J.B. Ball
J.B. Ball, who is in charge of plantations at the FAO project on high forest development in Nigeria, has extensive experience with pine plantations in Africa.
Pines grown in plastic containers in nurseries often develop coiling roots which strangle the tree and cause it to snap off at the base after planting. Standard practice, says the author, should be to cut these coiling roots before planting in the field.
Pine plantations have been rapidly expanded in many parts of the tropics in the last 15 years, generally in areas with a wet season of high rainfall followed by a dry period. The use of polythene tubes (open-ended) or pots (closed at one end) has given better survival of plants in the first year than is obtained with bed-raised stock because the nursery soil is retained around the roots.
However, it is now becoming apparent that the coiling of roots within the polythene container in the nursery may lead to stem strangulation and the breaking off of the butt at ground level some years after planting out in the field. In the nursery plant these roots may be easily seen, usually at the outside of the soil mass, and often near the bottom of the tube containers; in pots they may be even starting to grow upward again. After planting in the field, when the trees are 15 cm or more in diameter, the consequences become apparent. Trees snap off at ground level in a typical "ball and socket" formation, which I have called basal stem snap. So far no more than 2 to 3% of any stand that I have seen has been found to be affected, but the plantations have been no more than seven years old. The phenomenon may occur much more severely over a few hectares, and leave large gaps in the crop. It appears, too, to increase with age.
I have seen basal stem snap of Pinus caribaea, P. patula and P. taeda in Kenya, of P. elliottii and P. taeda in Malawi, and I have been told that it occurs in P. halapensis in Tunisia and in P. caribaea and P. oocarpa in Brazil. In Zambia I have seen P. kesiya deliberately kept in tubes after planting, but although there was severe root coiling there was no snap. In Nigeria I have seen two trees (P. oocarpa) two and a half years old that were just beginning to break off at the base.
Root coiling and stem strangulation are probably not caused only by the use of polythene containers, since planting stock of P. patula raised in boxes has also suffered from basal stem snap in Kenya. Rather, it appears to be the size of the plant that leads to root coiling where root pruning has been neglected in the nursery. However, the interaction of the size of the plant with the method of raising in the nursery appears to be greater with tube or pot plants than with bed-or box-raised stock.
Other causes of root coiling leading to root strangulation have been suggested. These include weed competition, particularly from grasses which inhibit the tree roots from expanding from the nursery soil, compaction of the planting hole walls from using a machete or similar tool to dig the hole, and the presence of a compacted "plough pan" beneath the level of mechanical cultivation. Deep planting has also been blamed for basal stem snap, but deep planting (i.e., with the nursery root collar down to 10 cm below ground level in the field) is usually important for survival. The size of the container is also important. Generally I have seen basal stem snap where a 10-cm lay-flat tube 15 cm long has been used. Even the colour of the container may be important. All these factors may contribute to root coiling and thus cause basal stem snap, but I am sure that the size of plant and its interaction with the polythene tube or pot are the main cause of root coiling.
Planting small (and therefore young) plants appears to be the easiest way to avoid root coiling, but below a certain size the plants may be too small for planting in the field as they will require a lot of tending. In my experience pines should be at least 20 cm tall, and preferably 30 cm, by which time root coiling has started. In this case two cuts should be made vertically on either side of the container before planting, and the bottom 2 cm should be cut off. This has been found to reduce root coiling considerably, without reducing survival. The polythene container should of course always be removed. If the plant is then put in a hole about 15 cm square and well weeded there should be little danger of root coiling.
Because of the widespread use of polythene containers for seedlings in the tropics, the incidence of coiling root may be quite frequent. Readers having experience with this problem are invited to write Mr. B. Ben Salem, Forestry Department, FAO, who is collating this information.
