Ecosystem Responses to Warming and Interacting Global Change Factors

Abstract

It is undeniable that ecosystems of the future will be subjected to multiple atmospheric and climatic changes. Research planning documents recognize this certainty and strongly promote the understanding of multi-factor interactions as a research imperative for the future (Committee on Global Change Research 1999). The research that we have highlighted here illustrates many of the chal- lenges of addressing this imperative, but it also shows the importance of attempting to meet those challenges. The net response to increased CO2 concentration, warming, altered precipitation, N deposition, and other changes may be a simple additive effect of the separate influences, but experimental results also demonstrate the possibility of complex interactions. Single-factor experi- ments, such as those in the GCTE Elevated CO2 network, are highly valuable for advancing understanding of the primary responses and how they are modified by other environmental factors. However, such experiments can- not be interpreted as providing predictions for ecosys- tem responses in a future climate. Single-factor ecosys- tem warming experiments are especially problematic, given the temporal and spatial variability of air tempera- ture and the greater uncertainty (compared to CO2) in future trajectories. Realistic warming treatments also are more difficult to carry out in experiments, especially in forests, and it has been harder to attain an integrated, ecosystem-scale understanding of all of the factors that will influence how ecosystems will respond to a warmer climate. Coupling the uncertain responses to warming to the simultaneous changes in other global change fac- tors would seem to be a daunting task. The experiments and modeling efforts highlighted in this chapter do not lead to a clear prediction of how eco- systems will be affected by various combinations of glo- bal change factors. They do, however, support several general principles about responses to climatic warming and multi-factor interactions: 1. Warming alone affects multiple pools and processes with different rate constants. Hence, responses are likely to change over time, as occurred in the response of soil respiration to soil warming at the Harvard Forest. 2. Whole-ecosystem warming experiments are necessary to address the complex interactions between below- ground and aboveground responses. Without a whole- ecosystem perspective, critical hypotheses such as the linkage between warming effects on N availability and aboveground production cannot be evaluated. 3. Stochastic events can strongly alter the trajectory of response to warming. In the TACIT experiment a severe summer heat spell led to a loss in productivity after 4 years despite generally positive effects of warming. Short-term experiments may over emphasize the importance of such events while under representing the possibility of their occurrence. 4. Elevated CO2 can sometimes ameliorate deleterious ef- fects of warming, and since higher CO2 concentrations are certain to be associated with future warming sce- narios, it is imperative that their combined effects be con- sidered when interpreting data. However, we cannot assume that the responses to elevated CO2 always are positive, as evidenced by the Jasper Ridge experiment. 5. The responses to combinations of factors often equal the simple additive effects of the individual factors. That being the case, single-factor experiments continue to be very informative and can be the basis for model simulations. However, complex interactions do occur, and they may or may not be predictable. Multi-factor experiments are important for reminding us of this complexity. 6. Ecosystem models that incorporate our best understanding of the modes of actions of the individual factors will also capture many of the major biogeochemi- cal interactions. Discrepancies between experimental data and model projections, such as for below-ground productivity in the Jasper Ridge experiment, indicate areas where model improvement is needed. The influence of stochastic events and unexplained year-to-year variation in the nature of interactions suggest that simulations should be expected only to provide an envelope of possible future responses. 7. The long-term net effect of elevated CO2 and temperature may be similar in different ecosystems, but the relative importance of the two global-change factors varies with site factors (e.g., water and N availability). Relatively modest N additions can overcome the soil-N feedback that can otherwise lead to C-sink saturation and loss of continued stimulation of NEP. These model results demonstrate why experiments must be conducted in a range of ecosystems under different conditions.