Why Do Trees Have Leaves On The North Side?

[Rev. Green]

Trees are symmetrical. Why haven't some evolved with more leaves in sunlight, the South side in the Northern hemisphere.

[sandyRoe]

Could it be a question of balance?

[Chipchopper]

I think that many trees will seem to search for light and therefore have more leaves on the side where there is the most light.

[Hopkirk]

Perhaps the leaves are efficient enough to utilise any light available, not just direct sunlight.

[IJKLM]

Trees have a number of strategies:
1. they try to grow taller than their neighbours to catch more sunlight (and deprive their neighbours of the light)
2. The branches carrying the leaves tend to grow toward areas of bright sunlight and away from shade
3. the leaves orient themselves toward the sunlight by moving around their stems during the day.

bear in mind that the sun moves from east to west during the day, and leaves on the north side can still capture light in the morning and evening.

Also, the leaves do not need direct sunlight: indirect light is enough for them to photosynthesise.

[theprof]

This entire question comes under the subject of phototropism.

Regardless of whether we’re discussing deciduous or evergreen trees in the northern hemisphere, there is little justification for evolution to take a hand here. An important factor has already been mentioned – the fact that the sun rotates around us in an East/West direction. The light requirements for photosynthesis to take place are very minimal and so as long as a reasonable amount of sunlight reaches the leaves for a few hours daily, that’s adequate for the tree to photosynthesise. It’s a misconception that bright sunlight is needed for the purpose.

Deciduous tree leaves do indeed rotate via their petioles to maximise the gathering of bright sunlight, but it varies from species to species and in general, the ability to do so is not vital for the tree. Conifers and other species demonstrate a similar but lesser ability to rotate their needles towards the sun. Nevertheless, leaving aside conifers, Holly species tend not to demonstrate leaf rotation and neither do cycads and most Eucalyptus species.

You also must consider trees that grow on the sides of steep, forest slopes where the individual trees gain more light from the downward angle than the upward angle. In these circumstances, there is little to gain from leaf or needle rotation as the trees behind others are effectively hiding the sunshine from those below them due to their higher elevation. The slope itself will also intercept the incident light. Vast forests throughout the world manage to photosynthesise with negligible phototropic response in these circumstances.

Phototropism is a vast and fascinating subject as are all tropisms and I’ve only skirted around the subject in my answer to be honest. Stem, branch and trunk phototropism is another matter.

[jno]

the north side of trees will have direct sunlight in summer, morning and evening.