Overwintering temperatures affect freezing temperatures of turions of aquatic plants

The effect of different overwintering temperatures (2.5 ± 1 °C in a refrigerator or outdoor natural overwintering on wet topsoil with weak frosts) on the freezing temperature and survival rate of turions of 10 aquatic plant species with different ecological traits (free-floating habit or bottom rooting) was studied using mini thermocouples. Dormant, non-hardened turions of 9 species exhibited freezing within a narrow temperature range of ?7.0 to ?10.2 °C, while Hydrocharis morsus-ranae froze at ?3.6 °C. The survival rate of the turions after the measurements was, however, very low (0–38%). In several species, the freezing temperature of turions at the beginning of germination was not significantly different (at p < 0.05) from the dormant ones. The mean freezing temperature of outdoor hardened turions of 6 species was within a very narrow range of ?2.8 to ?3.3 °C and was thus significantly higher by 4–7 °C (p < 0.0002) than that for the non-hardened turions. It is assumed that the freezing temperatures indicate freezing of the extracellular water. The hardened turions of all 7 species were able to survive mild winter frosts under the topsoil conditions at a rate of 76–100%. These characteristics suggest that the turions of aquatic species can be hardened by weak frosts and that their frost hardiness is based on the shift from frost avoidance in non-hardened turions to frost tolerance.     

Life cycle and growth ofPotamogeton crispus L. in a shallow pond,ojaga-ike

The life cycle and growth ofPotamogeton crispus L. were studied in a shallow pond, Ojaga-ike. With respect to the shoot elongation and seed and turion formations, the life cycle of this plant in the pond could be divided into following five stages: germination, inactive growth, active growth, reproductive and dormant stages. It was suggested that the plant showed these successive stages depending mainly upon water temperature. The turions germinated on the bottom in autumn when the water temperature fell below ca. 20 C. The plant showed hardly any growth during winter (December—early March) when the temperature was below 10 C. In the spring when the bottom water temperature rose to above 10 C (mid-March), the plant started to grow again and the shoot elongated rapidly at the rate of 4.2 cm day⁻¹ until the shoot apex reached the pond surface in late April. Both the increment of node number and the internodal elongation were associated with this rapid shoot growth. On 10 May (last sampling date), the mean values of shoot length, internodal length and the number of nodes estimated for 10 predominant plants were 238.2±5.6 cm, 7.1±0.8 cm and 34.9±4.0 cm, respectively. The turion formation and flowering occurred during the period from mid-April to mid-May when the surface water temperature ranged 19 and 22 C. The dry weight of a plant reached the maximum mean value of 1180 mg on 10 May. At its peak biomass, an individual plant produced 1–10 turions (5.5 on average) of which the mean individual turion dry weight was 53.2 mg. The turion dry weight accounted for ca. 42% of the total plant biomass m⁻² at that time.     

Cytochemical and ultrastructural aspects of aquatic carnivorous plant turions

Turions, which are modified shoot apices, are vegetative, dormant overwintering organs produced by perennial aquatic plants. In this study, the turion cytochemistry and ultrastructure of Aldrovanda vesiculosa, Utricularia vulgaris and U. stygia were compared with particular emphasis placed on storage substances. These three aquatic, rootless carnivorous plant species were studied at the end of their winter dormancy. At this stage, the turions of all species had starch as their main storage material. In contrast with A. vesiculosa, Utricularia turions were rich in protein storage vacuoles, and proteins were also accumulated as crystalline inclusions in the nuclei. All examined species accumulated lipid droplets in cells of epidermal glands.     

