Dwarf mistletoe affects whole-tree water relations of Douglas fir and western larch primarily through changes in leaf to sapwood ratios

Dwarf mistletoes induce abnormal growth patterns and extreme changes in the biomass allocation of their hosts as well as directly parasitizing them for resources. Because biomass allocation can affect the resource use and efficiency of conifers, we studied the influences of dwarf mistletoe infection on above-ground biomass allocation of Douglas fir and western larch, and the consequences of such changes on whole-tree water use and water relations. Sap flow, tree water potentials, leaf:sapwood area ratios (A_L:A_S), leaf carbon isotope ratios, and nitrogen content were measured on Douglas fir and western larch trees with various degrees of mistletoe infection during the summer of 1996 in western Montana. Heavy dwarf mistletoe infection on Douglas fir and western larch was related to significant increases in A_L:A_S. Correspondingly, water transport dynamics were altered in infected trees, but responses were different for the two species. Higher A_L:A_S ratios in heavily infected Douglas firs were offset by increases in sapwood area-based sap flux densities (Q_SW) such that leaf area-based sap flux densities (Q_L) and predawn leaf water potentials at the end of the summer did not change significantly with mistletoe infection. Small (but statistically insignificant) decreases of Q_L for heavily infected Douglas firs were enough to offset increases in leaf area such that whole-tree water use was similar for uninfected and heavily infected trees. Increased A_L:A_S ratios of heavily infected western larch were not offset by increases of Q_SW. Consequently, Q_L was reduced, which corresponded with significant decreases of water potential at the end of the summer. Furthermore, mistletoe-infection-related changes in A_L:A_S as a function of tree size resulted in greater whole-tree water use for large infected larches than for large uninfected trees. Such changes may result in further depletion of limited soil water resources in mature infected stands late in the growing season. Foliage from infected trees of both species had lower water use efficiencies than non-infected trees. Our results demonstrate substantial changes of whole-tree processes related to mistletoe infection, and stress the importance of integrating whole-tree physiological and structural processes to fully understand the mechanisms by which pathogens suppress forest productivity.     

Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees

We investigated how water transport capacity, wood density and wood anatomy were related to leaf photosynthetic traits in two lowland forests in Panama. Leaf-specific hydraulic conductivity (k_L) of upper branches was positively correlated with maximum rates of net CO₂ assimilation per unit leaf area (A_area) and stomatal conductance (g_s) across 20 species of canopy trees. Maximum k_L showed stronger correlation with A_area than initial k_L suggesting that allocation to photosynthetic potential is proportional to maximum water transport capacity. Terminal branch k_L was negatively correlated with A_area/g_s and positively correlated with photosynthesis per unit N, indicating a trade-off of efficient use of water against efficient use of N in photosynthesis as water transport efficiency varied. Specific hydraulic conductivity calculated from xylem anatomical characteristics (k_theoretical) was positively related to A_area and k_L, consistent with relationships among physiological measurements. Branch wood density was negatively correlated with wood water storage at saturation, k_L, A_area, net CO₂ assimilation per unit leaf mass (A_mass), and minimum leaf water potential measured on covered leaves, suggesting that wood density constrains physiological function to specific operating ranges. Kinetic and static indices of branch water transport capacity thus exhibit considerable co-ordination with allocation to potential carbon gain. Our results indicate that understanding tree hydraulic architecture provides added insights to comparisons of leaf level measurements among species, and links photosynthetic allocation patterns with branch hydraulic processes.     

Stem water storage capacity and efficiency of water transport: their functional significance in a Hawaiian dry forest

