The influence of externally supplied sucrose on phloem transport in the maize leaf strip

Sucrose (2,5–1000 mmol l^–1), labeled with [¹⁴C]sucrose, was taken up by the xylem when supplied to one end of a 30-cm-long leaf strip of Zea mays L. cv. Prior. The sugar was loaded into the phloem and transported to the opposite end, which was immersed in diluted Hoagland's nutrient solution. When the Hoagland's solution at the opposite end was replaced by unlabeled sucrose solution of the same molarity as the labeled one, the two solutions met near the middle of the leaf strip, as indicated by radioautographs. In the dark, translocation of ¹⁴C-labeled assimilates was always directed away from the site of sucrose application, its distance depending on sugar concentration and translocation time. When sucrose was applied to both ends of the leaf strip, translocation of ¹⁴C-labeled assimilates was directed toward the lower sugar concentration. In the light, transport of ¹⁴-C-labeled assimilates can be directed (1) toward the morphological base of the leaf strip only (light effect), (2) toward the base and away from the site of sucrose application (light and sucrose effect), or (3) away from the site of sucrose application independent of the (basipetal or acropetal) direction (sucrose effect). The strength of a sink, represented by the darkened half of a leaf strip, can be reduced by applying sucrose (at least 25 mmol l^–1) to the darkened end of the leaf strip. However, equimolar sucrose solutions applied to both ends do not affect the strength of the dark sink. Only above 75 mmol l^–1 sucrose was the sink effect of the darnened part of the leaf strip reduced. Presumably, increasing the sucrose concentration replenishes the leaf tissue more rapidly, and photosynthates from the illuminated part of the leaf strip are imported to a lesser extent by the dark sink.Supported by Deutsche Forschungsgemeinschaft     

Leaf dynamics and standing stocks of intertidal Zostera noltii Hornem. and Cymodocea nodosa (Ucria) Ascherson on the Banc d'Arguin (Mauritania)

Leaf dynamics and standing stocks of intertidal seagrasses were studied in the Baie d'Aouatif (Parc National du Banc d'Arguin, Mauritania) in April and September 1988. Standing stocks of Zostera noltii Hornem. suggest a unimodal seasonal curve similar to what is found for populations at higher latitudes. Also, leaf growth rates (0.03 cm² cm^–2 day^–1 on average) were similar to those found at higher latitudes in these months. Variation in leaf loss over tidal depth, time and different locations in the Baie d'Aouatif was larger and more often significant than variation in leaf growth. In general, Z. noltii beds in the Baie d'Aouatif had comparable leaf growth rates and standing stocks. In both months losses were almost always higher than or equal to growth.Variation in leaf loss over time was much higher in the plots that were situated high in the intertidal than in lower plots. This is explained by differences in susceptibility to sloughing, which is presumably higher in periods with low tide around noon for shallow depths.In an experiment using artificial shading nets, in situ leaf growth was affected negatively from 94% shading onwards. This shading was observed to reduce the light intensity reaching the seagrass bed to a level below the reported range of light compensation points for Z. noltii. Cymodocea nodosa (Ucria) Ascherson on average had higher leaf area and relative growth rates than Z. noltii and much lower loss rates, resulting in a positive net increase in September. Standing stocks were also higher than for Z. noltii. A mixed seagrass bed containing the above two species and Halodule wrightii Ascherson had the highest observed total biomass: 335 g m^–2 ash-free dry weight.     

