and Carson 1999). The functional-based hypothesis stated that species loss following N enrichment would be a function of specific functional traits having competition advantageous over other traits under N-enriched environment (Suding et al.
2005). For instance, N enrichment would favour species with higher growth rate or
taller stature that reduce light availability in the understory and thus lead to extinction of other species (Hautier et al. 2009).
For the biotic mechanism underlying diversity loss after N enrichment, plant
competition is the primary one (Tilman 1988). From a function-based perspective,
Pan et al. (2011) tested three different competition hypotheses (aboveground competition, belowground competition, and total competition) in driving the changes of
species richness in response to N enrichment using the first 2-year data from a N
addition experiment located at a temperate typical steppe. They investigated both
aboveground and belowground responses of plants at three different biological
organization levels, including individual level, species level, and community level.
Their results showed that plants differed significantly in their response to N addition
across all the three organization levels. The species loss at community level following N enrichment was mainly due to the losses of perennial grasses and forbs. The
relative abundance of bunchgrasses, Stipa grandis, Cleistogenes squarrosa, and
Agropyron cristatum, and that of the sedge, Carex korshinskyi, all decreased with
the increases of N addition rates. In contrast, two annuals (Axyris amaranthoides and
Chenopodium glaucum) increased their relative abundance in response to the
increasing N addition rates. They found no change of the relative abundance of
Leymus chinensis after 2-year N addition, maybe due to the short duration of their
experiment. Based on the results that the species relative abundance was not
significantly correlated with individual biomass but positively correlated with
aboveground allocation and negatively correlated with belowground allocation,
they concluded that increased aboveground competition was the key factor responsible for the plant species loss following N enrichment in the typical steppe.
In contrast, results from an N addition experiment in the alpine steppe from the
eastern Tibetan Plateau support the total competition hypothesis (Niu et al. 2008).
With a 2-year fertilization experiment, they explored the relationships between
individual and population responses. Nitrogen addition increased the individual
biomass of most grasses but had no significant impacts on that of forb species. At
plant functional group level, the responses of grass to N addition were stronger than
that of other groups though all three plant functional groups showed positive
responses to N fertilization in both years. The variation of species relative abundance
was positively correlated with changes of individual biomass response. Most species
enhanced their biomass allocation to leaf and decreased the allocation to stem even
to reproductive organs. Those results indicated that total competitive intensity was
increased under N enrichment. Combining the results of Niu et al. (2008) and Pan
et al. (2011), we concluded that biomass allocation strategy plays an important role
in driving plant species loss after N enrichment. The divergence between the findings
of Pan et al. (2011) and Niu et al. (2008) implies that the mechanisms of species loss
after N addition would be ecosystem-specific, given the substantial variations of
climate, soil, and biotic conditions across different grassland ecosystems in China.
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X. Lü et al.
2005). For instance, N enrichment would favour species with higher growth rate or
taller stature that reduce light availability in the understory and thus lead to extinction of other species (Hautier et al. 2009).
For the biotic mechanism underlying diversity loss after N enrichment, plant
competition is the primary one (Tilman 1988). From a function-based perspective,
Pan et al. (2011) tested three different competition hypotheses (aboveground competition, belowground competition, and total competition) in driving the changes of
species richness in response to N enrichment using the first 2-year data from a N
addition experiment located at a temperate typical steppe. They investigated both
aboveground and belowground responses of plants at three different biological
organization levels, including individual level, species level, and community level.
Their results showed that plants differed significantly in their response to N addition
across all the three organization levels. The species loss at community level following N enrichment was mainly due to the losses of perennial grasses and forbs. The
relative abundance of bunchgrasses, Stipa grandis, Cleistogenes squarrosa, and
Agropyron cristatum, and that of the sedge, Carex korshinskyi, all decreased with
the increases of N addition rates. In contrast, two annuals (Axyris amaranthoides and
Chenopodium glaucum) increased their relative abundance in response to the
increasing N addition rates. They found no change of the relative abundance of
Leymus chinensis after 2-year N addition, maybe due to the short duration of their
experiment. Based on the results that the species relative abundance was not
significantly correlated with individual biomass but positively correlated with
aboveground allocation and negatively correlated with belowground allocation,
they concluded that increased aboveground competition was the key factor responsible for the plant species loss following N enrichment in the typical steppe.
In contrast, results from an N addition experiment in the alpine steppe from the
eastern Tibetan Plateau support the total competition hypothesis (Niu et al. 2008).
With a 2-year fertilization experiment, they explored the relationships between
individual and population responses. Nitrogen addition increased the individual
biomass of most grasses but had no significant impacts on that of forb species. At
plant functional group level, the responses of grass to N addition were stronger than
that of other groups though all three plant functional groups showed positive
responses to N fertilization in both years. The variation of species relative abundance
was positively correlated with changes of individual biomass response. Most species
enhanced their biomass allocation to leaf and decreased the allocation to stem even
to reproductive organs. Those results indicated that total competitive intensity was
increased under N enrichment. Combining the results of Niu et al. (2008) and Pan
et al. (2011), we concluded that biomass allocation strategy plays an important role
in driving plant species loss after N enrichment. The divergence between the findings
of Pan et al. (2011) and Niu et al. (2008) implies that the mechanisms of species loss
after N addition would be ecosystem-specific, given the substantial variations of
climate, soil, and biotic conditions across different grassland ecosystems in China.
230
X. Lü et al.