Xylem hydraulic safety and construction costs determine tropical tree growth

Faster growth in tropical trees is usually associated with higher mortality rates, but the mechanisms underlying this relationship are poorly understood. In this study, we investigate how tree growth patterns are linked with environmental conditions and hydraulic traits, by monitoring the cambial growth of 9 tropical cloud forest tree species coupled with numerical simulations using an optimization model. We find that fast‐growing trees have lower xylem safety margins than slow‐growing trees and this pattern is not necessarily linked to differences in stomatal behaviour or environmental conditions when growth occurs. Instead, fast‐growing trees have xylem vessels that are more vulnerable to cavitation and lower density wood. We propose the growth ‐ xylem vulnerability trade‐off represents a wood hydraulic economics spectrum similar to the classic leaf economic spectrum, and show through numerical simulations that this trade‐off can emerge from the coordination between growth rates, wood density, and xylem vulnerability to cavitation. Our results suggest that vulnerability to hydraulic failure might be related with the growth‐mortality trade‐off in tropical trees, determining important life history differences. These findings are important in furthering our understanding of xylem hydraulic functioning and its implications on plant carbon economy.     

Interrelationships between water use and growth traits in biomass-producing willows

Abstract Water use, drought response and growth were examined under controlled conditions in four interbreeding willow species from different geographical origins (two clones of Salix viminalis L., one clone of S. viminalis × S. schwerenii E. Wolf and one clone of S. purpurea L.). The levels of soil water depletion that plants could sustain without wilting varied markedly between the clones. The level of drought resistance expressed this way was positively related to resistance to xylem cavitation, negatively related to the maximum stomatal conductance, and positively related to early stomatal closure. The rate of stomatal closure, however, was negatively related to the resistance to xylem cavitation. Prior to drought, there were no significant differences between leaf-specific hydraulic conductances of the clones when whole plants were considered. However, there were differences if the roots and shoots were considered separately. Drought resistance was negatively related to maximum growth yields. This is probably because resources were diverted away from leaf production to the production of denser wood (wood density was positively related to cavitation resistance), and, for one clone, to the growth of a larger root system. In addition, because the level of drought resistance was negatively related to the maximum stomatal conductance, growth may have been adversely affected as a result of reduced photosynthesis. Given its high water extraction ability, one of the clones started to wilt sooner than expected, although only lateral shoots were affected. This appeared to indicate a strategy of sacrificing expendable shoots.     

Trends in wood density and structure are linked to prevention of xylem implosion by negative pressure

Wood density (D_t), an excellent predictor of mechanical properties, is typically viewed in relation to support against gravity, wind, snow, and other environmental forces. In contrast, we show the surprising extent to which variation in D_t and wood structure is linked to support against implosion by negative pressure in the xylem pipeline. The more drought-tolerant the plant, the more negative the xylem pressure can become without cavitation, and the greater the internal load on the xylem conduit walls. Accordingly, D_t was correlated with cavitation resistance. This trend was consistent with the maintenance of a safety factor from implosion by negative pressure: conduit wall span (b) and thickness (t) scaled so that (t/b)² was proportional to cavitation resistance as required to avoid wall collapse. Unexpectedly, trends in D_t may be as much or more related to support of the xylem pipeline as to support of the plant.     

Impact of extreme drought on tree-ring width and vessel anatomical features of Chukrasia tabularis

Multiple sources of evidence suggest an increasing frequency of extreme climatic events during the past century. In Bangladesh, a country strongly influenced by the South Asian monsoon climate, the years 1999 and 2006 were the most severe droughts among the ten drought events identified over the last four decades. We investigated the impact of these two drought events on radial growth and xylem anatomical features of the brevi-deciduous tree species Chukrasia tabularis in a moist tropical forest in Bangladesh. Tree radial growth declined by 54% during the 1999 and 48.7% during the 2006 droughts, respectively. Among the wood anatomical features, the number of vessels (NV) showed the highest sensitivity to drought, with a 45% decrease in the 1999 drought year, followed by total vessel area (TVA) and mean vessel area (MVA). On the other hand, Vessel density (VD) increased by 13% during the 1999 drought but the increase in VD was very low in the drought year 2006. The decreasing vessel area and increasing vessel density indicate xylem hydraulic adaptation of C. tabularis to minimize drought induced cavitation risk and to avoid hydraulic failure. The significant correlations between the Standardized Precipitation Evapotranspiration Index (SPEI) and time series of tree-ring width and vessel variables imply that decline in radial growth and changes in vessel features in C. tabularis are likely to be caused by drought induced water stress. Our analyses suggest that radial growth and wood anatomical features of C. tabularis are highly sensitive to extreme drought events in South Asian moist tropical forests and can be used to reconstruct past droughts and to model tree response to drought stress under future climate conditions.     

