Tropical forest hydrology
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Reviews of selective forestry impacts:

Spatially-significant effects of selective tropical forestry on water, nutrient and sediment flows: a modelling-supported review

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 Niņo 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. Chappell, N.A., Nik, A.R., Yusop, Z., Tych, W and Kasran, B. 2004. Forests, Water and People in the Humid Tropics, Bonell M. and Bruijnzeel, L.A. (Eds), Cambridge University Press, Cambridge.

Minimising the hydrological impact of forest harvesting in Malaysia's rain forests

A process of formal certification of forest harvesting practices undertaken within Malaysia's rain forests is ongoing. The criteria* and indicators* used to assess the sustainability* of these practices within the Malaysian environment ('MC&I') specifically address measures to minimise damage to the hydrological system. This study details those MC&I and associated benchmarks (or Standards of Performance*) that help minimise hydrological impacts. Ability to meet the hydrological benchmarks during forest certification* was examined using a case study of an independent re-assessment of a Forest Management Unit* (i.e., Selangor State FMU) certified as 'sustainably managed'. Lastly, the consistency of the benchmark standards with current understanding of tropical forest hydrology was addressed, with strengths and weaknesses being identified. Thang H.C., and Chappell, N.A. 2004. Forests, Water and People in the Humid Tropics, Bonell M. and Bruijnzeel, L.A. (Eds),, Cambridge University Press, Cambridge.

Water pathways in humid forests: myths vs observations

This article is a scientist's perspective on the "popular beliefs" of the hydrological role of humid forests and the aquatic impacts of forestry within the tropical setting of Malaysia and temperate environment of the United Kingdom - two areas at either end of the Eurasian Continent. Over the last 50 years, selective harvesting of natural forests has been the predominant form of forestry in Malaysia, while within the UK over the same period, afforestation with exotic conifers and the subsequent patch clearcutting has dominated. The aim of the analysis of forest and forestry impacts within these two particular regions (somewhat different to Japan) is to provoke debate, rather than to provide a definite statement of well attested conclusions. Chappell, N.A. 2004. Suiri Kagaku, 37, in press (in Japanese and English).

Identification of land-use change impacts on tropical hydrological processes

It is widely believed that the impacts of tropical land-use change on hydrological processes are easily observable. Rigorous analysis of the few case studies that are available indicates that many impacts are often not significant at landscape-scales or are short-lived, and often obfuscated by data quality and natural dynamics. This presentation examines these issues, reviews some of the key impacts and presents a rigorous statistical analysis of key studies using Data-Based-Mechanistic modelling. Chappell, N.A. and Tych, W. 2003. Conference on Monsoon Environments: Agricultural and Hydrological Impacts of Seasonal Variability and Climate Change, ICTP, Trieste, 24-28 March 2003.


Dr Nick A Chappell <> 16/09/03