Bundle sheath extensions affect leaf structural and physiological plasticity in response to irradiance

Coordination between structural and physiological traits is key to plants' responses to environmental fluctuations. In heterobaric leaves, bundle sheath extensions (BSEs) increase photosynthetic performance (light‐saturated rates of photosynthesis, A_max) and water transport capacity (leaf hydraulic conductance, K_leaf). However, it is not clear how BSEs affect these and other leaf developmental and physiological parameters in response to environmental conditions. The obscuravenosa (obv) mutation, found in many commercial tomato varieties, leads to absence of BSEs. We examined structural and physiological traits of tomato heterobaric and homobaric (obv) near‐isogenic lines grown at two different irradiance levels. K_leaf, minor vein density, and stomatal pore area index decreased with shading in heterobaric but not in homobaric leaves, which show similarly lower values in both conditions. Homobaric plants, on the other hand, showed increased A_max, leaf intercellular air spaces, and mesophyll surface area exposed to intercellular airspace (S_mes) in comparison with heterobaric plants when both were grown in the shade. BSEs further affected carbon isotope discrimination, a proxy for long‐term water‐use efficiency. BSEs confer plasticity in traits related to leaf structure and function in response to irradiance levels and might act as a hub integrating leaf structure, photosynthetic function, and water supply and demand.     

Differential leaf expansion can enable hydraulic acclimation to sun and shade

Although leaf size is one of the most responsive plant traits to environmental change, the functional benefits of large versus small leaves remain unclear. We hypothesized that modification of leaf size within species resulting from differences in irradiance can allow leaves to acclimate to different photosynthetic or evaporative conditions while maintaining an efficient balance between hydraulic supply (vein density) and evaporative demand. To test this, we compared the function and anatomy of leaf hydraulic systems in the leaves of a woody angiosperm (Toona ciliata M. Roem.) grown under high and low irradiance in controlled conditions. Our results confirm that in this species, differential leaf expansion regulates the density of veins and stomata such that leaf hydraulic conductance and stomatal conductance remain proportional. A broader sample of field-grown tree species suggested that differences in leaf venation and stomatal traits induced by sun and shade were not regulated by leaf size in all cases. Our results, however, suggest that leaf size plasticity can provide an efficient way for plants to acclimate hydraulic and stomatal conductances to the contrasting evaporative conditions of sun and shade.     

Hydraulic analysis of water flow through leaves of sugar maple and red oak

Leaves constitute a substantial fraction of the total resistance to water flow through plants. A key question is how hydraulic resistance within the leaf is distributed among petiole, major veins, minor veins, and the pathways downstream of the veins. We partitioned the leaf hydraulic resistance (R(leaf)) for sugar maple (Acer saccharum) and red oak (Quercus rubra) by measuring the resistance to water flow through leaves before and after cutting specific vein orders. Simulations using an electronic circuit analog with resistors arranged in a hierarchical reticulate network justified the partitioning of total R(leaf) into component additive resistances. On average 64% and 74% of the R(leaf) was situated within the leaf xylem for sugar maple and red oak, respectively. Substantial resistance-32% and 49%- was in the minor venation, 18% and 21% in the major venation, and 14% and 4% in the petiole. The large number of parallel paths (i.e. a large transfer surface) for water leaving the minor veins through the bundle sheath and out of the leaf resulted in the pathways outside the venation comprising only 36% and 26% of R(leaf). Changing leaf temperature during measurement of R(leaf) for intact leaves resulted in a temperature response beyond that expected from changes in viscosity. The extra response was not found for leaves with veins cut, indicating that water crosses cell membranes after it leaves the xylem. The large proportion of resistance in the venation can explain why stomata respond to leaf xylem damage and cavitation. The hydraulic importance of the leaf vein system suggests that the diversity of vein system architectures observed in angiosperms may reflect variation in whole-leaf hydraulic capacity.     

