Natural Root Grafts in New Zealand Trees.
[Read before the Wellington Branch, May 28, 1941; received by the Editor, May 29, 1941; issued separately, December, 1941.]
Natural grafting has been recorded in a number of species of woody plants—stem grafts in beech, holly, lime, oak, Scots pine, willow, yew (Dallimore, 1917), elm (Caldwell, 1927), Hedera helix (Millner, 1932), Alnus oregana (Rigg and Harrar, 1931), Coccolobis laurifolia, Eugenia buxifolia, Quercus virginiana and Taxodium ascendens (Small, 1932); root grafts in Acer saccharinum, Betula lutea, Pinus strobus, Thuja occidentalis, Tilia americana, Ulmus americana (La Rue, 1934), Pinus radiata (Adams, 1940) and Pseudotsuga taxifolia (Rigg and Harrar, 1931).
As records of such phenomena in New Zealand trees have not been published, the following notes have been prepared to summarise the results of investigations made by the writer during the last six years, mostly in the Wellington Province south of Mount Holdsworth, but with supplementary observations in other districts of both North and South Islands. Attention has been concentrated on root grafts except where, at about ground level, woody roots and stems are involved in one fusion. Many specimens have been gathered. Some of these will be deposited in reference collections, in Auckland in the War Memorial Museum, in Wellington in the Dominion Museum, and in the Botany Division of the Plant Research Bureau.
Easily accessible examples may be seen in situ at Khandallah Domain (Fuchsia and Laurelia) and on the Wainui-o-Mata hills (Nothofagus). The best hunting ground was found to be in burnt second growth, for example where fire has been through regenerating beech forest. Where perhaps a hundred young seedlings are crammed into the space eventually to be occupied by one adult tree, root grafting is extremely common, and seems to be almost forced on the competing plants. In living forest, undergrowth usually hides the roots, but on dry ridges, especially after animal damage, they may be clearly exposed and comparatively easily examined.
In at least thirty species of woody plants indigenous to New Zealand definite grafts between root systems have been found. These have been arranged in list A, in sequence to indicate the relative frequency of grafting. In the first eleven, grafts are common, in the remainder only occasional. The six species in list B are strongly suspected, but in absence of definite proof are excluded from the main list. No doubt detailed search in other districts would add many more species.
Nothopanax arboreum, Nothofagus truncata, Laurelia novae-zelandiae, Brachyglottis repanda, Nothofagus solandri, N. cliffortioides, Leucopogon fasciculatus, Fuchsia excorticata, Pseudowintera
colorata, Beilschmiedia tawa, Aristotelia racemosa, Nothofagus menziesii, Corynocarpus laevigata, Myrtus bullata, Nothofagus fusca, Schefflera digitata, Suttonia australis, Knightia excelsa, Podocarpus totara, P. spicatus, P. ferrugineus, P. dacrydioides, Dacrydium cupressinum, Olea cunninghamii, Alectryon excelsum, Pennantia corymbosa, Metrosideros robusta, Weinmannia racemosa, Dysoxylum spectabile, Suttonia divaricata (the last in seedlings six years after being heeled into garden).
Shawia paniculata, Melicytus ramiflorus, Agathis australis, Macropiper excelsum, Vitex lucens, Metrosideros excelsa.
Types of Grafts.
Metrosideros robusta habitually encircles its host tree with lateral root bands that rejoin the main descending aerial root at about the same level (c.f. Laing and Blackwell, 1940, fig. 108). Matted aerial roots of M. excelsa probably graft together (c.f. Cockayne, 1928, fig. 4). Aerial roots of Nothopanax arboreum fuse about their host. One specimen shows roots of this species completely coalesced around a Podocarpus ferrugineus trunk 1½ inches in diameter.
Exposed root systems as in upturned Nothofagus, Nothopanax or Laurelia often show two roots from the same tree joining together at some distance from the butt, and carrying on as a single root. Grafts of this kind are extremely common in the three genera mentioned. These roots meeting the roots of an adjacent tree of the same
Fig. 1—Characteristic examples of root giafts. The thiee smaller specimens are Nothofus spp. The largest, Nothopanax arboieum, shows a head-on gratt at A. Diameter of larjest trunk 3 inches.
Fig. 2—Complex buttress graft between two trees of Laurelia noiue-zelandiae. Wairongomai River.—Photo, R. Wilson
Fig. 3—Specimen of Nothopanax arborcum, A, B, C, D, E, trunks; F, G, “parallel” joins; II, jug-handle.
species do not need to alter their habits very much to form grafts with the neighbour. More than two such root systems are often involved (text-fig. 1). A step beyond this is the case where roots of trees of different species of the same genus meet and fuse, e.g. Nothofagus solandri with N. truncata (text-fig. 2) or either of these with
Fig. 2—T.S. grafted butt region of Nothofagus. N. truncata on left, N. solandri on right. Parts of all recognisable growth rings are shown except at centres and where dotted lines indicate that only each fifth ring has been traced. Bark, both external and enclosed by graft, is stippled.
one of their numerous hybrids. All of these types have been seen in the present investigation and can be demonstrated. No union was found between roots of unrelated species as mentioned, though without
definite proof, by La Rue (1934). Romell (1919) describes an extraordinary graft between a young spruce branch and a Scots fir.
