北方澙湖大叶藻植株枚订移植法的效果评估与适宜性分析

2009年利用植株枚订移植法在我国北方典型澙湖——山东荣成天鹅湖逐月进行大叶藻(Zostera marina)植株移植, 并于当年逐月对移植植株的存活率、定居时间和生长进行监测, 分析该方法在我国北方澙湖的有效性和适宜性。结果显示: (1) 4-6月移植植株的存活率为76.5%-90.4%, 其中4月移植植株的存活率最低, 7-9月移植植株的存活率达到100%; (2) 6-9月移植植株的定居时间均为1个月, 5月移植植株的定居时间为2个月, 而4月移植植株的定居时间长达4个月; (3)除个别监测月份外, 移植植株的叶长和叶鞘长均显著小于天然植株, 而茎节直径和根长均与天然植株无明显差异; (4)我国北方澙湖较适宜大叶藻植株移植的区域为海水透明度高、水深不超过1 m的潮下带, 且底质为泥含量较高的泥砂底质海区, 9月份是适宜的移植时间。     

Effect of Planting Unit Size and Sediment Stabilization on Seagrass Transplants in Western Australia

Abstract The effect of increasing planting unit size and stabilizing sediment was examined for two seagrass planting methods at Carnac Island, Western Australia in 1993. The staple method (sprigs) was used to transplant Amphibolis griffithii (J. M. Black) den Hartog and the plug method was used to transplant A. griffithii and Posidonia sinuosa Cambridge and Kuo. Transplant size was varied by increasing the number of rhizomes incorporated into a staple and increasing the diameter of plugs. Planting units were transplanted into bare sand, back into the original donor seagrass bed, or into a meadow of Heterozostera tasmanica, which is an important colonizing species. Sprigs of A. griffithii were extracted from a monospecific meadow; tied into bundles of 1, 2, 5, and 10 rhizomes; and planted into unvegetated areas. Half the units were surrounded by plastic mesh and the remainder were unmeshed. All treatments were lost within 99 days after transplanting, and although larger bundles survived better than smaller ones, no significant differences could be attributed to the effects of mesh or sprig size. Plugs of P. sinuosa and A. griffithii were extracted from monospecific meadows using polyvinyl chloride pipe of three diameters, 5, 10, and 15 cm, and planted into unvegetated areas nearby. Half the units were surrounded by plastic mesh and the remainder were unmeshed. Posidonia sinuosa plugs were also placed within a meadow of H. tasmanica (Martens ex Aschers.) den Hartog. Only 60% of A. griffithii plug sizes survived 350 days after transplanting back into the donor bed; however, survival of transplants at unvegetated areas varied considerably, and analysis of variance indicated a significant two‐way interaction between treatment and plug size. Transplants survived better when meshed (90% survived) and survival improved with increasing plug size. Posidonia sinuosa transplants survived poorly (no plugs survived beyond 220 days in bare or meshed treatments) regardless of size. Survival of 10‐ and 15‐cm plugs was markedly better than the 5‐cm plugs in vegetated areas, including the H. tasmanica meadow. The use of large seagrass plugs may be appropriate for transplantation in high‐energy wave environments.     

A Comparative Test of Mechanized and Manual Transplanting of Eelgrass, Zostera marina, in Chesapeake Bay

