Leaves and needles: working surfaces

When you look at a bonsai with scissors in hand, you see a silhouette: the line of the trunk, the outline of the canopy, a crowded spot you would like to open up. The tree experiences something else. Every leaf or needle is a working surface that supplies the rest of the tree, and each one you remove leaves the tree less to work with, whatever happens to the outline.

What leaves and needles do

Foliage is the general word for a tree's leaves or needles. The three-lobed leaves of the trident maple, the glossy leaves of the ficus, and the short needles of the nursery juniper all do the same three linked jobs:

• They capture light, the energy behind nearly everything a tree builds.
• They carry out gas exchange with the air. In light they take in carbon dioxide and release oxygen, and they lose water vapor; their living cells also use oxygen for respiration, day and night.
• They make carbohydrates, the sugars and related compounds that become wood, roots, buds, and new leaves, and that fuel the tree's living cells.

Inside a leaf, layer by layer

A flat leaf, like the trident maple's, is a fraction of a millimeter thick, yet it is built in layers, each with a job. From the top down:

• The cuticle: a thin, waxy film that greatly slows water loss from the surface.
• The upper epidermis: usually a single layer of clear cells that protects the leaf and lets light through.
• The mesophyll, the leaf's working interior. Tall, tightly packed palisade cells directly under the upper epidermis catch most of the light. Below them, the looser spongy layer has air spaces between its cells, so gases can reach every cell.
• Veins, running through the mesophyll, each bringing water in through its xylem and carrying sugars out through its phloem, the transport tissues you traced through the trunk in Lesson 3.
• The lower epidermis, dotted with microscopic pores called stomata (one pore is a stoma). Two curved guard cells surround each pore and control how wide it opens. On many trees with flat leaves, most or all of the stomata are on the underside.

How chloroplasts capture light

Mesophyll cells hold many tiny green bodies called chloroplasts, most densely in the palisade layer. Chloroplasts are packed with chlorophyll, a pigment that absorbs red and blue light strongly and green light less well, so leaves reflect more green than other colors and look green.

Chloroplasts use that captured energy to join carbon dioxide from the air with water from the roots, building sugars and releasing oxygen. This is photosynthesis, the subject of Lesson 6. The leaf's layers bring its three supplies together in the same cells: light through the upper surface, water through the veins, and carbon dioxide through the stomata.

Needles and scales: same jobs, different packaging

A broadleaf tree, such as the trident maple or the ficus, has flat, usually wide leaves and bears its seeds in flowers and fruits. A conifer, such as a pine, juniper, or larch, usually has needle-like or scale-like leaves and bears its seeds in cones (berrylike ones, in junipers).

Conifer foliage has the same chloroplasts and stomata and does the same jobs, though a needle or scale usually has one or two veins along its center instead of a branching network. Much of it is built to lose less water. A thick cuticle slows water loss. The stomata are often sunken in small pits and lined up in rows; on many pines and junipers, those rows show as pale lines or bands along the needles, whitened by wax. Many conifers, including pines, also carry resin canals, a defense against insects.

Many junipers carry two forms: prickly, needle-like juvenile foliage and adult foliage of tiny scales pressed flat against the shoot. Some junipers, including the nursery juniper's cultivar, keep needle-like foliage as mature plants, so foliage shape alone does not show whether a juniper is old enough to produce cones.

A second pair of words describes how long leaves last. An evergreen keeps living foliage all year, replacing it gradually; a deciduous tree drops all its leaves for part of each year and grows a new set when growth resumes. The two pairs combine in every direction:

Tropical or temperate is a third, separate description, which Lesson 13 takes up. None of these labels, on its own, tells you how to care for a tree.

Stomata: one doorway, two directions

Photosynthesis needs carbon dioxide, and stomata are its way in: the cuticle that keeps water in also keeps gases out.

