Thresholds for boreal biome transitions

Abstract

Although the boreal region is warming twice as fast as the global average, the way in which the vast boreal forests and tundras may respond is poorly understood. Using satellite data, we reveal marked alternative modes in the frequency distributions of boreal tree cover. At the northern end and at the dry continental southern extremes, treeless tundra and steppe, respectively, are the only possible states. However, over a broad intermediate temperature range, these treeless states coexist with boreal forest (∼75% tree cover) and with two more open woodland states (∼20% and ∼45% tree cover). Intermediate tree covers (e.g., ∼10%, ∼30%, and ∼60% tree cover) between these distinct states are relatively rare, suggesting that they may represent unstable states where the system dwells only transiently. Mechanisms for such instabilities remain to be unraveled, but our results have important implications for the anticipated response of these ecosystems to climatic change. The data reveal that boreal forest shows no gradual decline in tree cover toward its limits. Instead, our analysis suggests that it becomes less resilient in the sense that it may more easily shift into a sparse woodland or treeless state. Similarly, the relative scarcity of the intermediate ∼10% tree cover suggests that tundra may shift relatively abruptly to a more abundant tree cover. If our inferences are correct, climate change may invoke massive nonlinear shifts in boreal biomes.

Fig. 1.

Frequency distribution of tree cover in the boreal zone (45°N–70°N) in 500 × 500 m grid cells. There are four distinct modes corresponding to forest, dense savanna-like woodland, sparse savanna-like woodland, and a treeless state (tundra or steppe). Tree cover percentage values have been transformed through the arcsine-squared-root transformation.

Fig. 2.

Relationship between mean July temperature averaged for the period 1961–2002 and the approximate position of alternative stable states of boreal tree cover (solid curves) inferred from minima in the computed stability landscapes (SI Appendix, Fig. S7) computed from the data (SI Appendix, SI Text and Fig. S7). The dashed curves correspond to maxima in the computed stability landscape that separate the basins of attraction of the alternative stable states. Dots represent the tree cover and mean July temperature in the grid cells we analyzed.

Fig. 3.

Distribution of boreal forest, dense savanna-like woodland, sparse savanna-like woodland (forest tundra), and the treeless tundra and steppe states as a function of mean July temperature (°C) and mean annual precipitation (mm⋅yr⁻¹) both averaged for the period 1961–2002 (A) and the geographical distribution of these states (Copyright 2011, Google Earth) (B).