A reciprocal transplant experiment within a climatic gradient in a semiarid shrub-steppe ecosystem: effects on bunchgrass growth and reproduction, soil carbon, and soil nitrogen

We investigated the effect of climate change on Poa secunda Presl. and soils in a shrub‐steppe ecosystem in south‐eastern Washington. Intact soil cores containing P. secunda were reciprocally transplanted between two elevations. Plants and soils were examined, respectively, 4.5 and 5 years later. The lower elevation (310 m) site is warmer (28.5 °C air average monthly maximum) and drier (224 mm yr^?1) than the upper elevation (844 m) site (23.5 °C air average monthly maximum, 272 mm yr^?1). Observations were also made on undisturbed plants at both sites. There was no effect of climate change on plant density, shoot biomass, or carbon isotope discrimination in either transplanted plant population. The cooler, wetter environment significantly reduced percent cover and leaf length, while the warmer, drier environment had no effect. Warming and drying reduced percent shoot nitrogen, while the cooler, wetter environment had no effect. Culm density was zero for the lower elevation plants transplanted to the upper site and was 10.3 culms m^?2 at the lower site. There was no effect of warming and drying on the culm density of the upper elevation plants. Culm density of in situ lower elevation plants was greater than that of the in situ upper elevation plants. Warming and drying reduced total soil carbon 32% and total soil nitrogen 40%. The cooler, wetter environment had no effect on total soil C or N. Of the C and N that was lost over time, 64% of both came from the particulate organic matter fraction (POM, > 53 µ m). There was no effect of warming and drying on the upper population of P. secunda while exposing the lower population to the cooler, wetter environment reduced reproductive effort and percent cover. With the warmer and drier conditions that may develop with climate change, total C and N of semiarid soils may decrease with the active fraction of soil C also rapidly decreasing, which may alter ecosystem diversity and function.     

Estuarine Restoration of Submersed Aquatic Vegetation: The Nursery Bed Effect

The historic decline of submersed aquatic vegetation (SAV) in mesohaline regions of Chesapeake Bay, United States involved a diversity of plant species. The recent modest recovery is mostly, however, associated with a single, prolific but ephemeral species, Ruppia maritima. Two previously abundant and more stable species, Potamogeton perfoliatus and Stuckenia pectinata, have shown virtually no evidence of recovery. Based on previous studies that demonstrated the ability of R. maritima stands to enhance water clarity and nutrient conditions for SAV growth, we hypothesized that these beds would serve as effective “nursery” areas to incite transplant success for other SAV. We conducted experiments in a two‐phase study at small and large spatial scales designed to explore this “nursery effect” as a restoration approach to increase plant species diversity. The first phase was conducted at small spatial scales to test effects of patch density by planting P. perfoliatus and S. pectinata into bare, sparse, and densely vegetated areas within three similar R. maritima beds in a tributary of Chesapeake Bay. Mean seasonal percent survivorship and shoot density were significantly higher in bare patches compared to vegetated patches. In the second phase of the study, P. perfoliatus was transplanted into separate R. maritima beds of different densities to test the effect of bed scale plant density on P. perfoliatus survival and growth. Transplant success of P. perfoliatus was positively correlated with the density of R. maritima among all sites.     

INFLUENCE OF ENVIRONMENT AND POPULATION ORIGIN ON SURVIVORSHIP AND REPRODUCTION IN RECIPROCAL TRANSPLANTS OF AMPHICARPIC PEANUTGRASS (AMPHICARPUM PURSHII)

Reciprocal transplants of both seeds and seedlings were utilized to determine whether populations of the annual grass Amphicarpum purshii have become locally adapted to specific habitats due to the consistent production of cleistogamous subterranean seeds from year to year. The hypothesis was that subterranean seeds placed in the same habitat as the parents will produce seedlings of greater vigor and adults of higher reproductive capacity than plants from seeds transplanted to a different habitat far removed from the parents. For both seed and seedling transplant experiments involving three sites in the Pine Barrens of New Jersey, the effects of site on shoot dry weight and production of aerial spikelets, subterranean spikelets, and seeds were generally much more significant than the effects of population origin. With one exception, there was no tendency for seedlings (or plants from seeds) replanted into their home sites to outperform alien seedlings (or plants from seeds) transplanted into these same sites. The overriding importance of environmental factors (relative to genetic differences among populations) in determining the phenotypic expression of life history characters, and selection occurring during succession at a site may retard the evolution of genetic adaptation to local habitat conditions in this species.     

Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river

1. Algal growth in lotic systems is controlled either from the bottom‐up (e.g. nutrients and light, which determine growth rates) or from the top‐down (e.g. grazing pressure, which reduces accumulated biomass). Nutrient‐enriched streams that support large and diverse grazing macroinvertebrate populations and those with shaded riparian corridors rarely suffer from excessive algal growth. 2. In this study, the density of benthic algivorous macroinvertebrates was experimentally manipulated in shaded and open nutrient‐enriched stream habitats of the Owennagearagh River, south‐west Ireland. The ability of macroinvertebrate grazers and riparian shade to control benthic algal growth [particularly the nuisance alga, Cladophora glomerata (L. Kütz)] was investigated. Three sites with markedly different concentrations of plant nutrients (one site upstream and two sites downstream of the sewage outfall) were selected. The density of grazing invertebrates colonising ceramic tiles was reduced using high‐voltage localised electric pulses. Replicates of treatment (grazer‐excluded) and control (grazed) tiles were deployed in open and shaded (<25 and >80% canopy cover, respectively) patches of stream bed, in each site. 3. After 2‐week Cladophora cover, periphytic chlorophyll a and biofilm ash‐free dry mass (AFDM) were quantified for all experimental tiles. Values for all three parameters were highest on grazer‐excluded tiles from open patches. Grazed tiles from open patches accrued little Cladophora and had significantly lower levels of chlorophyll a and AFDM. Nutrient inputs were found to have an impact on the density of grazing invertebrates, with higher densities of Baetis nymphs at the most nutrient‐enriched site. 4. Our results demonstrate that in eutrophic, high‐light streams, filamentous algae can quickly accumulate to nuisance levels in the absence of invertebrate grazers. In future, greater attention should be paid to the role of grazing invertebrates in controlling nuisance algae in streams, in addition to algal–nutrient relationships.     

Growth and architecture of small honey mesquites under jackrabbit browsing: overcoming the disadvantage of being eaten

Browsing is an important mortality factor in seedlings and small plants. However, the induced changes in the architecture of plant survivors may influence subsequent browsing, opening the possibility of compensating for the damage done. How jackrabbit (Lepus californicus) browsing affects the growth and architecture of small individuals of honey mesquite, Prosopis glandulosa var. torreyana, a tree/shrub that produces spines at every node, was explored. Naturally established mesquites of unknown age were selected in one site, and 2-year-old mesquites were transplanted in another site. In both cases, half of them were exposed to jackrabbits and the other half were excluded as controls. After 4 years, shoot production (height, length and number of derived shoots) and plant growth (height and cover) increased 1.4-2.5-fold in naturally established controls relative to exposed plants, depending on the measured variable. In the transplant experiment, the increases were 2.8-7.1-fold in controls relative to exposed plants 2 years after initiation of the experiment. The net loss of biomass in treatment vs. control plants in these experiments suggests a negative response to browsing which has been defined as under-compensation. Alternative architectures in honey mesquites were evident at the end of the exclusion experiments: controls had long branches and an extended crown cover, while exposed plants had short branches and a compact crown cover. Results indicated that mesquites were able to grow under browser pressure by packing many stems in a compact matrix armed with spines and producing one or more shoots tall and wide enough to escape from jackrabbits.     