Abstract The laborious process of manual seagrass transplanting has often limited the size of seagrass restoration efforts. This study tested the efficiency of a mechanized planting boat, previously used for transplanting Halodule wrightii, relative to manual transplanting methods for establishing Zostera marina in Chesapeake Bay. Eelgrass planting was conducted at two sites, one each in the Rappahannock and James rivers, in October 2001. The methods were evaluated by three criteria: (1) initial planting success = proportion of attempted planting units (PUs) initially established (number confirmed in sediment by divers/number attempted); (2) survival = proportion of the initially established PUs persisting over 1, 4, and 24 weeks; and (3) efficiency = labor (in person·seconds) invested in each surviving PU. Initial planting success was significantly lower for the planting boat (24 and 56% at the Rappahannock and James sites, respectively) than for manual transplanting (100% at both sites). At the Rappahannock site, survival of initially established PUs declined over time for both methods, but while mean survival was always higher for manually planted rows, differences in survival between methods were not statistically significant. At the James site, survival to 1 and 4 weeks was significantly lower for the machine than for the manual method, but survival to 24 weeks was not significantly different. While the machine was able to attempt PUs faster than the manual method (2.2 s/PU vs. 5.8 s/PU, respectively), this speed was offset by poorer planting success rates, resulting in a much greater total labor investment for each machine‐planted PU that persisted to 24 weeks than for each similarly persisting manually planted PU (40.6 person·seconds/PU and 22.4 person·seconds/PU, respectively, averaged across sites). In summary, those PUs successfully planted by the machine survived similarly to PUs planted by hand, but as a result of poorer initial planting success, the machine required a greater investment of labor and plant donor stock for each PU surviving to 24 weeks. Therefore, in its tested configuration this planting boat is not a significant improvement over the manual method for transplanting eelgrass.     

Seagrass Transplanting in Cockburn Sound, Western Australia: A Comparison of Manual Transplantation Methodology Using Posidonia sinuosa Cambridge et Kuo

Seagrass transplants (plant units [PUs]) were established to examine the feasibility of seagrass rehabilitation in Cockburn Sound, Western Australia. Five hundred and twenty plant units (plugs and sprigs) of Posidonia sinuosa Cambridge et Kuo were placed at seven locations at depths of 3, 5, and 9 m and monitored to determine the influence of transplant method, location, and depth upon survival and growth over 2 years. Depending on the site, more plugs had survived at the completion of the trial (mean survival 41%) than sprigs (mean survival 15%). Plug and sprig survival differed significantly with transplant depth, decreasing overall with increasing depth. Forty‐five percent of surviving plugs and 50% of sprigs exhibited horizontal rhizome extension. Mean rhizome extension after 2 years was 9.5 cm/plug (1–23 cm) and 18.3 cm/sprig (0.5–31 cm). Declines in PU survival and variable growth correlate with site‐specific variability in light climate. Plug transplantation was deemed the most suitable method for further manual seagrass rehabilitation, exhibiting higher survival across all sites and conditions; however, they are costly to deploy. Sprig PUs have greatest potential in shallow water with fine sands, moderate water movement, and maximum light availability. The low cost of deploying sprigs may outweigh their lower survival compared to plugs; further efforts should be directed to enhancing survival of sprig PUs under a wider range of conditions. Suitable locations for future rehabilitation efforts in Cockburn Sound were the Eastern and Western Banks and shallow areas off Woodman Point and Mangles Bay.     

Functional Anatomy of the Omasum in High Arctic Svalbard Reindeer (Rangifer tarandus platyrhynchus) and Norwegian Reindeer (Rangifer tarandus tarandus)

The structure and fill of the omasum was investigated in summer and in winter in adult female reindeer living on the polar desert and tundra of the high Arctic archipelago of Svalbard and in sub-Arctic mountain habitats in northern Norway The mean total mass of the omasum in non-lactating adult female Svalbard reindeer was 467 g (0.65 g per 100 g live body mass (BM)) in September and 477 g (1.03 g per 100 g BM) in April. By contrast, the mean mass of the omasum in non-lactating adult reindeer in northern Norway was 534 g (0.83 g per 100 g BM) in September but only 205 g (0.35 g per 100 g BM p<0.05) in late March, owing to a decrease in both tissue mass and the wet mass of the contents of the organ. The mean absorptive surface of the omasum in Svalbard reindeer was 2300 cm2 in September and 2023 cm2 in April. In Norwegian reindeer, by contrast, the absorptive surface area decreased from 2201 cm2 in September to 1181 cm2 (p<0.05) in late March. The marked seasonal decline of omasal tissue and contents in Norwegian reindeer probably results from intake of highly digestible forage plants, including lichens, in winter. Svalbard reindeer, a non-migratory sub-species, survive eating poor quality fibrous vascular plants in winter. The absence of any marked seasonal change in the mass, total absorptive surface area or filling of the omasum in Svalbard reindeer in winter despite a substantial decline in body mass presumably reflects their need to maintain maximum absorption of nutrients, including volatile fatty acids, when feeding on such poorly fermentable forage.     

