ABOVEGROUND CROP MASS AND SPECIES RICHNESS OF THE PRINCIPAL TYPES OF HERBACEOUS RICH-FEN VEGETATION OF LOWLAND ENGLAND AND WALES

(1) Eighty-six examples of rich-fen vegetation, representing the main types of herbaceous rich-fen plant communities of lowland England and Wales, were sampled to determine above-ground crop-mass components. (2) Above-ground standing crop (living material) (SC) and litter (dead material) was estimated by harvesting clipped material near the start (April) and end (September) of the main growing season and the increment of above-ground standing crop ('production') was calculated. (3) September standing crop and litter-mass values ranged from 80 to 2918 g m-2 and 37 to 2115 g m-2, respectively. The April-September net standing crop increment ranged from 58 to 2799 g m-2. Communities of highest production were almost always deciduous and had very small April standing crop mass. Conversely most low productivity stands had proportionately large April SC mass, reflecting their winter-green character. (4) Stands with greatest September standing crop usually had small proportions of September litter, probably because of rapid decomposition. An exception to this was some stands dominated by Cladium mariscus where large September standing crop and litter mass was combined. Some of these stands had the greatest total crop mass (maximum: 5011 g m-2). (5) All crop variables except April SC showed a significant negative relationship with species density (number of spp. 4 m-2) in regressions (log(c)-transformed variables). April-September SC increment (36%) and September SC (32%) accounted for most variation in species density in single regressions. A stepwise multiple-regression model included only the SC increment term. The poorer fit of September SC mass probably occurred because, unlike SC increment, some samples of high September SC were relatively species rich, partly a legacy of former, or present rotational, management. (6) When 'rare fen species score' (number of rare species 4 m-2 was the dependent variable, similar relationships were found but the crop mass variables generally accounted for rather less of its variation. The slopes of the regression equations were significantly (P < 0.05) steeper than those using species density, and samples with large crop mass had particularly few, if any, rare species. (7) Maintenance of low-growing swards of herbaceous fen vegetation is particularly important for the conservation of many rare fen species.