Seasonal changes in canopy photosynthesis and foliage respiration in a Rhizophora stylosa stand at the northern limit of its natural distribution

Gross photosynthesis and respiration rates of leaves at different canopy heights in a Rhizophora stylosa Griff. stand were measured monthly over 1 year at Manko Wetland, Okinawa Island, Japan, which is the northern limit of its distribution. The light-saturated net photosynthesis rate for the leaves at the top of the canopy showed a maximum value of 17 μmol CO₂ m⁻² s⁻¹ in warm season and a minimum value of 6 μmol CO₂ m⁻² s⁻¹ in cold season. The light-saturated gross photosynthesis and dark respiration rates of the leaves existing at the top of the canopy were 2−7 times and 3–16 times, respectively, those of leaves at the bottom of the canopy throughout the year. The light compensation point of leaves showed maximum and minimum peaks in warm season and cold season, respectively. The annual canopy gross photosynthesis, foliage respiration, and surplus production were estimated as 117, 49, and 68 t CO₂ ha⁻¹ year⁻¹, respectively. The energy efficiency of the annual canopy gross photosynthesis was 2.5%. The gross primary production GPP fell near the regression curve of GPP on the product of leaf area index and warmth index, the regression curve which was established for forests in the Western Pacific with humid climates.     

Carbon balance in a monospecific stand of an annual herb <Emphasis Type="Italic">Chenopodium album</Emphasis> at an elevated CO<Subscript>2</Subscript> concentration

Elevated CO₂ enhances carbon uptake of a plant stand, but the magnitude of the increase varies among growth stages. We studied the relative contribution of structural and physiological factors to the CO₂ effect on the carbon balance during stand development. Stands of an annual herb Chenopodium album were established in open-top chambers at ambient and elevated CO₂ concentrations (370 and 700 μmol mol⁻¹). Plant biomass growth, canopy structural traits (leaf area, leaf nitrogen distribution, and light gradient in the canopy), and physiological characteristics (leaf photosynthesis and respiration of organs) were studied through the growing season. CO₂ exchange of the stand was estimated with a canopy photosynthesis model. Rates of light-saturated photosynthesis and dark respiration of leaves as related with nitrogen content per unit leaf area and time-dependent reduction in specific respiration rates of stems and roots were incorporated into the model. Daily canopy carbon balance, calculated as an integration of leaf photosynthesis minus stem and root respiration, well explained biomass growth determined by harvests (r ² = 0.98). The increase of canopy photosynthesis with elevated CO₂ was 80% at an early stage and decreased to 55% at flowering. Sensitivity analyses suggested that an alteration in leaf photosynthetic traits enhanced canopy photosynthesis by 40–60% throughout the experiment period, whereas altered canopy structure contributed to the increase at the early stage only. Thus, both physiological and structural factors are involved in the increase of carbon balance and growth rate of C. album stands at elevated CO₂. However, their contributions were not constant, but changed with stand development.     

Predicting tropical plant physiology from leaf and canopy spectroscopy

A broad regional understanding of tropical forest leaf photosynthesis has long been a goal for tropical forest ecologists, but it has remained elusive due to difficult canopy access and high species diversity. Here we develop an empirical model to predict sunlit, light-saturated, tropical leaf photosynthesis using leaf and simulated canopy spectra. To develop this model, we used partial least squares (PLS) analysis on three tropical forest datasets (159 species), two in Hawaii and one at the biosphere 2 laboratory (B2L). For each species, we measured light-saturated photosynthesis (A), light and CO₂ saturated photosynthesis (A _max), respiration (R), leaf transmittance and reflectance spectra (400–2,500 nm), leaf nitrogen, chlorophyll a and b, carotenoids, and leaf mass per area (LMA). The model best predicted A [r ²  = 0.74, root mean square error (RMSE) = 2.9 μmol m⁻² s⁻¹)] followed by R (r ²  = 0.48), and A _max (r ²  = 0.47). We combined leaf reflectance and transmittance with a canopy radiative transfer model to simulate top-of-canopy reflectance and found that canopy spectra are a better predictor of A (RMSE = 2.5 ± 0.07 μmol m⁻² s⁻¹) than are leaf spectra. The results indicate the potential for this technique to be used with high-fidelity imaging spectrometers to remotely sense tropical forest canopy photosynthesis.     

