Why do leaves fall in autumn? The obvious answer is that there’s a high upkeep cost to them, and during winter there is little sunlight, so tree leaves should fall down then. That is, they should be deciduous trees. So trees in Montreal or London be deciduous. Whereas in places like the Mediterranean, tree leaves shouldn’t be deciduous, they should be evergreen. And it is so. Evergreen trees dominate in temperate, consistently sunny climates, whereas in places like the UK, only 10% of tree species are evergreen.
But that 10% figure is confusing. Doesn’t it imply that evergreen trees are also a successful strategy in places with (relatively) sunless seasons? What advantage do deciduous trees have then? Also, if sunless seasons → mostly deciduous trees, what’s up with the poles? They get way less sunlight.
AFAICT, evergreen trees near the equator are more like deciduous trees in the UK or Japan than they are like evergreen trees near the poles. In some ways, equatorial evergreens are like deciduous trees on steroids.
Deciduous leaves are optimized to photosynthesize as much as possible. Relative to evergreen leaves, they have ~ 3x higher upkeep costs in return for ~ 4x higher energy production per gram per unit time. So while they’re active for 1⁄2 the time, they make more energy, total, than evergreen leaves. But in autumn/winter, they cost too much in upkeep to be worth keeping. So deciduous trees re-absorb chlorophyll (freezing can denature proteins) and other nutrients from the leaves, leading to the leaves browning and falling off.
Numerically, these upkeep costs are 1⁄12 that of gross energy production for both deciduous and evergreen leaves. After accounting for the 3x increase in activity and 1⁄2 the year being leaf free compared to evergreens, that means deciduous trees pay 50% more in upkeep without accounting for the costs of re-creating the leaves each season.
In practice, this means deciduous leaves are pretty thin with high surface area, getting lots of sunlight for relatively small amounts of mass. There are relatively little support cells; most of the cells are for photosynthesis. Which makes them quite flimsy. They can collect lots of rainwater or snow, which is a big disadvantage in winter.
Compare this to evergreen leaves in trees originating around the poles. They’re much smaller, almost like needles. Whereas evergreen leaves are optimized for surviving the harsh winters. That means they have smaller surface areas/mass ratios for durability, have relatively fewer cells devoted to photosynthesis/gram, have waxy coatings to protect them from the cold but also reduce the sunlight they can absorb, and have anti-freeze inside of them to prevent damage in winter.
The whole “water freezes in winter” bit leads to another important factor in why leaves fall off deciduous trees. When water in the ground freezes, deciduous trees can’t absorb any water, but their energetic leaves could keep releasing large amounts of water. This would cause the tree to dry up and die. Better to jettison the leaves than risk that. This is less of a problem for evergreens as their leaves have low surface area/mass ratios, have coatings and close their pores in winter. But it’s still an issue.
Two notable things I haven’t mentioned yet. One, evergreen and deciduous leaves have the same energy production/construction cost ratios. In a really dumb model of energy production per gram (e), leaf life (T) and construction costs per gram (C) we find that:
e_D/e_E = T_E/T_D C_D /C_E
Evergreen leaves live 6x longer than deciduous leaves on average. We also know e_D/e_E ~ 3. So that implies so that implies C_D ~ 1⁄2 C_E. Which is somewhat surprising but I guess it makes sense in retrospect. Evergreen leaves are the low cost, low output, steady output, long lifetime counterpart to deciduous leaves.
I don’t think I’ve got an answer for why we see not insignificant numbers of evergreen trees in places like Montreal or London. But I do feel like it makes sense why we don’t see deciduous trees near the poles.
Another big complication I left out are the deciduous trees in (non frozen) deserts. What’s up with them?
Why do leaves fall in autumn? The obvious answer is that there’s a high upkeep cost to them, and during winter there is little sunlight, so tree leaves should fall down then. That is, they should be deciduous trees. So trees in Montreal or London be deciduous. Whereas in places like the Mediterranean, tree leaves shouldn’t be deciduous, they should be evergreen. And it is so. Evergreen trees dominate in temperate, consistently sunny climates, whereas in places like the UK, only 10% of tree species are evergreen.
But that 10% figure is confusing. Doesn’t it imply that evergreen trees are also a successful strategy in places with (relatively) sunless seasons? What advantage do deciduous trees have then? Also, if sunless seasons → mostly deciduous trees, what’s up with the poles? They get way less sunlight.
AFAICT, evergreen trees near the equator are more like deciduous trees in the UK or Japan than they are like evergreen trees near the poles. In some ways, equatorial evergreens are like deciduous trees on steroids.
Deciduous leaves are optimized to photosynthesize as much as possible. Relative to evergreen leaves, they have ~ 3x higher upkeep costs in return for ~ 4x higher energy production per gram per unit time. So while they’re active for 1⁄2 the time, they make more energy, total, than evergreen leaves. But in autumn/winter, they cost too much in upkeep to be worth keeping. So deciduous trees re-absorb chlorophyll (freezing can denature proteins) and other nutrients from the leaves, leading to the leaves browning and falling off.
Numerically, these upkeep costs are 1⁄12 that of gross energy production for both deciduous and evergreen leaves. After accounting for the 3x increase in activity and 1⁄2 the year being leaf free compared to evergreens, that means deciduous trees pay 50% more in upkeep without accounting for the costs of re-creating the leaves each season.
In practice, this means deciduous leaves are pretty thin with high surface area, getting lots of sunlight for relatively small amounts of mass. There are relatively little support cells; most of the cells are for photosynthesis. Which makes them quite flimsy. They can collect lots of rainwater or snow, which is a big disadvantage in winter.
Compare this to evergreen leaves in trees originating around the poles. They’re much smaller, almost like needles. Whereas evergreen leaves are optimized for surviving the harsh winters. That means they have smaller surface areas/mass ratios for durability, have relatively fewer cells devoted to photosynthesis/gram, have waxy coatings to protect them from the cold but also reduce the sunlight they can absorb, and have anti-freeze inside of them to prevent damage in winter.
The whole “water freezes in winter” bit leads to another important factor in why leaves fall off deciduous trees. When water in the ground freezes, deciduous trees can’t absorb any water, but their energetic leaves could keep releasing large amounts of water. This would cause the tree to dry up and die. Better to jettison the leaves than risk that. This is less of a problem for evergreens as their leaves have low surface area/mass ratios, have coatings and close their pores in winter. But it’s still an issue.
Two notable things I haven’t mentioned yet. One, evergreen and deciduous leaves have the same energy production/construction cost ratios. In a really dumb model of energy production per gram (e), leaf life (T) and construction costs per gram (C) we find that:
e_D/e_E = T_E/T_D C_D /C_E
Evergreen leaves live 6x longer than deciduous leaves on average. We also know e_D/e_E ~ 3. So that implies so that implies C_D ~ 1⁄2 C_E. Which is somewhat surprising but I guess it makes sense in retrospect. Evergreen leaves are the low cost, low output, steady output, long lifetime counterpart to deciduous leaves.
I don’t think I’ve got an answer for why we see not insignificant numbers of evergreen trees in places like Montreal or London. But I do feel like it makes sense why we don’t see deciduous trees near the poles.
Another big complication I left out are the deciduous trees in (non frozen) deserts. What’s up with them?