Problem with plotting a square 1 x 3 plot and placement of outer margin text

Dear R-helpers,

Please see the attached plot.

The problem is that I have too much space between the x-axis label
(which is mtext in an outer margin) and the plots.

My par settings for this plot are:

par(mfrow=c(1,3),oma=c(2,2,2,2),mar=c(5.1,4.1,4.1,2.1),pty="s")
#here is the code that produces the three plots, which I have deleted for simplicity
mtext("Log Wetland Area",side=1,outer=TRUE)

It works fine (less space between plots and outer margin text)) when I
set pty="m" but then I get very long and skinny rectangular plots. I
would like to keep the square plots.

Any help would be much appreciated!

Many thanks,

Mark