In addition to the 16th century example Lukc mentions, the US Civil War styles I'm using this month include some sparser tree representations. A zoomable Civil War Atlas
http://alabamamaps.ua.edu/historical...as/atlas2.html
(for which i needed to install a plugin - link for which appeared instead of the graphic, when I first tried to load one) shows what I mean:

Plate 21 on that page, the center-left inset has open (not tight-packed) scribbly trees, plus two kinds of marsh symbology on the same map - I take one to mean mostly water and some stuff sticking up, vs. mostly land and lots of puddles and swampy areas.

Plate 13 on that page has an open scribbly style of forests.

Plate 8 on that page varies from tightly scribbled to sparsely scribbled, to barely stippled - I suppose to show the density of coverage.

And plate 10 has maps with the widely-spaced single-tree symbology. I'll note that the cartographer felt a need to make those more identifiable tree shapes, even to a cast shadow -- if it takes that to make wide-scattered symbols look good, I'm going to be better off with massed scribbles :-).

On plate 56 of the next page of that site, on Map 6 there's some trees that are such random bits of green it almost looks like how you would stipple paint with a sponge. THERE's an act that would take supreme confidence - to painstakingly draw out a map and then go at it with a Really, Really random tool. Elswhere on the same map there are more complete scribbled-circle trees, so maybe the more random stuff was just light or incomplete printing. The adjacent map no. 5 also has some good cultivated-land symbology to use as an example.