Roles of succession,light, nutrients and disturbance on population vigor and maintenance of the rare plant Solidago shortii (Asteraceae)

Results of field and glasshouse experiments on Solidago shortii, and our observations on this species over many years, were used to construct a conceptual model of the roles of succession, light, soil nutrients and disturbance on population vigor and maintenance of this federal-endangered species. As cover of woody vegetation increased at a population site between 1986 and 1992, number of flowering ramets of S. shortii significantly decreased but number of vegetative ramets remained nearly constant. Adult plants transplanted into a redcedar thicket and those shaded in a glasshouse produced many fewer flowering ramets and capitula per flowering ramet and less biomass and had higher mortality than those in the open. Seedlings/juveniles shaded in a glasshouse had significantly less dry biomass; lower RGR, NAR, leaf area and root/shoot ratio and higher LAR, SLA and LWR than nonshaded ones. In a field site and glasshouse, fertilized plants (NPK) consistently had more flowering ramets and capitula per flowering ramet than nonfertilized ones. Hierarchy of dry weight of plants grown in a glasshouse in soils derived from five types of bedrock was phosphatic limestone > calcareous shale > sandstone > black shale = dolomite. Flowering and biomass production in the field-fertilizer and soil-type experiments were associated closely with levels of P. Number of flowering ramets significantly increased in plants transferred from shaded to nonshaded glasshouse conditions, but no such increase occurred after opening the canopy above plants in a thicket. Both high light and high nutrient levels apparently are necessary to maintain high vigor of S. shortii. In areas subject to invasion by woody plants, periodic high intensity disturbance may be required to prevent population extirpation.     

Environmental factors affecting dispersal,germination and distribution of <Emphasis Type="Italic">Stipa capensis</Emphasis> in the Negev Desert,Israel

  The effects of postmaturation ambient temperatures, light, as well as chemical and structural properties of the substrate on germination and patch distribution of Stipa capensis in a shrubland landscape were studied. This species is a dominant annual grass in exposed intershrub areas covered with biological loess soil crusts in the northern Negev Desert.Freshly matured caryopses do not germinate. After 7–8months of dry storage at high temperatures, there was a significant reduction in primary dormancy and an increase in the rate and percentage of germination of caryopses stored at high temperatures. The speed and percentages of germination were lower on various sterile substrates, such as organic matter and loose loess soil, in comparison with similar live substrates or the control on filter paper.In 3 of 4years of field observations in semi-arid shrubland, the density of Stipa capensis plants on soil crust in the exposed intershrub areas was significantly higher than below shrubs. The positive effects of higher temperatures on the dry soil during summer, before the season with rains, and of light during germination, can favor germination in exposed patches.Although germination responses to storage temperature and light regime differentiate between patch types before and during germination, other processes may be critical for the pattern of distribution of Stipa capensis. These include dispersal of caryopses, arrival, soil penetration and density of local seed banks, as well as substrate properties affecting germination and plant density, in different landscape patches.     

The effects of shading and N status on root proliferation in nutrient patches by the perennial grass Agropyron desertorum in the field

Competition for light can affect exploitation of spatially heterogeneous soil resources. To evaluate the influence of shoot status on root growth responses in nutrient-rich soil patches, we studied the effects of shading and whole-plant nitrogen status on root growth in N-enriched and nonenriched patches by mature Agropyron desertorum plants growing in the field with below-ground competition. Roots in enriched patches had greater length to weight ratios (specific root length, SRL), indicating increased absorptive surface areas, compared with roots in control patches. Increased SRL was due to increased production and length of higher order laterals rather than morphological changes in roots of the same branching order. Although the pattern of root growth rates in patches was the same for shaded and unshaded plants, the magnitude of this response to enriched patches was damped by shading. Root relative growth rates (RGR) in N-enriched patches were reduced by more than 50% by short-term shading treatments (60% reduction in photosynthetic flux density), while root RGR in unenriched patches was unaffected by shading. Unexpectedly, plants with higher nitrogen status had greater root RGR in enriched patches than plants that had not received nitrogen supplement, again with no detectable effect on root RGR in the unenriched patches. Therefore, while both shading and plant N status affected the ability of roots to exploit enriched patches by proliferation, there was no stimulation or suppression of root growth in the unenriched, control patches. Thus, plants already under competitive pressure above ground for light and below ground for nutrients should be less able to rapidly respond to opportunities presented in nutrient patches and pulses.     

