A rule of thumb is to light evergreens from an angle and light deciduous trees from below. But it really comes down to viewing areas, sight lines, desired effect, and how well all the lights fit together in the overall design. You may want a given tree to draw the eye more or less, and should adjust the angle of light accordingly. There is no right or wrong approach, just personal preference.

An evergreen tree lit from an angle, and a deciduous tree lit from directly below.
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The photo to the right shows an example of both options on side by side trees so you can compare.

The evergreen tree (left) is shown with the light cast at an angle from a short distance away from the tree. This highlights the tree’s surface and shape, and creates a texture of shadows within the tree itself. The farther the point of light, the less pronounced the shadow effect, so generally you want to find the “sweet spot” that maximizes the highlights. The thicker and more dense the tree, the more this effect stands out. This is often the only way to light an evergreen because its thick foliage will block too much of any light that comes from below.

The deciduous tree (right) is lit from directly below, with the light being cast up inside the tree. With large trees, this results in less of the tree’s overall volume being illuminated when the tree is in bloom, because the lower leaves block some of the light from getting through. You can see how the evergreen is catching more light than the deciduous tree at this point in the season. However, when the tree is not in bloom then the light still skims all the way up the trunk and beautifully light the branches, highlighting the tree’s unique shape and silhouette. This gives the tree different effects throughout the year, and much more impact during the months it is not in bloom. It’s also amazingly lovely for a tree that will be coated in snow or ice during winter, because the light will literally sparkle across its branches.

On the other hand, if the light on the deciduous was from a distance, the tree would “pop” more and be brighter when in bloom, especially in colourful Autumn. But it would fade more into the background in Winter and Spring. That’s the main thing to keep in mind: when selecting a lighting angle for a deciduous tree, you are effectively picking which seasons you want the tree to take centre stage.