Fatty acid-oxidizing consortia along a nutrient gradient in the Florida Everglades

The Florida Everglades is one of the largest freshwater marshes in North America and has been subject to eutrophication for decades. A gradient in P concentrations extends for several kilometers into the interior of the northern regions of the marsh, and the structure and function of soil microbial communities vary along the gradient. In this study, stable isotope probing was employed to investigate the fate of carbon from the fermentation products propionate and butyrate in soils from three sites along the nutrient gradient. For propionate microcosms, 16S rRNA gene clone libraries from eutrophic and transition sites were dominated by sequences related to previously described propionate oxidizers, such as Pelotomaculum spp. and Syntrophobacter spp. Significant representation was also observed for sequences related to Smithella propionica, which dismutates propionate to butyrate. Sequences of dominant phylotypes from oligotrophic samples did not cluster with known syntrophs but with sulfate-reducing prokaryotes (SRP) and Pelobacter spp. In butyrate microcosms, sequences clustering with Syntrophospora spp. and Syntrophomonas spp. dominated eutrophic microcosms, and sequences related to Pelospora dominated the transition microcosm. Sequences related to Pelospora spp. and SRP dominated clone libraries from oligotrophic microcosms. Sequences from diverse bacterial phyla and primary fermenters were also present in most libraries. Archaeal sequences from eutrophic microcosms included sequences characteristic of Methanomicrobiaceae, Methanospirillaceae, and Methanosaetaceae. Oligotrophic microcosms were dominated by acetotrophs, including sequences related to Methanosarcina, suggesting accumulation of acetate.     

Phylogeny of acetate-utilizing microorganisms in soils along a nutrient gradient in the Florida Everglades

The consumption of acetate in soils taken from a nutrient gradient in the northern Florida Everglades was studied by stable isotope probing. Bacterial and archaeal 16S rRNA gene clone libraries from eutrophic and oligotrophic soil microcosms strongly suggest that a significant amount of acetate is consumed by syntrophic acetate oxidation in nutrient-enriched soil.     

Biomass and nutrient allocation of sawgrass and cattail along a nutrient gradient in the Florida Everglades

Biomass and nutrient allocation in sawgrass (Cladium jamaicense Crantz) and cattail (Typha domingensis Pers.) were examined along a nutrient gradient in the Florida Everglades in 1994. This north to south nutrient gradient, created by discharging nutrient-rich agricultural runoff into the northern region of Water Conservatio ea 2A, was represented by three areas (impacted, transitional and reference). Contrasting changes of plant density and size along the gradient were found for communities of both species. For the sawgrass community, more small plants were found in ref ce areas, whereas few large plants were found in impacted areas. In contrast, for the cattail community, bigger plants were found in reference areas, and smaller plants were found in impacted areas. Both species allocated approximately 60% of their total biomass to leaves and 40% to belowground tissues. However, sawgrass biomass allocation to leaves, roots, shoot bases and rhizomes (65%, 19%, 11%, and 5%, respectively) was similar among the three areas. In contrast, cattail plants growing in referen reas showed higher root allocation (27.3%), but lower leaf allocation (51.1%) than those growing in impacted areas (14.6% and 65.8% for root and leaf allocation, respectively). Cattail had higher phosphorus concentrations than sawgrass in tissues associated with growth functions (leaves, roots, and rhizomes). In contrast, sawgrass had higher phosphorus and nitrogen concentrations than cattail in tissues primarily associated with resource storage (shoot bases). From impacted to reference areas, for sawgrass, there was a decrease of leaf TP from 605 to 248 (mg/kg), root TP from 698 to 181 (mg/kg), rhizome TP from 1,139 to 142 (mg/kg), and shoot base TP from 5,412 to 400 to (mg/kg). For cattail, leaf TP decreased from 1,175 to 556 (mg/kg), root TP de sed from 1,100 to 798 (mg/kg), rhizome TP decreased from 1390 to 380 (mg/kg), and shoot base TP decreased from 2,990 to 433 (mg/kg). N/P ratios of sawgrass in reference areas were 27, 63, 38, and 50 for leaves, roots, rhizomes, and shoot bases, respectively, whereas in impacted areas they were 11, 21, 6, and 2, respectively. The greatest TP storage was found in impacted areas. Differences in seed output, seed number, and mean seed weight were found for both species as well. Each cattail flower stalk duced approximately 105 tiny seeds (0.048 ± 0.001 mg) while each sawgrass flower stalk produced about 103 large seeds (3.13 ± 0.005 mg). These results suggest that phosphorus is a limiting resource in the Everglades and that the two species have different life history strategies. These data provide an ecological basis for making informed management and planning decisions to protect and restore the Everglades.     

