Fine-scale variables associated with the presence of native forbs in natural temperate grassland

Broad-scale threats to floristic diversity in native temperate grasslands are well-documented and include elevated soil nutrients, changes in disturbance regimes and exotic species. However, fine-scale variables associated with the presence of native forbs, such as gap size and biomass cover, have received relatively little attention. We conducted a case–control study to determine the relative influence of physical structural dimensions and other fine-scale variables associated with the presence of native forbs in a modified temperate grassland previously used for domestic grazing. We matched 145 case plots centred on 27 different species of native forbs with 290 control plots not centred on a native forb. For each percentage increase in ground litter cover, dead biomass cover, grass cover or exotic forb cover, or the area of bare ground within 30 cm, the relative odds that a native forb was present vs absent declined by a mean of 10–13%. Living and dead biomass reduces light availability, and the former can also reduce nutrient and water availability. Declines in the presence of native forbs associated with increasing total bare ground may suggest that gap sizes were too small or the soil surface condition too degraded. Our results add to a body of evidence suggesting that native forbs in temperate native grassland are likely to benefit from periodic removal of living and dead grass biomass and a reduction in the cover of exotic forbs.