Exotic invasive black willow (Salix nigra) in Australia: influence of hydrological regimes on population dynamics

Floodplain vegetation occupies disturbance prone habitats vulnerable to exotic plant invasion. Black willow (Salix nigra Marshall) is a flood-tolerant tree native to the southeastern United States and recently invasive in Australia, where it is considered detrimental to riparian environments. Effective management of S. nigra requires understanding the impacts of hydrological conditions on population dynamics at each stage of the plant’s life cycle. The influence of inundation events (timing, duration, and recession rates) on S. nigra stand dynamics and relative growth rate (RGR) was quantified across a hydrological gradient on the shorelines of Blowering Dam reservoir (NSW, Australia). Successful recruitment (sexual and asexual) and establishment is highly dependent on the position of S. nigra recruits above reservoir water levels and exposure to flooding during the first few years of establishment. Recruitment was ubiquitous along the hydrological gradient but a greater number of asexual recruits were found at higher elevations where inundation events are less frequent. Juvenile mortality was highest at lower elevations, which experienced greater depth and duration of inundation. Mean RGR was greatest in S. nigra stands experiencing no inundation over the study period and declined as inundation duration increased. Flooding events inhibit adult growth, but create suitable conditions for seedling recruits, provided the depth and duration of the flood does not exceed complete submersion for more than 30 days. To investigate relative tolerance to water table decline rooted S. nigra cuttings were grown in rhizopods, controlled growth devices that allow water-table manipulation. Water-table decline rates of 0, 1, 2, 4, and 8 cm day⁻¹ were applied and the response of leaf abscission measured. S. nigra cuttings were not significantly impacted at water-table decline rates of between 1 and 4 cm day⁻¹. Spatial location of S. nigra recruits in Australian floodplains represents a trade-off among hydrological conditions that promote recruitment without inducing mortality, resulting in a narrow spatial and temporal regeneration window critical to successful establishment. Future distribution and abundance of S. nigra in Australia will be determined by propagule availability and the degree to which current and future flow regimes create suitable hydrological conditions for successful establishment.     

Digging deep for diversity: riparian seed bank abundance and species richness in relation to burial depth

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Importance of low-flow and high-flow characteristics to restoration of riparian vegetation along rivers in arid south-western United States

1. Riparian vegetation in dry regions is influenced by low‐flow and high‐flow components of the surface and groundwater flow regimes. The duration of no‐flow periods in the surface stream controls vegetation structure along the low‐flow channel, while depth, magnitude and rate of groundwater decline influence phreatophytic vegetation in the floodplain. Flood flows influence vegetation along channels and floodplains by increasing water availability and by creating ecosystem disturbance. 2. On reference rivers in Arizona's Sonoran Desert region, the combination of perennial stream flows, shallow groundwater in the riparian (stream) aquifer, and frequent flooding results in high plant species diversity and landscape heterogeneity and an abundance of pioneer wetland plant species in the floodplain. Vegetation changes on hydrologically altered river reaches are varied, given the great extent of flow regime changes ranging from stream and aquifer dewatering on reaches affected by stream diversion and groundwater pumping to altered timing, frequency, and magnitude of flood flows on reaches downstream of flow‐regulating dams. 3. As stream flows become more intermittent, diversity and cover of herbaceous species along the low‐flow channel decline. As groundwater deepens, diversity of riparian plant species (particularly perennial species) and landscape patches are reduced and species composition in the floodplain shifts from wetland pioneer trees (Populus, Salix) to more drought‐tolerant shrub species including Tamarix (introduced) and Bebbia. 4. On impounded rivers, changes in flood timing can simplify landscape patch structure and shift species composition from mixed forests composed of Populus and Salix, which have narrow regeneration windows, to the more reproductively opportunistic Tamarix. If flows are not diverted, suppression of flooding can result in increased density of riparian vegetation, leading in some cases to very high abundance of Tamarix patches. Coarsening of sediments in river reaches below dams, associated with sediment retention in reservoirs, contributes to reduced cover and richness of herbaceous vegetation by reducing water and nutrient‐holding capacity of soils. 5. These changes have implications for river restoration. They suggest that patch diversity, riparian plant species diversity, and abundance of flood‐dependent wetland tree species such as Populus and Salix can be increased by restoring fluvial dynamics on flood‐suppressed rivers and by increasing water availability in rivers subject to water diversion or withdrawal. On impounded rivers, restoration of plant species diversity also may hinge on restoration of sediment transport. 6. Determining the causes of vegetation change is critical for determining riparian restoration strategies. Of the many riparian restoration efforts underway in south‐western United States, some focus on re‐establishing hydrogeomorphic processes by restoring appropriate flows of surface water, groundwater and sediment, while many others focus on manipulating vegetation structure by planting trees (e.g. Populus) or removing trees (e.g. Tamarix). The latter approaches, in and of themselves, may not yield desired restoration outcomes if the tree species are indicators, rather than prime causes, of underlying changes in the physical environment.     