Sex expression, skewed sex ratios, and microhabitat distribution in the dioecious desert moss Syntrichia caninervis (Pottiaceae)

The moss Syntrichia caninervis is the dominant soil bryophyte in a blackbrush (Coleogyne ramosissima) community in the southern Nevada Mojave Desert, with a mean cover of 6.3%. A survey of the 10-ha study site revealed an expressed ramet sex ratio of 14♀ : 1♂ (N = 890), with 85% of ramets not expressing sex over their life span, and an expressed population sex ratio of 40♀ : 2♂ : 1♀♂ (female : male : mixed-sex, N = 89), with 52% of populations not expressing sex. A greater incidence of sex expression was associated with shaded microsites, higher soil moisture content, and taller ramets. Shaded microsites had higher surface soil moisture levels than exposed microsites. In the exposed microhabitat, surface soil moisture was positively correlated with ramet height but not with sex expression. Male ramets and populations were restricted to shaded microhabitats, whereas female ramets and populations were found in both shaded and exposed microhabitats, suggesting gender specialization. The rarity of mature sporophytes, found in 0% of the ramets sampled and in only 3% of the populations, is probably due to the rarity of mixed-sex populations. We hypothesize that mixed-sex populations are rare because of factors relating to male rarity and that the differential cost of sex expression reduces the clonal growth capacity of male individuals.     

Phenotypic plasticity and population differentiation in seeds and seedlings of the grass Anthoxanthum odoratum

Summary Seeds of Anthoxanthum odoratum were transplanted reciprocally between a xeric and a mesic field population that were genetically differentiated in adult traits. In one experiment seeds were reciprocally buried in bags in the soil, in a second experiment seeds were reciprocally sown in small plots. For most traits, site effects were much larger than seed-source effects. Germination, emergence, mortality of buried seed and recruitment were significantly higher at the xeric site than at the mesic site, irrespective of population of origin. Seed dormancy, was significantly higher for seed originating from the mesic than from the xeric population. Seedling recruits originating from the xeric population tended to be larger at both sites. Fecundity of seedling recruits depended on the environment; fecundities of plants growing in the xeric site had more than double the fecundity of plants growing in the mesic site. Phenotypic plasticity rather than population differences determined variation in performance in the seed and seedling stages.     

Shading and the capture of localized soil nutrients: nutrient contents,carbohydrates, and root uptake kinetics of a perennial tussock grass

Summary The ability to exploit spatial and temporal heterogeneity in soil resources can be one factor important to the competitive balance of plants. Competition above-ground may limit selective plant responses to below-ground heterogeneity, since mechanisms such as root proliferation and alterations in uptake kinetics are energy-dependent processes. We studied the effect of shading on the ability of the perennial tussock grassAgropyron desertorum to take up nutrients from enriched soil microsites in two consecutive growing seasons. Roots of unshaded plants selectively increased phosphate uptake capacity in enriched soil microsites (mean increases of up to 73%), but shading eliminated this response. There were no changes in ammonium uptake capacity for roots in control and enriched patches for either shaded or unshaded plants. The 9-day shade treatments significantly reduced total nonstructural carbohydrate (TNC) concentrations for roots in 1990, but had no apparent effect on root carbohydrates in 1991 despite dramatic reductions in shoot TNC and fructan concentrations. Enrichment of the soil patches resulted in significantly greater phosphate concentrations in roots of both shaded and unshaded plants, with less dramatic differences for nitrogen and no changes in potassium concentrations. In many respects the shaded plants did surprisingly well, at least in terms of apparent nutrient acquisition. The effects of aboveground competition on nutrient demand, energy requirements, and belowground processes are discussed for plants exploiting soil resource heterogeneity.     

