Patterns of stomatal conductance among blue oak (Quercus douglasii) size classes and populations: implications for seedling establishment

Quercus douglasii Hook. & Arn. (blue oak) is a deciduous white oak that is currently failing to regenerate throughout much of its range in California, USA. Patterns of water use were observed in adult trees, saplings and seedlings to determine if ontogenetic changes in water use occur, which might be important in the establishment of this long-lived perennial species in a Mediterranean-type system. Seasonal and diurnal stomatal conductance (g(s)), late-season predawn xylem water potentials (Psi(pre)), carbon isotopic ratio (delta(13)C) and soil water status were compared among the three size classes at three sites differing in mean precipitation and soil water characteristics. Comparisons were also made between microsites with and without regeneration (defined by the presence or absence of saplings). Overall patterns of water use were consistent among the three sites, except that, at the site with the highest rainfall, Q. douglasii plants had higher g(s) and more positive Psi(pre) values. Although no differences in water use patterns were found between regeneration and non-regeneration microsites, the observed ontogenetic differences in water use may have important implications for Q. douglasii establishment. Compared with adult trees and saplings, seedlings had higher gas exchange rates during periods of high soil water content (early in the season and in the morning). Seedling g(s) was correlated with percent extractable soil water (ESW) throughout the season; adult tree and sapling g(s) was correlated with ESW between June and September. Despite experiencing greater water stress (indicated by more negative Psi(pre) values) than older trees, seedlings had more negative delta(13)C values, implying lower water-use efficiencies.     

Lifting and storing bareroot blue oak (Quercus douglasii) seedlings

In California today, several species of native oaks are not regenerating adequately. Artificial regeneration is a means of ensuring sufficient recruitment to replace trees that die or are harvested, and restoring areas where trees have been cleared. Until recently, however, no bareroot oak seedlings were being produced in the state and there was little information to guide nursery operators. This study evaluated the potential of bareroot blue oak seedlings to survive and grow after outplanting. Results indicated that 1-0 nursery stock performed well in the field as long as seedlings were planted early enough in the season to take advantage of a favorable growing environment. Late lifting and long storage resulted in planting at a time when soils were already becoming dry and temperatures were hot. As a result, seedlings grew slowly or even died. Seedlings lifted early in the season (December) grew best. Either one or two months of storage had little effect on seedlings lifted at this time of year. However, seedlings lifted in January, February, and even early March performed adequately as long as they were in the ground by early March. In this study, root growth capacity was not a good predictor of subsequent field performance.     

Seasonal fluctuations and temperature dependence in photosynthetic parameters and stomatal conductance at the leaf scale of Populus euphratica Oliv

A combined model to simulate CO? and H?O gas exchange at the leaf scale was parameterized using data obtained from in situ leaf-scale observations of diurnal and seasonal changes in CO? and H?O gas exchange. The Farquhar et al.-type model of photosynthesis was parameterized by using the Bayesian approach and the Ball et al.-type stomatal conductance model was optimized using the linear least-squares procedure. The results show that the seasonal physiological changes in photosynthetic parameters (e.g., V(cmax25), J(max25), R(d25) and g(m25)) in the biochemical model of photosynthesis and m in the stomatal conductance model should be counted in estimating long-term CO? and H?O gas exchange. Overall, the coupled model successfully reproduced the observed response in net assimilation and transpiration rates.     

Impact of an exceptionally hot dry summer on photosynthetic traits in oak (Quercus pubescens) leaves

Climatic constraints on diurnal variations in photosynthetic traits were investigated in oaks (Quercus pubescens Willd.) growing in the Swiss Alps. The measurement period included the summer of 2003, when central Europe experienced a record-breaking heat wave. During the summer, a combination of moderate heat and drought caused a reduction in photosynthetic CO(2) assimilation rate (P(n)) by mid-morning, which increased by the afternoon. More extreme drought and heat caused a sharp day-long reduction in P(n). These effects were closely related to changes in stomatal conductance (g(s)), but low g(s) was unaccompanied by low intercellular CO(2) concentrations (C(i)). Around midday, a combination of heat and drought increased C(i), indicating metabolic limitation of photosynthesis. Chlorophyll a (Chl a) fluorescence measurements revealed reversible down-regulation of photosystem (PS) II activity during the day, which was accentuated by heat and drought and correlated with diurnal variation in zeaxanthin accumulation. A combination of heat and drought reduced leaf Chl a + b concentrations and increased ratios of total carotenoids, xanthophyll-cycle carotenoids and lutein to Chl a + b. The combination of summertime heat and drought altered the 77 K Chl fluorescence emission spectra of leaves, indicating changes in the organization of thylakoid membranes, but it had no effect on the amounts of the major light-harvesting Chl-a/b-binding protein of PSII (LHCII), Rubisco, Rubisco activase, Rubisco-binding protein (cpn-60), phosphoribulokinase and chloroplast ATP synthase. The results demonstrate that Q. pubescens can maintain photosynthetic capacity under adverse summer conditions.     

