  TRAPPIST-1 is an ultra-cool dwarf hosting a system consisting of seven
planets. While orbital properties, radii and masses of the planets are nowadays
well constrained, one of the open fascinating issues is the possibility that an
environment hospitable to life could develop on some of these planets. Here we
use a simple formulation of an energy balance model that includes the
vegetation coverage to investigate the possibility of life affecting the
climate of the planets in the TRAPPIST-1 system. Results confirm that planet
T-e has the best chance for a habitable world and indicate that vegetation
coverage significantly affects the resulting temperatures and habitability
properties. The influence of vegetation has been evaluated in different
scenarios characterized by different vegetation types, land-sea distributions
and levels of greenhouse effect. While changes in vegetation type produce small
changes, about $0.1\%$, in the habitable surface fraction, different land-sea
distributions, by also affecting the vegetation growth, produce different
temperature distributions. Finally at latitudes where vegetation grows, the
lowering of local albedo still represents a relevant contribution in settling
the planetary temperature profiles even when levels of greenhouse effect higher
than the Earth-like case are considered.