Geometrical similarity analysis of photosynthetic light response curves, light saturation and light use efficiency

Light absorption and use efficiency (LAUE mol mol⁻¹, daily gross photosynthesis per daily incident light) of each leaf depends on several factors, including the degree of light saturation. It is often discussed that upper canopy leaves exposed to direct sunlight are fully light-saturated. However, we found that upper leaves of three temperate species, a heliophytic perennial herb Helianthus tuberosus, a pioneer tree Alnus japonica, and a late-successional tree Fagus crenata, were not fully light-saturated even under full sunlight. Geometrical analysis of the photosynthetic light response curves revealed that all the curves of the leaves from different canopy positions, as well as from the different species, can be considered as different parts of a single non-rectangular hyperbola. The analysis consistently explained how those leaves were not fully light-saturated. Light use optimization models, called big leaf models, predicted that the degree of light saturation and LAUE are both independent of light environment. From these, we hypothesized that the upper leaves should not be fully light-saturated even under direct sunlight, but instead should share the light limitation with the shaded lower-canopy leaves, so as to utilize strong sunlight efficiently. Supporting this prediction, within a canopy of H. tuberosus, both the degree of light saturation and LAUE were independent of light environment within a canopy, resulting in proportionality between the daily photosynthesis and the daily incident light among the leaves.     

Needle CO2 exchange, structure and defense traits in relation to needle age in Pinus heldreichii Christ – a relict of Tertiary flora

 Pinus heldreichii Christ is a long-lived, slow-growing Tertiary relict from the Balkans. In this study we evaluated the physiological characteristics of eight needle-age classes of P. heldreichii grown at the Arboretum of the Institute of Dendrology in Kórnik, Poland. At the end of the growing season, current-year foliage had the highest rates of mass-based light-saturated net photosynthesis (A_sat) of 33.5 nmol CO₂· g^–1· s^–1. A_sat decreased with needle age, but older needle classes retained from approximately 62 to 26% of the current needles’ rate. The relationship between leaf N and chlorophyll a concentration among all needle-age classes was highly significant (r = 0.96, P = 0.0006). The variation in A_sat of 1- to 7-year-old needles was linearly related to needle N concentration (r = 0.98, P = 0.0001). Needle dark respiration rates among these needle age classes ranged from 0.8 to 2.2 nmol · g^–1· s^–1 and decreased with needle age and nitrogen concentration. Total phenols and glucose concentrations increased linearly with needle age. A similar pattern was observed in acid buffering capacity and the pH of tissue homogenates. The water content ranged from 62% for the current needles to 51% for the 6-year-old needles. Greater investment in leaf structural tissue and increased chemical defense is associated with higher structural cost of older needles and may reduce their photosynthetic activity. Significant declines in water and nitrogen content with needle age and an increase in content of phenolics is most likely a defense adaptation of P. heldreichii related to the species’ long-lived leaves. Received: 8 January 1997 / Accepted: 4 March 1997     

Specific leaf area and leaf nitrogen concentration in annual and perennial grass species growing in Mediterranean old-fields

 We evaluated the hypothesis that photosynthetic traits differ between leaves produced at the beginning (May) and the end (November–December) of the rainy season in the canopy of a seasonally dry forest in Panama. Leaves produced at the end of the wet season were predicted to have higher photosynthetic capacities and higher water-use efficiencies than leaves produced during the early rainy season. Such seasonal phenotypic differentiation may be adaptive, since leaves produced immediately preceding the dry season are likely to experience greater light availability during their lifetime due to reduced cloud cover during the dry season. We used a construction crane for access to the upper canopy and sampled 1- to 2-month-old leaves marked in monthly censuses for six common tree species with various ecological habits and leaf phenologies. Photosynthetic capacity was quantified as light- and CO₂-saturated oxygen evolution rates with a leaf-disk oxygen electrode in the laboratory (O_2max) and as light-saturated CO₂ assimilation rates of intact leaves under ambient CO₂ (A_max). In four species, pre-dry season leaves had significantly higher leaf mass per unit area. In these four species, O_2max and A_max per unit area and maximum stomatal conductances were significantly greater in pre-dry season leaves than in early wet season leaves. In two species, A_max for a given stomatal conductance was greater in pre-dry season leaves than in early wet season leaves, suggesting a higher photosynthetic water-use efficiency in the former. Photosynthetic capacity per unit mass was not significantly different between seasons of leaf production in any species. In both early wet season and pre-dry season leaves, mean photosynthetic capacity per unit mass was positively correlated with nitrogen content per unit mass both within and among species. Seasonal phenotypic differentiation observed in canopy tree species is achieved through changes in leaf mass per unit area and increased maximum stomatal conductance rather than by changes in nitrogen allocation patterns. Received: 7 March 1996 / Accepted: 1 August 1996     

