Alpha to coverage for distance object

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DirectX

HI I am working on forest rendering engine. (3D model is of .X format)
i am using alpha to coverage for leaf plane rendering

HR(gd3dDevice->SetRenderState(D3DRS_ALPHATESTENABLE, true));
   HR(gd3dDevice->SetRenderState(D3DRS_ALPHAREF, 50));
HR(gd3dDevice-> SetRenderState(D3DRS_ADAPTIVETESS_Y, (D3DFORMAT)MAKEFOURCC('A', 'T', 'O', 'C')));

from near tree looks fine but as distance increases tree becomes almost transparent.