The restoration of gravelly floodplain vegetation and endemic plants to riparian habitat in a Japanese river

Aster kantoensis, an endangered and monocarpic perennial plant species, is endemic to the gravelly floodplain of a few rapid flowing rivers in eastern central Japan. In recent years, an extreme declining trend in the species has been accelerated due to the strong negative influence by invasion of an alien grass, Eragrostis curvula. A restoration project aimed at recovering the original condition of the floodplain in the Kinu River, central Japan, has been started. To determine the possibility of successful restoration as well as its habitat preferences, I carried out some seed sowing experiments. In April 2003, seeds collected from a seminatural habitat were sown (54,000 seeds) in the restoration site (1.2 ha), where flood frequency, substrate condition, and control of alien plants are combined to form different habitat conditions. Seedling survival, flowery, and seed production were subsequently monitored from 2003 to 2005. Seed cohorts completed their life cycles within 3 years, and mean fitness of 927 was achieved. Performance of A. kantoensis seedlings was generally greater for environmental variables of sandy-type substrate and/or with control of alien plants. In addition, there were significant negative correlations between percentage survival, percentage flowery, and seed production with vegetation cover and coverage of E. curvula. The results confirm that, if safe sites with sparse vegetation exist, irrespective of their substrate condition, as well as seed sources of river endemics in natural habitats, restoration of riparian vegetation including river endemics is possible. The aggressive alien species E. curvula should be taken into consideration.     

Hydrochory,seed banks,and regeneration dynamics along the landscape boundaries of a forested wetland

Following the environmental sieve concept, the setting in which the recruitment of Taxodium distichum occurs in, becomes increasingly restrictive from the seed to seedling stage in an impounded forested wetland. Although a wide elevational band of dispersing seed moves across the boundary of a swamp-field in the water sheet, the zone of germination is relegated to that portion of the forested wetland that draws down during the growing season. Seedling recruitment is further restricted to the uppermost zone of the winter water sheet. These patterns are likely applicable to other species of dominant swamp species, e.g., Cephalanthus occidentalis crossed the boundary of a forested wetland and abandonded field in winter flooding (November–December and November–March, respectively) in Buttonland Swamp. The elevation of the boundary was 101.3 m NGVD. While the seeds of at least 40 swamp species were dispersed across the boundary, few viable seeds were dispersed after the winter season. Kriged maps showed seeds of T. distichum and C. occidentalis dispersed in patches in the water depending on the position of the water sheet. Most species of both water- and gravity-dispersed species had a localized pattern of seed distribution (either spherical or exponential) and this indicated that seeds may not be dispersed for great distances in the swamp. Water-dispersed T. distichum and C. occidentalis had larger dispersal ranges (A ₀=225 and 195 m, respectively) than Bidens frondosa and B. discoidea (A ₀=14 and 16 m, respectively). Seed dispersal varied with season depending on the availability of seeds. In Buttonland Swamp, viable seeds typically were dispersed for T. distichum in November–June, and for C. occidentalis in November-July. Low water occurred in August 1993 and high in February 1994 (99.8 and 101.6 m NGVD, respectively). The seed banks along the landscape boundary varied in species composition according to elevation (r ² = 0.996). While the similarity of species richness between water-dispersed seeds and the seed bank at elevations that flooded (during June 1993 through May 1995) was high (10–17%), it was low between water-dispersed seeds and the seed bank at elevations that did not flood (5%). T. distichum seeds had a short germination window in that seeds germinated within a year following their production in zones that were flooded in the winter followed by drawdown during the next growing season. After 1 year, less than 5% of the T. distichum seeds remained viable on the surface of the soil. Germination of T. distichum was confined to specific elevations (above 99.3 but below 101.6 m NGVD) during this study with 4.1% of the seedlings surviving for more than 2 years at a mean of 101.4 m NGVD. All seedlings below this elevation died. To maximize natural regeneration along the boundaries of swamps in abandoned farm fields targeted for restoration, this study suggests a flood pulse regime consisting of high water in the winter to maximize dispersal of live seeds followed by low water in the summer to facilitate seed germination and seedling recruitment. Hydrologic restoration could assist in the natural recovery of damaged wetlands if a seed source exists nearby.     