Alterations in Rubisco activity and in stomatal behavior induce a daily rhythm in photosynthesis of aerial leaves in the amphibious-plant Nuphar lutea

Nuphar lutea is an amphibious plant with submerged and aerial foliage, which raises the question how do both leaf types perform photosynthetically in two different environments. We found that the aerial leaves function like terrestrial sun-leaves in that their photosynthetic capability was high and saturated under high irradiance (ca. 1,500 μmol photons m⁻² s⁻¹). We show that stomatal opening and Rubisco activity in these leaves co-limited photosynthesis at saturating irradiance fluctuating in a daily rhythm. In the morning, sunlight stimulated stomatal opening, Rubisco synthesis, and the neutralization of a night-accumulated Rubisco inhibitor. Consequently, the light-saturated quantum efficiency and rate of photosynthesis increased 10-fold by midday. During the afternoon, gradual closure of the stomata and a decrease in Rubisco content reduced the light-saturated photosynthetic rate. However, at limited irradiance, stomatal behavior and Rubisco content had only a marginal effect on the photosynthetic rate, which did not change during the day. In contrast to the aerial leaves, the photosynthesis rate of the submerged leaves, adapted to a shaded environment, was saturated under lower irradiance. The light-saturated quantum efficiency of these leaves was much lower and did not change during the day. Due to their low photosynthetic affinity for CO₂ (35 μM) and inability to utilize other inorganic carbon species, their photosynthetic rate at air-equilibrated water was CO₂-limited. These results reveal differences in the photosynthetic performance of the two types of Nuphar leaves and unravel how photosynthetic daily rhythm in the aerial leaves is controlled.     

Seasonal changes in the photosynthetic capacity of cones on a larch (Larix kaempferi) canopy

The seasonal changes of photosynthesis of cones of Japanese larch (Larix kaempferi Carr.) trees showed that gross photosynthetic rate of young cones (P _G) was 2–3 μmol m⁻² s⁻¹ at surface area unit and P _G/R _D (dark respiration of cones) peaked about 0.7 in the same period, indicating that 70 % of respiratory CO₂ was re-fixed. With maturation, P _G and P _G/R _D sharply decreased. Chlorophyll content in cones was 3–20 % of that in leaves, which made it a limiting factor for photosynthesis and its content was closely correlated with photosynthetic capacity. Although sunken and linearly arranged stomatal organs were found on the scale of young cones, differently from the significant regulation of leaf photosynthesis, these stomata tended to be non-functional since CO₂ is not limiting factor for cone photosynthesis. Thus photosynthesis of larch cones is an additional contribution to their development.     

Photosynthesis rates of selected tree species in lowland dipterocarp rainforest of Sabah, Malaysia

 Diurnal courses of net photosynthesis, transpiration and water potential of leaves of ten woody species from the natural lowland dipterocarp forests in Sabah (North Borneo, Malaysia) and one exotic tree species were studied in the field. The indigenous species represent different ecological niches and successional stages in the various layers of the dipterocarp forest, such as pioneers, trees of the understorey or main canopy and emergents. Diurnal changes in CO₂ exchange and transpiration reflected primarily differences in irradiance. The diurnal courses of water potential mainly tracked the rate of transpiratory water loss. Light-dependency describes most of the diurnal variations of leaves’ gas exchange. Light response curves of net photosynthesis of the investigated species of the Dipterocapaceae were almost equal (light saturated assimilation rate, A_max: 5.0–7.2 μmol CO₂ m^–2 s^–1), while those of the other species exhibited remarkable differences (A_max: 5.5–14.2 μmol CO₂ m^–2 s^–1). Leaf area, chlorophyll content and specific leaf dry weight as the reference parameters for assimilation gave a general ranking of the A_max, which is highest for the pioneering species, less for the understorey trees and lowest for emergents. Light compensation points and light saturation of net photosynthesis were attained mainly between 6 and 9 μmol photons m^–2 s^–1 and between 230 and 534 μmol photons m^–2 s^–1, respectively, but were higher for pioneering species. Photosynthetic performance may be a diagnostic feature of the successional and ecological status of species, i.e. to characterize pioneering species from understorey species or from emergents of the dipterocarp forest. Received: 3 March 1997 / Accepted: 15 December 1997     

