Changes in limiting resources in semiarid ecosystems enhance invasibility and composition of ephemeral plants

Background/Question/Methods

Climate change and nutrient deposition, as a consequence of human activity, are two major drivers of future global change. One prediction related to climate change is that precipitation variability will increase in arid semiarid ecosystems, where water is generally the main limiting resource. However, Soil nitrogen availability could also change, as consequence of atmospheric nitrogen deposition. Theoretical and experimental studies predict that increased resource availability opens windows of opportunity for exotic species to colonize and dominate susceptible ecosystems. Here we test the hypothesis that an increase in these two limiting factors will increase the chances of invasibility of semiarid plant communities and alter the coexistence of native and exotic ephemeral plants. We added water and soluble nitrogen to experimental plots in a long-term study site in semiarid Chile which was monitored for two decades previously, and analyzed their effects on density, cover, and species richness of native and exotic ephemerals. We used PERMANOVA and Species Indicator Analysis to determine the effect of the interaction of these factors on species composition and monitored the effects during two years

Results/Conclusions

We found that water addition has significant positive effects on total cover, density and species richness of native annual plants, but with a delay of one year. On the other hand, the interaction of water and nitrogen addition significantly affected annual plant species composition in the permanent plots, and the indicator species of these changes were mainly native ephemerals. Again, this effect was not detected immediately but after a lag of one year. As for ephemeral exotics, water did not significantly affect their cover, density or species richness. We conclude that the increase in both limiting resources did not affect invasibility of the semiarid ecosystem but it did affect native species composition, confirming that water but not nitrogen is the primary limiting factor for native ephemerals. We highlight the importance of the coupled interactions of these factors to predict ecosystem responses to future changes in climate and nitrogen supply.