Hydraulic conductance of Acacia phyllodes (foliage) is driven by primary nerve (vein) conductance and density

We determined effects of venation traits on hydraulic conductance of phyllodes (foliage), using an array of Acacia s.str. species with diverse phyllode morphologies as the source of variation. Measurements were made on phyllodes from 44 species, grown in common gardens but originating from different positions along a precipitation gradient. K(phyllode) varied 18-fold and was positively correlated with primary nerve hydraulic conductance, and with primary nerve (vein) density but not with minor nerve density, in contrast with previous studies of true leaves in other dicotyledons. Phyllodes with higher primary nerve density also had greater mass per area (PMA) and larger bundle sheath extensions (BSEs) from their minor nerves. We suggest that higher primary nerve conductivity and density may decrease the distance travelled in the high-resistance extra-xylem pathways of the phyllode. Further, larger BSEs may increase the area available for dispersion of water from the xylem to the extra-xylem tissue. High PMA phyllodes were more common in acacias from areas receiving lower annual precipitation. Maximizing efficient water movement through phyllodes may be more important where rainfall is meagre and infrequent, explaining relationships between nerve patterns and the climates of origin in Australian phyllodinous Acacia.     

Isotopes reveal contrasting water use strategies among coexisting plant species in a Mediterranean ecosystem

Variation in the stable carbon and oxygen isotope composition (δ(13) C, Δ(18) O) of co-occurring plant species may reflect the functional diversity of water use strategies present in natural plant communities. We investigated the patterns of water use among 10 coexisting plant species representing diverse taxonomic groups and life forms in semiarid southeast Spain by measuring their leaf δ(13) C and Δ(18) O, the oxygen isotope ratio of stem water and leaf gas exchange rates. Across species, Δ(18) O was tightly negatively correlated with stomatal conductance (g(s) ), whereas δ(13) C was positively correlated with intrinsic water use efficiency (WUE(i) ). Broad interspecific variation in Δ(18) O, δ(13) C and WUE(i) was largely determined by differences in g(s) , as indicated by a strong positive correlation between leaf δ(13) C and Δ(18) O across species The 10 co-occurring species segregated along a continuous ecophysiological gradient defined by their leaf δ(13) C and Δ(18) O, thus revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies ranging from 'profligate/opportunistic' (high g(s) , low WUE(i) ) to 'conservative' (low g(s) , high WUE(i) ). Coexisting species maintained their relative isotopic rankings in 2?yr with contrasting rainfall, suggesting the existence of species-specific 'isotopic niches' that reflect ecophysiological niche segregation in dryland plant communities.     

Variation in and adaptive plasticity of flower size and drought-coping traits

Although phenotypic plasticity of morphological and physiological traits in response to drought could be adaptive, there have been relatively few tests of plasticity variation or of adaptive plasticity in drought-coping traits across populations with different moisture availabilities. We measured floral size, vegetative size, and physiological traits in four field populations of Leptosiphon androsaceus (Polemoniaceae) that were distributed across a rainfall gradient in California, USA. Measurements were made over 5 years that varied in precipitation. We also conducted a growth chamber experiment in which half-sibs from three populations were divided equally among a well-watered and a drought treatment. We tested for selection on traits in each of the watering treatments, and evaluated whether traits exhibited plasticity. In the field, plant traits exhibited substantial variation across populations and years. Flower size, leaf size, and water-use efficiency (WUE) were generally higher for populations that received greater average rainfall. However, in dry years, we observed a decrease in flower and leaf size, but an increase in WUE across the populations. In the growth chamber experiment, leaf and physiological traits exhibited plasticity, with smaller leaves and higher WUE found in the drought, as compared to the well-watered treatment. Only specific leaf area exhibited differentiation in plasticity among populations. Although there was no observed plasticity in floral size, selection favored smaller flowers in the drought treatment and larger flowers in the well-watered treatment. Our results suggest that moisture availability has led to trait variation in L. androsaceus via a combination of selection and phenotypic plasticity.     