Morphological variability of Carex spicata Huds. utricles among plant communities

Differentiation in the size and number of seeds among populations or particular individuals of a given species may depend on genetic features and environmental conditions. The objective of our study was to answer two questions: whether any differences exist in the size and shape of utricles among Carex spicata populations growing in several plant communities and whether the hypothesized differences remain constant in ‘common garden’ conditions (i.e. if the sedges grown in different plant communities are evolving distinctly separate ecotypes). We studied utricle morphological traits (projected area, width, length, W/L ratio and projected perimeter) and number of utricles per spike collected from plants grown in five different plant communities (natural sites) and from plants transferred to common garden conditions. C. spicata utricles showed significant variability of morphological traits that depended on the plant community of origin. Among five plant communities, the largest utricles were found in Agropyron repens–Urtica dioica and Convolvulo arvensis–Agropyretum repentis communities, while the smallest ones were in plants from the Lolio-Plantaginetum community. The similarity of the analyzed populations regarding sedge utricle traits corresponded to the similarity of plant communities where the populations had grown. Moreover, the differences in utricle traits collected from natural conditions remained stable in common garden conditions. This indicated that differences in traits among utricles had a physiological dependence on different biotope conditions or different interspecific interactions prevailing in the particular plant communities. The relationship between the similarities in the diaspores of the populations studied and the similarities in the ecological conditions of the communities may also indicate that the variability of utricles is important for evolution and adaptation.The results support the hypothesis that C. spicata populations in different communities are producing separate ecotypes, i.e. specific species forms with genetically fixed traits adapted to narrowly determined habitat conditions.     

The status and future of the Lake Naivasha fishery,Kenya

I describe a laboratory system for investigating the role of light as a proximate cue for diel changes in locomotor activity and vertical location on the substrate of stream macro-invertebrates. The system consisted of computer-controlled halogen lamps positioned over a laboratory stream in which video-recordings were made of Stenonema modestum mayfly nymphs located on the undersides of unglazed tile substrates. Locomotor activity of study organisms in response to light changes were quantified during computer-programmed and reproducible light/dark (LD) cycles. The system provided the flexibility to simulate a variety of light environments so that the separate influences of light intensity and light change on diel activities of individuals and populations could be examined, which is difficult under natural light conditions. As a group, nymphs responded similarly to simulated twilight (light decrease from 7.9 × 10² to 6.9 × 10^–2 W cm^–2 at a constant –1.9 × 10^–3 s^–1 rate of relative light change) and to natural twilight, suggesting that proposed mechanisms of light control of diel activities in nature can be adequately tested in the simulated environment. However, locomotor activity and vertical movements among individual mayflies were highly variable under controlled conditions, suggesting that physiological differences influence their responses to environmental conditions.     

Callus induction and plant regeneration in the metallophyte <Emphasis Type="Italic">Silene vulgaris</Emphasis> (Caryophyllaceae)

Zinc tolerant and non-tolerant ecotypes of Silene vulgaris (Moench) Garcke were examined for their suitability to provide an efficient and reproducible callus formation and regeneration system. Successful and rapid regeneration of adventitious shoots from callus was achieved in leaf tissue but not root or apical meristematic tissue using concentrations of plant growth regulators that spanned a concentration range of (0.05–1 mg l^–1) NAA and (0.5–10 mg l^–1) BAP respectively. Large differences were observed between ecotypes regarding both callus formation and shoot regeneration on the different hormone concentrations. Leaf explants incubated on basal media with different concentrations of auxin/cytokinin demonstrated initial callus formation after 3 weeks of incubation. Callus initiation was seen to develop from the wounded margins of the leaf explants and, after 2 weeks the initially dark callus became more swollen and green. A mean of 6–8 shoots per leaf explant was observed and the survival rate of these regenerates was seen to be 90%. All regenerated plants that were transferred to soil after the emergence of roots, were seen to have no disturbed morphological characteristics. This study demonstrates the stability of the zinc tolerance traits in the regenerated explants and the potential use of this calli formation and regeneration system in Silene vulgaris. Further, this study is a necessary pre-requite for the development of a genetic transformation system with which to study the genetic basis of zinc and, other heavy metal tolerances in a species with a naturally selected high-level tolerance.     

