The effects of control on the biomass, carbohydrate content and bud reserves of bracken (Pteridium aquilinum (L.) Kuhn), and an evaluation of a bracken growth model

This paper examines the initial effects of bracken control on frond numbers and biomass, and the biomass, carbohydrate reserves and bud densities of bracken stands cut once per year, twice per year, subject to a single application of asulam or left untreated. The seasonal dynamics of these parameters are displayed; carbohydrate and biomass are both removed from the rhizome system to produce frond tissue, and are replenished at the end of the growing season. Asulam application reduced densities of both active and dormant buds, and both frond biomass and density. It did not significantly reduce rhizome biomass or carbohydrate reserves in the two years after treatment. Cutting, either once or twice per year reduced both rhizome biomass and rhizome carbohydrate reserves, as well as bud densities, though the latter were reduced in proportion to biomass. Cutting twice a year reduced the production of fronds, both in numbers and biomass. The collected data were used to evaluate a model of bracken growth, and subsequently to improve estimates of some of the model parameters. The model simulations of control treatments were compared to field data. The effects of cutting once per year and spraying with asulam were predicted accurately, but the bracken stand was more resilient to cutting twice per year than would be expected from model predictions. The combination of cutting and spraying is discussed as a potential tool in land management and the deficiencies of the model are discussed in relation to the need for future research into the biology of bracken.     

Do rhizome severing and shoot defoliation affect clonal growth of Leymus chinensis at ramet population level?

Leymus chinensis (Trin.) Tzvel. is a perennial species of Gramineae, usually subject to defoliation from grazing and mowing. We examined whether shoot defoliation and rhizome severing affected rhizome and ramet growth, and vegetative bud outgrowth of L. chinensis ramet populations. We also tested the hypothesis that clonal growth of the ramets subject to defoliation would benefit from clonal integration between interconnected ramets besides from possible compensatory growth. To 48 experimental plots, we applied six treatments resulting from interactions between two rhizome connection states (unsevered/severed) and three defoliation regimes (non-defoliated, mildly-defoliated and heavily-defoliated). Defoliation affected rhizome growth and bud outgrowth, but had little effect on shoot growth. Mild and heavy defoliation exerted similar effects on rhizome growth. Only heavy defoliation significantly reduced bud outgrowth while mild defoliation did not. The fact that shoot growth did not change after defoliation and that the bud numbers remained unchanged after mild defoliation suggest that the compensatory response enable the species to tolerate grazing to some extent. Neither rhizome severing nor the interaction of rhizome severing and defoliation had effect on any tested variables. Lack of the effect of rhizome severing falsified the first half of our hypothesis, that is, clonal integration was unimportant in our experiment. The probable reasons were suspected to be the short duration of the experiment and/or the buffer effect of carbohydrate reserves in rhizomes for shoot growth and bud production in time of defoliation.     

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.     

The importance of nitrogen and carbohydrate storage for plant growth of the alpine herb Veratrum album

We examined whether nitrogen (N) and carbohydrates reserves allow Veratrum album, an alpine forb, to start spring growth earlier than the neighbouring vegetation and to survive unpredictable disturbances resulting in loss of above-ground biomass. * Seasonal dynamics of plant reserves, soil N availability and vegetation growth were monitored. Veratrum album shoots were experimentally removed when carbohydrate reserves were at a seasonal minimum and the subsequent changes in biomass and reserves were compared with those in control plants. Reserves did not give V. album a competitive advantage in spring; however, they did function as a buffer against the impact of calamities. Shoot removal resulted in significantly lower root dry weight, higher N concentration in rhizome and roots and lower starch concentrations in rhizome and roots but no plant mortality was observed. Veratrum album used stored N reserves to supplement N uptake and establish high leaf N concentrations, which facilitated a rapid refilling of depleted carbohydrate reserves. The primary function of N reserves appears to be to allow V. album to complete the growing cycle in as short a period as possible, thus minimizing exposure to above-ground risks.     

