Vegetation Patterns in Spring-Fed Calcareous Fens: Calcite Precipitation and Constraints on Fertility

Publisher Website
Google Scholar
Abstract
(1) Some spring-fed calcareous fens in Britain [UK] have a distinct vegetation pattern, with stands of low-growing species-rich vegetation (= short-fen) abruptly joined by taller, less rich vegetation (+ taller-fen). The short-fen vegetation occupies the main seepage tracks, whereas the taller-fen community, though still irrigated by water seepage, flanks the most active seepage zones. (2) Four small spring-fed fens showing this distinct vegetation pattern were studied. At each site the short- and taller-fen zones were close together and separated by a sharp boundary. In all sites examined, both zones were on similar substrata, were irrigated from the same ''point-source'' of groundwater and had a similarly high (just subsurface) water level. Paired samples of short- and taller-fen were examined at each site. (3) Above-ground crop mass and net productivity were much smaller in short-fen than taller-fen zones. Juncus subnodulsus occurred in both short- and taller-fen zones at most sites, but its shoot mass was much less in the short-fen. Concentration of P in living leaves was also lower in shoots from short-fen zones, whilst concentrations of Ca, Mg, K and N were similar to, or higher than those in taller-fen living shoots of J. subnodulosus. (4) Growth (in controlled conditions) of two phytometer species (Epilobium hirsutum and J. subnodulosus) was less on short-fen than taller-fen substrata. This reduced growth was relieved by addition of P to short-fen substrata. (5) Within sample pairs, water pH values were significantly greater in short-fen than taller-fen, and dissolved concentrations of Ca2+, HCO3- and soluble reactive phosphorus (SRP) were significantly smaller. At one site the NO3- concentration was much greater in the short-fen sample than taller fen. (6) At rooting level there was less organic material and more calcite (X-ray diffractometry) in the short-fen substratum than in taller fen. Water in short-fen was on average slightly supersaturated with CaCO3 but saturation (SI values: -0.1 to 0.5) was well below levels (SI > 1.4) reported for heterogeneous nucleation, and calcite precipitation may be biogenic. SI values of taller-fen water samples were rather similar to short-fen values but generally slightly lower. (7) Low P concentration appears to limit fertility and productivity in the short-fen. Coprecipitation with calcite may immobilize P. (8) Mean values of partial pressure of CO2 calculated for water samples were significantly greater in taller-fen than short-fen. This may be caused by regassing of seepage water with CO2 as it percolates through the short-fen and may account for failure of calcite precipitation in taller-fen. (9) As P limits short-fen fertility, high NO3- concentrations in groundwater discharge do not increase fertility. The conservationally valuable short-fen vegetation may therefore relate to intrinsic geochemical processes rather than varying levels of agricultural eutrophication, though the ultimate capacity of the system to immobilize P is not known.