Inter- and intraspecific variability of plant individual growth and its role on species ranking in grasslands

Aims Individual growth constitutes a major component of individual fitness.However,measuring growth rates of herbaceous plants non-destructively at the individual level is notoriously difficult.This study,based on an accurate non-destructive method of aboveground biomass estimation,aims to assess individual relative growth rates(RGRs)of some species,identify its environmental drivers and test its consequences on community patterning.We specifically address three questions:(i)to what extent environmental conditions explain differences in individual plant growth between sites,(ii)what is the magnitude of intraspecific variability of plant individual growth within and between sites and(iii)do species-averaged(dis-)advantage of individual growth compared with the whole vegetation within a site correlate with species ranking at the community level?Methods We monitored the growth of individuals of four common perennial species in 18 permanent grasslands chosen along a large pedoclimatic gradient located in the Massif Central,France.We measured soil properties,levels of resources and meteorological parameters to characterize environmental conditions at the site level.This design enables us to assess the influence of environmental conditions on individual growth and the relative extent of inter-individual variability of growth explained within and between sites.We determined the ranking of each of the four species in each site with botanical surveys to assess the relationship between species-averaged growth(dis-)advantage relative to the whole community and species rank in the community.Important Findings We found that environmental conditions explain a significant proportion of individual growth variability,and that this proportion is strongly variable between species.Light availability was the main driver of plant growth,followed by rainfall amount and potential evapotranspiration,while soil properties had only a slight effect.We further highlighted a moderate to high within-site inter-individual variability of growth.We finally showed that there was no correlation between species ranking and species-averaged individual growth.

On the Simpson index for the Wright–Fisher process with random selection and immigration

Moran or Wright–Fisher processes are probably the most well known models to study the evolution of a population under environmental various effects.Our object of study will be the Simpson index which measures the level of diversity of the population,one of the key parameters for ecologists who study for example,forest dynamics.Following ecological motivations,we will consider,here,the case,where there are various species with fitness and immigration parameters being random processes(and thus time evolving).The Simpson index is difficult to evaluate when the population is large,except in the neutral(no selection)case,because it has no closed formula.Our approach relies on the large population limit in the“weak”selection case,and thus to give a procedure which enables us to approximate,with controlled rate,the expectation of the Simpson index at fixed time.We will also study the long time behavior(invariant measure and convergence speed towards equilibrium)of the Wright–Fisher process in a simplified setting,allowing us to get a full picture for the approximation of the expectation of the Simpson index.