F. C. Meinzer

Environmental and physiological regulation of transpiration in tropical forest gap species: the influence of boundary layer and hydraulic properties

Environmental and physiological regulation of transpiration were examined in several gap-colonizing shrub and tree species during two consecutive dry seasons in a moist, lowland tropical forest on Barro Colorado Island, Panama. Whole plant transpiration, stomatal and total vapor phase (stomatal + boundary layer) conductance, plant water potential and environmental variables were measured concurrently. This allowed control of transpiration (E) to be partitioned quantitatively between stomatal (gs) and boundary layer (gb) conductance and permitted the impact of invividual environmental and physiological variables on stomatal behavior and E to be assessed. Wind speed in treefall gap sites was often below the 0.25 m s−1 stalling speed of the anemometer used and was rarely above 0.5 m s−1, resulting in uniformly low gb (c. 200–300 mmol m−2 s−1) among all species studied regardless of leaf size. Stomatal conductance was typically equal to or somewhat greater than gb. This strongly decoupled E from control by stomata, so that in Miconia argentea a 10% change in gs when gs was near its mean value was predicted to yield only a 2.5% change in E. Porometric estimates of E, obtained as the product of gs and the leaf-bulk air vapor pressure difference (VPD) without taking gb into account, were up to 300% higher than actual E determined from sap flow measurements. Porometry was thus inadequate as a means of assessing the physiological consequences of stomatal behavior in different gap colonizing species. Stomatal responses to humidity strongly limited the increase in E with increasing evaporative demand. Stomata of all species studied appeared to respond to increasing evaporative demand in the same manner when the leaf surface was selected as the reference point for determination of external vapor pressure and when simultaneous variation of light and leaf-air VPD was taken into account. This result suggests that contrasting stomatal responses to similar leaf-bulk air VPD may be governed as much by the external boundary layer as by intrinsic physiological differences among species. Both E and gs initially increased sharply with increasing leaf area-specific total hydraulic conductance of the soil/root/leaf pathway (Gt), becoming asymptotic at higher values of Gt. For both E and gs a unique relationship appeared to describe the response of all species to variations in Gt. The relatively weak correlation observed between gs and midday leaf water potential suggested that stomatal adjustment to variations in water availability coordinated E with water transport efficiency rather than bulk leaf water status.

Stomatal control of transpiration

The role of stomata in regulating transpiration from vegetation has historically been controversial among those working either at the single leaf, or at the extensive canopy scales. Recently, the role of unstirred air layers surrounding leaves and canopies in limiting the impact of stomatal movements on transpiration has received renewed recognition. This has led to notable progress in quantitatively describing the effectiveness of stomata in controlling transpiration and in reconciling contrasting viewpoints concerning the role of stomata at the leaf, stand and regional scales. Considerable progress has also been made in understanding how variations in aerial factors such as evaporative demand and edaphic factors such as soil water availability are sensed and transduced into appropriate stomatal regulatory responses. These developments indicate that studies carried out at multiple scales of observation are needed to understand how external environmental factors and intrinsic plant properties interact to determine the role of stomata in regulating transpiration from different types of vegetation.

Partitioning of water resources among plants of a lowland tropical forest

Source water used by plants of several species in a semi-evergreen lowland tropical forest on Barro Colorado Island, Panama, was assessed by comparing the relative abundance of deuterium, D, versus hydrogen, H (stable hydrogen isotope composition, δD) in xylem sap and in soil water at different depths, during the dry season of 1992. Ecological correlates of source water were examined by comparing xylem water δD values with leaf phenology, leaf water status determined with a pressure chamber, and rates of water use determined as mass flow of sap using the stem heat balance method. Soil water δD values decreased sharply to 30 cm, then remained relatively constant with increasing depth. Average δD values were-13‰, for 0–30 cm depth and-36.7‰ for 30–100 cm depth. Soil water δD values were negatively associated with soil water content and soil water potential. Concurrent analyses of xylem water revealed a high degree of partitioning of water resources among species of this tropical forest. Xylem water δD of deciduous trees (average=-25.3±1.4‰) was higher than that of evergreen trees (average=-36.3±3.5‰), indicating that evergreen species had access to the more abundant soil water at greater depth than deciduous species. In evergreen shade-tolerant and high-light requiring shrubs and small trees, δD of xylem water was negatively correlated with transpiration rate and leaf water potential indicating that species using deeper, more abundant water resources had both higher rates of water use and more favorable leaf water status.