The influence of planting date,transgenic Bt maize and hybrid relative maturity on European corn borer Ostrinia nubilalis (Lepidoptera: Crambidae) ovipositional patterns

A three year study was carried out at Hoytville and at Wooster, Ohio, USA from 2006 to 2008 to investigate the influence of planting date, transgenic maize and hybrid maturity on Ostrinia nubilalis (Hubner) population dynamics and oviposition patterns. Maize plants were planted in late April or early May, mid‐May and early June during each year. The moth flight pattern showed bivoltine generations during the three years. The first moth flight peaked in June, with the populations declining during July. The second moth flight peaked in August and declined towards the end of September or early October. Egg mass density did not differ significantly between transgenic and non‐transgenic maize of different maturities. Significant differences were observed, however, among planting dates, sampling dates, and sampling date × planting date interactions. Generally higher numbers of egg masses from second generation moths were deposited on late planted maize than middle and early plantings.     

化学杀虫剂对不同类型棉田害虫、天敌种群的影响

研究了化学杀虫剂对6种不同类型棉田的害虫、天敌种群数量及其相互作用的影响。结果表明,化学防治使棉花苗期棉蚜数量下降,但不能抑制中后期棉蚜数量;除豆间棉田外,其他类型棉田化防区棉红蜘蛛和棉铃虫数量低于未防区。化学防治使各类型棉田天敌数量减少,以单作棉田天敌数量减少最明显,间套作棉田天敌数量下降幅度较小;此外,棉田不同天敌种群所受的影响存在一定差异,化学防治对瓢虫、捕食蝽类的影响显著,数量下降剧烈,但蜘蛛类群下降幅度比前两者小。化学防治对棉田害虫与天敌的相互关系的影响随棉田类型或害虫、天敌种类的不同而有所差异。     

Factors affecting the establishment of <Emphasis Type="Italic">Schoenoplectus</Emphasis><Emphasis Type="Italic">tabernaemontani</Emphasis> (C.C. Gmel.) Palla in urban lakeshore restorations

Despite their central role in lakeshore restoration, most littoral wetland plantings fail. The reasons for these failures are poorly understood, in part due to limited information on the effects of planting time, water depth, and propagation on the survival of emergent macrophyte plantings. We planted pots and prevegetated mats of softstem bulrush (Schoenoplectus tabernaemontani (C.C. Gmel.) Palla) at two different water depths (0–30 and 31–60 cm) in five lakes each month between May and September 2006 to evaluate the effects of planting month, water depth, and transplant type on the survival of planted S. tabernaemontani. Overall survival decreased from 73% at 30 days after planting to 40% pre-winter to 15% post-winter. The timing of planting was the most important factor influencing bulrush survival. Survival of bulrush planted later in the growing season is poor, regardless of the transplant type used, and should be avoided. During the optimal planting season of early-to-mid summer, transplants from pots are more likely to outperform mats, despite lower pre-planting biomass. Water depth is only important immediately after planting, after which time, its influence on successful establishment diminishes. Overall, our research indicated that key choices made by the practitioner can improve the likelihood that transplants establish in littoral wetland restorations.     

Age-specific seasonal storage dynamics of<Emphasis Type="Italic">Phragmites australis</Emphasis> rhizomes: a preliminary study