δ(13) C of leaf-respired CO(2) reflects intrinsic water-use efficiency in barley

Leaf intrinsic water-use efficiency (WUE), the ratio of photosynthetic rate to stomatal conductance (A/g(s) ), is a key plant trait linking terrestrial carbon and water cycles. A rapid, integrative proxy for A/g(s) is of benefit to crop breeding programmes aiming to improve WUE, but also for ecologists interested in plant carbon-water balance in natural systems. We hypothesize that the carbon isotope composition of leaf-respired CO(2) (δ(13) C(Rl) ), two hours after leaves are transferred to the dark, records photosynthetic carbon isotope discrimination and so provides a proxy for A/g(s) . To test this hypothesis, δ(13) C(Rl) was measured in four barley cultivars grown in the field at two levels of water availability and compared to leaf-level gas exchange (the ratio of leaf intercellular to ambient CO(2) partial pressure, C(i) /C(a) , and A/g(s) ). Leaf-respired CO(2) was more (13) C-depleted in plants grown at higher water availability, varied between days as environmental conditions changed, and was significantly different between cultivars. A strong relationship between δ(13) C(Rl) and δ(13) C of sucrose was observed. δ(13) C(Rl) was converted into apparent photosynthetic discrimination (Δ(13) C(Rl) ) revealing strong relationships between Δ(13) C(Rl) and C(i) /C(a) and A/g(s) during the vegetative stage of growth. We therefore conclude that δ(13) C(Rl) may provide a rapid, integrative proxy for A/g(s) in barley.     

Shifts in bryophyte carbon isotope ratio across an elevation?��?soil age matrix on Mauna Loa, Hawaii: do bryophytes behave like vascular plants?

The carbon isotope ratio (δ(13)C) of vascular plant leaf tissue is determined by isotope discrimination, primarily mediated by stomatal and mesophyll diffusion resistances and by photosynthetic rate. These effects lead to predictable trends in leaf δ(13)C across natural gradients of elevation, irradiance and nutrient supply. Less is known about shifts in δ(13)C for bryophytes at landscape scale, as bryophytes lack stomata in the dominant gametophyte phase, and thus lack active control over CO(2) diffusion. Twelve bryophyte species were sampled across a matrix of elevation and soil ages on Mauna Loa, Hawaii Island. We tested hypotheses based on previous findings for vascular plants, which tend to have less negative δ(13)C at higher elevations or irradiances, and for leaves with higher leaf mass per area (LMA). Across the matrix, bryophytes spanned the range of δ(13)C values typical of C(3) vascular plants. Bryophytes were remarkably similar to vascular plants in exhibiting less negative δ(13)C with increasing elevation, and with lower overstory cover; additionally δ(13)C was related to bryophyte canopy projected mass per area, a trait analogous to LMA in vascular plants, also correlated negatively with overstory cover. The similarity of responses of δ(13)C in bryophytes and vascular plants to environmental factors, despite differing morphologies and diffusion pathways, points to a strong direct role of photosynthetic rate in determining δ(13)C variation at the landscape scale.     

Evolution and Function of Leaf Venation Architecture: A Review

The leaves of extant terrestrial plants show highly diverseand elaborate patterns of leaf venation. One fundamental featureof many leaf venation patterns, especially in the case of angiospermleaves, is the presence of anastomoses. Anastomosing veins distinguisha network topologically from a simple dendritic (tree-like)pattern which represents the primitive venation architecture.The high degree of interspecific variation of entire venationpatterns as well as phenotypic plasticity of some venation properties,such as venation density, indicate the high selective pressureacting on this branching system. Few investigations deal withfunctional properties of the leaf venation system. The interrelationshipsbetween topological or geometric properties of the various leafvenation patterns and functional aspects are far from beingwell understood. In this review we summarize current knowledgeof interrelationships between the form and function of leafvenation and the evolution of leaf venation patterns. Sincethe functional aspects of architectural features of differentleaf venation patterns are considered, the review also refersto the topic of individual and intraspecific variation. Onebasic function of leaf venation is represented by its contributionto the mechanical behaviour of a leaf. Venation geometry anddensity influences mechanical stability and may affect, forexample, susceptibility to herbivory. Transport of water andcarbohydrates is the other basic function of this system andthe transport properties are also influenced by the venationarchitecture. These various functional aspects can be interpretedin an ecophysiological context. Copyright 2001 Annals of BotanyCompany Review, leaves, leaf venation, evolution, network, transport, flow, mechanical stabilization     

Carbon isotope compositions (δ13C) of leaf,wood and holocellulose differ among genotypes of poplar and between previous land uses in a short‐rotation biomass plantation

