Effects of soil substrate heterogeneity and moisture on interspecific competition between Alternanthera philoxeroides and four native species

Aims Clonal plant species have the potential for high relative performance in heterogeneous environments,and this might increase the com-petitive ability and invasiveness of introduced clonal plant species.It was hypothesized that clonal species whose performance responds more to heterogeneity of a resource have higher competitive ability in habitats where this resource is more heterogeneous and that this relationship is stronger when other resources are less limiting.Methods To test these hypotheses,the perennial clonal herb Alternanthera philoxeroides,which is invasive in China,was grown alone or with each of four clonal perennial,co-occurring herbs native to China,i.e.Alternanthera sessilis,Cynodon dactylon,Hemarthria altissima and Wedelia chinensis.Plants were given homogeneous or het-erogeneous soil substrate crossed with low and high levels of soil moisture.Important Findings Effects of heterogeneity on the accumulation of mass and ramets and on competitive effect and response of A.philoxeroides differed between native species and interacted with effects of soil moisture.A.philoxeroides reduced the final total mass or ramet number of the native species except A.sessilis,and the negative competitive effects on H.altissima and C.dactylon were more pronounced in heterogeneous than in homogeneous soil.Competitive response of A.philoxeroides was more negative to A.sessilis than to the other native species.Across native species,the competitive response of A.philoxeroides was more negative in heterogeneous than in homo-geneous soils at low moisture level,but the reverse was true at high moisture level.Results do not consistently support either hypoth-esis,but do suggest that competitive ability can be partly explained by individual species traits such as size,and that some competi-tive effects and responses are emergent properties of interspecific interactions.

Do endemic species always have a low competitive ability? A test for two Mediterranean plant species under controlled conditions

Aims Many observations concerning biological and ecological differentiation between narrow endemic and widespread congeneric plant species suggest that narrow endemic species are constrained to colonize marginal habitats because of a low tolerance to competition.Despite this topic being an important issue both for understanding evolutionary processes leading to endemism and for conservation purposes,few studies have been performed to compare competitive abilities between endemic and widespread species.Here,we present two independent experiments performed under controlled conditions using two different pairs of endemic and widespread congeneric species:Centaurea corymbosa/Centaurea maculosa and Arenaria provincialis/Arenaria serpyllifolia,both endemic species occurring in rocky calcareous habitats.Methods Mature seeds of C.corymbosa and C.maculosa were sown in pots containing ramets of the common grass,Brachypodium retusum.Pots were sorted in three treatments according to grass cover(low,intermediate and high).A control treatment(without competition)was also used.Germination,seedling survival and rosette growth were followed.For the comparisons between A.provincialis and A.serpyllifolia,seeds from natural populations were first sown without a competitor.One week after germination,healthy seedlings were transplanted in pots without Brachypodium seedling(control)or containing two Brachypodium seedlings(low competition)or four seedlings(high competition).We checked the number of capsules per individual,and we harvested the biomass after capsule maturation.Important Findings Despite differences in the protocol design,results are congruent,and in both cases,endemic species are highly affected by the presence of a competitor,as are the widespread species,although we did not detect any differences between species for response to competition.The results are discussed in relation to processes leading to endemism,suggesting that the specialist model is more likely for both the study species.The present study also contributes to guidelines for the conservation of rare species in relation to landscape modification in the Mediterranean area.