Age-specific seasonal rhizome storage dynamics of a wetland stand of Phragmites australis (Cav.) Trin. ex Steud. in Japan, were investigated from April to October 2000. For each sampling date, above- and below-ground biomass and age-specific rhizome bulk density, ?_rhiz were measured. Seven rhizome age classes were recognized, from <1 year to six years old, based on their position within the branching hierarchy as main criteria and rhizome color, condition of nodal sheaths and condition of the shoots attached to vertical rhizomes as secondary criteria. P. australis stand was moderately productive, having a net aerial and below-ground production of 1980 and 1240 g m^?2, respectively, and a maximum mean shoot height of 2.33 ± 0.12 m. In spring, shoot growth started at the expense of rhizome reserves, decreasing the rhizome biomass as well as ?_rhiz. Both parameters reached the seasonal minimum in May followed by a subsequent increase, indicating a translocation of reserves to rhizomes from shoots after they become self supporting. For each sampling date, ?_rhiz increased with rhizome age. Given that the quantity of reserves remobilized by the rhizomes for spring shoot growth, as assessed by the drop in bulk density from April to May, were positively correlated (r = 0.97, P < 0.05) with rhizome age, it is proposed that for spring shoot formation older rhizomes remobilize stored reserves more actively than younger ones. Given that the accumulation of rhizome reserves (rise in bulk density) from May to August, May to September or May to November was negatively correlated (r = 0.97, 0.92 and 0.87, respectively, P < 0.05) with rhizome age, it seemed possible that younger rhizomes were ‘recharged’ at a higher rate than older ones. These resource allocation mechanisms pertaining seasonal rhizome storage dynamics are of paramount importance in formulating management and conservation strategies of wetlands and aquatic habitats. Our results indicate that a harvest of above-ground biomass from May to June would be more effective in reducing the growth than a harvest in July to August or later, when rhizome reserves have already been replenished. However, the latter may remove a larger shoot bound nutrient stock, still preserving a healthy stand for the subsequent years.     

An Experimental Evaluation of Different Methods of Restoring Phyllospadix torreyi (Surfgrass)

The surfgrass Phyllospadix torreyi is an abundant seagrass found on rocky exposed shores of the Pacific coast of North America. In southern California surfgrass populations are adversely affected by a range of natural events and anthropogenic activities. Few attempts have been made to develop restoration methods for surfgrass, and none have investigated the efficacy of using different life stages. We evaluated several techniques for restoration in intertidal and subtidal habitats using: (1) laboratory‐reared seedlings transplanted to the field (2) sprigs (short lengths of rhizome containing a few shoots) transplanted from undisturbed populations, and (3) plugs (a cohesive clump of shoots and rhizomes) transplanted from undisturbed populations. We calculated the net change in the aerial coverage of surfgrass after 6 months, taking into account the recovery or additional losses from the donor population, and amount of effort involved in transplanting. Transplanted seedlings survived poorly and had minimal rhizome growth at both the intertidal and the subtidal sites, yet the individuals that did survive showed a 275% increase in leaf number. Survivorship of transplanted plugs was high in both habitats; however, physical disturbances to the donor populations exacerbated damage sustained at the time of collecting, yielding a substantial net loss in surfgrass. Sprigs transplanted to the subtidal had higher survivorship (71 versus 48%) and a greater increase in the aerial coverage of rhizome (86 versus 42%) than those transplanted to the intertidal. Of the three techniques, transplanted sprigs had the greatest overall increase in aerial coverage per unit effort, suggesting that this method may be the most effective approach for restoring P. torreyi.     

Cytokinin activity in oak (Quercus robur) with particular reference to transplanting

Cytokinin activity in sap collected under vacuum from field grown oaks (Quercus robur L.) was determined at monthly intervals throughout the year. A very low level in January was followed by an increase in February and March which reached its maximum 20–25 days before bud-break. Levels decreased through April and May during leaf expansion, rising once more in June, 10–15 days before a second shoot growth flush. The levels then progressively decreased to reach a minimum in November and December. Cytokinin activity in roots, 24 h and one week after root-tip removal did not differ significantly from that in intact control plants. Cytokinin activity in roots and shoots determined in relation to shoot growth flushes revealed no differences in the shoot while the activity in root extracts was lowered after a single flush of shoot growth. The effects of applying growth regulators at the time of transplanting were investigated by soaking entire transplants in hormone solution. Gibberellin promoted shoot growth at the expense of root growth, but root growth could be restored to normal levels by a simultaneous application of auxin. Cytokinin treatment increased leaf number, but resulted in small bushy plants. The seasonal changes in cytokinin activity are discussed with regard to successful transplanting.     