Vertical canopy gradients in δ13C correspond with leaf nitrogen content in a mixed-species conifer forest

Stable carbon isotope composition varies markedly between sun and shade leaves, with sun leaves being invariably more enriched (i.e., they contain more¹³C). Several hypotheses have emerged to explain this pattern, but controversy remains as to which mechanism is most general. We measured vertical gradients in stable carbon isotope composition (δ¹³C) in more than 200 trees of nine conifer species growing in mixed-species forests in the Northern Rocky Mountains, USA. For all species except western larch, δ¹³C decreased from top to bottom of the canopy. We found that δ¹³C was strongly correlated with nitrogen per unit leaf area (N _area), which is a measure of photosynthetic capacity. Usually weaker correlations were found between δ¹³C and leaf mass per area, nitrogen per unit leaf mass, height from the ground, or depth in the canopy, and these correlations were more variable between trees than for N _area. Gradients of δ¹³C (per meter canopy depth) were steeper in small trees than in tall trees, indicating that a recent explanation of δ¹³C gradients in terms of drought stress of upper canopy leaves is unlikely to apply in our study area. The strong relationship between N _area and δ¹³C here reported is consistent with the general finding that leaves or species with higher photosynthetic capacity tend to maintain lower CO₂ concentrations inside leaves. We conclude that photosynthetic capacity is a strong determinant of δ¹³C in vertical canopy profiles, and must be accounted for when interpreting δ¹³C values in conifer forests.     

Why does photosynthesis decrease with needle age in Pinus pinaster?

Rates of photosynthesis vary with foliage age and typically decline from full-leaf expansion until senescence occurs. This age-related decline in photosynthesis is especially important in species that retain foliage for several years, yet it is not known whether the internal conductance to CO₂ movement (g _i) plays any role. More generally, g _i has been measured in only a few conifers and has never been measured in leaves or needles older than 1 year. The effect of ageing on g _i was investigated in Pinus pinaster, a species that retains needle for 4 or more years. Measurements were made in autumn when trees were not water limited and after leaf expansion was complete. Rates of net photosynthesis decreased with needle age, from 8 μmol m⁻² s⁻¹ in fully expanded current-year needles to 4.4 μmol m⁻² s⁻¹ in 3-year-old needles. The relative limitation due to internal conductance (0.24–0.35 out of 1) was in all cases larger than that due to stomatal conductance (0.13–0.19 out of 1). Internal conductance and stomatal conductance approximately scaled with rates of photosynthesis. Hence, there was no difference among year-classes in the relative limitations posed by internal and stomatal conductance or evidence that they cause the age-related decline in photosynthesis. There was little evidence that the age-related decline in photosynthesis was due to decreases in contents of N or Rubisco. The decrease in rates of photosynthesis from current-year to older needles was instead related to a twofold decrease in rates of photosynthesis per unit nitrogen and V _cmax/Rubisco (i.e., in vivo specific activity).     

