). By highlighting similarities and differences between tension wood and TFW and by demonstrating that plants could have the ability to discriminate positive strains from negative strains, this work provides new insight into the mechanisms of mechanosensitivity in plants.

Drought Sensitivity of Norway Spruce at the Species' Warmest Fringe: Quantitative and Molecular Analysis Reveals High Genetic Variation Among and Within Provenances

Norway spruce (Picea abies) is by far the most important timber species in Europe, but its outstanding role in future forests is jeopardized by its high sensitivity to drought. We analyzed drought response of Norway spruce at the warmest fringe of its natural range. Based on a 35-year old provenance experiment we tested for genetic variation among and within seed provenances across consecutively occurring strong drought events using dendroclimatic time series. Moreover, we tested for associations between ≈1,700 variable SNPs and traits related to drought response, wood characteristics and climate-growth relationships. We found significant adaptive genetic variation among provenances originating from the species’ Alpine, Central and Southeastern European range. Genetic variation between individuals varied significantly among provenances explaining up to 44% of the phenotypic variation in drought response. Varying phenotypic correlations between drought response and wood traits confirmed differences in selection intensity among seed provenances. Significant associations were found between 29 SNPs and traits related to drought, climate-growth relationships and wood properties which explained between 11 and 43% of trait variation, though 12 of them were due to single individuals having extreme phenotypes of the respective trait. The majority of these SNPs are located within exons of genes and the most important ones are preferentially expressed in cambium and xylem expansion layers. Phenotype-genotype associations were stronger if only provenances with significant quantitative genetic variation in drought response were considered. The present study confirms the high adaptive variation of Norway spruce in Central and Southeastern Europe and demonstrates how quantitative genetic, dendroclimatic and genomic data can be linked to understand the genetic basis of adaptation to climate extremes in trees.

Phloem Girdling of Norway Spruce Alters Quantity and Quality of Wood Formation in Roots Particularly Under Drought

Carbon (C) availability plays an essential role in tree growth and wood formation. We evaluated the hypothesis that a decrease in C availability (i) triggers mobilization of C reserves in the coarse roots of Picea abies to maintain growth and (ii) causes modification of wood structure notably under drought. The 6-year-old saplings were subjected to two levels of soil moisture (watered versus drought conditions) and root C status was manipulated by physically blocking phloem transport in the stem at three girdling dates (GDs). Stem girdling was done before the onset of bud break [day of the year (doy) 77], during vigorous aboveground shoot and radial stem growth (GD doy 138), and after cessation of shoot growth (GD doy 190). The effect of blockage of C transport on root growth, root phenology, and wood anatomical traits [cell lumen diameter (CLD) and cell wall thickness (CWT)] in earlywood (EW) and latewood (LW) was determined. To evaluate changes in belowground C status caused by girdling, non-structural carbohydrates (soluble sugars and starch) in coarse roots were determined at the time of girdling and after the growing season. Although fine root mass significantly decreased in response to blockage of phloem C transport, the phenology of root elongation growth was not affected. Surprisingly, radial root growth and CLD of EW tracheids in coarse roots were strikingly increased in drought-stressed trees, when girdling occurred before bud break or during aboveground stem growth. In watered trees, the growth response to girdling was less distinct, but the CWT of EW significantly increased. Starch reserves in the roots of girdled trees significantly decreased in both soil moisture treatments and at all GDs. We conclude that (i) radial growth and wood development in coarse roots of P. abies saplings are not only dependent on current photosynthates, and (ii) blockage of phloem transport induces physiological changes that outweigh drought effects imposed on root cambial activity and cell differentiation.

Cambial pinning relates wood anatomy to ecophysiology in the African tropical tree Maesopsis eminii

A better understanding and prediction of the impact of changing climate on tree stem growth could greatly benefit from the combination of anatomical and ecophysiological knowledge, yet the majority of studies focus on one research field only. We propose an approach that combines the method of pinning (cambial wounding) to timestamp anatomical X-ray computed microtomography images with continuous measurements of sap flow and stem diameter variations. By pinning the cambium of well-watered and drought-treated young African tropical trees of the species Maesopsis eminii Engl. we could quantify wood formation during a specific period of time and relate it to tree physiology and prevailing microclimate. Integrating continuous plant measurements and high-frequency pinning proved very useful to visualize and quantify the effects on stem growth of drought in M. eminii. Wood formation completely stopped during drought, and was associated with a strong shrinkage in stem diameter. Next, an unexpected increase in stem diameter was observed during drought, probably caused by root pressure, but not accompanied by wood formation. Our proposed approach of combining continuous plant measurements with cambial pinning is very promising to relate ecophysiology to stem anatomy and to understand the mechanisms underlying tree stem growth and bridge the gaps between the two research fields.