Light response of hydraulic conductance in bur oak (Quercus macrocarpa) leaves

A four- to seven-fold enhancement of leaf hydraulic conductance by light has been reported in three temperate tree species. The enhancement occurs in the liquid-flow pathway between the petiole and the site of water evaporation. The enhancement occurs within 1 h, and dissipates in darkness over a period of 1 to 10 h depending on species. Here we report light-induced enhancement of leaf hydraulic conductance in a fourth species, bur oak (Quercus macrocarpa Michx.), the dependence of the effect on light flux and color, its absence in leaves of seedlings, and the impact on the response of leaf vein severance and several metabolic inhibitors. The light response of leaf hydraulic conductance approached saturation at a photosynthetic photon flux of 150 mumol m(-2) s(-1). Hydraulic enhancement was greater in response to blue and green light than to visible radiation of longer wavelengths, although at the same irradiance, the response to white light was greater than to light of any single color. Atrazine (a photosystem II inhibitor), fusicoccin (which stimulates plasma membrane-bound H(+)-ATPase) and HgCl(2) (an aquaporin blocker) reduced the light response of leaf lamina hydraulic conductance. When 2-mercaptoethanol was added following mercury treatment, the light response was totally suppressed. Our results are consistent with the notion that the effect of light on leaf lamina hydraulic conductance is controlled by factors acting outside the leaf veins, possibly through light-induced changes in membrane permeability of either mesophyll or bundle sheath cells, or both.     

Competition for soil water between perennial bunch-grass (Elymus glaucus B.B.) and blue oak seedlings (Quercus douglasii H. & A.)

The competition effects of the perennial bunch-grass (Elymus glaucus B.B.) on the growth and survival of the oak seedlings (Quercus douglasii H. & A.) were investigated. There were four levels of Elymus competition, replicated three times. The three densities ofElymus employed were zero (control), 50 (Low — ‘L’ -), 116 (Medium — ‘M’ -) and 199 (High — ‘H’ -) plants m^?2. Rates of soil water depletion, stomatal conductance, transpiration, shoot elongation and leaf expansion rates were measured between 23 March and 26 May 1988. Rates of soil water depletion, stomatal conductance and transpiration differed amongst the treatments and were higher in the control for the duration of the experiment. Shoot elongation rate (SER) and leaf expansion rate (LER) of blue oak seedling were directly related to soil water potentials. Zero values of LER rates for all treatments were observed at soil water potentials lower than?1.91 MPa, and concurrent reductions of stomatal conductance indicated stomatal closure due to the soil water deficit. In the control treatment, transpiration alone was not high enough to deplete soil moisture and to reduce LER of the oak seedlings. Leaf dessication occurred first in the H and M treatments (53% of seedlings dessicated) and two weeks later in the L treatment (37% dessicated) when the soil water potential was approximately ?4.0 MPa. The number of reproductive tillers and seed dry weight indicated thatElymus plants were under water stress from April 25 and concluded on May 25 with an early summer dormancy in all treatments. Data indicated that light intensity of 50% of ambient did not limit the development of oak seedlings. The results suggested that density of the perennial bunch-grassElymus glaucus lower than 50 plants m^?2 could allow survival and successful establishment of blue oak in understories.     

Experimental study of the effect of soaking temperature on the peeling parameters of two oak species (Quercus canariensis Willd. and Quercus afares Pomel)

ABSTRACT

Veneer checking is a common enough occurrence in woodworking. It appears as cracks in the veneer and generally following the grain. Their appearance and the variation of their depth and frequencies have tremendous impact on their utilization. Finding a means of identifying and characterizing the veneer checks is an important ongoing challenge. An automated device, named SMOF, was developed in LABOMAP (Arts et Metiers – France) and achieves this task. By using the SMOF device, the lathe checks occurred in veneers of two species: zeen oak (Quercus canariensis Willd.) and afares oak (Quercus afares Pomel) were detected, automatically imaged and then, the depth and intervals of checks measured. The results were described by statistical distributions that exhibited abnormalities, such as skewness and kurtosis, which were assessed by mode analysis. It has been established that hot soaking temperature (from 60°C to 90°C) reduce slightly the cutting forces for both species, no produce significant heart checks within the loose side of veneers. However, low soaking temperature (from 50°C to 65°C) allow avoiding deeper lathe checking, producing shallower ones acceptable for veneer production.     

