Within-tree and among-tree variation in growth of Epirrita autumnata on mountain birch leaves

Abstract.

• 1 We studied among-tree and within-tree variation in the growth of larvae of a geometrid, Epirrita autumnata, on mountain birch (Betula pubescens ssp. tortuosa) leaves at different hierarchical levels: among trees (genets), among ramets within trees, among branches within ramets, among shoots within branches and among leaves within shoots. We used only short shoot leaves, which burst simultaneously in spring.
• 2 Trees accounted for most of the variation in larval growth rate, but there was substantial variation also among ramets within trees, among branches within ramets, and among short shoots within branches. Variation among leaves within short shoots was negligible. When the probabilities from different experiments were combined, the differences were statistically significant at the tree, ramet and branch levels, and approached significance at the shoot level.
• 3 In different experiments, larval growth was from 9% to 54% lower on the worst tree than on the best tree of the experiment. On average, larval growth rate was in different experiments from 11% to 32% lower on the worst ramet than on the best ramet within tree, from 8% to 18% lower on the worst than on the best branch within ramet, and from 12% to 30% lower on the worst than on the best shoot within branch.
• 4 The amount of among-and within-tree variation shown by our results may have ecological and evolutionary implications: among-tree variation should select for discrimination by ovipositing females and dispersing larvae, within-tree variation should select also for optimal foraging behaviour of larvae.

     

Relationship between hydraulic resistance and leaf morphology in broadleaf Quercus species: a new interpretation of leaf lobation

We investigated the relationship between leaf shape and leaf hydraulic resistance in a set of broadleaf Quercus tree species (Q. cerris, Q. frainetto, Q. petraea, Q. pyrenaica, Q. robur, Q. rubra, Q. velutina). Seedlings of all the studied species were grown under uniform environmental conditions. A new high-pressure flowmeter was designed to measure leaf-blade hydraulic resistance. Leaf shape was characterised by the complexity of leaf outline which was regarded as an estimate of leaf lobation. This was done using the box-counting fractal dimension of the leaf silhouette. Leaf hydraulic resistance was negatively related to leaf lobation. It is suggested that the lower hydraulic resistance in deeply lobed leaves may constitute a mechanism for improving water balance under dry atmospheric conditions.     

Life history variation in Arabidopsis lyrata across its range: effects of climate,population size and herbivory

For plants with wide distributional areas, covering a wide range of ecologically distinct habitats, evolutionary divergence can lead to substantial phenotypic variation across a species’ range. These intraspecific trait differences can be very informative about the nature of the selective environment as they potentially reflect different environmental selection pressures while controlling for other species characteristics. In this study, multiple regression and structural equation models were used to examine the relative importance of environmental, ecological, population size and population density effects for variation in growth, reproduction and leaf morphology among 36 populations of the perennial plant Arabidopsis lyrata ssp. petraea across its northwest European range. Substantial variation in temperature, soil nutrient levels and herbivory was observed across the species’ range. In addition, large differences in flowering percentage and individual seed production were found. Leaf morphology varied considerably, with a substantial amount of variation in specific leaf area and trichome density among populations. Structural equation modeling suggested that this species is sensitive to small population sizes, eutrophication and herbivory. Reproductive output was negatively related to herbivory. In addition population size was negatively associated with soil nutrient concentrations. Leaf morphology was shown to be mainly associated with temperature and herbivory. Lower specific leaf areas and lower trichome densities were related to colder areas and high trichomes densities were related to high levels of herbivory. These model results are consistent with the interpretation that, in addition to changing environmental effects across its range, ecological effects such as herbivory contribute to the large variation in life history and morphology of this species. The results reveal a strong negative effect of herbivory on the reproductive output of this species, not only via direct effects of herbivory on flowers and seeds, but also indirectly via a shift in life history strategy.     

