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PubMed1996

Photosynthetic and stomatal responses to high temperature and light in two oaks at the western limit of their range.

Bur oak (Quercus macrocarpa Michx.) and chinquapin oak (Q. muehlenbergii Engl.) leaves were exposed to high temperatures at various photosynthetic photon flux densities under laboratory conditions to determine if species-specific responses to these factors were consistent with the distribution of these oaks in gallery forests in the tallgrass prairies of northeastern Kansas, USA. Measurements of the ratio of chlorophyll fluorescence decrease, R(fd), indicated that chinquapin oak maintained greater photosynthetic capacity than bur oak across all tested combinations of irradiance (100, 400, 700 and 1000 micro mol m(-2) s(-1)) and temperature (40, 42, 44, 46 and 48 degrees C). In both oak species, manipulation of leaf temperature to about 47 degrees C for 45 min in the field led to a 45% decrease in carbon assimilation up to one week after the heat treatment, and to sharp reductions in stomatal conductance. Photosynthetic recovery patterns indicated that bur oak took longer to recover from heat stress than chinquapin oak, suggesting that heat stress may be important in determining distribution patterns of these oak species. Based on a comparison of the results with data from other forest species, we conclude that the photosynthetic temperature tolerances of bur oak and chinquapin oaks facilitate their dominance at the western limit of the eastern deciduous forest.

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PubMed1997

Environmental controls over isoprene emission in deciduous oak canopies.

In summer 1992, isoprene emission was measured on intact leaves and branches of Quercus alba (L.) at two heights in a forest canopy. Isoprene emission capacity (measured at 30 degrees C and a photosynthetic photon flux density of 1000 micro mol m(-2) s(-1)) was significantly higher in sun leaves than in shade leaves when expressed on a leaf area basis (51 versus 31 nmol m(-2) s(-1); P < 0.01). Because leaf mass per unit area (LMA, g m(-2)) was higher in sun leaves than in shade leaves, emissions of sun and shade leaves expressed on a dry mass basis did not differ significantly (99 versus 89 micro g C g(DW) (-1) h(-1); P = 0.05). Similar measurements in 1995 were consistent with the 1992 data, but data from leaves in more shaded locations demonstrated that isoprene emission capacity decreased with decreasing growth irradiance, irrespective of units of expression. Isoprene emission capacity in leaves of Q. coccinea Muenchh. and Q. velutina Lam. also declined steeply with canopy depth. Emission capacity, on a dry mass basis, showed no obvious pattern with canopy position in Q. prinus L. There was no difference in the temperature response of sun versus shade leaves of Q. alba, but shade leaves exhibited a greater quantum efficiency and saturated at lower irradiance than sun leaves. Rates of isoprene emission measured on branches of Q. alba were approximately 60% of those measured on individual leaves, as a result of self-shading within branch enclosures. It is recommended that within-canopy variation in isoprene emission capacity be incorporated into regional emission models.

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PubMed1997

Stomatal conductance, growth and root signaling in young oak seedlings subjected to partial soil drying.

Leaf conductance, water relations, growth, and abscisic acid (ABA) concentrations in xylem sap, root apices and leaves were assessed in oak seedlings (Quercus robur L.) grown with a root system divided between two compartments and subjected to one of four treatments: (a) well watered, WW; (b) half of root system exposed to soil drying and half kept well watered, WD; (c) whole root system exposed to drought, DD; and (d) half of root system severed, RE. Sharp decreases in plant stomatal conductance, leaf water potential, hydraulic conductance and leaf growth were observed during DD treatment. No significant differences in plant leaf water potential and stomatal conductance were detected between the WW and WD treatments. Nevertheless, the WD treatment resulted in inhibition of leaf expansion and stimulation of root elongation only in the well-watered compartment. Abscisic acid concentrations did not change in leaves, root tips, or xylem sap of WD- compared to WW-treated plants. Increased concentrations of ABA were observed in xylem sap from DD-treated plant roots, but the total flux of ABA to shoots was reduced compared to that in WW-treated plants, because of decreases in transpiration flux. Similar plant responses to the WD and RE treatments indicate that the responses observed in the WD-treated plants were probably not triggered by a positive signal originating from drying roots.