Differential effects of light and nitrogen on production of hypericins and leaf glands in Hypericum perforatum

The effect of light intensity and root nitrogen supply on the levels of leaf hypericins was examined for St. John’s wort (Hypericum perforatum L.) grown in a sand culture system with artificial lighting. Increasing the light intensity illuminating St. John’s wort plants from 106 to 402 μmol·m^–2·s^–1 resulted in a continuous increase in the level of leaf hypericins. Using a leaf dissection approach, the association of hypericins with the dark glands on the leaves was shown, and it was found that increasing light intensity resulted in a parallel increase in the number of dark glands. In this respect, a linear relationship was observed between leaf gland number and the level of leaf hypericins (R = 0.901). While a decrease in nitrogen supply to St. John’s wort plants also yielded an increase in the level of leaf hypericins, this response occurred in a discontinuous manner over the range of nitrogen levels tested and no significant effect upon the number of dark leaf glands was observed. Overall, these effects of increased light intensity and decreased nitrogen supply on leaf hypericins appear to be independent and additive, and may reflect differences in the sites and processes where these environmental parameters impact production of these phytochemicals.     

Optimum growth conditions and light utilization efficiency of Spirulina platensis (= Arthrospira fusiformis) (Cyanophyta) from Lake Chitu, Ethiopia

Spirulina platensis (= Arthrospira fusiformis) was isolated from Lake Chitu, a saline, alkaline lake in Ethiopia, where it forms an almost unialgal population. Optimum growth conditions were studied in a turbidostat. Cultures grown in modified Zarrouk's medium and exposed to a range of light intensities (20–500 µmol photons m^–2s^–1) showed a maximum specific growth rate (µ_max) of 1.78 d^–1. Quantum yield for growth (µ) was 3.8% at the optimum light for growth of 330 µmol photons m^–2s^–1, and ranged from 2.8 to 9.4%. With increase in irradiance, the chlorophyll a concentration decreased, and the carotenoids/chlorophyll a ratio increased by a factor of 2.4. The phosphorus to carbon ratio (P/C) showed some variation, while the nitrogen to carbon ratio (N/C) remained relatively constant, thus causing fluctuations in the N:P ratio (7–11) of cells. An optimum N:P ratio of about 7 was attained in cells growing at the optimum light for growth. Results from the continuous culture experiments agreed well with maximum values of photosynthetic efficiency given in the literature for natural populations of S. platensis in the soda lakes of East Africa, Lake Arenguade (Ethiopia), and Lake Simbi (Kenya).     

N2 fixation by red alder (Alnus rubra) and scotch broom (Cytisus scoparius) planted under precommerically thinned Douglas-fir (Pseudotsuga menziesii)

Summary From acetylene reduction assays over a 10-month period starting in April 1979, nodule activities averaged 18.78 (se 4.67) moles C₂H₄ g nodule dw^–1 h^–1 forAlnus rubra and 59.95 (se 12.14) moles C₂H₄ g nodule dw^–1 h^–1 forCytisus scorparius. Plant rates were 1.91 (se. 47) moles C₂H₄ plant^–1 h^–1 forA. rubra and 0.55 (se. 17) moles C₂H₄ plant^–1 h^–1 forC. Scoparius. Plant activity and total leaf N were strongly correlated with the dw of other plant parts, but nodule activity and percent leaf N were not. Plant and nodule activities were not associated with temperature, moisture stress, precipitation events or percent light for either species over the growing season nor for 54A. rubra sampled in mid-season 1979 on one replication. After 5 to 6 growing seasons, 14A. rubra on the same site ranged from 30 to 332 cm in height and showed strong correlation between nodule dw, leaf dw, plant size and total leaf N. Results from this study and others indicate logistic equations may be modified to predict the effect of adding a N₂ fixing plant to a population of non N₂ fixing trees.     

