Martin J. Wassen

Endangered plants persist under phosphorus limitation

Nitrogen enrichment is widely thought to be responsible for the loss of plant species from temperate terrestrial ecosystems. This view is based on field surveys and controlled experiments showing that species richness correlates negatively with high productivity and nitrogen enrichment. However, as the type of nutrient limitation has never been examined on a large geographical scale the causality of these relationships is uncertain. We investigated species richness in herbaceous terrestrial ecosystems, sampled along a transect through temperate Eurasia that represented a gradient of declining levels of atmospheric nitrogen deposition—from ∼ 50 kg ha-1 yr-1 in western Europe to natural background values of less than 5 kg ha-1 yr-1 in Siberia. Here we show that many more endangered plant species persist under phosphorus-limited than under nitrogen-limited conditions, and we conclude that enhanced phosphorus is more likely to be the cause of species loss than nitrogen enrichment. Our results highlight the need for a better understanding of the mechanisms of phosphorus enrichment, and for a stronger focus on conservation management to reduce phosphorus availability.

SPECIES RICHNESS-PRODUCTIVITY PATTERNS DIFFER BETWEEN N-, P-, AND K-LIMITED WETLANDS

We evaluated whether the kind of nutrient limitation (N, P, or K) may affect species richness–productivity patterns and subsequently may explain variation in species richness and in richness of threatened species. We present a data set from previous studies in wetlands in Poland, Belgium, and The Netherlands and examine species richness–productivity patterns for vascular plants in all 150 sites together as well as for N-, P-, and K-limited sites separately. The kind of nutrient limitation was assessed by N:P, N:K, and K:P ratios in the vegetation. Critical values for these ratios were derived from a literature review of fertilization experiments. The kind of nutrient limitation influenced species richness–productivity patterns in our 150 sites through large differences in productivity. P (co)-limitation occurred only at low productivity, K (co)-limitation up to intermediate productivity, and N limitation along the entire productivity gradient. There was a decreasing trend in species richness with increasing productivity for K (co)-limited sites, whereas for both the N-limited sites and P (co)-limited sites a sort of “filled hump-shaped curve” was observed. The species richness–productivity relationship for threatened species was restricted to a much narrower productivity range than that for all species. Richness of threatened species was higher in P (co)-limited sites than in N-limited sites, suggesting that increased P availabilities in wetlands may be particularly important in causing disappearance of threatened species in western Europe. The role of nutrient limitation in species richness–productivity relationships not only reveals mechanisms that may explain variation in species richness and occurrence of threatened species, but it also may be important for nature management practice.

Calibrating Ellenberg indicator values for moisture, acidity, nutrient availability and salinity in the Netherlands

A general calibration of Ellenberg indicator values for moisture, acidity, nutrient availability and salinity was carried out on a large database of relevées and environmental variables from a variety of ecosystems in the Netherlands.Satisfying relationships with Ellenberg indicator values for moisture, acidity and salinity were found for mean groundwater level in spring time, soil pH and chloride concentration in groundwater. For mean groundwater level in spring and chloride concentration in groundwater subdivision of the database led to clearer relationships with indicator values. For the Ellenberg indicator value for nutrient availability satisfying calibration results were only achieved with data on standing crops and N stock in standing crop. The relationship with soil chemical variables was less clear.Although the correlation between indicator and measured values is obvious, the variation around the regression lines is considerable. However, because of the size and composition of the database, it is unlikely that our calibration results can be much improved by adding more (Dutch) data.The calibration results will be applied in the multi-stress model SMART-MOVE, developed to predict changes in species composition due to acidification, eutrophication and the effects of lowering groundwater.

Fens and floodplains of the temperate zone: Present status, threats, conservation and restoration

ABSTRACT This Special Feature focuses on lowland fens and flood plains. In this introduction we discuss the most important mire-related terms, present status, threats and conservation and restoration attempts. Floodplains and especially lowland fens are rare and vulnerable ecosystems. They are highly threatened all over the world because of direct conversion to agricultural land and especially the lack of appropriate management and altered catchment hydrology. Finally we present a framework for the conservation and restoration of these ecosystems. This consists of (1) optimising abiotic conditions; (2) safeguarding propagule availability of the target species; (3) creating and maintaining conditions for (re)establishment of these species, and (4) appropriate management to keep the conditions suitable.

