Sap rising

[LeMartinPecheur]

A few weeks ago I was concerned that if our garden's big beech trees came quickly into leaf after the warmish winter, they might get blown down by a late equinoctial gale - we were having quite a few at the time down here in Cornwall.

It hasn't happened so far, but in watching the advance of green shoots on trees this year I've been asking myself a question that hadn't occurred to me before: Why do small trees get their leaves so much earlier than full-grown ones?

My wife thinks it's purely an effect of gravity: sap taking longer to rise all the way up a big tree. But I've been wondering if trees have 'learnt' that they take a big risk by having too many leaves too early, and have therefore evolved some mechanism whereby they come into leaf later as they get bigger.

A tiny detail that I think may support the latter hypothesis is that I don't actually see much evidence that a big tree's lower branches come into leaf before the very top ones, which surely ought to be the case if it's all down to gravity.

[umslopogaas]

Hmmm ... this puts me in a rather uncomfortable position, because as a plant biologist I should know the answer, and I dont. However, one possible reason is exposure. These functions like leaf expansion are temperature dependent, and small trees will be a bit warmer than big ones, because they are sheltered from cold winds and also are protected by the big ones from radiating heat to clear skies.

There is also the mysterious question of root pressure. I lost count of how many times I had to write an essay on How Water Gets to the Top of Tall Trees, but I never found a fully satisfactory explanation. Anyway, however they do it, big trees need to work harder to get the necessary water to the buds to make the leaves expand: they need more water, and they need to pump it higher.

There may also be an advantage in light exploitation. Small trees are overshadowed by big ones, which steal their light, so reducing the amount of energy available for growth. By coming into leaf earlier, the overshaded small trees can make a head start before the big ones get going.

[umslopogaas]

Hmmm ... this puts me in a rather uncomfortable position, because as a plant biologist I should know the answer, and I dont. However, one possible reason is exposure. These functions like leaf expansion are temperature dependent, and small trees will be a bit warmer than big ones, because they are sheltered from cold winds and also are protected by the big ones from radiating heat to clear skies.

There is also the mysterious question of root pressure. I lost count of how many times I had to write an essay on How Water Gets to the Top of Tall Trees, but I never found a fully satisfactory explanation. Anyway, however they do it, big trees need to work harder to get the necessary water to the buds to make the leaves expand: they need more water, and they need to pump it higher.

There may also be an advantage in light exploitation. Small trees are overshadowed by big ones, which steal their light, so reducing the amount of energy available for growth. By coming into leaf earlier, the overshaded small trees can make a head start before the big ones get going.

[umslopogaas]

Woops ... ignore repeat, I clicked something by mistake.

[ardcarp]

Why do small trees get their leaves so much earlier than full-grown ones?

Probably good ole evolution at work, Kingfisher. I wish I could feel my sap rising. The Spring of the soul hasn't Sprung yet. Perhaps that's evolution too. The world doesn't need out-of-control geriatrics cavorting around en masse.

[jean]

I wonder why trees of roughly the same size -beech trees in particular? - come into leaf at different times.

There used to be a beech hedge planted all at the same time around an open air swimming pool I frequent, and I used to enjoy watching the individual trees greening over a period of weeks.

But then, the pool's owners noticed that the beech leaves fell into the water over a long period too, and they chopped it down.

[umslopogaas]

Jean, its because they are genetically variable. Hedge trees would be raised from seed, rather than cuttings or grafts. Each one is therefore slightly different in its genetic make up. Each one will therefore react slightly differently to the environmental stimuli like temperature and day-length. Its the same as us, each one of us is genetically distinct and we reach differently: some people tan in response to sunlight, others (like me) go pink and then peel.

[french frank]

I know nothing about trees but was astonished to see that my broad beans which have been hideously slow and miles away from flowering have apparently had a 'growth spurt' over night. I thought it was rather cold last night.

[umslopogaas]

ff, that is rather hard to explain, because growth is definitely temperature-dependent. Are you sure that the growth didnt actually occur the previous day, but that you didnt notice it until the following one? At this time of year when days are warming up, if you take your eye off a plant for a couple of days it can certainly seem to have suddenly jumped out of the ground.

The only thing I can think of that might explain overnight growth is that a warm sunny afternoon might have heated the soil sufficiently that the plant was able to benefit from the soil warmth during the night even if the air temperature was quite cold.

[french frank]

The only thing I can think of that might explain overnight growth is that a warm sunny afternoon might have heated the soil sufficiently that the plant was able to benefit from the soil warmth during the night even if the air temperature was quite cold.

I'll buy that one I was busy yesterday. I don't usually take my eye off the beans: the runners were sown too early and I'm leaving them out overnight, partly to harden them, partly because they'll have to be planted far too early and will probably then be lost. Such a pain being an ignorant gardener.

[umslopogaas]

I'll buy that one I was busy yesterday. I don't usually take my eye off the beans: the runners were sown too early and I'm leaving them out overnight, partly to harden them, partly because they'll have to be planted far too early and will probably then be lost. Such a pain being an ignorant gardener.

Its worse if people think you are a knowledgeable gardener, because things still go wrong with monotonous regularity and you havent even got the excuse of ignorance. I am in exactly the same position with runner beans: I planted them under glass too early, now they are growing very rapidly but its too early to put them out. I guess I just put them in bigger pots and risk the conservatory becoming a bean jungle.

[french frank]

I am in exactly the same position with runner beans: I planted them under glass too early, now they are growing very rapidly but its too early to put them out.

(sorry ) I feel better. I'll try and hold them until the second week in May and then they'll have to take their chance. If I had a hair drier I'd be out trying to warm the earth.

Someone suggested popping a bean in beside each plant, so that if the plant failed the new bean might get a bit of a head start.

[jean]

Broad beans you can overwinter if you sow them outdoors in October.

Not runner beans though - but I'm surprised to hear you say it's too early now; everyone down the allotments has got theirs planted out already and they look fine, except for the ones the slugs have eaten.

But all the ones planted at the same time seem to develop at the same rate. Unlike the trees!

[french frank]

I'm surprised to hear you say it's too early now; everyone down the allotments has got theirs planted out already and they look fine, except for the ones the slugs have eaten.

Everyone knows their plot, I suppose, and what they normally do, but I wouldn't risk, even down here, until mid-late May. Even if the frost risk is over, the earth isn't usually warm enough.

No harm in taking a risk, but one of my plants has already taken a bit of a knock through being left out at night (about a week ago).

[umslopogaas]

Broad beans you can overwinter if you sow them outdoors in October.

Not runner beans though - but I'm surprised to hear you say it's too early now; everyone down the allotments has got theirs planted out already and they look fine, except for the ones the slugs have eaten.

But all the ones planted at the same time seem to develop at the same rate. Unlike the trees!

You Liverpudlians are an optimistic lot! Either that, or Liverpool is a lot warmer than I thought it was. Runners wont stand frost, so I guess its OK to put them out now providing you keep a beady eye on the weather forecast and are prepared to rush out with old sheets and bubble wrap to protect them if frost is forecast. I'm in east Devon, which I think is milder than Liverpool, but I think I'll keep mine under cover for a couple of weeks yet. And my strawberries are about to flower, so I need all the old sheets to keep the frost off them.