Chappell, Tych, W., Yusop, Z. N.A. Rahim, and Kasran, B. 2004 Spatially-significant effects of selective tropical forestry on water, nutrient and sediment flows: a modelling-supported review. In Forests-Water-People in the Humid Tropics, Cambridge University Press, Cambridge. p 513-532.



There is an increasing desire to improve the environmental sustainability of timber harvesting within tropical natural forests. These efforts are, however, hampered by the dearth of robust interpretations of the impacts of selective (non-clearfell) forms of tropical forestry on water, nutrient and sediment flows at meaningful, landscape-scales. This review, therefore, seeks to (a) assimilate the results of studies of selective forestry impacts on catchment-scale, ecohydrological flows in tropical natural forests (as opposed to tropical plantation forests), and (b) examine the value of data-based (time-series) modelling in the assimilation of the most reliable case studies.

The findings indicate that very few studies have measured catchment-scale water, nutrient or sediment flows within natural tropical forests undergoing selective logging. Most of the studies that do exist have been undertaken within one country - Malaysia. Statistical modelling and numerical modelling (using Data-Based-Mechanistic or 'DBM' approaches) of the limited data that does exist, indicate that: (i) catchment water-yield is increased but by less than a factor of two, (ii) the rate of migration of rainfall through a catchment to a river and hence the 'river responsiveness' may be increased, but only slightly and for a short period, (iii) nutrient flows increase by a factor of 1 to 6 in the harvesting year, (iv) river sediment flows are significantly increased by a factor of 2 to 50, though more precise explanation of this range remains unclear, (v) the effect of natural cycles (e.g., El Niño Southern Oscillation) in the rainfall may exert as strong an influence on the river records as the selective forestry activities, (vi) differences in the physical impact of different forms selective forestry remain poorly quantified at the landscape-scale, and (vii) the rate of recovery of the catchment system from selective forestry impacts, in terms of water-yield, nutrient loss and sediment delivery, also remains poorly quantified at the landscape-scale.

Further catchment-scale studies to support, scientifically, explanations of the differential impacts of different selective forestry operations in tropical natural forests, particularly on sediment delivery where the greatest relative changes are seen, are urgently needed. Further, the extension of such water, nutrient and sediment studies over 10 to 30 years is needed to quantify the recovery and hence environmental sustainability of such natural forest systems within the tropics. Data-based modelling approaches, such as DBM, may assist further rainforest catchment studies by allowing us to (i) undertake quality assurance of the existing and new river data, and (ii) extract selective forestry impacts from natural cycles and trends. Once such data-based modelling approaches have indicated that changes have taken place, then process studies (even undertaken at small scales) and more process-based models would be useful in further developing guidelines of more sustainable forestry operations.

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