X-ray computed microtomography characterizes the wound effect that causes sap flow underestimation by thermal dissipation sensors

Insertion of thermal dissipation (TD) sap flow sensors in living tree stems causes damage of the wood tissue, as is the case with other invasive methods. The subsequent wound formation is one of the main causes of underestimation of tree water-use measured by TD sensors. However, the specific alterations in wood anatomy in response to inserted sensors have not yet been characterized, and the linked dysfunctions in xylem conductance and sensor accuracy are still unknown. In this study, we investigate the anatomical mechanisms prompting sap flow underestimation and the dynamic process of wound formation. Successive sets of TD sensors were installed in the early, mid and end stage of the growing season in diffuse- and ring-porous trees, Fagus sylvatica (Linnaeus) and Quercus petraea ((Mattuschka) Lieblein), respectively. The trees were cut in autumn and additional sensors were installed in the cut stem segments as controls without wound formation. The wounded area and volume surrounding each sensor was then visually determined by X-ray computed microtomography (X-ray microCT). This technique allowed the characterization of vessel anatomical transformations such as tyloses formation, their spatial distribution and quantification of reduction in conductive area. MicroCT scans showed considerable formation of tyloses that reduced the conductive area of vessels surrounding the inserted TD probes, thus causing an underestimation in sap flux density (SFD) in both beech and oak. Discolored wood tissue was ellipsoidal, larger in the radial plane, more extensive in beech than in oak, and also for sensors installed for longer times. However, the severity of anatomical transformations did not always follow this pattern. Increased wound size with time, for example, did not result in larger SFD underestimation. This information helps us to better understand the mechanisms involved in wound effects with TD sensors and allows the provision of practical recommendations to reduce biases associated with wounding in field sap flow measurements.

Anatomical explanations for acute depressions in radial pattern of axial sap flow in two diffuse-porous mangrove species: implications for water use

Mangrove species have developed uniquely efficient water-use strategies in order to survive in highly saline and anaerobic environments. Herein, we estimated the stand water use of two diffuse-porous mangrove species of the same age, Sonneratia apetala Buch. Ham and Sonneratia caseolaris (L.) Engl., growing in a similar intertidal environment. Specifically, to investigate the radial patterns of axial sap flow density (Js) and understand the anatomical traits associated with them, we measured axial sap flow density in situ together with micromorphological observations. A significant decrease of Js was observed for both species. This result was accompanied by the corresponding observations of wood structure and blockages in xylem sapwood, which appeared to influence and, hence, explained the acute radial reductions of axial sap flow in the stems of both species. However, higher radial resistance in sapwood of S. caseolaris caused a steeper decline of Js radially when compared with S. apetala, thus explaining the latter's more efficient use of water. Without first considering acute reductions in Js into the sapwood from the outer bark, a total of ~55% and 51% of water use would have been overestimated, corresponding to average discrepancies in stand water use of 5.6 mm day-1 for S. apetala trees and 2.5 mm day-1 for S. caseolaris trees. This suggests that measuring radial pattern of Js is a critical factor in determining whole-tree or stand water use.

Coexisting oak species, including rear-edge populations, buffer climate stress through xylem adjustments