Respiration of turions and winter apices in aquatic carnivorous plants

Basic respiration characteristics were measured in turions of six aquatic plant species differing greatly in their ecological and overwintering characteristics both before and after overwintering, i.e., in dormant and non-dormant state: non-carnivorous Hydrocharis morsus-ranae and Caldesia parnassifolia and carnivorous Aldrovanda vesiculosa, Utricularia australis, U. ochroleuca, and U. bremii, and in non-dormant winter apices of three Australian (sub)tropical populations of Aldrovanda and of two temperate North American Utricularia species, U. purpurea and U. radiata. Respiration rate of autumnal (dormant) turions at 20°C ranged from 0.36 to 1.3 μmol O₂ kg⁻¹ (FM) s⁻¹ and, except for U. bremii, increased by 11–114% after overwintering. However, this increase was statistically significant only in two species. Respiration Q₁₀ in dormant turions ranged within 1.8–2.6 and within 2.3–3.4 in spring (non-dormant) turions. Turions of aquatic plants behave as typical storage, overwintering organs with low respiration rates. No relationship was found between respiration rate of turions and overwintering strategy. In spite of their low respiration rates, turions can usually survive only from one season to another, due to their limited reserves of respiratory substrates for long periods. Contrary to true turions, respiration rates in non-dormant winter apices both in Australian Aldrovanda populations and temperate U. radiata and U. purpurea, in sprouting turions, and growing shoot apices of Aldrovanda were high and ranged from 2.1 to 3.1 μmol kg⁻¹ (FM) s⁻¹, which is comparable to that in aquatic plant leaves or shoots.     

The introduction of the aquatic carnivorous plantAldrovanda vesiculosa to new potential sites in the Czech Republic: A five-year investigation

Five microsites in shallow dystrophic waters were selected for the introduction of a critically endangered aquatic carnivorous plantAldrovanda vesiculosa in the Třeboň region in S Bohemia, the Czech Republic. The selected sites were fen pools close to two hypertrophic fishponds, Ptačí blato (four microsites) and Domanínsky (one microsite). In June 1995, 30 plants from an allochthonous population in E Poland were introduced in each of the microsites. Water chemistry and plant growth dynamics were investigated at the microsites throughout the 1995–1996 seasons, and the maximum size of the subpopulations were estimated during the 1997–1999 seasons. The selected waters may be characterized as oligo-mesotrophic. The warm and dry 1995 season resulted in a low water level (ca. 10 cm below average) and plant propagation was poor at all but one microsite. However, the rainy and colder 1996 season resulted in a high water level, and thus rapid plant propagation occurred at all four microsites at the Ptačí blato fishpond and between 841 and 2669 turions were formed. Here, the doubling time of the apices was between 13.5–19.3 days in the summer. At the Domanínsky fishpond, however, the growth was relatively poor in 1996. The overwintering rate of turions (19–100%) was found as being high enough to keep the introduced plant populations stable. At each of the four microsites at Ptačí blato, the maximum numbers of shoot apices were estimated to be between 1,000–20,000 in the 1997–1999 seasons. Depth of free water appears to be the crucial factor for the rapid growth ofAldrovanda at some selected sites; water depth below 5 cm is unfavourable. Overall,Aldrovanda was successfully established in an intensively agricultural landscape in the Třeboň region.     

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.     

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.     

黄土丘陵区燕沟流域典型植物叶片C、N、P化学计量特征季节变化

以黄土丘陵区燕沟流域为例,分析了流域8种典型植物叶片C、N、P化学计量特征的季节变化。结果发现,8种植物叶片C含量分布范围在370.2-566.9 mg/g,N含量在9.2-39.0 mg/g,P含量在0.81-2.35 mg/g,C:N在10.5-52.9,C:P在186.8-667.5,N:P在5.7-23.0。叶片平均C、C:N和C:P在5月小于7月和9月(P<0.05),而在7月和9月差异不显著;N在5月大于7月和9月(P<0.05),7月和9月差异不显著;P在7月小于5月和9月(P<0.05),5月和9月差异不显著;N:P在9月明显小于5月和7月(P<0.05),5月和7月差异不显著。叶片C含量受季节因素影响显著,而在物种间差异不显著;叶片N、P、C:N、C:P、N:P受物种和季节因素影响均显著。因此,8种植物中沙棘、黄刺梅和虎榛子采用防御性的生活史策略;刺槐、柠条和狼牙刺采用竞争性生活史策略,铁杆蒿和茭蒿的生活史策略介于上述二者之间;尽管叶片N:P随生长季节发生明显变化,但研究区植物生长的限制性元素未随生长季节变化而改变。     

Seed germination and seedling growth of Zostera marina L. (eelgrass) in the chesapeake bay

