Stress, vigor, and recovery

Picture two nursery junipers side by side on a garden-center bench (an instructional example). Both are a deep, even green, dense and full.

Below the soil, they differ. One root ball is laced with firm roots and fresh white tips. The other sat at the dry end of the bench, missed by the sprinklers through a hot week; its root ball dried out completely, and most of its fine roots died (Lesson 2). Watered again, it looks just like its neighbor. Unless new roots grow, it will brown, but it may stay green for weeks or even months first.

If green cannot tell these two apart, what can?

Foliage color is a lagging sign. A leaf stays green as long as its cells keep their chlorophyll, and that can outlast the roots that supply it. Conifers are the extreme case, especially in cool weather, because their needles lose water slowly. Broadleaf trees usually show trouble sooner, by wilting, yellowing, or dropping leaves, though a symptom says only that something is wrong, not what.

Growth is a better witness, especially growth that continues. A swelling bud or a first flush of shoots can run on stored reserves, so on its own it shows only that the tree had reserves, not that its roots are working now. Shoots that keep lengthening and then harden, new leaves that reach full size, and root tips that advance over several weeks show a tree building new tissue with working roots and leaves.

What vigor means

In this course, vigor means a tree's observed growth performance compared with what that kind of tree normally does at that stage of the season.

• Observed. You see vigor in shoots, buds, leaves, and roots; it is not a hidden force.
• Compared with what that kind of tree normally does. In mid-spring, a healthy trident maple can add several pairs of leaves per shoot within a few weeks; a healthy juniper shows modest, paler green tips. Each is vigorous by its own standard.
• At that stage of the season. A trident maple with bare twigs in midwinter is resting, not weak (Lesson 13). The same bare twigs in early summer would be a serious warning.

Because vigor shows in how a tree changes, judge it from several observations over time, alongside what has recently been done to the tree.

Growers also call trees strong or weak, out of balance, or low on energy. In observable terms, strong and weak describe growth compared with what is expected. Balance usually means where growth goes, toward the parts with the most light (Lesson 12). Energy means carbohydrate supply and use: the sugars leaves make in light (Lesson 6) and the reserves stored in roots, trunk, and branches (Lesson 9), which you infer from how the tree has grown, never read from a photograph.

What to look for, and when

Five kinds of evidence are worth a minute at every check:

• Shoot extension. New shoots lengthening, then hardening (firming and turning woody from the base), at this tree's normal pace for the stage.
• Buds. Plump, firm new buds forming where leaves join the shoot and at the tips (Lesson 5). Small, shriveled, or missing buds suggest little to spare.
• Foliage. Color, size, and density, compared with this tree's past growth. Leaves that come out paler or smaller with each round of growth, for no reason you know of, say more than any single large leaf.
• Roots, when visible. White, active root tips at the drainage holes or along the pot edge show roots that are growing (Lesson 2); seeing none is an unknown, not a bad sign. Routine checks need nothing more, and a recently potted root ball can break apart if lifted out. When a root problem is suspected, a careful look becomes a diagnostic step (Course 07, Tree Health and Diagnosis).
• Recent history. What was done to the tree, such as repotting, pruning, wiring, or a move, and how it has responded since.

At each check, note the date and what you see, and take a photo from the same spot. Changes too slow to notice day to day, such as shoots a little shorter than last month, become obvious when dated photos sit side by side, and the record shows you this tree's own normal.

Weigh the evidence

In Is this tree ready for more work?, you judge three instructional-example trees. For each, reveal the observations, look back through the dated Observation record, and choose what the evidence supports. Two of the choices mention elective work: styling, pruning, or root work that you choose to do, as opposed to urgent care.

The three trees earned three different answers, and none came from a single observation. The maple's case rests on four weeks of agreeing records, and even then the answer is proportionate work: one planned job, not everything at once. The juniper's green foliage is only “Weak on its own,” and its unknowns are a reason to keep watching, not bad news. The ficus is the opposite: every card is marked “Sign of stress,” and the stresses are connected.

How stresses add up

Stress is any condition that pushes a tree's functioning outside its normal range and limits growth or survival: drought, waterlogging, heat, cold, poor light, root loss, pests, or heavy pruning. A healthy tree can usually recover from one moderate stress, given time and good conditions. Trouble comes when a stress is severe or several stack up, as they did for the ficus in case C, because they strain the same connected systems.

