by
O. O. Okoro
Forestry Research Institute of Nigeria, P.M.B. 5054,
Ibadan, Nigeria
INTRODUCTION
Pinus caribaea var. hondurensis Barr. and Golf is being grown in Nigeria from latitude 4°54'N to 10°37'N, with daylength of 11.91 – 12.34 hours at the lower latitude and 11.56 – 12.70 at the higher latitude, and over an altitudinal range of 30 – 1,100m (Okoro, 1981). About 60% of the plantation sites lie below 300 m altitude. Apart from the provenance trials, the seed sources of most plantings were British Honduras, Guatemala, Belize and Australia (introduced material).
ATTAINMENT OF REPRODUCTIVE GROWTH
In the nursery a couple of plants out of thousands of one-year-old seedlings may produce one or more female cones. Potted rooted cuttings and air-layers from adult donor trees flower within the first one year (Okoro, 1980b) and nine months (Okoro, 1984b) of potting respectively. Generally in the high forest and derived savanna zones, field grown trees attain reproductive growth in the third year of planting, rooted needle fascicles being most precocious with about 66% trees attaining reproductive growth in the third year at latitude 7°22'N and altitude 200 m in Ibadan (Okoro, 1980b). The rooted needle fascicles produce predominantly pollen cones initially. Probably this is to be expected because pollen cones normally occur in place of needle fascicles. For plantations raised from the usual planting stock, reproductive growth is about 100% by age 15 (Okoro, 1984a) but could be attained earlier in the derived savanna at latitude 6°26'N and altitude 400 m (Enugu-Ngwo). In the Northern Guinea savanna, at 10°37'N and 600 m (Afaka), reproductive growth is attained at age five years. Usually seed cones are the first to form and many abort in the first few years. Pollen cones are often formed much later and their production becomes consistent with advancing age (Okoro 1984 a and b). In field grown trees by age fifteen years, abundant pollen cones are produced. However, on a good flowering site such as Enugu Ngwo, pollen cone production is abundant by age 8 – 10 years.
PHENOLOGY
Seed cone production spreads over the whole year, whereas pollen cone production is more restricted in time (Oje, 1980 and Okoro, 1984a). This is attributable to the fact that this species makes three to four flushes in one calendar year (Okoro, 1980a) and about three of these are accompanied by seed cone production, whereas usually one (the second or third) is accompanied by pollen cone production. Thus seed cone production starts as early as January – March, while production of the main pollen cone crop starts in June and is over by about October. There are slight variations within this range from tree to tree and from location to location. The life span of a seed cone is about 30 months, while that of a pollen cone is about 12 weeks and anthesis on a given shoot lasts about 10 days.
Consequently, only one or two sets of seed cones are sufficiently exposed to pollination, usually the second and/or third set. On most sites, both high forest and savanna, cones ripen for harvest from August to late September but, surprisingly, at about latitude 7°22'N (Ibadan and Iwo) cones ripen in February to March.
Although pollen cones are produced during the rains and anthesis occurs during this season, pollen has been observed to fly nevertheless throughout the planting range, especially during dry spells. In years of abundant pollen cone production, site differences in the relative abundance of pollen in the air occur. It is remarkable to note that pollen-fly per unit area is most abundant at 6°26'N and 400 m with mean annual rainfall of 2,000 mm and temperature of 21 – 28°C (Enugu-Ngwo). It is pertinent to add that, on this site, the stand size is far bigger than on other sites studied and this boosts the size of the pollen cloud. Also the stand is situated on an exposed slope with a river in the valley. A lot of natural regeneration is frequently observed in this stand and a satisfactory occurrence of productive seed cones has been obtained. There is evidence of pollination occurring on sites with a smaller size of pollen cloud, as evident in the presence of pollen grains in acetolysed young seed cones. Also some natural regeneration has been observed in some stands.
