Complex interactions between plant diversity, succession and elevated CO 2

Abstract

Considerable optimism and a certain disdain for all things past are distinctive characteristics of quickly developing fields of science. This is true of research into plant responses to elevated atmospheric CO2 and on biodiversity effects on ecosystem processes, two areas of research that have tended to ignore both each other and some fundamental community processes that are well understood within the framework of traditional plant ecology. Highly simplistic experimental arrangements and strong responses to treatments – sometimes leading to dramatic projections to the real world – were characteristics of early studies in these areas.Recently, there has been an increased interest in moving into more realistic growth conditions and plant assemblages, based on accumulating evidence that responses of natural systems tend to be strongly influenced by resource conditions, plant traits and interactions, and might involve complex positive and negative feedbacks. The new article by Pascal Niklaus and colleagues1xA long-term field study on biodiversity x elevated CO2 interactions in grassland. Niklaus, P.A. et al. Ecol. Monogr. 2001; 71: 341–356See all References1 is a rare example of this new research trend.Together with Reich et al.2xPlant diversity enhances ecosystem responses to elevated CO2 and nitrogen deposition. Reich, P.B. et al. Nature. 2001; 410: 809–812Crossref | PubMed | Scopus (320)See all References2, Niklaus et al.1xA long-term field study on biodiversity x elevated CO2 interactions in grassland. Niklaus, P.A. et al. Ecol. Monogr. 2001; 71: 341–356See all References1 are one of the first to show bi-directional interactive effects of elevated CO2 and plant diversity on plant community productivity. The authors tested whether the ecosystem responses to elevated CO2 changed when specific sets of species were lost from nutrient-poor calcareous grasslands. They set up a five-year field experiment in northwestern Switzerland, constructing communities composed of five, 12 and 31 plant species assembled from the native species pool, and submitting them to either ambient or elevated atmospheric CO2.As reported elsewhere2xPlant diversity enhances ecosystem responses to elevated CO2 and nitrogen deposition. Reich, P.B. et al. Nature. 2001; 410: 809–812Crossref | PubMed | Scopus (320)See all References2, Niklaus et al. found that diversity influenced plant-community response to CO2 enrichment. Elevated CO2 increased biomass production only in communities with the highest number of species. Interestingly, as recently established communities, the assemblages underwent compositional changes during the experiment. Following a classic secondary succession process, there was a gradual replacement of fast-growing short-lived species by slower-growing conservative species. At different successional stages, increased biomass production by elevated CO2 in more diverse communities was due to the presence of responsive species that were absent in species-poorer communities, and these responsive species were different at different successional stages. This suggests that, although response to elevated CO2 is more dependent on plant species traits than on species number, a rich species pool might be important to maintain high productivity under changing environmental conditions.CO2 enrichment also affected biodiversity. Elevated CO2 altered succession by reducing species extinction and increasing coexistence between early- and late-successional species. The fact that elevated CO2 might slow down secondary succession is relevant in a world where disturbance makes early-successional communities increasingly common.The findings of Niklaus et al. might be bad news for those wishing to obtain simple principles to predict the future. Species composition is unlikely to be just a passive response variable in the face of global change. Self-feeding, nonlinear processes can occur, with different communities having different responses to CO2 enrichment, and elevated CO2 affecting the composition of these communities. However, this article shows how fruitful it can be to integrate CO2 and biodiversity research in long-term studies, and combine cutting-edge enquiry with good old-fashioned natural history.