Impacts of the invasive plant <Emphasis Type="Italic">Fallopia japonica</Emphasis> (Houtt.) on plant communities and ecosystem processes

Fallopia japonica (Japanese knotweed) invades riparian areas and roadsides in New England. This large clonal species drastically alters the appearance of habitats by forming highly productive near-monocultures. To understand how these invasions affect ecosystem processes in New England, we quantified the impacts of F. japonica on species diversity, primary productivity, and nitrogen cycling at five locations in central Massachusetts, USA. In stands of F. japonica and in adjacent uninvaded areas, we recorded the cover of each plant species and measured the aboveground biomass and nitrogen (N) concentrations in plants, along with N retranslocation from F. japonica leaves and several soil characteristics. In addition, we severed rhizomes of peripheral F. japonica shoots to determine if clonal integration contributes to the species’ rapid spread and dominance. Stands of F. japonica had lower species diversity, but greater aboveground biomass and standing N than uninvaded areas. Nitrogen and carbon concentrations in biomass and N mineralization rates in soil did not differ between stands and adjacent areas. Rhizome severing temporarily reduced growth of F. japonica, suggesting that retranslocation of photoassimilates and/or nutrients between shoots via rhizomatal connections may maximize stand level growth rates and facilitate dominance by F. japonica.     

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.     

Mechanisms of Central Die-back of Reynoutria japonica in the Volcanic Desert on Mt. Fuji. A Stochastic Model Analysis of Rhizome Growth

Reynoutria japonica (Polygonaceae) is a pioneer clonal herbcolonising the volcanic desert on Mt. Fuji (height 3776 m),central Japan; establishment of secondary successional speciesoccurs only in the central die-back parts of the clonal standsofR. japonica. Clonal stands were excavated and the morphologyand growth pattern of the rhizomes were investigated with specialreference to the mechanisms of central die-back. Four morphologicalparameters, length of mother rhizomes, and number, branchingpositions and branching angle of daughter rhizomes on respectivemother rhizomes were measured or counted, and their roles inrhizome growth were examined employing a stochastic computersimulation model of the whole stand development. The simulationsclarified that, of these four parameters, the branching anglewas the most influential determinant of the whole pattern ofshoot distribution and that the central die-back was producedwhen the mean branching angle was 40° or smaller. Theseresults strongly infer that the onset of central die-back iscaused intrinsically byR. japonica itself by having the meanbranching angle of 40°. The adaptive significance of thegrowth pattern is discussed in relation to resource acquiringand habitat exploitation strategies of this species. Below-ground morphology; branching angle; central die-back; clonal plant; computer simulation; Japanese knotweed;Reynoutria japonica Houttuyn; rhizome growth pattern     

Biomass and net production in a bambooPhyllostachys bambusoides stand

Biomass and net production were measured in aPhyllostachys bambusoides stand in Kyoto Prefecture, central Japan, which had carried out gregarious flowering in 1969 and has been recovering vegetatively. The culm density fluctuated around an average value of 12 040 ha⁻¹ during the research period (1985–91). Annual recruirment and mortality rates of culms were 1340 and 1133 ha⁻¹, respectively. The mean diameter at breast height increased from 7.28 cm in 1985 to 8.68 cm in 1991, and the biomass of culms increased from 71.3 to 111.6t ha⁻¹ over the same time period. Branch and leaf biomasses were almost constant, 10.0 and 9.4t ha⁻¹ on average, respectively. The leaf area index of the stand was 11.6 ha ha⁻¹, which is one of the largest values found in Japanese forests. The belowground biomass of 32.6t ha⁻¹ for rhizomes and 14.8t ha⁻¹ for fine roots resulted in the smaller ratio of aboveground parts to the root system (2.38) than those determined for forest stands. The amount of litterfall, excluding culms and large branches, was large (9.13t ha⁻¹ year⁻¹), corresponding to those measured in equatorial stands. The aboveground net production was 24.6t ha⁻¹ year⁻¹, larger than the average value reported for forest stands under similar weather conditions.     

