Biome boundaries across gradients: the sagebrush steppe-temperate forest ecotone

Background/Question/Methods

Regional climate change and widespread forest disturbances (e.g., drought, fire and beetle mortality) may combine to substantially alter the location of forests in the western US. Western forests are currently experiencing mortality at unprecedented rates. Meanwhile, non-forest ecosystems such as sagebrush steppe are predicted to move upslope and northward due to climate change. These dynamics will unfold at the forest-nonforest ecotone (lower tree line). To better understand how disturbances and climate change will impact these ecotones, we tested a conceptual framework based on work of Goesz, Cadenasso, and Yarrow, which describes relationships of spatial arrangement of ecosystems across elevational gradients. Specifically, we tested (i) for a negative relationship between the width of ecotones and elevational slope and (ii) for a shift of the frequency distribution of patch sizes from larger to smaller patches across the ecotones. We tested these relationships on sagebrush steppe – temperate forest ecotones based on data extracted with GIS for the extent of Wyoming using elevation (NED) and vegetation cover (GAP) at 30-m resolution. We measured the characteristics of ecotones, which we located based on 5,000 random points and three neighborhood sizes (5, 10, and 50 km).

Results/Conclusions

Our GIS analysis yielded 662 transects across sagebrush steppe – temperate forest ecotones in Wyoming. The ecotones span an elevational gradient from 2070 to 2370 m. The mean slopes ranged from close to 0º to 29º with a mean of 4.4 º. The projected ecotone width varied between 30 m and 43 km with a mean of 5.3 km. Despite such wide variations among ecotone transects, we found a significant negative relationship between ecotone width and mean slope. We also found a significant shift of the frequency distribution of patch sizes across the ecotones both for sagebrush steppe and for temperate forest. For sagebrush steppe, medium sized patches (between 100 m2 and 1 km2) per unit length transect were the most common both below and within the ecotones, but more common within. For temperate forest, medium sized patches were most common below, medium and large patches most common within, and large patches most common above the ecotones. Our analysis demonstrates the complexity of ecotone boundaries in topographically rich areas and the need for improved understanding to evaluate the future conditions of ecosystem boundaries, because potential shifts in ecotone structure or location is likely influenced by current ecotone structure.