Seed germination and seedling growth of Zostera marina L. were monitored in the Chesapeake Bay in 1979 and 1980. Harvested seeds were placed in small acrylic tubes at several sites representing the salinity range of Z. marina distribution. Seed germination was observed first in late September and continued through May, with peaks in the fall and spring. The majority of seeds that germinated (66%) did so between December and March when water temperatures ranged from 0–10°C. There was no correlation between sites (different salinity regimes) and frequency of germination rates, indicating that salinity was not a major factor in the germination process in this study. Additional information on seed germination was available for seeds collected in 1977 and 1980 and subsequently monitored for germination at only one site. These data were similar to germination frequency recorded in 1979–1980.Seedling growth was measured from individuals collected from an existing Zostera marina bed. Seedlings were collected from November through May, at which time we could no longer distinguish seedlings from existing vegetative stock. Growth was characterized by the increased length of the primary shoot, number of leaves per shoot and numbers of shoots per plant. Seedling growth was slow during the winter months (water temperature ? 10°C) but rapidly increased in the spring (temperatures > 10°C). The size range of the harvested seedlings indicated that seed germination in the field probably occurred from October through April, corroborating evidence from the seed germination experiments.     

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.     

Seasonal growth and reproduction of Ruppia maritima L. s.l. in Lake Pontchartrain,Louisiana, USA

Ruppia maritima L. s.l. is an important submerged aquatic plant in Lake Pontchartrain, a water quality impaired oligohaline estuary located north of New Orleans, Louisiana, USA. Efforts to restore Lake Pontchartrain are in progress, and a major restoration goal is the re-establishment of historic submerged macrophyte beds that have declined since first studied in 1953. This study was part of a long-term monitoring program to distinguish natural from anthropogenic causes of macrophyte population changes. Ruppia populations were monitored monthly for a 2-year period to obtain information on seasonal growth, peak biomass periods, flowering, seed production, and germination. We found that Lake Pontchartrain R. maritima had relatively high peak biomass and two seasonal peaks of biomass and flowering due to high water temperature and absence of competition from other macrophytes. However, biomass peaks were often disrupted by storms. Shoot biomass was high from May through November, and low from December through April. Spring shoot growth from seeds, roots, and rhizomes was rapid. Plants matured and produced abundant inflorescences and seeds between May and October. Reproductive shoots bearing seeds were detached in May and June. Remaining shoots formed dense beds through vegetative propagation during the summer. Turion production occurred during the winter.     

Life cycle of Egeria densa planch., an aquatic plant naturalized in Japan

The ecophysiological life cycle of Egeria densa Planch., in a lotic irrigation ditch was evaluated. Phenological and quantitative measurements were made from August 1984 to July 1985. Lateral shoots with roots developed and elongated to the surface of the water when the bottom-water temperature increased above c. 15°C. Root crown developed simultaneously as the plant biomass increased. Dense shoot crown was formed under the water surface after the shoot reached the water surface. Plant biomass had two maxima in August and December-January. The bimodal curve in plant biomass was caused by the difference of growth attributed to the winter-type and summer-type plants. Relative growth rate, in length, of summer-type plants occurred at an optimum temperature of 20.7°C in culture. The seasonal activity of photosynthesis and respiration was measured in March, August and December. The optimum temperature of net photosynthesis of the summer-type plants reached a high 35°C similar to that of the C₄ plant. The compensation for light intensity at 35°C was 340 lux. Each photosynthesis-temperature curve suggested that Egeria had the ability to adapt to the seasonal changes in temperature in the natural habitat. The maximum starch concentrations reached 25.4% in the leaf and 22.6% in the stem in December. The shortage in the balance of organic matter for overwintering was found to be maintained by stored starch in the leaf and the stem.     

Dormancy and germination in axillary turions ofHydrilla verticillata

Effects of environmental conditions and growth regulators on release from dormoncy of axillary turions inHydrilla verticillata were investigated. Coll treatment at 2 C for 33 days produced the most complete release from dormancy. One week of 2 C treatment was sufficient for the germination; however, longer cold periods produced more rapid growth in shoot or root lengths as well as a shorter lag time for germination. Dormancy in turions could be broken by a photoperiod of 16 hr but not by on of 8 or 12 hr, nor by continuous lighting. When a cold treatment was applied turions grew out in response to all of the photoperiodic conditions. Red and far-red irradiation during the incubation after a cold treatment promoted gremination; blue and green light markedly inhibited the germination. At 10⁻⁴ and 10⁻⁵ M, gibberellic acid broke dormancy of non-cold treated turions, but was toxic at 10⁻⁴ M to the development after germination. Gibberellic acid promoted growth of cold treated turions even at 10⁻⁶ M. Indoleacetic acid at 10⁻⁴, 10⁻⁵ and 10⁻⁶ M induced outgrowth of both non-cold treated and cold treated turions. Apparently normal growth and development was observed in a high concentration of indoleacetic acid.     