Ontogenetic photosynthetic changes, dispersal and survival of Thalassia testudinum (turtle grass) seedlings in a sub-tropical lagoon

Northward expansion of Thalassia testudinum (turtle grass) in Laguna Madre is occurring faster than can be explained by rhizome growth. We hypothesized that seedling establishment can account for the measured rates of meadow expansion and that seedling carbohydrate reserves are utilized until the plant is photosynthetically self-sufficient. To address seedling establishment, we estimated seed output, seedling dispersal and survival. Carbon dynamics were calculated from measurements of biomass allocation, non-structural carbohydrate carbon reserves and photosynthetic parameters in relation to T. testudinum seedling age. Potential seed production calculated for 1996 was consistent with field observations and was estimated at 66±14 seeds m⁻² bare area. Fruits can be positively buoyant for up to 10 days, while seeds were generally buoyant for <1 day. Water current measurements, made at about the time of seed release, indicate a positive net transport of 1.5 km d⁻¹ to the north. Seedling survival in laboratory culture after 6 months was 96% compared to 11% in the field after 1 year. The average root:rhizome+seed:leaf ratio changed from 0:11:1 for a 1 week old plant to 1:3:1 for a 15 month old plant. Seedlings used to determine whole plant photosynthesis ranged in age from about 1 week (0.25 months) to 15 months. Gross P_max increased from 80 to 220 μmol O₂ gdw sht⁻¹ h⁻¹, while whole plant respiration decreased from 170 to 60 μmol O₂ gdw sht⁻¹ h⁻¹. As the photosynthetic parameters changed, the average non-structural carbohydrate carbon (NSCC) reserves of the seeds decreased from 24 to 3.0 mg NSCC plant⁻¹. Subsequent increases in NSCC were the result of rhizome development. Daily carbon balance, assessed using H_sat periods of 8–18 h d⁻¹, predicts that T. testudinum seedlings become photosynthetically self-sufficient between 2 and 6 months. The unique characteristics of T. testudinum, including seed buoyancy, high seed production and survival rates, coupled with ontogenetic changes in carbon allocation and production imply that sexual reproduction can be important in the long distance dispersal and colonization for this species.     

Rhizome dynamics and resource storage in Phragmites australis

Seasonal changes in rhizome concentrations of total nonstructural carbohydrates (TNC), water soluble carbohydrates (WSC), and mineral nutrients (N, P and K) were monitored in two Phragmites australis stands in southern Sweden. Rhizome biomass, rhizome length per unit ground area, and specific weight (weight/ length ratio) of the rhizomes were monitored in one of the stands.Rhizome biomass decreased during spring, increased during summer and decreased during winter. However, changes in spring and summer were small (< 500 g DW m-2) compared to the mean rhizome biomass (approximately 3000 g DW m^–2). Winter losses were larger, approximately 1000 g DW m-2, and to a substantial extent involved structural biomass, indicating rhizome mortality. Seasonal changes in rhizome length per unit ground area revealed a rhizome mortality of about 30% during the winter period, and also indicated that an intensive period of formation of new rhizomes occurred in June.Rhizome concentrations of TNC and WSC decreased during the spring, when carbohydrates were translocated to support shoot growth. However, rhizome standing stock of TNC remained large (> 1000 g m^–2). Concentrations and standing stocks of mineral nutrients decreased during spring/ early summer and increased during summer/ fall. Only N, however, showed a pattern consistent with a spring depletion caused by translocation to shoots. This pattern indicates sufficient root uptake of P and K to support spring growth, and supports other evidence that N is generally the limiting mineral nutrient for Phragmites.The biomass data, as well as increased rhizome specific weight and TNC concentrations, clearly suggests that reloading of rhizomes with energy reserves starts in June, not towards the end of the growing season as has been suggested previously. This resource allocation strategy of Phragmites has consequences for vegetation management.Our data indicate that carbohydrate reserves are much larger than needed to support spring growth. We propose that large stores are needed to ensure establishment of spring shoots when deep water or stochastic environmental events, such as high rhizome mortality in winter or loss of spring shoots due to late season frost, increase the demand for reserves.     

