The growth and ultimate form of English Elm
Perhaps the most frequently asked of all questions about the modern elm cultivars is: will they look like English Elm?
The aim of this note is to provide some insights into why English Elm looked as it did – typically a figure of eight shape – and to warn that characteristics currently sought by the tree nursery trade may actually be those least likely to produce any approach to a look-alike.
But first,back to the question. Will the new elms look like English Elm? The straightforward answer is that they almost certainly will not. However, this is a moment at which the glass may be perceived to be either half empty or half full. On the one hand, not even Ulmus minor, the field elm, (of which English Elm is merely a variety) tends to look identical to English Elm. On the other hand, even an elm as obviously unlike English Elm as Sapporo Autumn Gold can add an elm-like note to a landscape more effectively than any other species. English Elm understood as an object is unlikely ever to be precisely copied through the breeding of new clones. English Elm, if understood as thelandscape presence of cohesive crowns of terminal twigs roofing distance without blocking it, can certainly be approximated.
What follows would ideally be illustrated by 150 annual photographs of a given tree from its first year onward. Time being short, words will largely have to do.
The new elms which appeal to the nursery industry are monopodial (they have a strong central trunk and light branching from the outset) and have relatively narrow crowns. These are trees whose growth is predictable, proportioned to the restricted space of urban sites, unlikely to cause costly problems, and easy to handle in the nursery. There is little economic incentive for industry investment in trees which do not meet those criteria, regardless of their putative value to the open landscape. English Elm, even if it stood a chance of survival nowadays, would certainly be passed over.
The crucial characteristic of English Elm is that in its very early years it is not obviously a monopodial tree. At the end of its first year, it will be a more or less upright shoot, probably slightly recurved at its tip. However in the following year, it is likely that the terminal buds will develop into co-dominant leaders. At this stage it seems improbablethat a single straight trunk will be formed.
Nevertheless, by the end of the tree’s second year, the influence which will enable it despite present appearances to form a towering vertical trunk may already be very subtly apparent – one of the leading shoots is likely to be further from the vertical than the other, and may have made slightly less growth. The other is prevailing; the phenomenon of apical dominance is beginning to make its mark.
In the third year, both branches are likely to produce two or three strongly growing shoots. At least one of those from the year 2 branch which was the more leaning may now be at a very lax angle, and its weight will force that parent branch further from the vertical. The tree’s form would now seem ruined; but from the more vertical year 2 branch one of the new shoots will probably be close to vertical. It will exert apical dominance over all other growth, and the shoot which bore it will now gain in girth over its rival.
The form of the sapling is opening up. The same process is repeated for perhaps 20to 30years. Almost always, one shoot will correct or over-correct the deviation from vertical in the preceding year’s shoot from which it arises. This will be the shoot which tends to make the most vigorous growth. A trunk of sorts is beginning to form. It will not be a straight vertical, but a series of near approaches to the vertical quite possibly with one or two abrupt kinks. As the trunk gains more girth, the lowest kinks will begin to disappear.
The ultimately monopodial character of English Elm is thus the product of successive indirections and subsequent corrections.
Meanwhile, the would-be leaders which lost the competition have been forced into growing outwards from the trunk, and although they remain vigorous branches they are beginning to make less growth than before. It may be horizontal growth or below horizontal, because the bulk of the young tree above is beginning to cast a depressiveshade. Shoots from what are now side-branches will dart out in whatever direction light is to be had, but because of their angle they will have reduced vigour. Were they over-vigorous, as in Dutch Elm, that habit would drastically affect the shape of the mature tree, leading to a broadly spherical shape and an open, ragged crown.
The very oldest and lowest side shoots are now shaded out completely, and will either die or at least cease extension growth.The young tree is now a bulkyand amorphous pyramid, ragged at the bottom and broad a little higher up, where the side branches have had longest to reach out. One would look in vain at this stage for any hint of what the structure is to become. That should be borne in mind when assessing modern elm cultivars.The best of them are likely to go through a similarly amorphous phase.
Depending on soil, hydration, climate, exposure , latitude, and the proximity of other trees of similar height (and leaving out of account extreme situations), this phase may see the tree reach 30 to 60 feet in height. That assembly of variables determines the energy which the tree has for extension growth before it switches to maximising its photosynthetic potential with a view to the production of seed.
Extension growth and seed production are in complex balance. The setting of seed, even if it is in fact infertile,is debilitating and reduces the plant’s vigour; in fact in some years seed-set has apparently been so heavy that observers reported the near-death of elms from the effort involved. Seeding is nevertheless needed for reproduction (the fact that English Elm has actually evolved to be self-sterile is irrelevant).Meanwhile, early extension growth is essential in order to produce the largest viable structure for the laterproduction of seed, and the requisite exposure to light.
