Agroforestation of Grasslands in Southeast Asia: WaNuLCAS Model Scenarios for Shade-Based Imperata Control During Tree Establishment - Smallholder Tree Growing for Rural Development and Environmental Services - page 135

concentration in the foliage is quite low (and thus the competition for N not
excessive), the subsequent decomposition of Imperata residues (leaf or root lit-
ter) involves immobilization of mineral soil nitrogen by microbes, further
depleting the generally poor soils (Snelder 2001).
3. Fire: the dry aboveground biomass in dry periods is a well connected, well-aer-
ated fuel that readily burns and in which fire spreads easily. While Imperata
itself survives such fires (and may benefit from their nutrient mobilizing effects
before other plants respond), the trees' survival depends on their size, the height
above the ground of the growing tips, the thickness of the bark which protects
lateral buds, and the nature of the fire (height of the flames, temperatures
reached, spread of the fire to the tree crown).
4. Allelopathy: the roots and rhizomes of Imperata release organic compounds that
inhibit the germination and early growth of many plant species; the effect of
these compounds after the establishment phase of other plants tends to enhance
the effects of N-immobilization in Imperata soils; enriching the soil with nitro-
gen tends to reduce the 'allelopathic' effect.
Aspect 3 (fire) is in practice the most problematic consequence of the presence of
Imperata in agroforestry systems, because a single fire can waste several years of
investment in establishment of trees. Control of Imperata is thus an important target
for tree establishment. Four types of control have been developed in practice:
1. Mechanical control: soil tillage which exposes the rhizomes to the sun and dries
them out has to be repeated several times to be effective; part of the rhizomes
may survive below the usual depth of hoeing or animal-drawn ploughs, but
mechanized ploughing can reach the required depth.
2. Herbicide: the most popular and cheapest currently is glyphosate, available
under a range of trade names.
3. Pressing of the aboveground biomass has a remarkable effect on slowing down
the regrowth by mechanisms that are not fully understood (Terry et al. 1997). The
technique can be used selectively around newly planted trees (Murniati 2002).
4. Shade-based control: reducing the growth rate and, in combination with removal
of aboveground biomass, gradually depleting the rhizome capacity for regenera-
tion (Purnomosidhi et al. 2005). Imperata biomass decreased drastically when
relative light intensity of 20 percent was reached (Hairiah et al. 2000). When
more than 20 percent of sunlight reaches the ground, Imperata still has a chance
in these agroforestry systems.
In practice, a combination of these techniques will have to be used in the various
stages of a developing agroforestry system (Fig. 6.1):
A. Land preparation for food crops using tillage, herbicides or (preferably) a com-
bined approach.
B. Close to the tree and in the stage where there is too much shade for profitable
intercrops but too little for Imperata control, a combination of the pressing tech-
nique and/or selective use of herbicides.