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PubMed1997

Acclimation of shade-developed leaves on saplings exposed to late-season canopy gaps.

We hypothesized that photoinhibition of shade-developed leaves of deciduous hardwood saplings would limit their ability to acclimate photosynthetically to increased irradiance, and we predicted that shade-tolerant sugar maple (Acer saccharum Marsh.) would be more susceptible to photoinhibition than intermediately shade-tolerant red oak (Quercus rubra L.). After four weeks in a canopy gap, photosynthetic rates of shade-developed leaves of both species had increased in response to the increase in irradiance, although final acclimation was more complete in red oak. However, photoinhibition occurred in both species, as indicated by short-term reductions in maximum rates of net photosynthesis and the quantum yield of oxygen evolution, and longer-term reductions in the efficiency of excitation energy capture by open photosystem II (PSII) reaction centers (dark-adapted F(v)/F(m)) and the quantum yield of PSII in the light (phi(PSII)). The magnitude and duration of this decrease were greater in sugar maple than in red oak, suggesting greater susceptibility to photoinhibition in sugar maple. Photoinhibition may have resulted from photodamage, but it may also have involved sustained rates of photoprotective energy dissipation (especially in red oak). Photosynthetic acclimation also appeared to be linked to an ability to increase leaf nitrogen content. Limited photosynthetic acclimation in shade-developed sugar maple leaves may reflect a trade-off between shade-tolerance and rapid acclimation to a canopy gap.

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PubMed1998

Leaf distribution in large trees and stands of the floodplain forest in southern Moravia.

Vertical distributions of leaf dry mass (M(d)) and leaf area (A(f)) were related to relative irradiance (I(r); I(r) above the stand = 1) in closed-canopy, old-growth stands of the floodplain forest in southern Moravia composed largely of Quercus, Fraxinus and Tilia species. Foliage area and mass at any given canopy height were converted to solar equivalent leaf area (A(s)) and mass (M(s)) by multiplying actual values at a given level in the canopy by the relative irradiance at that position. Stand leaf area index (LAI) was 5 (7 including shrub and herb layer), and solar equivalent parameters reached about 25% of that amount. In all species, vertical profiles of both relative irradiance and leaf dry mass to area ratio (LMA) were sigmoidal and the two variables were linearly related. The dominant, upper canopy species had a larger proportion of solar equivalent foliage than suppressed understory species. For individual trees of all species, the upper canopy had a larger proportion of solar equivalent foliage than the lower canopy. Light compensation points at both the leaf and whole-tree level were defined according to leaf or tree position, size and structure. I conclude that optimization of A(s) for forest stands may be used as a basis for determining thinning schedules and evaluating tree survival after damage to tree crowns by various factors.

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PubMed1998

Water relations of several hardwood species in response to throughfall manipulation in an upland oak forest during a wet year.

We investigated the effects of altered precipitation on leaf osmotic potential at full turgor (Psi(pio)) of several species in an upland oak forest during the 1994 growing season as part of a Throughfall Displacement Experiment at the Walker Branch Watershed near Oak Ridge, Tennessee. The main species sampled included overstory chestnut oak (Quercus prinus L.), white oak (Q. alba L.), red maple (Acer rubrum L.); intermediates sugar maple (A. saccharum L.) and blackgum (Nyssa sylvatica Marsh.); and understory dogwood (Cornus florida L.) and red maple. The precipitation treatments were: ambient precipitation; ambient minus 33% of throughfall (dry); and ambient plus 33% of throughfall (wet). Except in late September, midday leaf water potentials (Psi(l)) were generally high in all species in all treatments, ranging from -0.31 to -1.34 MPa for C. florida, -0.58 to -1.51 MPa for A. rubrum, and -0.78 to -1.86 MPa for Q. prinus. Both treatment and species differences in Psi(pio) were evident, with oak species generally exhibiting lower Psi(pio) than A. saccharum, A. rubrum, C. florida, and N. sylvatica. The Psi(pio) of C. florida saplings declined in the dry treatment, and Q. prinus, Q. alba, and A. saccharum all exhibited a declining trend of Psi(pio) in the dry treatment, although Psi(pio) of Q. prinus leaves increased in late August, corresponding to a recovery in soil water potential. Cornus florida exhibited osmotic adjustment with the largest adjustment coinciding with the period of lowest soil water potential in June. The only other species to exhibit osmotic adjustment was Q. prinus, which also maintained a lower baseline Psi(pio) than the other species. We conclude that a 33% reduction of throughfall is sufficient both to alter the water relations of some species in the upland oak forest and to enable the identification of those species capable of osmotic adjustment to a short-term drought during a wet year.