Nitrogen dynamics of a boreal black spruce wildfire chronosequence

This study examined the nitrogen (N) dynamics of a black spruce (Picea mariana (Mill.) BSP)-dominated chronosequence in Manitoba, Canada. The seven sites studied each contained separate well- and poorly drained stands, originated from stand-killing wildfires, and were between 3 and 151 years old. Our goals were to (i) measure total N concentration ([N]) of all biomass components and major soil horizons; (ii) compare N content and select vegetation N cycle processes among the stands; and (iii) examine relationships between ecosystem C and N cycling for these stands. Vegetation [N] varied significantly by tissue type, species, soil drainage, and stand age; woody debris [N] increased with decay state and decreased with debris size. Soil [N] declined with horizon depth but did not vary with stand age. Total (live + dead) biomass N content ranged from 18.4 to 99.7 g N m⁻² in the well-drained stands and 37.8–154.6 g N m⁻² in the poorly drained stands. Mean soil N content (380.6 g N m⁻²) was unaffected by stand age. Annual vegetation N requirement (5.9 and 8.4 g N m⁻² yr⁻¹ in the middle-aged well- and poorly drained stands, respectively) was dominated by trees and fine roots in the well-drained stands, and bryophytes in the poorly drained stands. Fraction N retranslocated was significantly higher in deciduous than evergreen tree species, and in older than younger stands. Nitrogen use efficiency (NUE) was significantly lower in bryophytes than in trees, and in deciduous than in evergreen trees. Tree NUE increased with stand age, but overall stand NUE was roughly constant (∼ ∼150 g g⁻¹ N) across the entire chronosequence.     

Nitrogen and phosphorus economy of the riparian shrub <Emphasis Type="Italic">Salix gracilistyla</Emphasis> in western Japan

Salix gracilistyla is one of the dominant plants in the riparian vegetation of the upper-middle reaches of rivers in western Japan. This species colonizes mainly sandy habitats, where soil nutrient levels are low, but shows high potential for production. We hypothesized that S.␣gracilistyla uses nutrients conservatively within stands, showing a high resorption efficiency during leaf senescence. To test this hypothesis, we examined seasonal changes in nitrogen (N) and phosphorus (P) concentrations in aboveground organs of S. gracilistyla stands on a fluvial bar in the Ohtagawa River, western Japan. The concentrations in leaves decreased from April to May as leaves expanded. Thereafter, the concentrations showed little fluctuation until September. They declined considerably in autumn, possibly owing to nutrient resorption. We converted the nutrient concentrations in each organ to nutrient amounts per stand area on the basis of the biomass of each organ. The resorption efficiency of N and P in leaves during senescence were estimated to be 44 and 46%, respectively. Annual net increments of N and P in aboveground organs, calculated by adding the amounts in inflorescences and leaf litter to the annual increments in perennial organs, were estimated to be 9.9 g and 0.83 g m⁻² year⁻¹, respectively. The amounts released in leaf litter were 6.7 g N and 0.44 g P m⁻². These values are comparable to or larger than those of other deciduous trees. We conclude that S. gracilistyla stands acquire large amounts of nutrients and release a large proportion in leaf litter.     

Population dynamics,productivity and biomass allocation ofZizania latifolia in an aquatic-terrestrial ecotone

The population and production ecology of aZizania latifolia stand at a sheltered shore of the Hitachi-Tone River were investigated. Shoot emergence was observed twice a year; the fist was a synchronized shoot emergence in April and the second was from August to October. Aboveground biomass was mostly occupied by leaves and peaked at 1500 g dry weight m⁻² in August. The belowground biomass also reached its peak, 750 g dry weight m⁻², in August. The secondary shoots were small in spite of their high density. Leaves were produced continuously throughout the season. The leaf life span was as short as 55.6 days for cohorts that emerged from May through to September. Total annual net production ofZ. latifolia could be more than 3400 g dry weight m⁻². Shoot clusters of several centimeters were observed in April. The following self-thinning caused a regular distribution of the remaining shoots in August. Most shoots produced in August to October were found near a shoot persisting since April. They showed more concentrated distribution than shoots in April. A large biomass allocation to leaves and the ability to produce many clump shoots during the late growing period may facilitate dominance ofZ. latifolia in relatively sheltered sites.     