Physiological basis of spacing effects on tree growth and form in Eucalyptus globulus

We examine the effects of spacing and layout on the growth and form of 3- to 4-year-old Eucalyptus globulus in a farm forestry context. Four planting layouts were chosen. These represented the range commonly in use in farm forestry: block plantings (2Ǹ m), triple rows (2Ǹ m) at 10-m intervals, single rows (2᎒ m) and isolated trees (10᎒ m). The physiological significance of key results is interpreted in terms of changes in the parameters of a simple plantation growth model. Under conditions where levels of direct light are high, for example during summer, block-planted trees intercepted only 38% of the light intercepted by isolated trees. On a stand basis, however, the combination of incident radiation and ground coverage declined with lower stand densities. While stand leaf area index declined from around 6 to 1 with increased spacing, individual tree leaf areas rose from around 50 m² in block plantings to 150 m² in isolated trees. The proportion of above-ground biomass found in stems declined with increasing spacing as the mass in foliage and branches increased. Stems accounted for 65% of above-ground biomass in block-planted trees but only 35% in isolated trees. The contributions of leaves and branches correspondingly rose from 19% to 35% and from 16% to 29%, respectively. Changes in biomass distribution were accompanied by increasing branch number, branch thickness, flatter branch angles and the longer retention of lower branches with greater spacing. These changes have implications for the merchantability of the timber. The efficiency of above-ground radiation conversion was constant at 0.67 g MJ^-1 irrespective of spacing. We estimated that foliar maintenance respiration (R_m) accounted for about 90% of above-ground R_m. On a stand basis R_m costs block plantings 23.90 t DM ha^-1 year^-1 (50% annual above-ground photosynthetic production) compared with 6.22 t DM ha^-1 year^-1 (40% annual above-ground photosynthetic production) in stands of isolated trees.     

Chlorophyll fluorescence and photosynthetic response to light in 1-year-old needles during spring and early summer in <Emphasis Type="Italic">Pinus leucodermis</Emphasis>

Ten-year-old trees from four Italian populations of Pinus leucodermis (populations A, B, C and D), which were collected from different sites at different altitudes, were grown near Florence, Italy. Needle CO₂ gas exchange and chlorophyll fluorescence response to increasing light intensities were evaluated; gas exchange and chlorophyll fluorescence variation between April and July were also monitored. Populations A, B and C showed a similar photosynthetic response to increasing photosynthetic photon flux density (PPFD) intensities, while at various light intensities population D, which originated from the highest altitude, showed the highest photosynthetic rates. In this population photosynthesis was saturated at PPFDs higher than 900 µmol m^-2s^-1 and a slow decrease of effective photosystem II quantum yield and F'_V/F'_M in response to increasing PPFDs were found. The same trees also showed a faster recovery in photosynthesis from limitations induced by winter temperatures than the other three populations. This work showed that photosynthetic response to light in population D was different from the other populations; trees from this population were probably naturally selected to prevent photoinhibition due to excess light.     

Estradiol distribution during the development and expression of reproductive structures in<Emphasis Type="Italic"> Populus tremuloides</Emphasis> Michx.

Estradiol (E₂) was measured in spring and winter tissues in several populations of dioecious Populus tremuloides Michx. and in four trees with perfect flowers. A correlation between the concentration of E₂ and sex expression was observed. In catkins, E₂ concentrations increased before anthesis, peaked during flowering, and decreased as flowers matured. The increases were concurrent with sporogenesis and development of gametophytes. Statistically significant seasonal variation in E₂ concentration was also evident. Dormant winter tissues showed significantly lower E₂ concentrations than spring tissues. Branches exposed to relatively more light had significantly higher E₂ content in both spring and winter. Seasonal variation in E₂ content was also observed in trees of the same sex, among branches of the same tree, and among different organs.     

Variation in seed production schedule among individual trees of a deciduous oak species <Emphasis Type="Italic">Quercus serrata</Emphasis>: its relation to seed characteristics

Intraspecific variation in seed production schedule and its effects on seed characteristics were examined in the deciduous oak species Quercus serrata in Iwate Prefecture, northern Japan. Seed production schedule differed markedly among 31 individual trees. The median dates of seed production differed by over 1 month between the earliest and latest individuals. Relationships between seed production schedule and seed characteristics were investigated at two different scales: the among-tree scale and the within-tree scale. At the within-tree scale, analyses revealed that the seeds dispersed by a tree later in the season tended to be larger in size and lower in tannin concentration. Similarly, at the among-tree scale, analysis revealed that trees with a later seed production schedule produced on average larger acorns. However, no clear tendency was observed between seed production schedule and mean tannin concentration. The observed variation in seed production schedule is likely to affect seed fate per se, and also via changes in seed characteristics.     