COMPARISON OF INTRASPECIFIC VARIATIONS IN THE REPRODUCTION AND PHOTOSYNTHESIS OF AN UNDERSTORY HERB,ARNICA CORDIFOLIA

Intraspecific variations in the reproduction of individual ramets and monospecific patches of the understory herb Arnica cordifolia Hook. were compared with variations in photosynthesis and understory light conditions. Ramets and patches were compared from three microhabitats (open, intermediate, and shaded) that differed in daily integrated irradiance. Individual ramets from open microhabitats (> 12 MJ m⁻² d⁻¹) had 23% more total dry wt and produced twice as many seeds, when compared to ramets from shaded locations (< 5 MJ m⁻² d⁻¹). In addition, monospecific patches from open locations were 63% more dense, and estimates of seed and vegetative patch reproductive effort were 4 and 2 times greater, respectively, when compared to shaded plant patches. For all measurements, ramets and patches from intermediate understory locations (6–10 MJ m⁻² d⁻¹) were intermediate in reproductive capacity between those of open and shaded locations. In addition, A. cordifolia seeds from open microhabitats germinated significantly better (45%) than either intermediate or shaded location seeds under high light and only seeds from shaded microhabitats germinated (14%) in the dark. Compared to shaded location plants, the greater total dry weight and seed production of individual ramets and the greater estimated reproductive effort of patches from open locations corresponded to a greater maximum photosynthetic rate (16.9 μmoles m⁻² s⁻¹) and daily carbon gain (12.2 g m⁻² d⁻¹). Possibly, a greater photosynthetic capacity may make more photosynthetic resources available for reproduction by A. cordifolia plants in open locations. Thus, intraspecific variation in physiology may contribute to intraspecific variation in reproduction.     

The spatial scale of adaptive population differentiation in a wide‐spread,well‐dispersed plant species

Adaptation to the specific conditions at different sites may contribute strongly to the wide distribution of a plant species. However, little is known about the scale at which such adaptation occurs in common species. We studied population differentiation, plasticity and local adaptation of the short‐lived perennial Hypochoeris radicata, a widespread and common plant whose seeds are well‐dispersed. We reciprocally transplanted seedlings among several populations of different size within and among three European regions (in the northwest Czech Republic, central Germany and the central Netherlands) and studied several fitness‐related traits over two growing seasons. The region in which the reciprocal transplant experiment was carried out had no influence on the performance of seedlings, indicating that there were no differences in overall habitat quality. In contrast, the site within region, and the plot within site strongly influenced mean plant performance. Plants from different populations of origin differed in their performance, indicating genetic variation among populations, but performance strongly depended on the specific combination of population of origin and transplant site. Plants that grew at their home site produced on average almost twice the number of seeds per transplant (a multiplicative fitness measure) than foreign plants originating from other sites. Survival, rosette size and multiplicative fitness all decreased with increasing distance from the home site to the transplant site. The size of the population of origin did not influence overall plant performance or the strength of local adaptation. In conclusion, our results indicate that the common and widespread H. radicata consists of locally adapted genotypes within its European range at a relatively small scale. Thus a large potential for gene flow by seeds and a high density of populations do not appear to be sufficient to prevent population differentiation by selection.     

Ecological genetic interactions between a clonal host plant (Spartina pectinata) and associated rust fungi Puccinia seymouriana and Puccinia sparganioides

The spatial scale of genetic diversity among patches of a host plant could affect the likelihood of pathogen adaptation to the host. If host patches are genetically distinct, pathogen adaptation to local host genotypes may occur. To study this issue, we focused on the ecological and genetic interactions between two rust fungi, Puccinia seymouriana and P. sparganioides, and the clonal prairie grass, Spartina pectinata. In a field transplant experiment, disease levels differed among plants from different patches, suggesting variation in resistance. Over a 4.5-km scale, disease levels were not higher on plants transplanted back into their source patch as opposed to other locations, providing no evidence for local adaptation in the pathogen. However, on the spatial scales examined (ranging from 0.2 km to 120 km), there was no relationship between the physical distance separating host patches and their similarity in isozyme banding patterns, implying that plants from more distant patches are not necessarily more genetically distinct than plants from nearby patches. Plants derived from the most distant location had, on average, the lowest mean number of pustules at the end of the summer, suggesting the need for reciprocal transplant studies to be performed on a larger spatial scale.     