Nitrogen fixation in sediments and the rhizosphere of the seagrassZostera capricorni

Rates of nitrogen fixation in seagrass beds (Zostera capricorni) were determined with¹⁵N and reduction of acetylene in intact cores of sediment and seagrass. There was good agreement in the results from the two techniques, with a molar ratio of 3∶1.9 ethylene: ammonia produced. Fixed nitrogen was rapidly utilized by the plants, with significant amounts of¹⁵N found in the roots and rhizomes and 50% of fixed¹⁵N apparently translocated to the leaves. Rates of fixation were high in summer (25 to 40 mg N m⁻² day⁻¹) and lower in winter (10 mg N m⁻² day⁻¹) and were estimated to supply between one-third and one-half of the nitrogen requirements of the seagrass. Rates of nitrogen fixation were greater in the light than in the dark, and in cores of intact seagrass than in defoliated cores, indicating that the bacteria were dependent on organic compounds secreted by the plants.     

Estimating nitrogen budgets of Typha angustifolia by considering the regrowth shoot productivity and nitrogen content after harvesting aerial organs in different growing seasons

A dynamic model that includes regrowth after harvesting aerial shoots of an emergent macrophyte, Typha angustifolia L., was applied to evaluate the nitrogen (N) budget and the N uptake by the plant from sediment in Shibakawa Pond, Japan. Under natural conditions (control/uncut stands), the analysis showed that the annual uptake of N from sediment was 26.6 gN/m² and harvesting Typha shoots at their growth peak removed 29.0 gN/m² from the system (142 days in summer). Harvesting in winter after weathering of leaves removed only 13.9 gN/m². To evaluate the N budget considering regrowth shoot characteristics, three sets of harvesting experiments were done on 16 May, 8 July, and 5 August 2003. Our study revealed that May, July, and August harvesting removed 9.4, 21.9, and 16.3 gN/m², respectively. Further, combining the first harvesting from spring to summer and the second harvesting in autumn (before the start of senescence of regrowth shoots), the annual total N removals in stands cut in May and autumn and July and autumn were 34.7 and 36.0 gN/m², respectively—higher than that in stands cut in August and autumn (22.2 gN/m²) or that in uncut stands (13.9 gN/m²). At the same time, the amounts stored in rhizomes by stands cut in May and autumn, July and autumn, and August and autumn were 9.1, 8.4, and 4.4 gN/m², respectively, lower than that in uncut stands (18.8 gN/m²). Our results suggest that summer harvesting, especially in July to August, improves N removal efficiency and decreases the translocation of N from primary shoots to rhizomes, which is important for the sustainable management of Typha-dominated wetlands. Combined summer and autumn harvests further increase the removal efficiency but drastically reduce the storage of N. This might be useful when we need to control the plants properly.     

Rhizome development ofPhragmites australis in a reed community

Variations in the height, shoot density, biomasses of above- and below-ground parts and rhizome distributions ofPhragmites australis were investigated along a line-transect in a reed community at Yufutsu Mire, Hokkaido. Relationships of performance of the reed plants to soil conditions and species compositions were also examined. Three types of rhizome development were recognized in reed plants; (1) the central part of the reed community, characterized by well developed rhizomes and dense aerial shoots, (2) the intermediate part, characterized by development of rhizomes along both the peat and surface layers and very dense aerial shoots, and (3) the marginal part, characterized by development of rhizomes only along the peat layer and sparse aerial shoots. Observation showed that rhizomes in the surface layer actively produced aerial shoots, whereas rhizomes in the peat layer contributed to the spreading of their distribution range. With the growth of rhizomes, organic debris originating from dead rhizomes and roots accumulated in the mineral soil to promote organic soil formation. In dense parts of the reed stand, species composition was poor because of the shading and litter accumulation by reed plants. The effects of microtopography and water level on the establishment of reed seedlings were also considered.     