Responses of leaf photosynthesis,pigments and chlorophyll fluorescence within canopy position in a boreal grass (<Emphasis Type="Italic">Phalaris arundinacea</Emphasis> L.) to elevated temperature and CO<Subscript>2</Subscript> under varying water regimes

The effects of elevated growth temperature (ambient + 3.5°C) and CO₂ (700 μmol mol⁻¹) on leaf photosynthesis, pigments and chlorophyll fluorescence of a boreal perennial grass (Phalaris arundinacea L.) under different water regimes (well watered to water shortage) were investigated. Layer-specific measurements were conducted on the top (younger leaf) and low (older leaf) canopy positions of the plants after anthesis. During the early development stages, elevated temperature enhanced the maximum rate of photosynthesis (P _max) of the top layer leaves and the aboveground biomass, which resulted in earlier senescence and lower photosynthesis and biomass at the later periods. At the stage of plant maturity, the content of chlorophyll (Chl), leaf nitrogen (N_L), and light response of effective photochemical efficiency (Φ_PSII) and electron transport rate (ETR) was significantly lower under elevated temperature than ambient temperature in leaves at both layers. CO₂ enrichment enhanced the photosynthesis but led to a decline of N_L and Chl content, as well as lower fluorescence parameters of Φ_PSII and ETR in leaves at both layers. In addition, the down-regulation by CO₂ elevation was significant at the low canopy position. Regardless of climate treatment, the water shortage had a strongly negative effect on the photosynthesis, biomass growth, and fluorescence parameters, particularly in the leaves from the low canopy position. Elevated temperature exacerbated the impact of water shortage, while CO₂ enrichment slightly alleviated the drought-induced adverse effects on P _max. We suggest that the light response of Φ_PSII and ETR, being more sensitive to leaf-age classes, reflect the photosynthetic responses to climatic treatments and drought stress better than the fluorescence parameters under dark adaptation.     

Structure and productivity along a tree height gradient in a Kandelia obovata mangrove forest in the Manko Wetland, Okinawa Island, Japan

Tree height (H) of Kandelia obovata trees decreased sharply from 5 m at the forest interior behind the terrestrial forest to 1.5 m at the forest edge near the river bank according to an increase in the yearly waterlogged period along a belt transect. The decreasing tree stature was attributed to a decrease in the asymptote of H in the D _0.1 (stem diameter at H/10)-H relationship toward the edge. The K. obovata trees were well classified into interior and edge types using a discriminant function based on the habitat-specific D _0.1–H relationships. Allometric equations, as a function of D _0.1² H, differed significantly between the interior and edge types in the estimation of the phytomasses of stems and leaves, and the leaf area per tree. On the other hand, common allometric equations were successfully established in the estimation of respective phytomasses of aboveground parts and branches. Biomass and leaf area index decreased toward the forest edge. The biomass allocation to stems decreased toward the edge, whereas those to branches and leaves increased. A dramatic change in stem diameter increment resulted in differences in the D _0.1–H relationship along the tree height gradient. Relative growth rate of biomass and light-saturated net photosynthesis, which paralleled net assimilation rate from the interior to the edge, showed their maximum peaks in the middle of the belt transect. This indicates that there exists an optimal environmental condition for growth of K. obovata trees. Leaf nitrogen content tended to increase to the edge with increasing waterlogged period.     

Coordinated changes in photosynthesis,water relations and leaf nutritional traits of canopy trees along a precipitation gradient in lowland tropical forest

We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian forests with contrasting mean annual precipitation (1,800, 2,300, 3,100 and 3,500 mm). There was near complete turn-over of dominant canopy tree species among sites, resulting in greater dominance of evergreen species with long-lived leaves as precipitation increased. Mean structural and physiological traits changed along this gradient as predicted by cost–benefit theories of leaf life span. Nitrogen content per unit mass (N_mass) and light- and CO₂-saturated photosynthetic rates per unit mass (P_mass) of upper canopy leaves decreased with annual precipitation, and these changes were partially explained by increasing leaf thickness and decreasing specific leaf area (SLA). Comparison of 1,800 mm and 3,100 mm sites, where canopy access was available through the use of construction cranes, revealed an association among extended leaf longevity, greater structural defense, higher midday leaf water potential, and lower P_mass, N_mass, and SLA at wetter sites. Shorter leaf life spans and more enriched foliar ¹⁵N values in drier sites suggest greater resorption and re-metabolism of leaf N in drier forest. Greater dominance of short-lived leaves with relatively high P_mass in drier sites reflects a strategy to maximize photosynthesis when water is available and to minimize water loss and respiration costs during rainless periods. Overall, our study links coordinated change in leaf functional traits that affect productivity and nutrient cycling to seasonality in lowland tropical forests.     