Humic Substances Delay Aging of the Photosynthetic Apparatus of Chara hispida

In freshwaters, dissolved humic substances (HSs) distinguish apparently HS‐avoiding Charophytes from apparently HS‐tolerant ones, but the underlying mechanisms so far remain obscure. In this contribution, we tested direct and indirect effects of HSs on Chara hispida (L.) Hartm. Using Rhodamine B, we showed that C. hispida is able to adsorb or even uptake and, subsequently, desorb and depurate organic compounds in the molecular mass range of the applied fulvic acids. To classify direct and indirect HS‐mediated effects due to reduced light quantities, or to effects more strongly elicited by red relative to blue light, plants were exposed to HSs directly as well as through a neutral foil, or shaded by means of an external HS‐containing reservoir (low‐light variant). We showed that the apparently HS‐tolerant C. hispida exhibited reduced lipid peroxidation and non photochemical quenching of chlorphyll fluorescence when exposed to HSs. Plants directly exposed to HSs were significantly different from control as well as to foil‐shaded plants in terms of chl a+b, VAZ/chl, and β‐Car/chl; yet, in low‐light plants these variables did not differ from control and HS‐exposed plants, suggesting that the shift in favor of red lights in the low‐light variant led to a reduction in its cells' internal antioxidant content. However, the Fv/Fm ratio in HS‐exposed plants decreased more slowly than in all other exposure variants, indicating that the photosynthetic apparatus aged more slowly, by a mechanism yet to be discovered. Our study indicates that both direct and indirect effects contribute to the HS tolerance of C. hispida.     

Ecological and evolutionary importance of neighbors in the grass Anthoxanthum odoratum

Summary A field experiment was performed to estimate the relative importance of neighbors and the rest of the environment for the growth, mortality and reproductive output of cloned individuals of the perennial bunchgrass Anthoxanthum odoratum. Single cloned Anthoxanthum tillers (targets) were reciprocally transplanted between a xeric and a mesic grassland site with one of four neighbor treatments: (1) no neighbors, (2) Anthoxanthum neighbors transplanted from the xeric site, (3) Anthoxanthum neighbors from the mesic site, and (4) Holcus lanatus neighbors. Targets without neighbors had a twofold higher two year reproductive output (RO) than those with neighbors, but there was no difference among neighbor treatments. No overall site effect on two year RO was found, because the site with the highest mortality among targets produced larger plants, with more inflorescences. Neighborhood competition was more intense at the xeric site than at the mesic site. The effects of environmental and neighborhood variation on Anthoxanthum were additive, rather than interactive. Population origin did not affect target performance significantly. Anthoxanthum neighbors of different origin did respond differentially to transplant site. There was a strong target genotype x site interaction, but no genotype x neighborhood interaction.     

Structure of biological soil crust communities in Callitris glaucophylla woodlands of New South Wales,Australia

Question: What is the nature of the relationships between cover, diversity and abundance of biological soil crusts, cover and diversity of vascular plants, and annual rainfall, soil texture and forestry practices in Callitris glaucophylla woodlands? Location: Arid and semi‐arid Callitris glaucophylla‐domi‐nated woodlands of eastern Australia. Methods: We documented soil crust‐forming mosses, lichens and liverworts at 83 woodland sites along a gradient of declining rainfall. Linear and non‐linear regression were used to examine relationships between soil crust species and attributes of vascular plant communities, and a similarity matrix (species abundance X sites) was subjected to Non‐metric Multi‐Dimensional Scaling (MDS), and Analysis of Similarities (ANOSIM) to show the degree of association between groups of taxa, and soil texture, rainfall classes and forestry practices. Results : We collected 86 taxa. Mosses were dominated by the family Pottiaceae, and lichens were dominated by squamulose forms. Average annual rainfall was highly correlated with soil crust community composition, and loamy soils supported a greater cover and diversity of taxa compared with sandy soils. Increases in tree cover were associated with significant, though weak, increases in abundance, but not diversity, of crusts. Crusts tended to be more diverse in areas that (1) had a sparse cover of ground‐storey plants; (2) were relatively stable ‐ as indicated by the proportion of perennial and/or native plants; (3) had more stable soil surfaces; and (4) were unlogged. Litter cover, overstorey thinning, and livestock grazing had no appreciable effect on crust diversity or cover. Conclusions : Callitris glaucophylla woodlands provide substantial habitat for soil crust organisms, and the dense tree cover and closed canopies of Callitris do not appear to have a major influence on the structure of biological crust communities. Unlike other woodland systems, relatively few patches would be required to reserve a high diversity of crust species.     