A Putative Mechanism for Bog Patterning

The surface of bogs commonly shows various spatial vegetation patterning. Typical are “string patterns” consisting of regular densely vegetated bands oriented perpendicular to the slope. Here, we report on regular “maze patterns” on flat ground, consisting of bands densely vegetated by vascular plants in a more sparsely vegetated matrix of nonvascular plant communities. We present a model reproducing these maze and string patterns, describing how nutrient‐limited vascular plants are controlled by, and in turn control, both hydrology and solute transport. We propose that the patterns are self‐organized and originate from a nutrient accumulation mechanism. In the model, this is caused by the convective transport of nutrients in the groundwater toward areas with higher vascular plant biomass, driven by differences in transpiration rate. In a numerical bifurcation analysis we show how the maze patterns originate from the spatially homogeneous equilibrium and how this is affected by changes in rainfall, nutrient input, and plant properties. Our results confirm earlier model results, showing that redistribution of a limiting resource may lead to fine‐scale facilitative and coarse‐scale competitive plant interactions in different ecosystems. Self‐organization in ecosystems may be a more general phenomenon than previously thought, which can be mechanistically linked to scale‐dependent facilitation and competition.

The relationship between fen vegetation gradients, groundwater flow and flooding in an undrained valley mire at Biebrza, Poland.

(1) Relationships between vegetation composition and nutrient and major ion concentrations in groundwater and peat were examined in 58 stands of fen vegetation in the Biebrza mire, Poland. The 58 stands included rich fens with a large standing biomass, rich fens with lower standing biomass and poor fens. (2) The highly productive rich fen (i.e. rich fen with large standing biomass) receives moderately eutrophic river water. The rheophilous part of the mire that is not, or only occasionally flooded contains gradients from quite productive rich fens to less productive rich fens and poor fens. In this part of the gradient, occasional river flooding appeared to be an important source of K and was probably responsible for the presence of quite productive rich fen vegetation. (3) Upward seepage of calcium-rich and phosphate-poor groundwater keeps phosphate concentration low in the superficial mire water and the peat from the lower productive rich fen. Phosphate concentration is higher in the mire water and the peat from the poor fen which is fed by downward-flowing rainwater. (4) Nitrogen concentration in mire water and peat is only poorly correlated with species composition and vegetation structure.

Patterns in vegetation, hydrology, and nutrient availability in an undisturbed river floodplain in Poland

In the undisturbed floodplain of the Biebrza river (N.E. Poland) wecompared vegetation composition, standing crop and the nutrients in standingcrop to site factors such as flood duration and inundation depth during springfloods, summer water levels and concentrations of chemical constituents inwaterand nutrient release rates from peat. Our analysis shows a number of clearspatial patterns of biotic and abiotic variables in the ca. 1 kmwide river marginal wetland. The distribution of vegetation types follows acertain pattern: Glycerietum maximae close to the river,followed by respectively Caricetum gracilis andCaricetum elatae and finally Calamagrostietumstrictae at the margin of the river plain. Species richnessincreasesand standing crop decreases from the river towards the margin. The elevation ofthe ground surface gently rises with increasing distance from the river; floodduration and flooding depths in spring decrease in the same direction.Groundwater tables in summer are less correlated to the elevation gradient buttend to be closer to the ground surface at the valley margin. These differencesalso lead to a higher amplitude close to the river and a fairly stable watertable far away from the river. Concentrations of major ions and ammoniumincrease towards the river. Nutrient release rates are also higher closer totheriver. Absence and presence of species and the variation in species compositionof the vegetation was explained best by flood variables; variables fromgroundwater explained much less of the variance. Variations in standing cropandnutrients in standing crop corresponded better to the rates of nutrient releasefrom the organic soil than to nutrient concentrations in the soil water. Weconcluded that river hydrology and nutrient release from the soil are clearlyrelated to vegetation composition, species richness and productivity of thevegetation.