Syntrophic-Methanogenic Associations along a Nutrient Gradient in the Florida Everglades

Nutrient runoff from the Everglades Agricultural Area resulted in a well-documented gradient of phosphorus concentrations in soil and water, with concomitant ecosystem-level changes, in the northern Florida Everglades. It was recently reported that sulfate-reducing prokaryote assemblage composition, numbers, and activities are dependent on position along the gradient (H. Castro, K. R. Reddy, and A. Ogram, Appl. Environ. Microbiol. 68:6129-6137, 2002). The present study utilized a combination of culture- and non-culture-based approaches to study differences in composition of assemblages of syntrophic and methanogenic microbial communities in eutrophic, transition, and oligotrophic areas along the phosphorus gradient. Methanogenesis rates were much higher in eutrophic and transition regions, and sequence analysis of 16S rRNA gene clone libraries constructed from samples taken from these regions revealed differences in composition and activities of syntroph-methanogen consortia. Methanogens from eutrophic and transition regions were almost exclusively composed of hydrogenotrophic methanogens, with approximately 10,000-fold-greater most probable numbers of hydrogenotrophs than of acetotrophs. Most cultivable strains from eutrophic and transition regions clustered within novel lineages. In non-culture-based studies to enrich syntrophs, most bacterial and archaeal clones were either members of novel lineages or closely related to uncultivated environmental clones. Novel cultivable Methanosaeta sp. and fatty acid-oxidizing bacteria related to the genera Syntrophomonas and Syntrophobacter were observed in microcosms containing soil from eutrophic regions, and different lines of evidence indicated the existence of novel syntrophic association in eutrophic regions.     

Fatty Acid-Oxidizing Consortia along a Nutrient Gradient in the Florida Everglades

The Florida Everglades is one of the largest freshwater marshes in North America and has been subject to eutrophication for decades. A gradient in P concentrations extends for several kilometers into the interior of the northern regions of the marsh, and the structure and function of soil microbial communities vary along the gradient. In this study, stable isotope probing was employed to investigate the fate of carbon from the fermentation products propionate and butyrate in soils from three sites along the nutrient gradient. For propionate microcosms, 16S rRNA gene clone libraries from eutrophic and transition sites were dominated by sequences related to previously described propionate oxidizers, such as Pelotomaculum spp. and Syntrophobacter spp. Significant representation was also observed for sequences related to Smithella propionica, which dismutates propionate to butyrate. Sequences of dominant phylotypes from oligotrophic samples did not cluster with known syntrophs but with sulfate-reducing prokaryotes (SRP) and Pelobacter spp. In butyrate microcosms, sequences clustering with Syntrophospora spp. and Syntrophomonas spp. dominated eutrophic microcosms, and sequences related to Pelospora dominated the transition microcosm. Sequences related to Pelospora spp. and SRP dominated clone libraries from oligotrophic microcosms. Sequences from diverse bacterial phyla and primary fermenters were also present in most libraries. Archaeal sequences from eutrophic microcosms included sequences characteristic of Methanomicrobiaceae, Methanospirillaceae, and Methanosaetaceae. Oligotrophic microcosms were dominated by acetotrophs, including sequences related to Methanosarcina, suggesting accumulation of acetate.     