实验室条件下两种生物土壤结皮对荒漠植物种子萌发的影响

生物土壤结皮广泛分布在干旱、半干旱区,深刻影响着土壤表层特性,进而对植物种子散布、萌发和定居产生极大的影响。到目前为止,生物土壤结皮与植物关系的研究很少见到,并且这些有限的研究所得出的结论存在着争议。研究了不同年龄的两种生物土壤结皮（苔藓结皮和藻类结皮）对油蒿（Artemisia ordosica）和雾冰藜（Bassia dasyphylla）种子萌发的影响,同时也研究了这两种结皮在失去活性前后对油蒿、雾冰藜和小画眉草（Eragrostis poaeoides）种子萌发的影响。苔藓和藻类结皮的出现对油蒿和雾冰藜种子的萌发均有显著的促进作用,而结皮年龄对植物种子的萌发没有显著的差异。对于不同的植物种,结皮类型和活性对种子的萌发具有不同的作用。雾冰藜在两种结皮上的萌发有显著的差异而油蒿和小画眉草在两种结皮上的萌发没有显著差异。活藻类显著地增加了三种植物的种子萌发,活苔藓仅增加了油蒿和雾冰藜种子萌发量而对小画眉草种子的萌发没有作用,研究表明,生物土壤结皮对一些植物种子萌发具有明显的促进作用。     

A genetic assessment of a successful seagrass meadow (Posidonia australis) restoration trial

Seagrass meadows are in decline globally. Although numerous experimental methods have been implemented to restore meadows, few have been successful in the long term. Poor decisions on the sourcing of transplants from donor sites, including poor genetic integration and/or low genetic diversity, may impact on restoration success. However, despite evidence to suggest a positive association between genetic diversity and ecological resilience, there is usually little or no input from genetic data to inform on the genetic management of ecological restoration. Cockburn Sound has seen a 77% decline in seagrass cover since 1967. A transplant trial was conducted between 2004 and 2008 with sprigs of Posidonia australis being planted into a bare sand area. Survival was monitored annually, and in 2012, we compared genetic diversity in this transplant area with the original donor site. Genetic diversity in the restored meadow was very high and comparable to the donor site, with no genetic differentiation detected. The high level of genetic diversity and choice of site may have played an important role in the success of this restoration trial. The observed natural recruits around the site after establishment of transplants suggest that local restoration efforts may improve seafloor habitat and facilitate natural expansion of the meadow.     

THE RELATIONSHIPS BETWEEN SEASONAL VARIATION IN LIGHT INTENSITY,RAMET SIZE,AND SEXUAL REPRODUCTION IN NATURAL AND EXPERIMENTAL POPULATIONS OF ASTER ACUMINATUS (COMPOSITAE)

We conducted experiments to evaluate the effects of different patterns of seasonal light availability on ramet size and sexual reproduction in the understory herb, Aster acuminatus. In April 1982, 200 potted rhizomes were divided equally between a high light and a shaded site. Twice during the growing season subsets of pots were reciprocally transferred between the sites, resulting in eight different light treatments. There was significant variation among treatments in mean ramet weight (0.41–1.21 g), height (15.0–23.4 cm), number of flowers (0.4–7.9) and percentage of ramets flowering (14–95). Periods of high light increased ramet height, weight, rhizome production and flowering, but high light earlier in the season increased ramet height more than did high light later in the season, while high light later had pronounced effects on the other three traits. Measurements taken in nine natural patches experiencing different patterns of seasonal light availability are consistent with the experimental results. The results provide new insights into the general phenomenon of size-dependent reproduction in herbs and illustrate how the phenology of biomass allocation can affect growth and reproduction.