We investigated the contribution of internal water storage and efficiency of water transport to the maintenance of water balance in six evergreen tree species in a Hawaiian dry forest. Wood‐saturated water content, a surrogate for relative water storage capacity, ranged from 70 to 105%, and was inversely related to its morphological correlate, wood density, which ranged between 0·51 and 0·65 g cm^?3. Leaf‐specific conductivity (k_L) measured in stem segments from terminal branches ranged from 3 to 18 mmol m^?1 s^?1 MPa^?1, and whole‐plant hydraulic efficiency calculated as stomatal conductance (g) divided by the difference between predawn and midday leaf water potential (Ψ_L), ranged from 70 to 150 mmol m^?2 s^?1 MPa^?1. Hydraulic efficiency was positively correlated with k_L (r² = 0·86). Minimum annual Ψ_L ranged from ? 1·5 to ? 4·1 MPa among the six species. Seasonal and diurnal variation in Ψ_L were associated with differences among species in wood‐saturated water content, wood density and k_L. The species with higher wood‐saturated water content were more efficient in terms of long‐distance water transport, exhibited smaller diurnal variation in Ψ_L and higher maximum photosynthetic rates. Smaller diurnal variation in Ψ_L in species with higher wood‐saturated water content, k_L and hydraulic efficiency was not associated with stomatal restriction of transpiration when soil water deficit was moderate, but avoidance of low minimum seasonal Ψ_L in these species was associated with a substantial seasonal decline in g. Low seasonal minimum Ψ_L in species with low k_L, hydraulic efficiency, and wood‐saturated water content was associated with higher leaf solute content and corresponding lower leaf turgor loss point. Despite the species‐specific differences in leaf water relations characteristics, all six evergreen tree species shared a common functional relationship defined primarily by k_L and stem water storage capacity.     

Comparative water use and nitrogen relationships in a mistletoe and its host

Summary The impact of the xylem-tapping mistletoe Phoradendron juniperinum on the nitrogen and water relations of its host Juniperus osteosperma was investigated under natural field conditions. Leaf conductance, leaf water potential, and leaf Kjeldahl nitrogen contents were followed through the growing season on mistletoes, infected junipers (separating infected from uninfected stems) and uninfected junipers. Infected trees experienced lower leaf water potentials than uninfected trees and also had lower leaf conductances and lower leaf nitrogen contents. Infected juniper stems had higher conductances than uninfected stems. Mistletoes had higher leaf nitrogen contents than their hosts and much of this nitrogen appeared as arginine, a potential nitrogen storage compound. Photosynthetic rates (per unit leaf area) were significantly higher in junipers than in the mistletoe, and higher in the uninfected than infected junipers. Water use efficiencies as estimated by carbon isotope ratios were significantly lower in mistletoes than in their hosts. Increased mistletoe infestation appeared to increase absolute water use efficiency of both host and mistletoe.Dedicated to Professors D. Wiens (Salt Lake City) and H. Ziegler (München) for initiating our curiosity in mistletoes     

A test of the hydraulic limitation hypothesis in fast-growing Eucalyptus saligna

The hydraulic limitation hypothesis proposes that (1) reduced growth in taller trees is caused by decreased photosynthesis resulting from a decrease in hydraulic conductance promoted by a longer root‐to‐leaf flow path, and (2) this mechanism reduces stand productivity after canopy closure. This hypothesis was tested by comparing the physiology of 7 m (1 year) and 26 m (5 year) Eucalyptus saligna plantations where above‐ground productivity for the 26 m trees was approximately 69% of that for the 7 m trees, and water and nutrients were not limiting. The study compared whole tree physiology [water flux (Q_l), average crown conductance (G_T), crown hydraulic conductance per unit leaf area (K_L), carbon isotope discrimination (δ¹³C)] and leaf physiology under light saturation (leaf water potential at the canopy top (Ψ_LEAF), photosynthetic capacity (A_max), and photosynthesis (A) and stomatal conductance (g_s). K_L was 50% lower in the taller trees, but whole tree Q_l and G_T were the same for the 7 m and 26 m trees. Photosynthetic capacity was the same for leaves at the canopy top, but δ¹³C was ?1.8‰ lower for the 26 m trees. A and g_s were either lower in the taller trees or equal, depending on sampling date. The taller trees maintained 0.8 MPa lower Ψ_LEAF during the day and had 2.6‐times higher sapwood area per unit leaf area; these factors compensated for the effects of increased height and gravitational potential in the taller trees to maintain higher G_T. The hydraulic limitation hypothesis (as originally stated) failed to explain the sharp decline in net primary productivity after canopy closure in this study. The effects of increased height appear to be a universal hydraulic problem for trees, but compensation mitigated these effects and maintained Q_l and G_T in the present study. Compensation may induce other problems (such as lower Ψ_LEAF or higher respiratory costs) that could reduce carbon gain or shift carbon allocation, and future studies of hydraulic limitation should consider compensation and associated carbon costs. In this study, the combination of similar G_T and lower δ¹³C for the 26 m trees suggests that total crown photosynthesis was lower for the 26 m trees, perhaps a result of the lower Ψ_LEAF.     

