Two juniper species, Ashe juniper and Eastern redcedar, are invading open grasslands across vast portions of the Great Plains, USA. This biome-level transformation from grassland to juniper forest is threatening the sustainability and profitability of livestock production, precipitating declines in grassland-obligate wildlife, reducing available water resources, and threatening human safety by increasing wildfire risk. Juniper invasion has been linked to a reduction in the intensity and spatial extent of fires following European settlement of the Great Plains. However, the extent of cedar invasion varies across the region. We hypothesize that this variability is driven by differences in laws and policies regarding fire use, which prescribe the conditions under which fire management can occur and the resulting range in fire intensities and scales of application available for meeting management goals. To explore the influence of law and policy on juniper invasion, we built on previous research that linked well-established models from applied fire physics with a widely applied ecological positive feedback model of woody plant encroachment. We introduced laws as constraints on different components of this coupled physical-ecological model.
Our modeling exercise identified the relative influence of laws and policies on the transition from grassland to juniper forest. Laws limited both the temporal window available to managers for conducting fires, and the spatial extent of fire application. For instance: 1) stringent liability standards decrease the number of managers who are willing to burn, thereby decreasing the spatial extent of fire in an area and allowing greater spread of juniper into uninvaded areas; 2) burning restrictions during periods of drought limit opportunities to burn under conditions where high fire intensities and attendant reductions in cover of mature juniper can be achieved. Understanding the effects of laws and policies on juniper invasion is necessary to assess trade-offs associated with legislatively limiting the scale and intensity of fire available to mangers and to avoid unintended consequences when developing legislation aimed at protecting human health and safety and the resource base upon which we depend.