Influence of some plant growth regulators on the growth and organogenesis ofCymbidium aloifolium (L.) Sw. seed-derived rhizomesin vitro

Summary An efficient procedure is outlined forin vitro regeneration of an epiphytic orchid,Cymbidium aloifolium (L.) Sw. using rhizomes developed from seeds. Murashige and Skoog's (1962) medium (MS) containing indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), or 1-naphthaleneacetic acid (NAA) stimulated growth and proliferation of rhizomes with NAA being most effective at 5.0 mg.l⁻¹ (27.0 μM). Shoot bud differentiation was induced in the apical portions of the rhizomes on MS medium containing kinetin (Kn) or N⁶-benzyladenine (BA). The highest frequency of shoot regeneration (91.5%) and the maximum number of shoot buds formed (3.5 shoots/rhizome) were recorded with BA at 1.0 mg.l⁻¹ (4.4 μM). NAA (0.1 mg.l⁻¹, 0.54 μM), whenever added to the medium in conjunction with BA (1.0 mg.l⁻¹, 4.4 μM), slightly enhanced the frequency of shoot bud regeneration (92.6%) and the number of shoot buds formed (5.2 shoots/rhizome). Moreover, an NAA-BA combination induced rooting in regenerated shoots thereby producing complete plantlets in one step. Shoots developed on cytokinin-supplemented medium were rooted on MS containing NAA at 1.0 mg.l⁻¹ (5.4 μM). Regenerated plantlets were acclimated and eventually established in a garden.     

Decomposition of aboveground and belowground organs of wild rice (Zizania latifolia): mass loss and nutrient changes

Decomposition of aboveground and belowground organs of the emergent macrophyte Z.latifolia was investigated using a litterbag technique for a period of 359 days in a freshwater marsh in Japan. Aboveground parts were classified into: leaves, sheaths and stems. Belowground parts were classified into: horizontal rhizomes (new rhizome, hard rhizome, soft rhizome) and vertical rhizome (stembase). The decay rate (k) was 0.0036 day⁻¹, 0.0033 day⁻¹ and 0.0021 day⁻¹ for leaves, sheaths and stems, respectively. For belowground parts, the decay rate varied considerably from 0.0018 day⁻¹ to 0.0079 day⁻¹, according to differences in the initial chemical compositions of rhizomes. After 359 days of decay, new rhizomes lost 94% of their original dry mass, compared with a loss of 48–84% for the other rhizomes. There was a significant positive relationship between litter quality and decay rate for horizontal rhizomes. For the new rhizomes, which had an internal nitrogen content of 24.2 mg N g⁻¹ dry mass, the mass loss was 40% higher than that of soft rhizomes, which had an internal N content of 9.8 mg N g⁻¹ dry mass. Over the period of 359 days, the nitrogen concentration in all rhizome types decreased to levels lower than initial values, but the phosphorus concentration remained constant after an initial leaching loss. Most nitrogen and phosphorus were lost during the first 45 days of decay. Changes in carbon to nitrogen (C:N) and carbon to phosphorus (C:P) ratios basically followed inversed trends of the nitrogen and phosphorus concentrations.     

Expansion of <Emphasis Type="Italic">Phragmites australis</Emphasis> (Cav.) Trin. ex Steud. (Common Reed) into <Emphasis Type="Italic">Typha</Emphasis> spp. (Cattail) Wetlands in Northwestern Indiana,USA

Expansion of Phragmites australis (Cav.) Trin. ex Steud. (common reed) into stands of Typha spp. (cattail; Typha australis L. and T. x glauca) is common in the wetlands of northwestern Indiana (USA). To understand this phenomenon better, we investigated the production of shoot sprouts and proportional allocation of biomass as well as a potential role for the water table in the relative dominance of each species. The reduction in sprouts from rhizomes upon vegetative expansion of Phragmites appeared to be the most likely process causing the decline of Typha. The latter had a shoot density of 39/m² in plots without Phragmites, but this dropped to 13 shoots m⁻² in plots that had been invaded by Phramites. Such a decline was likely caused by reduced reserves; e.g., the belowground biomass of Typha decreased from 11.3 g m⁻² without Phragmites to 8.1 g m⁻² with Phragmites. The latter also reduced its belowground biomass but not its shoot density in the presence of Typha. The mean weight of Phragmites shoots was 2.9 g, and nearly all produced inflorescences. Meanwhile, Typha failed to develop spadices despite its shoots having a greater biomass (7 g). This suggests that Phragmites is more efficient than Typha in shoot growth. Springtime flooding appeared to promote the sprout of Typha shoots from shallow rhizomes (≈18 cm below the soil surface), whereas the shoot density of Phragmites showed no correlation with water level in that season. Deep-rooted Phragmites (≈39 cm) occurred on both high and low water-table sites, whereas the shallow-rooted Typha was limited to only the former. Phragmites will likely continue its expansion, by vegetative sprouts from rhizomes, into Typha wetlands.     