The vertical distribution of nitrogen and photosynthetic activity at different plant densities in Carex acutiformis

Shoots of the monocotyledonous perennial Carex acutiformis were grown in open (28 shoots m⁻²) and dense stands (280 shoots m⁻²). For fully grown stands the distribution of relative PPFD and leaf nitrogen per unit leaf area over canopy depth was determined. Light response of photosynthesis was measured on leaf segments sampled at various heights in the stands. Relations between parameters of these curves and leaf nitrogen were investigated. Simulations showed that in the open stand daily canopy photosynthesis was not affected by nitrogen redistribution in the canopy. For the dense stand however, a uniform nitrogen distribution would lead to only 73% of the maximum net carbon gain by the stand under optimal nitrogen distribution. The actual canopy photosynthesis was only 7% less than this theoretical maximum; the actual nitrogen distribution of the dense stand clearly tended to the optimal distribution. The vertical pattern of the nitrogen distribution was to a large extent determined by the minimum leaf nitrogen content. The relatively high minimum leaf nitrogen content found for Carex leaves may perhaps be necessary to maintain the physiological function of the basal parts of the leaves.     

Comparative study of leaf carbon gain in saplings ofThujopsis dolabrata var.hondai andQuercus mongolica var.grosseserrata in a cool-temperate deciduous forest

Seasonal courses of leaf CO₂ gas exchange in a growing season were examined in saplings ofThujopsis dolabrata var.hondai andQuercus mongolica var.grosseserrata in a cool temperate deciduous forest. Between the two tree species there were no large differences in the light compensation point of leaf photosynthesis, except for the season of new leaf expansion. However, light-saturated rates of net photosynthesis were obviously high inT. dolabrata var.hondai. EvergreenT. dolabrata var.hondai saplings had large photosynthetic production in two seasons, before the emergence of new foliage and after foliage fall of the overstory deciduous trees, because of the significantly high solar radiant energy penetrating under the forest canopy during the seasons. Saplings of deciduousQ. mongolica var.grosseserrata were heavily shaded throughout the growing season by foliage of the overstory trees, which resulted in a low daily surplus production. The annual surplus production of leaves in the growing season was estimated to be 2300 mmol CO₂ m⁻² inT. dolabrata var.hondai and −100 mmol CO₂ m⁻², slightly negative, inQ. mongolica var.grosseserrata. These results supported the high survivability ofT. dolabrata var.hondai saplings and the high mortality ofQ. mongolica var.grosseserrata in the deciduous forest.     

EFFECTS OF CANOPY POSITION AND IRRADIANCE ON THE LEAF PHYSIOLOGY AND MORPHOLOGY OF PENTACLETHRA MACROLOBA (MIMOSACEAE)

Pentaclethra macroloba (Willd.) Kuntze (Mimosaceae) is a dominant late-successional tree species in the Atlantic lowland forests of Costa Rica. Leaves of P. macroloba from three heights in the forest canopy were compared with leaves of seedlings grown in controlled environment chambers under four different irradiance levels. Changes in leaf characteristics along the canopy gradient paralleled changes resulting from the light gradient under controlled conditions. The effect of light or canopy position on light-saturated photosynthesis was small, with maximum photosynthesis increasing from 5 to 6.5 μmol m^−-2 s^−-1 from understory to canopy. Both chamber grown and field leaves showed large adjustments in photosynthetic efficiency at low light via reductions in dark respiration rates and increases in apparent quantum yields. Light saturation of all leaves occurred at or below 500 μmol m^−-2 s^−-1. Leaf thickness, specific leaf weight, and stomatal density increased to a greater extent than saturated photosynthesis with higher irradiance during growth or height in the canopy. As a result, there was a poor correspondence between leaf thickness and light-saturated photosynthesis on an area basis. It is concluded that Pentaclethra macroloba possesses the characteristics of a typical shade-tolerant species.     

Can light-saturated photosynthesis in lowland tropical forests be estimated by one light level?