The efficiency of water use to produce biomass is a key trait in designing sustainable bioenergy‐devoted systems. We characterized variations in the carbon isotope composition (δ¹³C) of leaves, current year wood and holocellulose (as proxies for water use efficiency, WUE) among six poplar genotypes in a short‐rotation plantation. Values of δ¹³C_wood and δ¹³C_{holocellulose} were tightly and positively correlated, but the offset varied significantly among genotypes (0.79–1.01‰). Leaf phenology was strongly correlated with δ¹³C, and genotypes with a longer growing season showed a higher WUE. In contrast, traits related to growth and carbon uptake were poorly linked to δ¹³C. Trees growing on former pasture with higher N‐availability displayed higher δ¹³C as compared with trees growing on former cropland. The positive relationships between δ¹³C_leaf and leaf N suggested that spatial variations in WUE over the plantation were mainly driven by an N‐related effect on photosynthetic capacities. The very coherent genotype ranking obtained with δ¹³C in the different tree compartments has some practical outreach. Because WUE remains largely uncoupled from growth in poplar plantations, there is potential to identify genotypes with satisfactory growth and higher WUE.     

欧洲3种常见乔木幼苗在两种光环境下叶片的气体交换、叶绿素含量和氮素含量

在室内测定了分别栽培于全光照和20％光照条件下的垂枝桦Betulapendula,欧洲水青冈Fagussylvatica和欧洲白栎Quercusrobur幼苗叶片的光合作用－光响应曲线、叶片气孔导度、胞间二氧化碳浓度、水分利用效率,叶绿素含量和氮素含量,并分析叶片叶绿素含量和净光合速率的回归关系．20％光照条件引起净光合速率的光饱和点下降,叶片气孔导度和水分利用效率以及单位叶面积叶绿素含量降低,叶片的光合物质积累减少,但氮素含量上升．回归分析结果表明,叶片叶绿素含量与净光合速率成正相关．3种植物的幼苗对荫蔽条件有一定的适应性,其中B．pendula和Q．robur的耐荫能力比Fsylvatica强.     

The relationship between anatomy and photosynthetic performance of heterobaric leaves

Heterobaric leaves show heterogeneous pigmentation due to the occurrence of a network of transparent areas that are created from the bundle sheaths extensions (BSEs). Image analysis showed that the percentage of photosynthetically active leaf area (Ap) of the heterobaric leaves of 31 plant species was species dependent, ranging from 91% in Malva sylvestris to only 48% in Gynerium sp. Although a significant portion of the leaf surface does not correspond to photosynthetic tissue, the photosynthetic capacity of these leaves, expressed per unit of projected area (Pmax), was not considerably affected by the size of their transparent leaf area (At). This means that the photosynthetic capacity expressed per Ap (P*max) should increase with At. Moreover, the expression of P*max could be allowing the interpretation of the photosynthetic performance in relation to some critical anatomical traits. The P*max, irrespective of plant species, correlated with the specific leaf transparent volume (lambda(t)), as well as with the transparent leaf area complexity factor ((CF)A(t)), parameters indicating the volume per unit leaf area and length/density of the transparent tissues, respectively. Moreover, both parameters increased exponentially with leaf thickness, suggesting an essential functional role of BSEs mainly in thick leaves. The results of the present study suggest that although the Ap of an heterobaric leaf is reduced, the photosynthetic performance of each areole is increased, possibly due to the light transferring capacity of BSEs. This mechanism may allow a significant increase in leaf thickness and a consequent increase of the photosynthetic capacity per unit (projected) area, offering adaptive advantages in xerothermic environments.     

Ectopic divisions in vascular and ground tissues of Arabidopsis thaliana result in distinct leaf venation defects

Leaf venation patterns vary considerably between species and between leaves within a species. A mechanism based on canalization of auxin transport has been suggested as the means by which plastic yet organized venation patterns are generated. This study assessed the plasticity of Arabidopsis thaliana leaf venation in response to ectopic ground or procambial cell divisions and auxin transport inhibition (ATI). Ectopic ground cell divisions resulted in vascular fragments between major veins, whereas ectopic procambial cell divisions resulted in additional, abnormal vessels along major veins, with more severely perturbed lines forming incomplete secondary and higher-order venation. These responses imply limited vascular plasticity in response to unscheduled cell divisions. Surprisingly, a combination of ectopic ground cell divisions and ATI resulted in massive vascular overgrowth. It is hypothesized that the vascular overproduction in auxin transport-inhibited wild-type leaves is limited by simultaneous differentiation of ground cells into mesophyll cells. Ectopic ground cell divisions may negate this effect by providing undifferentiated ground cells that respond to accumulated auxin by differentiation into vascular cells.     