Nutrient concentrations in mire vegetation as a measure of nutrient limitation in mire ecosystems

Abstract. The above-ground standing crop and nutrient concentrations in plant material were examined in 45 stands of mire vegetation in the Biebrza peatland, Poland. The stands included flood-plains, rich fens, transitional fens and bogs. The pattern in nutrient concentrations in the above-ground plant material resembled the pattern in nutrient concentrations in peatwater and peat which had been investigated in an earlier study. Concentrations of N were quite uniform along the gradient. P-concentrations were highest in the transitional fen. Critical nutrient concentrations were defined on the basis of a review of nutrient concentrations in plant material from peatlands in which a fertilization experiment had been carried out. Defined critical values for phanerogams were: 13-14 and 0.7 mg/g dry wt for N and P respectively. Concentrations lower than these values indicate deficiency. P/N ratios ≥ 0.07 indicate N-deficiency and P/N ratios ≤ 0.04 — 0.05 indicate P-deficiency. According to these values the Biebrza fens and bogs appear to be primarily deficient in N. The growth of the flood-plain vegetation does not appear to be restricted by nutrients.

N, P, AND K BUDGETS ALONG NUTRIENT AVAILABILITY AND PRODUCTIVITY GRADIENTS IN WETLANDS

Nutrient enrichment in Western Europe is an important cause of wetland deterioration and the concomitant loss of biodiversity. We quantified nitrogen, phosphorus, and potassium budgets along biomass gradients in wet meadows and fens (44 field sites) to evaluate the importance of various nutrient flows (atmospheric deposition, flooding, groundwater flow, leaching, soil turnover rates) for availability of the growth-limiting nutrient(s). From the nutrient budgets, we assessed N, P, and K availabilities for plants and compared them with N, P, and K in aboveground biomass. Also, potential long-term effects of annual hay harvesting on nutrient limitation were assessed. Comparing N, P, and K availabilities with N, P, and K amounts in the vegetation revealed that (1) the assessed availabilities could explain amounts and variation of nutrients in the vegetation along the biomass gradients, and (2) N was likely the major limiting nutrient along the gradients and P and K could (co)limit growth in some of the sites. Increasing N availabilities along the biomass gradients were caused by increasing N turnover rates in the soil. The contribution of atmospheric N deposition (43 kg N·ha 21 ·yr 21 at all sites) to N availability varied from ;63-76% in low-productivity meadows and fens to 24-42% in highly productive meadows and fens. P and K availabilities along the biomass gradients were primarily influenced by soil processes, as indicated by soil extractable nutrient pools. Flooding could explain 20-30% of K in aboveground higher plants but was less important for P or N availabilities. Nutrient input and output by groundwater flow were more or less negligible for nutrient availability. At low-productivity sites, N output by hay harvesting just accounted for N input from atmospheric deposition, whereas there was net output of P and K. At highly productive sites, there was net output of all three nutrients. Compared to total N, P, and K pools in the top soil, net K output (1-20% of soil K pool) was at many sites much larger than that of P (generally 0.5-3%) or N (0-3%). Hay harvesting particularly seems to create K limitation. Our results indicate that conservation or restoration of low productivity wetlands in Western Europe requires (1) stable site conditions controlling low N, P, and K turnover rates in the soil, and (2) in case of N limitation, annual removal of biomass by harvesting hay, or another management measure to counterbalance the N input from atmospheric deposition. Key words: atmospheric deposition; eutrophication; flooding; groundwater; mineralization; na- ture management; nitrogen; nutrient cycling; phosphorus; potassium; soil nutrient turnover; wetlands.

Nutrients and hydrology indicate the driving mechanisms of peatland surface patterning.

Peatland surface patterning motivates studies that identify underlying structuring mechanisms. Theoretical studies so far suggest that different mechanisms may drive similar types of patterning. The long time span associated with peatland surface pattern formation, however, limits possibilities for empirically testing model predictions by field manipulations. Here, we present a model that describes spatial interactions between vegetation, nutrients, hydrology, and peat. We used this model to study pattern formation as driven by three different mechanisms: peat accumulation, water ponding, and nutrient accumulation. By on‐and‐off switching of each mechanism, we created a full‐factorial design to see how these mechanisms affected surface patterning (pattern of vegetation and peat height) and underlying patterns in nutrients and hydrology. Results revealed that different combinations of structuring mechanisms lead to similar types of peatland surface patterning but contrasting underlying patterns in nutrients and hydrology. These contrasting underlying patterns suggest that the presence or absence of the structuring mechanisms can be identified by relatively simple short‐term field measurements of nutrients and hydrology, meaning that longer‐term field manipulations can be circumvented. Therefore, this study provides promising avenues for future empirical studies on peatland patterning.