An experimentally controlled extreme drought in a Norway spruce forest reveals fast hydraulic response and subsequent recovery of growth rates

Key message

An experimental drought treatment, exacerbated by a natural drought event, compromised growth in Norway spruce, but more cavitation-resistant xylem was produced and no long-term growth reductions were observed.

Abstract

An experimental drought treatment in a mature Norway spruce forest that coincided with a rare drought event in southern Sweden in 1992, allowed us to study how such forests may respond to similar extreme events in the future. Immediately after the onset of the drought treatment, height and diameter growth decreased compared to control treatments. New xylem cells had smaller lumen und thicker walls, resulting in a more safety-orientated water transport system. The maximum growth and hydraulic system response of the 1990–1996 drought treatment coincided with the 1992 summer drought event. After the drought treatment ended, all measured traits recovered to control and irrigation treatment values after 3 years. While height and diameter growth recovered with delay, wood structure and hydraulic properties showed fast recovery. We conclude that a highly plastic response of the hydraulic system indicates a notable degree of resilience to droughts that are expected to become more common under climate change. Our results do not imply, however, that survival and productivity of Norway spruce plantations would not be compromised under drier conditions in the future, and they apply to site conditions equivalent to the studied system.     

Genotypic variation in wood properties and growth traits of triploid hybrid clones of Populus tomentosa at three clonal trials

To better understand the genetic control of growth traits (tree height, dbh, and stem volume) and wood properties (basic wood density and fiber length) in triploid hybrid clones of Populus tomentosa, genetic relationships among selected wood properties with growth traits were examined in 5-year-old clonal field trials located in Yanzhou, Gaotang, and Xiangfen, northern China. In total, 180 trees from 10 clones were sampled from the three sites. The site had a moderate effect on basic wood density (BWD), stem wood dry weight (DWT), and tree growth and had a highly significant effect on fiber length (FL) (P?<?0.001). Clonal effects were also significant (P?<?0.05) for all studied traits (except for diameter at breast height (DBH) and stem volume (SV)). Clone × site interaction was significant for all the studied traits except for FL. The estimated repeatability of clonal means for FL (0.91) was higher than for BWD (0.71), DWT (0.62), tree height (0.62), DBH (0.61), and SV (0.55). Intersite genetic correlation estimates indicated that wood properties were more stable than growth traits. Phenotypic correlation estimates between SV and BWD ranged from ?0.29 to ?0.10.     

Enhanced growth and ethylene increases spiral grain formation in Picea abies and Abies balsamea trees

Spiral grain angle in Norway spruce (Picea abies) trees and balsam fir (Abies balsamea) seedlings was investigated in relation to growth rate, endogenous and applied ethylene. Trees from stands of Norway spruce, which were irrigated and fertilised in order to enhance growth, and trees having different growth rates in non-treated stands were studied. Stem growth rate at the stand level (m³ ha^-1 year^-1) was measured annually, or by means of microscopy on stem sections as the number and size of tracheids produced. Enhanced growth increased ethylene evolution and maintained a high level of left-handed spiral grain angle in comparison to slower-growing trees. An increased number of earlywood tracheids in fast growing trees was correlated to a more left-handed spiral grain angle. Ethrel, applied to stems of balsam fir seedlings, increased the internal ethylene levels in parallel with increased left-handed spiral grain angle. The results indicate that ethylene regulates the extent of spiral grain angle.     