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PubMed1998

Drought tolerance, xylem sap abscisic acid and stomatal conductance during soil drying: a comparison of young plants of four temperate deciduous angiosperms.

Patterns of water relations, xylem sap abscisic acid (ABA) concentration ([ABA]) and stomatal aperture were compared in drought-sensitive black walnut (Juglans nigra L.) and black willow (Salix nigra Marsh.), less drought-sensitive sugar maple (Acer saccharum Marsh.) and drought-tolerant white oak (Quercus alba L.). Strong correlations among reduction in predawn water potential, increase in xylem sap [ABA] and stomatal closure were observed in all species. Stomatal response was more highly correlated with xylem [ABA] than with ABA flux. Xylem sap pH and ion concentrations appeared not to play a major role in the stomatal response of these species. Stomata were more sensitive to relative changes in [ABA] in drought-sensitive black walnut and black willow than in sugar maple and white oak. In the early stages of drought, increased [ABA] in the xylem sap of black walnut and black willow was probably of root origin and provided a signal to the shoot of the water status of the roots. In sugar maple and white oak, leaf water potential declined with the onset of stomatal closure, so that stomatal closure also may have occurred in response to the change in leaf water potential.

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PubMed1998

Drought tolerance, xylem sap abscisic acid and stomatal conductance during soil drying: a comparison of canopy trees of three temperate deciduous angiosperms.

Patterns of water relations, xylem sap abscisic acid concentration ([ABA]) and stomatal aperture were characterized and compared in drought-sensitive black walnut (Juglans nigra L.), less drought-sensitive sugar maple (Acer saccharum Marsh.) and drought-tolerant white oak (Quercus alba L.) trees co-occurring in a second-growth forest in Missouri, USA. There were strong correlations among reduction in predawn leaf water potential, increased xylem sap [ABA] and stomatal closure in all species. Stomatal conductance was more closely correlated with xylem sap ABA concentration than with ABA flux or xylem sap pH and cation concentrations. In isohydric black walnut, increased concentrations of ABA in the xylem sap appeared to be primarily of root origin, causing stomatal closure in response to soil drying. In anisohydric sugar maple and white oak, however, there were reductions in midday leaf water potential associated with stomatal closure, making it uncertain whether drought-induced xylem sap ABA was of leaf or root origin. The role of root-originated xylem sap ABA in these species as a signal to the shoot of the water status of the roots is, therefore, less certain.

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PubMedDate not supplied

A review of whole-plant water use studies in tree.

