A Unasylva technical note: Yield model of plantations in the tropics

D. Pandey

D. Pandey is Lecturer at State Forest Service? College, Forest Research Institute and Colleges, Dehra Dun, India.

· Forest plantations have been established in the tropical zone at a progressively increasing rate, particularly since 1970. Before that, the rate of establishment was less than 0.25 million ha/year as compared with the current plantation rate (during 1980-85) of 1.1 million ha/ year. This rate is likely to grow because of increasing industrial and fuelwood demand and shrinking supply from the natural forests. In most cases, the tree species being raised are on short rotation ranging from five to 20 years and are commonly fast-growing to produce maximum volume rather than large-sized timber. This note describes the yield, and factors affecting it, of some commonly planted species as an aid to rational planning in forestry, and to identify some fundamental questions on which little or no information seems to exist.

Since the yield of a tree stand is subject to great variability and dependent upon several factors, it is essential that these factors be properly taken into account before applying any yield prediction model. The definition of what constitutes yield is of paramount importance. This may be the volume of standing timber with or without bark, or the volume of all wood products including timber, pulpwood, poles, branchwood and small wood. With the increasing degree of utilization, the common practice in tropical countries now is to measure all the wood products down to 5 cm in diameter.

Once a correct choice of species for planting in a particular locality has been made, the main factors which govern yield are:

· Productive capacity of the site
· Stocking of the stand
· Silvicultural treatment given to the crop.

Productive capacity of the site effects both the volume of production per unit of area and the age of maximum mean annual increment (MAI). Poorer sites take a longer time to reach MAI culmination age as compared with a good site. For example, Gmelina arborea planted in Nigeria attains maximum MAI of 34 m³/ha at 4.5 years of age on a good site and of 10 m³/ha at 8.5 years on a poor site. The determination of productive potential of a site by direct method such as quantifying climates, soil properties and indicator plants, is not a common practice in tropical countries. Instead, top-height/age relationship is used to determine site classification.

Stocking of the stand (number of stems planted per unit of area) also has a direct bearing on the yield. A fully stocked stand has greater volume production than a poorly stocked one. The number of stems per hectare in a fully stocked stand depends on the form and growth rate of tree species and also on the object of the plantation. However, for maximum volume production, the number of stems per hectare for narrow-crowned species is generally more than for wide-crowned species. When the number of stems per hectare increases beyond a specified limit, the growth of the trees is likely to be suffocated and a lower yield may result. Very little research information is available about the optimum spacing/number of stems/hectare required for any species on a given site to produce maximum volume.

Stocking of the stand also affects the MAI culmination age on a given site. Utilization of site productivity is faster in high-density plantations as compared with low-density; hence rotation for a dense plantation would be shorter. For example, Eucalyptus tereticornis planted on good sites in India attains maximum MAI of 15.5 m³/ha at eight years with 1200 stems/ha and 21 m³/ha at six years with 2900 stems/ha.

The traditional silvicultural treatments such as weeding, soil working, climber cutting and thinning play an important role in the establishment of the plantations and on the size and quality of the trees produced, but they have a relatively minor effect on the total yield. In recent years the use of fertilizers and irrigation in plantations has attracted particular attention, however, and a greater yield of wood is expected.

But what type of fertilizer or how much irrigation is best for a given species being raised on a particular site, at what stage of its life and in what quantity? The economics of fertilizing and irrigating must also be taken into account since increased investment ought to be repaid by greater yields of wood. Studies carried out in the tropics on these factors are very limited and show varied results. Therefore it is not possible to be specific on the application of fertilizers and irrigation and consequent increase of yield.

The data on 19 species (see the Table) are based on the yield tables, yield figures, and estimated yield published/unpublished by planting countries. Of the 19 species, seven belong to the humid tropics, four to tropical high lands and eight to semiarid areas. While indicating yield of the species, a very modest approach has been adopted. The mean annual increment (m³/ha/year) mentioned indicates yield only on average sites without using fertilizer and irrigation, and at the rotation age appropriate for the number of stems per hectare commonly planted. Extreme cases (i.e. yield on the best and poorest sites) have been excluded. Information about the major climatic variables where species are being raised has also been provided.

Yield model

Humid tropics	Rotation age (in years)	Wood yield MAI (m³/ha)	No. of Stems per hectare	Latitudinal range (m)	Mean annual rainfall (mm)	No. of dry months
Acacia auriculiformis	10-5	8-10	1000-2500	0-500	1300-1700	4-6
Albizia falcataria	8-10	25-40	800-1200	0-1200	2000-4000	0-2
Casuarina equisetifolia	7-10	6-10	1600-2500	0-1400	750-1400	3-4
Eucalyptus tereticornis	8-10	10-12	2500-3000	0-1000	1000-1500	34
Gmelina arborea	5-10	20-30	1000-1600	0-800	1000-2500	2-4
Sesbania grandiflora	5-10	20-25	2500-3000	0-500	1000-2500	few
Syzygium cumini	20-25	6-9	800-1200	0-500	1500-2500	n.a.
Tropical high land
Acacia mearnsii	8-10	20-25	1000-1500	1000-2500	500-1600	3-4
Eucalyptus globulus	8-12	20-25	1600-2500	1500-3000	900-1800	2-3
Eucalyptus grandis	8-10	20-25	1600-2500	0-2100	1000-4000	0-2
Grevillea robusta	10-15	10-12	800-1200	800-2100	700-1200	2-6
Semi-arid areas
Acacia nilotica	15-20	3-5	700-1000	0-500	250-750	5-8
Acacia tortilis	10-12	22-41	800-1200	Low land	Upto 1000	6-10
Albizia lebbek	10-15	5-7	1100-1600	0-1400	500-1000	2-6
Azadirachta indica	8-10	4-6	1200-1600	0-500	450-1000	5-7
Cassia siamea	7-10	8-10	1600-2500	0-1000	650-950	4-6
Dalbergia sissoo	15-20	5-8	1100-1600	0-1000	750-2000	3-5
Eucalyptus camaldulensis	10-15	5-8	800-1200	500-2000	400-1000	4-6
Eucalyptus microtheca	8-10	7-10	1600	0-1000	250-500	5-7

¹ With reference to the above species, the following minimum and maximum temperatures prevail: humid tropics, 1635° C; tropical high lands, 4-32° C; and semi-arid areas. 4-40° C. ² Dried weight in tonnes/ha/yr.

Bibliography

ANON. 1980 Firewood crops. Washington, D.C., USA, National Academy of Sciences.

LANLEY, J.P. 1982 Tropical forest resources. FAO Forestry Paper No. 30. Rome.

PANDEY, D. 1983 Growth and yield of plantation species in the tropics. Misc. publication. Rome, FAO.

SHARMA, R.P. 1978 Yield table for Eucalyptus hybrid (plantation) for various levels of stocking. The Indian Forester, Vol. 104, No. 6.

SINGH, S.P. 1982 Growth performance of Syzygium cumini. The Indian Forester, Vol. 108, No. 11.

SUTTER, H. 1982 The construction of a tariff table and a yield table for Gmelina arborea in the Omo and Oluwa Forest Reserves. UNDP/FAO, Nigeria Project working document No. 10. FO: NIR/77/008.

WEBB, O.B. 1980 et al. A guide to species selection for tropical and subtropical plantations, Tropical Forestry Paper No. 15, Oxford Forestry Inst., Oxford, UK.