Persisting soil drought reduces leaf specific conductivity in Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens)

Leaf specific conductivity (LSC; the ratio of stem conductivity (K(P)) to leaf area (A(L))), a measure of the hydraulic capacity of the stem to supply leaves with water, varies with soil water content. Empirical evidence for LSC responses to drought is ambiguous, because previously published results were subject to many confounding factors. We tested how LSC of similar-sized trees of the same population, under similar climatic conditions, responds to persistently wet or dry soil. Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) trees were compared between a dry site and a wet site in the Valais, an inner alpine valley in Switzerland. Soil water strongly influenced A(L) and K(P) and the plant components affecting K(P), such as conduit radius, conduit density and functional sapwood area. Trees at the dry site had lower LSC than trees with the same stem diameter at the wet site. Low LSC in trees at the dry site was associated with a smaller functional sapwood area and narrower conduits, resulting in a stronger reduction in K(P) than in A(L). These observations support the hypothesis that trees maintain a homeostatic water pressure gradient. An alternative hypothesis is that relatively high investments in leaves compared with sapwood contribute to carbon gain over an entire season by enabling rapid whole-plant photosynthesis during periods of high water availability (e.g., in spring, after rain events and during morning hours when leaf-to-air vapor pressure deficit is small). Dynamic data and a hydraulic plant growth model are needed to test how investments in leaves versus sapwood and roots contribute to transpiration and to maximizing carbon gain throughout entire growth seasons.     

Drought tolerance and transplanting performance of holm oak (Quercus ilex) seedlings after drought hardening in the nursery

Drought stress is the main cause of mortality of holm oak (Quercus ilex L.) seedlings in forest plantations. We therefore assessed if drought hardening, applied in the nursery at the end of the growing season, enhanced the drought tolerance and transplanting performance of holm oak seedlings. Seedlings were subjected to three drought hardening intensities (low, moderate and severe) for 2.5 and 3.5 months, and compared with control seedlings. At the end of the hardening period, water relations, gas exchange and morphological attributes were determined, and survival and growth under mesic and xeric transplanting conditions were assessed. Drought hardening increased drought tolerance primarily by affecting physiological traits, with no effect on shoot/root ratio or specific leaf mass. Drought hardening reduced osmotic potential at saturation and at the turgor loss point, stomatal conductance, residual transpiration (RT) and new root growth capacity (RGC), but enhanced cell membrane stability. Among treated seedlings, the largest response occurred in seedlings subjected to moderate hardening. Severe hardening reduced shoot soluble sugar concentration and increased shoot starch concentration. Increasing the duration of hardening had no effect on water relations but reduced shoot mineral and starch concentrations. Variation in cell membrane stability, RT and RGC were negatively related to osmotic adjustment. Despite differences in drought tolerance, no differences in mortality and relative growth rate were observed between hardening treatments when the seedlings were transplanted under either mesic or xeric conditions.     

Canopy stomatal conductance and xylem sap abscisic acid (ABA) in mature Scots pine during a gradually imposed drought

We investigated the effect of drought on canopy stomatal conductance (g(c)), and examined the hypothesis that g(c) is controlled by the chemical messenger abscisic acid (ABA) produced in roots. Beginning in November 1994, we subjected a mature stand of Scots pine (Pinus sylvestris L.) to an imposed 11-month drought. Control plots were maintained at average-season soil water content. Xylem sap was extracted from shoots at regular intervals from April to November 1995. Soil water, sap flow and leaf water potentials (predawn to dusk) were recorded at the same time. Canopy stomatal conductance was calculated from sap flow data and xylem sap ABA concentrations ([ABA(xyl)]) were measured by radioimmunoassay. Mean [ABA(xyl)] in control trees was 250 micromol m(-3). No diurnal variation in [ABA(xyl)] was detected. With soil drying, [ABA(xyl)] increased to a maximum in summer (600 micromol m(-3)), but decreased again toward autumn; however, no significant increase in ABA flux to the leaves occurred. A decline in g(c) was detected when volumetric soil water content declined below 0.12. The decline in g(c) could not have been mediated by increasing [ABA(xyl)] because stomatal closure appeared to precede any increase in [ABA(xyl)]. Peak sap flow velocity data were used to estimate delivery times for root-to-shoot signals in 15-m tall trees. Under normal field conditions, a signal would take 12 days to travel from the site of production (roots) to the presumed site of action (shoots). However, under drought conditions it may take a chemical signal in excess of 6 weeks. We conclude that a feedforward model of short-term stomatal response to soil drying, based solely on the action of a chemical messenger from the roots, is not applicable in mature conifer trees because signal transmission is too slow.     

