Saturday, January 11, 2014

Precocious is Precious, Part II

"The sun, with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else in the universe to do."  -Galileo Galilei

The general consensus regarding precociousness is that genetics plays a large part, but that certain environmental conditions must be in place to enable the expression of those precocious genes.

Seedlings from precocious strains of Quercus bicolor (Swamp White Oak) have been known to produce acorns in the 4th leaf.  Picture from HERE

It is the Sun, Silly

As the bud develops, it "decides" whether to be vegetative or fruitful based on the level of sugars (carbohydrates).  A high level of carbohydrates tips the bud toward fruitfulness.  A low level of carbohydrates tips the bud toward vegetative expression.

Sun + Leaves = Sugar = Acorns or Chestnuts

Grow a generous canopy as quickly as possible.  Take steps to ensure that the canopy is amply exposed to the sun and you will make many acorns and chestnuts.

It is almost that simple.

Plug the drain in the bottom sink

The tricky part is that the trees have many carbohydrate sinks.  Examples of carbohydrate sinks are leaves and branches that are shaded,  growing points (i.e. shoots that are extending) and developing nuts.

Picture from Genetic Improvement of Oak Trees

Certain tree forms are more favorable for precocious production.  Good tree form is not sufficient to create precociousness, but it can help enable it.
Picture from HERE
Pear trees are notorious for producing many side branches that have upright growth and compete with the central leader.  Those competing branches mutually shade each other.  Pear growers will frequently allow the tree to size up like the tree on the left, but will then prune out many of the branches and spread the remaining branches to create a tree that looks like the one on the right.  The tree on the right intercepts more sun than the tree on the left and no branch is significantly shaded by any other branch.  Virtually every leaf on the spreading tree is merrily photosynthesizing carbohydrates every hour of daylight. 

Few of us are likely to clamber about a 40' tall oak tree to spread tree limbs.  But we can manage the natural variation in the population by selecting for individuals that show desirable traits.  It is prudent to cull based on tree shape if/when no other selection criterion is available.

So the ideal tree shape for precociousness would be similar to a Pin Oak (Q. palustris) with moderate diameter limbs growing nearly perpendicular to the trunk.  But unlike a Pin Oak, there would be far fewer (like one-third to one-fifth as many) of the side branches to facility the penetration of sunlight into the crown.

Sawtooth oak exhibiting central leader, sparse side branching and flat branch angles

It might not be coincidence that many Sawtooth Oak (Q. acutissima) exhibit that kind of tree shape.  Sawtooth Oak is famous for producing many precocious individuals.

Carbohydrate - Nitrogen ratio

Fruit growers talk about managing fruit load by controlling the Carbohydrate - Nitrogen ratio.  That terminology is a little bit misleading.  The fruit grower can control the nitrogen level by varying the amount of fertilizer used, by adding or withholding irrigation and/or by managing competition from the sod on the orchard floor.

Adding nitrogen increases shoot extension and "sinks" more of the available carbohydrates.  Withholding water or by allowing the sod in the aisles between the trees to encroach into the herbicide zone reduces N availability and decreases shoot extension which results in the carbohydrate level rising.

So "nitrogen management" is the gas and brake pedal that the driver manipulates.  The actual heavy lifting of moving and stopping the car are done by the fuel injections and the brake pads.  Essentially, we control the nitrogen in an effort to manipulate the carbohydrate levels.

The dilemma is that one needs to make enough nitrogen available for the health of the tree and yet to not create so much shoot extension that the tree fails to set significant numbers of nuts.

Bottom line

Breed the best to the best or graft. Grow them quick.  Grow them big and strong. Be mindful of tree form.  Cull ruthlessly to enable sunlight to penetrate the crown from the side.  Back off the nitrogen after the tree(s) size  up.  If the soil has extreme natural fertility plant a cover crop to sponge up the excess N starting in early-summer.

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