I don't know that there is any place on Niku I'd call a Savanah. There are some generally open areas down in Aukaraime South, particularly North and South of Baureke passage, but they aren't "tropical grassland", they are barren coral rubble. Not much soil there to be found, and I think it is because this is an area of overwash during large storms, lots of washing away the fine grained stuff, not to mention salt water.
It could also be that what Maude saw was entirely different than what we see today as we know that Aukaraime South had planting activities, and parts of it were even bulldozed by the Coast Guard at one point, so what may have been open area seen by Maude, may now be a dense thicket of scaevola.
amck