Root–shoot competition interactions cause diversity loss after fertilization:a field experiment in an alpine meadow on the Tibetan Plateau

Aims A decrease in species diversity after fertilization is a common phenomenon in grasslands;however,the mechanism causing it remains highly controversial.The light competition hypothesis to explain loss of diversity has received much attention.The aim of the present paper was to test this hypothesis.Methods Fertilization was used to control above-and belowground resources simultaneously,while shade was used to control aboveground resource in an alpine meadow on the Tibetan Plateau.Univariate general linear models was used to estimate the effects of fertilization and shade on above-and belowground vegetation characteristics,including photosynthetically active radiation(PAR)in the understory,aboveground biomass,belowground biomass,R:S ratio,species richness and Simpson’s diversity index.Important findings PAR was similar in the understory of shaded and fertilized plots,but only fertilization reduced species richness and diversity,suggesting that light competition alone could not explain diversity loss after fertilization.The root biomass and R:S ratio had a significant increase in shaded plots,but the richness and diversity did not change,suggesting that root competition alone also could not explain diversity loss after fertilization in this community.Our results illustrated that the root–shoot competition interactions,investigated from a functional groups perspective,should be the most reasonable explanation leading to the diversity loss due to fertilization.

Density Effects on Plant Height Growth and Inequality in Sunflower Populations

Comparisons between competing and non-competing sunflower （Helianthus annuus L.） populations demonstrate pronounced effects of density on plant height growth, height-to-crown width ratio, and s popuiaUon＇s height inequality. In the present study, non-destructive measurements of height and the prolected crown area of sunflower plants were taken at seven times from emergence to fruit maturation in even-aged monospeclflc stands with initial densities of 1, 4, 16, and 64 plants/m^2. The mean height of populations Increased and then decreased with increasing population density; the height Inequalities of uncrowded populations decreased during stand growth, whereas the height inequaiiUes of crowded popuisUons decreased first and then increased during stand development. The interindlvidual relationships between the relative height growth rate and height within uncrowded populations became significantly negative during population growth, whereas these relationships were negative first and then became positive during the development of crowded populations. In the uncrowded populations, the static Interindlvldual relationship between height-to-crown width ratio and volume was positive, whereas for the crowded population these relationships became negative with increasing competition for light. The data suggest that the plastic responses of plant height and height-to-crown width ratio to light competition will become more Intense with increasing competition Intensity. The results of the present study argue strongly for the Importance of size-dependent Individual-level plastic responses due to size-asymmetric light competition In generating the variations in population height inequality.

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.