Maximum yield and the C-D effects in natural white birch stands

The competition-density effect for natural Japanese white bitch (Betula platyphylla var.japonica) stands, located in the northern part of Hokkaido Island, was examined for various growth stages to establish mean tree weight-density relationships over a considerable extent of stand growth of white birch. Aboveground yields of investigated stands ranged from 16.0 to 161.7 t·ha⁻¹, and it was recognized that maximum aboveground yield of this species was about 160 t·ha⁻¹. ConstantsA andB of the reciprocal equation of the C-D effect, 1/w=AN+B (w, mean tree weight; N, tree density), were related to mean height classes of birch stands. ConstantB was related to the entire mean height classes as the allometric relationship. On the other hand, constantA had an allometric relationship to mean height classes under 16 m, but became fixed over the 16 m class. White birch stands can come up to the maximum yield approximately at the stage of the 16 m class. The asymptotic tendency seemed to be appropriate for constantA-mean height class relationship. The maximum yield of white birch stands was predicted by the asymptotic value ofA. The trend of mean tree weight-density relationships with stand growth also changed with constantA. It was considered that maximum yield was an important characteristic for practical applications of the C-D effect to agriculture and forestry.     

Litter pool sizes,decomposition, and nitrogen dynamics in <Emphasis Type="Italic">Spartina alterniflora</Emphasis>-invaded and native coastal marshlands of the Yangtze Estuary

Past studies have focused primarily on the effects of invasive plants on litter decomposition at soil surfaces. In natural ecosystems, however, considerable amounts of litter may be at aerial and belowground positions. This study was designed to examine the effects of Spartina alterniflora invasion on the pool sizes and decomposition of aerial, surficial, and belowground litter in coastal marshlands, the Yangtze Estuary, which were originally occupied by two native species, Scirpus mariqueter and Phragmites australis. We collected aerial and surficial litter of the three species once a month and belowground litter once every 2 months. We used the litterbag method to quantify litter decomposition at the aerial, surficial and belowground positions for the three species. Yearly averaged litter mass in the Spartina stands was 1.99 kg m⁻²; this was 250 and 22.8% higher than that in the Scirpus (0.57 kg m⁻²) and Phragmites (1.62 kg m⁻²) stands, respectively. The litter in the Spartina stands was primarily distributed in the air (45%) and belowground (48%), while Scirpus and Phragmites litter was mainly allocated to belowground positions (85 and 59%, respectively). The averaged decomposition rates of aerial, surficial, and belowground litter were 0.82, 1.83, and 1.27 year⁻¹ for Spartina, respectively; these were 52, 62 and 69% of those for Scirpus litter at corresponding positions and 158, 144 and 78% of those for Phragmites litter, respectively. The differences in decomposition rates between Spartina and the two native species were largely due to differences in litter quality among the three species, particularly for the belowground litter. The absolute amount of nitrogen increased during the decomposition of Spartina stem, sheath and root litter, while the amount of nitrogen in Scirpus and Phragmites litter declined during decomposition for all tissue types. Our results suggest that Spartina invasion altered the carbon and nitrogen cycling in the coastal marshlands of China.     

