Indirect effects of global change on the alpine tundra: Linking plant community and ecosystem responses
Global change affects ecosystems in complex ways. Global change drivers may interact with one another, producing non-additive effects that are not predictable by knowledge of single factor effects alone. Global change drivers may also have effects that propagate through interacting components in the system, leading to indirect effects. Indirect effects can amplify or counteract direct effects, being as or more important than the direct effects of global change. In a long-term (6-year) factorial global change experiment at Niwot Ridge, we test how increased nitrogen (N), winter precipitation (snow), and elevated temperatures affect ecosystem and plant community properties in the alpine tundra. We hypothesize that 1) global change drivers will have interactive effects on the ecosystem and plant community, and 2) global change drivers will have indirect effects on ecosystem properties through changes in dominant plant species.
We found that ecosystem properties (nitrogen availability, microbial biomass, productivity) and the plant community (richness, evenness, abundance of dominant species) responded primarily to nitrogen and snow manipulations; temperature only affected microbial biomass. Both ecosystem properties and plant communities showed lags in their response to global change drivers, with effects manifesting after three to four years. Very few interactive effects of global change drivers were found. Indirect effects were common: specifically, global change drivers indirectly affected richness, evenness, microbial biomass and N mineralization through their effects on the abundance of dominant species and productivity. Results suggest that interactive effects of global change drivers may not necessarily be important in all systems and that indirect effects due to interactions among ecosystem components are common and lead to coordinated response of ecosystem properties and the plant community to similar global change drivers.