The ability of trees to cope with climate change is a pivotal feature of forest ecosystems, especially for rear-edge populations facing warm and dry conditions. To evaluate current and future forests threats, a multi-proxy focus on the growth, anatomical and physiological responses to climate change is needed. We examined the long-term xylem adjustments to climate variability of the temperate Quercus robur L. at its rear edge and the sub-Mediterranean Quercus pyrenaica Willd. Both species coexist at a mesic (ME, humid and warmer) and a xeric (XE, dry and cooler) site in northern Spain, the latter experiencing increasing temperatures in recent decades. We compared xylem traits at each site and assessed their trends, relationships and responses to climate (1960-2008). Traits included basal area increment, earlywood vessel hydraulic diameter, density and theoretical-specific hydraulic conductivity together with latewood oxygen (δ18O) stable isotopes and δ13C-derived water-use efficiency (iWUE). Quercus robur showed the highest growth at ME, likely through enhanced cambial activity. Quercus pyrenaica had higher iWUE at XE compared with ME, but limited plasticity of anatomical xylem traits was found for the two oak species. Similar physiological performance was found for both species. The iWUE augmented in recent years especially at XE, likely explained by stomatal closure given the increasing δ18O signal in response to drier and sunnier growing seasons. Overall, traits were more correlated at XE than at ME. The iWUE improvements were linked to higher growth up to a threshold (~85 μmol mol-1) after which reduced growth was found at XE. Our results are consistent with Q. pyrenaica and Q. robur coexisting at the central and dry edge of the climatic species distribution, respectively, showing similar responses to buffer warmer conditions. In fact, the observed adjustments found for Q. robur point towards growth stability of similar rear-edge oak populations under warmer climate conditions.

Functional relationships between wood structure and vulnerability to xylem cavitation in races of Eucalyptus globulus differing in wood density

Wood density can be considered as a measure of the internal wood structure, and it is usually used as a proxy measure of other mechanical and functional traits. Eucalyptus is one of the most important commercial forestry genera worldwide, but the relationship between wood density and vulnerability to cavitation in this genus has been little studied. The analysis is hampered by, among other things, its anatomical complexity, so it becomes necessary to address more complex techniques and analyses to elucidate the way in which the different anatomical elements are functionally integrated. In this study, vulnerability to cavitation in two races of Eucalyptus globulus Labill. with different wood density was evaluated through Path analysis, a multivariate method that allows evaluation of descriptive models of causal relationship between variables. A model relating anatomical variables with wood properties and functional parameters was proposed and tested. We found significant differences in wood basic density and vulnerability to cavitation between races. The main exogenous variables predicting vulnerability to cavitation were vessel hydraulic diameter and fibre wall fraction. Fibre wall fraction showed a direct impact on wood basic density and the slope of vulnerability curve, and an indirect and negative effect over the pressure imposing 50% of conductivity loss (P50) through them. Hydraulic diameter showed a direct negative effect on P50, but an indirect and positive influence over this variable through wood density on one hand, and through maximum hydraulic conductivity (ks max) and slope on the other. Our results highlight the complexity of the relationship between xylem efficiency and safety in species with solitary vessels such as Eucalyptus spp., with no evident compromise at the intraspecific level.

Adaptations of white spruce to climate: strong intraspecific differences in cold hardiness linked to survival

Understanding local adaptation of tree populations to climate allows the development of assisted migration guidelines as a tool for forest managers to address climate change. Here, we study the relationship among climate, a wide range of physiological traits, and field performance of selected white spruce provenances originating from throughout the species range. Tree height, survival, cold hardiness, hydraulic, and wood anatomical traits were measured in a 32-year-old common garden trial, located in the center of the species range. Provenance performance included all combinations of high versus low survival and growth, with the most prevalent population differentiation for adaptive traits observed in cold hardiness. Cold hardiness showed a strong association with survival and was associated with cold winter temperatures at the site of seed origin. Tree height was mostly explained by the length of the growing season at the origin of the seed source. Although population differentiation was generally weak in wood anatomical and hydraulic traits, within-population variation was substantial in some traits, and a boundary analysis revealed that efficient water transport was associated with vulnerable xylem and low wood density, indicating that an optimal combination of high water transport efficiency and high cavitation resistance is not possible. Our results suggest that assisted migration prescriptions may be advantageous under warming climate, but pronounced trade-offs between survival and cold hardiness require a careful consideration of the distances of these transfers.

Effects of Age and Size on Xylem Phenology in Two Conifers of Northwestern China

The climatic signals that directly affect the trees can be registered by xylem during its growth. If the timings and duration of xylem formation change, xylogenesis can occur under different environmental conditions and subsequently be subject to different climatic signals. An experimental design was applied in the field to disentangle the effects of age and size on xylem phenology, and it challenges the hypothesis that the timings and dynamics of xylem growth are size-dependent. Intra-annual dynamics of xylem formation were monitored weekly during the growing seasons 2013 and 2014 in Chinese pine (Pinus tabulaeformis) and Qilian juniper (Juniperus przewalskii) with different sizes and ages in a semi-arid region of northwestern China. Cell differentiation started 3 weeks earlier in 2013 and terminated 1 week later in 2014 in small-young pines than in big-old pines. However, differences in the timings of growth reactivation disappeared when comparing the junipers with different sizes but similar age. Overall, 77 days were required for xylem differentiation to take place, but timings were shorter for older trees, which also exhibited smaller cell production. Results from this study suggest that tree age does play an important role in timings and duration of growth. The effect of age should also be considered to perform reliable responses of trees to climate.