Changes in nitrogen and base cation concentrations in soil water due to the tree cutting in a wetland alder forest in the Kushiro Wetland, northern Japan

As part of the Kushiro Wetland Restoration Project, we evaluated the effect of the tree-cutting (February 2003) on soil water chemistry for 3 years in an alder (Alnus japonica) shrubland on the peat soil in the Kushiro Wetland, the largest wetland in Japan. The alder stand was divided into two types; low stature and high stature stands, mean heights being 1.5 and 2.6 m, respectively. The treatment plot with tree cutting and the reference plot measuring 25 m × 25 m each were established in both stands, and a soil incubation experiment was also conducted by trenching the root zone of wetland vegetation in the treatment plots in both stands in the summer of 2004. The tree cutting did not substantially increase the concentrations of ammonium ion (NH₄ ⁺) and nitrate ion (NO₃ ⁻) in the soil water, although a gradual and slight increase of NO₃ ⁻ concentration was found after the tree-cutting only in the high stature stand (<10 μmol/l). This increase in the NO₃ ⁻ concentration in the high stature stand was probably due to the rhizosphere oxidation of wetland herbaceous vegetation, and was accompanied by the increase of Ca²⁺ concentration. The soil incubation experiment showed no significant change of the nitrogen concentration, suggesting that inorganic nitrogen was immobilized and NO₃ ⁻ was denitrified intensively in this wetland peat soil. The results of this study suggested that this wetland ecosystem had a high potential to stabilize the soil water chemistry.     

Central Die-back of Monoclonal Stands of Reynoutria japonica in an Early Stage of Primary Succession on Mount Fuji

Reynoutria japonica is a common perennial pioneer species onJapanese volcanoes. In a volcanic desert (1500m above sea level)on Mount Fuji (3776m), central Japan, this species forms circularstands (patches). As a patch develops, shoot density decreasesin its centre (‘central die-back’). The centraldie-back has been considered a key process in the early stagesof primary succession, though its mechanism has been unknown. The pattern of patch development, population dynamics of aerialshoots, and growth patterns of below-ground organs were analysedin order to investigate the mechanism of die-back, and the followingtraits are clarified: (1) central die-back areas occur in mostsmall patches (approx. 1m²) without later successional species;(2) shoot characteristics are dependent both on their positionwithin a patch and on patch size; (3) despite the large differencesin shoot density, neither time course of shoot growth nor theirmortality differs between the centre and periphery of patches;and (4) rhizomes ofR. japonicagrow outwards with regular sympodialbranching. From these results, it is concluded that neither interspecificnor intraspecific competition is likely to be a primary causeof the die-back phenomenon, but that central die-back is broughtabout intrinsically by the growth pattern of the rhizome systems.We also discuss the importance of the central die-back in facilitatingestablishment of later successional species in the early stagesof primary succession. Clonal plant; central die-back; competition; facilitation; Japanese knotweed; Mount Fuji; primary succession;Reynoutria japonica ; rhizome growth; volcanic desert     

Carbon, nitrogen and phosphorus in volcanic soils following afforestation with native birch (Betula pubescens) and introduced larch (Larix sibirica) in Iceland

Afforestation has become an important tool for soil protection and land reclamation in Iceland. Nevertheless, the harsh climate and degraded soils are growth-limiting for trees, and little is know about changes in soil nutrients in maturing forests planted on the volcanic soils. In the present chronosequence study, changes in C, N and total P in soil (0–10 and 10–20 cm depth) and C and N in foliar tissue were investigated in stands of native Downy birch (Betula pubescens Enrh.) and the in Iceland introduced Siberian larch (Larix sibirica Ledeb.). The forest stands were between 14 and 97 years old and were established on heath land that had been treeless for centuries. Soils were Andosols derived from basaltic material and rhyolitic volcanic ash. A significant effect of tree species was only found for the N content in foliar tissue. Foliar N concentrations were significantly higher and foliar C/N ratios significantly lower in larch needles than in birch leaves. There was no effect of stand age. Changes in soil C and the soil nutrient status with time after afforestation were little significant. Soil C concentrations in 0–10 cm depth in forest stands older than 30 years were significantly higher than in heath land and forest stands younger than 30 years. This was attributed to a slow accumulation of organic matter. Soil N concentrations and soil P_tot were not affected by stand age. Nutrient pools in the two soil layers were calculated for an average weight of soil material (400 Mg soil ha⁻¹ in 0–10 cm depth and 600 Mg soil ha⁻¹ in 10–20 cm depth, respectively). Soil nutrient pools did not change significantly with time. Soil C pools were in average 23.6 Mg ha⁻¹ in the upper soil layer and 16.9 Mg ha⁻¹ in the lower soil layer. The highest annual increase in soil C under forest compared to heath land was 0.23 Mg C ha⁻¹ year⁻¹ in 0–10 cm depth calculated for the 53-year-old larch stand. Soil N pools were in average 1.0 Mg N ha⁻¹ in both soil layers and did not decrease with time despite a low N deposition and the uptake and accumulation of N in biomass of the growing trees. Soil P_tot pools were in average 220 and 320 kg P ha⁻¹ in the upper and lower soil layer, respectively. It was assumed that mycorrhizal fungi present in the stands had an influence on the availability of N and P to the trees. Responsible Editor: Hans Lambers.     