Xylem cavitation caused by drought and freezing stress in four co-occurring Juniperus species

Previous studies indicate that conifers are vulnerable to cavitation induced by drought but in many cases, not by freezing. Rarely have vulnerability to drought and freezing stress been studied together, even though both influence plant physiology and the abundance and distribution of plants in many regions of the world. We studied vulnerability to drought- and freezing-induced cavitation, along with wood density, conduit reinforcement, tracheid diameter, and hydraulic conductivity, in four Juniperus species that typically occupy different habitats, but uniquely co-occur at the same site in Arizona, AZ. We combined drought with a freeze-thaw cycle to create freezing-induced vulnerability curves. All four species demonstrated greater vulnerability to drought + freezing- than to drought-induced cavitation alone ( P  < 0.0001). Mean tracheid diameter was correlated with vulnerability to drought + freezing-induced cavitation (r = 0.512, P  = 0.01). The vulnerability to cavitation of each species followed expected rankings based on relative moisture within each species' natural distribution. Species with naturally drier distributions showed greater resistance to both drought- and drought + freezing-induced cavitation. Even conifer species with relatively small tracheid diameters can experience xylem embolism after a single freeze-thaw cycle when under drought stress.     

树木高生长限制的几个假说

树木生长到一定年龄后高生长停滞,对这一现象的解释存在很多争议.成熟假说认为树木顶端分生组织分裂活性下降导致树木高生长减慢.营养限制假说认为土壤中营养元素(特别是氮素)在植物活体或枯落物中积累使土壤中可利用的养分含量降低,细根生物量增加和叶片光合能力下降导致了地上部分生长的减缓.呼吸假说认为边材呼吸消耗随个体发育的增加使投入到高生长的碳减少.水力限制假说认为水分运输阻力随高度增加而增加导致了叶片总光合碳同化下降,分配到高生长的生物量减少.树木发展假说认为植物用多种调节机制克服随个体发育增加的水力阻力,包括叶片结构和生理特征的变化,叶/边材面积比降低,边材渗透性和树干储水能力的增加等.水力限制假说得到了较多的关注,对不同高度树木的叶比导率、光合特征和树干生长量等测定结果支持这一假说.但对这一假说也存在很多的争议,主要表现在:水力阻力是否确实随高度的增加而增加,水力阻力的分布,补偿机制的作用和生物量分配转变等.本文综述了树木高生长限制的4个假说以及争论的焦点,并总结了目前研究的热点问题和今后的研究方向.     

树木高生长限制的几个假说

树木生长到一定年龄后高生长停滞, 对这一现象的解释存在很多争议。成熟假说认为树木顶端分生组织分裂活性下降导致树木高生长减慢。营养限制假说认为土壤中营养元素(特别是氮素)在植物活体或枯落物中积累使土壤中可利用的养分含量降低, 细根生物量增加和叶片光合能力下降导致了地上部分生长的减缓。呼吸假说认为边材呼吸消耗随个体发育的增加使投入到高生长的碳减少。水力限制假说认为水分运输阻力随高度增加而增加导致了叶片总光合碳同化下降, 分配到高生长的生物量减少。树木发展假说认为植物用多种调节机制克服随个体发育增加的水力阻力, 包括叶片结构和生理特征的变化, 叶/边材面积比降低, 边材渗透性和树干储水能力的增加等。水力限制假说得到了较多的关注, 对不同高度树木的叶比导率、光合特征和树干生长量等测定结果支持这一假说。但对这一假说 也存在很多的争议, 主要表现在: 水力阻力是否确实随高度的增加而增加, 水力阻力的分布, 补偿机制的作用和生物量分配转变等。本文综述了树木高生长限制的4个假说以及争论的焦点, 并总结了目前研究的热点问题和今后的研究方向。     

Drought tolerance in faster- and slower-growing black spruce (Picea mariana) progenies: I. Stomatal and gas exchange responses to osmotic stress

Sixteen years growth in height and basal stem diameter of full-sib black spruce [ Picea mariana (Mill.) B. S. P.] progenies varied with soil moisture availability. The responses to water stress of two faster-growing progenies under drought were compared with two slower-growing progenies to determine the physiological basis of drought tolerance. Six-month-old seedlings were stressed using an osmoticum, polyethylene glycol-3350 (PEG). Seedlings were passed through a series of increasing concentration: 10, 18 and 25% PEG (w/v) each for 3 days to provide solution water potentials of -0.4, -1.0, and -2.0 MPa, respectively. The stress was then relieved by returning the seedlings to nutrient solution without PEG for 24 hours. Gas exchange and water relation parameters were similar in the 4 progenies prior to the imposition of the stress but varied during the stress and after stress relief. The two progenies which grew more vigorously on the driest site maintained significantly higher stomatal conductance, leaf transpiration rate, and net photosynthesis rate during mild (10% PEG) and moderate (18% PEG), but not severe (25% PEG) osmotic stress, and also recovered faster after release of the stress than the other two slower-growing progenies. Black spruce progenies did not differ in xylem water potential or water use efficiency. Progenies capable of faster growth under drought stress were thus characterized by a greater dehydration tolerance, rather than postponement, compared with slower-growing progenies.     