Continuous ethanol production byZymomonas mobilis andSaccharomyces cerevisiae in biofilm reactors

Continuous ethanol fermentations were performed in duplicate for 60 days withZymomonas mobilis ATCC 331821 orSaccharomyces cerevisiae ATCC 24859 in packed-bed reactors with polypropylene or plastic composite-supports. The plastic composite-supports used contained polypropylene (75%) with ground soybean-hulls (20%) and zein (5%) forZ. mobilis, or with ground soybean-hulls (20%) and soybean flour (5%) forS. cerevisiae. Maximum ethanol productivities of 536 gL^–1 h^–1 (39% yield) and 499 gL^–1 h^–1 (37% yield) were obtained withZ. mobilis on polypropylene and plastic composite-supports of soybean hull-zein, respectively. ForZ. mobilis, and optimal yield of 50% was observed at a 1.92h^–1 dilution rate for soybean hull-zein plastic composite-supports with a productivity of 96gL^–1h^–1, whereas with polypropylene-supports the yield was 32% and the productivity was 60gL^–1h^–1. With aS. cerevisiae fermentation, the ethanol production was less, with a maximum productivity of 76gL^–1h^–1 on the plastic composite-support at a 2.88h^–1 dilution rate with a 45% yield. Polypropylene-support bioreactors were discontinued due to reactor plugging by the cell mass accumulation. Support shape (3-mm chips) was responsible for bioreactor plugging due to extensive biofilm development on the plastic composite-supports. With suspensionculture continuous fermentations in continuously-stirred benchtop fermentors, maximum productivities of 5gL^–1h^–1 were obtained with a yield of 24 and 26% withS. cerevisiae andZ. mobilis, respectively. Cell washout in suspensionculture continuous fermentations was observed at a 1.0h^–1 dilution rate. Therefore, for continuous ethanol fermentations, biofilm reactors out-performed suspension-culture reactors, with 15 to 100-fold higher productivities (gL^–1h^–1) and with higher percentage yields forS. cerevisiae andZ. mobilis, respectively. Further research is needed with these novel supports to evaluate different support shapes and medium compositions that will permit medium flow, stimulate biofilm formation, reduce fermentation costs, and produce maximum yields and productivities.This is Journal Paper No. J-16357 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 3253     

The effect of light on the number of chloroplast nucleoids in daughter cells of the algaScenedesmus quadricauda

Summary Synchronous cultures of the green algaScenedesmus quadricauda (Turp.) Bréb. grown at mean irradiances 25Wm^–2, 75Wm^–2, and 130Wm^–2 PhAR were exposed to different illumination regimes (ratio of light to dark interval varied from 2:22 to 24:0 hours). The populations of daughter cells released under these conditions differed markedly in their progress in the cell cycle. The cells from these populations were stained with DAPI and the shape, localization and number of chloroplast nucleoids were examined. The nucleoids were of spherical shape, divided asynchronously having dumbbell shape during fission. In the chloroplast, nucleoids were located symmetrically about the transverse axis of the cells. The mean number of nucleoids varied from two in the least developed daughter cells to 16 in the daughter cells of the highest developmental stage. The progress of these cells and thus also the number of nucleoids were proportional to the portion of the light energy amount which these daughter cells shared from the total light energy amount obtained by their mother cells.Abbreviations DAPI 4, 6-diamidino-2-diphenylindole - PhAR photosynthetically active radiation (400–700 nm)     

Morphological plasticity of regeneration subject to different levels of canopy cover in mixed-species, multiaged forests of the Romanian Carpathians

Morphological plasticity was studied for advanced regeneration trees in different light environments of the mountainous, mixed-species forests in the Carpathian Mountains of Romania. The primary species in these mixtures were very shade tolerant silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.), and midtolerant Norway spruce (Picea abies (L.) Karst). Seedlings/saplings of these species were selected for measurements in different stands from two different geographical locations. Various morphological traits (specific leaf area, live crown ratio, crown width to length ratio, terminal to lateral ratio, number of internodal shoots, number of shoots in terminal whorl, stem symmetry, stem orientation, stem forking) for each regenerating tree were measured during summers of 2001 and 2002. Percentage of above canopy light and stand basal area measures were used to assess the available growing space for each seedling/sapling. Regression relationships were developed for the different morphological indicators as a function of these two variables. All species adapted their morphology along the gradient in light and basal area. Spruce seemed to be less adapted to low light conditions than both fir and beech. However, no significant differences in terms of shade tolerance were detected using the above indicators. In really dense stand conditions (less than 20% above canopy light and stand basal area above 36 m² ha⁻¹), probability for stem forking in beech increased. In open, all three species adapted their morphology for vigorous growth. Under such conditions, spruce was better adapted than fir.     