Phylogeny of Acetate-Utilizing Microorganisms in Soils along a Nutrient Gradient in the Florida Everglades

The consumption of acetate in soils taken from a nutrient gradient in the northern Florida Everglades was studied by stable isotope probing. Bacterial and archaeal 16S rRNA gene clone libraries from eutrophic and oligotrophic soil microcosms strongly suggest that a significant amount of acetate is consumed by syntrophic acetate oxidation in nutrient-enriched soil.     

Structure and function of methanogens along a short-term restoration chronosequence in the Florida Everglades

The removal of plants and soil to bedrock to eradicate exotic invasive plants within the Hole-in-the-Donut (HID) region, part of the Everglades National Park (Florida), presented a unique opportunity to study the redevelopment of soil and the associated microbial communities in the context of short-term primary succession and ecosystem restoration. The goal of this study was to identify relationships between soil redevelopment and activity and composition of methanogenic assemblages in HID soils. Methane production potentials indicated a general decline in methanogenic activity with restoration age. Microcosm incubations strongly suggested hydrogenotrophic methanogenesis as the most favorable pathway for methane formation in HID soils from all sites. Culture-independent techniques targeting methyl coenzyme M reductase genes (mcrA) were used to assess the dynamics of methanogenic assemblages. Clone libraries were dominated by sequences related to hydrogenotrophic methanogens of the orders Methanobacteriales and Methanococcales and suggested a general decline in the relative abundance of Methanobacteriales mcrA with time since restoration. Terminal restriction fragment length polymorphism analysis indicated methanogenic assemblages remain relatively stable between wet and dry seasons. Interestingly, analysis of soils across the restoration chronosequence indicated a shift in Methanobacteriales populations with restoration age, suggesting genotypic shifts due to site-specific factors.     

Structure and Function of Methanogens along a Short-Term Restoration Chronosequence in the Florida Everglades

The removal of plants and soil to bedrock to eradicate exotic invasive plants within the Hole-in-the-Donut (HID) region, part of the Everglades National Park (Florida), presented a unique opportunity to study the redevelopment of soil and the associated microbial communities in the context of short-term primary succession and ecosystem restoration. The goal of this study was to identify relationships between soil redevelopment and activity and composition of methanogenic assemblages in HID soils. Methane production potentials indicated a general decline in methanogenic activity with restoration age. Microcosm incubations strongly suggested hydrogenotrophic methanogenesis as the most favorable pathway for methane formation in HID soils from all sites. Culture-independent techniques targeting methyl coenzyme M reductase genes (mcrA) were used to assess the dynamics of methanogenic assemblages. Clone libraries were dominated by sequences related to hydrogenotrophic methanogens of the orders Methanobacteriales and Methanococcales and suggested a general decline in the relative abundance of Methanobacteriales mcrA with time since restoration. Terminal restriction fragment length polymorphism analysis indicated methanogenic assemblages remain relatively stable between wet and dry seasons. Interestingly, analysis of soils across the restoration chronosequence indicated a shift in Methanobacteriales populations with restoration age, suggesting genotypic shifts due to site-specific factors.     

Peat accretion and phosphorus accumulation along a eutrophication gradient in the northern Everglades

