Fact Brief: Tree-planting for watershed management

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1 UWASNET Fact Brief: Tree-planting for watershed management One of the most common and widely promoted means of catchment protection in Uganda is tree planting. Its proponents laud it as a quick fix to most of the environmental issues, like soil & riverbank erosion, flooding, water conservation (as bringers of rain ) and last but not least also an income generation opportunity. Especially the latter, which in practice usually means the creation of tree plantations of exotic species like Eucalyptus, has also resulted in a fair share of critics, claiming environmental damage and the destruction of biodiversity. What is needed however is a more balanced view on the issue of tree-planting and on many cases also a better understanding of the scientific basis. The positive or negative (side) effects of planting trees or even if they serve the intendedd purpose at all, very much depends of the specific location and tree species. Furthermore, economic opportunities and compatibility with modern lifestyles need to be balanced with the conservation of natural biodiversity and traditional uses. Maybe the most often heard general misconception is that trees increase rainfall or water availability, while in fact they might just do the opposite depending on the specific site. As usual there is some truth to it however, and the general perception probably stems from two facts: The (remaining) forest cover is usually found on mountain sides that exhibit a natural geographic increase in rainfall due to uplifting of moist air ( orographic lift ) Due to the slowdown of winds and the higher evapotranspiration in woodlots, the micro-climate is a lot moister and large forest areas can lead to cloud formation and subsequent localized rain (from recycled evapotranspiration). However this does not mean there is more water in the form of rain or otherwise. In fact due to the higher transpiration of densely vegetated areas and the interception of rain and following direct evaporation on the large surface area formed by the canopies, more water is transferred to the atmosphere than in non-forested areas (although if that can be counted as a net loss is under debate). On the other hand, forest areas intercept and retain more of the rainfall, which would otherwise be lost to surface run-off, which would not only cause soil-erosion but potentially also floods (although these usually occur when the storage capacity of woodlots is already fully exhausted, thus only minor floods

2 can be prevented). Overall the benefits of a mature forest cover in relation to water can be summarized as follows: A slowdown in water run-off that both lessens erosion and the peak of floods. An overall more equal release of water over the dry periods even though the total amount of available water is not increased and can be locally even less. At a regional level, large forest-areas are likely impacting the overall availability of water through capturing it in a continuous recycling cycle. Large scale deforestation is therefore believed to severely threaten the water security of a region in the long run. There can be other indirect beneficial effects of tree-cover however; By slowing down wind-speeds, trees planted in strategic locations can both prevent wind erosion (one of the main drivers of desertification in dry areas) and also lessen evaporation on close by open-water bodies (like valley dams). Larger scale afforestation projects in dry areas should however be avoided, especially with fast growing trees like Eucalyptus, which consume a lot of water (see box). Dry-land Acacia trees from which fuel wood and Gum Arabic can be harvested are a potential option however. Specific issues of tree plantations Compared to natural forests, which once lost can only be restored to their full ecosystem value over centuries, tree plantations can be described as biological deserts. Their biodiversity is very low, with hardly any species other than the densely planted trees able to survive within. The dense canopies of the preferred fast growing exotic species like pines or eucalyptus prevent any undergrowth and their leaf litter is full of slowly degrading substances that can spoil the soils. This lack of vegetation on the ground around the trunks of the trees can actually worsen soil-erosion and propagate gully formation and subsequent sediment pollution in nearby open water bodies. Dense tree plantations with exotic species are therefore not a suitable land-management practise from a watershed management perspective. However these species are usually quite robust and can grow on heavily degraded soils, thus if the alternative are bare degraded hill-slopes, these can be a better than nothing option. Controversy around Eucalyptus It is common amongst environmentalists to advocate against planting of Eucalyptus trees, which have been introduced in Uganda from Australia in 1912 and are very popular for commercial forestry due to their quick growth and easy management. Based on various studies, Eucalyptus is believed to use too much water, drains nutrients from soils and increasingly displaces native tree species. Most of these claims are to some extend true; yet again it very much depends of the specific conditions. Relatively speaking Eucalyptus is actually using water very efficiently, that is the water needed for the amount of wood produced is quite low compared to other trees. Its fast growing properties, coupled with its ability to partially drain soils with especially high moisture contents (wetland boundaries) and the deep tapping root do however result in ecologically undesirable high water use under certain conditions. Furthermore, densely planted Eucalyptus forests barely sustain any other wildlife and their thick canopies prevent undergrowth which has a negative effect of the retention of soil. All in all, when well managed, Eucalyptus forests can be a good choice as a cash crop and much needed source of wood. It is however seldom the best choice when catchment protection is the main objective.

