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 134

mosaics. The risk of fire, however, remains a concern for such transformations
(Wibowo et al. 1997; Murdiyarso et al. 2002; Chokkalingam et al. 2007) and inte-
grative models are needed (Van Noordwijk et al. 2001).
Technically, Imperata as a weed can be controlled and Imperata grasslands can
be converted to more productive systems. However, before any so-called rehabilita-
tion takes place a few questions should be answered: Is land conflict related to cur-
rent fire frequency (Suyanto 2007) and maintenance of the grasslands? Will anybody
currently using the grasslands lose out as a result of rehabilitation? Is labour a con-
straint for rehabilitation? Is there a lack of market opportunities for the products of
intensified systems? Are there any other major policy constraints? If the answer is
yes to any of the above, perhaps that issue should be resolved first before proceeding
with the Imperata rehabilitation per se. The agendas of government and develop-
ment agencies are often not grounded in a proper understanding of the local human
and bio-physical ecology of grasslands or of successful local agroforestry practices;
and research on many of the most important dimensions of grassland management
is poorly conducted and/or utilized (de Foresta and Michon 1997; Dove 2004).
Imperata cylindrica (alang-alang, cogon grass) is an efficient colonizer of open
spaces on a wide range of soils (MacDonald 2004). Once it has established its rhi-
zomes the grass is tenacious and able to survive repeated fires that usually don't kill
the growth tips of the leaves at surface level. If they do, Imperata still has the capac-
ity to regenerate from buds on rhizomes in deeper soil layers and establish ground
cover again, before most other plants. This capacity to rapidly regenerate from rhi-
zomes also allows the grass to survive soil tillage (ploughing), unless a repeated
cycle of ploughing, drying the soil and re-ploughing the soil is used. The ecological
success of Imperata has given it the reputation of being among the worlds ten worst
weeds, even though it provides soil cover on soils that otherwise would experience
high rates of erosion. While fast-growing leguminious cover crops such as Mucuna
pruriens can lead to initial control, they may not provide sustained shade to reduce
the vigour of Imperata rhizomes (Hairiah et al. 1993).
The first steps in technically controlling Imperata in the agroforestation of grass-
lands can be achieved by either mechanical or chemical control. Farmers employ a
range of techniques from herbicide or soil tillage to 'pressing', depending on their
resources and the current cost of the technique (Purnomosidhi et al. 2005). Food
crops can be used in the first few years of most tree crops or agroforestry systems
to maintain income and pay for the suppression of Imperata regrowth. However, the
gap between the last food crop interplanting and canopy closure leads to a major
risk of Imperata regrowth and fire occurrence (Bagnall-Oakeley et al. 1997;
MacDicken et al. 1997; Van Noordwijk et al. 1997).
Four aspects of Imperata can influence the growth and performance of trees,
although other mechanisms remain debated in the literature (Christopher and Ervin
2006):
1. Light: the capture of light by the grass (1.0 to 2.0 m high) affects small trees.
2. Water/nutrients: the capture of water and nutrients (N, P, K) by the grass reduces
what is available for trees (Van Noordwijk et al. 2004b); while the nitrogen