Photosynthetic performance along a light gradient as related to leaf characteristics of a naturally occurring <Emphasis Type="Italic">Cypripedium flavum</Emphasis>

Photosynthesis, leaf structure, nitrogen content and nitrogen allocation in photosynthetic functions of Cypripedium flavum were studied in a naturally varying light regime. Light-saturated leaf net photosynthetic rate (A (max)) was strongly correlated with leaf dry mass per area (LMA), mesophyll conductance (g (m)) and area-based leaf nitrogen content (N(area)), with all variables increasing with increasing irradiance. Such coordinate variation of all these parameters illustrates the plastic response of leaf structure to high light (HL). Leaf N(area) was greater under HL than in low light (LL). The fractions of leaf nitrogen partitioning in carboxylation (P (R)) and bioenergetics (P (B)) were positively related to LMA. In contrast, P (R) and P (B) decreased with increasing mass-based leaf nitrogen content (N(mass)). However, no correlation was found between leaf nitrogen investment in light harvesting (P (L)) and either LMA or N(mass). Like maximum rate of carboxylation (V (cmax)) and electron transport (J (max)), the J (max)/V (cmax) ratio, which was strongly correlated to LMA, also increased significantly with irradiance. Under HL, leaf maximum photosynthetic nitrogen efficiency (ANUE) and intrinsic water use efficiency (WUE) were greater than in LL conditions, despite a small difference in WUE. This suggests that a functional balance in the photosynthetic machinery favors leaf photosynthetic plasticity of C. flavum in response to different light conditions. Given an ample soil nitrogen supply, C. flavum may offset its susceptibility to HL by efficient nitrogen use and higher stomatal and mesophyll conductance against photoinhibition so as to keep leaf photosynthesis positive.     

Comparison of water-use efficiency and internal leaf carbon dioxide concentration in juvenile leaves and phyllodes of Acacia koa (leguminosae) from Hawaii,estimated by two methods

To gain further insight into comparative ecophysiology of different leaf types, water-use efficiency (WUE) and internal leaf carbon dioxide concentration (C_i) were estimated in the field for juvenile leaves and phyllodes of Acacia koa by carbon dioxide and water vapor exchange using a closed system infrared gas analyzer and humidity sensor, and by δ¹³C measurements. Both methods indicate that phyllodes possessed higher WUE and lower C_i than juvenile leaves. However, C_i predicted by δ¹³C for juvenile leaves and phyllodes was lower than the average gas exchange estimated values of C_i and closer to minimal gas exchange estimated values of C_i. It is suggested that δ¹³C may be influenced more during times of maximal assimilation and leaf expansion than during maintenance.     

The rapid light response of leaf hydraulic conductance: new evidence from two experimental methods

Previous studies have shown a rapid enhancement in leaf hydraulic conductance (K(leaf)) from low to high irradiance (from <10 to >1000 micromol photons m(-2) s(-1)), using the high-pressure flow meter (HPFM), for 7 of 14 tested woody species. However, theoretical suggestions have been made that this response might arise as an artifact of the HPFM. We tested the K(leaf) light response for six evergreen species using refined versions of the rehydration kinetics method (RKM) and the evaporative flux method (EFM). We found new evidence for a rapid, 60% to 100% increase in K(leaf) from low to high irradiance for three species. In the RKM, the leaf rehydration time constant declined by up to 70% under high irradiance relative to darkness. In the EFM, under higher irradiance, the flow rate increased disproportionately to the water potential gradient. Combining our data with those of previous studies, we found that heterobaric species, i.e. those with bundle sheath extensions (BSEs) showed a twofold greater K(leaf) light response on average than homobaric species, i.e. those without BSEs. We suggest further research to characterize this substantial dynamic at the nexus of plant light- and water-relations.     

Direct and indirect selection on flowering time,water‐use efficiency (WUE, δ13C), and WUE plasticity to drought in Arabidopsis thaliana

