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Although data on productivity are rare, the analysis of the yield data contained
in Appendices 2-4 seems to suggest that quantity prevails over quality.
The remarks made to explain individual yield figures, and more so the ones
which are not made, reveal the weaknesses and limitations of the data provided.
From the many shortcomings concerning the reliability of the yield
figures the most obvious are listed hereunder:
- With regard to yields on fodder leaves, the following aspects are often left
unexplained:
• whether the figures refer to dry matter or green weight
• at what age and stage of development the trees were when the measurements
were taken
• how big the sample was of which (average) yield data were obtained
• whether yield data were obtained from manipulated or unmanipulated
trees/shrubs
• whether leaf fodder harvest operations, on which yield data are based,
included the total leaf biomass or were restricted to browse height, etc.
- With regard to pod and fruit production for either human or livestock
consumption, the following additional clarifications are often required:
• frequency of harvesting. Are the yields indicated per tree obtainable
every year? If not, as is the case with many species, are the yields
given reduced to an annual average?
• Are the pods or fruits harvested in total or only partially, i.e. only those
which can be reached in the process of harvesting?
Genetic variance and bio-physical differences of growing sites may account
in part for the often enormous differences in yields indicated in different
records for the same species. However, answers to the questions posed above
would certainly explain much further the wide range of yields as shown in
Table 2 on page 9.
Some of the shortcomings outlined above are partially due to some inadequacy
of the questions asked on yield figures in the course of obtaining
site-specific information for the Multipurpose Tree and Shrub Data Base.
The large number of considerations, important to produce compatible yield
data, constitute a serious obstacle to data collection in such a broad approach
as the one being conducted in the case of ICRAF's inventorial type of data
base. In an attempt to strike a balance between what is desirable to obtain
and what can be expected to be answered by volunteer informants, many of
the questions demanding additional required information to place yield data
in the right perspective had to be omitted.
There is no doubt that these shortcomings mentioned earlier to some
extent limit the acceptability of the yield figures as presented in Table 2 and
Appendices 2-4. But nevertheless they provide indications useful for the
comparison of the productive potential of different species.
What has been said so far with regard to the inadequacy of some of the
yield data presented also raises questions of a more fundamental nature.