Overstory #45 - Vegetative Erosion Barriers in Agroforestry
In this edition of The Overstory, special guest author Michael Pease, Coordinator of the European and Mediterranean Vetiver Network, describes the uses of vegetative erosion barriers, highlighting their use in agroforestry.
Vegetative Erosion Barriers
The problems of retaining soil, water and plant nutrients where they can support timber growth and fruit production are critical to productive agroforestry. Yet we are steadily losing our soil resource and much of our water and plant nutrients is lost in run-off.
Previously, solutions to problems of soil erosion and to soil and water run-off have been sought mainly from earthworks and engineered constructions. However, such structures often prove to be costly, ineffective, and unsustainable. The solution lies in permanent, vegetative barriers planted on the contour. There is an upright, tufted, deep-rooted and very dense grass that is proving to be one of the prime tools in providing a solution to these problems, particularly in tropical countries. It is called Vetiveria zizanioides or vetiver grass.
Highlight of Vetiver Grass
Vetiver has been grown in various countries for many centuries but principally for the value of the essential oil that is produced from its roots rather than for its soil conservation properties. Vetiver grass originates from Northern India. Within the past 15-20 years it has been planted in some 100 countries. Erosion control barriers using vetiver grass is a low-cost, simple and effective technology, proven by much practical application and research data.
Vetiver can survive extremes of drought, heat, water logging, pH, and, to some extent, cold. It is insensitive to photo period and grows year-round where temperatures permit. It does not harbour pests and diseases to any significant extent. Most importantly, most cultivated varieties of vetiver do not produce viable seed as has been shown in trials in a number of countries including Hawaii. So it is not a weed and not invasive. Any reader interested in growing vetiver grass should check carefully that the plants are of a genotype (genetic selection) that either produce no seed, or produce only sterile seed.
Vetiver is a bunch grass with very rapid growth in warm, moist conditions growing to more than 2m in height and with a remarkably dense and vertical rooting system descending for 3-5m. It is a very strong plant, its roots having a tensile strength equivalent to one sixth of mild steel, and when planted on the contour as a hedge presents a virtually impenetrable barrier through which soil can hardly pass and through which water only passes with much reduced velocity. As silt builds up behind the plant it is capable of shooting from the nodes and rising above the deposited silt to form a natural terrace. Weeds and undesirable foreign grasses, are unable to penetrate through a well-established hedge. It is cheap and easy to plant and maintenance is minimal. Overall, it is not detrimental to crops or forestry grown between hedgerows due to the vertical nature of its rooting system.
Vetiver is tolerant to a wide range of climatic and soil conditions. For instance it has been shown to flourish in temperatures from -9ºC to 50ºC and in areas with rainfall as low as 300mm per annum or as a high as 6,000mm per annum. It survives pH levels as low as 3.3 and as high as 10.6. It has been successfully grown to stabilise soils on the slimes surrounding mining sites in South Africa and tin mine wastes in Malaysia and Australia where it tolerates very high levels of heavy metals. It has also been used for degraded wasteland rehabilitation. It will grow in saline and sodic conditions and even in acid sulphate soils or pure laterite. It survives fire well, pushing out new green shoots shortly after fire has ravaged an area. Consequently, when in its green state, it can be used as a firebreak. Whilst vetiver is growing effectively at 40º North in some countries its primary area of growth is in the tropics and sub-tropics. It is commonly grown to control soil erosion and retain water on slopes in excess of 100%.
Vetiver acts as a pioneer plant growing where other plants would not survive and providing micro-climatic conditions where native species may become established. A good example of this is in Murcia region of Spain where vetiver has been successfully established to control erosion under conditions of 300mm annual precipitation and on very poor soils.
At a time when there is much concern regarding global warming and CO2 emissions, 18,000 ha. of land protected by vetiver grass hedgerows will provide a CO2 sink for the CO2 produced by 100,000 automobiles travelling 20,000 km. per annum!
In the United States, the National Research Council reviewed the potential of the grass and reported that, "vetiver hedges were a cheap, safe, and effective technique to slow erosion and retain soil moisture."
Multiple Uses of Vetiver
Vetiver has a myriad of on-farm and other uses besides those mentioned above, including, for example, use as a living fence and boundary marker. The leaves and roots of vetiver can be used for an extensive range of handicrafts and are excellent for thatching. The young leaves are palatable to livestock and have about the same nutritive value as Napier grass (Pennisetum purpureum). It is used domestically in cooking and insect control and also has medicinal properties.
Vetiver in Agroforestry: Fruit, Timber, and Mixed Systems
There are conflicting views as to how best to use vetiver hedges to aid increased production from fruit trees. As a semi-circle around fruit trees, about 3m from each tree on the down-hill side, it has been shown to be an effective technique with lychees in Thailand where increased yields exceeding 20% were recorded. However, if the hedge forms a complete circle round the tree it prevents moisture reaching the tree rather than retaining it for use by the tree's roots. The technique provides a windbreak that benefits young tree growth in the early stages. Using vetiver hedges planted on the contour with the fruit trees also planted on the contour between the hedges is probably the most dependable method. Competition between the vetiver and adjacent fruit trees is minimal because of the vertical nature of the vetiver rooting system as has been shown in Vietnam. In Trinidad's Maracas Valley it was noted that mango trees benefited from the presence of vetiver hedge barriers due to increase in soil organic matter and moisture. In the Philippines vetiver was shown to be tolerant to heavy shade, but the converse is reported elsewhere. In Malaysia, under rubber and oil palm, it was shown that vetiver is tolerant to moderate levels of shade intensity. Vetiver is only tolerant to heavy shade if it has first been established in full sunlight before being shaded out, as is the case in sugarcane closing over the hedge.