Leaf-level net photosynthesis (A_n) estimates and associated photosynthetic parameters are crucial for accurately parameterizing photosynthesis models. For tropical forests, such data are poorly available and collected at variable light conditions. To avoid over- or underestimation of modeled photosynthesis, it is critical to know at which photosynthetic photon flux density (PPFD) photosynthesis becomes light-saturated. We studied the dependence of A_n on PPFD in two tropical forests in French Guiana. We estimated the light saturation range, including the lowest PPFD level at which A_sat (A_n at light saturation) is reached, as well as the PPFD range at which A_sat remained unaltered. The light saturation range was derived from photosynthetic light-response curves, and within-canopy and interspecific differences were studied. We observed wide light saturation ranges of A_n. Light saturation ranges differed among canopy heights, but a PPFD level of 1,000 µmol m⁻² s⁻¹ was common across all heights, except for pioneer trees species that did not reach light saturation below 2,000 µmol m⁻² s⁻¹. A light intensity of 1,000 µmol m⁻² s⁻¹ sufficed for measuring A_sat of climax species at our study sites, independent of the species or the canopy height. Because of the wide light saturation ranges, results from studies measuring A_sat at higher PPFD levels (for upper canopy leaves up to 1,600 µmol m⁻² s⁻¹) are comparable with studies measuring at 1,000 µmol m⁻² s⁻¹.     

Water relations and gas exchange in Fagus sylvatica L. and Quercus petraea (Mattuschka) Liebl. in a mixed stand at their southern limit of distribution in Europe

Water status and gas exchange of beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Mattuschka) Liebl.] were studied in a mixed stand in the Montejo de la Sierra forest (central Iberian Peninsula), one of the southernmost locations of both species in Europe. Gas exchange and water potential were measured in leaves at different canopy levels over several days in two growing seasons. The daily variation pattern was established with the measurements of three selected dates per year, representative of the soil moisture content situations in early, mid- and late summer. A similar daily time course of leaf water potential was found for the two species. Nevertheless, beech showed a most noticeable decrease of water potential at midmorning and maintained lower leaf water potential than oak in the early afternoon. In 1994 the sessile oak saplings showed higher values of predawn water potential (Ψ_pd) than beech at the end of summer, when soil moisture content was lowest (20 cm depth). Beech showed a significantly lower net assimilation rate (A) than sessile oak for leaves under the same PPFD. Maximum net photosynthesis values (A _max) for beech and sessile oak on sunny leaves were 10.1±0.4 μmol m^–2 s^–1 and 17.8±1.7 μmol m^–2 s^–1 respectively, and those for water vapour stomatal conductance (g _wv) were 265±31 mmol m^–2 s^–1 and 438±74 mmol m^–2 s^–1. Differences in A and g _wv between the two species were maintained throughout the day on all measurement dates. No clear relationship was found between water status of saplings and stomata performance; there was only a negative correlation between Ψ_pd and g _wvmid in beech. Nevertheless, a significant response to the air vapour pressure gradient between leaf and air was translated into stomata closure on an hourly basis, more intensively in beech. Received: 4 March 1999 / Accepted: 21 December 1999     

Comparison of leaf water use efficiency of oak and sycamore in the canopy over two growing seasons

The seasonal trends in water use efficiency of sun and shade leaves of mature oak (Quercus robur) and sycamore (Acer pseudoplatanus) trees were assessed in the upper canopy of an English woodland. Intrinsic water use efficiency (net CO₂ assimilation rate/leaf conductance, A/g) was measured by gas exchange and inferred from C isotope discrimination (δ¹³C) methods. Shade leaves had consistently lower δ¹³C than sun leaves (by 1–2‰), the difference being larger in sycamore. Buds had distinct sun and shade isotopic signatures before bud break and received an influx of ¹³C-rich C before becoming net autotrophs. After leaf full expansion, δ¹³C declined by 1–2‰ gradually through the season, emphasising the importance of imported carbon in the interpretation of leaf δ¹³C values in perennial species. There was no significant difference between the two species in the value of intrinsic water use efficiency for either sun or shade leaves. For sun leaves, season-long A/g calculated from δ¹³C (72–78 μmol CO₂ [mol H₂O]⁻¹) was 10–16% higher than that obtained from gas exchange and in situ estimates of leaf boundary layer conductance. For shade leaves, the gas exchange–derived values were low, only 10–18% of the δ¹³C-derived values. This is ascribed to difficulties in obtaining a comprehensive sample of gas exchange measurements in the rapidly changing light environment.