Reduced performance of native infauna following recruitment to a habitat-forming invasive marine alga

Despite well-documented negative impacts of invasive species on native biota, evidence for the facilitation of native organisms, particularly by habitat-forming invasive species, is increasing. However, most of these studies are conducted at the population or community level, and we know little about the individual fitness consequences of recruitment to habitat-forming invasive species and, consequently, whether recruitment to these habitats is adaptive. We determined the consequences of recruitment to the invasive green alga Caulerpa taxifolia on the native soft-sediment bivalve Anadara trapezia and nearby unvegetated sediment. Initially, we documented the growth and survivorship of A. trapezia following a natural recruitment event, to which recruitment to C. taxifolia was very high. After 12 months, few clams remained in either habitat, and those that remained showed little growth. Experimental manipulations of recruits demonstrated that all performance measures (survivorship, growth and condition) were significantly reduced in C. taxifolia sediments compared to unvegetated sediments. Exploration of potential mechanisms responsible for the reduced performance in C. taxifolia sediments showed that water flow and water column dissolved oxygen (DO) were significantly reduced under the canopy of C. taxifolia and that sediment anoxia was significantly higher and sediment sulphides greater in C. taxifolia sediments. However, phytoplankton abundance (an indicator of food supply) was significantly higher in C. taxifolia sediments than in unvegetated ones. Our results demonstrate that recruitment of native species to habitat-forming invasive species can reduce growth, condition and survivorship and that studies conducted at the community level may lead to erroneous conclusions about the impacts of invaders and should include studies on life-history traits, particularly juveniles.     

Alien seedling recruitment as a response to altitude and soil disturbance in the mountain grasslands of central Argentina

Climate and disturbance are considered key factors in explaining plant invasion, mainly by their effect on alien seedling recruitment. We tested whether soil disturbance by digging and altitude, as a sub-set of climate, affected the seedling recruitment of two alien species (Cirsium vulgare and Melilotus alba) in the mountain grasslands of central Argentina. We performed a seed-addition factorial experiment with altitude as a fixed factor co-variable (6 levels), time since sowing (6 levels) as an autoregressive co-variable, and soil disturbance by digging (two levels) and seed addition (two levels) as fixed factors. Seeds of the studied species were sown in replicated undisturbed and disturbed soil plots, in grassland stands located every 200 m from 1200 to 2200 m a.s.l. The number of emerging seedlings at each plot was recorded during a period of 8 months, at intervals of 30–60 days. Seedlings of both alien species emerged at all the studied altitudes, but only the recruitment of C. vulgare increased linearly with altitude. Moreover, the time window of seedling recruitment for this species increased with altitude. Soil disturbance produced a significant decrease in overall seedling emergence of both species; however, an increase in emergence was observed for C. vulgare at an altitude over 2000 m a.s.l. Natural emergence inside control plots without seed addition was only observed for Cirsium vulgare, which indicates that this species was not seed limited. Our results show that altitude and the absence of disturbance do not restrict the recruitment of C. vulgare and M. alba in these natural mountain grasslands. Moreover, these alien species seem to be tolerant to and even favoured by conditions occurring at higher altitudes and in absence of micro-site soil disturbances in the Córdoba mountain grasslands.     

