def roessler(r, tree='beech'):
    """Estimate bark thickness

    Returns the estimated bark thickness as published by Rössler (2008)

    Parameters
    ----------
    r : float
        tree radius at breast height (in cm)
    tree : str
        Tree name, for which to calculate bark thickness.
        Can be one of ['beech', 'oak']

    Returns
    -------
    db : float
        bark thickness (in mm)
    
    Raises
    ------
    NotImplementedError : if tree is not in ('oak', 'beech')

    References
    ----------
    Rössler, G.: Rindenabzug richtig bemessen, Forstzeitung, 4, p. 21, 2008.
    """
    if tree == 'beech':
        db = 2.61029 + 0.28522 * 2 * r
    elif tree == 'oak':
        db = 9.88855 + 0.56734 * 2 * r
    else:
        raise NotImplementedError("Only 'beech' and 'oak' supported.")
    return db / 10
