section 3 : regional examples

Agroclimatic zones of Zimbabwe and potential for agroforestry

K.W. Nyamapfene

Department of Land Management
University of Zimbabwe
 P. O. Box MP167, Harare, Zimbabwe

Abstract

Zimbabwe is considered to be divisible into five agroclimatic zones which are really agroecological zones. The paper describes the main agroclimatic features of each of these zones and evaluates the possible significance of each of the major features on agroforestry. It is recognised that only the availability of moisture in the form of rain fall, at the right time, is the main limiting factor for agriculture in general and agroforestry in particular. To a limited degree, temperatures are also considered limiting but only as far as excessively high temperatures in the arid and semi-arid parts of the country increase the possibility of moisture stress. Although frost occurs in parts of the country during the cool, dry season, low temperatures are not a serious limitation to agro-forestry. The areas that appear to be climatically most suited to agro-forestry are, for a variety of reasons, also the areas in which there seems to be the least need for it. The semi-arid and arid areas that are marginal for most other forms of agriculture appear to be the areas in which the greatest benefit could be derived from agroforestry. They are also the areas in which it would appear great difficulty may be experienced in trying to establish a useful and practical agro-forestry system. This last problem is the challange which the paper poses for the agroforestry researcher.

Introduction

The agroclimatic zones which will be used in this paper are based on a combination of Zimbabwe's rainfall map and the agro-ecological zones described by Vincent and Thomas (1960). An examination of the two maps (Figures 1 and 2) shows that the agro-ecological zones are very closely related to rainfall. The agro-ecological zonation, however, was based on several parameters such as soil type, vegetation as an indicator of agricultural potential, rainfall and, in particular, the effectiveness of the rainfall. Paramount importance is therefore attached to the availability of moisture in the right quantity and at the right time.

There is no doubt that climatic factors are a sine qua non for any agricultural activities but their impact will vary according to other physical as well as human conditions. For the purposes of this discussion it is proposed to put the impact of agroclimatic factors in the context of three other factors. These are:

1. the existence or absence of a tradition of agroforestry;

2. the desirability of it; and

3. the feasibility of it.

Figure 1 Mean annual rainfall.

Figure 2 Agroclimatic zones.

The writer firmly believes that the existence of a tradition of agroforestry provides conditions which facilitate the re-introduction or modification of agroforestry systems and probably constitutes the most important guarantee in terms of potential for agroforestry. In many cases where there has been a tradition of agroforestry, agroclimatic conditions are often conducive to the development of a variety of such systems as is the case in most high-rainfall tropical zones.

Desirability of agroforestry has to do with those conditions in which it is perceived that agroforestry will result in a significant improvement in the physical condition of the environment or in the improvement of the social condition of the residents. This is often the case in arid and semi-arid regions. However, in such regions, agroclimatic considerations can be limiting or even overriding. It is felt that desirability of the system will create the necessary will without which no practice can be adopted regardless of how conducive the physical environment may be.

Feasibility simply refers to the presence of optimum conditions that would minimize impediments to the establishment of agroforestry, i.e., is it physically possible, socially acceptable and economically sensible?

The case of Zimbabwe

On the whole, Zimbabwe may be described as a semi-arid country in which more than 85% of the area receives less than 800 mm of rain per year. The rain is concentrated in the five months from November to March followed by a marked dry season.

Although parts of the country are susceptible to frost in the middle of the dry season, temperatures are not a major limiting factor for plant growth, except in isolated areas where ground frost can be a problem in the cooler months of the year. On the whole, therefore, Zimbabwe's potential can be considered to be largely a function of rainfall amount and distribution.

Agroclimatic zone I

This is a region of relatively high elevation with cool temperatures throughout the year and a rainfall generally well in excess of 1,000 mm per year. The rainfall is fairly reliable and although it is concentrated in the five summer months from November to March, this zone has the additional advantage that, in the dry winter months, it experiences light drizzles, called 'guti' in local parlance. This, together with the relatively cool temperatures at that time that minimize evaporation losses, means that the soil profile is moist throughout the year.

Most of the zone is on mountainous or hilly terrain with steep slopes that make soil erosion a serious hazard especially with the high silt and clay contents of the soils commonly found there. The soils are mostly Ultisols and Ultic or Oxic subgroups of Alfisols and some Oxisols. They are therefore of relatively low agricultural potential.

