  We report on high-temperature electron spin resonance studies of a detwinned
LaMnO$_3$ single crystal across the Jahn-Teller transition at $T_{\rm JT}$ =
750 K. The anisotropy of the linewidth and g-factor reflects the local
Jahn-Teller distortions in the orbitally ordered phase. A clear jump in the
linewidth accompanies the Jahn-Teller transition at $T_{\rm JT}$ = 750 K
confirming that the transition is of first order. Already at $T^*$ = 550 K a
significant decrease of the reduced linewidth is observed. This temperature
scale is discussed with respect to the interaction of the $e_g$-electrons of
the Mn$^{3+}$-ions and the elastic field of the cooperative distortions. Our
results support a partial melting of the orbital order along the
antiferromagnetically coupled $b$-axis at $T^*$. The remaining two-dimensional
orbital ordering within the ferromagnetically coupled $ac$-plane finally
disappears together with the cooperative distortion at $T_{\rm JT}$. Moreover
in our discussion we show that elastic strain field interactions can explain
the melting of the orbital order and, thus, has to be taken into account to
explain the orbital ordering in LaMnO$_3$.
