Effects of CO2 elevation on canopy development in the stands of two co-occurring annuals

Elevated CO₂ may increase dry mass production of canopies directly through increasing net assimilation rate of leaves and also indirectly through increasing leaf area index (LAI). We studied the effects of CO₂ elevation on canopy productivity and development in monospecific and mixed (1:1) stands of two co-occurring C₃ annual species, Abutilon theophrasti, and Ambrosia artemisiifolia. The stands were established in the glasshouse with two CO₂ levels (360 and 700 l/l) under natural light conditions. The planting density was 100 per m² and LAI increased up to 2.6 in 53 days of growth. Root competition was excluded by growing each plant in an individual pot. However, interference was apparent in the amount of photons absorbed by the plants and in photon absorption per unit leaf area. Greater photon absorption by Abutilon in the mixed stand was due to different canopy structures: Abutilon distributed leaves in the upper layers in the canopy while Ambrosia distributed leaves more to the lower layers. CO₂ elevation did not affect the relative performance and light interception of the two species in mixed stands. Total aboveground dry mass was significantly increased with CO₂ elevation, while no significant effects on leaf area development were observed. CO₂ elevation increased dry mass production by 30–50%, which was mediated by 35–38% increase in the net assimilation rate (NAR) and 37–60% increase in the nitrogen use efficiency (NUE, net assimilation rate per unit leaf nitrogen). Since there was a strong overall correlation between LAI and aboveground nitrogen and no significant difference was found in the regression of LAI against aboveground nitrogen between the two CO₂ levels, we hypothesized that leaf area development was controlled by the amount of nitrogen taken up from the soil. This hypothesis suggests that the increased LAI with CO₂ elevation observed by several authors might be due to increased uptake of nitrogen with increased root growth.     

Nitrogen concentration of cauliflower organs as determined by organ size,N supply,and radiation environment

Data from field experiments carried out in three consecutive years under contrasting N supply and radiation environment altered by artificial shading were used to identify (a) the relationship between N concentration and organ size under conditions of unrestricted N supply and (b) critical levels of soil nitrate (Nmin_crit), where nitrogen concentration of cauliflower organs begin to decline because of N limitations. The decline of N concentrations in cauliflower was analysed at different levels of morphological aggregation, i.e., the whole shoot level, the organ level (leaves, stem, and curd), and within different leaf groups within the canopy. Nmin_crit values (0–60 cm soil depth) for total nitrogen concentration of cauliflower organs leaves, stem and curd were estimated at 85, 93 and 28 kg N ha^–1, respectively. Within the canopy, Nmin_crit values for total N of leaves increased from the top to the bottom from 44 to 188 kg N ha^–1. Nmin_crit values for protein N in leaves from different layers of the canopy were much lower at around 30 kg N ha^–1, without a gradient within the canopy. It is discussed that these differences in Nmin_crit values are most likely a consequence of N redistribution associated with nitrogen deficiency. The decline of average shoot nitrogen concentrations, [Nm] (%N DM), with shoot dry matter, W _sh, (t ha^–1) under conditions of optimal N supply was [Nm]= 4.84 (±0.071) W _sh ^{–0.089(± 0.011)}, r ²=0.67 (±S.E.). The reduction of radiation intensity by artificial shading (60% of control) had no significant influence on total nitrogen concentrations of leaves and only a small influence on protein nitrogen concentrations in lower layers of the canopy. The leaf nitrate nitrogen fraction of nitrogen, f _nitr (–), within the canopy decreased linearly with increased average incident irradiance in different canopy layers (I _av, W PAR m^–2) (f _Nitr. = 0.2456(±0.0188) – 0.0023(±0.0004)I _av, r ² = 0.67.     

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.     