Inheritance of isozymes in Brassica campestris L. and genetic divergence among different species of Brassiceae

Electrophoretic investigations of Brassica campestris revealed 8 polymorphic loci that had not been described earlier. The inheritance of 7 of them was determined by crosses between different accessions of B. campestris. A phylogeny of B. campestris, B. nigra, B. alboglabra, Sinapis arvensis and S. alba was estimated from the analysis of 15 isozyme loci. The phylogeny showed a close relationship between B. nigra and S. arvensis and a less close relationship between B. campestris and B. oleracea. S. alba was remotely-related to all of the other species.     

15N natural abundance in plants of the Amazon River floodplain and potential atmospheric N2 fixation

Summary The¹⁵N natural abundance values of various Amazon floodplain (várzea) plants was investigated. Samples of young leaf tissues were collected during three different periods of the river hydrography (low water, mid rising water and high water) and during one period in the Madeira River (high water). A large variation of¹⁵N abundance was observed, both among the different plant types and between the different flood stages. This variation probably, reflected, in part, the highly variable nature of the floodplain, sometimes dry and oxygenated and at other times inundated and anaerobic and, in part, changes in plant nitrogen metabolism. Comparison of the nitrogen isotopic composition of leguminous plants with that of non-leguminous plants showed that, on average, the¹⁵N abundance was lower in the legumes than non-legumes, suggesting active N-fixation. Also, the¹⁵N natural abundance in aquatic grasses of the generaPaspalum, was in general, lower than the¹⁵N abundance of aquatic grasses of the generaEchinochloa. As both of these grasses grow in the same general habitat, it appears thatPaspalum grasses may also be nitrogen fixers.     

Alterations in flood frequency increase exotic and native species richness of understorey vegetation in a temperate floodplain eucalypt forest

The delivery of environmental flows for biodiversity benefits within regulated river systems can potentially contribute to exotic weed spread. This study explores whether exotic plants of a floodplain forest in Victoria, Australia, are characterised by specific functional groups and associated plant traits linked to altering hydrological conditions over time. Permanently marked 20 × 20 m² plots from five wetland sites in Eucalyptus camaldulensis floodplain forest were sampled twice, first in the early 1990s (1993–1994) and then 15 years later (2007–2008). Species cover abundance data for understorey vegetation communities were segregated by season and analysed using ordination analysis. Exotic species richness was modelled as a function of site flooding history and native species richness using general linear models. Site ordinations by detrended correspondence analysis showed differential community compositions between survey dates, but native and exotic species were not clearly differentiated in terms of DCA1 scores. Most exotics belonged to functional groups containing annual species that germinate and reproduce under drier conditions. Exotics reproducing under wetter conditions were in the minority, predominantly perennial and capable of both sexual and asexual reproduction. Site flooding history and native species richness significantly predicted exotic species richness. Vegetation changes are partially structured by reduced flood frequency favouring increased abundance of exotic, sexually reproducing annuals at drier sites. Sites of low flood frequency are more sensitive to future exotic weed invasion and will require targeted management effort. Flow restoration is predicted to benefit propagule dispersal of species adopting dual regeneration strategies, which are predominantly natives in this system.     