Vessel diameter is related to amount and spatial arrangement of axial parenchyma in woody angiosperms

Parenchyma represents a critically important living tissue in the sapwood of the secondary xylem of woody angiosperms. Considering various interactions between parenchyma and water transporting vessels, we hypothesize a structure-function relationship between both cell types. Through a generalized additive mixed model approach based on 2,332 woody angiosperm species derived from the literature, we explored the relationship between the proportion and spatial distribution of ray and axial parenchyma and vessel size, while controlling for maximum plant height and a range of climatic factors. When factoring in maximum plant height, we found that with increasing mean annual temperatures, mean vessel diameter showed a positive correlation with axial parenchyma proportion and arrangement, but not for ray parenchyma. Species with a high axial parenchyma tissue fraction tend to have wide vessels, with most of the parenchyma packed around vessels, whereas species with small diameter vessels show a reduced amount of axial parenchyma that is not directly connected to vessels. This finding provides evidence for independent functions of axial parenchyma and ray parenchyma in large vesselled species and further supports a strong role for axial parenchyma in long-distance xylem water transport.

Cambial response of Norway spruce to modified carbon availability by phloem girdling

We tested the hypothesis that increase in carbon (C) availability in Norway spruce saplings (Picea abies (L.) Karst.) intensifies cambial cell division and increases cell lumen diameter (CLD) and cell wall thickness (CWT) when water availability is adequate. To accomplish this, we experimentally subjected 6-year-old P. abies saplings (n = 80 trees) to two levels of soil humidity (watered versus drought conditions) and manipulated tree C status by physically blocking phloem transport at three girdling dates (GDs). Stem girdling occurred in mid-March (day of the year (doy) 77) and in mid-May (GD doy 138), i.e., ~4 weeks before the onset of bud break and during vigorous stem growth, respectively, and in early July (GD doy 190), i.e., 6 and 4 weeks after cessation of radial growth in drought-stressed trees and shoot growth in both soil humidity (SH) treatments, respectively. In response to phloem blockage a striking increase in the number of xylem cells at all GDs and reactivation of cambial activity in drought-stressed trees was detected above the girdling zone, while below girdling xylem formation stopped in both SH-treatments. Although girdling differently affected wood anatomical parameters (CLD, CWT and CLD:CWT ratio) during earlywood and latewood formation, GD had a minor effect on cambial cell division and xylem cell differentiation. Results also revealed that phloem girdling outweighed drought effects imposed on cambial activity. We explain our findings by accumulation of carbohydrates, osmotically active sugars and/or C based signaling compound(s) in response to girdling. Altogether, we conclude that wood formation in P. abies saplings is limited by C availability, which is most likely caused by high C demand belowground especially under drought.

Linking wood traits to vital rates in tropical rainforest trees: Insights from comparing sapling and adult wood

PREMISE OF THE STUDY

Wood density is the top predictor of growth and mortality rates (vital rates) but with modest explanatory power at best. Stronger links to vital rates are expected if wood density is decomposed into its anatomical properties at sapling and adult stages, since saplings and adults differ in wood traits and vital rates. We examined whether anatomical determinants of wood density and strength of the relationship between wood traits and vital rates shift between saplings and adults.

METHODS

Using wood segments from near pith (sapling) and near bark (adult) for 20 tree species (three adults each) from Barro Colorado Island, Panama, we quantified wood traits. Vital rates for saplings and adults were obtained from an earlier study.

KEY RESULTS

Anatomical predictors of wood density were similar for sapling and adult wood, with wood density variation largely explained by fiber lumen area and fiber wall fraction. In sapling wood only, growth rates decreased with fiber wall fraction and increased with fiber lumen area, while mortality rates increased with vessel area but decreased with fiber wall fraction and vessel density.

CONCLUSIONS

Wood traits of sapling trees provide functional insight into the growth-mortality tradeoff. Sapling wood with relatively large fiber lumen area and wide vessels, enabling faster hydraulic transport but less mechanical strength, is associated with fast growth and high mortality. Sapling wood with relatively more fiber wall and many narrow vessels, enabling greater mechanical strength but slower hydraulic transport, is associated with slow growth and low mortality.