Factors governing survival of black willow (Salix nigra) cuttings in a streambank restoration project

A field study was conducted at Little Topashaw Creek in northern Mississippi, aimed at expanding the limited database on the survivorship of Salix nigra (black willow) cuttings planted on riparian restoration sites. We tested the hypothesis that sediment moisture availability (deficit, excess) as mediated by sediment texture and depth to the prevailing water table is a major factor governing black willow survival during the initial stage of establishment following transplanting. Replicated plots were established across elevational gradients and a range of soil texture. Each plot contained 16 planted cuttings (2.5 cm diameter × 2.5 m length). Plot depth to water table, soil texture, and soil redox potential were measured. Plant gas exchange, leaf chlorophyll content, growth, and survival were monitored periodically over two growing seasons. Survival was best at low elevation compared to cuttings planted at mid- and high elevations. Poor survival and growth were noted for cuttings that encountered sediment moisture deficits in plots with coarse texture while the best cutting survival was recorded for intermediate sand content plots. Results indicated that plot location on the bank and soil texture are two important factors that influence riverbank restoration success. Therefore, any riparian restoration plan should include careful assessment of these factors prior to undertaking such efforts.     

Effects of seeding date and canola species on seedling damage by flea beetles in three ecoregions

Flea beetles ( Phyllotreta striolata (Fabricius), Phyllotreta cruciferae (Goeze) and Psylliodes punctulata Melsheimer) (Coleoptera: Chrysomelidae) are chronic threats to canola seedling establishment throughout the Northern Great Plains of North America. We conducted field studies in AB, Canada, from 2001 to 2004 at Lethbridge, Lacombe and Beaverlodge in the southern, central and northern regions of the province, respectively, to assess the impacts of seeding date (fall, April, May), and canola species ( Brassica rapa L., Brassica napus L.) on flea beetle damage to canola. Flea beetle damage was generally low at Lacombe but reached economic threshold levels at other sites. Although our results were not consistent in all years, seeding date had opposite effects on flea beetle damage to canola seedlings at the southern and northern sites. In the south, both canola species planted in April escaped flea beetle damage unlike the May-seeded plots, which required insecticide spraying in 2003. The numbers of flea beetles on sticky cards sampled during the seedling stage of canola were also lower in early-planted plots than in those planted in May. In the north, flea beetle damage was lower in the May-planted plots compared with those planted earlier. To maximize yields, canola growers need to plant as early as possible in all agro-ecoregions; our study showed that crops in the central and northern regions will be at greater risk of flea beetle damage than in southern regions.     

转基因抗草甘膦抗虫棉的荒地生存竞争能力

【背景】随着转基因作物的大量种植,其环境安全性已成为目前的研究热点,其中转基因作物的荒地生存竞争能力,是其环境安全性评价的重要内容之一。【方法】以2种转双价抗草甘膦抗虫基因（EPSPS＋crylAc）棉为试验品种,非转基因棉中棉所49为对照品种,分别于4月底和5月底在荒地用地表撒播和3cm深播2种方式播种,并于4～9月份对棉花存活率、株高、真叶数、茎直径、果枝数、蕾铃数等生长参数进行比较,以检测、评价其荒地竞争能力及杂草化风险。【结果】无论是撒播还是深播,荒地中棉花长势均较弱,表现为生长缓慢、植株矮小、生育期延迟。4月底播种的棉花有极个别植株能够结铃,但铃数极少且小,无成熟的种子;5月底播种的棉花长势更弱,至9月底未见蕾铃。【结论与意义】转基因抗草甘膦抗虫棉在荒地中无竞争优势,不能繁殖传代,无杂草化风险。本研究为转基因棉花环境安全评价技术的完善提供了理论依据。     

