Native upland species, Nardus stricta, Eriophorum
vaginatum,
Erica cinerea and Vaccinium vitis-idaea were given
3 or 60 kg N ha−1 yr−1, over 2 yr,
applied as a mist (NH4NO3). The high N treatment
increased above-ground
biomass in all four species, but only significantly in E. cinerea,
E. vaginatum and N. stricta. Biomass increases in
E. vaginatum and N. stricta resulted from enhanced tiller
production rather than shoot elongation. Root growth
increased in N. stricta, so that root[ratio ]shoot ratio
in this species was unchanged by N. Root growth in E. vaginatum,
E. cinerea and V. vitis-idaea did not respond to N and
their root[ratio ]shoot ratios decreased. Tissue N concentrations
increased in both shoots and roots of all species in response to N. The
accumulated foliar N did not increase the
proportion of N allocated to Rubisco and the photosynthetic capacities
of
N. stricta, E. vaginatum and V. vitis-idaea
were unchanged. Thus growth responses to N were due to altered allocation
rather than increased rate of
photosynthesis per unit leaf area. The high N treatment increased flower
production significantly in E. cinerea but
not in the other species. Although in this experiment dwarf shrubs were
more responsive than graminoids to N,
in the field at current N inputs the enhanced tillering of the graminoids
may be more competitively advantageous,
especially where gaps develop in the canopy. Thus increasing N deposition
may lead to increased grassiness of
upland heath, and in particular, a spread of N. stricta.