Hydraulic responses to height growth in maritime pine trees

As trees grow taller, decreased xylem path conductance imposes a major constraint on plant water and carbon balance, and is thus a key factor underlying forest productivity decline with age. The responses of stomatal conductance, leaf area: sapwood area ratio (A_L : A_S) and soil–leaf water potential gradient (ΔΨ_S–L) to height growth were investigated in maritime pine trees. Extensive measurements of in situ sap flow, stomatal conductance and (non‐gravitational) needle water potential (_L = Ψ_L ? ρ_wgh) were made during 2 years in a chronosequence of four even‐aged stands, under both wet and dry soil conditions. Under wet soil conditions, _L was systematically lower in taller trees on account of differences in gravitational potential. In contrast, under dry soil conditions, our measurements clearly showed that _L was maintained above a minimum threshold value of ?2.0 MPa independently of tree height, thus limiting the range of compensatory change in ΔΨ_S–L. Although a decrease in the A_L : A_S ratio occurred with tree height, this compensation was not sufficient to prevent a decline in leaf‐specific hydraulic conductance, K_L (50% lower in 30 m trees than in 10 m trees). An associated decline in stomatal conductance with tree height thus occurred to maintain a balance between water supply and demand. Both the increased investment in non‐productive versus productive tissues (A_S : A_L) and stomatal closure may have contributed to the observed decrease in tree growth efficiency with increasing tree height (by a factor of three from smallest to tallest trees), although other growth‐limiting responses (e.g. soil nutrient sequestration, increased respiratory costs) cannot be excluded.     

Climate-driven changes in biomass allocation in pines

Future increases in air temperature resulting from human activities may increase the water vapour pressure deficit (VPD) of the atmosphere. Understanding the responses of trees to spatial variation in VPD can strengthen our ability to predict how trees will respond to temporal changes in this important variable. Using published values, we tested the theoretical prediction that conifers decrease their investment in photosynthetic tissue (leaves) relative to water‐conducting tissue in the stem (sapwood) as VPD increases. The ratio of leaf/sapwood area (A_L/A_S) decreased significantly with increasing VPD in Pinus species but not in Abies, Pseudotsuga, Tsuga and Picea, and the average A_L/A_S was significantly lower for pines than other conifers (pines: 0.17 m² cm^?2; nonpines: 0.44 m² cm^?2). Thus, pines adjusted to increasing aridity by altering above‐ground morphology while nonpine conifers did not. The average water potential causing a 50% loss of hydraulic conductivity was ?3.28 MPa for pines and ?4.52 MPa for nonpine conifers, suggesting that pines are more vulnerable to xylem embolism than other conifers. For Pinus ponderosa the decrease in A_L/A_S with high VPD increases the capacity to provide water to foliage without escalating the risk of xylem embolism. Low A_L/A_S and plasticity in this variable may enhance drought tolerance in pines. However, lower A_L/A_S with increasing VPD and an associated shift in biomass allocation from foliage to stems suggests that pines may expend more photosynthate constructing and supporting structural mass and carry less leaf area as the climate warms.     

Coupled response of stomatal and mesophyll conductance to light enhances photosynthesis of shade leaves under sunflecks

Light gradients within tree canopies play a major role in the distribution of plant resources that define the photosynthetic capacity of sun and shade leaves. However, the biochemical and diffusional constraints on gas exchange in sun and shade leaves in response to light remain poorly quantified, but critical for predicting canopy carbon and water exchange. To investigate the CO₂ diffusion pathway of sun and shade leaves, leaf gas exchange was coupled with concurrent measurements of carbon isotope discrimination to measure net leaf photosynthesis (A_n), stomatal conductance (g_s) and mesophyll conductance (g_m) in Eucalyptus tereticornis trees grown in climate controlled whole‐tree chambers. Compared to sun leaves, shade leaves had lower A_n, g_m, leaf nitrogen and photosynthetic capacity (A_max) but g_s was similar. When light intensity was temporarily increased for shade leaves to match that of sun leaves, both g_s and g_m increased, and A_n increased to values greater than sun leaves. We show that dynamic physiological responses of shade leaves to altered light environments have implications for up‐scaling leaf level measurements and predicting whole canopy carbon gain. Despite exhibiting reduced photosynthetic capacity, the rapid up‐regulation of g_m with increased light enables shade leaves to respond quickly to sunflecks.     