3 Economic advantages of wood growing By itself, commercial tree growing has many potential advantages even for small holder farmers. Fast growing tree varieties like Eucalyptus or Pines can give farmers a very good return of investment in a timeframe that is not outside typical individual considerations. While the trees might take thirty or so years to fully mature to be used as high quality trunk wood, plenty of lesser quality wood is produced much earlier through proper woodlot management. Trees are usually planted very densely to promote strait and upwards grow, but as the trees grow there is a need to thin out the plantation to give space to the highest quality trees. Much of this earlier cut wood can be used as simple poles for construction purposes or as fire-wood and for charcoal production. Other relevant factors making commercial tree growing relevant are: Trees are a relatively stable investment over a longer time frame, making them act as a sort of tree-bank or even tree-insurance which allows one to save up for a later time should the need or opportunity for investment arise. In certain instances they can even be used as collateral for a commercial bank loan. Many trees can be planted quite reliably on land that is less suitable for food crops, yet can yield a similar overall profit. Compared to other cash-crops that are mostly produced for export (and thus subject to large world market price fluctuations), wood has a relatively stable domestic demand. Trees can be a form of securing land tenure for transient populations spending significant time working in nearby cities, however reverting to farmland later on is somewhat difficult. Depending on the tree species, other profitable side effects can be gained. There are direct ones like those of fruit and rubber trees (incl. Gum Arabic production in drier areas), or less direct ones like flowering for honey production etc. Carbon-credit schemes can generate a small but continuous income even without cutting the trees. However, as much as tree-planting can be a beneficial mean of livelihood diversification one should not be blinded by its potential economic benefits over other important environmental considerations. Permaculture (e.g. Agroforestry) Intercropping trees with agricultural plants like ground-nuts etc. can be a suitable land-management practise especially in sloped areas. It has been shown however that it is not a suitable practise (previously promoted under the name Taungya ) for commercial tree growing in Uganda. If done, the emphasis should be therefore on tree species that offer other benefits like fruit trees or products like bark-cloth and traditional medicine. It can be also be a good practise for cocoa growing, as these plants benefit from the shadowing and cooling trees can provide.

4 Tree planting and nursery establishment It is generally advisable to establish local tree nurseries before commencing a larger tree-planting effort. Not only does this ensure Very detailed instructions and troubleshooting guides for tree the participation of the local communities (including some income nursery establishment are generation possibilities) and the availability of local tree varieties, but it also prevents too high stress of seedlings due to longer available by the FAO for free online. Local sources of expertise are for example the Tooro transport and lack of water. Botanical Garden in Fort Portal or Establishing a tree nursery is relatively easy and can be done with minimal cash input. There is s need for some means of providing (for commercial tree varieties) the NFA and the SPGS. shade and sufficient water and some high quality soil like compost that can be easily compartmented in small plastic bags or tin cans. For commercial trees high quality seeds need to be supplied in addition, but local ones or those that can regenerate themselves from a cut stem (so called vegetative propagation) can usually be sourced more easily near the nursery. It should be noted however that not all trees are easy to grow and might require expert input on the specific procedures. Another specialized technique which can be learned by local nursery caretakers is graftingg of trees. This technique of combining different trees together is especially popular with fruit trees, where for example the root-stock of a very robust citrus tree can be used as a base for a high yielding and tasty orange tree that would otherwise not grow well. Other common grafting methods include joining special bred mango trees with a local rootstock. When planting trees along riverbeds, timing and the use of well established older seedlings (of specific species) is of utmost importance. It has to be ensured that the planted trees have sufficiently grown to survive the first flooding event, or else the planted trees will be lost. Another often overlooked issue when planting trees with the specific objective of preventing soilhas closed and sufficient erosion on steep slopes is thatt during the first few years until the canopy undergrowth with a dense root network has formed, young trees alone have very little effect. Such interventions thus need to be always complemented by other faster growing plants with the risk that the trees are overgrown by them if those are not cut regularly.

5 Alternatives to trees Growing bamboo can be a very good alternative to planting trees. While it does not provide high value logs, it grows even faster and (with the right techniques) can be a good source of wood-fuel and construction material. It is also very easy to propagate (some running varieties even spread quickly by themselves) and can be continuously harvested over the year. Its relatively shallow but very dense roots are also good at stabilizing top-soils and the quickly closing canopy protects the ground from heavy rain erosion very reliably and shortly after planting. If the prevention of uphill soil-erosion is the main objective, local bamboo species should be likely the preferred option over tree planting especially because they yield actual results more quickly. Summary From a water resources management perspective, tree planning is by far not the optimal solution as it sometimes believed to be. Their impact on the prevention of soil erosion and floods is probably overestimated, with other methods of sustainable agriculture being more beneficial when done properly. Tree planting also has a long lag-phase before giving the intended effect as found in mature wood-lots and the risk of premature cutting is high. Never the less it can be an important supplementing activity, potentially resulting in improving nutrition (fruit trees) and diversifying livelihoods (commercial tree planting). In addition it can lessen the pressure on natural forests, which do show many of the intended effects often falsely attributed to tree plantations. When implementing a watershed conservation project, emphasis should be on restoring and protecting parts of the natural highly bio-diverse vegetation, which amongst other efforts can include the planting of indigenous tree species; preferably those that have a product that is of use for the surrounding communities. Recommended readings (available at the UWASNET resource centre) UK Forestry Commission, Water Use by Trees Information Note 2005 Uganda National Forest Authority, Uganda s Forests, Functions and Classification June 2005 Disclaimer: This fact brief was prepared in 2013 in coordination with the UWASNET IWRM, Climate Change and Environment Thematic working-group. The Author (Krischan Makowka, Technical Advisor) was supported financially by HORIZONT3000.