Physiological comparisons of two soybean cultivars differing in canopy photosynthesis. II. Variation in specific leaf weight,nitrogen, and protein components

Cultivar differences in canopy apparent photosynthesis (CAP) have been observed in soybean (Glycine max (L.) Merr.) but little is known about the physiological mechanisms which are responsible for such differences. This study was initiated to determine if variation in ribulose 1,5-bisphosphate carboxylase (RuBPCase) and soluble protein exists among cultivars which differ in CAP during reproductive growth. In addition, the relationship between specific leaf weight (SLW) and leaf protein was examined. Two Maturity Group VI cultivars, Tracy (high CAP) and Davis (low CAP), were grown in the field during 1979, 1980, and 1981 and in a greenhouse experiment. Leaves located at two canopy positions (topmost, fully expanded leaf and eighth node from the top) in 1979 and three canopy positions (those mentioned, plus the fourth node from the top) in 1980 and 1981 were sampled. Leaves at the two upper canopy positions exhibited greater SLW, RuBPCase m^–2, and soluble protein m^–2 than found at the eighth node down. Photosynthetic capacity of leaves at inner canopy regions was therefore affected by both light penetration into the canopy and leaf protein status. Over the three year period, the SLW was 23 percent and the soluble protein m^–2 leaf 21 percent greater in Tracy than in Davis. Although the trend in RuBPCase m^–2 leaf was not significant, it was consistently greater in Tracy in the field and greenhouse. No cultivar differences were observed when the proteins were expressed on a unit of leaf dry weight. The quantity of RuBPCase per unit leaf area was positively correlated with SLW with significant partial correlation coefficients of 0.62, 0.67, 0.35, and 0.82 for 1979, 1980, 1981, and the greenhouse study, respectively. Since these cultivars have similar leaf area indices during September, the greater SLW of Tracy is translated into more photosynthetic proteins per unit ground area and higher CAP rate.Abbreviations AP Leaf Apparent Photosynthesis - CAP Canopy Apparent Photosynthesis - DAP Days After Planting - DTT Dithothreitol - HEPES N-2-hydroethylpiperazine N-2 ethanesulfonic acid - LAI Leaf Area Index - LSD Least Significant Difference - PPFD Photosynthetic Photon Flux Density - PVP-40 Polyvinylpolypyrroledone (molecular weight, 4000) - RuBPCase Ribulose 1,5-bisphosphate Carboxylase - SLW Specific Leaf Weight     

Canopy structure within a Quercus ilex forested watershed: variations due to location,phenological development,and water availability

Spatial and temporal changes in canopy structure were studied in 1988 and 1989 in a Mediterranean Quercus ilex forest in north-eastern Spain. Due to differences in precipitation patterns the 1989 growing season was drier than the 1988 growing season. Sampling was conducted in parallel at two sites which represent endpoints along a slope gradient within a watershed (ridge top at 975 m, and valley bottom at 700 m). At both sites, similar inter-annual changes in canopy structure were observed in response to differences in water availability. Samples harvested in the upper 50 cm of the canopy during 1989 exhibited a decrease in both average leaf size and the ratio of young to old leaf and stem biomass relative to samples obtained in 1988. At the whole canopy level, a decrease in leaf production efficiency and an increase in the stem to leaf biomass ratio was observed in 1989. Temporal changes in canopy leaf area index (LAI) were not statistically significant. Average LAI values of Q. ilex at the two sites were not significantly different despite differences in tree stature and density (4.6 m² m^–2 at the ridge top, and 5.3 m² m^–2 at the valley bottom). Vertical distribution of leaves and stems within the canopy was very similar at the two locations, with more than 60% of the total LAI in the uppermost metre of the canopy. The possible significance of such an LAI distribution on the canopy carbon budget is discussed.     

