There have been times when I’ve idly wondered if there was ever an artist who spent their life painting just one thing. The closest I could come was Monet’s serial obsessions with the cathedral at Rouen or the Houses of Parliament, which he painted dozens of times at different intensities of light, but those were only tributaries in a lifelong flood of paintings that encompassed all sorts of topics.

Now comes Sylvia Plimack Mangold, the mother of the film director James Mangold (“Walk The Line,” “Girl Interrupted,” “3:10 to Yuma,” etc.), in a new and cumulatively moving exhibit at the Norton who has more or less done just that - spent 30 years painting a small copse of trees outside her Hudson River studio window.

Mangold didn’t start out painting trees - she earned early praise for work centering on floors and mirrors - but foliage and a lack of foliage has become her central focus in the last 30 years.

Perhaps the two crucial paintings are “Winter Writing,” from 1984, and “Trees at Pond” from a year earlier, if only because these are long shots rather than closeups. The clump of trees is in the center of an expansive rural composition, but over the years she moves closer and closer, the better to examine the geometrics of the branches, which in turn means that Mangold often paints winter scenes, so that the leaves can’t obscure the complicated connections between the main trunk and its offshoots.

All the later paintings are framed tightly, so that there’s no overall sense of the surrounding geography. The looseness of the watercolors gives them a more optimistic feel than is strictly warranted by the constricted, frigid winter light. The atmosphere is forbidding, but subtly so - the work has a stylized, slightly Japanese feel to it. Mangold’s palette is limited but compelling because the paintings have a sense of weight, of the kind of observed detail that results in emotion.

Relief from the starkness is provided by the occasional summer painting, which provide splashes of green or even yellow, although in the context of the overall exhibition they seem slightly beside the point.

The series concludes with a painting called “The Maple Tree (Summer)” from 2011. There’s a new lushness to this work; it’s painted more loosely, with a sensuality missing from the earlier paintings. Mangold’s line is gentler, more forgiving. Perhaps it’s just that she and the trees have grown together, and she’s now able to relax with them.

At long last, they’re old friends.