This combination of factors makes this zone highly suitable for agroforestry practices. Such practices are also desirable as they would ensure increased cover to protect the fragile lands. The introduction of agroforestry systems could be initiated in the areas where dairying is currently taking place with beneficial effect, especially if leguminous trees could be introduced for the benefit of the pastures. At present, the only form of agroforestry that has been attempted so far has been the growing of shrub and tree crops such as coffee and tea together with various larger trees for shade and for firewood to be used in the processing of the tea or coffee. However, one practice which has shown interesting possibilities is the practice of integrating coffee with the cultivation of silverleaf desmodium ( Desmodium uncinatum ) and fine stem stylo (Stylosanthes guianensis var. intermedia). These pasture species provide a thick cover on the surface which apparently helps to conserve moisture while at the same time providing the coffee crop with nitrogen. In turn, the pastures are grazed by sheep at regular intervals thus keeping their growth in check and at an acceptable level with regard to potential competition for water and other nutrients.

Although climatically this zone is ideal for agroforestry and the practice would result in desirable, beneficial effects, the zone covers only 0.4% of the total land surface of Zimbabwe. With its ideal climate, competition for land here is very high and plantation agriculture and horticulture enjoy a high preference over any other form of land use.

Agroclimatic zone II

This zone has a rainfall in the region of 800 -1,000 mm. It is also an area of relatively high elevation and concomitant cool temperatures. Rainfall is somewhat less reliable, particularly its onset. Some good, fertile soils, mainly Alfisols, typify the region. Climatically, this region is also suitable for the introduction of a wide range of trees and tree crops. However, this being probably the best agricultural land in the country, and also because of the proximity of most of the region to Harare, the capital, land here is at a premium and there has been strong resistance to any form of agroforestry.

A strong tradition of rotational monocultures revolving around maize, wheat, tobacco and, sometimes, groundnuts, is the norm here. There is also a considerable amount of dairy activity and it is in the area of dairying that some possibilities for agroforestry could be examined since this zone suffers some very high temperatures in the three months preceeding the rainy season, namely September, October and November. If nothing else, the introduction of certain tree species could provide shade during that period which is also a period when the 'veld' for grazing is of very poor quality. The selection of tree species for such a practice would have to take into account the severe frosts that occur in this zone.

Agroclimatic zone III

The rainfall here is moderate but comes mainly as infrequent heavy showers which, together with the generally high temperatures, reduces the effectiveness of the rain. The region is therefore subject to severe mid-season droughts. In their agro-ecological zonation, Vincent and Thomas (1960) point out that the region is

'....marginal for maize and tobacco production or for enterprises based on crop production alone... The farming  system  in  conformity  with  the  natural  conditioning  factors  should therefore be based on both livestock (assisted by the production of fodder crops) and cash crops, each playing a varying but approximately equal part.'

This zone probably has the most ideal combination of desirability and feasibility for agroforestry. The farming systems recommended for the zone lend themselves well to the possible inclusion of agroforestry. This could take the form of plants that have a rapid rate of growth and high nutritional value for livestock. At the same time the tree species thus introduced, if carefully selected, could, after the first few years of growth, prove to be hardier and more viable than any of the cash crops currently being produced there.

Agroclimatic zone IV

The rainfall is low (450 - 600 mm yr^-1) and tends to be erratic. Periodic seasonal droughts and severe dry spells during the rainy season are characteristic of this region. Except for the production of some drought-resistant crops in favoured localities, this is a zone in which cattle ranching appears to be the most viable use of land possible. The potential for agroforestry here will be discussed jointly with that of Zone V.

Agroclimatic zone V

Rainfall here is too low and erratic even for the production of drought-resistant fodder crops.

Zones IV and V offer the possibility of introducing some agroforestry systems integrated with livestock production especially as these are areas of extremely high temperatures for most of the year. High temperatures are known to have adverse effects on animal metabolism and therefore on the productivity of the animals. Although rainfall is low, it would be necessary to ensure an adequate supply of water for the trees only in the first year or two because most of the area covered by Zones IV and V has extensive reserves of ground water. Trees would be able to exploit this ground water better than most other crops. In time, the change in vegetation could result in desirable micro-climatic effects.

The possibility also exists to introduce commercial tree crops in these regions. For example, in the Zambezi Valley, some of the local people have successfully grown cashew nuts for their own consumption but the possibility of larger scale, organized production on scientific lines has not been investigated seriously.

Conclusion

Zimbabwe has a wide range of both macro- and micro-climatic conditions which offer reasonable possibilities for agroforestry. In those areas where climatic conditions are optimum, agroforestry faces unfair competition from traditional farming systems which are considered to be a more intensive and more economic use of the land. Some agroforestry systems appear to be possible in the low rainfall- high temperature zones. Here the introduction of agroforestry practices can increase the diversity of agricultural practices and ameliorate the hostility of the environment for other agricultural production systems. Thus the economic potential of these otherwise inimical environments can be increased.

References

Vincent, V. and R.G. Thomas, 1960. An agro-ecological survey of Southern Rhodesia, Part I: Agroecological survey. Government Printer (Federal).