THE ANNUAL CARBOHYDRATE CYCLE OF ALPINE PLANTS AS RELATED TO GROWTH

Mooney, H. A., and W. D. Billings. (Duke U., Durham, N. C.) The annual carbohydrate cycle of alpine plants as related to growth. Amer. Jour. Bot. 47(7): 594–598. Illus. 1960.—Analyses were made of the carbohydrate content of roots, rhizomes, and shoots of certain plants in the alpine tundra region of the Medicine Bow Mountains, Wyoming, from the beginning of one growing season until the start of the following one. Principal species investigated were Saxifraga rhomboidea, Polygonum bistortoides, and Geum turbinatum. Results were correlated with phenological events as observed in the field. Growth of these plants was found to be very rapid, commencing in some instances under a cover of old snow. The underground organs contained relatively large amounts of carbohydrate reserves. A great part of this stored carbohydrate was utilized in growth prior to snowmelt and during the grand period of shoot growth immediately following snowmelt. In Polygonum, 50% of the rhizome reserves was used in a 1-wk. period in early growth. Except for this short period of rapid depletion in rhizomes and roots during early growth, high carbohydrate levels were maintained both in the shoot and in the underground organs during most of the growing season. Generally, the lowest carbohydrate reserve level in both root and shoot occurred before flowering, a relatively high level was maintained in the shoot from flowering until after fruiting, while peak storage in underground parts was reached at the start of fall dormancy. The carbohydrate cycle in these alpine plants is quite similar to that in certain arctic plants.     

Seasonal changes in shoot regrowth potential in Calamagrostis canadensis

Summary A series of laboratory experiments was conducted to examine seasonal change in shoot regrowth potential following disturbance in Calamagrostis canadensis. On several dates during the 1988 and 1989 growing seasons, soil cores were collected from field sites dominated by this grass. Shoot regrowth from cores after clipping at the soil surface was monitored under dark or light laboratory conditions at 20°C. seasonal changes in field concentrations of total nonstructural carbohydrate and nitrogen in rhizomes largely accounted for the observed seasonal change in etiolated regrowth potential of shoots in laboratory experiments. In contrast, shoot regrowth potential in the light showed a very different seasonal pattern. The ratio of shoot biomass regrowth 20 d after clipping in the light versus dark treatment showed a gradual seasonal decrease from 12:1 in the early May experiment to near 1:1 in the September experiment. However, the rate of photosynthesis of regrowing shoots in the light was highest in experiments conducted late in the growing season. This may indicate a strong seasonal decrease in the proportion of current photosynthate of regrowing shoots that is allocated to new shoot growth. Alternatively, mobilization of rhizome carbohydrate reserves for shoot regrowth may have been inhibited during the re-establishment of photosynthesis in the light treatment. Either mechanism would explain why shoot regrowth in the light is poorly correlated with levels of belowground carbohydrate reserves, even under controlled laboratory conditions.     

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.     

Interspecific and intraspecific variation in tree seedling survival: effects of allocation to roots versus carbohydrate reserves

We examined interspecific and intraspecific variation in tree seedling survival as a function of allocation to carbohydrate reserves and structural root biomass. We predicted that allocation to carbohydrate reserves would vary as a function of the phenology of shoot growth, because of a hypothesized tradeoff between aboveground growth and carbohydrate storage. Intraspecific variation in levels of carbohydrate reserves was induced through experimental defoliation of naturally occurring, 2-year-old seedlings of four northeastern tree species –Acer rubrum, A. saccharum, Quercus rubra, and Prunus serotina– with shoot growth strategies that ranged from highly determinate to indeterminate. Allocation to root structural biomass varied among species and as a function of light, but did not respond to the defoliation treatments. Allocation to carbohydrate reserves varied among species, and the two species with the most determinate shoot growth patterns had the highest total mass of carbohydrate reserves, but not the highest concentrations. Both the total mass and concentrations of carbohydrate reserves were significantly reduced by defoliation. Seedling survival during the year following the defoliation treatments did not vary among species, but did vary dramatically in response to defoliation. In general, there was an approximately linear relationship between carbohydrate reserves and subsequent survival, but no clear relationship between allocation to root structural biomass and subsequent survival. Because of the disproportionate amounts of reserves stored in roots, we would have erroneously concluded that allocation to roots was significantly and positively related to seedling survival if we had failed to distinguish between reserves and structural biomass in roots. Received: 14 December 1999 / Accepted: 2 June 1999     