小叶锦鸡儿天然居群叶形态性状变异研究

为揭示小叶锦鸡儿(Caragana microphylla)天然居群叶形态性状的变异规律及其生态适应性特征,该研究以10个小叶锦鸡儿天然居群为对象,通过多重比较、巢式方差分析、相关性分析、聚类分析和主成分分析等方法,对7个叶形态性状进行分析。结果表明：(1)小叶锦鸡儿叶形态性状在居群内和居群间均存在极显著差异(P < 0.01),平均变异系数为10.13%,不同性状的变异幅度为6.23%~12.78%;平均叶形态性状的表型分化系数为43.62%,居群内变异(30.09%)大于居群间变异(24.91%),说明居群内是其叶形态性状变异的主要来源。(2)相关性分析表明,环境因子对小叶锦鸡儿的叶形态性状变异有很大的影响,在地理空间上主要呈现出沿海拔梯度的变异模式;主成分分析的结果显示,小叶宽、叶柄宽和叶柄长对小叶锦鸡儿叶形态变异起主导作用;利用欧式距离对小叶锦鸡儿居群进行UPGMA聚类分析结果显示,基于叶形态性状和环境因子可分别将小叶锦鸡儿10个居群分为3类和2类,Mantel检验结果表明,小叶锦鸡儿的叶形态性状变异不存在地理连续性。研究结果为小叶锦鸡儿的适应性进化和开发利用提供了理论依据。     

Determination of the quantum efficiency of photosystem II and of non-photochemical quenching of chlorophyll fluorescence in the field

A newly developed portable chlorophyll fluorometer in combination with a special leaf clip holder was used for assessing photosynthetic activity of attached sun leaves of Fagus sylvatica and Cucurbita pepo under field conditions. During diurnal time courses, fluorescence yield, photosynthetic photon flux density (PPFD) incident on the leaf plane, and leaf temperature were measured and quantum efficiency of photosystem II (PS II), apparent relative electron transport rates, and non-photochemical fluorescence quenching (NPQ) calculated. In both species, quantum efficiency followed closely the incident PPFD and no hysteresis could be observed during the day. Apparent electron transport rate showed light saturation above a PPFD of 700 mol m^–2 s^–1 in F. sylvatica, while in C. pepo no saturation was visible up to 1400 mol m^–2 s^–1. NPQ was closely correlated to excessive PPFD calculated from the PS II quantum yield. Maximal NPQ observed was 3.3 Although the beech leaf was exposed for a considerable time to PPFD values of 1400–1500 mol m^–2 s^–1 and leaf temperatures between 30 and 35°C, no obvious signs for sustained photodamage could be observed. The data demonstrate the potential of chlorophyll fluorescence measurements to analyse photosynthetic performance under field conditions with minimal disturbance of the plant. Potential error sources due to the geometry of the leaf clip holder used are discussed.Dedicated to Prof. Dr. F.-C. Czygan on the occasion of his 60th birthday     

Relationship between morphological and physiological responses to waterlogging and salinity in Sporobolus virginicus (L.) Kunth

The effects of waterlogging and salinity on morphological and physiological responses in the marsh grass Sporobolus virginicus (L.) Kunth were investigated in a 4×2 factorial experiment. Plants were subjected to four salinity levels (0, 100, 200 and 400 mol m^–3 NaCl) and two soil inundation conditions (drained and flooded) for 42 days. Flooding at 0 mol m^–3 NaCl caused initiation of adventitious surface roots, increased internal acration and plant height, induced alcohol dehydrogenase activity (ADH), and decreased belowground biomass and the number of culms per plant. Salinity increase from 0 to 400 mol m^–3 NaCl under drained conditions increased leaf and root proline concentrations and decreased photosynthesis, aboveground biomass, number of culms per plant and number of internodes per culm. Concurrent waterlogging and salinity induced ADH activity and adventitious surface roots but decreased plant height and aboveground biomass. Internal air space increased with waterlogging from 0 to 100 mol m^–3 NaCl but further increases in salinity to 400 mol m^–3 reduced air space. Combined waterlogging and salinity stresses, however, had no effect on photosynthesis or on the concentrations of proline in leaves or roots. These results are discussed in relation to the widespread colonization by S. virginicus of a wide range of coastal environments varying in soil salinity and in the frequency and intensity of waterlogging.     

