Taxonomic and phylogenetic composition show biotic resistance to exotic invasion in acid seep springs

Background:Few studies have incorporated the evolutionary insights provided by analysis of phylogenetic structure along with community composition to assess the effects of exotic invasion on freshwater wetlands.Here,we assess the taxonomic and phylogenetic relationships among acid seep springs to investigate the potential homogenization or resistance of communities due to invasion of an exotic grass.Results:Multivariate community analyses indicated differences in community and phylogenetic composition and dispersion among acid seep springs,associated with gradients in soil moisture,canopy cover,and phylogenetic diversity.By contrast,univariate analyses showed differences in taxonomic diversity but not phylogenetic diversity among acid seep springs.Conclusions:Despite exotic invasion,individual acid seep springs remained taxonomically and phylogenetically distinct from each other.Taxonomic and phylogenetic diversity metrics revealed different aspects of composition,reinforcing the importance of including both in analyses of plant communities for understanding community assembly following exotic invasion and for management purposes.Within acid seep springs,taxonomic and phylogenetic composition appear to be driven more through environmental filtering by light and moisture than by the competitive effects following invasion of an exotic grass in support of Elton’s biotic resistance hypothesis.

Mowing and fertilizer effects on seedling establishment in a successional old field

Aims We used a 10-year field experiment that consisted of mowing and fertilizer treatments to evaluate the role of niche limitation in seedling establishment of species from different functional groups and of varying local abundance in an old field undergoing succession.Methods Seedlings of nine different species were planted into a successional field subjected to mowing and fertilizer treatments for 10 years that resulted in different plant communities and resource availability.Species representative of the factorial combination of three functional groups(C4 grasses,C3 grasses and legumes)and three abundance categories(abundant,present,or absent in the old field)were planted in four treatments resulting from the factorial combination of annual spring mowing(mowed and unmowed)and fertilizer application(annually fertilized and unfertilized).Survivorship,relative growth rate(RGR)and biomass were measured to determine the role of niche limitation on recruitment and growth.Important Findings Mowing increased the establishment success of seedlings.Fertilization had little influence on seedling performance and survivorship.C3 grasses had the highest survivorship,while C4 grasses and legumes had equivalent RGRs,but higher than C3 grasses.By contrast,survivorship of legumes was unrelated to mowing or fertilizer,suggesting that establishment of this functional group was dependent on other,unmeasured conditions or processes.Species already present,but at low abundance,performed better than locally abundant or absent species.Propagule limitation may restrict the arrival of a species.However,recruitment and establishment was subject to niche limitation,which varied among species,functional groups and whether a species is already resident at the site and its abundance.Thus,species interactions restrict establishment during old-field succession,supporting the niche limitation hypothesis.

How reproductive allocation and flowering probability of individuals in plant populations are affected by position in stand size hierarchy,plant size and CO_(2) regime

Aims We investigate the effect of position within a size-structured population on the reproductive allocation(RA)and flowering probability of individual plants of Sinapis arvensis.We also assess the effects of plant size and changing level of CO_(2) on both responses.Methods Sinapis arvensis L.,(field mustard),an annual agricultural weed,was grown in monoculture at six densities under ambient and elevated CO_(2) in a study with 84 stands.Individual aboveground biomass and reproductive biomass were measured.Varying density produced a wide range of mean plant sizes across stands and size hierarchies within stands.Many(;40%)individuals had zero reproductive biomass.Employing a novel modelling approach,we analysed the joint effects of position in stand size hierarchy,plant size and CO_(2) on RA and flowering probability of individuals.Important Findings We found a strong effect of position within the size hierarchy of individuals in a population:for an individual of a given size,greater size relative to neighbours substantially increased RA and flowering probability at a single harvest time.There was no other effect of plant size on RA.We found a positive effect of elevated CO_(2) on RA regardless of position within the size hierarchy.These observed patterns could impact doubly on the reproductive biomass(R)of small individuals.First,because RA is not affected by size,smaller plants will have smaller R than larger plants;and second,for smaller plants lower down in a population size hierarchy,their RA and hence R will be further reduced.These results suggest that size relative to neighbours may be independent of and more important than direct abiotic effects in determining RA.Further studies are required to evaluate how these observed patterns generalize to other populations in non-experimental conditions.