How Bonsai Grow: Secondary Growth

Its been a busy week at the day job, dealing with a quick turn-around database job, reviewing Iraq S&T proposals, keeping the Plone sites spinning along, and handling some political issues surrounding WACSI. Somewhere in there were repairmen all over the house, keeping an eye on Terrance, watching the new transplants in the gardens what with 97 F weather, and the usual groceries and laundry. No matter, I still had time to update the Travel Schlepp website, moving all the podcasts onto a Comcast filespace and fixing the permalinks and xml.

So here it is 1:30 AM and what better time to continue our discussion of secondary growth in woody dicots. Since the last installment, I came across an excellent PowerPoint reference for those who would like a well illustrated, basic explanation of how plants grow.

On a more technical side, Groove and Robischon at the USFS have published a detailed review of the current state of knowledge. Three key areas are identified plus the interactions among them:

• Transcriptional regulation (microRNAs, differential gene activity, etc.)
• Phytohormones (auxins, cytokinins, and gibberellins)
• Cell wall regulation (hydrolysis, Xylogen induction, etc.)

Cytokinin is primarily produced in the roots, although probably all meristems generate some. From tissue culture experiments we know that low cytokinin to auxin ratios promote root growth; high C/A ratios promote shoot and lateral meristem growth. Considering that auxins are produced in the apices, this makes perfect sense. One important observation is that over-expressing a cytokinin gene reduces xylem formation.

From the point of view of growing a thick trunk on a bonsai, one would probably want to keep cytokinin production relatively low. However, to enhance the above ground portion of the tree, we would like to see a high C/A ratio. That means reducing the auxin supply as well.

When you consider that managing a bonsai involves tip pruning as well as root pruning, this begins to come together. When I keep the rootball small and vigorously nip the apices of my junipers, I not only am reducing apical dominance and getting a bushier tree, I am probably controlling the C/A ratio to enhance secondary thickening. By maintaining the plant in a shallow pot, we further spread out the surface roots and develop the much sought after nebari.

Bonsai as Ecology

It occurs to me that a useful start to my ecological/botanical blog would be to take a systematic look at some convenient microcosm and discuss it thoroughly. My bonsai trees seem tailor-made for this. So here we go with a series of essays on "How Bonsai Grow."

In the front courtyard under the shade of an old Photinia tree along the portal, I display my ragged little collection of mostly temperate bonsai during the spring, summer and fall. In winter they all get packed away in the back of the garage underneath a friendly skylight. Right now they're all doing well except a Ponderosa pine I harvested from the wild two months ago and a Taxus that has been struggling to hold on in New Mexico's low humidity for more than a year.

The tropicals usually find them selves on the back porch where they get higher temperatures, a good shot of morning sun, and some dappled sunlight in the afternoons. In the winter they must come all the way indoors, being secreted all over the house.

Bonsai means "growing in a tray," referring to the generally low profile of the traditional pots. Some styles, notably cascades and semi-cascades, use pots with higher aspect ratios, but by and large one is looking for a pot that is roughly the same height as the thickness of the tree's trunk.

Meanwhile, the volume of the pot is going to be approximately 1/3 of the volume of the tree's canopy. The pot's proportions are dictated by traditional formulae for esthetics and their relationship to the height and width of the branches. As you can guess, this doesn't leave us much room to work with.

So, back to the topic at hand, how do bonsai grow? Fortunately, there are only four mechanisms in plants and one of those is restricted to monocots, which are rare in bonsai. These are:

1. Primary apical shoot meristems -- the growing tips of the branches, producing new shoots, flowers, and leaves (some would count floral meristems separately)
   
2. Primary apical root meristems -- the growing tips of roots
3. Intercalary meristems and primary thickening -- the means of increasing height and diameter in horsetails and monocots; primary thickening meristems are found in arborescent monocots, palms and such
   
4. Secondary thickening meristems -- lateral meristems or cambium that increase the diameter of the plant (these produce xylem and phloem as well as bark)
   

Of course, the object of growing bonsai is, as an art form, to evoke an emotion in the viewer. Usually the bonsai artist attempts to capture the age and grace of a vast and ancient forest dweller in miniature.

This in turn usually means that we want relatively short trees (typically under 3 or 4 feet) with very wide trunks. The Japanese term that describes a desirable wide flaring of the trunk into obvious surface roots is nebari.

To grow trees with these characteristics, one has two choices: harvest a naturally formed beauty from the wild or to carefully cultivate nursery stock to develop the desired form. Therefore, we must control growth such that secondary thickening of the trunk is encouraged and primary apical growth is limited.

We'll continue in a later posting on the topic of the control of meristematic activity, what stimulates them and what suppresses them. This will lead us to shoot-root balance, control of primary vs secondary growth, and plant nutrition.

Introducing Yet Another Blog

I've been developing for the Web since '94 and blogging since '99 (although I didn't know it by that name then). At the moment I've got two blogs going:

1. Plone Metrics for my Plone advocacy and marketing thoughts, and
2. Travel Schlepp's Dispatches for my travel and children's tales.

That has left a gap that is now being filled by this series. Here I want to address things biological, horticultural, ecological, and systematic. This is where I'll be posting my thoughts on backyard biology, urban gardening, NM high desert ecology, and systematics of society and culture. I'll even be able to tie in botany, plant anatomy, plant physiology, and growing bonsai.

Stay tuned for items on how Terrance, my resident ornate western box turtle, is doing, how I'm reducing water use in a typical yard in Albuquerque, which birds are passing through, how the bees are bouncing back, and what ecology and systematic biology can tell us about the presidential election process.