Leaflet morphometric variation of service tree (Sorbus domestica L.) in the Balkan Peninsula

Abstract

In most European countries, the service tree (Sorbus domestica L.) is a rare and threatened species and its conservation has been recognised as a priority. The aim of this study was to asses its morphologic variation in the western and central part of the Balkan Peninsula and in southern Central Europe. Three populations were analysed: one in Serbia, one in Bosnia and Herzegovina and one in Slovenia. In each population 30 trees were selected, and from each tree 30 leaves were collected for morphometric analysis based on nine leaflet morphological traits. Univariate (ANOVA) and multivariate (MANOVA) analysis of variance were used to estimate the variation within- and between populations and a discriminant analysis was performed to examine the structure of the between-population differences. The values of particular morphological traits found in our study did not differ considerably from the values reported elsewhere. The results revealed significant within- and between population variation. Variation within populations was highly significant for all the scored leaf morphological traits, while variation between populations was significant for all the studied traits except for the leaflet length. The discrimination between the three populations was significant. High percentages of correctly classified samples demonstrate good discriminating employability of the analysed leaf morphological traits and indicate differentiation of the analysed populations.     

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.     

Geometric morphometric analyses of leaf variation in four oak species of the subgenus Quercus (Fagaceae)

Abstract

This paper defines a protocol for the study of leaf morphology and morphometric relationships among Quercus frainetto, Quercus robur, Quercus petraea and Quercus pubescens. Leaf shapes were investigated in an attempt to characterize and discriminate among these four species. Landmark and outline data were compared in order to ascertain their relative efficacy for describing variation in leaf morphology. MANOVA was performed to assess intra‐specific differentiation and to compare the distribution of variance in individuals, populations and species. A strong correlation was found between landmark and outline datasets of the four species: both methods proved capable of discriminating among the four species. Finally, a Two‐Block Partial Least‐Squares analysis was performed for each species in order to assess the correlation between intra‐specific shape variation and environmental factors.     

Effects of range position,inter-annual variation and density on demographic transition rates of <Emphasis Type="Italic">Hornungia petraea</Emphasis> populations

The central-marginal model assumes unfavourable and more variable environmental conditions at the periphery of a species’ distribution range to negatively affect demographic transition rates, finally resulting in reduced population sizes and densities. Previous studies on density-dependence as a crucial factor regulating plant population growth have mainly focussed on fecundity and survival. Our objective is to analyse density-dependence in combination with the effect of inter-annual variation and range position on all life stages of an annual plant species, Hornungia petraea, including germination and seed incorporation into the seed bank. As previous studies on H. petraea had revealed a pattern opposite to existing theory with lower population densities at the distribution centre in Italy than at the periphery in Germany, we hypothesised that (1) demographic transition rates are lower, (2) the inter-annual variation in demographic transition rates is higher and (3) the intensity of density-dependence is weaker in Italy than in Germany. To analyse demographic transition rates, we used an autoregressive covariance strategy for repeated measures including density and inter-annual variation. All the three hypotheses were confirmed, but the impact of range position, density-dependence and inter-annual variation differed among the transition steps. All transition rates except fecundity were higher in the German populations than in the Italian populations. Germination rate and incorporation rate into the seed bank were strongly density-dependent. Central populations showed a larger inter-annual variation in fecundity and winter survival rate. Winter survival rate was the only transition step with a stronger density-dependence in peripheral populations. In most cases, these differences between distribution centre and periphery would not have emerged without taking density-dependence and inter-annual variation into account. We conclude that including range position, inter-annual variation and density-dependence in one single statistical model is an important tool for the interpretation of demographic patterns regarding the central-marginal model. Electronic Supplementary Material Supplementary material is available for this article at     

