The use of Eucalyptus in agroforestry systems of southern Rwanda : to integrate of segregate

Eucalyptus tree species widely grown in the tropics and subtropics predominate in the Rwandan landscape and primarily serve to supply the firewood and charcoal, timber and other non-timber forest products that are in high demand. They also play a significant role in the conservation of the environment by protecting soils against the erosion and landslides that are common in the study area. The trees are managed in woodlots of varying sizes owned by farmers, private institutions and local governments. The scattered rural households countrywide influence the distribution of eucalypt woodlots and cropped fields, creating a mosaic distribution of the two landuse types. This results in an intimate interaction between annual crops and the woodlot trees next to the fields. Eucalyptus trees are known to be very competitive, suppressing crops in such settings to the extent that crops fail in the field zones nearest the woodlots. On the other hand, farmers get firewood, timber and other wood products as well as income from sales of timber and firewood and prefer to retain these very competitive trees on their fields. Controversy about Eucalyptus in other areas led to policymakers restricting the growing Eucalyptus, especially near wet areas. This thesis aimed to study the interaction between Eucalyptus woodlot trees and crops in fields next to the woodlots in southern Rwanda. General allometric equations were developed relating tree diameter and height to total and component aboveground tree biomass in coppice and planted stands and these were used to quantify aboveground tree biomass. A competition experiment was run in Huye and Gisagara district (southern Rwanda) to investigate competition between woodlots and crops, with maize as the example field crop. Environmental resources of soil moisture, soil nutrients and solar radiation were assessed in a spatial gradient from tree–crop interface. Variation of the resources with distance from the tree–crop interface was also assessed. This was related to maize growth and grain yield at the different distances from the tree–crop interface. Using the resulting biomass equations, woodlot biomass production was quantified and an economic evaluation was done using the net present value approach to make trade-offs of growing combinations of Eucalyptus woodlot for producing timber, poles and firewood and maize, considering varying area proportions and growing sole maize. Extra woodlot biomass produced as a result of the edge effect was estimated and compared to the loss in grain yield due to competition. Eucalyptus water use was estimated using the leaf area–tree diameter allometric relationship and the rate was compared to that of Eucalyptus reported in other areas and that of key crops grown in the area. General equations were derived for predicting aboveground total and component tree biomass. It was found that soil moisture, nutrients and light were significantly reduced in the crop fields next to the woodlots in a zone of up to 10.5 m width. Here, grain yield was 80% lower. However, the affected zone represents 10.5% of a 1 ha cropped field and when the loss is spread over the whole crop field area, the actual grain yield loss per hectare falls to 8.4%. Trees growing within 8 m of the perimeter of the woodlots had 17.8% bigger diameter at breast height and 34.5% more aboveground biomass than trees further away from the perimeter. Combining maize cropping with eucalypt woodlots was more profitable than solely cultivating maize, and revenue from extra wood gains due to edge effects exceeded the corresponding revenue losses in maize yield. Crop field size and orientation were very important. The smaller the field the more the edge effects since the proportion of the total field affected becomes more significant as the field size decreases. Rectangular crop fields will suffer more when longer sides adjoin woodlots. Woodlots, especially if small, also suffered from edge effects. Farmers with bigger landholdings are likely to benefit more than those with very small landholdings and they are likely to prioritise maize production (a less profitable option) over woodlots, for food security reasons. Recorded potential tree transpiration (3 mm d-1) was 10% below local annual precipitation. The observed dry month water deficit could be covered by reductions in leaf area, stomatal closure and changes in soil water storage. A sensitivity analysis showed that 50% leaf area reduction corresponded to a 32.8 mm decline of potential tree transpiration. The moderate eucalypt water use rate observed may be a function of trees’ small size and low tree stocking. The observed eucalypt water use rate is smaller than the range reported for eucalypts in Africa and was also smaller than that of key annual crops in the study area. It is concluded that Eucalyptus woodlots are worth maintaining in the study area to ensure the supply of firewood and charcoal, timber and non-timber forest products, and for soil erosion and landslide control, as well as for generating income for the local communities. They should, however, be managed as woodlots, since managing them as single trees in cropland may lead to severe tree–crop competition. The current practice of not planting Eucalyptus trees in the major catchment areas in the country should be maintained. This should be extended to riparian zones and near wetland zones, since eucalypt water use is reported to increase with water availability. Managing woodlots as short rotations and increasing initial tree spacing may contribute to resolving issues related to catchment hydrology associated with eucalypt plantations. This could be enhanced by planting mixed stands of Eucalyptus and other tree species, preferably N-fixing, as these would also be useful in improving soils and woodlot productivity. It is also recommended that eucalypt species be selected to allow for undergrowth development in the woodlots that improves soil erosion control and biodiversity. Finally, a holistic, integrated planning and management approach involving different stakeholders in natural resource management is recommended, to achieve agricultural production that is more compatible with the protection of ecosystems without impairing farmers’ livelihoods.