Impact of eastern dwarf mistletoe (Arceuthobium pusillum) on host white spruce (Picea glauca) development,growth and performance across multiple scales

Infection by eastern dwarf mistletoe (Arceuthobium pusillum) modifies needle and branch morphology and hastens white spruce (Picea glauca) mortality. We examined potential causal mechanisms and assessed the impacts of infection‐induced alterations to host development and performance across scales ranging from needle hormone contents to bole expansion. Needles on infected branches (IBs) possessed higher total cytokinin (CK) and lower abscisic acid contents than needles on uninfected branches (UBs). IBs exhibited greater xylem growth than same‐aged UBs, which is consistent with the promotive effect of CKs on vascular differentiation and organ sink strength. Elevated CK content may also explain the dense secondary and tertiary branching observed at the site of infection, i.e. the formation of ‘witches’ brooms' with significantly lower light capture efficiencies. Observed hormone perturbations were consistent with higher rates of transpiration, lower water use efficiencies (WUEs) and more negative needle carbon isotope ratios observed for IBs. Observed reductions in needle size allowed IBs to compensate for reduced hydraulic conductivity. Severe infections resulted in dramatically decreased diameter growth of the bole. It seems likely that the modifications to host hormone contents by eastern dwarf mistletoe infection led white spruce trees to dedicate a disproportionate fraction of their photoassimilate and other resources to self‐shaded branches with low WUE. This would have decreased the potential for fixed carbon accumulation, generating a decline in the whole‐tree resource pool. As mistletoe infections grew in size and the number of IBs increased, this burden was manifested as increasingly greater reductions in bole growth.     

Host specific variation in photosynthesis of an obligate xylem‐tapping mistletoe Dendrophthoe curvata in a Bornean heath forest

Most mistletoe–host ecophysiological studies have paid attention to the influence of parasites on host performance. This paper explored the impact of varying hosts on the photosynthesis of a single mistletoe species. Here, we studied an obligate xylem‐tapping tropical mistletoe (Dendrophthoe curvata (Blume) Miquel) parasitizing four different hosts (Acacia auriculiformis A. Cunn. Ex Benth, Andira inermis (W. Wright) DC., Mangifera indica L. and Vitex pinnata L.) in a homo geneous tropical heath forest patch in Brunei Darussalam. We compared photosynthetic capacity and photosynthesis‐related characteristics of the mistletoe on four different hosts to evaluate the overall impact of hosts on the parasite. Results showed that the mistletoe–host patterns of CO₂ assimilation rates, transpiration rates and water use efficiency varied significantly based on the host. In the D. curvata–Vitex pinnata association, the mistletoe exhibited significantly lower CO₂ assimilation rates but showed no significant variations in transpiration rates and water use efficiency when compared to the host. In D. curvata–Andira inermis and D. curvata–Mangifera indica associations, the mistletoe showed significantly higher photosynthetic rates than the hosts, whereas in the D. curvata–Acacia auriculiformis association, there was no significant difference in photosynthetic rates between the counterparts. Host specificity also significantly influenced some mistletoe photosynthetic parameters such as light saturated photosynthesis, specific leaf area, leaf chlorophyll content, CO₂ assimilation rates, stomatal conductance, transpiration rates and water use efficiency. Different tree hosts intrinsically offer different resources to their obligate mistletoe parasites based on their physiology and environmental parameters. We argue that host‐specific responses have driven these intra‐specific variations in mistletoe physiology. This study provides background for future investigation on potential host‐regulated mechanisms that drive functional changes in host‐dependent mistletoes.     

Leaf δ18O of remaining trees is affected by thinning intensity in a semiarid pine forest

Silvicultural thinning usually improves the water status of remaining trees in water‐limited forests. We evaluated the usefulness of a dual stable isotope approach (δ¹³C, δ¹⁸O) for comparing the physiological performance of remaining trees between forest stands subjected to two different thinning intensities (moderate versus heavy) in a 60‐year‐old Pinus halepensis Mill. plantation in semiarid southeastern Spain. We measured bulk leaf δ¹³C and δ¹⁸O, foliar elemental concentrations, stem water content, stem water δ¹⁸O (δ¹⁸O_{stem water}), tree ring widths and leaf gas exchange rates to assess the influence of forest stand density on tree performance. Remaining trees in low‐density stands (heavily thinned) showed lower leaf δ¹⁸O, and higher stomatal conductance (g_s), photosynthetic rate and radial growth than those in moderate‐density stands (moderately thinned). By contrast, leaf δ¹³C, intrinsic water‐use efficiency, foliar elemental concentrations and δ¹⁸O_{stem water} were unaffected by stand density. Lower foliar δ¹⁸O in heavily thinned stands reflected higher g_s of remaining trees due to decreased inter‐tree competition for water, whereas higher photosynthetic rate was largely attributable to reduced stomatal limitation to CO₂ uptake. The dual isotope approach provided insight into the early (12 months) effects of stand density manipulation on the physiological performance of remaining trees.     

