Bl. Foster et Kl. Gross, Species richness in a successional grassland: Effects of nitrogen enrichment and plant litter, ECOLOGY, 79(8), 1998, pp. 2593-2602
We conducted a field experiment in a successional grassland to investigate
the short-term effects of nitrogen enrichment and plant litter on plant spe
cies richness and on the establishment of a native grass (Andropogon gerard
i) that was experimentally introduced to the study site as seed. Additions
of nitrogen fertilizer to experimental plots over two growing seasons incre
ased plant production as indicated by increases in both living plant biomas
s and litter biomass. Increased productivity reduced species richness by ef
fectively preventing the seedling establishment of the subordinate forb spe
cies and reduced the recruitment of Andropogon gerardi by inhibiting both g
ermination and survival. Litter removals carried out at each of two levels
of nitrogen enrichment (no nitrogen, nitrogen added) showed that litter sig
nificantly reduced species richness by the same amount in fertilized and un
fertilized plots, suggesting that living biomass and litter were purely add
itive, rather than interactive in their effects. In contrast, the results o
f litter additions suggested that the declines in richness associated with
fertilization could largely be due to the suppressive effects of increased
litter biomass alone. As a whole, the results indicate that litter and livi
ng biomass are largely substitutable in their inhibitory effects on species
richness in highly productive successional grasslands due to their indepen
dent and equivalent capacities to attenuate light to very low levels. This
study highlights the combined roles of competition and plant litter in infl
uencing the diversity of grasslands through effects on seedling establishme
nt.