Now best known as a Neo- and Post-Impressionist figurative painter, Théo van Rysselberghe was a keen yachtsman, painter of coastal scenes, and painted quite a few trees in his landscapes too.
Other Neo-Impressionists also painted trees, but van Rysselberghe was fortunate enough to paint through and after the First World War, leaving us many works in vibrant colour. He was also a key figure among the Post-Impressionists in the south of France, who were so influential to painting in the twentieth century.
Biography
I have written a more extensive account of his career in this article.
Théophile (Théo) van Rysselberghe (1862-1926) was born in Ghent, Belgium, and studied at the Academy of Fine Arts in Ghent, then the Royal Academy of Fine Arts in Brussels. His early paintings were well-received, and in 1882-3 he overwintered in Spain and visited Tangier, Morocco; he returned to Morocco in 1883-4, and painted many orientalist works during those visits.
In 1886, he aligned with Les XX, and experimented with impressionist styles before becoming a Divisionist and Neo-Impressionist during another visit to Morocco in 1887-8. He settled in Brussels, then in 1892 sailed with Signac to the Mediterranean via the Canal du Midi. By the summer of 1898, he wanted to get closer to nature, and started to migrate from strict Neo-Impressionism. In 1903, he joined Signac, Cross, Denis, Pierre Bonnard, and others, in an ‘inaugural’ exhibition at Druet’s gallery in Paris.
He was spending more time in the south of France, where he helped Cross during the final years of his life. In his own later years, he abandoned all traces of Divisionism, concentrating instead on light and colour, or ‘luminism’. He settled in Saint-Clair on the Côte d’Azur in 1911, from where he painted extensively along the Mediterranean coast. The area was just starting to be visited more, and to attract those from the north, with the advent of high-speed train connections with Paris.
Divisionist trees
Relatively few trees of any significance appeared in his work during his early realist and impressionist periods, and it was not until he became a Divisionist in 1888 that van Rysselberghe appears to have made trees of importance in his paintings.

In The Schelde Upstream from Antwerp: After the Fog (1892) he has adhered to anatomical construction and the rigorous discipline of Divisionism, applying fine marks of contrasting colour throughout the trees. Unlike some other Divisionists, he used dark colours and dense marks to produce this contrasting shade effect, with some patches of light on the trunks.

This study for his Canal in Flanders (1894) used coarser marks for speed, and established the underlying colour contrasts to be developed in the finished work below

The finished version of Canal in Flanders (1894) is an amazing combination of radical perspective projection, finely observed and composed trees (carefully adjusted from his sketch), and meticulous reflections, all constructed using tiny marks of contrasting colours. It should surely be rated as a classic depiction of trees, and one of the great Divisionist paintings.

In Pointe Saint-Pierre, Saint-Tropez (1896) he used the same technique to model his first pines in the hot light of the Mediterranean coast. Their anatomy is explicit, each tree assembled from its hundreds of small marks laid along branches, then giving rise to foliage.
Pines and the move from Divisionism

In the early years of the twentieth century, he started to paint recurrent motifs of a loose group of pines behind the beach. In The Mediterranean at Le Lavandou (1904) there are no bathers or other figures, just the curved and twisting trunks and branches, with their brilliant foliage and dark blue shadows.

This study for In the Shade of the Pines (1905) shows the more vibrant colours which he was to use for later finished works, continuing his progression away from formal Divisionism.

The finished version of In the Shade of the Pines (1905) introduced a couple of female nudes, and his earlier fine marks have been lost. Instead he built the trunks, branches, foliage, and other vegetation with orientated brushstrokes, in a manner not unlike that of Vincent van Gogh before his marks organised into whorls and swirls. He thus enhanced the contrast in textures between trees, rocks, sea, and skin.

Other paintings of this period show more evidence of his previous Divisionism: in Pines and Eucalyptus at Cavalière (1905) he aligned his marks with the structure of foliage and trunks, but they were laid as tiles of colour on the beach and sea.

Bathers under the Pines at Cavalière (1905) shows another pines-beach-bathers motif, with a different intermediate style between Neo- and Post-Impressionism, and the heightened colours of his later works.

Following the death of Cross in 1910, van Rysselberghe abandoned his last traces of Divisionism. Pines at Pointe Layet (1912) progressed his earlier motifs of pines and the sea, but now on a steep cliff and without beach or bathers. His colours are more sombre again, and his brushstrokes worked in to model texture in the trunks.

Painted half way through the First World War, Sunset (1916) was a further progression, its paint applied in coarse marks, and the cliff now explicitly vertiginous.

Pine by the Mediterranean Sea (1916) is altogether brighter, lighter, and less impending, with similar textures in the bark and foliage as in Pines at Pointe Layet (1912).
Colour and the 1920s

At the end of the war, van Rysselberghe’s colours became as strong as those of the Fauves. In Almond Trees in Blossom (Morning) (1918) the more delicate pinks of the flowers pale in comparison with his full reds and blues – even down to the blue horse pulling the plough.

He eased those colours a little in Large Tree near the Sea (1919), and used less regular marks for the fields and ground vegetation.

Noon Landscape (c 1921) is even more conventionally realist, with smoother bark and leaves stroked in individually.

Les Fonds de Saint-Clair (1924) appears more likely to have been painted in a single plein air session, with areas where the application of paint has been so light that the canvas texture shows through.

Probably his final variation on his pines-beach-bathers motifs, Bathers under the Pines by the Sea (1926) uses high chroma colours in its shadows, with near-primary red marking the highlights on the rough bark of the pine trunks, ground, and flesh, and livid green over much of the rest of the flesh. It is as if those colour contrast dots of Divisionism had ruptured, releasing their intensity over larger areas.
Summary
Théo van Rysselberghe painted in a wide range of styles, from realism to near-Fauvism, and several genres, particularly portraits and other figurative work. He also painted many fine examples of trees in landscapes, particularly of pines on the Mediterranean coast, especially the combination of pines, beach, and female bathers, which recurred from 1904 to 1926, when he died.
Among his Divisionist (Neo-Impressionist) works, Canal in Flanders (1894) is as masterly as any painting in that style, a virtuoso combination of perspective, finely-depicted poplars, and meticulous reflections, crafted laboriously using tiny marks.
His series of paintings of pines on the Mediterranean coast also merits further examination; although most of those paintings are now in private collections, they would surely make an ideal theme for an exhibition.
References
Wikipedia
WikiArt
Beyond the French Impressionists (this blog)
Supplements and corrections to the Catalogue Raisoné.
Holberton P ed (2006) Théo van Rysselberghe, Mercatorfonds, Centre for Fine Arts and Belgian Art Research Institute. ISBN 978 9 057 79080 5.