Influence of nutrient versus water supply on hydraulic architecture and water balance in Pinus taeda

We investigated the hydraulic consequences of a major decrease in root‐to‐leaf area ratio (A_R:A_L) caused by nutrient amendments to 15‐year‐old Pinus taeda L. stands on sandy soil. In theory, such a reduction in A_R:A_L should compromise the trees’ ability to extract water from drying sand. Under equally high soil moisture, canopy stomatal conductance (G_S) of fertilized trees (F) was 50% that of irrigated/fertilized trees (IF), irrigated trees (I), and untreated control trees (C). As predicted from theory, F trees also decreased their stomatal sensitivity to vapour pressure deficit by 50%. The lower G_S in F was associated with 50% reduction in leaf‐specific hydraulic conductance (K_L) compared with other treatments. The lower K_L in F was in turn a result of a higher leaf area per sapwood area and a lower specific conductivity (conducting efficiency) of the plant and its root xylem. The root xylem of F trees was also 50% more resistant to cavitation than the other treatments. A transport model predicted that the lower A_R:A_L in IF trees resulted in a considerably restricted ability to extract water during drought. However, this deficiency was not exposed because irrigation minimized drought. In contrast, the lower A_R:A_L in F trees caused only a limited restriction in water extraction during drought owing to the more cavitation resistant root xylem in this treatment. In both fertilized treatments, approximate safety margins from predicted hydraulic failure were minimal suggesting increased vulnerability to drought‐induced dieback compared with non‐fertilized trees. However, IF trees are likely to be so affected even under a mild drought if irrigation is withheld.     

The effect of tree height on crown level stomatal conductance

Variation in stomatal conductance is typically explained in relation to environmental conditions. However, tree height may also contribute to the variability in mean stomatal conductance. Mean canopy stomatal conductance of individual tree crowns (G_Si) was estimated using sap flux measurements in Fagus sylvatica L., and the hypothesis that G_Si decreases with tree height was tested. Over 13 d of the growing season during which soil moisture was not limiting, G_Si decreased linearly with the natural logarithm of vapour pressure deficit (D), and increased exponentially to saturation with photosynthetic photon flux density (Q_o). Under conditions of D = 1 kPa and saturating Q_o, G_Si decreased by approximately 60% with 30 m increase in tree height. Over the same range in height, sapwood‐to‐leaf area ratio (A_S:A_L) doubled. A simple hydraulic model explained the variation in G_Si based on an inverse relationship with height, and a linear relationship with A_S:A_L. Thus, in F. sylvatica, adjustments in A_S:A_L partially compensate for the negative effect of increased flow‐path length on leaf conductance. Furthermore, because stomata with low conductance are less sensitive to D, gas exchange of tall trees is reduced less by high D. Despite these compensations, decreasing hydraulic conductance with tree height in F. sylvatica reduces carbon uptake through a corresponding decrease in stomatal conductance.     

Functional relationships between hydraulic traits and the timing of diurnal depression of photosynthesis

The hydraulic coordination along the water transport pathway helps trees provide adequate water supply to the canopy, ensuring that water deficits are minimized and that stomata remain open for CO₂ uptake. We evaluated the stem and leaf hydraulic coordination and the linkages between hydraulic traits and the timing of diurnal depression of photosynthesis across seven evergreen tree species in the southern Andes. There was a positive correlation between stem hydraulic conductivity (k_s) and leaf hydraulic conductance (K_Leaf) across species. All species had similar maximum photosynthetic rates (A_max). The species with higher k_s and K_Leaf attained A_max in the morning, whereas the species with lower k_s and K_Leaf exhibited their A_max in the early afternoon concurrently with turgor loss. These latter species had very negative leaf water potentials, but far from the pressure at which the 88% of leaf hydraulic conductance is lost. Our results suggest that diurnal gas exchange dynamics may be determined by leaf hydraulic vulnerability such that a species more vulnerable to drought restrict water loss and carbon assimilation earlier than species less vulnerable. However, under stronger drought, species with earlier CO₂ uptake depression may increase the risk of hydraulic failure, as their safety margins are relatively narrow.     