Xylem and Leaf Functional Adjustments to Drought in Pinus sylvestris and Quercus pyrenaica at Their Elevational Boundary

Climatic scenarios for the Mediterranean region forecast increasing frequency and intensity of drought events. Consequently, a reduction in Pinus sylvestris L. distribution range is projected within the region, with this species being outcompeted at lower elevations by more drought-tolerant taxa such as Quercus pyrenaica Willd. The functional response of these species to the projected shifts in water availability will partially determine their performance and, thus, their competitive success under these changing climatic conditions. We studied how the cambial and leaf phenology and xylem anatomy of these two species responded to a 3-year rainfall exclusion experiment set at their elevational boundary in Central Spain. Additionally, P. sylvestris leaf gas exchange, water potential and carbon isotope content response to the treatment were measured. Likewise, we assessed inter-annual variability in the studied functional traits under control and rainfall exclusion conditions. Prolonged exposure to drier conditions did not affect the onset of xylogenesis in either of the studied species, whereas xylem formation ceased 1-3 weeks earlier in P. sylvestris. The rainfall exclusion had, however, no effect on leaf phenology on either species, which suggests that cambial phenology is more sensitive to drought than leaf phenology. P. sylvestris formed fewer, but larger tracheids under dry conditions and reduced the proportion of latewood in the tree ring. On the other hand, Q. pyrenaica did not suffer earlywood hydraulic diameter changes under rainfall exclusion, but experienced a cumulative reduction in latewood width, which could ultimately challenge its hydraulic performance. The phenological and anatomical response of the studied species to drought is consistent with a shift in resource allocation under drought stress from xylem to other sinks. Additionally, the tighter stomatal control and higher intrinsic water use efficiency observed in drought-stressed P. sylvestris may eventually limit carbon uptake in this species. Our results suggest that both species are potentially vulnerable to the forecasted increase in drought stress, although P. sylvestris might experience a higher risk of drought-induced decline at its low elevational limit.

The importance of hydraulic architecture to the distribution patterns of trees in a central Amazonian forest

Species distributions and assemblage composition may be the result of trait selection through environmental filters. Here, we ask whether filtering of species at the local scale could be attributed to their hydraulic architectural traits, revealing the basis of hydrological microhabitat partitioning in a Central Amazonian forest. We analyzed the hydraulic characteristics at tissue (anatomical traits, wood specific gravity (WSG)), organ (leaf area, specific leaf area (SLA), leaf area : sapwood area ratio) and whole-plant (height) levels for 28 pairs of congeneric species from 14 genera restricted to either valleys or plateaus of a terra-firme forest in Central Amazonia. On plateaus, species had higher WSG, but lower mean vessel area, mean vessel hydraulic diameter, sapwood area and SLA than in valleys; traits commonly associated with hydraulic safety. Mean vessel hydraulic diameter and mean vessel area increased with height for both habitats, but leaf area and leaf area : sapwood area ratio investments with tree height declined in valley vs plateau species. [Correction added after online publication 29 March 2017: the preceding sentence has been reworded.] Two strategies for either efficiency or safety were detected, based on vessel size or allocation to sapwood. In conclusion, contrasting hydrological conditions act as environmental filters, generating differences in species composition at the local scale. This has important implications for the prediction of species distributions under future climate change scenarios.

Evolution of wood anatomical characters in Nepenthes and close relatives of Caryophyllales

BACKGROUND AND AIMS

Nepenthes attracts wide attention with its spectacularly shaped carnivorous pitchers, cultural value and horticultural curiosity. Despite the plant's iconic fascination, surprisingly little anatomical detail is known about the genus beyond its modified leaf tip traps. Here, the wood anatomical diversity of Nepenthes is explored. This diversity is further assessed with a phylogenetic framework to investigate whether the wood characters within the genus are relevant from an evolutionary or ecological perspective, or rather depend on differences in developmental stages, growth habits, substrates or precipitation.

METHODS

Observations were performed using light microscopy and scanning electron microscopy. Ancestral states of selected wood and pith characters were reconstructed using an existing molecular phylogeny for Nepenthes and a broader Caryophyllales framework. Pairwise comparisons were assessed for possible relationships between wood anatomy and developmental stages, growth habits, substrates and ecology.