Leaf phenology is associated with soil water availability and xylem traits in a tropical dry forest

In tropical dry forests, spatial heterogeneity in soil water availability is thought to determine interspecific differences in key components of resource use strategies, such as leaf phenology and xylem function. To understand the environmental drivers of variation in leaf phenology and xylem function, we explored the relation of soil water potential to topographic metrics derived from a digital elevation model. Subsequently, we compared nine xylem hydraulic, mechanical and storage traits in 18 species in three phenological classes (readily deciduous, tardily deciduous, and evergreen) in the dry tropical forest of Chamela, Mexico. Soil water potential was negatively correlated with elevation, insolation and water flow accumulation. Evergreen species characterized low-elevation moist sites, whereas deciduous species dominated hills and dry sites. Overall, evergreen species had lower xylem specific conductivity than deciduous species, and tardily deciduous species were different from readily deciduous and evergreen species in five of eight xylem traits. In dry tropical forests, water availability promotes divergence in leaf phenology and xylem traits, ranging from low wood density, evergreen species in moist sites to a combination of low wood density, readily deciduous species plus high wood density, tardily deciduous species in dry sites.     

Stomatal factors and vulnerability of stem xylem to cavitation in poplars

The relationships between the vulnerability of stem xylem to cavitation, stomatal conductance, stomatal density, and leaf and stem water potential were examined in six hybrid poplar (P38P38, Walker, Okanese, Northwest, Assiniboine and Berlin) and balsam poplar (Populus balsamifera) clones. Stem xylem cavitation resistance was examined with the Cavitron technique in well-watered plants grown in the greenhouse. To investigate stomatal responses to drought, plants were subjected to drought stress by withholding watering for 5 (mild drought) and 7 (severe drought) days and to stress recovery by rewatering severely stressed plants for 30 min and 2 days. The clones varied in stomatal sensitivity to drought and vulnerability to stem xylem cavitation. P38P38 reduced stomatal conductance in response to mild stress while the balsam poplar clone maintained high leaf stomatal conductance under more severe drought stress conditions. Differences between the severely stressed clones were also observed in leaf water potentials with no or relatively small decreases in Assiniboine, P38P38, Okanese and Walker. Vulnerability to drought-induced stem xylem embolism revealed that balsam poplar and Northwest clones reached loss of conductivity at lower stem water potentials compared with the remaining clones. There was a strong link between stem xylem resistance to cavitation and stomatal responsiveness to drought stress in balsam poplar and P38P38. However, the differences in stomatal responsiveness to mild drought suggest that other drought-resistant strategies may also play a key role in some clones of poplars exposed to drought stress.     

Limits to water transport in Juniperus osteosperma and Pinus edulis: implications for drought tolerance and regulation of transpiration

1. An air-injection method was used to study loss of water transport capacity caused by xylem cavitation in roots and branches of Pinus edulis (Colorado Pinyon) and Juniperus osteosperma (Utah Juniper). These two species characterize the Pinyon–Juniper communities of the high deserts of the western United States. Juniperus osteosperma can grow in drier sites than P. edulis and is considered the more drought tolerant.
2. Juniperus osteosperma was more resistant to xylem cavitation than P. edulis in both branches and roots. Within a species, branches were more resistant to cavitation than roots for P. edulis but no difference was seen between the two organs for J. osteosperma . There was also no difference between juveniles and adults in J. osteosperma ; this comparison was not made for P. edulis .
3. Tracheid diameter was positively correlated with xylem cavitation pressure across roots and stems of both species. This relation suggests a trade-off between xylem conductance and resistance to xylem cavitation in these species.
4. During summer drought, P. edulis maintained higher predawn xylem pressures and showed much greater stomatal restriction of transpiration, consistent with its greater vulnerability to cavitation, than J. osteosperma .
5. These results suggest that the relative drought tolerance of P. edulis and J. osteosperma results in part from difference in their vulnerability to xylem cavitation.     

