CO_(2)enrichment accelerates successional development of an understory plant community

Aims Rising concentrations of atmospheric carbon dioxide([CO_(2)])may influence forest successional development and species composition of understory plant communities by altering biomass production of plant species of functional groups.Here,we describe how elevated[CO_(2)](eCO_(2))affects aboveground biomass within the understory community of a temperate deciduous forest at the Oak Ridge National Laboratory sweetgum(Liquidambar styraciflua)free-air carbon dioxide enrichment(FACE)facility in eastern Tennessee,USA.We asked if(i)CO_(2)enrichment affected total understory biomass and(ii)whether total biomass responses could be explained by changes in understory species composition or changes in relative abundance of functional groups through time.Materials and methods The FACE experiment started in 1998 with three rings receiving ambient[CO_(2)](aCO_(2))and two rings receiving eCO_(2).From 2001 to 2003,we estimated species-specific,woody versus herbaceous and total aboveground biomass by harvesting four 130.5-m subplots within the established understory plant community in each FACE plot.In 2008,we estimated herbaceous biomass as previously but used allometric relationships to estimate woody biomass across two 535-m quadrats in each FACE plot.Important findings Across years,aboveground biomass of the understory community was on average 25%greater in eCO_(2)than in aCO_(2)plots.We could not detect differences in plant species composition between aCO_(2)and eCO_(2)treatments.However,we did observe shifts in the relative abundance of plant functional groups,which reflect important structural changes in the understory community.In 2001-03,little of the understory biomass was in woody species;herbaceous species made up 94%of the total understory biomass across[CO_(2)]treatments.Through time,woody species increased in importance,mostly in eCO_(2),and in 2008,the contribution of herbaceous species to total understory biomass was 61%in aCO_(2)and only 33%in eCO_(2)treatments.Our results suggest that rising atmospheric[CO_(2)]could accelerate successional development and have longer term impact on forest dynamics.

Resource availability and plant diversity explain patterns of invasion of an exotic grass

Aims in this study,we examine two common invasion biology hypotheses-biotic resistance and fluctuating resource availability-to explain the patterns of invasion of an invasive grass,Microstegium vimineum.Methods We used 13-year-old deer exclosures in great smoky mountains National Park,USA,to examine how chronic disturbance by deer browsing affects available resources,plant diversity,and invasion in an understory plant community.using two replicate 1 m2 plots in each deer browsed and unbrowsed area,we recorded each plant species present,the abundance per species,and the fractional per-cent cover of vegetation by the cover classes:herbaceous,woody,and graminoid.For each sample plot,we also estimated overstory canopy cover,soil moisture,total soil carbon and nitrogen,and soil pH as a measure of abiotic differences between plots.Important Findings We found that plant community composition between chronically browsed and unbrowsed plots differed markedly.Plant diversity was 40%lower in browsed than in unbrowsed plots.at our sites,diver-sity explained 48%and woody plant cover 35%of the variation in M.vimineum abundance.in addition,we found 3.3 times less M.vimineum in the unbrowsed plots due to higher woody plant cover and plant diversity than in the browsed plots.a parsimonious explanation of these results indicate that disturbances such as herbivory may elicit multiple conditions,namely releasing available resources such as open space,light,and decreasing plant diversity,which may facilitate the proliferation of an invasive species.Finally,by testing two different hypotheses,this study addresses more recent calls to incorporate multiple hypotheses into research attempting to explain plant invasion.