Decomposition of Cynodon dactylon (L.) Pers. in the seasonally flooded areas of the Pongolo river floodplain: pattern and significance of dry matter and nutrient loss

Cynodon dactylon meadows occupying seasonally inundated areas of the Pongolo river floodplain, South Africa, decompose during periods of submergence. The loss of dry matter and nutrients from both fresh and dried material enclosed in litter bags was studied. The pattern of loss of dry matter was diphasic. Loss was more rapid from dried than from fresh material, half the mass being lost in 18 and 28 days respectively. Nutrient loss followed an exponential pattern. Nitrogen, phosphorus and magnesium were lost more rapidly from dried than from fresh material whilst rates of loss of sodium, potassium and calcium were similar. The significance of Cynodon decomposition in the functioning of the floodplain is assessed from estimates of the above- and below-ground standing crop around one small lake before and after submergence. It is concluded that decomposition of plants growing in seasonally inundated areas of the floodplain may contribute significantly to the productivity of the system, but that the extent of the contribution can be greatly influenced by the flooding regime. The completion of the Pongolapoort dam upstream of the floodplain will alter the natural flooding pattern, effecting a reduction in floodplain productivity, unless a programme for flood release can be initiated.     

Pollen-pistil interactions in Populus: β-galactosidase activity associated with pollen-tube growth in the crosses P. nigra x P. nigra and P. nigra x P. alba

Summary In order to understand the nature of interspecific barriers in Populus, we have explored pollen/pistil interactions in intra- and interspecific crosses Populus nigra x P. nigra and P. nigra x P. alba. The kinetics of pollen-tube growth demonstrated that P. nigra and P. alba pollen tubes have distinct behavioral patterns inside P. nigra pistils. P. alba pollen tubes exhibit an unique S-shaped growth curve and an arrested growth site near the sylodium. P. nigra pollen tubes exhibit two growth phases, in the stigmatic tissues and in the ovarian cavity respectively. P. nigra and P. alba curves diverge 5 h after controlled pollination and could be related to a change in the physiology of the P. nigra pollen tube, which shifts from an autotrophic to a heterotrophic type of nutrition. Protein analysis of pollinated stigmatic extracts (0,6 and 20 h after pollination) revealed qualitative and quantitative differences that are related to the presence of either P. nigra or P. alba pollen tubes inside the stigmatic tissues. Increasing numbers of protein bands were detectable from 0 to 20 h after pollination only in intraspecific cross. Glycoproteins were detected, and the differences observed were dependent of the cross. -Galactosidase activity was found in pollinated stigmas, but an increase in its activity (one isozyme of pHi 4.2) between 6 h and 20 h after pollination was detected only in the intraspecific cross. This enzyme could play a role in heterotrophic pollen-tube nitrition, and its activity could be the final result of a series of interactions started by the initial pollen-stigma dialog.     

A comparison of genetic variation among wild and cultivated Morus Species (Moraceae: Morus) as revealed by ISSR and SSR markers

In this study, inter-simple sequence repeats (ISSR) ans simple sequence repeat (SSR) markers were used to investigate genetic diversity of 27 mulberry accessions including 19 cultivated accessions (six M. multicaulis, three M. alba, two M. atropurpurea, two M. bombycis, one M. australis, two M. rotundiloba, one M. alba var. pendula, one M. alba var. macrophylla, and one M. alba var. venose) and 8 wild accessions (two M. cathayana, two M. laevigata, two M. wittiorum, one M. nigra and one M. mongolica). ISSRs and SSRs were compared in terms of their informativeness and efficiency in a study of genetic diversity and relationships among 27 mulberry genotypes. SSRs presented a higher level of polymorphism and greater information content. All index values of genetic diversity both markers analyzed using Popgene 32 software indicated that within wild species had higher genetic diversity than within cultivated species. Cultivation may caused the lose of genetic diversity of mulberry compared with wild species revealed by ISSR and SSR markers. The mean genetic similarity coefficients among all mulberry genotypes ascribed by ISSR and SSR matrices were 0.7677 and 0.6131, respectively. For all markers a high similarity in dendrogram topologies was obtained although some differences were observed. Cluster analysis of ISSR and SSR using UPGMA method revealed that the wild species are genetically distant from the domesticated species studied here. The correlation coefficients of similarity were statistically significant for both marker systems used. Principal coordinates analysis (PCA) for ISSR and SSR data also supports their UPGMA clustering. These results have an important implication for mulberry germplasm characterization, improvement, molecular systematics and conservation.     

