  The integrable quantum group $spl_q(2,1)$-invariant supersymmetric t-J model
with open boundaries is studied via an analytic treatment of the Bethe
equations. An $su(2)$ feature is seen to hold for states at or close to
half-filling. For these states the eigenvalues of the transfer matrix of the
t-J model satisfy a set of $su(2)$ functional relations. The finite-size
corrections to the relevant eigenvalues, and thus the surface effect on the
spin excitations, have been calculated analytically by solving the functional
relations.
