Eucalypts were introduced to South Africa by colonial government officials, missioners, and settlers in the early nineteenth century [
Location of the Tchianga species introduction experiment in the Central Angolan Highlands. The analysed sample plots of the six
Since the 1950s, interest in pulp production increased as the national and international paper market needed extensive raw material sources. The Companhia de Celulose do Ultramar Portugues (CCUP), owned by the colonial Portuguese government, started the acquisition of
Although
Following the independence, 27 years of civil war ensued. During this period, no new research was conducted, and no experiments were established. In addition, the existing research experiments suffered from extensive illegal logging, especially close to the main cities due to the lack of any other secure fuel source. The Tchianga campus remained the only place with preserved forest stands [
The natural habitats of
Eucalypts are extensively used in large scale plantations in temperate regions, but more commonly in tropical and subtropical regions around the world. They are mainly used as pulp raw material and fuel wood, as well as sawn timber and reconstituted wood products.
Many species introduction experiments, similar to the one in Tchianga, were established in Africa in the early 1950s and 1960s. Widely studied species include
The purpose of the present research was to report the growth performance of six
The Central Highland region of Angola covers approximately 7.9 million ha. The region is mainly a flat plateau at 1500 to 1800 m altitude, including some hills and mountains up to 2600 m, which is the elevation of Angola’s highest peak, Moco [
The experiment was set up in the rainy season of 1966/1967 and is located in the Tchianga research station (12°43′S, 15°48′E) in Huambo region. It lies at an altitude of 1650 m and has a typical flat Oxisol. The annual precipitation is 1400 mm. The species introduction experiment includes 22
The plot size, excluding two border lines on each side, was 1031 m2 (27.5 m × 37.5 m), and stand density varied from 294 to 833 trees/ha. Diameter at breast height (dbh) was measured on each tree, while height and bark thickness were measured on 12 trees per plot covering the whole range of tree sizes. Five-mm thick radial cores were taken from each tree measured in the Tchianga experiment with an increment borer.
Once the cores were air-dried, cleaned, and fixed onto sheets, they were scanned with a high resolution X-ray device. The X-ray images were used to measure annual rings creating a radial increment file. An X-ray densitometer, Itrax, device was used to measure the increments (Figure
An example of X-ray image and density profile of an increment core.
A species-specific model was fitted between single bark thickness (bark, mm) and diameter (
Stand development was backtracked to reconstruct the growth history of each tree and plot based on the measured radial increments. This required information on each year’s growth for every tree in the stand. Since some radial growth measurements were missing, a linear model was fitted separately for each year and plot, in order to predict the widths of those annual rings which were not available as a measurement. The model was as follows:
The tree and stand characteristics one year ago were calculated as follows: (i) obtain underbark diameter by multiplying overbark diameter with the u/o ratio; (ii) calculate underbark diameter in year
In the absence of temporal mortality information, only the average mortality rate of each plot could be calculated, using the planting density,
Stand characteristics of six tropical
Species | Number of plots | Volume | Mortality rate | |||||
Yield1 | Height2 | trees/ha | m2 ha−1 | cm | m | m3 ha−1 | % year−1 | |
2 | 14 | 652 | 67.8 | 44.4 | 41.3 | 1427 | 2.1 | |
2 | 2 | 531 | 34.2 | 32.2 | 39.0 | 680 | 2.8 | |
2 | 2 | 302 | 25.8 | 36.6 | 38.8 | 511 | 3.9 | |
1 | 1 | 294 | 25.0 | 36.8 | 33.1 | 423 | 3.9 | |
1 | 1 | 833 | 55.0 | 34.4 | 30.6 | 858 | 1.5 | |
2 | 2 | 476 | 34.6 | 35.5 | 57.1 | 1006 | 2.9 |
1Number of plots used in growth and yield analysis.
2Number of plots used for mean height modelling.
Parameter
The annual mortality rate calculated in this way made it possible to gradually increase the number of survivors when stand development was backtracked from 2009 to the year of stand establishment. The additional trees, that is, the mortality, were assumed to be the most suppressed and among the smallest trees of the stand. Their diameter was assumed to be a weighted average of the minimum and mean diameter of the measured trees (survivors in 2009) in the respective backtracking year
Past stand basal area and mean diameter calculated both with (
To calculate the mean tree height in the previous years, the following “guide curve” was fitted for the average dominant height development of eucalypts in the region:
The predicted mean tree height made it possible to calculate stand volume, which made comparisons to earlier research easier as compared to results expressed in stand basal area. The stand volume was calculated from:
There were clear differences between the species regarding stand volume, basal area, and other characteristics (Table
Stand volume, stand basal area, and mean diameter of six
The mean annual mortality rate was quite high for
The stand basal area increased fast at young ages, slowing down after 10 to 20 years (Figure
Volume, basal area, and mean diameter development of the analysed eucalypts in the Tchianga experiment.
Most species had a rather constant increment in mean tree diameter. One reason for the continuing increment is that the smallest most suppressed trees were gradually dying with a consequence that mean diameter increased although the largest trees were growing less (see Figure
Mean annual increment (MAI) of the six
The presented methodology to describe stand development, based on analysing the annual rings, has the advantage of being quick and reasonably straightforward. It proved to be highly convenient for completing information from permanent plots where periodical measurements have not been made. It is a scientifically sound methodology, since the results obtained agree with field measurements. Verzino et al. [
The methodology employed in the current study may be more accurate than the one used in Delgado-Matas and Pukkala [
The current study suffers from certain data limitations, specifically the small number of plots on only one site, and only one to two repetitions per species. The plots used in the study have never been systematically thinned, but illegal logging has been done in some plots and uncontrolled fires have also killed trees. Other factors including the presence of false rings and small annual growths in recent years in suppressed trees may have created errors in the measurements. On the other hand, the region of the Angolan Highlands is quite uniform, and the results therefore represent a large area [
Stand volume was calculated in order to compare our results with previous studies in the region. The growth rates were calculated using a form factor, stand basal area, and predicted mean stand height. The form factor used, 0.51, is commonly accepted for eucalypt plantations in South Africa [
The traditional belief in Angola is that
The eucalypt species planted in Tchianga showed growth rates substantially higher than in their natural ranges. Zobel et al. [
The
Burgess [
The studied species are adapted to summer rains to uniform rainfall year around, with
The species included
Burgess [
Borough et al. [
Traditionally in Angola the rotation lengths of
The outcomes of the present study suggest that further studies are required on provenances and hybrids of eucalypts in Angolan Highland conditions. Those studies should be in line with recent successful initiatives in other intertropical regions. In addition, the results of this study should be extended to timber and wood characteristics. The results of this study suggest that
The authors gratefully acknowledge the logistic support and helpful suggestions of Engineer Caetano, Dr. Kiala Kalusinga, and technicians in the Instituto de Desenvolvimento Florestal (IDF) and Tchianga Research Station.