Is hemiepiphytism an adaptation to high irradiance? Testing seedling responses to light levels and drought in hemiepiphytic and non‐hemiepiphytic Ficus

The epiphytic growth habit in many Ficus species during their juvenile stages has commonly been hypothesized to be an adaptation for avoiding deep shade in the forest understory, but this has never been tested experimentally. We examined growth and ecophysiology in seedlings of three hemiepiphytic (Hs) and three non‐hemiepiphytic (NHs) Ficus species grown under different irradiance levels. Both Hs and NHs exhibited characteristics of high light requiring species, such as high plasticity to growth irradiance and relatively high maximum photosynthetic assimilation rates. Diurnal measurements of leaf gas exchange showed that Hs have much shorter active photosynthetic periods than NHs; moreover, leaves of Hs have lower xylem hydraulic conductivity but stronger drought tolerance as indicated by much lower rates of leaf diebacks during the drought treatment. Seedlings of NHs had 3.3‐ and 13.3‐fold greater height and biomass than those of Hs species after growing in the nursery for 5 months, indicating a trade‐off between growth and drought tolerance due to the conflicting requirements for xylem conductivity and cavitation resistance. This study does not support the shade‐avoidance hypothesis; rather, it suggests that the canopy regeneration in Hs is an adaptation to avoid alternative terrestrial growth‐related risks imposed to tiny Ficus seedlings. The NHs with terrestrial regeneration reduce these risks by having an initial burst of growth to rapidly gain relatively large seedling sizes, while in Hs seedlings more conservative water use and greater drought tolerance for surviving the canopy environment are intrinsically associated with slow growth.     

Hydraulic failure and tree dieback are associated with high wood density in a temperate forest under extreme drought

Catastrophic hydraulic failure will likely be an important mechanism contributing to large‐scale tree dieback caused by increased frequency and intensity of droughts under global climate change. To compare the susceptibility of 22 temperate deciduous tree and shrub species to hydraulic failure during a record drought in the southeastern USA, we quantified leaf desiccation, native embolism, wood density, stomatal conductance and predawn and midday leaf water potential at four sites with varying drought intensities. At the two driest sites, there was widespread leaf wilting and desiccation, and most species exhibited predawn leaf water potentials of ≤3 MPa and >60% loss of xylem conductivity in branches. Although species with high wood density were more resistant to cavitation, they had higher levels of native embolism and greater canopy dieback than species with low wood density. This unexpected result can be explained by the failure of species with dense wood to avert a decline in water potential to dangerous levels during the drought. Leaf water potential was negatively correlated with wood density, and the relationship was strongest under conditions of severe water deficit. Species with low wood density avoided catastrophic embolism by relying on an avoidance strategy that involves partial drought deciduousness, higher sensitivity of stomata to leaf water potential and perhaps greater rooting depth. These species therefore maintained water potential at levels that ensured a greater margin of safety against embolism. These differences among species may mediate rapid shifts in species composition of temperate forests if droughts intensify due to climate change.     

Morphology, wood structure and cell wall composition of rolC transgenic and non-transformed aspen trees

Morphology, wood structure and cell wall composition of 35S-rolC transgenic hybrid aspen (P. tremula2tremuloides) were compared with non-transformed control trees. The transgenics are characterised by stunted growth, altered physiological parameters and light green leaves of reduced size. Histometric measurements revealed thinner fibre walls as compared to the controls. UV microspectrophotometry of individual wall layers did not reveal distinctive differences in the lignification of xylem cells, but in the extremely thin-walled fibres of the transgenics the secondary walls were less lignified as revealed by KMnO₄ staining in transmission electron microscopy. In the transgenics the formation of xylem cells was delayed and the differentiation zone reduced to only a few rows. Immunocytochemical analyses revealed the deposition of lignins in less differentiated xylem cells as compared to the controls. The first labelling of condensed lignin appeared in cell corners and of non-condensed lignin in secondary walls near cell corners during the deposition of S1 polysaccharides. Because of alterations in the formation and differentiation of xylem cells, 35S-rolC transgenic aspen may be useful for studies on molecular factors controlling the differentiation continuum.     