Flowering time and water-use efficiency (WUE) are two ecological traits that are important for plant drought response. To understand the evolutionary significance of natural genetic variation in flowering time, WUE, and WUE plasticity to drought in Arabidopsis thaliana, we addressed the following questions: (1) How are ecophysiological traits genetically correlated within and between different soil moisture environments? (2) Does terminal drought select for early flowering and drought escape? (3) Is WUE plasticity to drought adaptive and/or costly? We measured a suite of ecophysiological and reproductive traits on 234 spring flowering accessions of A. thaliana grown in well-watered and season-ending soil drying treatments, and quantified patterns of genetic variation, correlation, and selection within each treatment. WUE and flowering time were consistently positively genetically correlated. WUE was correlated with WUE plasticity, but the direction changed between treatments. Selection generally favored early flowering and low WUE, with drought favoring earlier flowering significantly more than well-watered conditions. Selection for lower WUE was marginally stronger under drought. There were no net fitness costs of WUE plasticity. WUE plasticity (per se) was globally neutral, but locally favored under drought. Strong genetic correlation between WUE and flowering time may facilitate the evolution of drought escape, or constrain independent evolution of these traits. Terminal drought favored drought escape in these spring flowering accessions of A. thaliana. WUE plasticity may be favored over completely fixed development in environments with periodic drought.     

Experimental Study and Mathematical Simulation of Water Use Efficiency in Wheat Leaves Affected by Some Environmental Factors

By incorporating Ball-Berry model of stomatal conductance into the models of photosynthesis and transpiration, a model of leaf water use efficiency (WUE) as affected by several environmental variables [irradiance (Ⅰ), vapor pressure deficit (VPD) and atmospheric CO2 concentration (Ca) ] was constructed. Because the environmental variables influenced the photosynthetic rate and transpiration rate in different ways, the changes of leaf WUE with these factors were quite complicated. The rates of photosynthesis and transpiration of wheat leaves were also measured in the phytotron where the environmental variables were kept within certain ranges, and leaf WUE was calculated therefrom. The results of simulation fit quite well with the measurements except at high Ca.     

Effects of two passive back-support exoskeletons on postural balance during quiet stance and functional limits of stability

While occupational back-support exoskeletons (BSEs) are considered as potential workplace interventions, BSE use may compromise postural control. Thus, we investigated the effects of passive BSEs on postural balance during quiet upright stance and functional limits of stability. Twenty healthy adults completed trials of quiet upright stance with differing levels of difficulty (bipedal and unipedal stance; each with eyes open and closed), and executed maximal voluntary leans. Trials were done while wearing two different BSEs (SuitX™, Laevo™) and in a control (no-BSE) condition. BSE use significantly increased center-of-pressure (COP) median frequency and mean velocity during bipedal stance. In unipedal stance, using the Laevo™ was associated with a significant improvement in postural balance, especially among males, as indicated by smaller COP displacement and sway area, and a longer time to contact the stability boundary. BSE use may affect postural balance, through translation of the human + BSE center-of-mass, restricted motion, and added supportive torques. Furthermore, larger effects of BSEs on postural balance were evident among males. Future work should further investigate the gender-specificity of BSE effects on postural balance and consider the effects of BSEs on dynamic stability.     

Plant growth-form alters the relationship between foliar morphology and species shade-tolerance ranking in temperate woody taxa

Variation in leaf size (area per leaf) and leaf dry weight per area (LWA) in relation to species shade- and drought-tolerance, characterised by Ellenberg's light (ELD) and water demand (EWD) values, respectively, were examined in 60 temperate woody taxa at constant relative irradiance. LWA was independent of plant size, but leaf size increased with total plant height at constant ELD. Canopy position also affected leaf morphology: leaves from the upper crown third had higher LWA and were larger than leaves from the lower third. Leaf size and LWA were negatively correlated, and leaf size decreased and LWA increased with decreasing species shade-tolerance. Mean LWA was similar for trees and shrubs, but trees had larger leaves than shrubs. Furthermore, all relationships were altered by plant growth-form: none of the qualitative tendencies was significant for trees. This implies the considerably lower plasticity of foliar parameters in trees than those in shrubs. Accordingly, shade-tolerance of trees, having relatively constant leaf structure, may be most affected by the variability in biomass partitioning and crown geometry which influence foliage distribution and spacing and finally determine canopy light absorptance. Alteration of leaf form and investment pattern for construction of unit foliar surface area which change the efficiency of light interception per unit biomass investment in leaves, is a competitive strategy inherent to shrubs. EWD as well as wood anatomy did not control LWA and leaf size, though there was a trend of ring-porous tree species to be more shade-tolerant than diffuse-porous trees. Since ring-porous species are more vulnerable to cavitation than diffuse-porous species, they may be constrained to environments where irradiances and consequently evaporative demand is lower.