Carbohydrate storage and use in an alpine population of the perennial herb, Oxytropis sericea

I tested hypotheses for ecological roles of storage carbohydrates in perennating organs (roots and branches) of alpine Oxytropis sericea, a leguminous herb. In naturally growing plants, total nonstructural carbohydrates achieved their maximal concentration in the fall, declined during winter, and reached minimal levels immediately after growth initiation in the spring. Experimental manipulation of carbon sink-source relations through shading of leaves of reproductive plants revealed that the normally unused portion of these carbohydrates is largely available for withdrawal. In another experiment, plants subjected to carbohydrate depletion through shading suffered decreased leaf growth after winter dormancy and had a lower probability of flowering and decreased inflorescence biomass. The dependence of reproductive growth on stored carbohydrates, however, was limited to its initial stages, because accumulation of storage carbohydrates occurred simultaneously with inflorescence expansion, flowering, and fruiting. Moreover, the whole-plant photosynthetic rate, estimated from gas exchange measurements also peaked at the time of inflorescence growth. To address whether stored reserves allow compensatory regrowth following defoliation, plants were subjected to experimental removal of leaves and inflorescences. Defoliated O. sericea partly regrew the lost leaves but withdrawal of stored carbohydrates was limited. Similarly, in a second defoliation experiment where infructescences were left intact, the plants used little stored carbohydrate and only partly compensated for fruit growth. However, carbohydrate accumulation was negatively affected by defoliation. While the ecological importance of stored nonstructural carbohydrates cannot be attributed to any function in isolation, winter respiration, leaf regrowth after winter, and early reproductive growth in O. sericea all depend to a significant extent on stored reserves. Maintaining a large storage pool may protect these functions in years when carbon status is less favorable than during this study. Received: 13 May 1998 / Accepted: 24 November 1998     

羊草根茎的贮藏碳水化合物及对氮素添加的响应

 为了研究氮素对羊草(Leymus chinensis)根茎碳水化合物贮藏的影响,在中国科学院内蒙古草原生态系统定位研究站的羊草样地,设计了不同水平和不同时期的氮素添加试验。采用高效液相色谱（High Performance Liquid Chromatography, HPLC）对羊草根茎中的贮藏性碳水化合物进行了测定。结果表明,羊草根茎中的贮藏碳水化合物组分包括果聚糖、甘露糖醇、蔗糖、葡萄糖和果糖。其中果聚糖是最主要贮藏碳水化合物,约占60%;其次是甘露糖醇,约占20%。氮素添加量对羊草根茎中的贮藏碳水化合物有显著影响。在0～17.5 g N·m－2范围内,随着氮素添加量的增加,碳水化合物总量、果聚糖、甘露糖醇的含量均逐渐升高。氮素添加时期对羊草根茎中的贮藏碳水化合物的含量亦有显著影响。在7月初添加氮素比4月份添加氮素更有利于贮藏碳水化合物的积累。 关键词     

The morphology of bracken (Pteridium aquilinum (L.) Kuhn) in the North York Moors—a comparison of the mature stand and the interface with heather (Calluna vulgaris (L.) Hull) 2. The rhizome