Opening a stoma has a cost. The inside of a leaf is moist and the air is usually drier, so whenever a stoma opens to let carbon dioxide in, water vapor escapes through the same opening. This loss of water vapor is transpiration, and it is not small: a leaf loses hundreds of water molecules for each molecule of carbon dioxide it takes in, and most of the water a tree absorbs leaves this way (Lesson 8).

So every leaf and needle lives with a tradeoff, and its guard cells manage it: they swell and open the pore when they take up water, and close it when they lose water. Answer the prediction at the top of the simulation first. Then change one condition at a time, watching the guard cells and the Carbon dioxide entering and Water vapor leaving meters, and try Night and Drought.

Three tendencies drive what you saw. Light opens stomata: wide in bright light, part way in dim light, and mostly closed in darkness, when no carbon dioxide comes in. A shortage of water at the roots narrows them through a hormone signal that builds up at the guard cells; when water is very short, they close almost completely, even at midday. Dry air pulls water out faster through any opening without bringing in any more carbon dioxide, and wind sweeps away the moist air at the leaf surface, which speeds water loss while doing little for carbon dioxide uptake. Many leaves respond to dry air by narrowing their stomata part way.

Narrowing always trades one gas against the other. On the hot, dry, windy afternoon, the pores narrowed to a slit. Water loss was still higher than on the calm morning, but far below what wide-open pores would lose in that air, and carbon dioxide fell from high to low, so sugar-making slowed. A bonsai in a small pot can run short of water quickly on such a day, and its stomata may narrow well before any leaf wilts. A tree often short of water makes less to grow with, even if it never looks stressed.

Which foliage makes the most sugar

How much carbohydrate a tree can make depends on its working surface: how much healthy foliage it has, how much light that foliage receives, and what condition it is in.

Test your prediction on the model tree. Remove pad 10, deep in the shade, then restore it and remove pad 1 at the top. Then compare Remove 3 shaded pads with Remove 3 sunny pads, and switch Tree condition to the weak tree.

The two pictures disagree on purpose. Removing the three shaded pads left the outline unchanged and barely moved production; removing the three sunny pads changed the outline and cut production sharply. Each layer of foliage shades the one beneath, so the top and outside of a canopy make the largest share, while leaves deep inside may get only a small fraction of full sun. Switching Tree condition changed the outlook but not the production meter, for reasons the next section explains.

Shading works the same way without a cut. Foliage that gets less light, whether under dense growth above or in a dim spot indoors, makes less, though the outline does not change. Inner shoots that stay in deep shade tend to weaken, and on bonsai they are often the first to thin and die back (Course 14, Refinement and Long-Term Stewardship).

What this means when you shape a tree

When you remove foliage, the tree loses what that foliage was making. It rebuilds from the foliage that remains and from reserves stored in its wood and roots, and each new leaf imports sugars until it is well grown (Lesson 9). On many conifers, needles work for several years, so a branch's foliage builds up over several seasons and is slow to replace. Fertilizer cannot fill the gap: it supplies mineral nutrients, not carbohydrates.

How much the loss matters depends on the tree's condition. A healthy, vigorous tree, one growing strongly for its kind and the season, generally has the reserves and remaining foliage to rebuild, though new growth tends to come where light is good. A weak or recently repotted tree may not: its new roots already depend on sugars from its leaves, so the same removal deepens its shortfall. Judge condition from growth seen over several observations (Lesson 14), not from leaf color alone, which has many possible causes (Course 07, Tree Health and Diagnosis). Deliberately removing most or all of the leaves is a technique for particular vigorous trees and aims, not routine maintenance; Course 14 will cover it.

Where the foliage sits matters too. Take an instructional example: you strip the inner half of a branch and leave a tuft of foliage at its tip. The trident maple can generally regrow along that bare stretch later. The nursery juniper may never do so, because like many junipers it does not reliably produce new buds on bare, older wood (Lesson 5). Interior foliage makes little sugar, but it may be where future growth has to come from.