International Trials of Acacia and Prosopis: Overview of results

bullet1 Introduction

bullet2 Characteristics measured

The following characteristics were evaluated. Not all characteristics were evaluated at each trial. Characteristics 1 - 6 were measured by non-destructive means. Characteristics 7 - 8 were measured by destructive sampling of selected trees.

(1) Survival
Survival is regarded as one of the key variables when analysing tree provenance trials, since it indicates the adaptability of the provenance to the environment at the trial site. It should be noted that survival reflects only the conditions experienced during the growth of the first few years of the trial and not necessarily the climatic extremes and conditions that may be experienced during the life-span of a tree in the field

(2) Height
Height is usually considered an important variable in the evaluation of species and provenances. This of course depends on the main uses of the trees. Apart from indicating productivity, height may also be seen as a measure of the adaptability of trees to the environment, tall provenances/trees usually being better adapted to the site than short provenances/trees. This interpretation need not always be true, however: Cases have been observed where the tallest provenances are suddenly affected by stress with a subsequent death of the trees.

(3) Crown area
Crown area indicates the ability of the trees to cover the ground. The character is of importance in shading for agricultural crops, in evaluating the production of fodder and in the protection of the soil against erosion.

(4) Number of stems
The number of stems gives an indication of the growth habit of the species. Trees with a large number of stems are considered bushy, whereas trees with only one stem have a more tree-like growth.

(5) Basal area of the mean tree
The basal area is often used as a measure of the productivity of stands, since it is correlated with the production of wood. The basal area of the mean tree is calculated on the live trees only and gives an indication of the potential basal area production of the provenance provided that all trees survive.

(6) Total basal area per hecatare
In comparison to the basal area of the mean tree, the total basal area accounts for missing trees and is thus a better measure of the actual (total) production on the site. The figures are calculated on the basis of the spacing used in the trial, which was usually 3x3 metres but varied from 2x2 to 4x4 metres.

(7) Dry weight of the mean tree
Dry weight was measured for selected trees of eight species. For each species a correlation between basal area and dry weight was made, enabling calculation of dry weight for all trees of these species. The dry weight of the mean tree is comparable to the basal area of the mean tree in that they both are calculated on the live trees only and thus serve as a measure of the potential production at the site, provided that all trees survive. Furthermore, the two variables are linked closely together, as the basis for estimation of the dry weight is the basal area. However, an important difference is that the dry weight includes a cubic term (in comparison to basal area having only a square term), meaning that large trees with a large dry mass are weighted heavily in this variable. The dry weight of the mean tree is thus the best estimate for the production potential of biomass at the site.

(8) Total dry weight per hectare
As with the total basal area, the total dry weight account for survival (since missing trees are included in the plot sum) and thus gives the best measure of the total production on the site. As for the basal area, the figures are calculated on the basis of the spacing used in the trial which varied from 2x2 to 6x6 metres.

(9) Damage / health

Where there was damage to the trees (say from browsing) this was measured in on a four point scale: 0 indicating no damage, to 3 for severe damage. The same scale was used where trees had been attacked by insects.

(10) Fodder quality:  

Leaf and pod fodder quality traits were measured on selected trees by collection of samples, followed by laboratory and in-vivo investigations.

(11) Phenology:

For all species, notes were made of seasonal development of foliage, flowering and fruiting and registered.