Weighing lysimeters, large-tree potometers, ventilated chambers, radioisotopes, stable isotopes and an array of heat balance/heat dissipation methods have been used to provide quantitative estimates of whole-tree water use. A survey of 52 studies conducted since 1970 indicated that rates of water use ranged from 10 kg day(-1) for trees in a 32-year-old plantation of Quercus petraea L. ex Liebl. in eastern France to 1,180 kg day(-1) for an overstory Euperua purpurea Bth. tree growing in the Amazonian rainforest. The studies included in this survey reported whole-tree estimates of water use for 67 species in over 35 genera. Almost 90% of the observations indicated maximum rates of daily water use between 10 and 200 kg day(-1) for trees that averaged 21 m in height. The thermal techniques that made many of these estimates possible have gained widespread acceptance, and energy-balance, heat dissipation and heat-pulse systems are now routinely used with leaf-level measurements to investigate the relative importance of stomatal and boundary layer conductances in controlling canopy transpiration, whole-tree hydraulic conductance, coordinated control of whole-plant water transport, movement of water to and from sapwood storage, and whole-plant vulnerability of water transport to xylem cavitation. Techniques for estimating whole-tree water use complement existing approaches to calculating catchment water balance and provide the forest hydrologist with another tool for managing water resources. Energy-balance, heat dissipation and heat-pulse methods can be used to compare transpiration in different parts of a watershed or between adjacent trees, or to assess the contribution of transpiration from overstory and understory trees. Such studies often require that rates of water use be extrapolated from individual trees to that of stands and plantations. The ultimate success of this extrapolation depends in part on whether data covering short time sequences can be applied to longer periods of time. We conclude that techniques for estimating whole-tree water use have provided valuable tools for conducting basic and applied research. Future studies that emphasize the use of these techniques by both tree physiologists and forest hydrologists should be encouraged.

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PubMedDate not supplied

Fine roots dynamics in a Mediterranean forest: effects of drought and stem density.

Summer drought is a characteristic trait of Mediterranean climates and can limit primary production. The holm oak (Quercus ilex L.) forest at the Prades Experimental Complex of Catchments (PECC, NE Spain) was managed by clearcutting until the 1950s. As a result of this practice, the forest has a high density (13000 stems ha(-1)) that is causing growth stagnation and intense competition for water. A thinning experiment was conducted to release resources, reduce stem competition and improve forest productivity. Minirhizotrons were installed in two plots in each of the thinned and control stands and fine root dynamics quantified to a soil depth of 60 cm over a 2-year period (June 1994-May 1996). Soil water content was higher and soil water penetrated deeper in the soil (30-50 cm) in the thinned plots than in the control plots. In the control plots, soil temperature did not vary substantially with depth (mean = 11.65 degrees C), whereas soil temperature was higher in shallow horizons compared with deep horizons in the thinned plots. Mean soil temperature was one degree higher in the thinned plots than in the control plots (12.9 versus 11.9 degrees C). There were always more roots in the top 20 cm of soil in thinned plots than in control plots. In the thinned plots, more than 50% of root density (root no. cm(-2)) was located in the top 20 cm of soil, whereas root density never reached 50% in the 0-20 cm soil layer of the control plots. Roots longevity varied from 35 to 471 days, and there were significant differences between soil depths. Root longevity was greater in the control plots than in the thinned plots.

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PubMed1999

Seasonal variations in leaf area index, leaf chlorophyll, and water content; scaling-up to estimate fAPAR and carbon balance in a multilayer, multispecies temperate forest.

Seasonal differences in phenology between coniferous and deciduous tree species need to be considered when developing models to estimate CO(2) exchange in temperate forest ecosystems. Because seasonal variations in CO(2) flux in temperate forests are closely correlated with plant phenology, we quantified the phenology of forest species in a multilayered forest with patches of Scots pine (Pinus sylvestris L.) and oak (Quercus robur L.) in Brasschaat, Belgium. A scaling-up modeling approach was developed to simulate reflectance at the leaf and canopy scales over a one-year cycle. Chlorophyll concentration, water content, specific leaf area and leaf area index of the forest species were measured throughout an entire year (1997). Scaling-up from the leaf to canopy was achieved by linking the PROSPECT and SAIL models. The result is the annual progression of the fraction of absorbed photosynthetically active radiation (fAPAR) in a 1 km(2) forest area, which can be directly related to high-resolution, remotely sensed data.

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PubMed1999

Interspecific and environmentally induced variation in foliar dark respiration among eighteen southeastern deciduous tree species.