The rhizome system of mature bracken (Pteridium aquilinum (L.) Kuhn) contains large reserves of both biomass (mean 8.63 kg m?² fr. wt) and buds (mean 565 m^-2) which are largely responsible for both its persistence and its often rapid rates of vegetative encroachment. Within areas such as the North York Moors the spread of bracken into areas previously dominated by heather and grass is considered undesirable because of reduced land value in terms of both agriculture and ecological diversity. In this paper we describe the morphology of bracken rhizome within a mature bracken stand, and at advancing and stationary stand margins where bracken-heather interfaces occur. Stationary margins, i.e. those where bracken is not encroaching into heather at a significant rate, often have morphological characteristics intermediate to those of a mature stand and an advancing margin. In the mature stand rhizome biomass is dominated by carbohydrate-storing long shoots which comprise 63% of the total fresh weight, whilst the majority of rhizome buds (89% of all active and 86% of all dormant buds) are found on frond-bearing short shoots. At the margins of a bracken stand the proportion of rhizome which is composed of long shoots is even greater, and that of short shoots small relative to that in the mature stand. More transitional shoots are also found at the stand margins. Hence close to the margin a greater proportion of fronds is found on transitional rhizome than is the case in more mature parts of the stand. The majority of buds on all types of rhizome are in a dormant state. The proportion of buds which are active is, however, greater on long and transitional shoots than on short shoots. Hence, a larger proportion of buds are active close to the margin where the rhizome is composed less of short shoots than is the case further into a mature stand. The differences in the morphology of bracken in a mature stand and at the stand margins which are identified here support the idea of controlling bracken at stand margins in preference to the spraying of large areas of dense, mature bracken. Morphological differences include an increased proportion of active buds, greater frequency of fronds per unit rhizome biomass, reduced biomass reserves. Improved conditions for the re-invasion and re-establishment of alternative vegetation are also available at stand margins in comparison with the centre of a dense bracken stand.     

松嫩平原野古草种群构件结构动态

野古草是根茎型无性系禾草,在松嫩平原草甸经常形成单优种群落。采用单位面积挖掘取样、分株按营养繁殖世代划分龄级、根茎按实际生活年限划分龄级的方法,对松嫩平原单优群落和混生群落的野古草种群构件结构进行了调查与分析。结果表明,在生长季初期两群落野古草种群均以春性分株和根茎芽占优势,且分株及芽构件结构相对稳定,芽库的输出率单优群落为80.4%,混生群落为62.5%;整个生长季分株由2—3个龄级组成,1a分株数量是2a的2.9—10.2倍,其生物量各月份所占比例平均为93%,随着龄级的增加依次明显减少,呈增长型年龄结构;根茎由3—4个龄级组成,根茎累积长度及生物量均以2a占绝对优势,为稳定型年龄结构;分株生产力1a明显高于2a,对种群贡献最大;根茎贮藏力除个别月份以3a、4a最高外,两群落大部分以2a最高,在生长季后期,1a根茎物质积累的速率最快。     

Predicting the Effectiveness of <Emphasis Type="Italic">Phragmites</Emphasis> Control Measures using a Rhizome Growth Potential Bioassay

In the last century, Phragmites australis (common reed) has expanded from a minor component of the mid-Atlantic tidal wetlands to a dominant species in many locations. Expansion of Phragmites results in decreased plant diversity and alterations to the tidal characteristics of the marsh, resulting in decreased wetland value. Management efforts have used a variety of strategies in an attempt to control its expansion. We tested a greenhouse bioassay that provided insight into the rhizome vitality of six herbicide-treated sites in the Alloway Creek Watershed, NJ well in advance of the growing season. At three sites, rhizomes were exhumed and classified by depth (0–25 cm and 25–75 cm) and appearance (color and firmness). Concurrently, the same protocol was followed, but conducted on an areal basis at three additional sites. Material was grown in sand under greenhouse conditions void of nutrient supplements for 70 days, after which shoots were removed and the rhizomes replanted for 30 days. Effectiveness of control strategies was quantified by examining rhizome color, vitality, and shoot densities in the field. Color was indicative of quality of rhizome reserves. Less than 0.2% of the firm, brown rhizomes produced shoots upon initial planting and none produced shoots upon replanting, whereas 50.9% of white rhizomes produced shoots on initial planting. Rhizome vitality was quantified by examining shoot emergence and the morphology of the shoots. Coupling rhizome vitality with observed field densities resulted in a predictive capability, and shoot density and biomass predictions were compared to field measurements in July 2001. We tested and accurately predicted the relative shoot densities and shoot biomass of the three sites for which we collected rhizome material on an areal basis. The result is a rapid, valuable, and cost-effective monitoring tool that can quickly quantify the effects of past control methods and predict future growth potential.     

