  Building on previous work of [BPS] we investigate $\sigma$-closed partial
orders of size continuum. We provide both an internal and external
characterization of such partial orders by showing that (1) every
$\sigma$-closed partial order of size continuum has a base tree and that (2)
$\sigma$-closed forcing notions of density $\mathfrak c$ correspond exactly to
regular suborders of the collapsing algebra $Coll(\omega_1, 2^\omega)$.
  We further study some naturally ocurring examples of such partial orders.