Kinetics of leaf temperature fluctuation affect isoprene emission from red oak (Quercus rubra) leaves

Because the rate of isoprene (2-methyl-1,3-butadiene) emission from plants is highly temperature-dependent, we investigated natural fluctuations in leaf temperature and effects of rapid temperature change on isoprene emission of red oak (Quercus rubra L.) leaves at the top of the canopy at Harvard Forest. Throughout the day, leaves often reached temperatures as much as 15 degrees C above air temperature. The highest temperatures were reached for only a few seconds at a time. We compared isoprene emission rates measured when leaf temperature was changed rapidly with those measured when temperature was changed slowly. In all cases, isoprene emission rate increased with increasing leaf temperature up to about 32 degrees C and then decreased with higher temperatures. The temperature at which isoprene emission rates began to decrease depended on how quickly measurements were made. Isoprene emission rates peaked at 32.5 degrees C when measured hourly, whereas rates peaked at 39 degrees C when measurements were made every four minutes. This behavior reflected the rapid increase in isoprene emission rate that occurred immediately after an increase in leaf temperature, and the subsequent decrease in isoprene emission rate when leaf temperature was held steady for longer than 20 minutes. We concluded that the observed temperature response of isoprene emission rate is a function of measurement protocol. Omitting this parameter from isoprene emission models will not affect simulated isoprene emission rates at mild temperatures, but can increase isoprene emission rates at high temperatures.     

Sprouts of shoot-clipped oak (Quercus alba and Q. robur) germinants show morphological and photosynthetic acclimation to contrasting light environments

New Forests - Sprouting by woody plants can increase species resilience to disturbance and foster regeneration during periods with little recruitment from seed. Though sprouting often plays a...     

Effects of partial defoliation on carbon and nitrogen partitioning and photosynthetic carbon uptake by two-year-old cork oak (Quercus suber) saplings

At the end of the growing season in late July, 20-month-old cork oak (Quercus suber L.) saplings were partially defoliated (63% of leaf area) to evaluate their ability to recover leaf area after defoliation. At 18 and 127 days after defoliation, changes in starch and nitrogen pools were determined in leaves and perennial organs, and variations in photosynthetic carbon uptake were investigated. To determine the role of stored nitrogen in regrowth after defoliation, plant nitrogen was labeled in the previous winter by enriching the nutrient solution with 15N. Plants recovered the lost leaf area in 127 days. Although there was remobilization of starch and nitrogen from leaves and perennial organs, the availability of resources for growth in the following spring was not decreased by defoliation. On the contrary, starch concentration in coarse roots was higher in defoliated saplings than in control saplings, presumably as a result of the higher net CO2 exchange rate in newly developed leaves compared with pre-existing leaves.     

Response of stomatal conductance to drought in ponderosa pine: implications for carbon and ozone uptake

To gain insight into the limitations imposed by a typical Mediterranean-climate summer drought on the uptake of carbon and ozone in the ponderosa pine (Pinus ponderosa Dougl. ex Laws.) ecosystem, we compared diurnal trends in leaf physiology of young trees in a watered and a control plot located in the Sierra Nevada Mountains, CA, USA (Blodgett Forest, 38 degrees 53' N, 120 degrees 37' W, 1315 m elevation). Predawn water potential of trees in the watered plot remained above -0.3 MPa throughout the growing season, whereas it dropped in the control plot from -0.24 to -0.52 MPa between late May and mid-August. Photosynthesis and stomatal conductance of trees in the watered plot were relatively insensitive to atmospheric vapor pressure deficit (VPD), whereas gas exchange of trees in the control plot varied with changes in soil water, VPD and temperature. Although the 1998 growing season was abnormally wet, we saw a pronounced drought effect at the control site. Over the 2 months following the onset of watering, carbon and ozone uptake were measured on three days at widely spaced intervals. Carbon uptake per unit leaf area by 1-year-old foliage of trees in the control plot was 39, 35 and 30% less, respectively, than in the watered plot, and estimated ozone deposition per unit leaf area (ozone concentration times stomatal conductance) was 36, 46 and 41% less.     