Light quality affects photosynthesis and leaf anatomy of birch plantlets in vitro

Cultures in vitro of Betula pendula Roth were subjected to light of different spectral qualities. Photosynthetic capacity was highest when the plantlets were exposed to blue light (max recorded photosynthesis, 82 mol CO₂ dm^–2 h^–1) and lowest when irradiated with light high in red and/or far-red wave lengths (max recorded photosynthesis, 40 mol CO₂ dm^–2 h^–1). Highest chlorophyll content (2.2 mg dm^–2 leaf area) was found in cultures irradiated with blue light, which also enhanced the leaf area. Morphometric analysis of light micrographs showed that the epidermal cell areas were largest in plantlets subjected to blue light and smallest in those subjected to red light. Morphometric analysis of electron micrographs of palisade cells, showed that the functional chloroplast area was largest in chloroplasts of leaves subjected to blue light and smallest in those exposed to red light. We suggest that light quality affects photosynthesis both through effects on the composition of the photosynthetic apparatus and on translocation of carbohydrates from chloroplasts.     

Involvement of chloroplasts in the programmed death of plant cells

The effect of cyanide, an apoptosis inducer, on pea leaf epidermal peels was investigated. Illumination stimulated the CN^–-induced destruction of guard cells (containing chloroplasts and mitochondria) but not of epidermal cells (containing mitochondria only). The process was prevented by antioxidants (-tocopherol, 2,5-di-tret-butyl-4-hydroxytoluene, and mannitol), by anaerobiosis, by the protein kinase C inhibitor staurosporine, and by cysteine and serine protease inhibitors. Electron acceptors (menadione, p-benzoquinone, diaminodurene, TMPD, DCPIP, and methyl viologen) suppressed CN^–-induced apoptosis of guard cells, but not epidermal cells. Methyl viologen had no influence on the removal of CN^–-induced nucleus destruction in guard cells under anaerobic conditions. The light activation of CN^–-induced apoptosis of guard cells was suppressed by DCMU (an inhibitor of the electron transfer in Photosystem II) and by DNP-INT (an antagonist of plastoquinol at the Q_o site of the chloroplast cytochrome b ₆ f complex). It is concluded that apoptosis initiation in guard cells depends on the simultaneous availability of two factors, ROS and reduced quinones of the electron transfer chain. The conditions for manifestation of programmed cell death in guard and epidermal cells of the pea leaf were significantly different.     

Variation in growth and ion accumulation between two selected populations of Trifolium repens L. differing in salt tolerance

Two divergent populations of T. repens cv. Haifa developed from two generations of recurrent selection for shoot chloride concentration, were grown in the greenhouse at 0 and 40 mol m^–3 NaCl. Over two harvest cycles at 40 mol m^–3 NaCl, the population selected for a low concentration of chloride in the shoot maintained a significantly lower chloride and sodium concentration compared with those plants selected for a high shoot chloride concentration. The distribution of chloride in the shoots was further examined in a subsample of plants from both populations. In all plants, concentrations of chloride were lower in the expanding and fully expanded leaves than in the older leaf tissue or petioles.While there were no significant differences in the photosynthetic rates between lines, shoot yields and relative leaf expansion rates were higher in the low chloride population. Plant death was greater in plants selected for high shoot chloride. These results suggest that selections based on measurements of low shoot chloride concentrations may be successful in developing a cultivar of T. repens with improved salt tolerance.     

