QGIS - Counting vertexes in radius from each feature in line layer

Assuming your roads layer has no respective field yet:

• mark the layer and open the Field Calculator
  • toggle Create a new field
  • choose an Output field name
  • select Output field type: Whole number (integer)
  • paste this expression:
    aggregate( '<tree_layer_name>', 'count', '', distance( geometry( @parent ), $geometry ) <= <dist> )
    
    with <tree_layer_name> replaced with the actual name of the tree layer, and <dist> replaced with the search radius around the line feature
    
    Note:
    The <dist> value will be treated as units of CRS, meaning that, if your layers are referenced in EPSG:4326 (or any geographic reference), these will be degrees. For it to be treated as meters, make sure your layers are projected in a suitable metric projection!