Recent rates of peat accretion (as determined by¹³⁷Cs) and N, P, organic C, Ca and Na accumulation were measured along a 10 km eutrophication gradient in the northern Everglades area of Water Conservation Area 2A (WCA 2A) that has received agricultural drainage from the Hillsboro canal for the past 25–30 yrs. Rates of peat accretion were highest at sampling locations closest to the Hillsboro canal, 1.6 km downstream, (5.67 ± 0.50 mm/yr) and decreased to 2.01 ± 0.31 mm/yr at distances of 7.1 to 10.7 km downstream. Phosphorus and Na accumulation were a function of both peat accretion and soil P and Na concentrations. The concentration and accumulation of P in peat deposited in the past 26 years was highest near the Hillsboro canal (1478 ± 67 ug/g, 0.66 ± 0.06 g/m²/yr) and decreased to 560 ± 20 ug/g and 0.10 ± 0.02 g/m²/yr at distances of 8.8 to 10.7 km downstream. Like phosphorus, the concentration and rate of Na accumulation was highest near the Hillsboro canal (3205 ± 1021 ug/g, 1.48 ± 0.53 g/m²/yr). Although sodium enrichment of the peat was limited to 1.6 km downstream of the Hillsboro canal, increased rates of Na accumulation penetrated 5.2 km downstream of the Hillsboro canal, the extent of the area of enhanced peat accretion. In contrast to P and Na, there was no difference in the concentration of soil organic C, N and Ca along the eutrophication gradient. However, there was a gradient of organic C, N and Ca accumulation corresponding to the area of enhanced peat accretion. The highest rates occurred 1.6 km south of the Hillsboro canal (212 ± 5 g organic C/m²/yr, 14.1 ± 0.4 g N/m²/yr, 22.1 ± 5.2 g Ca/m²/yr). Accumulation of organic C, N and Ca at distances of 7.1–10.7 km downstream averaged 87 ± 11, 6.3 ± 0.7 and 6.5 ± 0.9 g/m²/yr, respectively. The areal extent of enhanced peat accretion and organic C, N, Ca and Na accumulation encompasses approximately 7700 ha of the northern part of WCA 2A. The area of enhanced P accumulation is larger, covering 11,500 ha or 26% of the total area of WCA 2A. The 11,500 ha area has functioned as a sink for P for the past 25–30 yr removing 74% (49.3 MT/yr) of the 67 MT/yr that enters via agricultural drainage and rainfall. Moreover, P accumulation along the gradient was related to mean (1989–1990) surface water P concentration, decreasing as surface water P decreases. These findings suggest that P accumulation is dependent on the P concentration in the water column and that decreasing P loadings per unit area result in less P storage per unit area. The potential longterm equilibrium of the 11,500 ha area as a sink for P is based on a mean annual loading of 67 metric tons P/yr. Input rates exceeding this loading rate could result in an expansion of the 11,500 ha area until a new equilibrium size is reached.     

Interactions between nutrient availability and hydroperiod shape macroinvertebrate communities in Florida Everglades marshes

Hydroperiod and nutrient status are known to influence aquatic communities in wetlands, but their joint effects are not well explored. I sampled floating periphyton mat and flocculent detritus (floc) infaunal communities using 6-cm diameter cores at short- and long-hydroperiod and constantly inundated sites across a range of phosphorus (P) availability (total phosphorus in soil, floc and periphyton). Differences in community structure between periphyton and floc microhabitats were greater than any variation attributable to hydroperiod, P availability, or other spatial factors. Multivariate analyses indicated community structure of benthic-floc infauna was driven by hydroperiod, although crowding (no. g⁻¹ AFDM) of individual taxa showed no consistent responses to hydroperiod or P availability. In contrast, community structure of periphyton mat infauna was driven by P availability, while densities of mat infauna (no. m⁻²) were most influenced by hydroperiod (+correlations). Crowding of mat infauna increased significantly with P availability in short-hydroperiod marshes, but was constant across the P gradient in long-hydroperiod marshes. Increased abundance of floating-periphyton mat infauna with P availability at short-hydroperiod sites may result from a release from predation by small fish. Community structure and density were not different between long-hydroperiod and constantly inundated sites. These results have implications for the use of macroinvertebrates as indicators of water quality in wetlands and suggest the substrate sampled can influence interpretation of ecological responses observed in these communities.     