PHYSIOLOGICAL AND POPULATION ECOLOGY OF INTERTIDAL AND SUBTIDAL ASCOPHYLLUM NODOSUM (PHAEOPHYTA)1

Morphological, demographic and physiological characteristics of Rhode Island intertidal and subtidal populations of Ascophyllum nodosum (L.) Le Jolis were compared in order to examine factors influencing vertical distribution. The two populations had distinctive morphologies: subtidal plants were narrower (more terete) and highly branched compared with intertidal plants. The subtidal population showed signs of necrosis and breakage, which was reflected in significantly shorter mean plant size. High survivorship and low recruitment of both population resulted in relatively constant densities, averaging 91 and 50 plants per m² in the intertidal and subtidal habitats, respectively. Intertidal plants had higher mean annual growth rates (25 cm.yr^?1) than subtidal plants (2 cm.yr^?1). In general, intertidal plants had higher photosynthetic capacity and nutrient (NO₃^?) uptake rates than the subtidal population but maintained lower light-harvesting pigment and tissue nitrogen concentrations. Although Ascophyllum nodosum is capable of survival and growth in subtidal as well as intertidal areas, results of this study suggest that different selective pressures affect persistence in each habitat. The scarcity of plants in the subtidal environment may be due to the lack of a critical balance between algal production, allocation of photosynthate, and the negative effects of grazers or competitors.     

Effect of sand cover on the survival and vigor ofRosa rugosa Thunb.

Rugosa rose,Rosa rugosa Thunb., is adapted to the northeast coast of the United States. It can be used to trap blowing sand or planted for beautification. Unpredictable weather conditions during the year of establishment have caused survival to be generally poor. Field observations have indicated that rugosa rose spruted from old plants when covered with 30 to 60 cm of sand. To test this observation, rugosa rose was planted at three depths: 0, 15 and 30 cm deeper than the normal planting depth. Roses planted 30 cm deeper than normal developed expanded leaves sooner than did those planted at the normal depth. Peat moss mixed in the planting hole tended to delay leaf formation for the 0 cm treatment but had no effect on survival at any depth. Survival during the first year was 100% for the 30 cm depth and 93% for 0- and 15-cm treatments. The deeper planted roses were more vigorous at the end of the season than were those set at the normal depth. Rose hips developed on plants set at the two deeper treatments. No hips were produced on those plants for the normal depth. New roots grew on the buried stems. These new roots were near the sand surface rather than uniformly distributed along the buried stems.Presented at the Seventh International Biometeorological Congress, 17–23 August 1975, College Park, Maryland, USA.     

Restoring Eelgrass (Zostera marina L.) Habitats Using a Simple and Effective Transplanting Technique

Eelgrass beds in coastal waters of China have declined substantially over the past 30 years. In this study, a simple new transplanting technique was developed for eelgrass (Zostera marina L.) restoration. To assist in anchoring single shoots, several rhizomes of rooted shoots were bound to a small elongate stone (50–150 g) with biodegradable thread (cotton or hemp), and then the bound packet was buried at an angle in the sediments at a depth of 2–4 cm. This stone anchoring method was used to transplant eelgrass in early November 2009 and late May 2010 in Huiquan Bay, Qingdao. The method led to high success. Three month survivorship of the transplanted shoots at the two transplant sites was >95%. From April 20 to November 19, 2012, the following characteristics of the 2009 and 2010 transplanted eelgrass beds were monitored: morphological changes, shoot density, shoot height, leaf biomass, and sediment particle size. Results showed that the sexual reproduction period of the planted eelgrass was from April to August, and vegetative reproduction reached its peak in autumn. Maximum shoot height and biomass were observed in June and July. After becoming established, the transplanted eelgrass beds were statistically equal to natural eelgrass beds nearby in terms of shoot height, biomass, and seasonal variations. This indicates that the transplant technique is effective for eelgrass restoration in coastal waters.     