Field growth characteristics of two aquatic carnivorous plants,Aldrovanda vesiculosa andUtricularia australis

Basic growth characteristics of two species of free-floating submerged carnivorous plants, the very rare and stenotopicAldrovanda vesiculosa and the very common and eurytopicUtricularia australis, were investigated in a 10/11-day field growth experiment within three nylon enclosures at two artificialAldrovanda sites in the T?eboň region, S Bohemia, Czech Republic, at the peak of a growing season. Growth ofAldrovanda was best at a meso-eutrophic site (biomass doubling time,T ₂, 8.4–10.7 days, mean growth of new leaf whorls 0.96 whorls days^?1, 1.6 developed branches per shoot) and slower at an oligo-mesotrophic site (T ₂ 17.2–21.5 days, growth of whorls 1.01 whorls days^?1, 0.1–0.5 branches per shoot). Growth ofUtricularia was similar at both sites (T ₂ 19.8–33.2 days or 9.1–16.8 days, growth of whorls 3.1 or 2.7 whorls days^?1, 1.5–2.1 or 0.8–1.4 developed branches per shoot at the former or latter site, respectively). Throughout the experiment, both species at the meso-eutrophic site allocated relatively more biomass to the production and growth of branches, than to that of new whorls. The results show thatAldrovanda, although usually considered as competitively weaker, can grow faster during the growing season peak thanUtricularia due to frequent branching and the subsequent rapid growth and separation of daughter shoots. Very rapid growth of rootless aquatic carnivorous plants in nutrient-poor habitats allows the consideration of ecophysiological adaptations that enable the plants to gain limiting mineral nutrients. These adaptations include carnivory, efficient nutrient reutilization from senescent shoots, and very high affinity for mineral nutrient uptake from water. Comparison of growth rates of rare and stenotopicA. vesiculosa and very common and eurytopicU. australis shows that differences in their rarity do not seem to be based on differences of growth rate.     

Comparative growth and propagule production byHydrilla verticillata grown from axillary turions or subterranean turions

Hydrilla verticillata (L.f.) Royle produces two types of vegetative propagules, subterranean turions and axillary turions. After 8 or 12 weeks growth under similar conditions, plants grown from subterranean turions weighed 1.7 to 2 times as much as plants grown from axillary turions. Subterranean turion-derived plants produced more propagules (by weight and number) per plant than plants from axillary turions. Characteristics of weight frequency distributions (median, minimum, maximum, and coefficient of variation) for new subterranean turions were also influenced by the type of propagule from which the parent plant was derived. The number of root crowns per plant which reflects the plant's ability to expand horizontally was significantly greater for subterranean turion-derived plants (by 2 times) than for plants from axillary turions. These results support the hypothesis that following colonization of an area the impact ofHydrilla on resident species changes over time.     

Comparative physiology of young and old cohorts of bay scallop Argopecten irradians irradians (Lamarck): mortality,growth, and oxygen consumption

The bay scallop Argopecten irradians (Lamarck) undergoes rapid population decline in its second year of life. Pre-(1st-yr) and postreproductive (2nd-yr) bay scallops were held in cages at two sites on Long Island, New York, U.S. Survival, growth, and metabolic rates of the two cohorts were compared monthly throughout the autumn and winter. Second-year scallops, the harvestable crop of the year, maintained a positive energy balance until late November–December. Both age classes experienced similar relative tissue weight losses during overwintering (9–11% at the site where milder environmental conditions prevailed, and 24–25% at the more stressful site). Ambient water temperature explained a significant proportion (93%) of the seasonal variation in the rate of oxygen consumption. Thus the northern bay scallop A.i. irradians shows a limited ability to acclimatise oxygen consumption to seasonal temperature changes over the range of 1–23°C. A significant increase in oxygen uptake was associated with increased gametogenic activity of young scallops in May. Metabolic rate at this time was 50% higher than that predicted based on temperature, providing an estimate of the metabolic cost of reproduction in this species. The weight-normalized oxygen uptake rate of senescent scallops was significantly lower than that of young scallops. Mass natural mortality of the older cohort occurred during the winter, before the onset of a second gametogenesis; only 50% of the population survived beyond late January in 1985. Mortality was delayed by 2.5 months during a similar experiment conducted in 1986. Results of this study suggest that senescent mortality of New York bay scallop populations is not directly linked to the energy drain of a second reproductive event following overwintering stress.     