Nitrogen cycling in dense plantings of hybrid poplar and black alder

Summary Nitrogen cycling was studied during the third growing season in pure and mixed plantings (33×33 cm spacing) of hybrid poplar and black alder in southeastern Canada. After 3 years, hybrid poplar growth and N content of living tissues in a plot and of individual hybrid poplar plants increased with the proportion of black alder in a planting. No differences were detected among N contents of individual alder plants regardless of plot treatment. Black alder allocated a larger portion of its N to roots than hybrid poplar. Symbiotic nitrogen fixation was estimated to account for 80% of the nitrogen in aboveground alder tissues in the pure treatment using natural¹⁵N dilution. N return in leaf litter was estimated to be 70kg ha^–1 in the pure alder treatment and decreased to a minimum of 20 kg ha^–1 in the pure hybrid poplar plots. No difference was detected among treatments for throughfall N content. Nitrogen concentration in roots and leaf litterfall of black alder was higher than hybrid poplar. Significant soil N accretion occurred in mixed plantings containing two alders to one poplar and pure black alder plantings. Nitrogen availability (NO₃–N) increased with the amount of black alder in a plot. Results suggest that the early increase in nitrogen accumulation of hybrid poplar in mixed treatments can be attributed to an increase of total soil N availability resulting from the input of large amounts of N from easily mineralizable alder tissue.     

Intracanopy variation in nitrogen cycling through leaves is influenced by irradiance and proximity to developing fruit in mature walnut trees

Intracanopy variation in net leaf nitrogen (N) resorption and N cycling through leaves in mature walnut (Juglans regia L. cv Hartley) trees were monitored in 3 different years. Differential irradiance among the spurs sampled was inferred from differences among leaves in dry weight per unit area (LW/LA) which varied from 4.0 mg · cm^–2 to 7.0 mg · cm^–2 in shaded (S) and exposed (E) canopy positions, respectively. Our results, using ¹⁵N-depleted (NH₄)₂SO₄ validated the concept that N influx and efflux through fully expanded leaves occurred concurrently during the period of embryo growth. Additionally, it also suggested that N influx into leaves was substantially greater in exposed as compared with shaded canopy positions. Because of its well documented phloem immobility, leaf Ca accumulation was used to better estimate the relative influx of N into exposed and shaded leaves. N cycling varied locally within the tree canopy, i. e. Ca (and presumably N) influx was 100% greater in exposed than shaded tree canopy positions, but influx was not influenced significantly by the proximity of developing fruit. In contrast, both the amount and percentage N efflux was significantly greater during embryo growth in fruit-bearing than defruited spurs. Net leaf N resorption averaged 2–4 times greater (25–30%) in fruit-bearing spurs than the 5–10% decrease in the leaf N content in defruited spurs. Since about 90% of leaf N content reportedly occurs as protein, fruit N demand apparently influenced protein turnover and catalysis in associated spur leaves. The amount of leaf N resorption was greater in exposed than shaded positions in the tree canopy in 2 of the 3 years of data collection. Our data show that like leaf N content, N influx, N efflux and net leaf N resorption vary throughout mature walnut tree canopies under the combined local influences of fruiting and irradiance.     

Nodulation potential of soils from red alder stands covering a wide age range

Red alder (Alnus rubra Bong.) stands in the Pacific Northwest are the common first stage in succession following disturbance. These stands are highly productive and contribute a large amount of N to the soils as a result of their N₂-fixing symbiosis with Frankia. As these alder stands age, the soils not only increase in total N, but concentrations of NO^– ₃ increase and pH decreases as a result of nitrification. The objective of this study was to determine how the nodulation capacity of Frankia varies as red alder stands age and if differences in nodulation capacity are related to changes in soil properties. Nodulation capacity was determined by a red alder seedling bioassay for soils from red alder stands in the Oregon coast range covering a wide range of ages. Six chronosequences were sampled, each containing a young, an intermediate, and an older alder stand. Soil total N, total C, NO^– ₃, NH⁺ ₄, and pH were measured on the same soil samples. These factors as well as alder stand characteristics were compared with nodulation capacity in an attempt to identify soil characteristics typical in developing alder stands that most strongly affect nodulation capacity. Soil pH and NO^– ₃ concentration were highly correlated with nodulation capacity and with each other. Cluster analysis of the sites using these two variables identified two groups with distinctly different nodulation capacities. The cluster with the higher nodulation capacity was lower in NO^– ₃ and higher in pH than the other cluster, which included the majority of sites. There was substantial overlap in the age ranges for the two clusters and there was no significant correlation between age and nodulation capacity. Thus nodulation capacity appears to be most closely related to soil properties than to stand age.     