Size-dependent variability of leaf and shoot hydraulic conductance in silver birch

Variation in leaf and shoot hydraulic conductance was examined on detached shoots of silver birch (Betula pendula Roth), cut from the lower third (shade leaves) and upper third of the crown (sun leaves) of large trees growing in a natural temperate forest stand. Hydraulic conductances of whole shoots (K _S), leaf blades (K _lb), petioles (K _P) and branches (i.e. leafless stem; K _B) were determined by water perfusion using a high-pressure flow meter in quasi-steady state mode. The shoots were exposed to irradiance of photosynthetic photon flux density of 200–250 μmol m⁻² s⁻¹, using different light sources. K _lb depended significantly (P < 0.001) on light quality, canopy position and leaf blade area (A _L). K _lb increased from crown base to tree top, in parallel with vertical patterns of A _L. However, the analysis of data on shade and sun leaves separately revealed an opposite trend: the bigger the A _L the higher K _lb. Leaf anatomical study of birch saplings revealed that this trend is attributable to enhanced vascular development with increasing leaf area. Hydraulic traits (K _S, K _B, K _lb) of sun shoots were well co-ordinated and more strongly correlated with characteristics of shoot size than those of shade shoots, reflecting their greater evaporative load and need for stricter adjustment of hydraulic capacity with shoot size. K _S increased with increasing xylem cross-sectional area to leaf area ratio (Huber value; P < 0.01), suggesting a preferential investment in water-conducting tissue (sapwood) relative to transpiring tissue (leaves), and most likely contributing to the functional stability of the hydraulic system, essential for fast-growing pioneer species.     

Short- and long-term responses to seasonal drought in ponderosa pines growing at different plantation densities in Patagonia, South America

Trees drought responses could be developed in the short- or in the long-term, aiming at sustaining carbon fixation and water use efficiency (WUE). The objective of this study was to examine short- and long-term adjustments occurring in different size Pinus ponderosa Dougl. ex P. & C. Laws trees in response to seasonal drought when they are growing under different competition level. The following variables were studied: branch and stem hydraulic conductivity, canopy and stomatal conductance (gc, gs), transpiration (E), photosynthesis (A _max), wood δ¹³C (as a proxy of intrinsic WUE), leaf to sapwood area ratio (A _L:A _s) and growth in the biggest (B) and the smallest (S) trees of high (H) and low (L) density stands. A _L:A _s was positively correlated with tree size and negatively correlated with competition level, increasing leaf hydraulic conductance in H trees. Accordingly, higher gc and E per unit A _L were found in H than in L trees when soil water availability was high, but decreased abruptly during dry periods. BL trees maintained stable gc and E values even during the summer drought. The functional adjustments observed in H trees allow them to maintain their hydraulic integrity (no apparent k _s losses), but their stem and leaf growth were severely affected by drought events. _iWUE was similar between all tree groups in a wet season, whereas it significantly decreased in SH trees in a dry season suggesting that when radiation and water are co-limiting gas exchange, functional adjustments not only affect absolute growth, but also WUE.     

Spatial variation in sapwood area to leaf area ratio and specific leaf area within a crown of silver birch

Spatial variation in sapwood area to leaf area ratio (Huber value, HV) and specific leaf area (SLA) was examined in branches of closed-canopy trees of silver birch (Betula pendula Roth). HV increased basipetally within a crown and decreased with increasing branch order, but exhibited no significant radial trend along a primary branch. HV was primarily determined by branch position in a crown and branch diameter at the sampling point, being independent of the size of the tree and branch. Greater HV in the lower-crown branches is considered a means to mitigate differences in hydraulic transport capacity between the branches located in different canopy layers. Beside branch position and sampling location on a branch, SLA depended significantly on several other variables characterising tree and branch size. SLA increased basipetally within a crown and along a primary branch, but exhibited no significant trend with branch orders. Because height caused leaf area (A_L) to diminish more rapidly than leaf dry weight, A_L primarily determined the vertical variation in SLA.     