Bacteria and algae in stream periphyton along a nutrient gradient

1. Stream riffles in southern Ontario and western Quèbec were sampled for biomass (58 stations from 51 streams) and production (22 stations from 21 streams) of algae and bacteria in periphyton to test the hypothesis that bacteria in benthic biofilms compete with algae for nutrients. 2. Algal and bacterial biomass were positively correlated, as were algal and bacterial production. Bacterial production was also positively correlated to algal and bacterial biomass, but the relationship was not significant. The ratio of algal to bacterial biomass did not vary with nutrients whereas algal production tended to increase with nutrients more rapidly than bacterial production. 3. Instream nitrogen concentrations explained 38–58% of the variability in algal biomass and production. Bacterial abundance explained an additional 9–29% of the residual variance in algal production and biomass. However, the relationship between bacterial abundance and algal production and biomass, once nutrients were taken into account, was positive, in contrast to the predicted effect of competition. 4. Hence, we reject our original hypothesis that bacteria in biofilms compete with algae for nutrients and instead suggest that bacteria and algae in biofilms coexist in an association that offers space and resources to sustain production of both groups of organisms.     

Biogeochemical processes along a nutrient gradient in a tropical ombrotrophic peatland

Biogeochemical properties, including nutrient concentrations, carbon gas fluxes, microbial biomass, and hydrolytic enzyme activities, were determined along a strong nutrient gradient in an ombrotrophic peatland in the Republic of Panama. Total phosphorus in surface peat decreased markedly along a 2.7 km transect from the marginal Raphia taedigera swamp to the interior sawgrass swamp, with similar trends in total nitrogen and potassium. There were parallel changes in the forest structure: basal area decreased dramatically from the margins to the interior, while tree diversity was greatest at sites with extremely low concentrations of readily-exchangeable phosphate. Soil microbial biomass concentrations declined in parallel with nutrient concentrations, although microbes consistently contained a large proportion (up to 47%) of the total soil phosphorus. Microbial C:P and N:P ratios and hydrolytic enzyme activities, including those involved in the cycles of carbon, nitrogen, and phosphorus, increased towards the nutrient-poor wetland interior, indicating strong below-ground nutrient limitation. Soil CO₂ fluxes and CH₄ fluxes did not vary systematically along the nutrient gradient, although potential soil respiration determined on drained soils was lower from nutrient-poor sites. Soil respiration responded strongly to drainage and increased temperature. Taken together, our results demonstrate that nutrient status exerts a strong control on above and below-ground processes in tropical peatlands with implications for carbon dynamics and hence long term development of such ecosystems.     

Changes in semi-arid plant species associations along a livestock grazing gradient

In semi-arid ecosystems, vegetation is heterogeneously distributed, with plant species often associating in patches. These associations between species are not constant, but depend on the particular response of each species to environmental factors. Here, we investigated how plant species associations change in response to livestock grazing in a semi-arid ecosystem, Cabo de Gata-Níjar Natural Park in South East Spain. We established linear point-intercept transects at four sites with different grazing intensity, and recorded all species at each point. We investigated plant associations by comparing the number of times that each pair of species occurred at the same spatial point (co-occurrences), with the expected number of times based on species abundances. We also assessed associations for each shrub and grass species by considering all their pairs of associations and for the whole plant community by considering all pairs of associations on each site. At all sites, the plant community had a negative pattern of association, with fewer co-occurrences than expected. Negative association in the plant community increased at maximum grazing intensity. Most species associated as expected, particularly grass species, and positive associations were most important at intermediate grazing intensities. No species changed its type of association along the grazing gradient. We conclude that in the present plant community, grazing-resistant species compete among themselves and segregate in space. Some shrub species act as refuges for grazing-sensitive species that benefit from being spatially associated with shrub species, particularly at intermediate grazing intensities where positive associations were highest. At high grazing intensity, these shrubs can no longer persist and positive associations decrease due to the disappearance of refuges. Spatial associations between plant species and their response to grazing help identify the factors that organize plant communities, and may contribute to improving management of semi-arid ecosystems.     

