Dirk Boeye

Nutrient limitation in species‐rich lowland fens

. Nitrogen, phosphorus and potassium were supplied to some Belgian fens of varying nutrient status and productivity. Plant growth in the lowest productive fen with a species-rich Caricion davallianae vegetation was strongly P-limited. N was ineffective when applied alone, but increased the effect of P-addition when applied together. Summer biomass and plant nutrient concentrations were monitored for four years, and showed partial recovery of nutrient limitation. In a more productive fen dominated by Carex lasiocarpa and in a fen meadow, nutrient limitation was less strong. N limited growth in the productive fen, and N and K were co-limiting in the fen meadow. The P-concentration in the productive fen vegetation showed a marked increase after P-fertilization, but it did not result in higher standing crop. The significance of P-limitation for the conservation of species rich low productive fens is discussed. P-limitation may be an essential feature in the conservation of low productive rich fens: because it is less mobile in the landscape than N and/or because it is an intrinsic property of this vegetation type. Plant nutrient concentrations and N:P-ratios may be used as an indication for the presence and type of nutrient limitation in the vegetation. We found N:P-ratios of 23 to 31 for a P-limited site and 8 to 15 in N-limited sites. This was in agreement with critical values from the literature: N:P > ca. 20 for P-limitation and N:P < 14 for N-limitation. Thus, this technique appears valid in the vegetation types that were studied here.

Nutrient limitations in an extant and drained poor fen : implications for restoration

In a species-rich poor fen (Caricetum nigrae) and a species-poor drained fen, the difference in nutrient limitation of the vegetation was assessed in a full-factorial fertilization experiment with N, P and K. The results were compared to the nutrient ratios of plant material and to chemical analysis of the topsoil. A rewetting experiment with intact sods was carried out in the glasshouse and the results are discussed in view of restoration prospects of drained and degraded peatlands. In the undrained poor fen the above-ground biomass yield was N-limited while the vegetation of the drained fen was K-limited. Experimental rewetting of intact turf samples, taken in the drained site, did not change the biomass yield or the type of nutrient limitation. It was concluded that mire systems which have been subjected to prolonged drainage are inclined to pronounced K-deficiency, probably due to washing out of potassium and harvesting the standing crop. This may hamper restoration projects in degraded peat areas where nature conservation tries to restore species-rich vegetation types with a high nature value.

The relation between vegetation and soil chemistry gradients in a ground water discharge fen

. The soil chemistry of a headwater valley fen is influenced by local ground water discharge that supplies base cations and alkalinity to the fen. An irrigation canal just upward of the fen is the source of this alkalinity. The ecological consequences of this artificial system are studied both on the soil and vegetation level. Rich-fen species of the alliance Caricion davallianae are connected to soil water alkalinity and soil base status. They depend directly on the alkaline ground water discharge. In addition, the local input of this water causes a gradient-rich pattern from poor to rich fen, and it is therefore concluded that it is responsible for the presence of intermediate fen vegetation too. High nutrient levels in the irrigation water have not influenced the fen until now. This case study illustrates the possibility for rich fen restoration after acidification. Irrigation with alkaline water is efficient if excess nutrients can be removed.

Experimental investigation of drought induced acidification in a rich fen soil

Intact rich fen soil cores with controlled water levels near thesurface were installed in an open greenhouse. To simulate short termsummer drought, water levels were lowered (20 cm) after two weeks inhalf of the cores (experimental cores) and remained near the surface inthe other half (blanks). After two more weeks, the water levels werebrought back to the surface in the experimental cores and remainedthere for another two weeks. In the blanks, reduction and alkalinizationof the top peat layer occurred. In the experimental soil cores oxidationand acidification started within one week after drawdown. An indicationfor a drought induced rise in soluble reactive phosphorus has beenfound. The velocity of the acidification process illustrates the dynamicnature of the hydrochemical conditions in fen soils during drought. Theprocesses controlling the acid/base status of rich fen, the effect ofdrought induced acidification on P availability and the significance forthe vegetation are discussed.

Phosphorus fertilization in a phosphorus‐limited fen: effects of timing

Phosphorus availability in low P-soils is primarily controlled by soil processes that are subject to seasonal fluc- tuation. There is evidence that summer drought causing low fen-water levels induces temporal high P-availability. We investigate here whether and how fen vegetation responds to P-pulses at different times in the season. Plots were fertilized with equal P-doses at three different times in the season. Four conclusions can be drawn from the experiment: 1. The vegetation is able to increase P-uptake independent of the timing of fertilizer application; 2. Early-season fertilization stimulates growth and increases P-concentration in above- ground tissue while late-season fertilization does not stimu- late growth but strongly increases above-ground tissue con- centration; 3. Timing effects cause differential species re- sponses: the response of Carex demissa, a stress-tolerant species, is truly independent of timing, while Juncus articulatus, a CSR-type species (sensu Grime) profits more from early- season fertilization; 4. Timing effects persist over several years. The differences between the experimental treatment and the events it aimed to simulate are discussed. We expect that higher frequencies of drought events, that may be induced by climate change, will cause a shift from nutrient-stress tolerant towards stress-tolerant-competitor fen species.

Hydrochemical variation in a ground-water discharge fen

In a ground-water discharge fen that supports species-rich herbaceous vegetation, the chemistry of the shallow ground water was monitored over two distinct periods. The fen is situated on a thick sandy aquifer in a region that is known for its acid soils. An irrigation ditch supplies the aquifer with alkaline water just upstream of the fen. Three water types of different origin contribute to the hydrochemical pattern in the fen. Recently infiltrated acid ground water is present on higher grounds, neutral-to-alkaline irrigation water discharges at the upper fen margin, and deep ground water that is rich in iron discharges in the lower parts. The soil chemistry and species distribution are in accordance with this pattern: rich fen at the upper margin and poor fen in lower parts. However, we detected considerable hydrochemical variability due to varying influence of water types and temporal acidification of the shallow ground water. The latter leads to anionic dominance shifts from HCO3 to SO42− and the release of base cations in the soil solution. This hydrochemical variability is an additional source of environmental heterogeneity and may thus be partly responsible for the species richness.

A local rich fen fed by calcareous seepage from an artificial river water infiltration system

The habitat of low growing, herbaceous rich-fen vegetation (Caricion davallianae) is now threatened throughout Western Europe. Its conservation, restoration and even recreation receives increasing attention. In the Campine area of Belgium a rich fen, that depends entirely on an artificial river water infiltration system for its alkaline discharge, is present. The rich fen vegetation cannot be older then a century, i.e. the age of the infiltration system. This illustrates that poor-fen precursors can be transformed into rich fen by alkaline ground-water discharge. The possibilities for restoration and the difficulties with use of allochtonous water for nature conservation are discussed. Finally, the hydrochemical consequences of the alkaline discharge in the fen soil are considered: in dry summers periodical acidification of the peat soils occurs.

The Marshy heathland of's Gravendel (Belgium) : trophic gradients in relation to the vegetation, with special reference to Littorellion communities

Abstract s Gravendel is a marshy heathland of high botanical interest. Its amphiphytic communities of the Eleocharitetum multicaulis association (Littorelletea) contain almost all typical taxa, nowadays a rare situation. In this paper the hydrology of the nature reserve is studied. The combined effect of the complex microtopography and historical flooding results in some interesting trophic gradients which provide the basis for the species richness of the reserve. Intensive agriculture in the immediate surroundings of the reserve has as yet apparently had no negative effects. Problems may, however, occur in the future.