The Effect of Summer Harvesting of Phragmites australis on Growth Characteristics and Rhizome Resource Storage

The effect of harvesting the aboveground biomass on the growth of Phragmites australis in the subsequent growing season was investigated following cutting in June or July. Seasonal changes in rhizome biomass and total nonstructural carbohydrate (TNC) in seven age categories, from newly formed to six-years-old, were monitored for the two treatment stands and a control stand. The growth of the stands, as indicated by the aboveground biomass, showed a significant decline due to cutting in June but did not show a significant difference due to cutting in July, compared to that of the control stand. The timing of harvesting of aboveground biomass affected the annual rhizome resource allocation. A similar trend was observed for the pattern of resource allocation, as described by biomass variation of different rhizome-age categories for July-cut and control stands. However, the biomass of June-harvested rhizome categories tended to be smaller than the other two stands, indicating substantially reduced resource storage as a direct result of harvesting the aboveground biomass during the previous growing season. This implies that cutting of aboveground biomass in June is a better option for control of P. australis stands than cutting later in summer.     

A soil heat and water transfer model to predict belowground grass rhizome bud death in a grass fire

Question: Grasses often resprout from surviving belowground buds following a fire in which aboveground matter is consumed. We used a soil heat and water transport model to present a general method for determining the potential mortality of rhizome buds due to fire for three tallgrass species (Andropogon gerardii, Sorghastrum nutans, and Panicum virgatum). Methods: Soil heating was described by physical processes that include heat conduction through the soil and heating and evaporation of soil water. We considered the following factors: soil moisture, texture, mineral thermal conductivity, maximum surface temperature, and fire residence time. Simulated soil temperature profiles were combined with measured belowground bud distributions to determine the proportion of buds expected to be heated to lethal temperatures under various conditions. Location: Wisconsin, USA. Results: Lethal temperatures for buds do not occur below ～2 cm, and at least 30% of rhizome buds remain below lethal temperatures, even under extreme conditions. Conclusions: The model explains the possible mechanisms for grass belowground rhizome bud survival in fires. Changes in fire and soil conditions do not notably impact soil temperatures and rhizome bud survival.     

Detecting the impacts of notorious invaders: experiments versus observations in the invasion of eelgrass meadows by the green seaweed <Emphasis Type="Italic">Codium fragile</Emphasis>

Biological invasions can vary in the extent of their effects on indigenous communities but predicting impacts for particular systems remains difficult. In coastal marine ecosystems, the green seaweed Codium fragile ssp. fragile is a notorious invader with its reputation based on studies conducted largely on rocky shores. The green seaweed has recently invaded soft-bottom eelgrass communities by attaching epiphytically to eelgrass (Zostera marina) rhizomes, thereby creating the potential for disruption of these coastal habitats through competition or disturbance. We investigated the effect of this invader on various aspects of eelgrass performance (shoot density and length, shoot growth, above- and below-ground biomass, carbohydrate storage) using both small-scale manipulative and large-scale observational experiments. Manipulative experiments that varied Codium abundance demonstrated clear negative effects over a 4-month period on shoot density and carbohydrate reserves, but only for high, but realistic, Codium biomass levels. Light levels were much lower under canopies for high and medium density Codium treatments relative to low and control Codium cover treatments, suggesting that shading may influence eelgrass growing under the algal cover. In contrast, these effects were either not detectable or very weak when examined correlatively with field surveys conducted at larger spatial scales, even for sites that had been invaded for over 4 years. It is premature to extend generalizations of Codium’s impact derived from studies in other systems to eelgrass communities; further efforts are required to assess the long-term threats that the alga poses to this ecosystem. This study demonstrates the need to investigate impacts of invasions over multiple scales, especially those that incorporate the temporal and spatial heterogeneity of the invader’s abundance.     

