Abstract

The specific objectives of this study were to: (a) determine the growth (cambial) periodicity and factors influencing it during juvenile wood production, including false ring formation, in teak (Tectona grandis); (b) determine the age at which teak stops producing juvenile wood and begin to form mature wood; and (c) evaluate the differences in size and proportion of different secondary xylem (wood) elements, microfibrillar angle, specific gravity and bending strength between juvenile and mature wood. Cambial periodicity was followed for two consecutive annual cycles (1994-95), in the marked trees in plantations of of 4 age groups (7, 13, 20 and 40 yr) from 3 locations with diverse environmental conditions in Kerala. To determine the age of demarcation between juvenile and mature wood, wood samples were collected from 65-yr-old plantations located at 3 other locations. Cambial activity was influenced by tree age. Cambium was active for a longer period of the annual cycle in juvenile wood formationthan in mature wood production. Premonsoon showers during March-April influenced the cambial activity and broke dormancy earlier. Irrespective of age and location, a peak period of wood production was recorded during June-July, when the cambium was 15-20layered, correlating with the highest rain fall of the year. Narrowing of cambial zone was observed from September onwards, and by the time the cambium had attained dormancy during November-December, the zone was reduced to 6-10 layers of radially flattened cells. Wider growth rings, and a larger amount of growth ring were produced in juvenile 7- and 13-yr age groups; this is attributed to the prolonged period of cambial activity of juvenile trees. The late onset of dormancy influenced the quality of wood as evidenced by a higher proportion of latewood in the growth ring. Artificial drought induction and field planting interrupted active growth causing false ring formation in juvenile wood of 2- and 3-yr-old seedlings. Insect defoliation in 8-yr-old trees did not induce false ring formation, and decreased growth rate of the trees without affecting specific gravity, size and proportion of vessels and cell wall percentage. The data obtained from both cross-dating of plantations and controlled experimentsincluding induced drought, showed that rain fall during the dry period, drought during the active growing season, and polybag/field transplanting of seedlings are the important causative factors of frequent false ring formation. The age of demarcationbetween juvenile and mature wood was around 20-25 yr depending on growth rate. The fast growing trees had a tendency to prolong the period of juvenile wood formation. Juvenile wood was characterized by wide growth rings, a wide microfibrillar angle, a small diameter and low percentage of vessels, and a high percentage of cell walls with short fibres, in comparison with mature wood. Because mechanical properties such as bending strength of slow grown trees and compression strength (parallel to grain) didnot differ consistently between juvenile and mature wood, juvenile wood is not necessarily always inferior in strength to mature wood