We measured variations in leaf dark respiration rate (Rd) and leaf nitrogen (N) across species, canopy light environment, and elevation for 18 co-occurring deciduous hardwood species in the southern Appalachian mountains of western North Carolina. Our overall objective was to estimate leaf respiration rates under typical conditions and to determine how they varied within and among species. Mean dark respiration rate at 20 degrees C (Rd,mass, micromol CO2 per kg leaf dry mass per s) for all 18 species was 7.31 micromol per kg per s. Mean Rd,mass of individual species varied from 5.17 micromol per kg per s for Quercus coccinea Muenchh. to 8.25 micromol per kg per s for Liriodendron tulipifera L. Dark respiration rate varied by leaf canopy position and was higher in leaves collected from high-light environments. When expressed on an area basis, dark respiration rate (Rd,area, micromol CO2 per kg leaf dry area per s) showed a strong linear relationship with the predictor variables leaf nitrogen (Narea, g N per square m leaf area) and leaf structure (LMA, g leaf dry mass per square m leaf area) (r squared = 0.62). This covariance was largely a result of changes in leaf structure with canopy position; smaller thicker leaves occur at upper canopy positions in high-light environments. Mass-based expression of leaf nitrogen and dark respiration rate showed that nitrogen concentration (Nmass, mg N per g leaf dry mass) was only moderately predictive of variation in Rd,mass for all leaves pooled (r squared = 0.11), within species, or among species. We found distinct elevational trends, with both Rd,mass and Nmass higher in trees originating from high-elevation, cooler growth environments. Consideration of interspecies differences, vertical gradients in canopy light environment, and elevation, may improve our ability to scale leaf respiration to the canopy in forest process models.

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PubMed1999

Foliar temperature-respiration response functions for broad-leaved tree species in the southern Appalachians.

We measured leaf respiration in 18 eastern deciduous forest tree species to determine if there were differences in temperature-respiration response functions among species or among canopy positions. Leaf respiration rates were measured in situ and on detached branches for Acer pensylvanicum L., A. rubrum L., Betula spp. (B. alleghaniensis Britt. and B. lenta L.), Carya glabra (Mill.) Sweet, Cornus florida L., Fraxinus spp. (primarily F. americana L.), Liriodendron tulipifera L., Magnolia fraseri Walt., Nyssa sylvatica Marsh., Oxydendrum arboreum L., Platanus occidentalis L., Quercus alba L., Q. coccinea Muenchh., Q. prinus L., Q. rubra L., Rhododendron maximum L., Robinia psuedoacacia L., and Tilia americana L. in the southern Appalachian Mountains, USA. Dark respiration was measured on fully expanded leaves at 10, 15, 20, 25, and 30 degrees C with an infrared gas analyzer equipped with a temperature-controlled cuvette. Temperature-respiration response functions were fit for each leaf. There were significant differences in response functions among species and by canopy position within species. These differences were observed when respiration was expressed on a mass, nitrogen, or area basis. Cumulative nighttime leaf respiration was calculated and averaged over ten randomly selected nights for each leaf. Differences in mean cumulative nighttime respiration were statistically significant among canopy positions and species. We conclude that effects of canopy position and species on temperature-respiration response functions may need to be considered when making estimates of whole-tree or canopy respiration.

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PubMed1999

Influence of increased atmospheric CO(2) concentration on quality of plant material and litter decomposition.

Nitrogen (N) and lignin concentrations in plant tissues and litter of plants grown in greenhouses or open-top chambers in elevated atmospheric CO(2) concentration were compared with those of plants grown in ambient air in short-term studies. We also compared the N concentration of plant material of Quercus ilex L. and Q. pubescens Willd. growing in the vicinity of natural CO(2)-springs with that of the same species growing at a control site. In the short-term studies, elevated CO(2) caused significant decreases in tissue N concentration and the extent of the decrease varied with species. Nitrogen amendment of the soil lessened the CO(2)-enrichment effect. Lignin concentration was modified by elevated CO(2) and the effect was species specific, but no general positive or negative trend was evident. A comparison of trees growing under natural conditions near a natural CO(2)-spring and at a control site revealed no site differences in N concentration of the plant material. A comparison of published results on decomposition rates of litter produced in elevated atmospheric CO(2) and in ambient air indicated that CO(2) enrichment can cause both enhancements and decreases of carbon mineralization. We conclude that (1) long-term responses to elevated CO(2) could differ from the results obtained from short-term studies and that (2) biodiversity could be an important factor altering the sign of the feedback on atmospheric CO(2) concentration. We also discuss the implications of our finding of a long-term, inhibitory effect of the initial N concentration of litter on the decomposition rate of litter and its consequence on ecosystem feedback.