Ulva rigida (Ulvales,Chlorophyta) tank culture as biofilters for dissolved inorganic nitrogen from fishpond effluents

Ulva rigida was cultivated in 7501 tanks at different densities with direct and continuous inflow (at 2, 4, 8 and 12 volumes d^–1) of the effluents from a commercial marine fishpond (40 metric tonnes, Tm, of Sparus aurata, water exchange rate of 16 m³ Tm^–1) in order to assess the maximum and optimum dissolved inorganic nitrogen (DIN) uptake rate and the annual stability of the Ulva tank biofiltering system. Maximum yields (40 g DW m^–2 d^–1) were obtained at a density of 2.5 g FW 1^–1 and at a DIN inflow rate of 1.7 g DIN m^–2 d^–1. Maximum DIN uptake rates were obtained during summer (2.2 g DIN M^–2 d^–1), and minimum in winter (1.1 g DIN m^–2 d^–1) with a yearly average DIN uptake rate of 1.77 g DIN m^–2 d^–1 At yearly average DIN removal efficiency (2.0 g DIN m^–2 d^–1, if winter period is excluded), 153 m² of Ulva tank surface would be needed to recover 100% of the DIN produced by 1 Tm of fish.Abbreviations DIN= dissolved inorganic nitrogen (NH _inf4 ^sup+ + NO _inf3 ^sup– + NO _inf2 ^sup– ); - FW= fresh weight; - DW= dry weight; - PFD= photon flux density; - V= DIN uptake rate     

Production ecology of phyto- and zooplankton in a eutrophic pond dominated byChaoborus flavicans (Diptera: Chaobolidae)

Primary production of phytoplankton and secondary production of a daphnid and a chaoborid were studied in a small eutrophic pond. The gross primary production of phytoplankton was 290 gC m⁻² per 9 months during April–December. Regression analysis showed that the gross primary production was related to the incident solar radiation and the chlorophylla concentration and not to either total phosphorus or total inorganic nitrogen concentration. The mean chlorophylla concentration (14.2 mg m⁻³), however, was about half the expected value upon phosphorus loading of this pond. The mean zooplankton biomass was 1.60 g dry weight m⁻², of whichDaphnia rosea and cyclopoid copepods amounted to 0.69 g dry weight m⁻² and 0.61 g dry weight m⁻², respectively. The production ofD. rosea was high during May–July and October and the level for the whole 9 months was 22.6 g dry weight m⁻².Chaoborus flavicans produced 10 complete and one incomplete cohorts per year. Two consecutive cohorts overlapped during the growing season. The maximum density, the mean biomass, and the production were 19,100 m⁻², 0.81 g dry weight m⁻², and 11.7 g dry weight m⁻²yr⁻¹, respectively. As no fish was present in this pond, the emerging biomass amounted to 69% of larval production. The production ofC. flavicans larvae was high in comparison with zooplankton production during August–September, when the larvae possibly fed not only on zooplankton but also algae.     