X and Y grafts are the commonest both within a system and between systems. X junctions where the roots cross at a wide angle and are fused only at the intersection are presumably of later development in the lives of both the roots concerned than Y junctions, where, beyond the place of fusion, only a single root carries on. This single root may be composite, with equal or unequal contributions from the two sources, or the tip of the smaller root may abort and only the larger one persist beyond the graft. Loop or jug-handle joins within a system are common enough (pl. 35, fig. 3 H). Head-on grafts, often oblique, like those described between the branches of Taxodium ascendens (Small, 1932) occur (pl. 35, fig. 1). One specimen of Schefflera digitata shows parallel roots, both grafted, but very unequal in size, connecting the two individuals. A similar parallel connection can be shown in Nothofagus truncata and equivalent double ones appear in Nothofagus arboreum (pl. 35, fig. 3 F, G).
Conditions of Grafting.
Grafts occur at any time after the root becomes woody, or, perhaps, with Y junctions, even before that. The angle of meeting and the respective age and size of the parent trees, or even of the uniting roots, seem unimportant, though pairs are often similar. The distance from the trunk is likewise immaterial, though the majority of the specimens dissected out were, for practical reasons, neither far from the butt nor much below the surface of the ground. The number of plants that may be involved seems unlimited (see text-fig. 1 and pl. 35, fig. 3, and compare La Rue—eleven Thuja orientalis in one complex). In one day's hunt on ridges with Nothofagus truncata dominant, I found two groups of four trees joined together and five groups of three, while pairs were plentiful.
A fusion sometimes involves the whole butt region, both root and stem, and may be the natural sequel to an early graft between two short roots. Buttresses of Laurelia graft as easily and as frequently as underground roots of other trees (pl. 35, fig. 2).
Suckering may in some cases produce results not easily distinguished from root grafts, and for this reason species prone to send up branches from superficial roots have been relegated to the doubtful list until unquestionable grafts are found. The grain of the wood and the inclusion of fragments of bark along the line of junction are convincing proof when a real graft is sectioned.
The occurrence of natural grafting is generally admitted and is adequately demonstrated by photographs accompanying this and other published records. In some cases grafts have been attributed to swaying of trees in the wind, leading to attrition of the bark with fusion following when meristematic areas of different members are brought into contact. According to La Rue (1934) the soil plays a part in the grafting of roots by serving as a support that prevents growth in diameter from pushing apart the roots which have come
into contact with each other. Millner (1932) concluded from experimental work that slight wounding, causing a rupture of the bark, is the stimulus that initiates grafting.
In New Zealand, wind action would be expected to be of very minor importance in a thick stand of young sapling beeches, for instance, and pressure of soil would be least near the surface where so many root grafts have been found.
Rigg and Harrar (1931) state that “it seems probable that the root fusions within the system in trees growing in sphagnum give rise to increased rigidity in anchorage and thus increase the mechanical efficiency of these organs.” It is possible that on wind-swept ridge tops, root grafts of such plants as Pseudowintera colorata, Nothofagus menziesii, Fuchsia excorticata, and Aristotelia racemosa may be of some value in resisting gales.
The present limited investigation indicates that root grafts are widespread in New Zealand forests. Adams (1940) records in Australian plantations of Monterey pine as many as 21 root fusions between an individual tree and its neighbours. If such organic connections between trees could provide paths for the migration of disease organisms like heart rot fungi as they do for the conduction of liquids (Millner, 1932), more detailed study of this phenomenon might be advocated, especially in commercial timber trees, both native and exotic.
Thanks are due to Dr. H. H. Allan for help and encouragement in the early stages of the work, and to Miss L. B. Moore for assistance in the preparation of the manuscript.
Adams, A. J. S., 1940. Observations on Root Fusions in Monterey Pine, Australian Forestry, 5, 78–80.
Caldwell, J., 1927. An Unusual Case of a Natural Graft, Nature, 120, 513.
Cockayne, L., 1928. The Vegetation of New Zealand. Leipzig.
Dallimore, W., 1917. Natural Grafting of Branches and Roots, Kew Bull., 303–306.
Laing, R. M., and Blackwell, E. W., 1940. Plants of New Zealand. Wellington.
La Rue, C. D., 1934. Root Grafting in Trees, Amer. Jour. Bot., 21, 121–126.
Millner, M. E., 1932. Natural Grafting in Hedera helix, New Phytol., 31, 1–25.
Rigg, G. B., and Harrar, E. S., 1931. The Root Systems of Trees growing in Sphagnum, Amer. Jour. Bot., 18, 391–39.
Romell, L. G., 1919. Sammanvaxning och Naturympning, Skogen, 6, 133.
Small, J. K., 1932. Natural Grafts, Jour. New York Bot. Gard., 33, 213–219.