Seasonal dynamics of <Emphasis Type="Italic">Zostera noltii</Emphasis> Hornem. in two Mediterranean lagoons

The density, biomass and shoot morphology of two populations of Zostera noltii were monitored from January 1998 to July 1999 at two shallow Mediterranean lagoons of Biguglia and Urbino, which differ in hydro-morphological conditions and nutrient loading. Monitoring included the principal biological and foliar parameters (shoot density, aboveground and belowground biomass, length, width and number of leaves, LAI and coefficient A: percentage of leaves having lost their apex), the organic matter contents of the sediment and the environmental conditions (salinity, turbidity, temperature, nutrient concentrations and dissolved oxygen levels). The two populations of Z. noltii displayed seasonal changes in density (1600–19600 m²), aboveground biomass (11–153 g. DW. m⁻²), leaf length (33–255 mm), and leaf width (0.9–1.8 mm). Temperature and turbidity were significant environmental factors influencing the temporal changes observed in the Z. noltii meadows studied. Conversely, the belowground biomass, the number of leaves per shoot and the LAI did not undergo any seasonal changes. In the Biguglia lagoon, the functioning dynamics of the Z. noltii seagrass beds are determined by the catchment area and the inputs of nutrients derived from it, whereas in the Urbino lagoon the dynamics of the Z. noltiibeds depend on low levels of water turbidity.     

Dynamics of leaf minerals,bdleaf area,and biomass from hardwoods intensively grown on a peat bog

Summary Water willow and grey alder were grown on a raised sphagnum bog in central Sweden. The stands were intensively treated by daily irrigation and fertilization during the growing period in order to improve site fertility. After a 2-year establishment period high production rates were achieved in willow stands, 0.8 kg stem dry weight m^–2 year^–1 on current plus one (C+1) year old shoots. In these stands the canopy was closed with a leaf area index (LAI) that peaked at approximately 7. The canopy in the alder stand did not close during the initial 3 years of growth and the measured production rate was relatively low, at approximately 0.4 kg dry weight m^–2 year ^–1 in the last year. The leaf nitrogen content was 3%–4% of dry weight during the summer and the other studied mineral elements were in almost optimal proportion to nitrogen. This was considered to be an effect of the intensive fertilization regime. Above-ground production close to maximum yield was attained in the prevailing conditions of soil, climate and biomass partitioning.     

Leaf litter decomposition and nutrient dynamics in a subtropical forest after typhoon disturbance

Decomposition of typhoon-generated and normal leaf litter and their release patterns for eight nutrient elements were investigated over 3 yr using the litterbag technique in a subtropical evergreen broad-leaved forest on Okinawa Island, Japan. Two common tree species, Castanopsis sieboldii and Schima wallichii, representative of the vegetation and differing in their foliar traits, were selected. The elements analyzed were N, P, K, Ca, Mg, Na, Al, Fe and Mn. Dry mass loss at the end of study varied in the order: typhoon green leaves > typhoon yellow leaves > normal leaves falling for both species. For the same litter type, Schima decomposed faster than Castanopsis. Dry mass remaining after 2 yr of decomposition was positively correlated with initial C:N and C:P ratios. There was a wide range in patterns of nutrient concentration, from a net accumulation to a rapid loss in decomposition. Leaf litter generated by typhoons decomposed more rapidly than did the normal litter, with rapid losses for N and P. Analysis of initial quality for the different litter types showed that the C:P ratios were extremely high (range 896 – 2467) but the P:N ratios were < 0.05 (range 0.02 – 0.04), indicating a likely P-limitation for this forest. On average 32% less N and 60% less P was retranslocated from the typhoon-generated green leaves than from the normal litter for the two species, Castanopsis and Schima. An estimated 2.13 g m^–2 yr^–1 more N and 0.07 g m^–2 yr^–1 more P was transferred to the soil as result of typhoon disturbances, which were as high as 52% of N and 74% of P inputted from leaf litter annually in a normal year. Typhoon-driven maintenance of rapid P cycling appears to be an important mechanism by which growth of this Okinawan subtropical forest is maintained.