You can use multiple blending shaders; each controlling an element of the final blending shader mask. For instance, this is a simple setup for making clumps of trees.
Get Texture coordinates Constant Scalar
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Voronoi Cell Scalar
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Power Fractal
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The power fractal is the blending shader. The Voronoi Cell Shader makes the clumps. Get Texture coordinates provides coordinates for the voronoi noise. The constant scalar is for scale. I use the same value for the constant scalar and the power fractals scale. The default smallest scale of the power fractal is usually fine. The lead in scale of the power fractal controls the spacing distance of the clumps.
You can also adjust the coverage using the color controls of the power fractal.
