  We consider a semi-linear parabolic problem in a model plane thick fractal
junction $\Omega_{\varepsilon}$, which is the union of a domain $\Omega_{0}$
and a lot of joined thin trees situated $\varepsilon$-periodically along some
interval on the boundary of $\Omega_{0}.$ The trees have finite number of
branching levels. The following nonlinear Robin boundary condition
$\partial_{\nu}v_{\varepsilon} + \varepsilon^{\alpha_i}
\kappa_i(v_{\varepsilon}) = \varepsilon^{\beta_i} g^{(i)}_{\varepsilon}$ is
given on the boundaries of the branches from the $i$-th branching layer;
$\alpha_i$ and $\beta_i$ are real parameters. The asymptotic analysis of this
problem is made as $\varepsilon\to0,$ i.e., when the number of the thin trees
infinitely increases and their thickness vanishes. In particular, the
corresponding homogenized problem is found and the existence and uniqueness of
its solution in an anizotropic Sobolev space of multi-sheeted functions is
proved. We construct the asymptotic approximation for the solution
$v_\varepsilon$ and prove the corresponding asymptotic estimate in the space
$C\big([0,T]; L^2(\Omega_\varepsilon) \big) \cap L^2\big(0, T;
H^1(\Omega_\varepsilon)\big)$, which shows the influence of the parameters
$\{\alpha_i\}$ and $\{\beta_i\}$ on the asymptotic behavior of the solution.
