ThomagicRenderer/TreeStreamer.cs

using UnityEngine;

namespace Assets.ThoMagic.Renderer
{
    public class TreeStreamer : InstanceStreamer
    {
        private readonly Terrain _terrain;
        private Bounds worldBounds;
        private float maxTreeDistance;

        public TreeStreamer(Terrain terrain)
          : base()
          => _terrain = terrain;

        public void Build(float maxDistance)
        {
            maxTreeDistance = maxDistance;

            Vector3 position = _terrain.GetPosition();
            worldBounds = new Bounds(_terrain.terrainData.bounds.center + position, _terrain.terrainData.bounds.size);       
        }

        public override bool IsInRange(Camera referenceCamera, Plane[] planes)
        {
            if (!_terrain.editorRenderFlags.HasFlag(TerrainRenderFlags.Trees))
                return false;

            var eyePos = referenceCamera.transform.position;

            if ((eyePos - worldBounds.ClosestPoint(eyePos)).magnitude <= Mathf.Min(referenceCamera.farClipPlane, maxTreeDistance))
                return true;

            return false;
        }
    }
}