Effects of elevated CO<Subscript>2</Subscript> on foliar chemistry of saplings of nine species of tropical tree

This study examined the effects of elevated CO₂ on secondary metabolites for saplings of tropical trees. In the first experiment, nine species of trees were grown in the ground in open-top chambers in central Panama at ambient and elevated CO₂ (about twice ambient). On average, leaf phenolic contents were 48% higher under elevated CO₂. Biomass accumulation was not affected by CO₂, but starch, total non-structural carbohydrates and C/N ratios all increased. In a second experiment with Ficus, an early successional species, and Virola, a late successional species, treatments were enriched for both CO₂ and nutrients. For both species, nutrient fertilization increased plant growth and decreased leaf carbohydrates, C/N ratios and phenolic contents, as predicted by the carbon/nutrient balance hypothesis. Changes in leaf C/N levels were correlated with changes in phenolic contents for Virola (r=0.95, P<0.05), but not for Ficus. Thus, elevated CO₂, particularly under conditions of low soil fertility, significantly increased phenolic content as well as the C/N ratio of leaves. The magnitude of the changes is sufficient to negatively affect herbivore growth, survival and fecundity, which should have impacts on plant/herbivore interactions.     

Growth-unit structure in trees: effects of branch category and position on Nothofagus nervosa, N. obliqua and their hybrids (Nothofagaceae)

In plants with rhythmic growth, a branch segment extended in one event is known as growth unit (GU). GU structure, resulting from the resources allocated to stem length, volume and mass, and to leaf area and mass, is relevant for understanding branch functioning in the context of plant development. This study compares GU structure between main branches and short branches positioned at low and high positions on nursery-grown trees of three closely related genetic entities: Nothofagus nervosa, N. obliqua and natural hybrids between these species. GUs of short branches, compared to those of main branches, had lower length, diameter and number of leaves, and higher specific leaf area (SLA), stem density and proportional mass in leaves than in stems. GUs at high position on the trees had a higher proportion of their mass in stem than in leaves and a lower SLA than those at low position. Stem density was higher for N. nervosa and the hybrid trees than for N. obliqua. Most other GU traits did not differ statistically between the considered genetic entities. The three genetic entities exhibited distinct patterns of variation in leaf size with leaf position along main-branch GUs. The individual tree had a significant effect on most variables. GU structure would have a major ontogenetic component and would play a relevant role in the architecture of Nothofagus species and their adaptation to different environmental conditions.     

Gas exchange and hydraulic properties in the crowns of two tree species in a Panamanian moist forest

Hydraulic properties and gas exchange were measured in branches of two tropical tree species (Simarouba amara Aubl. and Tapirira guianensis Aubl.) in a moist lowland forest in Panama. Branch-level sapflow, leaf-level stomatal conductance, and water potential measurements, along with measurements of specific hydraulic conductivity of stems in crown tops, were used to relate hydraulic parameters to leaf conductance in two individuals of each species. Branches of the taller trees for each species (28 m, 31 m) showed much higher leaf-specific hydraulic conductance and leaf vapor-phase conductance than those of the smaller trees (18m, 23m). This was probably related to the leaf-to-sapwood area ratio in branches of taller trees, which was less than half that in branches of smaller trees. Dye staining showed evidence of massive cavitation in all trees, indicating that stomata do not control leaf water potential to prevent xylem cavitation in these species. Stomatal conductance of intact leaves also appeared to be insensitive to leaf area removal treatment of nearby foliage. Nevertheless, a simple mass-balance model of water flux combining hydraulic and vapor transport was in close agreement with observed maximal vapor-phase conductance in the four trees (r²=0.98, P=0.006). Our results suggest that the major organismal control over water flux in these species is by structural (leaf area) rather than physiological (stomatal) means.     