Flowering in 1982 was abundant but scanty in 1983 and the onset was delayed by the protracted drought of that year. The pollen cloud in the latter year was also scanty.
SEED PRODUCTION
Of the four kinds of ovuliferous scales found in a ripe seed cone, only the large middle zone scales are productive and are most abundant, but not all bear seeds. The size of ripe seed cones varies a great deal and it has been observed that more of the larger seed cones are productive than of the smaller. The proportion of these larger productive seed cones increases in older plantations. Consequently, at 7°22'N 3°58'E and 200 m (Ibadan), optimum seed production is expected from age 15 years onwards when most trees in the plantation have attained reproductive growth, supply of pollen is adequate and production of larger and more productive seed cones is certain (Okoro 1984a). Only a few existing plantations have attained this age, as the survival of plantations, especially the younger ones, is constantly threatened by fire (Okoro, 1983) and weeds.
The percentage of seed cones which is productive is about 33 – 43% at the lower latitudes of 4°58'N – 5°59'N with very low altitudes of 50 – 100 m; and 75 to 100% at higher altitudes above 200 m. Seed content per cone varies greatly, with a range of 1 – 75 seeds but with a modest average of 10 seeds per cone.
On a happy note, 325g of Pinus caribaea seeds was collected from a 1961 stand in 1974 and seedlings raised and planted out (Shado, 1980). From observations made so far, it is expected that if yearly cone collection from all available plots is organized, some seeds to support at least part of the planting programme can be obtained. It is worth staking resources on this venture when one considers the enormous amount of money spent in foreign exchange on procuring seeds from abroad.
FLOWER INDUCTION
Attempts at flower induction have been made with hormone and fertilizer application. Using GA4/7 in combination with NAA spray on rooted air-layers from graft donors has produced, so far, only pollen cones, especially when combined with the cultural treatment of girdling. The compound fertilizer NPK and potassium applied singly, were used. It is rather early to make deductions from the fertilizer trials but potassium seems to hold promise on very humid sites.
REFERENCES
Oje, S.A. 1980 Preliminary Observation on the Flowering and Seed Production of Pinus caribaea var. hondurensis Barr. and Golf. in Northern Nigeria. Proc. 10th Ann. Conf. For. Ass. Nigeria, Sokoto, Nov., 1980
Okoro, O. O. 1980a Impact of Some Climatic Factors on Shoot Elongation in Pinus caribaea. In: Control of Shoot Growth in Trees. Proc. IUFRO Workshop of WP on Xylem and Shoot Growth Physiology, Fredericton, New Brunswick, Canada, 1980.
Okoro, O. O. 1980b Potentials of Vegetative Propagules of Pinus caribaea in Nigerian Forestry. Proc. 10th Ann. Conf. For. Ass. Nigeria, Sokoto, Nov., 1980.
Okoro, O. O. 1981 The Problems of Seed Production in Pinus caribaea var. hondurensis Barr. and Golf. in Nigeria. Nig. J. For. 11 (2): 25 – 31.
Okoro, O. O. 1983 A Potential Nigerian Seed Source for Pinus caribaea var. hondurensis destroyed by fire. Proc. Workshop on Forest Fires: Ecology and Environment, Ibadan, 1983.
Okoro, O. O. 1984a Influence of Flowering Habit of Pinus caribaea var. hondurensis Barr. and Golf. on Seed Yield. Proc. lnaug. Meeting Bot. Society of Nigeria, Ilorin, July, 1984.
Okoro, O. O. 1984b Preliminary Studies on Flowering in Potted Plants of Pinus caribaea var. hondurensis Barr. and Golf. Proc. 14th Ann. Conf. of For. Ass. of Nigeria, Port Harcourt, Dec., 1984.
Shado, M.B. 1980 Procurement of Pine Seeds: Towards Self Sufficiency. Proc. 10th Ann. Conf. For. Ass. Nigeria, Sokoto, Nov., 1980.
Article received in February 1985
