At the beginning of combat on your turn, target creature you control gets +X/+0 until end of turn, where X is this creature's power.
If Brambleguard Captain’s power is negative as its ability resolves, the target creature will get +0/+0 until end of turn. At least they’ll know Brambleguard Captain appreciates them.
The value of X is calculated only once, as Brambleguard Captain’s ability resolves.