Water relations of coastal and estuarine Rhizophora mangle : xylem pressure potential and dynamics of embolism formation and repair

Physiological traits related to water transport were studied in Rhizophora mangle (red mangrove) growing in coastal and estuarine sites in Hawaii. The magnitude of xylem pressure potential (Px), the vulnerability of xylem to cavitation, the frequency of embolized vessels in situ, and the capacity of R. mangle to repair embolized vessels were evaluated with conventional and recently developed techniques. The osmotic potential of the interstitial soil water (πsw) surrounding the roots of R. mangle was c. –2.6±5.52×10–3 and –0.4±6.13×10–3xa0MPa in the coastal and estuarine sites, respectively. Midday covered (non-transpiring) leaf water potentials (ΨL) determined with a pressure chamber were 0.6–0.8xa0MPa more positive than those of exposed, freely-transpiring leaves, and osmotic potential of the xylem sap (πx) ranged from –0.1 to –0.3xa0MPa. Consequently, estimated midday values of Px (calculated by subtracting πx from covered ΨL) were about 1xa0MPa more positive than ΨL determined on freely transpiring leaves. The differences in ΨL between covered and transpiring leaves were linearly related to the transpiration rates. The slope of this relationship was steeper for the coastal site, suggesting that the hydraulic resistance was larger in leaves of coastal R. mangle plants. This was confirmed by both hydraulic conductivity measurements on stem segments and high-pressure flowmeter studies made on excised leafy twigs. Based on two independent criteria, loss of hydraulic conductivity and proportions of gas- and liquid-filled vessels in cryo-scanning electron microscope (cryo-SEM) images, the xylem of R. mangle plants growing at the estuarine site was found to be more vulnerable to cavitation than that of plants growing at the coastal site. However, the cryo-SEM analyses suggested that cavitation occurred more readily in intact plants than in excised branches that were air-dried in the laboratory. Cryo-SEM analyses also revealed that, in both sites, the proportion of gas-filled vessels was 20–30% greater at midday than at dawn or during the late afternoon. Refilling of cavitated vessels thus occurred during the late afternoon when considerable tension was present in neighboring vessels. These results and results from pressure-volume relationships suggest that R. mangle adjusts hydraulic properties of the water-transport system, as well as the leaf osmotic potential, in concert with the environmental growing conditions.

Forest growth along a rainfall gradient in Hawaii: Acacia koa stand structure, productivity, foliar nutrients, and water- and nutrient-use efficiencies

We tested whether variation in growth of native koa (Acacia koa) forest along a rainfall gradient was attributable to differences in leaf area index (LAI) or to differences in physiological performance per unit of leaf area. Koa stands were studied on western Kauai prior to Hurricane Iniki, and ranged from 500 to 1130 m elevation and from 850 to 1800 mm annual precipitation. Koa stands along the gradient had basal area ranging from 8 to 42 m2/ha, LAI ranging from 1.4 to 5.4, and wood increment ranging from 0.7 to 7.1 tonnes/ha/year. N, P, and K contents by weight of sun leaves (phyllodes) were negatively correlated with specific leaf mass (SLM, g m-2) across sites; on a leaf area basis, N increased whereas P and K decreased with SLM. LAI, aboveground woody biomass increment, and production per unit leaf area (E) increased as phyllode δ13C became more negative. The δ13C data suggested that intrinsic water-use efficiency (ratio of assimilation to conductance) increased as water availability decreased. In five of the six sites, phyllode P contents increased as LAI increased, but biomass increment and E were not correlated with phyllode nutrient contents, suggesting that productivity was limited more by water than by nutrient availability. Because vapor pressure deficits increased with decreasing elevation, actual water-use efficiency (ratio of assimilation to transpiration) was lower at drier, low-elevation sites. There was a trade-off between intrinsic water-use efficiency and production per unit of canopy N or P across the gradient. In summary, koa responds to water limitation both by reducing stand LAI and by adjusting gas exchange, which results in increased intrinsic water-use efficiency but decreased E.

Regulation of transpiration in field-grown sugarcane: Evaluation of the stomatal response to humidity with the Bowen ratio technique

Abstract Parallel time courses of net radiation, canopy conductance and evapotranspiration in a sugarcane canopy demonstrate the interaction of environmental and physiological factors in regulating water loss. The Bowen ratio-energy balance technique was used to evaluate the stomatal response to humidity in the absence of potential artifacts associated with porometer and leaf chamber measurements. Canopy conductance ( g c ) responded to leaf-air vapor pressure difference ( V ), supporting the validity of available physiological data showing clear stomatal responses to V in porometers and leaf chambers. Several factors were found to obscure the stomatal response to V under field conditions. The stomatal closing stimulus, V , and the opening stimulus, photon flux density ( I ), were positively correlated. This caused offsetting stomatal responses to V and I , which reduced the net stomatal response when V changed with radiation-driven changes in leaf temperature. Normalization of g c by I revealed the expected hyperbolic decline of g c with increasing V . Low boundary layer conductance attenuated V imposed at the leaf surface relative to V measured with reference to humidity in the bulk atmosphere. This reduced the stimulus for stomatal response to V , below that indicated by agrometeorological measurements. Use of air saturation deficit ( D ) rather than V to express evaporative demand overestimates V , minimizing apparent stomatal sensitivity to evaporative demand, when leaves are cooler than the air. These factors may result in small stomatal responses to V that escape detection by indirect measures of stomatal movement, such as those based on the temperature of exposed leaves.