Deposition of ammonia to the forest floor under spruce and beech at the Höglwald site

Laboratory and field measurements of the flux of ammonia to forest floor canopies of spruce and beech stands at the Höglwald site in southern Bavaria are reported. Measurements were performed with an open chamber method. A linearity between ammonia concentration and ammonia flux from the atmosphere to the ground floor canopy was detected. Deposition of ammonia showed no saturation even at air concentrations up to 50 g NH₃ m^–3 air. Temperature, water content and the moss layer of the ground floor canopy had a minor influence on the deposition velocity in laboratory experiments. Deposition velocity of ammonia was higher to the spruce (1.3 cm s^–1), and limed spruce ground floor canopy (1.17 cm s^–1) compared to the beech stand (0.79 cm s^–1). In field studies, a diurnal course of the deposition velocity was detected with highest velocities in midday and minor during night times, but not in the climatic chamber. The flux of ammonia to the ground floor canopy was estimated of app. 10 kg N ha^–1 yr^–1 for the soil under spruce, 9 kg N ha^–1 yr^–1 for the limed spruce and 6 kg N ha^–1yr^–1 for the soil under beech. The fluxes are interpreted as fluxes from the atmosphere to the ground canopies of the stands.     

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Summary Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day^–1 tree^–1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m² of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s^–1 that is equivalent to 112 mmol m^–2 s^–1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m^–2 s^–1 at the top and 150 mmol m^–2 s^–1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was –27.76±0.27 (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.This paper is dedicated to Prof. Dr. O.L. Lange on the occasion of his 65th birthday     

Production of Gammarus pulex L. (Amphipoda) in a small Danish stream

Quantitative samples of Gammarus pulex L. taken from a small Danish stream during 1975 showed mean annual population densities varying from 500 m^–2 in early May to 5 500 m^–2 in late September. The mean annual biomass was 1.5 g dry weight m^–2. No discrete cohorts could be distinguished from the size frequency distributions. Annual production, estimated by the size-frequency method, was 3.9 g dry weight M^-2 and P/B ratio was 2.6. The contribution to trout energetics may have been as much as 17%.     

Canopy characteristics of the brown alga Sargassum muticum (Fucales,Phaeophyta) in Lake Grevelingen,southwest Netherlands

The present distribution of the invasive brown alga Sargassum muticum in the southwest Netherlands is updated. Populations of the alga were found to remain at their 1985 level in Lake Grevelingen, with a small eastward expansion into the Eastern Scheldt estuary. A new population for the brackish, non-tidal Lake Veere is reported. Within Lake Grevelingen S. muticum forms a persistent, extensive canopy of 100% cover (4,442.5 ± 525.6 g fresh wt m^–2, 640.3 ± 75.8 g dry wt m^–2) that has a marked effect upon the penetration of photosynthetically active radiation (PAR) (reduced by 97% at 0.1 m). Surface sea water temperatures can be elevated by 2.7 °C above water not associated with a Sargassum canopy; furthermore, the dense canopy shades and hence reduces water temperatures below 0.1 m depth. Productivity studies indicate that assimilation occurs in the upper levels of the canopy (57.09 µg C mg dry wt^–1 m^–2 at a mean PAR rate of 106.7 J cm^–2 h^–1). Self-shading and a resultant decrease in the rate of assimilation was evident below the canopy.     

Effects of nitrogen supply and elevated carbon dioxide on construction cost in leaves of Pinus taeda (L.) seedlings

Seedlings of loblolly pine (Pinus taeda L.) were grown under varying conditions of soil nitrogen and atmospheric carbon dioxide availability to investigate the interactive effects of these resources on the energetic requirements for leaf growth. Increasing the ambient CO₂ partial pressure from 35 to 65 Pa increased seedling growth only when soil nitrogen was high. Biomass increased by 55% and photosynthesis increased by 13% after 100 days of CO₂ enrichment. Leaves from seedlings grown in high soil nitrogen were 7.0% more expensive on a g glucose g^–1 dry mass basis to produce than those grown in low nitrogen, while elevated CO₂ decreased leaf cost by 3.5%. Nitrogen and CO₂ availability had an interactive effect on leaf construction cost expressed on an area basis, reflecting source-sink interactions. When both resources were abundant, leaf construction cost on an area basis was relatively high (81.8±3.0 g glucose m^–2) compared to leaves from high nitrogen, low CO₂ seedlings (56.3±3.0 g glucose m^–2) and low nitrogen, low CO₂ seedlings (67.1±2.7 g glucose m^–2). Leaf construction cost appears to respond to alterations in the utilization of photoassimilates mediated by resource availability.     