Stem hydraulic supply is linked to leaf photosynthetic capacity: evidence from New Caledonian and Tasmanian rainforests

A strong relationship between hydraulic supply of water to leaves and maximum photosynthetic capacity was found in a group of seven conifers and 16 angiosperm species, including two vessel‐less taxa, from similar rainforest communities in New Caledonia and Tasmania (Australia). Stem hydraulic supply was expressed as the hydraulic conductivity of branches in terms of leaf area supplied (K_L) and leaf photosynthetic capacity was measured as the mean quantum yield of PSII (Ø_PSII) in leaves exposed to full sun, as determined by chlorophyll fluorescence analysis. A single, highly significant linear regression (r² = 0·74) described the relationship between hydraulic conductivity and quantum yield in all species. This suggests that the maximum photosynthetic rate of leaves is constrained by their vascular supply. In both rainforest locations, the K_L of conifer wood overlapped broadly with that of associated vessel‐bearing and vessel‐less angiosperms indicating a degree of hydraulic convergence in these forests.     

Contrasting leaf characteristics of trees and lianas in secondary and mature forests in southwestern China

We compared variation in sun-canopy leaf anatomy, morphology and photosynthetic rates of coexisting woody species (trees and lianas) in an 8-year-old secondary forest (SF) and mature forest (MF) in the wet season in Xishuangbanna, SW China. Variability of leaf traits of 66 species within growth-form groups in each forest was quantified using coefficients of variation (CV). For the mean values, the woody species in the SF had significantly higher leaf thickness and stomatal density, but lower nonmesophyll/mesophyll ratios than those in the MF. The average leaf area and leaf mass area (LMA) in the studied woody species did not change greatly during the successional process, but differed significantly between the growth forms, with trees having higher values than lianas. The light-saturated photosynthetic rate per unit leaf area (A _a) of the woody species in the SF ranged from 11.2 to 34.5 μmol m⁻² s⁻¹, similarly to pioneer tree species from literature data in southeast Asia. The A _a and photosynthetic nitrogen-use efficiency (PNUE) were significantly higher than those in the MF; whereas A _a in the MF ranged between 9 to 21 μmol m⁻² s⁻¹, with similar values between lianas and trees. For all woody species in both SF and MF, there were no significant differences in the average values of the CV of all measured variables for both lianas and trees. However, considerable variation in leaf anatomy, morphology, and photosynthetic rates within both growth forms and forests existed, as well as substantial variation in leaf size and stomatal density. We concluded that the tropical woody species formed a heterogeneous functional group in terms of leaf morphology and physiology in both secondary and mature forests.     

Seasonal differences in leaf-level physiology give lianas a competitive advantage over trees in a tropical seasonal forest

Lianas are an important component of most tropical forests, where they vary in abundance from high in seasonal forests to low in aseasonal forests. We tested the hypothesis that the physiological ability of lianas to fix carbon (and thus grow) during seasonal drought may confer a distinct advantage in seasonal tropical forests, which may explain pan-tropical liana distributions. We compared a range of leaf-level physiological attributes of 18 co-occurring liana and 16 tree species during the wet and dry seasons in a tropical seasonal forest in Xishuangbanna, China. We found that, during the wet season, lianas had significantly higher CO₂ assimilation per unit mass (A _mass), nitrogen concentration (N _mass), and δ¹³C values, and lower leaf mass per unit area (LMA) than trees, indicating that lianas have higher assimilation rates per unit leaf mass and higher integrated water-use efficiency (WUE), but lower leaf structural investments. Seasonal variation in CO₂ assimilation per unit area (A _area), phosphorus concentration per unit mass (P _mass), and photosynthetic N-use efficiency (PNUE), however, was significantly lower in lianas than in trees. For instance, mean tree A _area decreased by 30.1% from wet to dry season, compared with only 12.8% for lianas. In contrast, from the wet to dry season mean liana δ¹³C increased four times more than tree δ¹³C, with no reduction in PNUE, whereas trees had a significant reduction in PNUE. Lianas had higher A _mass than trees throughout the year, regardless of season. Collectively, our findings indicate that lianas fix more carbon and use water and nitrogen more efficiently than trees, particularly during seasonal drought, which may confer a competitive advantage to lianas during the dry season, and thus may explain their high relative abundance in seasonal tropical forests.