Stem water storage capacity and diurnal patterns of water use were studied in five canopy trees of a seasonal tropical forest in Panama. Sap flow was measured simultaneously at the top and at the base of each tree using constant energy input thermal probes inserted in the sapwood. The daily stem storage capacity was calculated by comparing the diurnal patterns of basal and crown sap flow. The amount of water withdrawn from storage and subsequently replaced daily ranged from 4 kg d -1 in a 0·20-mdiameter individual of Cecropia longipes to 54 kg d -1 in a 1·02-m-diameter individual of Anacardium excelsum, representing 9-15% of the total daily water loss, respectively. Ficus insipida, Luehea seemannii and Spondias mombin had intermediate diurnal water storage capacities. Trees with greater storage capacity maintained maximum rates of transpiration for a substantially longer fraction of the day than trees with smaller water storage capacity. All five trees conformed to a common linear relationship between diurnal storage capacity and basal sapwood area, suggesting that this relationship was species-independent and size-specific for trees at the study site. According to this relationship there was an increment of 10 kg of diurnal water storage capacity for every 0·1 m 2 increase in basal sapwood area. The diurnal withdrawal of water from, and refill of, internal stores was a dynamic process, tightly coupled to fluctuations in environmental conditions. The variations in basal and crown sap flow were more synchronized after 1100 h when internal reserves were mostly depleted. Stem water storage may partially compensate for increases in axial hydraulic resistance with tree size and thus play an important role in regulating the water status of leaves exposed to the large diurnal variations in evaporative demand that occur in the upper canopy of seasonal lowland tropical forests.

The present study examines the manner in which several whole-tree water transport properties scale with speciesspecific variation in sapwood water storage capacity. The hypothesis that constraints on relationships between sapwood capacitance and other water relations characteristics lead to predictable scaling relationships between intrinsic capacitance and whole-tree behaviour was investigated. Samples of sapwood from four tropical forest canopy tree species selected to represent a range of wood density, tree size and architecture, and taxonomic diversity were used to generate moisture release curves in thermocouple psychrometer chambers, from which species-specific values of sapwood capacitance were calculated. Sapwood capacitance was then used to scale several whole-tree water transport properties determined from measurements of upper branch and basal sap flow, branch water potential, and axial and radial movement of deuterated water (D 2 O) injected into the base of the trunk as a tracer. Sapwood capacitance ranged from 83 to 416 kg m ----3 MPa ----1 among the four species studied and was strongly correlated with minimum branch water potential, soil-to-branch hydraulic conductance, daily utilization of stored water, and axial and radial movement of D 2 O. The species-independent scaling of several wholetree water transport properties with sapwood capacitance indicated that substantial convergence in plant function at multiple levels of biological organization was revealed by a simple variable related to a biophysical property of water transport tissue.

The savannas of central Brazil (cerrado) form the second most extensive plant formation in South America. In Brazil, the 2·0 × 10 6 km 2 of land area covered by cerrado vegetation is exceeded only by the Amazonian rain forest (Ratter 1992). The cerrado is characterized by a markedly seasonal rainfall regime with a 4-5 month dry season, old oligotrophic soils and frequent fires. The fires occur either naturally, or as part of a land-management system applied to increase the abundance and palatability of the relatively shallow-rooted grasses for cattle (Skole et al. 1994). The impact of the resulting decrease in density of the more deeply rooted trees and shrubs on ecosystem water fluxes is unknown.Cerrado comprises three principal physiognomic subtypes ranging from cerradão, medium to tall woodlands with closed or semi-closed canopies, to cerrado sensu stricto, a savanna woodland with 3. The dependence of maximum whole-plant sap flow rates on sapwood area was similar among all four species during both the wet and dry seasons. When total daily sap flow on a leaf area basis was normalized by the daily average air saturation deficit (ASD), only one of the four species showed significantly greater water use during the wet season. 4. Although seasonal differences in regulation of transpiration were not pronounced, strong stomatal limitation of both maximum daily transpiration rates and total daily transpiration was evident during both the wet and dry seasons. Sap flow typically increased sharply in the morning, briefly attained a maximum value by about 09.30-10.30 h, then decreased sharply, despite steadily increasing solar radiation and atmospheric evaporative demand. 5.The total leaf area-specific apparent hydraulic conductance of the soil/leaf pathway (G t ) varied among plants and diurnally. The identical linear dependence of transpiration and stomatal conductance (g s ) on G t among the four study species suggested that stomatal adjustment to variation in G t limited transpiration over the entire range of G t observed. 6. When g s was normalized for daily variation in G t , about 80% of the remaining variation in g s was associated with variation in ASD. The results suggested that transpiration in these species was not limited by soil water availability per se, but by high atmospheric evaporative demand and hydraulic constraints possibly arising from their deep rooting habit.