Artemia monica cyst production and recruitment in Mono Lake,California, USA

Annual egg production was determined for Artemia monica in Mono Lake, California, from 1983 to 1987. Annual oviparous (overwintering cyst) production was 3 and 7 million cysts m^–2 yr^–1 in 1986 and 1987, respectively, as measured by in situ sediment traps. Cyst production for the entire five year period was calculated using Artemia census data and inter-brood duration derived from mixolimnetic temperature. These estimates ranged from 2 to 5 million cysts m^–2 yr^–1. This method underestimated annual production by 30%, when compared to estimates using sediment traps. Cyst production was similar during 1983–1986 and showed a significant increase in 1987, which was due primarily to a larger reproductive population later in the year. Recruitment into the adult populations of the following spring ranged between 1.4 to 3.2%. Overall abundance of this generation reflected the patterns in annual cyst production. Compensatory effects must operate on the second generation of each year, since summer populations were similar in all years despite differences in cyst production.     

Autumnal changes in tissue nitrogen of autumn olive,black alder and eastern cottonwood

Two experiments were conducted to determine patterns of N change in tissues of autumn olive (Elaeagnus umbellata Thunb.) and black alder (Alnus glutinosa [L.] Gaertn.) during autumn in central Illinois, U.S.A. In the first study leaf nitrogen concentrations of autumn olive decreased 40% at an infertile minespoil site and 39% at a fertile prairie site throughout autumn whereas nitrogen concentrations in respective bark samples increased by 39% and 37%. Salt-extractable protein concentrations increased in bark and decreased in leaves over the sampling period. Free amino acid concentrations of autumn olive leaves decreased over the course of the experiment from peak concentrations in August. Asparagine, glutamic acid and proline were major constituents of the free amino acid pools in leaves. Total phosphorus concentrations of autumn olive leaves declined by 40–46% during autumn while bark concentrations of P did not significantly change.In the second experiment non-nodulated seedlings of alder receiving a low level of N-fertilization did not exhibit net resorption of leaf N during autumn whereas foliar N concentration of contrasting nonactinorhizal cottonwood plants (Populus deltoides Bartr. ex. Marsh) under the same fertilization regime decreased by 27% after the first frost. A gradual but significant decrease of 38% in foliar N concentration of nodulated alder seedlings grown under a low N-fertilization regime was associated with the cessation of nitrogenase activity during autumn in nodules. Compared with the low N fertilization regime, the higher level of N-fertilization resulted in smaller autumnal decreases of foliar N concentration in nodulated alder (17%) and in cottonwood (20%); but there was no decrease in foliar N concentration in non-nodulated alder. The higher level of N-fertilization promoted a greater accumulation of N in the roots than in the bark of both tree species after the first frost.Our results suggest that black alder lackingFrankia symbionts does not exhibit net leaf N resorption and that autumnal decreases in leaf N ofFrankia-nodulated black alder result primarily from declining foliar N import relative to export due to low temperature inhibition of N₂ fixation. In contrast, autumn olive exhibited greater and more precipitous autumnal declines in foliar N concentration than those of alder, and the pattern of N decline was unaffected by site fertility.     

