The challenge of HIV/AIDS: Where does agroforestry fit in? - World ...

Chapter 12: Watershed functions in productive agricultural landscapes109Depth (cm)Depth (cm)ForestMultistrata coffeemain processes are captured in simulationmodels that have reached considerablepredictive ability, the dynamics of soilstructure in terms of decay and recoveryare still largely a black box, constrainingfurther precision of models of water balancefor example. The WaNuLCAS model(van Noordwijk et al. 2004c) uses theempirical reference value for bulk density,BDref , as a ‘fall-back’ value to which soilstructure decay reverts in the absence ofspecific macropore creation activities,which create macropores directly (vanNoordwijk et al. 2004d). This model descriptionsuggests that the most importantparts of a tree for land rehabilitation are thedead leaves that it sheds and the fine andcoarse root turnover it induces.Simple shade coffeein conditions where the soil remains farfrom saturated. Where surface phenomenasuch as crusting rather than soil compactiondominate in the soil physical degradationprocess, recovery may be faster: anytype of mulch that protects the soil fromthe direct impact of rain and sunshine andthat stimulates soil biological activity maylead to recovery in a timeframe of months.Sun coffeeFigure 4. Infiltration patterns for a dye that leaves a dark trace in all macropores it passesthrough. This simulates what may happen during heavy rainfall on four types of land usein the Sumberjaya benchmark area in West Lampung, Indonesia; see Hairiah et al. (2004)and Widianto et al. (2004) for details on the methods and sites.It is thus important to correctly diagnosewhat type of degradation dominates in agiven location, as this will influence thetimeframe for potential recovery. Avoidingcompaction at sites that are still in a naturalforest condition is probably more effectivethan trying to rehabilitate degraded sites.Where surface processes dominate, however,rapid gains by mulch-based restorationactivities can be expected.Standard soil physical textbooks and handbookof methods specify how BD can bemeasured – but not how the data can be interpreted.Bulk density is strongly related tosoil texture and soil organic matter content(which in itself depends on texture), so fora valid interpretation in the context of compaction,we need to derive a reference valuefor a soil with the same texture. A simplescheme is available in spreadsheet form onwww.ICRAF.org/sea as part of the ecologicalmodels that can be freely downloaded.While the water, nutrient and carbon balanceof soils are well understood, and theA further complication arises when werealize that surface litter, depending onits size and weight, is prone to be carriedaway by wind or overland flow of water,leading to a differentiation of the land intomutually enhancing zones of high infiltrationwith deposition of surface mulch,and zones of crusted soil with high runoff.Classification of litter sources by their propensityto transport is only just starting.A macro version of the transport–depositioneffect is known as the ‘tiger bush’striped pattern in semi-arid lands – wherethe degraded zones act as water harvestingsource areas for the vegetated parts.Land rehabilitation can aim at strategicallymodifying the scale of this pattern, but notat a fully homogeneous state.For a full understanding of the tradeoffsbetween productivity (or profitability) ofland use and the implication for watershedfunctions we thus have a reasonably wellequippedtool kit. There are complications,however, such as the differences in time