In Johnson's Book about D-branes he gets the Tension of a D brane:
$$\tau_p = \frac{\sqrt{\pi}}{16 \kappa_0 g_s}(4\pi^2\alpha')^{(11-p)/2}$$
By performing a string amplitude calculation and comparing it with the QFT analogous (in the same fashion as in Polchinski lecture).
The tension of the brane is of order $g_s^{-1}$ and it seems that the conclusion is that the brane is a Non-perturbative object, what is the reason behind this? and which are the consecuences of a brane being non-perturbative?