Effects of the allelopathic plant Artemisia herba-alba Asso on the soil seed bank of a semi-arid plant community

Aims Seed bank strongly influences composition and structure of above-ground vegetation.Little attention has been paid to the role of allel-opathy driving soil seed bank dynamics,even though allelochemicals released by allelopathic plants might determine whether a species can or cannot germinate from the soil seed bank and,therefore,to co-exist in the neighborhood of the allelopathic species.Hence,the effects of allelopathic plants on community organization through the effects of allelochemicals on soil seed bank remain largely unknown.In this study,we assessed spatial heterogeneities in soil seed bank caused by the presence of allelopathic plants,and evaluated allelo-pathic effects on seed bank germination and seedling mortality.Methods We examined the effects of the allelopathic shrub Artemisia herba-alba Asso on the spatial structure(in terms of species richness and seed density)of the soil seed bank of a semi-arid ecosystem in NE Spain.Specifically,we evaluated seed bank richness and density at three microsites:under the canopy of A.herba-alba individuals,under the canopy of Salsola vermiculata L.(a non-allelopathic shrub)individuals,and in bare soil.In addition,we assessed the effects of aqueous extract of A.herba-alba on soil seed bank germination(seed-ling emergence and emergence timing)and seedling mortality.Important Findings We found that seed bank richness and density were higher under shrub canopy than they were in bare soil.A.herba-alba and S.vermiculata microsites had similar seed bank richness.However,seed bank density was higher under A.herba-alba because of the high abundance of A.herba-alba seeds.Aqueous extract reduced seedling emergence from the seed bank by 50%,and affected mortality of some emerged seedlings.On the other hand,aqueous extract did not influence the emergence timing of seedlings.We conclude that,although the presence of allelo-pathic plants does not cause relevant changes in seed bank struc-ture relative to similar non-allelopathic shrubs,their presence can actually result in a reduction of seedling emergence from the seed bank,which leads to low plant species richness and density nearby.To our knowledge,this is the first study to examine the effect of an allelopathic plant on the structure and germination in an entire soil seed bank.

Shrub interactions drive vegetation succession of subalpine grasslands under two climatic conditions

Aims The abandonment of agronomic practices in subalpine systems has led to shrub encroachment,which has transformed grasslands into woodlands.The competitive and facilitative interactions among incoming shrub species might influence vegetation succession and might be affected by climate change.This study aimed to identify the nature of shrub species interactions between an early colonizer and a secondary succession shrub in the successional encroach-ment process and whether it differs between two contrasting cli-matic conditions.Methods The interactions between the two main encroaching shrub spe-cies in the Central Pyrenees(Spain),Buxus sempervirens and Echinospartum horridum,were studied at the current upper limit of the altitudinal range of B.sempervirens,where it coexists with E.horridum(high site),and in a milder location at lower eleva-tion(low site).in the studied areas,E.horridum is the first shrub species that encroaches upon the abandoned grasslands and B.sempervirens enters the system after E.horridum has become established.A neighbor-removal experiment was used to assess the bidirectional interactions between both species,and whether those interactions differed among the life stages of B.sempervirens(seed-lings,juveniles and adults),at the two study sites.Survival,annual crown growth and developmental instability(error in development caused by stress)were the performance indices used to quantify plant responses.Differences in air temperatures and soil nutrient amelioration were evaluated as possible mechanisms of facilitation of E.horridum on B.sempervirens.Important Findings E.horridum enhanced the survival of B.sempervirens seedlings under its crown,whereas,B.sempervirens adults outcompeted E.horridum.Therefore,the invasion of the grasslands by E.horridum facilitated the settlement and expansion of B.sempervirens,pro-moting vegetation succession at the Pyrenees.in addition,climatic differences associated with elevation affected shrub interactions.Under low temperatures at the upper limit of B.sempervirens alti-tudinal range,the presence of E.horridum enhanced the growth of B.sempervirens juveniles.At low elevation,under a milder climate,B.sempervirens exerted a high degree of competition toward E.hor-ridum.An increase in temperatures associated with climate change would benefit B.sempervirens juveniles growth which might affect the successional process because it would increase their competi-tive ability when B.sempervirens juveniles become adults.