def store_trees(prefix=None, start_year=2015, tree_dict={}):
    """Saves values of `BaseStorageTree` objects. The file is saved into the 'data' directory
    in the current working directory. If there is no 'data' directory, one is created. 

    Parameters
    ----------
    prefix : str, optional 
        prefix to be added to file_name
    start_year : int, optional
        start year of analysis
    **kwargs 
        arbitrary keyword arguments of `BaseStorageTree` objects

    """
    if prefix is None:
        prefix = ''
    for name in tree_dict.keys():
        tree_dict[name].write_columns(prefix + 'trees', name, start_year)
