Fire is an important factor in the Imperata grassland ecosystem. It prevents or slows down the natural succession to shrubs and/or secondary forest vegetation and is a major threat to (agro)forestry options for Imperata grassland rehabilitation. Forest fires can also be a primary cause of the extension of Imperata grasslands. In this review an attempt is made to integrate biophysical and socioeconomic aspects of the causation of fires in a conceptual model. Fire effects on vegetation are examined. The management options at the level of a farmer, a village community and a national government are analyzed.
Tag: imperata cylindrica
Soil fertility management for reclamation of imperata grasslands by smallholder agroforestry
Imperata cylindrica grasslands are widely believed to indicate poor soil fertility. Soil fertility improvement may have to be an important component of a reclamation strategy. Data for Sumatra, Indonesia indicate, however, that Imperata occurs on a broad range of soil types and is not confined to the poorest soils. A direct role of Imperata in soil degradation cannot be ascertained. In many instances, however, Imperata soils are low in available P and effective N supply. The use of rock phosphate in combination with erosion control (‘fertility traps’) and legume cover crops can be effective in restoring soil fertility. Case studies for a number of sites in Sumatra have confirmed the practical possibility of reclaiming grasslands for food and tree crops.
Food-crop-based production systems as sustainable alternatives for Imperata grasslands?
Purely annual crop-based production systems have limited scope to be sustainable under upland conditions prone to infestation by Imperata cylindrica if animal or mechanical tillage is not available. Farmers who must rely on manual cultivation of grassland soils can achieve some success in suppressing Imperata for a number of years using intensive relay and intercropping systems that maintain a dense soil cover throughout the year, especially where leguminous cover crops are included in the crop cycle. However, labour investment increases and returns to labour tend to decrease in successive years as weed pressure intensifies and soil quality declines. Continuous crop production has been sustained in many Imperata-infested areas where farmers have access to animal or tractor draft power. Imperata control is not a major problem in such situations. Draft power drastically reduces the labour requirements in weed control. Sustained crop production is then dependent more solely upon soil fertility management. Mixed farming systems that include cattle may also benefit from manure application to the cropped area, and the use of non-cropped fallow areas for grazing. In extensive systems where Imperata infestation is tolerated, cassava or sugarcane are often the crops with the longest period of viable production as the land degrades. On sloping Imperata lands, conservation farming practices are necessary to sustain annual cropping. Pruned tree hedgerows have often been recommended for these situations. On soils that are not strongly acidic they may consistently improve yields. But labour is the scarcest resource on small farms and tree-pruning is usually too labour-intensive to be practical. Buffer strip systems that provide excellent soil conservation but minimize labour have proven much more popular with farmers. Prominent among these are natural vegetative strips, or strips of introduced fodder grasses. The value of Imperata to restore soil fertility is low, particularly compared with woody secondary growth or Compositae species such as Chromolaena odorata or Tithonia diversifolia. Therefore, fallow-rotation systems where farmers can intervene to shift the fallow vegetation toward such naturally-occurring species, or can manage introduced cover crop species such as Mucuna utilis cv. cochinchinensis, enable substantial gains in yields and sustainability. Tree fallows are used successfully to achieve sustained cropping by some upland communities. A variation of this is rotational hedgerow intercropping, where a period of cropping is followed by one or more years of tree growth to generate nutrient-rich biomass, rehabilitate the soil, and suppress Imperata. These options, which suit farmers in quite resource-poor situations, should receive more attention.