东北草地异质生境芦苇种群根茎芽年龄结构及输出规律

为明确异质生境条件下芦苇种群根茎芽年龄结构及输出规律,揭示芦苇种群的营养繁殖特性,采用单位土体挖掘取样,分别计数各龄级根茎芽的调查与统计方法,对东北草甸草原草甸土和盐碱土两个生境单优群落芦苇种群根茎芽动态进行比较分析。结果表明,两个生境芦苇种群根茎芽库主要均由6个龄级组成;草甸土生境在6—10月均为增长型年龄结构;盐碱土生境6—7月份为衰退型年龄结构,8月份为稳定型年龄结构,9—10月份为增长型年龄结构。根茎芽数量1—4a普遍以草甸土生境高于盐碱土生境,5—6a普遍以盐碱土生境高于草甸土生境,各龄级根茎芽数量与月份之间均符合y=a+bx直线关系(P0.05)。随着龄级的增加,休眠芽比率呈逐渐下降趋势,而萌发芽比率则呈逐渐上升趋势,5个生育期的休眠芽比率和萌发芽比率与龄级之间均符合y=a+bx直线关系(P0.01)。各龄级根茎的休眠芽具有一个相对稳定的萌发输出过程,草甸土生境根茎休眠芽按每年11%的比率萌发输出,而盐碱土生境根茎休眠芽按每年7%的比率萌发输出。虽然芦苇种群根茎芽年龄结构及年龄谱在异质生境中存在显著差异,但却有着相同的季节变化规律,均以不断形成新根茎的芽来维持着种群的营养繁殖更新。     

扎龙湿地不同生境芦苇种群根茎数量特征及动态

采用单位土体取样,计测长度和生物量的调查与统计方法,对扎龙湿地保护区4个生境芦苇种群根茎数量特征进行比较分析。结果表明,芦苇5月10日左右返青后进入营养生长期,根茎长度6—8月份缓慢增加,8—10月份显著增加,后期是前期的3.5—10.3倍,生长季中后期是种群新根茎补充和生长的主要时期,不仅实现了种群空间扩展,并为营养繁殖储备更多繁殖芽;根茎生物量和干物质贮量6—8月份逐渐减少,8—10月份又逐渐增加,均以生长季末期的10月份最大,并均显著地(P0.05)高于其他月份,种群根茎养分的消耗主要供给根茎芽的萌发和幼株生长,根茎养分的储藏又为翌年种群的更新及扩展提供物质保障,种群对地下根茎存在明显的养分"超补偿性"贮藏现象。种群根茎长度和生物量均以湿生生境最大,依次为旱生生境、水生生境,盐碱生境最小,根茎干物质贮量以旱生生境最大,依次为湿生生境、水生生境,盐碱生境最小。种群根茎长度与返青后实际生长时间之间均较好地符合直线函数关系,种群根茎生物量和干物质贮量与生长时间之间较好地符合二次曲线函数关系,R2在0.804—0.997之间,拟合方程均达到了显著或极显著(P0.01)水平。4个生境芦苇种群在根茎长度、生物量、干物质贮量等数量特征均表现出由遗传因素控制的比较稳定的季节动态规律,在生境间的差异及其差异序位又均基本稳定,均表现出明显的土壤因子环境效应,其中土壤含水量、有机质、速效氮为正向驱动,p H、速效磷为负向驱动,土壤含水量、p H对根茎数量特征的驱动作用更突出。