through-flow before the crop cover is established in the first 2-3 weeks of the monsoon season. One of the important objectives of the jhum (shifting agriculture) farmer is therefore to capture this transient resource base as quickly and effectively as possible through mixed cropping and weed management. The jhum farmer ensures effective use of the nutrient gradient on the steep hill slope by employing species that have a high nutrient-use efficiency along the nutrient-poor top of the slope, and the less efficient ones along the base.
Interactions between biodiversity and function at both plot and landscape scale can be seen in examples of the role of key plant species in regulating nutrient cycles in fallow-based systems. Increases in perturbation regimes under shortened agricultural cycles lead to biological invasion of the fallow as well as the crop plots (Drake et al. 1989) by Latin American species. These exotic C3 species, such as Eupatorium spp. and Mikania micrantha, largely occupy nutrient-rich microsites on the hill slope, avoiding competition from the more frequent C4 natives that are nutrient-use efficient. Apart from the fact that the weedy fallows quickly help in checking nutrient losses from the agricultural piot, weed communities at very early stages tend to conserve phosphorus, and a species such as Mikania micrantha concentrates potassium in the plant biomass. This is particularly significant under short, shifting agricultural cycles of about 5 years; what this exotic weed does for potassium conservation under shorter cycles is done by a bamboo species like Dendrocaiamus hamihonii under longer cycles of 10-30 years. Indeed, many bamboo species of the early stages of forest succession (10-50 years) have been demonstrated to conserve nitrogen, phosphorus and potassium in the system, making these nutrients more available at the cropping phase (Ramakrishnan 1994). Among the nitrogen fixers, the Nepalese alder (Alnus nepalertsis) of the early successional forests is particularly significant because the jhum farmer tends to protect it in his jhum plots for socio-cultural reasons. Some of these keystone species thus have a major role in regulating nutrient cycling within the jhum plots, apart from their role in the fallows themselves. This is apart from the plot-landscape interactions related to movement of nutrients from cropping systems under varied intensities and frequencies of perturbation.
The limited number of cultivated species at the plot level represents only a small proportion of the biodiversity of the whole agroecosystem over the landscape. Clearing of forests on the top of the loposequenee can have a profound influence on the hydrology of the farms in the lowest part of the landscape. For example, decades of extensive tree and bush clearance in southwestern Australia for arable cultivation and pasture production has resulted in a dramatic rise in the waterlable, leading to serious salinisation (Peck and Hurle 1973), becausc the removal of deep-rooted woody species has resulted in a major reduction in the overall transpiration by the vegeta-
tion. For these reasons, current effort in Australia is aimed at the re-introduction of Eucalyptus and Acacia trees to reduce the watertable.
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