Summary 1.We employed standardized measurement techniques and protocols to describe the size dependence of whole-tree water use and cross-sectional area of conducting xylem (sapwood) among several species of angiosperms and conifers. 2. The results were not inconsistent with previously proposed 3/4-power scaling of water transport with estimated above-ground biomass. However, for a given size, angiosperms transported considerably greater quantities of water than conifers. 3. In the angiosperms studied, the scaling of water transport with sapwood area, stem diameter and above-ground biomass was best described by sigmoid functions rather than a power function, consistent with the previously reported size dependence of other processes such as growth. 4. At least three distinct species groupings for relationships between sapwood area and stem basal area were observed. Scaling of sapwood area with stem radius was well described by a power function of the form Y = Y 0 X b . However, exponents obtained for two of the three species groups differed significantly from a recently proposed theoretical value of 2·33.

1. Metrosideros polymorpha (O’hia), the dominant tree species in Hawaiian forest ecosystems, grows from sea level to treeline (2500 m). Consistent changes in its morphology and anatomy occur along this altitudinal/temperature gradient. Patterns of variation in photosynthetic gas exchange, leaf nitrogen content, nitrogen‐use efficiency, δ13C, and morphological and anatomical characteristics were determined across the elevational gradient. In addition, on‐line carbon isotope discrimination studies of high and low elevation M. polymorpha were performed. 2. Observed trends with increasing altitude were: (1) progressively higher carboxylation efficiency, leaf N content on an area basis, leaf mass per unit area (LMA), less negative foliar δ13C and (2) progressively smaller leaf size. Net CO2 assimilation (A) expressed on an area basis, leaf dry mass and N content per leaf remained relatively constant along the gradient. 3. Foliar δ13C became less negative with increasing elevation (– 30‰ at low elevation to – 24‰ at high elevation) and was strongly correlated with foliar N and LMA. Foliar δ13C was also correlated with variations in the ratio of intercellular to ambient partial pressure of CO2 (pi/pa), as determined by field gas‐exchange studies. 4. Results from on‐line fractionation experiments suggested that the relatively large internal resistance to CO2 diffusion did not differ between high and low elevation populations, despite differences in LMA. Less negative values of δ13C at high elevations and corresponding lower values of pi/pa were associated with increased carboxylation efficiency and N content on a unit leaf area basis. 5. Two major homeostatic responses in M. polymorpha plants along elevational/temperature gradients were observed: (1) maintenance of similar photosynthetic rates per unit leaf surface area despite suboptimal conditions for CO2 assimilation at high elevation and (2) similar N content per leaf despite lower soil N availability at high elevations. These homeostatic mechanisms allow M. polymorpha to maintain a relatively high level of growth‐related activities at high elevation, despite limiting environmental conditions.

Concurrent, independent measurements of stomatal conductance (g^), transpiration [E) and microenvironmental variables were used to characterize control of crown transpiration in four tree species growing in a moist, lowland tropical forest. Access to the upper forest canopy was provided by a construction crane equipped with a gondola. Estimates of boundary layer conductance (g,,) obtained with two independent methods permitted control of E to be partitioned quantitatively between gâ nd gh using ^ dimensionless decoupling coefficient (12) ranging from zero to 1. A combination of high g^ (c. 300-600 mmol m~^ s~') and low wind speed, and therefore relatively low g,, (c. 100-800 mmol m~^ s"'), strongly decoupled E from control by stomata in all four species (Q = 0-7-0-9). Photosyntbetic water-use efficiency was predicted to increase rather than decrease with increasing gs because g,, was relatively low and internal conductance to CO2 transfer was relatively high. Responses of g, to humidity were apparent only when the leaf surface, and not the bulk air, was used as the reference point for determination of external vapour pressure. However, independent measurements of crown conductance (g^.), a total vapour phase conductance that included stomatal and boundary layer components, revealed a clear decline in ge with increasing leaf-to-bulk air vapour pressure difference (V^), because the external reference points for determination of ge and V,, were compatible. The relationships between g^ and V,, and between g, and V, appeared to be distinct for each species. However, when gj, and gj were normalized by the branch-specific ratio of leaf area to sapwood area (LA/SA), a morphological index of potential transpirational demand relative to water transport capacity, a common relationship between conductance and evaporative demand for all four species emerged. Taken together, these results implied that, at a given combination of LA/SA and evaporative demand scaled to the appropriate reference point, the vapour phase conductance and therefore transpiration rates on a leaf area basis were identical in all four contrasting species studied.

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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