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PubMed1986

Carbon-nitrogen interactions in CO(2)-enriched white oak: physiological and long-term perspectives.

The responses of forest trees to atmospheric CO(2) enrichment will depend in part on carbon-nutrient linkages. Insights into the possible long-term ecological consequences of CO(2) enrichment can be gained from studying physiological responses in short-term experiments. One-year-old white oak (Quercus alba L.) seedlings were grown in unfertilized forest soil for 40 weeks in controlled-environment chambers with ambient (362 microl l(-1)) or elevated (690 microl l(-1)) CO(2). As previously reported, seedling dry weight was 85% greater in the elevated CO(2) environment, despite severe nitrogen deficiency in all seedlings. The increase in growth occurred without a concomitant increase in nitrogen uptake, indicating an increase in nitrogen-use efficiency in elevated CO(2). The weight of new buds was greater in elevated CO(2), suggesting that shoot growth in the next year would have been enhanced relative to that of seedlings in ambient CO(2). However, there was less translocatable nitrogen in perennial woody tissue in elevated CO(2); thus, further increases in nitrogen-use efficiency may not be possible. The leaves that abscised from seedlings in elevated CO(2) contained higher amounts of soluble sugars and tannin and a lower amount of lignin compared with amounts in abscised leaves in ambient CO(2). Based on lignin:N and lignin:P ratios, the rates of litter decomposition might not be greatly affected by CO() enrichment, but the total amount of nitrogen returned to soil would be lower in elevated CO(2).

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PubMed1986

Alkaline runoff, soil pH and white oak manganese deficiency.

Rainfall, surface runoff, and root zone pH values were determined in an effort to explain symptoms of decline in white oaks (Quercus alba L.) growing in, and downslope from, a parking lot. The soil was a Morley silt loam (Typic Hapludalf). Root zones within and downslope from the lot were dominated by alkaline soil reactions. Soil pH values along 15-m transects radiating from a white oak tree in a healthy stand upslope from the lot were predominantly acid beneath the crown, averaging between pH 5.5 and 6.1 to a depth of 90 cm within a 1.5 m radius of the tree trunk. Rainfall pH values varied from 4.3 to 5.9. Rainwater runoff from the parking lot and roadway varied from pH 7.5 to 8.0. Sampled in late summer, foliage of stressed trees in, and downslope from, the parking lot were all deficient in manganese (26-38 mg kg(-1)). It was concluded that Morley soils can be alkalinized by alkaline runoff from surfaces of calcium carbonate-containing structures, thereby reducing the solubility of soil manganese to a degree sufficient to cause deficiency of this element in white oaks.

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PubMed2000

Differences in leaf gas exchange and water relations among species and tree sizes in an Arizona pine-oak forest.