自然干旱梯度下的酸枣表型变异

表型变异是植物应对环境变化的一种策略。酸枣植物从中国东部沿海到内陆腹地均有分布,其表型性状的变异可能解释其对自然干旱梯度的适应机制。为验证这一假说,以烟台、石家庄、银川、吐鲁番4个自然干旱梯度生境中生长的酸枣三年生植株的12个表型性状为调查研究对象,采用变异系数和巢式方差分析对酸枣的表型变异进行分析。结果表明:(1)从烟台到银川,叶面积、叶长、叶周长和叶柄长总体呈减小的趋势,而比叶面积呈增大的趋势;(2)随着干旱程度的增强,二次枝的长度、二次枝的基部粗、二次枝的枣吊数、茎比密度和茎水分含量均呈减小趋势,并且种子重和种子短轴长也均呈减小的趋势;(3)对沿干旱梯度分布的4个酸枣种群而言,叶性状的平均变异系数(33.70%)枝性状的平均变异系数(32.41%)种子性状的平均变异系数(9.07%),并且酸枣性状间存在很强的协变。结果表明酸枣的地上部分形态性状沿干旱梯度表现出很强的变异,推测在未来的气候变化下,酸枣将通过这种表型变异的有效组合来适应环境变化。     

Characteristics of oviposition behaviour of the pollen beetle, Meligethes aeneus on four different host plants

Oviposition behaviour of Meligethes aeneus F. (Coleoptera: Nitidulidae) is characterised and quantified on four different plant species. Six behavioural components are identified: W—walking, WA—walking with abdomen on surface, R—resting, B—biting, AOH—placing abdomen over the bite hole and OVI—oviposition. Comparison of host acceptance behaviours on Brassica napus L., Brassica juncea (L.) Czern, Brassica nigra (L.) Koch and Sinapis alba L. showed that S. alba was accepted as a host only after a long exposure to the plant. Behaviour on the Brassica species was similar, however on B. nigra beetles spent a high proportion of time actually ovipositing. We conclude that important cues for oviposition are located both on the bud surface and inside the bud.     

Different Responses of Sexual and Asexual Reproduction of Arundinella hirta to Flooding

The sexual and asexual reproductive features of the graminoid species Arundinella hirta growing at riversides of the Jialing River were analyzed. It was found that the total seed mass, seed number per plant, and sexual reproductive allocation of A. hirta decreased with decreasing bank elevation, and plants growing at the lowest elevations of banks subjected to intense flooding did not show sexual reproduction. The total plant biomass and the number of ramifications per plant increased with flooding intensity, which implies that, contrary to sexual reproduction, asexual reproduction of A. hirtawas enhanced by flooding. Translated from the Journal of Southwest China Normal University (Natural Science), 2005, 30(2) (in Chinese)     

Impact of habitat fragmentation on genetic population structure of roach, Rutilus rutilus, in a riparian ecosystem

During the last 200 years, the riparianecosystem along major rivers has been reducedto a few scattered remnants. Important elementsof the riparian ecosystem are water bodieswhich were originally connected to the mainriver channel by annual floodings. Due to riverregulations many of these remnants are nowvirtually isolated. In an allozyme analysisusing roach, Rutilus rutilus, as a studyspecies we demonstrate that the geneticdiversity (number of alleles per locus,expected heterozygosity) of populations livingwithin floodplain water bodies is not severelyimpoverished compared to the genetic diversitywithin the main river channel. However, wefound slight differences in the allelefrequencies of flood plain water bodies and themain river channel. Nevertheless, fishpopulations in floodplain water bodies mayserve as reservoirs of autochthonous geneticmaterial for restoration of fish populations inthe main river channel after populationextinction due to catastrophic accidents (e.g.industrial pollution).     