Molecular characterization of Cladium peat from the Florida Everglades: biomarker associations with humic fractions

The accumulation and preservation of peat soils in Everglades freshwater marshes and mangrove swamps is an essential process in the ecological functioning of these ecosystems. Human intervention and climate change have modified nutrient dynamics and hydroperiod in the Everglades and peat loss due to such anthropogenic activities is evident. However, not much is known on the molecular level regarding the biogeochemical characteristics, which allow peat to be preserved in the Everglades. Lipid biomarkers trapped within or bound to humic-type structures can provide important geochemical information regarding the origin and microbial transformation of OM in peat. Four lipid fractions obtained from a Cladium peat, namely the freely extractable fraction and those associated with humin, humic acid, and fulvic acid fractions, showed clear differences in their molecular distribution suggesting different OM sources and structural and diagenetic states of the source material. Both, higher plant derived and microbial lipids were found in association with these humic-type substances. Most biomarker distributions suggest an increment in the microbial/terrestrial lipid ratio from the free to humin to humic to fulvic fractions. Microbial reworking of lipids, and the incorporation of microbial biomarkers into the humic-type fractions was evident, as well as the preservation of diagenetic byproducts. The lipid distribution associated with the fulvic acids suggests a high degree of microbial reworking for this fraction. Evidence for this 3D structure was obtained through the presence of the relatively high abundance of α,ω-dicarboxylic acids and phenolic and benzenecarboxylic compounds. The increment in structural complexity of the phenolic and benzencarboxylic compounds in combination with the reduction in the carbon chain length of the dicarboxylic acids from the free to fulvic fraction suggests the latter to be structurally the most stable, compacted and diagenetically altered substrate. This analytical approach can now be applied to peat samples from other areas within the Everglades ecosystem, affected differently by human intervention with the aim to assess changes in organic matter preservation.     

Tree island pattern formation in the Florida Everglades

The Florida Everglades freshwater landscape exhibits a distribution of islands covered by woody vegetation and bordered by marshes and wet prairies. Known as “tree islands”, these ecogeomorphic features can be found in few other low gradient, nutrient limited freshwater wetlands. In the last few decades, however, a large percentage of tree islands have either shrank or disappeared in apparent response to altered water depths and other stressors associated with human impacts on the Everglades. Because the processes determining the formation and spatial organization of tree islands remain poorly understood, it is still unclear what controls the sensitivity of these landscapes to altered conditions. We hypothesize that positive feedbacks between woody plants and soil accretion are crucial to emergence and decline of tree islands. Likewise, positive feedbacks between phosphorus (P) accumulation and trees explain the P enrichment commonly observed in tree island soils. Here, we develop a spatially-explicit model of tree island formation and evolution, which accounts for these positive feedbacks (facilitation) as well as for long range competition and fire dynamics. It is found that tree island patterns form within a range of parameter values consistent with field data. Simulated impacts of reduced water levels, increased intensity of drought, and increased frequency of dry season/soil consuming fires on these feedback mechanisms result in the decline and disappearance of tree islands on the landscape.     

Pattern of nutrient availability and plant community assemblage in Everglades Tree Islands, Florida, USA

We address the relative importance of nutrient availability in relation to other physical and biological factors in determining plant community assemblages around Everglades Tree Islands (Everglades National Park, Florida, USA). We carried out a one-time survey of elevation, soil, water level and vegetation structure and composition at 138 plots located along transects in three tree islands in the Park’s major drainage basin. We used an RDA variance partitioning technique to assess the relative importance of nutrient availability (soil N and P) and other factors in explaining herb and tree assemblages of tree island tail and surrounded marshes. The upland areas of the tree islands accumulate P and show low N concentration, producing a strong island-wide gradient in soil N:P ratio. While soil N:P ratio plays a significant role in determining herb layer and tree layer community assemblage in tree island tails, nevertheless part of its variance is shared with hydrology. The total species variance explained by the predictors is very low. We define a strong gradient in nutrient availability (soil N:P ratio) closely related to hydrology. Hydrology and nutrient availability are both factors influencing community assemblages around tree islands, nevertheless both seem to be acting together and in a complex mechanism. Future research should be focused on segregating these two factors in order to determine whether nutrient leaching from tree islands is a factor determining community assemblages and local landscape pattern in the Everglades, and how this process might be affected by water management.     