Influence of temperature and leaf-to-air vapor pressure deficit on net photosynthesis and stomatal conductance in red spruce (Picea rubens)

The roles of temperature (T) and leaf-to-air vapor pressure deficit (VPD) in regulating net photosynthesis (A(net)) and stomatal conductance (G(s)) of red spruce (Picea rubens Sarg.) were investigated in a field study and in a controlled environment experiment. Both A(net) and G(s) exhibited a relatively flat response to temperatures between 16 and 32 degrees C. Temperatures between 32 and 36 degrees C markedly decreased both A(net) and G(s). Vapor pressure deficits above 2 kPa had significant effects on both A(net) and G(s). The influence of VPD on A(net) and G(s) fit a linear response model and did not interact significantly with T effects.     

Low temperature during winter elicits differential responses among populations of the Mediterranean evergreen cork oak (Quercus suber)

Populations of cork oak (Quercus suber L.) were assessed for seasonal and inter-population variability in, and temperature responses of, the ratio between light-induced variable and maximum fluorescence of chlorophyll, Fv/Fm, considered a surrogate for the maximum photochemical efficiency of photosystem II (PSII). Seedlings from 10 populations throughout the distribution range of Q. suber in the Mediterranean basin were grown in a common garden in central Spain. The Fv/Fm ratio of dark-adapted leaves was measured at dawn every month for 2 years. Air temperature was recorded at a nearby climatic station. During the summer, when maximum air temperatures reached 40 degrees C, there were no significant differences in Fv/Fm among populations, but significant differences were seen during the winter. In colder months, Fv/Fm ranged in all populations between 0.5-0.6 and 0.2-0.3 in 2001 and 2002, respectively. The variance explained by the population effect was greatest during winter months, especially in 2002, reaching a peak value of 10% when minimum air temperature was below -10 degrees C. Populations originating from warmer sites showed the largest decline in Fv/Fm between the end of 2001 and the beginning of 2002. Thus, a negative linear relationship was established between mean annual temperature at the population source and population mean Fv/Fm recorded in the coldest month in 2002 and normalized by the Fv/Fm spring measurement.     

Seasonal photosynthetic responses to light and temperature in white spruce (Picea glauca) seedlings planted under an aspen (Populus tremuloides) canopy and in the open

Photosynthetic light and temperature response curves were measured seasonally in seedlings of white spruce (Picea glauca (Moench.) Voss) grown for two years in the understory of aspen (Populus tremuloides Michx.) or in the open in central Alberta. Light-saturated rate of net photosynthesis, the optimum temperature for net photosynthesis, transpiration rate, photochemical efficiency, and stomatal and mesophyll conductances increased from spring to summer and declined thereafter, whereas dark respiration rate and compensation and saturation points were highest in spring. Depression of photosynthetic parameters was greater in open-grown seedlings than in understory seedlings during the periods in spring and autumn when night frosts were common. Net photosynthetic rates were similar in understory and open-grown seedlings in summer, but they were significantly lower in open-grown seedlings in spring and autumn. Significantly lower transpiration rates and stomatal conductances in open-grown seedlings than in understory seedlings were also observed at 15 and 25 degrees C in the autumn. Shoot and needle growth were less in open-grown seedlings than in understory seedlings. In summer, when irradiances were low in the aspen understory, understory white spruce seedlings maintained a positive carbon balance by decreasing their compensation and saturation points and increasing their photochemical efficiency compared to spring and autumn.     

Provenance-specific growth responses to drought and air warming in three European oak species (Quercus robur, Q. petraea and Q. pubescens)

Provenance-specific growth responses to experimentally applied drought and air warming were studied in saplings of three European oak species: Quercus robur, Quercus petraea and Quercus pubescens. Four provenances of each species were grown in large open-top chambers and subjected to four climates: control, periodic drought, air warming or their combination in 3 subsequent years. Overall growth responses were found among species and provenances, with drought reducing shoot height growth and stem diameter growth and air warming stimulating shoot height growth but reducing stem diameter growth and root length growth. Differential growth responses in shoots, stems and roots resulted in altered allometric growth relations. Root length growth to shoot height growth increased in response to drought but decreased in response to air warming. Stem diameter growth to shoot height growth decreased in response to air warming. The growth responses in shoots and stems were highly variable among provenances indicating provenance-specific sensitivity to drought and air warming, but this response variability did not reflect local adaptation to climate conditions of provenance origin. Shoot height growth was found to be more sensitive to drought in provenances from northern latitudes than in provenances from southern latitudes, suggesting that genetic factors related to the postglacial immigration history of European oaks might have interfered with selective pressure at provenance origins.     

Underplanting to sustain future stocking of oak (Quercus) in temperate deciduous forests

New Forests - Oaks (Quercus spp.) are one of the most important tree taxa in the northern hemisphere. Although they are dominant in mixed species forests and widely distributed, there are frequent...