KEY RESULTS

Wood anatomy of Nepenthes is diffuse porous, with mainly solitary vessels showing simple, bordered perforation plates and alternate intervessel pits, fibres with distinctly bordered pits (occasionally septate), apotracheal axial parenchyma and co-occurring uni- and multiseriate rays often including silica bodies. Precipitation and growth habit (stem length) are linked with vessel density and multiseriate ray height, while soil type correlates with vessel diameter, vessel element length and maximum ray width. For Caryophyllales as a whole, silica grains, successive cambia and bordered perforation plates are the result of convergent evolution. Peculiar helical sculpturing patterns within various cell types occur uniquely within the insectivorous clade of non-core Caryophyllales.

CONCLUSIONS

The wood anatomical variation in Nepenthes displays variation for some characters dependent on soil type, precipitation and stem length, but is largely conservative. The helical-banded fibre-sclereids that mainly occur idioblastically in pith and cortex are synapomorphic for Nepenthes , while other typical Nepenthes characters evolved convergently in different Caryophyllales lineages.

Acclimation of leaf water status and stem hydraulics to drought and tree neighbourhood: alternative strategies among the saplings of five temperate deciduous tree species

Adjustment in leaf water status parameters and modification in xylem structure and functioning can be important elements of a tree's response to continued water limitation. In a growth trial with saplings of five co-occurring temperate broad-leaved tree species (genera Fraxinus, Acer, Carpinus, Tilia and Fagus) conducted in moist or dry soil, we compared the drought acclimation in several leaf water status and stem hydraulic parameters. Considering the extremes in the species responses, Fraxinus excelsior L. improved its leaf tissue hydration in the dry treatment through osmotic, elastic and apoplastic adjustment while Fagus sylvatica L. solely modified its xylem anatomy, which resulted in increased embolism resistance at the cost of hydraulic efficiency. Our results demonstrate the contrasting response strategies of coexisting tree species and how variable trait plasticity among species can be. The comparison of plants grown either in monoculture or in five-species mixture showed that the neighbouring species diversity can significantly influence a tree's hydraulic architecture and leaf water status regulation. Droughted Carpinus betulus L. (and to a lesser extent, Acer pseudoplatanus L.) plants developed a more efficient stem hydraulic system in heterospecific neighbourhoods, while that of F. sylvatica was generally more efficient in conspecific than heterospecific neighbourhoods. We conclude that co-occurring tree species may develop a high diversity of drought-response strategies, and exploring the full diversity of trait characteristics requires synchronous study of acclimation at the leaf and stem (and possibly also the root) levels, and consideration of physiological as well as morphological and anatomical modifications.

Force of habit: shrubs, trees and contingent evolution of wood anatomical diversity using Croton (Euphorbiaceae) as a model system

Background and Aims

Wood is a major innovation of land plants, and is usually a central component of the body plan for two major plant habits: shrubs and trees. Wood anatomical syndromes vary between shrubs and trees, but no prior work has explicitly evaluated the contingent evolution of wood anatomical diversity in the context of these plant habits.

Methods

Phylogenetic comparative methods were used to test for contingent evolution of habit, habitat and wood anatomy in the mega-diverse genus Croton (Euphorbiaceae), across the largest and most complete molecular phylogeny of the genus to date.

Key Results

Plant habit and habitat are highly correlated, but most wood anatomical features correlate more strongly with habit. The ancestral Croton was reconstructed as a tree, the wood of which is inferred to have absent or indistinct growth rings, confluent-like axial parenchyma, procumbent ray cells and disjunctive ray parenchyma cell walls. The taxa sampled showed multiple independent origins of the shrub habit in Croton , and this habit shift is contingent on several wood anatomical features (e.g. similar vessel-ray pits, thick fibre walls, perforated ray cells). The only wood anatomical trait correlated with habitat and not habit was the presence of helical thickenings in the vessel elements of mesic Croton .

Conclusions

Plant functional traits, individually or in suites, are responses to multiple and often confounding contexts in evolution. By establishing an explicit contingent evolutionary framework, the interplay between habit, habitat and wood anatomical diversity was dissected in the genus Croton . Both habit and habitat influence the evolution of wood anatomical characters, and conversely, the wood anatomy of lineages can affect shifts in plant habit and habitat. This study hypothesizes novel putatively functional trait associations in woody plant structure that could be further tested in a variety of other taxa.