How do i tune this a Billboard Shader into a Tree Billboard shader?

i just want it to stop rotating up and down and just rotate sideways to always face the player without goin up or down. the billboard shader im using is this one:

Shader "Unlit/StandardBillboard"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "RenderType"="Transparent" "Queue"="Transparent" }
        Blend SrcAlpha OneMinusSrcAlpha
        LOD 100

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            // make fog work
            #pragma multi_compile_fog

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                UNITY_FOG_COORDS(1)
                float4 vertex : SV_POSITION;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;

            v2f vert (appdata v)
            {
                v2f o;

                // o.vertex = UnityObjectToClipPos(v.vertex);

                float4 origin = float4(0,0,0,1);
                float4 world_origin = mul(UNITY_MATRIX_M, origin);
                float4 view_origin = mul(UNITY_MATRIX_V, world_origin);
                float4 world_to_view_translation = view_origin - world_origin;

                float4 world_pos = mul(UNITY_MATRIX_M, v.vertex);
                float4 view_pos = world_pos + world_to_view_translation;
                float4 clip_pos = mul(UNITY_MATRIX_P, view_pos);

                o.vertex = clip_pos;

                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                UNITY_TRANSFER_FOG(o,o.vertex);
                return o;

            }

            fixed4 frag (v2f i) : SV_Target
            {
                // sample the texture
                fixed4 col = tex2D(_MainTex, i.uv);
                // apply fog
                UNITY_APPLY_FOG(i.fogCoord, col);
                return col;
            }
            ENDCG
        }
    }
}