Multiyear Population Dynamics of Ditylenchus dipsaci Associated with Phlox subulata

Field population densities of Ditylenchus dipsaci associated with shoot tissue of Phlox subulata were monitored during two consecutive growing seasons and intervening periods of overwintering and plant storage. The population density increased significantly through four peaks during the first growing season, and decreased significantly during storage at 5-7 C or overwintering in the field. During the second growing season, there was only a single increase to a moderate population density, followed by a severe population decline associated with the poor physiological condition of the host. A simple model is proposed to explain the population dynamics of D. dipsaci during the first growing season.     

Carbon dioxide fluxes in a spatially and temporally heterogeneous temperate grassland

Landscape position, grazing, and seasonal variation in precipitation and temperature create spatial and temporal variability in soil processes, and plant biomass and composition in grasslands. However, it is unclear how this variation in plant and soil properties affects carbon dioxide (CO₂) fluxes. The aim of this study is to explore the effect of grazing, topographic position, and seasonal variation in soil moisture and temperature on plant assimilation, shoot and soil respiration, and net ecosystem CO₂ exchange (NEE). Carbon dioxide fluxes, vegetation, and environmental variables were measured once a month inside and outside long-term ungulate exclosures in hilltop (dry) to slope bottom (mesic) grassland throughout the 2004 growing season in Yellowstone National Park. There was no difference in vegetation properties and CO₂ fluxes between the grazed and the ungrazed sites. The spatial and temporal variability in CO₂ fluxes were related to differences in aboveground biomass and total shoot nitrogen content, which were both related to variability in soil moisture. All sites were CO₂ sinks (NEE>0) for all our measurments taken throughout the growing season; but CO₂ fluxes were four- to fivefold higher at sites supporting the most aboveground biomass located at slope bottoms, compared to the sites with low biomass located at hilltops or slopes. The dry sites assimilated more CO₂ per gram aboveground biomass and stored proportionally more of the gross-assimilated CO₂ in the soil, compared to wet sites. These results indicate large spatio-temporal variability of CO₂ fluxes and suggest factors that control the variability in Yellowstone National Park.     

Effects of CO(2) and O(2) on the Photosynthetic O(2) Evolution of Spirodela polyrrhiza Turions

Net photosynthetic rates of Spirodela polyrrhiza turions, at low O₂ levels, were 6.2 and 38.8 micromoles O₂ per gram fresh weight per hour at 1 millimolar HCO₃⁻ and CO₂ saturation, respectively, and much lower in a regular low-pH growth solution. Air equilibration O₂ concentrations decreased rates considerably, except at CO₂ saturation. The surfacing rate of turions in various inorganic carbon surroundings correlated positively with their photosynthetic rates, but were the same at high and low O₂ levels. The relevance of these findings in relation to environmental conditions conductive to germination of autotrophically growing turions is discussed.     

荒漠草原区柠条固沙人工林地表草本植被季节变化特征

研究荒漠草原人工林固沙区地表草本植被季节变化特征及其和柠条林龄的关系,对于分析柠条人工林地表草本植物的季节适应性和制订合理的人工林管理措施均具有重要的科学意义。选择6、15、24年生和36年生柠条人工林为研究对象,通过调查每个样地5月、8月和10月地表草本植物密度、物种数、盖度和高度,分析了荒漠草原区柠条人工固沙林生长过程中地表草本植被季节变化特征及其影响因素。结果表明,地表草本植物物种数在柠条林龄6和15a时受季节改变的影响较小,在24a之后受到季节变化的显著影响(P0.05)。地表草本植物密度在柠条林龄6a时受季节改变的影响较小,但在15a之后季节变化显著影响地表草本植物个体数分布(P0.05),而且在10月具有最多的地表草本植物个体数。地表草本植被盖度和高度均受到季节变化的显著影响(P0.05),而受林龄的影响较小;不同年龄林地地表草本植被盖度和高度均表现为10月和8月较高,5月较低。研究表明,荒漠草原柠条人工林固沙区,柠条林发育生长和灌木形态的改变不仅影响土壤营养条件,而且还可以调控由于季节改变而引起的土壤温湿度变化,柠条林龄和季节更替二者交互作用,共同影响地表草本植被的季节变化特征。