In vitro regeneration of Alstroemeria cv. ‘Yellow King’ by direct organogenesis

The common techniques for the in vitro production of Alstroemeria plants are based on rhizomes as explants, which have low multiplication rates and a high risk of carrying viral diseases. To overcome these problems, we developed a protocol for the in vitro regeneration of Alstroemeria cv.‘Yellow King’, by testing for shoot induction several explant sources (leaf, stem apices, rhizomes and immature inflorescence apices), temperature and light/dark regimes, hormone and salt concentrations. For shoot multiplication and rooting, several hormone concentrations were tested. We found that only the young floral apices produced adventitious shoots by direct organogenesis. The highest shoot induction rate (10.4 shoots per explant) was obtained by incubation in the dark for 15 days at 8 °C followed by 15 days at 25 °C and a 16-h/8-h light/dark regime, on a Murashige and Skoog (1962) liquid medium at 50% of the salt concentration, supplemented with 2.5 mg l⁻¹ KIN, 1.5 mg l⁻¹ BA and 1.0 mg l⁻¹ NAA, using a piece filter paper to support the explant. The highest shoot multiplication rate (9 shoots per explant) was obtained on a liquid MS medium at full strength supplemented only with BA at 1.0 mg l⁻¹. In vitro rooting of shoots was induced also on a liquid MS medium, either with or without plant hormones.     

Variation in shoot mortality within crowns of severely defoliated <Emphasis Type="Italic">Betula maximowicziana</Emphasis> trees in Hokkaido,northern Japan

To clarify mortality patterns of current-year shoots within the crown of Betula maximowicziana Regel after severe insect herbivory in central Hokkaido, northern Japan, we investigated the degree of defoliation, pattern of shoot development, shoot mortality, and leaf tissue-water relations. One hundred current-year long shoots growing in a B. maximowicziana plantation were observed for defoliation and mortality in June 2002. An outbreak of herbivorous insects (Caligula japonica and Lymantria dispar praeterea) occurred in the stand in mid-to-late June, and the monitored shoots were defoliated to various degrees. Within 1 month of defoliation, some of the severely defoliated shoots had produced new leaves on short shoots that had emerged from axillary buds. Stepwise logistic regression revealed that the probability that current-year long shoots would put out axillary short shoots with leaves is closely related to the degree of defoliation. To evaluate the water relations of the leaves, we determined pressure–volume curves for the leaves that survived the herbivorous insect outbreak and the new leaves that emerged after defoliation. The water potential at turgor loss (Ψ_l,tlp) and the osmotic potential at full turgidity (Ψ_π,sat) were higher for the new leaves than for the surviving leaves, indicating a lower ability to maintain leaf cell turgor against leaf dehydration in the new leaves. Of the 100 shoots, 13 died after the emergence of new leaves. Stepwise logistic regression revealed that the probability that the long shoots would die generally increased with the emergence of new leaves, with increasing shoot height. This result suggests that the combined effect of the vulnerability of newly emerged leaves and low water availability, associated with higher shoot positions within the crown, caused shoot mortality. Based on our results, some possible mechanisms for mortality in severely defoliated B. maximowicziana are discussed.