The physiological implications of primary xylem organization in two ferns

Xylem structure and function are well described in woody plants, but the implications of xylem organization in less‐derived plants such as ferns are poorly understood. Here, two ferns with contrasting phenology and xylem organization were selected to investigate how xylem dysfunction affects hydraulic conductivity and stomatal conductance (g_s). The drought‐deciduous pioneer species, Pteridium aquilinum, exhibits fronds composed of 25 to 37 highly integrated vascular bundles with many connections, high g_s and moderate cavitation resistance (P₅₀ = ?2.23 MPa). By contrast, the evergreen Woodwardia fimbriata exhibits sectored fronds with 3 to 5 vascular bundles and infrequent connections, low g_s and high resistance to cavitation (P₅₀ = ?5.21 MPa). Xylem‐specific conductivity was significantly higher in P. aqulinium in part due to its wide, efficient conduits that supply its rapidly transpiring pinnae. These trade‐offs imply that the contrasting xylem organization of these ferns mirrors their divergent life history strategies. Greater hydraulic connectivity and g_s promote rapid seasonal growth, but come with the risk of increased vulnerability to cavitation in P. aquilinum, while the conservative xylem organization of W. fimbriata leads to slower growth but greater drought tolerance and frond longevity.     

Differential impacts of long-term (CO<Subscript>2</Subscript>) and O<Subscript>3</Subscript> exposure on growth of northern conifer and deciduous tree species

The long-term effects of elevated CO₂ and CO₂+O₃ concentrations on the growth allocation in northern provenances of Norway spruce [Picea abies (L.) Karst.], Scots pine [Pinus sylvestris (L.)] and pubescent birch clones (Betula pubescens Ehrh.) were examined in open-top chambers after a 4-year-long experiment. The total biomass responses of the tree seedlings to increased CO₂ and CO₂+O₃ concentrations were not statistically significant and varied between the provenances and species. The seedlings of northern origin were the least sensitive in their response to treatments. The total biomass of the Norway spruce seedlings slightly decreased in response to CO₂ in three provenances. Scots pine from the local provenance had a slight biomass increase after elevated CO₂+O₃ treatment. The slower-growing birch clone seemed to benefit from elevated CO₂, whereas in the faster-growing clone, reductions in biomass accumulation were seen. The combined CO₂+O₃ treatment reduced the positive effects of elevated CO₂, especially in the slower-growing birches. Observations of significant effects were limited to a few parameters. Carbon dioxide treatment decreased needle dry weight of Norway spruce in one northern provenance. The needle and wood dry weight increased (CO₂ + O₃) in local Scots pine. Significant birch response was limited to increased fine root density (O₃ + CO₂) in the inland clone. The diverse effects of elevated CO₂ and CO₂ +O₃ on seedling growth and biomass provide evidence that exposure of northern trees to the enhanced variable CO₂ and O₃ concentrations of the future will have varied effects on the growth of these species. The direction and magnitude of those effects will differ depending on species and origins.     

Functional and ecological xylem anatomy

Cohesion-tension transport of water is an energetically efficient way to carry large amounts of water from the roots up to the leaves. However, the cohesion-tension mechanism places the xylem water under negative hydrostatic pressure (P_x), rendering it susceptible to cavitation. There are conflicts among the structural requirements for minimizing cavitation on the one hand vs maximizing efficiency of transport and construction on the other. Cavitation by freeze-thaw events is triggered by in situ air bubble formation and is much more likely to occur as conduit diameter increases, creating a direct conflict between conducting efficiency and sensitivity to freezing induced xylem failure. Temperate ring-porous trees and vines with wide diameter conduits tend to have a shorter growing season than conifers and diffuse-porous trees with narrow conduits. Cavitation by water stress occurs by air seeding at interconduit pit membranes. Pit membrane structure is at least partially uncoupled from conduit size, leading to a much less pronounced trade-off between conducting efficiency and cavitation by drought than by freezing. Although wider conduits are generally more susceptible to drought-induced cavitation within an organ, across organs or species this trend is very weak. Different trade-offs become apparent at the level of the pit membranes that interconnect neighbouring conduits. Increasing porosity of pit membranes should enhance conductance but also make conduits more susceptible to air seeding. Increasing the size or number of pit membranes would also enhance conductance, but may weaken the strength of the conduit wall against implosion. The need to avoid conduit collapse under negative pressure creates a significant trade-off between cavitation resistance and xylem construction cost, as revealed by relationships between conduit wall strength, wood density and cavitation pressure. Trade-offs involving cavitation resistance may explain the correlations between wood anatomy, cavitation resistance, and the physiological range of negative pressure experienced by species in their native habitats.