The effects of cambial age and position within the stem on specific conductivity in Douglas-fir (Pseudotsuga menziesii) sapwood

Specific conductivity (k_s, m²s^-1MPa^-1) describes the permeability of xylem and is determined by all aspects of xylem anatomy that create resistance to the flow of water. Here we test the hypothesis that k_s is a function of radial and vertical position within the stem, rather than solely a function of cambial age (ring number from the pith), by measuring k_s on samples excised from 35-year-old Douglas-fir [Pseudotsuga menziesii var. menziesii (Mirb.) Franco] trees at six heights and two or three radial positions. Sapwood k_s decreased from the cambium to the heartwood boundary, and the difference between outer and inner sapwood increased with height in the tree. Beneath the live crown, inner sapwood had 80-90% the k_s of outer sapwood, but only 55% just 10 m higher in the stem (about 10 nodes down from the tree top). Outer sapwood k_s peaked near the base of the crown and declined toward both the base and top of the stem. These patterns can be explained by two superimposed effects: the effect of cambial age on the dimensions of tracheids as they are produced, and the effect of xylem aging, which may include accumulation of emboli and aspiration of bordered pits. Tracheid lumen diameter and earlywood and latewood density and width, all factors known to vary with cambial age, were measured on different trees of the same age and from the same stand. Lumen diameter increased with cambial age, whereas the proportion of latewood and growth ring density increased after an initial decrease in the first 5 years. Our results suggest that the effect of cambial age on xylem anatomy is not sufficient to explain variation in k_s. Instead, physical position (both vertical and radial) in the stem and cambial age must be considered as determinants of conductivity.     

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.     

The genetic resolution of juvenile canopy structure and function in a three-generation pedigree of Populus

 A genetic approach to the understanding of tree architecture is to cross trees of contrasting features and to study their segregating F₂ progenies. For this purpose, members of a 3-generation pedigree, combining Populus trichocarpa, P. deltoides, and their F₁ and F₂ offspring, were grown side by side in a clonally replicated plantation. At 2 and 3 years of growth, tree architecture was analyzed at the stem, branch, and leaf levels. In all generations, proleptic branches were more numerous, longer, and had more and larger leaves than sylleptics initiated in the same year. The analysis of variance revealed significant genotypic effects on growth, branch and leaf biometrics in the F₂ family, with broad-sense heritabilities (H²) ranging from 0.50 to 0.80 for most traits. For branch and leaf traits, the H² values were found to vary among branch types and crown positions. In year 2, the degree of genetic control was stronger for sylleptics than proleptics and for upper than lower crown positions. These patterns were followed in year 3, except that H² values were more a function of position within crown, as a consequence of increased competition among trees. The genetic correlations between branch/leaf morphology and stem growth were also a function of branch type and crown position. Generally, traits on proleptics or at upper positions were more tightly correlated with height growth, whereas those on sylleptics or at lower positions, with basal area growth. By year 3, proleptic traits showed increased genetic correlations with both height and radial growth. The implications of these results for the construction of ideotypes are discussed. Received: 1 December 1995     

Morphological and genetic differentiation in populations of the dispersal‐limited coco de mer (Lodoicea maldivica): implications for management and conservation

Aims Developing plant conservation strategies requires knowledge of ecological and genetic processes underlying population dynamics. We aimed to quantify morphological and genetic differentiation among remnant populations of the iconic coco‐de‐mer palm Lodoicea maldivica. We hypothesized that limited gene flow among widely spaced populations would result in high genetic variation and large phenotypic differences among populations. Location Islands of Praslin and Curieuse (CU), Seychelles, Indian Ocean. Methods We conducted an extensive population survey and recorded morphological parameters for 447 Lodoicea in the main populations at Vallée de Mai (VM) and Fond Ferdinand (FF) on Praslin, and on CU. We collected leaf material from 180 trees in these populations for DNA genotyping using amplified fragment length polymorphisms. Results A total of 16,766 Lodoicea trees were recorded in the three populations (72.6% of Lodoicea on both islands). Lodoicea trees at VM and FF showed similar morphology, but differed in most parameters from those at CU, which were shorter, grew more slowly and produced fewer seeds. Mean overall genetic diversity was 0.337, and percentage of polymorphic loci was 91.1. Genetic diversity of the CU population was lower than that at VM and FF. There was weak genetic differentiation between CU and Praslin populations, but 99% of all genetic diversity was within populations. Main conclusions Trees on CU differed in growth and morphology from those of the two Praslin populations. These phenotypic differences, however, were not mirrored in the genetic structure of the populations. All populations were relatively genetically diverse with remarkably little differentiation among populations. This suggests that the capacity of Lodoicea to dominate across a range of habitats may be because of high phenotypic plasticity. High genetic connectivity may be maintained through long‐distance wind pollination. Given the uncertainty about the extent of underlying adaptive variation, we recommend that restoration projects avoid transferring seeds between island populations.     