Agroforestation of grasslands in Southeast Asia: WaNulCAS model scenarios for shade-based imperata control during tree establishment
In the stage of land use evolution where smallholder tree-based systems are desirable as replacement of Imperata cylindrica (and similar) grasslands, agroforestry can provide a gradual and rewarding approach to the transition. There tends to be, however, a gap between the last opportunity for food crop interplanting and canopy closure providing shade-based control of grass and weed growth. In such period, regrowth of Imperata enhances the risk of fire and failure of tree establishment. We analyzed the duration of this ‘Imperata regrowth window’, for a range of planting patterns and choice of tree species in Lampung (Indonesia) and northern Mindanao (the Philippines). Simulations of agroforestation scenarios with the WaNuLCAS model (‘water, nutrient and light capture in agroforestry systems’) focuss on the Imperata regrowth window as the period between 50 percent and 15 percent of ground-level light availability. The simulation results first of all confirm a well-known fact: young trees of most species are not able to compete with Imperata and partial weeding around the tree stem base is absolutely necessary to get most trees started, with the possible exception of Paraserianthes falcataria. Although Acacia mangium is a fast growing tree, a more intensive weeding regime will double tree growth. The improvement of initial tree growth speeds up tree canopy closure and reduces subsequent Imperata regrowth window by two to more than five years according to the model, with periods longer than five years associated with slow initial growth rates. There is, according to the model, only limited opportunity to reduce risk exposure by modifying tree spacing.
Can rehabilitation of Imperata grasslands help to protect the remaining rain forests?
Forest conversion for unsustainable land use practices in the humid tropics often results in the formation of coarse grasslands, dominated by imperata cylindrica. Rehabilitation of these grasslands may help to alleviate the pressure on further forest conservation. Evidence in favour and against this hypothesis in review on the basis of results of phase 1of the global Alternatives to slash-and-Burn project in Indonesia.
Agroforestry approaches or rehabilitating degraded lands after tropical deforestation
There are three main types of degraded lands that result from slash and burn agriculture: degraded fallows, degraded pastures and Imperata cylindrica grasslands. Together they may cover as much as 250 million hectares, and have the advantage for recuperation of being close to road, market and urban infrastructure. Land degradation can be due to one or more causes: physical (e.g. soil compaction, erosion), chemical (e.g. increases in soil acidity, decline in available nutrients) and biological degradation (e.g. loss of microsymbionts, weed encroachment). The use of bulldozers to clear land accelerates degradation and generally requires more labour, energy and/or purchased inputs for reclamation. The current research knowledge base has focused upon alternatives to slash and burn that start with fertile, cleared forests rather than degraded systems. In the past, agroforestry approached for reclaiming the three types of degraded systems. Int he past, agroforestry approaches for reclaiming the three types of degraded lands were employed but most of the experiences are not based on sound research. The establishment of Acacia mangium on Imperata grasslands is one success story. In the case of degraded fallows and pastures, land regeneration strategies have been identified (e.g. economically and biologically improved fallows; silvopastoral systems) but the essential research is still to be done. Fundamental concepts of restoration ecology with emphasis on nutrient cycling, plant succession and weed dynamics should set the state for understanding how to turn degraded areas into productive lands. Coupled with appropriate policies and other socio-economic considerations, such work will provide viable alternatives to land abandonment after tropical deforestation.
Imperata economics and policy
Should policymakers — or anyone else — care about millions of ha of Imperata grasslands The answer depends on the balance between costs of conversion to other uses and the net benefits produced in economic growth, poverty alleviation, and protection of the environment. The first section on Imperata economics sets up the analytical framework to address this question and draws on the wider development economics literature to consider whether growth and poverty alleviation are conflicting or complementary objectives. Although evidence is limited, it suggests smallholder-based agroforestry could provide the same economic growth with greater poverty alleviation than large-scale forestry estates. There is, however, no substitute for project appraisal for specific settings. The second section on Imperata policy reviews whether policy distortions and market failures provide a sufficient rationale for direct policy intervention to promote tree planting on Imperata grasslands. Estimates of imputed values of carbon sequestration to alleviate global warming are presented for Acacia mangium and rubber agroforestry. The conclusion summarizes the policy research agenda and examines the desirability and feasibility of policy intervention to promote carbon sequestration through Imperata grassland conversion to tree-based systems.