Stem photosynthesis in Psorothamnus spinosus (smoke tree) in the Sonoran desert of California

SummaryStem photosynthetic responses to environmental parameters were investigated with Psorothamnus spinosus in the Sonoran Desert of California. Light saturation of stem photosynthesis was equal to maximum midday summer irradance (1600–2000 μmol·m-2·s-1). The optimum temperature for stem photosynthesis was 39°C, and lower stem temperatures (27–35°C) caused significant decreases (up to 50%) in stem photosynthesis. Positive stem photosynthesis was maintained up to 51°C. Stem photosynthesis was relatively insensitive to increasing vpd up to 5 kPa; However, stem conductance decreased by 25% at a vpd of 5 kPa. At vpd greater than 5 kPa stem photosynthesis decreased relatively more than that of stem conductance causing a decrease in water use efficiency and an increase an intercellular carbon dioxide concentration. Maximum stem photosynthetic rates were low (6.2–10.6 μmol·m-2·s-1) on a stem surface area, but, stem photosynthetic rates of young shoots were substantially higher (19.5–33.3 μmol· m-2·s-1) on a projected area basis.

Energy balance and latent heat flux partitioning in coffee hedgerows at different stages of canopy development

The energy balance of drip-irrigated coffee (Coffea arabica L. cv. Yellow Catuai) hedgerows was evaluated at different stages of canopy development using the Bowen ratio-energy balance technique. Simultaneous measurements of mass flow of water through the coffee stems using the heat balance method allowed total latent heat flux (λE) to be partitioned between crop canopy (λEc) and soil and interrow vegetation (λEs) components. The average Bowen ratio decreased from 0.92 at leaf area index (L) of 1.4 to 0.36 at L = 6.7. Differences in the Bowen ratio between 2 consecutive years appeared to be related to stomatal response to leaf-to-air vapor pressure difference (V) and variations in net radiation (Rr). Latent heat loss was the most important component of the energy balance at all stages of canopy development, while sensible heat flux (H) remained relatively constant. Soil heat flux (G) invariably declined as L increased. λEc was the major form of latent heat loss at all stages of canopy development except at L = 1.4, where λEs accounted for 60% of λE. However, the magnitude of λEs declined with canopy development, with λEc accounting for nearly 100% of total λE at L = 6.7. Withholding irrigation dramatically influenced the partitioning of energy, strongly reducing λEs and increasing H. After irrigation was discontinued λEc dropped rapidly, and the contribution of λEs to λE became more important. After irrigation was resumed, a rapid recovery of all energy balance components to their previous values was observed. At low L, energy balance and latent heat partitioning of coffee hedgerows grown under wide spacing resembled the behavior of sparse row crops, but the development of a tall, dense canopy caused available energy to be partitioned in a way typically found in closed canopies such as those of forests.

Determinants of thermal balance in the Hawaiian giant rosette plant, Argyroxiphium sandwicense

The effects of leaf pubescence and rosette geometry on thermal balance were studied in a subspecies of a Hawaiian giant rosette plant, Argyroxiphium sandwicense. This species, a member of the silversword alliance, grows above 2000 m elevation in the alpine zone of two Hawaiian volcanoes. Its highly pubescent leaves are very reflective (absorptance in the 400–700 nm waveband=0.44). Temperature of the expanded leaves was very similar to, or even lower than, air temperature during clear days, which was somewhat surprising given that solar radiation at the high elevation sites where this species grows can exceed 1100 W m−2. However, the temperature of the apical bud, which is located in the center of the parabolic rosette, was usually 25°C higher than air temperature at midday. Experimental manipulations in the field indicated that incoming solar radiation being focussed towards the center of the rosette resulted in higher temperatures of the apical bud. Attenuation of wind speed inside the rosette, which increased the thickness of the boundary layer surrounding the apical bud, also contributed to higher temperatures. The heating effect on the apical bud of the large parabolic rosette, which apparently enhances the rates of physiological processes in the developing leaves, may exclude the species from lower elevations by producing lethal tissue temperatures. Model simulations of apical bud temperatures at different elevations and laboratory estimates of the temperature threshold for permanent heat injury predicted that the lower altitude limit should be approximately 1900 m, which is reasonably close to the lower limit of distribution of A. sandwicense on Haleakala volcano.