Photosynthetic characteristics in canopies of Quercus rubra, Quercus prinus and Acer rubrum differ in response to soil water availability

Photosynthesis and related leaf characteristics were measured in canopies of co-occurring Quercus rubra L. (red oak), Quercus prinus L. (chestnut oak) and Acer rubrum L. (red maple) trees. Mature (20+ m tall) trees were investigated at sites of differing soil water availability within a catchment (a drier upper site and a wetter lower site). Leaf photosynthetic characteristics differed significantly between species and in response to site and position in the canopy. Photosynthetic capacity (A_max) was significantly greater at the wetter site in all canopy strata in A. rubrum but not in Q. rubra or Q. prinus. Our findings for A. rubrum are generally consistent with those predicting that species with higher specific leaf area (SLA) will have higher A_max per unit leaf nitrogen (N) and that species with leaves with lower SLA (e.g. Q. rubra and Q. prinus) will have shallower slopes of the A_max-N relationship. Importantly, the relationships between A_max and N_area (and by implication photosynthetic nitrogen-use efficiency, PNUE) differed in A. rubrum between the sites, with PNUE significantly lower at the drier site. The lower photosynthetic capacity and PNUE must substantially reduce carbon acquisition capacity in A. rubrum under these field conditions. Maximum stomatal conductance (g_smax) differed significantly between species, with g_smax greatest in Q. rubra and Q. prinus. In Q. rubra and Q. prinus, g_smax was significantly lower at the upper site than the lower site. There was no significant response of g_smax to site in A. rubrum. These stomatal responses were consistent with the C_i/C_a ratio, which was significantly lower in leaves of Q. rubra and Q. prinus at the upper site, but did not differ between sites in A. rubrum. Leaf '¹³C was significantly lower in A. rubrum than in either Q. rubra or Q. prinus at both sites. These findings indicate differences in stomatal behaviour in A. rubrum which are likely to contribute to lower water use efficiency at both sites. Our results support the hypothesis that the two Quercus species, in contrast to A. rubrum, maintain photosynthetic capacity at the drier site whilst minimising transpirational water loss. They also suggest, based primarily on physiological evidence, that the ability of A. rubrum to compete with other species of these deciduous forests may be limited, particularly in sites of low moisture availability and during low rainfall years.     

Does foliage on the same branch compete for the same water? Experiments on Douglas-fir trees

Do branchlets within a branch have autonomous water supplies, or do they share a common water supply system? We hypothesized that if branchlets shared a common water supply, then stomatal conductance (g_s) on sunlit foliage would increase with reduced transpiration of competing foliage on the branch. We reduced transpiration of other foliage on the branch through bagging and shading, and we monitored the gas-exchange responses of the remaining sunlit foliage on the branch relative to control branches for several age classes of Douglas-fir trees (aged ~10 years, 20 years, and 450 years old). Contrary to our hypothesis, we found no increases in g_s in either young or old trees following transient reductions in the amount of transpiring leaf area. The diurnal change in water potential, mid-day stomatal closure and associated photosynthetic decline occurred at the same time and were of the same magnitude on both treated and untreated branches, with the exception of photosynthesis in one 450-year-old tree. Hydraulic conductance measurements of branch junctions indicate that xylem within branches is only partially interconnected which would reduce the effectiveness of shading as a means of increasing water supply to the remaining sunlit foliage. The lack of a response implies that when a branch is in partial shade, the remaining sunlit foliage has no advantage with respect to water status over foliage on a branch completely in the sun.