Increased community compositional dissimilarity alleviates species loss following nutrient enrichment at large spatial scales

Aims Anthropogenic activities have drastically increased nutrient availabil-ity,resulting in declines in species richness in many plant communi-ties.However,most previous studies have explored only species-loss patterns and mechanisms over small sampling areas,so their results might overestimate species loss at larger spatial scales.The aim of this research was to explore species diversity change patterns and species-loss rates at multiple scales in alpine meadow communities following nutrient enrichment.Specifically,we asked two closely related ques-tions:(i)do changes in species diversity and species-loss patterns differ among spatial scales?and(ii)how does community compositional dissimilarity and species turnover change among spatial scale?Methods This study was implemented in an alpine meadow community,which is regarded as one of the most sensitive and vulnerable ter-restrial ecosystems to anthropogenic nutrient enrichment.We conducted a fertilization experiment that involved the addition of nitrogen(N),phosphorus(P)and a mixture of both to a series of quadrats ranging from 1 to 16 m2 over 5 years to study the variations in the patterns of species diversity in response to nutrient additions at different spatial scales.Important Findings Our results showed that the changes in species diversity and species loss were dependent on the type of fertilization and the spatial scale.After N and NP fertilization,species diversity significantly decreased at the small scale but not at the large scale,and the rate of species loss decreased as the spatial scale increased.In contrast,the differences between the P addition and control communities were negligible at both the small and large spatial scales.N fertilization caused species to be lost from the small sampling scale,but because different species were lost from dif-ferent samples,there was an increase in compositional dissimilarity at larger spatial scales,which reduced the total number of species lost when measured at larger scales.These findings highlight spatial scale in evalu-ating the biodiversity loss after fertilization and suggest that the compo-sitional dissimilarity might play an important role in mediating species loss after fertilization.Our study significantly improved our understanding of changes in species diversity and species loss at different spatial scales under nutrient-enrichment scenarios.

Species richness loss after nutrient addition as affected by N:C ratios and phytohormone GA_(3)contents in an alpine meadow community

Aims From the light-competition hypothesis,competition for light is asymmetric and the observed increases in plant-size variability with increasingly denser canopies are primarily due to competition for light.Greater plant height provides pre-emptive access to light and produces increased height differences among species.The question is what produces these differences in plant height or height growth response among species in response to fertilization.Methods In 2009,a field experiment of N,P and N+P enrichments at three levels each was initiated in an alpine meadow on the northeast Qinghai-Tibet Plateau.Effects of fertilization on species richness,aboveground net primary production(ANPP),relative light intensity and plant height of different plant functional groups were determined.Festuca ovina(grass),Kobresia humilis(sedge),Oxytropis ochrocephala(legume),Taraxacum lugubre(rosette forb)and Geranium pylzowianum(upright forb)were selected as exemplars of each of the indicated functional groups.The N:C ratios in aboveground biomass,gibberellic acid(GA_(3))concentrations in leaves,plant heights and height relative growth rate(RGR)of these exemplar species were analyzed in detail.Important Findings Species richness of grasses significantly increased with increasing N+P levels.Species richness of legumes and upright forbs decreased after N and N+P additions.P addition had no significant effect on species richness.The effects of N+P addition on species richness and ANPP were consistently stronger than those of the single N or P fertilization.Reductions in species richness caused by nutrient addition paralleled the increases in ANPP and decreases in light intensity under the canopies,indicating indirect effect of nutrient addition on species richness via ANPP-induced light competition.The exemplar species that responded most positively to fertilization in height and RGR also displayed stronger increases in their GA_(3) content and N:C ratios.GA_(3) concentrations and N:C ratios were positively correlated with height RGR when the data were pooled for all species.The tallest and the fastest-growing grass,F.ovina,had the largest increase in N:C ratios and the highest leaf GA_(3) concentrations after nutrient addition.These results indicated that differential responses of GA_(3) concentrations and N:C ratios to fertilization were related to the inequality in plant heights among species.