In trials in China it has been shown that vetiver planted in citrus orchards increased soil organic matter from 0.4% to 1.8%. Soil bulk density decreased whilst porosity, organic matter, various trace elements and some 20 amino acids increased. Vetiver hedgerows have also been effectively demonstrated in citrus plantations in Costa Rica. Overall, where vetiver hedges are planted in fruit tree orchards increased yields can be expected due to reduced run-off, the retention of organic material and run-off behind the hedges and protection of tree seedlings against wind.
In the case of timber production it is best to plant the vetiver hedge barriers at the same time or before planting the trees. This allows the vegetative barriers to become well formed and for erosion to be controlled and soil moisture retained to benefit the young tree growth. It is possible that eventually the tree canopy may become so dense that the vetiver will die due to lack of sunlight. However, by then it will have done its work and its initial planting will have been justified through increased timber or fruit production. As a rough 'rule of thumb' it can be said that vetiver will take at least 50% shade after establishment, though this figure is regarded as very conservative by Dr. Julio Alegre, Coordinator at ICRAF in Peru. Over time, silt will build up behind the vetiver hedge and this should be taken into consideration when planting the nearest row of trees above the hedge.
In eucalyptus and teak plantations for example, where run-off is high, plantation layout could be so designed as to allow unshaded vetiver hedges to be sited at strategic points to break the velocity of rainfall run-off. Essentially, the vetiver hedge will provide an understory that complements the tree production.
In areas of high rainfall intensity such as Peru, which may receive in excess of 400mm in a month, ICRAF is promoting the agroforestry practice with farmers of planting each row of fast-growing native tree species such as Inga, Erythrina, Gliricidia and Cassia combined with another row of vetiver at an interval of 4-6m, depending upon slope. Resultant terrace formation is fast and farmers can grow crops between the rows that benefit from the build-up of silt and plant nutrients.
At this early stage in the development of Vetiver Grass Technology (VGT) data is scarce regarding increased timber production resulting from the establishment of vetiver hedge barriers. However, it stands to reason that with increased soil moisture, retention of plant nutrients and reduction in erosion tree growth will be more rapid and overall production will be higher.
Vetiver produces an excellent long-lasting, absorbent mulch which, when placed around fruit trees reduces evaporation and aids in the long-term, build-up of mycorrhizae. The leaves are unattractive to insects. In India it was recorded that 90% of tree seedlings survived when mulched with vetiver as against 30% in an untreated control nursery.
In Guangdong Province, China, an experiment in eucalyptus plantations was commenced in 1991 and observations recorded over a 50 month period by Dr. Liao Baowen of The Research Institute of Tropical Forestry. Two rows of trees were inter-planted, one with vetiver and the other with stylostanthes guyanensis; a third plot was left as control with trees only. The results indicated that the surface liquid run-off in the vetiver plot was 15.7% lower than the stylo and 18% lower than the control plot. The surface solid run-off on the vetiver plots was 30.3% less than the stylo and 54.6% less than control. A subsequent measurement of five-year old eucalyptus showed a 15.9% increase in diameter at breast height, compared to control. Overall, increased tree growth as a result of the vetiver hedge barriers may be as much as 20%.
As mentioned above, vetiver is a pioneer plant but this role can be enhanced when it is planted in conjunction with tree species. In China it has been shown that such practices: assist in the stabilization of slopes; alleviate sheet erosion, especially under storm conditions; provide a more aesthetically pleasing effect than vetiver on its own and; result in greater economic benefits.
Michael Pease is Coordinator of the European and Mediterranean Vetiver Network, a non-profit organisation registered in Portugal. Their address is: Quinta das Espargoas, Odiaxere, 8600-250 Lagos, Algarve, Portugal. Tel/fax: 351-82-79.84.66.
Newsletters produced by The Vetiver Network, 15 Wirt St. N.W., Leesburg, VA, 20176 USA.
Vetiver Grass - A Thin Green Line Against Erosion. Board on Science and Technology for International Development, National Research Council, Washington D.C. 1993.
Baowen L, Dezhand Z and Songfa Z, Research Institute of Tropical Forestry of Chinese Academy of Forestry. 'Studies on the effects of soil and water conservation of vetiver grass in eucalyptus plantations' 1991.
Tam TT, Phien T, Tu Siem N, National Institute for soils and fertilisers, Vietnam 'A promising grass for soil conservation in Vietnam'. Khao Hin Som Royal Development Study Centre, Thailand 'Study on cultivation of vetiver grass to maintain moisture for fruit crops'.
Le LC, Zhenyan G, Shizong L, Hanping X, Forest Research Institute, Guangdong province, China. Grimshaw R.G. & Helfer L., Vetiver Grass for soil and Water Conservation, Land Rehabilitation and Embankment Stabilization. World Bank Technical Paper No 273, Washington DC 1995
Hanping X., Huixiu A., Shizhong L, and Daoqian H South China Institute of Botany, Academia Sinica, Guangzhou, China.
Private communications from: Grimshaw R.G., Truong P., Elevitch C., Greenfield J., and Alegre J.
Related Editions to The Overstory
- The Overstory #73--Buffers, Common-Sense Conservation
- The Overstory #29--Tropical Green Manures/Cover Crops
- The Overstory #22--Pioneering
- The Overstory #17--Microcatchment