At the stage in the tree’s development when seeding begins in earnest (notably late in the case of English Elm and sparse in comparison to some other elms), its capacity for vigorous extension growth lessens, and apical dominance diminishes. The result seems to be that amongst the fan of new shoots at the tree’s apex the tendency for the most vertical to predominate over the others is far less than it was in the younger plant. Branches developing at a considerable angle to the vertical are able to take almost as much of the tree’s energy as do those which are more erect. In consequence a dome-like crown begins to form. In it, a continuation of the main trunk may be found, or it may not, depending on the tree’s vigour. Persistence of a central trunk high into the canopy was frequent in the ideal growing conditions of the Vale of Gloucester, parts of Berkshire, and Middlesex. In the somewhat less favourable environment of Wiltshire or Devon (for example), a crown lacking a central axis was the rule.
Whichever is the case, the tree has reached its junction. Below is the central trunk surrounded by initially strong side branches depressed later in their development by the assertion of dominance at the tree’s apex, and by its shade. Above, the crown begins to develop, now that apical dominance is waning.
The advantage of a crown is that it achievesan array of twigs with maximum exposure to light for the production of seed. The branching which begins to constitute it is soon carried on not by post- juvenile extension growth, but by mature shoots which have seeding as their primary function. They will grow in any direction in which they find access to light, be it horizontally or even downwards if that gains them any advantage of exposure. This continual competition for light now leads the twigs which will form the scaffolding of the crown to grow in haphazard directions, so that as they mature, the branches are crooked. Terminal twigs will develop efficiently to fill any space in the array. Apical dominance now gone for good, their growth will be slow and uniform, so that the characteristic bounding line of the elm’s canopy begins to form.
The establishment of the canopy has a profound effect on two sectors of the tree’s lower foliage. Immediately below the junction, the lateral growth from the main trunk is now substantially shaded and much slowed. This is why the lower bowl of the figure of eight narrows towards its top. However, lateral branches lower down the trunk are less shaded, able to continue their extension, and the lower half of the tree’s silhouette now notably bulges at its midpoint. The most vigorous side branches may extend sufficiently far from the trunk to escape much of the crown’s shade. In fact the angles adopted by these “successful” side branches can often be seen to correspond closely to phases of the crown’s development. Relatively vertical growth is initially made by the juvenile branch, before the crown’s development reduces light from above. A phase of horizontal extension follows, corresponding to the continued availability of lateral light. Once the branch has extended sufficiently to escape the shade of the crown, it will turn upwards again.
At the junction and immediately above it, at the base of the crown, the shade of the canopy above is extreme. Here, most growth will die. This is why the base of the branches arising from the junction tends to be visible while the tree’s scaffolding above and below is completely concealed by foliage.
The shape of the tree now approximates to two crowns more or less distinctly divided – a lower and an upper, though the most successful side branches may mediate between the two (forming what is often called the “pie-crust” outline of English Elm). This hierarchy of crowns is articulated by the junction zone which is relatively free of foliage. As a result, it is possible in a sense to see through an English Elm at its midpoint. In addition, the openness at the base of the upper crown may make it appear less like a globe of foliage and more like a parasol or roof, though that tendency was much influenced by local conditions. It was often seen in an extreme form in Cornish Elm, due in part to the effects of wind and salt.
If this account is correct, English Elm owed its form (a) to a tendency for the current year’s growth to fan outwards in the form of co-dominant leaders of which one would later prevail through apical dominance; (b) to the ability of its lateral branches to grow at a wide range of angles to the vertical in search of light, to which the tree was highly sensitive; and (c) to the comparatively late onset of seeding (sexual maturity) which favoured the formation of a tall central trunk before seeding caused it to give way to crown development.
This apparent hyper-sensitivity to the relative intensity of light is not surprising in a tree growing far north of its natural range, now generally thought to be central Italy. The uniquely “English” silhouette of fully developed English Elm actually arose in part from the tree’s limited adaptation to this latitude.
The nursery trade, however, tends to select clones of a relatively fastigiate or pyramidal habit providing guaranteed upright growth from the outset. Thisresults in weak side branches which curve upwards as far as space allows to mimicthe vertical growth of the leader.Examples from across the range of breeding programmes are Columella, Cathedral, the very vigorous San Zanobi seedling FL634, and to an extent Lobel and Vada. The consequence is a tree in which there is actually insufficient distinction of character between trunk and lateral growth; the latter is merely a weaker (perhaps much weaker) and off-centre version of the former. Clones of this type are not likely to display a hierarchy of form at maturity, nor the coherent crown so often seen in English Elm. Clones of apparently more unruly early growth, such as FL 493 or Morfeo, have a better chance of developing into trees which will recall the place of English Elm in the open landscape.