• Root loss cuts water and mineral uptake (Lesson 10) and removes some stored reserves. After root work, water loss can outrun uptake on a sunny, windy day (Lesson 8).
• Foliage loss cuts sugar production. Leaves supply the sugars that build new roots, and roots supply the water that leaves need, so losing a lot of both at once leaves each part waiting on the other.
• Environmental stress adds on top. Poor light lowers sugar production even when every leaf remains; heat and wind raise water demand; cold and pests bring their own damage.

The same logic explains why heavy root work, heavy pruning, and wiring with strong bends in one session can be more than a tree can support: root work reduces uptake and reserves, pruning removes sugar-making leaves, and strong bending can crack living tissue that must then be walled off and grown over.

Costs that surface a season later are common, because a tree can draw on reserves for a while. When stress continues, shoot tips may die back. This dieback, the progressive death of shoots or roots from the tips toward older parts, is a symptom with many possible causes. Several stresses together can lead to decline, a gradual loss of health and growth over months or years. Its early signs, slightly shorter shoots and smaller, paler leaves each season that recent work does not explain, are easiest to catch in a dated record.

Recovery takes leaves, roots, and time

Recovery is a tree's return toward normal growth and function after stress or work. It draws on what the tree has left: leaves to make sugars, roots to take up water and minerals, and reserves to bridge the gap. It needs conditions that let those parts work, such as good light and a root zone that holds both water and air, and it needs time: new roots and leaves take weeks to grow, and rebuilding reserves can take a growing season or more.

You can support recovery, but you cannot hurry it past what roots and light allow. Fertilizer supplies mineral nutrients, not energy (Lesson 10). It can correct a shortage, but a tree short of roots can absorb only so much, a tree short of light lacks sugars that no fertilizer supplies, and a strong dose around a reduced root system can harm the roots that remain.

What helps is correcting the conditions, holding off on elective work, and watching for the signs of vigor to return. No one can promise how long that takes; read it from the tree, not the calendar.

Trees seal wounds; they do not heal them

Every pruning cut, broken branch, or deep wire scar is a wound.

A tree responds in two ways. First, it walls the damage off. Wood exposed by the cut is open to air and to decay organisms. Within weeks, the living wood around it changes chemically and structurally, forming boundaries that resist the spread of decay inward, up and down, and sideways. This response is called compartmentalization.

Second, it grows over the wound. The vascular cambium, the thin living layer that makes new wood and bark (Lesson 3), is gone across the cut but survives at its edges. There it makes woundwood, often called callus: a rounded roll of new wood and bark that creeps over the exposed surface year by year. Wound closure is complete when the rolls from the edges meet.

In A pruning wound over five years, press Play with a vigorous tree and a small wound, and compare the front view with section A, a slice across the trunk at the dashed line. Once the wound is covered, set Section A to Along the trunk. Then press Replay with a Weak tree, and again with a Large wound.

On the surface, the story ends with a seam: within a few years, the small wound on the vigorous tree looks nearly whole. Inside, the discolored wood is still there, sealed behind its boundary and buried under new growth rings. The lengthwise section showed it reaching farther up and down than inward, because the boundaries that resist spread up and down, along the wood's water-conducting cells, are the weakest.

The weak tree made thinner rolls and narrower rings, so its wound stayed open longer, and its discolored zone can be larger. The large wound was still open after five years, even on the vigorous tree. Real closure speed varies widely with the kind of tree, its vigor, and the wound's size and position, and some large wounds never close fully.

For bonsai, two consequences follow. A vigorous tree covers wounds faster, one more reason to judge vigor before cutting. And a large cut permanently changes the trunk: it takes years to cover, if it ever does, and the damage stays inside. Where and how to cut, and whether to use a wound dressing, which growers debate, belong to Course 09, Pruning and Managing Growth.

Read several signs before more work

This lesson completes How Trees Work and its model of a tree: roots that need water and air, leaves that build sugars from light, separate routes for water and sugars, and growth that follows the seasons. After the synthesis checkpoint, Course 02, Choosing a Tree and Creating Its Growing Environment, will put that model to work, and Course 03, Watering and Daily Observation, will turn this observation habit into a routine; both are being prepared.