Biological invasions change ecosystem functioning along secondary succession in the Inland Pampas

LAURA YAHDJIAN; SPIRITO, FLORENCIA; TOGNETTI, PEDRO; CHANETON, ENRIQUE

Rosario

Congreso; IX International Rangelands Congress; 2011

Rangeland Society

Biological invasions changes ecosystem functioning of old field grasslands in the Inland Pampa  Introduction Land uses changes due to anthropogenic activities put in danger the natural ecosystems and the services they provide, allowing for biological invasions (Mack et al. 2000). Plant invasions leads to changes in plant species composition and ecosystem functioning, that in turn, modifies the trajectory of succession. Exotic species may affect plant-soil interactions that in turn promotes the persistence of exotic species by a positive feedback mechanism. The study of post-agricultural succession (i.e. old field) in Inland Pampa grasslands has shown that the recovery of natural communities is affected by the presence of different exotic plant species (Omacini et al. 1995, Tognetti et al. 2010). However, little is known relating changes in ecosystem processes with changes in species composition along secondary succession and the impacts of exotic grasses invasions. The objective of this study was to evaluate key  ecosystem process relating species composition along post- agricultural succession in Inland Pampa grasslands to understand the causes of exotic plant species persistence.  Materials and methods We established a field experiment inan experimental site of the Universty of Buenos Aires, in the Inland Pampa, Argentina(35º 53’ S, 61º 12’ W). We evaluated litter decomposition of dominant species and common litter, soil respiration, and nutrient turnover in plots that were sown by different plant assemblages and  that represented different successional stages (early, intermediate, and late successional species, including native and exotic species). We had 4 treatments, replicated 6 times in a randomized block design. In addition, we established 10 plots behind the successional plotsto assess litter quality effects on litter decomposition process in acommon garden.  Results and discussion Litter decomposition of the dominant species in early succesional stages decomposed significantly faster than litter from intermediate and latter’sspeciesof secondary succession stages. Soil respiration rates also decreased from early to late successional stages (P<0.05). However, when the plant community was dominated by perennial exotic grasses common of late succesional stages, litter decomposition and soil respiration rates were higher than in native communities and similar to rates measured at intermediate successional stages. The differences in litter decomposition rates were explained mainly by differences in the litter quality  of dominant species along secondary succession. Litter decomposition rates in the common garden were significantly and negatively correlated with litter C:N (r2=0.56, P<0.05). We observed that litter decomposition of the common substrate was marginal higher in latter stages dominated by exotic than those dominated by native perennial species(P<0.10), independently of litter quality effects. We did not detect changes in nitrogen turnover rates. Conclusions Biological invasions of perennial grasses modify key ecosystem processes in successional grasslands of the Inland Pampa, giving to faster turnover rates common to early successional stages. These impacts on biogeochemical processes as litter decomposition and soil respiration may explain the persistence of exotic perennial grasses in this ecosystem, due to change in plant-soil interactions that may promote the exotic species and might represent one of the factors that hamper the recovery of native perennial grasses. In addition, plant species invasions may change the capacity of Inland Pampas grasslands to store carbon and consequently may impact key ecosystems services in face of global changes scenarios. References Mack, R. N., D. Simberloff, W. Mark Lonsdale, H. Evans, M. Clout, and F. A. Bazzaz. 2000. Biotic invasions: causes, epidemiology, global consequences, and control. Ecological Applications 10:689-710. Omacini, M., E. J. Chaneton, R. J. C. León, and W. B. Batista. 1995. Old-field successional dynamics on the Inland Pampa, Argentina. Journal of Vegetation Science 6:309-316. Tognetti, P. M., E. J. Chaneton, M. Omacini, H. J. Trebino, and R. J. C. León. 2010. Exotic vs. native plant dominance over 20 years of old-field succession on set-aside farmland in Argentina. Biological Conservation 143:2494-2503.