def recko(r, hydra=False):
    """sap-wood thickness after Račko et al. (2018)

    Calculates sap-wood thickness as a function of the tree radius 
    at breast height.

    Parameters
    ----------
    r : float
        tree radius at breast height (in cm)
    hydra : bool
        selects if only the hydrated area is returned (when True)

    Returns
    -------
    sapwood : float
        sapwood thickness (in cm)
    
    References
    ----------
    Račko, V., O. Mišíková, P. Hlaváč, and V. Deáková (2018), 
    Can bark stripping cause red heartwood formation in beech stems? 
    iForest - Biogeosciences and Forestry, 11(2), 251–258, doi:10.3832/ifor2147-011.
    """
    if hydra:
        return 0.34 * r - 2.714
    else:
        return 0.3748 * r
