Although the niche concept was initially developed in the context of species, Whittaker (1970) extended it to classify global biome patterns—the highest organizational unit of vegetation—suggesting that the spatial limits (boundary) of biomes are deterministically predictable by climate. Recent empirical works, however, show that biomes, such as tropical savanna and forest, are not always predictable by climate. Although forests are more common in high rainfall areas and savannas in low rainfall areas, the boundary between these biomes occurs over a wide range of rainfall values . Explaining variations in the spatial limits of savanna and forest biomes with respect to climate remains an open challenge. 
Whittaker's (1970) biome classification map based on climatic niches.
The position of the savanna-forest boundary (brown line) and its relationship to rainfall. 
Contrary to Whittaker's biome theory, the savanna-forest boundary is not solely predictable by a particular rainfall contour PM (grey line; commonly known as Maxwell precipitation), but also by its geometrical shape. When PM is curved, the boundary deviates from PM (left) because of the imbalance between influx and outflux of seeds, resulting in the nondeterministic relationship between rainfall and tree cover (right).