Competition between Australian native and introduced grasses along a nutrient gradient

Abstract Seven grass species were grown in monocultures and in multispecies mixtures along a gradient of total nutrient levels that ranged from 1/64 to 16× the normal level of nutrient solution. The seven grasses represented three ecological groups: (i) three perennial species native to Australia (Themeda triandra, Poa labillardieri and Danthonia carphoides); (ii) two introduced annuals (Vulpia bromoides and Hordeum leporinum); and (iii) two introduced perennials (Lolium perenne and Dactylis glomerata). We hypothesized that the native grasses would prove less competitive when grown at increased nutrient levels than those introduced from Europe. Results supported the hypothesis. The native species were unable to compete in mixtures even at the lowest nutrient level, where T. triandra was the most productive species in monoculture. Lolium perenne and Dactylis glomerata dominated mixtures at intermediate nutrient levels. The responses of the annual introduced grasses differed in that Vulpia bromoides showed an optimum at intermediate nutrient levels in both monoculture and in mixtures, whereas Hordeum leporinum dominated at the highest nutrient levels in mixture but was suppressed by V. bromoides, L. perenne and D. glomerata at intermediate levels. The results are discussed in terms of predicting species responses in mixtures from their performance in monocultures as well as in terms of previous observations on the sequential changes in botanical composition of south‐eastern Australian grasslands after 150 years of continuous grazing by sheep.     

The gradostat: A model of competition along a nutrient gradient

The general mathematical theory of the gradostat is presented for two competitors. The gradostat provides a mechanism for studying competition along a nutrient gradient. In the two vessel case, the results are complete and the conditions are testable. In then-vessel case, the relevant conditions are stated in terms of the stability modulii of certain matrices and are testable for any specific case.     

Competition and coexistence of rhizomatous perennial plants along a nutrient gradient

I studied competition and coexistence of three tall clonal perennial plant species, Calamagrostis epigejos (L.) Roth, Solidago canadensis L., and Tanacetum vulgare L. along a gradient of soil productivity over five years. A replacement series field experiment was conducted with high, moderate and low fertility levels in 1m×1m plots. There were significant effects of soil type on ramet density (P<0.001), mean height (P<0.01), and total biomass (P<0.01). Ramet density, mean height, and total biomass increased with increasing soil fertility. There were also significant effects of mixture on ramet density (P<0.01), but not on mean height and total biomass for all species. Significant neighbor effects on ramet density and total biomass (P<0.01) were found for Solidago, showing that it is important whether Tanacetum or Calamagrostis is its neighbor within mixtures. During the five years there was only one case of competitive exclusion: Calamagrostis excluded Solidago on the most fertile substrate in the fifth growing season. In most cases species coexisted over the five years. Each of the three species was able to dominate in at least one combination of substrate type and mixture. The experiment showed that asymmetric competition for light on substrates of high fertility, symmetric competition for nutrients on nutrient-poor soil and positive interactions especially on substrates of intermediate fertility played a role. A founder effect was evident in aggregated mixtures of Calamagrostis and Solidago on the nutrient-rich substrate. A conceptual model of the relative importance of root competition for soil nutrients, shoot competition for light, and positive interactions along the fertility gradient is presented. The model emphasizes that positive interactions play an important role over a broad range of the productivity scale with a peak at intermediate levels of fertility. On the substrate of high productivity shoot competition for light is more important than positive interactions and root competition for soil nutrients as well. The competitive superiority of Calamagrostis on the most productive substrate was evident only in the long run. Rare events like extreme summer drought or selective herbivore pressure caused a switch in dominance in mixtures with Solidago, respectively Tanacetum. The guerrilla growth strategy of Calamagrostis and interference competition through a dense cover of aboveground biomass and litter could further cause competitive exclusion.