We compared leaf gas exchange and water potential among the dominant tree species and major size classes of trees in an upland, pine-oak forest in northern Arizona. The study included old-growth Gambel oak (Quercus gambelii Nutt.), and sapling, pole, and old-growth ponderosa pines (Pinus ponderosa var. scopulorum Dougl. ex Laws.). Old-growth oak had higher predawn leaf water potential (Psi(leaf)) than old-growth pine, indicating greater avoidance of soil water stress by oak. Old-growth oak had higher stomatal conductance (G(w)), net photosynthetic rate (P(n)), and leaf nitrogen concentration, and lower daytime Psi(leaf) than old-growth pine. Stomatal closure started at a daytime Psi(leaf) of about -1.9 MPa for pine, whereas old-growth oak showed no obvious reduction in G(w) at Psi(leaf) values greater than -2.5 MPa. In ponderosa pine, P(n) and G(w) were highly sensitive to seasonal and diurnal variations in vapor pressure deficit (VPD), with similar sensitivity for sapling, pole, and old-growth trees. In contrast, P(n) and G(w) were less sensitive to VPD in Gambel oak than in ponderosa pine, suggesting greater tolerance of oak to atmospheric water stress. Compared with sapling pine, old-growth pine had lower morning and afternoon P(n) and G(w), predawn Psi(leaf), daytime Psi(leaf), and soil-to-leaf hydraulic conductance (K(l)), and higher foliar nitrogen concentration. Pole pine values were intermediate between sapling and old-growth pine values for morning G(w) and daytime Psi(leaf), similar to sapling pine for predawn Psi(leaf), and similar to old-growth pine for morning and afternoon P(n), afternoon G(w), K(l), and foliar nitrogen concentration. For the pines, low predawn Psi(leaf), daytime Psi(leaf), and K(l) were associated with low P(n) and G(w). Our data suggest that hydraulic limitations are important in reducing P(n) in old-growth ponderosa pine in northern Arizona, and indicate greater avoidance of soil water stress and greater tolerance of atmospheric water stress by old-growth Gambel oak than by old-growth ponderosa pine.

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PubMed2000

Temporal variability in basal isoprene emission factor.

Seasonal variability in basal isoprene emission factor (&mgr;g C g(-1) h(-1) or nmol m(-2) s(-1), leaf temperature at 30 degrees C and photosynthetically active radiation (PAR) at 1000 &mgr;mol m(-2) s(-1)) was studied during the 1998 growing season at Duke Forest in the North Carolina Piedmont. Emissions from eight upper-canopy white oak (Quercus alba L.) leaves were measured periodically from the onset of isoprene emission on Day of Year (DOY) 119 (April 29) to leaf senescence in late October (DOY 299). Emissions from four leaves were measured under basal conditions with a controlled-environment cuvette system equipped with 10-ml gas-tight syringes and a reduction gas detector. Emissions from the other four leaves were measured under ambient conditions with the same system. Emission rates from the four leaves measured under ambient conditions were adjusted to basal conditions based on the PAR and leaf temperature algorithms of Guenther et al. (1993). The seasonal onset of isoprene emission was in agreement with previous studies where cumulative degree days from the date of the last spring frost were used to estimate bud break, leaf expansion, and increase in basal emission factor (EF). Between DOY 141 (May 21) and 240 (August 28), mean meteorological conditions 6 to 18 h prior to the EF measurements (ambient PAR and temperature) explained up to 78% of the variability in mean basal EF between measurement periods. Summertime mean isoprene emission potential was reached on DOY 141 (May 21) and was maintained until DOY 240 (August 28), when isoprene emission began to decline monotonically as leaf senescence approached. The mean value for leaves measured under ambient conditions and adjusted to basal conditions for DOY 141-240 was 75.6 &mgr;g C g(-1) h(-1) (74.2-79.1), whereas the mean value for leaves measured under basal conditions was 72.9 &mgr;g C g(-1) h(-1) (64.7-88.9). Between DOY 141 and 240, daily mean isoprene EFs varied from 54 to 96 &mgr;g C g(-1) h(-1) (27 to 49 nmol m(-2) s(-1)). In agreement with previous work at this and other sites, basal isoprene emission rates of fully exposed leaves at the crown apex of this tree were about 20% higher than those of the selected leaves. The length of the period prior to measurement of isoprene emission, during which meteorology was correlated with basal EF, appeared to be related to the timing and periodicity of meteorological change, and probably explains quantitative differences in the length of this period among studies. The empirical equation that we derived for this effect explained variability in midday EFs at the study site, but its general applicability remains to be tested. Strong diurnal changes in EF (as high as a factor of 2) are implied in this study, and should be examined further.

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