Surface and submarine pollination in the seagrass Zoster a marina L

COX, P. A., LAUSHMAN, R. H. & RUCKLESHAUS, M. H., 1992. Surface and submarine pollination in the seagrass Zostera marina L. Hydrophilous plants can be divided into three ecological categories depending upon whether their pollen is transported above, on, or under the water surface. A mixed mode of submarine and surface hydrophilous pollination occurs in the seagrass Zostera marina L. In the surface mode of pollination, pollen rafts or 'search vehicles' which superficially resemble snowflakes, form at low tide and are transported on the surface of the sea by winds and water currents. Some of the search vehicles collide with the floating female stigmas, effecting pollination. In the submarine mode of pollination, small pollen masses resembling whisk brooms travel beneath the surface of the water. Although we failed to observe a submarine pollination event, SEM analysis of stigmas from subtidal populations confirms that submarine pollination does occur in Z. marina . However, observations of stigmas positioned at and below the surface of the water show surface pollination to be highly efficient. Electrophoretic evaluation of both subtidal and intertidal populations indicates significant genetic variation between populations. Given the high flux rates of surface-borne pollen and pollen viability in excess of 5 h, it is likely that surface-borne pollen is a major source of gene flow in Zostera marina populations.     

Dynamics of growth,carbon and nutrient translocation in Zizania latifolia

We studied the seasonal resource dynamics between organs of wild rice (Zizania latifolia (Griseb.) Turcz. ex Stapf.) to obtain a better understanding of its growth dynamics, carbon and nutrient translocation. The results of observation from January 2002 to February 2004 showed the shoot density markedly increased after emergence of shoots at the end of March until May (up to 800 ind/m²). However the shoot mortality due to self-thinning reduced the total new shoots by more than 70% by the end of July. Thereafter, the shoot density was nearly constant with the aboveground biomass peaking at the end of August. In the late winter, the rhizome biomass declined by respiration loss to about 25% of its peak value. Meanwhile the decline in rhizome reserves from January to the end of April was about 20%. This small reduction compared with other perennial emergent species implies that there is a lower contribution of rhizome reserves to support new shoot formation. The initial heterotrophic growth of new shoots based on the rhizome resources lasted for a short period, then switched to autotrophic growth at the end of April or the beginning of May. Thus, in most periods of foliage development, nutrients were obtained mostly from soil through uptake by roots, not through resource allocation of the rhizome. In autumn, the standing dead shoots retained most of the nutrients and carbohydrates without translocating downwards. This suggests that in practice, the plant can remove nutrients from sediment more efficiently than other emergent plants.     

Reproduction and recruitment of the seagrass Halophila stipulacea

The small seagrass species, Halophila stipulacea is abundant in the subtidal zone of the Bay of Eilat, Red Sea, southern Israel. Early life history characteristics of this species were investigated in summer 2002 by means of field surveys and outdoor experiments. Monospecific stands were found at depths of between 2 and 20 m. Reproduction began in late May and ripe pericarps were found for 1 month starting from the beginning of August. The ratios of female versus male plants were 0.9 at depths of between 2.5 and 10 m and 0.5 at depths of between 12.5 and 15 m. The proportion of reproductive branches was significantly larger in the shallow (2–5 m) than in the deep (7–15 m) populations, i.e., 20 ± 11% versus 6 ± 10%, respectively. Ripe seeds were predominantly produced at depths of between 2 and 5 m. Experimental studies demonstrated that full sunlight completely inhibited seedling growth at a depth of 30 cm; no macroscopic seedlings could be observed after 40-day exposure to full sunlight. If exposed to 90% photosynthetic active radiation (PAR) but protected from ultraviolet radiation (UVR), the number of macroscopic seedlings increased to 7.4 ± 2.3% of the planted seeds. If protected from both UVR and 80% of the PAR, the number of macroscopic seedlings increased to 22.5 ± 4.0% of the planted seeds. UVR exclusion and 80% PAR reduction also significantly increased the rhizome growth rates of seedlings in the first month after germination (0.14 ± 0.04 mm day⁻¹) compared with only UVR exclusion (0.04 ± 0.02 mm day⁻¹). The absence of H. stipulacea from the uppermost part of the subtidal zone (depths of 0–2 m) may be due to light inhibition of germling growth and uprooting by occasional storms.