Linkage between seasonal gas exchange and hydraulic acclimation in the top canopy leaves of Fagus trees in a mesic forest in Japan

We investigated how leaf gas exchange and hydraulic properties acclimate to increasing evaporative demand in mature beech trees, Fagus crenata Blume and Fagus japonica Maxim., growing in their natural habitat. The measurements in the top canopy leaves were conducted using a 16-m-high scaffolding tower over two growing seasons. The daily maxima of net photosynthetic rate for the early growing season were close to the annual maximum value (11.9 mol m^–2 s^–1 in F. crenata and 7.7 mol m^–2 s^–1 in F. japonica). The daily maxima of water vapor stomatal conductance were highest in the summer, approximately 0.3 mol m^–2 s^–1 in F. crenata and 0.15 mol m^–2 s^–1 in F. japonica. From the early growing season to the summer season, the leaf-to-air vapor pressure deficit increased and the daily minima of leaf water potentials decreased. However, there was no loss of leaf turgor in the summer as a result of effective osmotic adjustment. Both the soil-to-leaf hydraulic conductance per unit leaf area and the twig hydraulic conductivity simultaneously increased in the summer, probably as a result of production of new vessels in the xylem. These results suggest that both osmotic adjustment and increased hydraulic conductance resulted in the largest diurnal maximum of stomatal conductance in the summer, resulting in the lowest relative stomatal limitation on net photosynthetic rate, although the leaf-to-air vapor pressure deficit was highest. These results indicate that even in a mesic forest, in which excessive hydraulic stress does not occur, the seasonal acclimation of hydraulic properties at both the single leaf and whole plant levels are important for plant carbon gain.     

Light-associated nitrogen distribution profile in flowering canopies of sunflower (Helianthus annuus L.) altered during grain growth

In vegetative canopies of many species, the vertical gradient of lamina nitrogen concentration (NW) parallels the profile of light distribution in such a way that the actual nitrogen partitioning approaches the optimum pattern for canopy photosynthesis. This paper evaluates the hypothesis that a strong sink for nitrogen, viz. growing grain, affects the pattern of lamina nitrogen distribution usually described for vegetative canopies. The light and NW profiles of sunflower (Helianthus annuus L.) crops were characterised from anthesis to physiological maturity. The factorial combination of two plant populations (2.4 and 4.8 plants m^–2) and two levels of nitrogen supply (0 and 5 g N m^–2) were the sources of variation for NW and light profiles. Before the onset of nitrogen accumulation in grain, the pattern of NW was similar to that described for other species and it was related to the distribution of light in the canopy. Important changes in the profile of NW occurred during grain filling that were unrelated to the light regime. Nitrogen was mobilised from leaves in all positions in the canopy and the rate of NW change was greater in leaves closer to the grain, which were also the leaves where nitrogen was more concentrated. It is concluded that the physiological mechanisms involved in determining the distribution of leaf nitrogen in vegetative canopies do not apply to sunflower during grain filling.     

Seasonal growth and biomass ofTrapa japonica Flerov in Ojaga-Ike Pond,Chiba, Japan

In order to determine the seasonal growth and biomass ofTrapa japonica Flerov, field observations were carried out at Ojaga-ike Pond, Chiba, Japan, during 1979 and 1980. In spring, the plant showed exponential growth (c. 0.080 g g⁻¹ day⁻¹) and shoot elongation was as rapid as 10 cm day⁻¹. The plant attained its maximum biomass (380.5±35.1 g m⁻²) in late August, and about 50% of this was concentrated in the topmost 30-cm stratum (645.7±33.1 g m⁻³); maximum total stem length exceeded 6m. The plant produced large (500–800 mg per fruit), but small numbers of nut-like fruit (maximum, 5 fruits per rosette). Defoliation occurred almost linearly with time at a rate of 30.6 leaves m⁻² day⁻¹; annual net leaf production was estimated to be about twice as large as the seasonal maximum leaf biomass. While the number of leaves per rosette showed moderate seasonal change, rosette density, rosette area and leaf dry weight changed considerably during the year. From the negative log-log correlation between mean total leaf dry weight per rosette and rosette density, density-dependent rosette growth was assumed. The cause of the wide spread of this species in aquatic habitats is briefly discussed in terms of its seed size and morphology.