Recruitment and local persistence of a freshwater bryozoan in stream riffles

Recruitment in clonal organisms often involves both sexual and asexual processes whereby new individuals are added to adult populations. During 1988–1990,1 examined local distribution and abundance patterns, substrate colonization, and recruitment in riffle populations of the freshwater bryozoan Plumatella emarginata. Abundance levels of P. emarginata at eight selected study sites were strongly dependent on substrate size but not on overall substrate availability or stream position. Colonization of large, P. emarginata-free substrates at one site resulted in an aggregated dispersion pattern of short-lived colonies and the production of locally persistent sessoblasts. Sessoblast production rates were dependent of colony size and number. However, the estimated single season rates of colonization and recruitment were insufficient to account for the high resident population on large substrates. I conclude that sessoblast persistence is essential for escaping mortality and promoting future recruitment and local population growth and that the population dynamics of P. emarginata are strongly nonequilibrial being influenced by unpredictable disturbances as well as by rates of recruitment which increase with local density.     

Group Recruitment and Early Survival of Mazzaella Laminarioides

Several phycocolloid-producing Rhodophyta of significant economic importance are coalescing species, able to fuse with conspecifics during recruitment, reach larger sizes and increase their survival. In these species spores are needed to start cultivation (e.g. Gigartina, Mazzaella) or to increase the seed stocks, to renew senescent clones or to enlarge the base of genetic variation of vegetatively propagated species (e.g. Chondrus, Gracilaria, Eucheuma). This study uses Mazzaella laminarioides to evaluate some key features that influence recruitment success. Field measurements indicate that in any recruitment event a variable amount of the spores reaching a given place may form groups of 2 to over 100 coalescing spores, while field experiments support the idea that early recruitment success is a function of the number of coalescing spores forming the individual, as multisporic, coalescing recruits have higher survival rates than sporelings formed by one or a few spores. Therefore, group recruitment (spores settling and recruiting in close spatial proximity) appears as a prerequisite for sporeling coalescence and early recruitment success. In turn, laboratory experiments suggest that the frequency of group recruitment and coalescence increases with increasing spore abundance and with slight Ca⁺⁺ additions to the culture medium. These last two factors could be handled by farmers to improve the success of spore inoculations of coalescing species.     

Factors affecting the regeneration and distribution of riparian woodlands along a northern prairie river: the Red Deer River,Alberta, Canada

Abstract. Flooding patterns and variations in the composition and successional trends of riparian vegetation in the upper and lower Red Deer River in southern Alberta, Canada, were studied in order to establish which flood regimes were most important in the regeneration and maintenance of riparian vegetation communities, with a particular focus on riparian poplars. The dominant riparian tree in the upper river is Populus balsamifera with some Picea glauca and in the lower river Populus deltoides (the plains cottonwood). Dendrochronological studies of the poplars along the river show that major periods of regeneration correspond with major flood events during the record period. Extensive cottonwood regeneration occurred in the period 1900–20, corresponding with a series of floods, some as high as the 1 in 100-year event. In addition, just prior to and during this period there was a significant reduction in use of the floodplain by bison, clearance of forests and a higher incidence of fires in upper reaches of the river and a series of high rainfall years. A series of floods greater than the 1 in 10-year flood occurred in the 1950s and stimulated the most extensive regeneration of poplars experienced since the 1920s. Parts of the lower Red Deer floodplain are now at elevations well above the 1 in 100-year flood event. It is suggested that fringe replenishment of riparian poplars is currently the dominant form of regeneration and that the large stands of mature poplars found on the floodplain, initiated during the end of the last century and first decades of this century, are unlikely to be replaced unless large floods (>1 in 50-year events) occur again. Construction of the Dickson Dam above the city of Red Deer in 1983 has led to attenuation of floods and a reduced likelihood that extensive flooding and poplar regeneration will occur again. A number of recommendations are made regarding flow management to both retain the fringe regeneration of poplars currently occurring and to stimulate more widespread regeneration of poplars on the floodplain.