Research: modelling

Steppes and forests as alternative states and integration of field studies
Researchers: Dr. Bazartseren Boldgiv, Dr. Peter Petraitis

This project integrates the separate research projects. We suspect that steppe grasslands and taiga forests may represent alternative stable states in which small changes in grazing and/or evapotranspiration in Mongolia could cause abrupt shifts between steppe grasslands and taiga forests. For example, adult larch trees may increase recruitment of larch seedlings by limiting desiccation stress. This benefit, however, may be an all-or-nothing effect, because a critical minimum number of adult plants may be needed for any improvement in recruitment. If the critical number of adults depends on environmental conditions then species composition (steppe grasslands vs. larch forests) could then show an abrupt shift even with a very small change in rates of evapotranspiration, which is driven by grazing pressure and global warming. Moreover, after such a shift, grasslands or forest would not be recoverable by small reversals in grazing pressure or warming trends.

Theory suggests the switch between alternative states occurs at a critical threshold with a step-like transition between the types of communities, and this has two implications for larch forests and steppe grasslands in Mongolia. First, the community we see in a natural ecosystem today - for example, steppe grasslands - may not be the product of current ecological and anthropogenic processes but rather the result of past historical events that set the stage. Second, the duration of a past event - e.g., a bout of overgrazing - may be quite brief but still cause the switch between community states because a critical threshold is crossed. Large, discontinuous and irreversible changes have been seen in lakes, coral reefs, arid grasslands, and fisheries, and often the existence of alternative stable states has been suggested as the underlying cause.

Data from the GEF project are being used to develop the first round of models, which are state and transition models. These models provide a way to organize and evaluate our research goals. We will use insights derived from our initial round of modeling and data from our research projects to develop more mechanistic models.

Other PIRE Mongolia Research Projects