Response of dwarf shrubs to neighbour removal and nutrient addition and their influence on community structure in a subalpine heath

1 A 3-year experiment involving nutrient addition and removal of one of two coexisting dwarf shrub species was conducted in two community types in a s ubalpine heathland on the northern Apennines (Italy). Vaccinium uliginosum occurred at all sites but was associated with the deciduous Vaccinium myrti llus at more sheltered nutrient-rich sites (HV community), and with the eve rgreen Empetrum hermaphroditum where the habitat was poorer and more expose d (EV community). Length of current-year shoots and fruit production of eac h species were determined in each of the 3 years, and standing crop at the beginning and end of the experiment. 2 The length of current-year shoots of both deciduous species, but not of t he evergreen, varied considerably between years, presumably due to varying temperatures at the beginning of the three growing seasons. Fruit productio n also varied. 3 Fertilization promoted an increase in the length of V. uliginosum shoots at the HV community but not at the EV community. The removal of V. uliginos um enhanced shoot elongation in V. myrtillus (HV community) but reduced sho ot elongation in E. hermaphroditum (EV community). Neighbour removal did no t affect shoot length of V. uliginosum at either community. There were few treatment effects on fruiting of these clonal species. 4 The standing crop in untreated stands did not change during the experimen tal period. Changes in shoot length resulting from environmental manipulati ons were not accompanied by consistent variation in the standing crop of an y species. Standing crop increased only for V. myrtillus after removing V. uliginosum at the HV community (same direction as shoot length). The standi ng crop of E. hermaphroditum did not change after removing V. uliginosum at the EV community, although shoot length was significantly reduced. The sta nding crop of V. uliginosum was unaffected by neighbour removal and was dec reased by nutrient addition at both communities. 5 None of the shrubs appeared able to utilize more abundant resources to in crease above-ground biomass. However, increased shoot length in nutrient-ri ch habitats gave V. uliginosum a superior ability to capture light compared with V. myrtillus, the latter being more successful when the coexisting co mpetitively superior species was removed. This would support Grime's theory , indicating that competition becomes more important as soil resource level s increase. Positive interactions appeared to play a role in the more extre me habitat, where E. hermaphroditum normally benefited from the shelter of the V. uliginosum canopy but was able to adapt its architecture when expose d.