Hydrological functions of tropical forests: not seeing the soil for the trees?
Introduction
In their introductory paper to this special issue, Tomich et al. (this volume) described the so-called ‘environmental issue cycle’ which consists of seven consecutive stages (Winsemius, 1986). During the first three stages there is a growing awareness and public acceptance of the seriousness of a certain environmental problem, with gradually mounting pressure for action by the responsible authorities. Since this may challenge the effectiveness of existing government policies on the issue, this is usually followed by a debate on the validity of the available evidence on causes and effects. Once a cause and effect chain has been established equivocally in stage 4, options for mitigation of the problem can be considered, negotiated and implemented during the remaining three stages. Naturally, the latter half of the environmental issue cycle hinges on a decisive outcome of such a debate. It is quite possible, however, that the process comes to a halt because of perceived gaps in the understanding of the problem or the quantification of its impacts. Different interest groups may then apply evidence selectively and advocate a position servicing their own interests (Tomich et al., this volume). The environmental impacts of tropical forest clearance and conversion to other land uses, particularly the effect on low flows, represent a case in point, with seemingly mutually excluding viewpoints being expressed not only by different interest groups but also by different representatives of the scientific community. The ‘traditional’ stance is aptly summarised by the following excerpt from Valdiya and Bartarya (1989) with respect to environmental conditions prevailing in the Kumaun Himalaya in northern India:
‘The increasingly greater use of land resources has a telling effect on the quality of life of the people of the region. Springs are drying up or becoming seasonal, and the difference in the volume of water flowing down the rivers during the dry and rainy seasons is commonly more than 1000 times, resulting in the too-little-and-too-much-water syndrome, a common feature of desert country. The evaporation of moisture in the soil on the tree-less slopes is very high, and xerophytes (cacti) are beginning to take foothold on naked slopes. These features could be described as harbingers of the onset of desertification’.
The authors go on to say that:
‘It is reasonable to attribute the change in the conditions of weather as reflected in the decline of rainfall and moisture deficiency in the soil to the deterioration—and decimation in some parts—of the forests. For the forests are known to have great influence on rainfall’.
Similarly, there is a widespread belief that logging of upland forested catchments is the root cause of floods occurring in the lowlands and that flood damage can be eliminated by large-scale reforestation. For example, in the words of Sharp and Sharp (1982): ‘Overlogging is now officially recognised as the cause of the July 1981 severe flooding of the Yangtze’ in China. Similar statements were issued after the devastating floods of 1998 in the same area. A central concept in the ‘traditional’ view of the role of forests is the ‘sponge’ effect of the tree roots, forest litter and soil. It has been claimed even that the roots (sic!) soak up water during wet periods and release it slowly and evenly during the dry season to maintain water supplies (Spears, 1982, Myers, 1983). With this in mind, planting trees in degraded areas is often expected to restore the reliability of streams (Eckholm, 1976, Sharp and Sharp, 1982, Nooteboom, 1987, Bartarya, 1989).
The traditional line of thinking on the hydrological role of forest came under scrutiny in the early 1980s when L.S. Hamilton and others began to question the validity of some of the underlying assumptions.
A heated battle on the hydrological role of forest was fought on the pages of the forestry journal of the former Dutch East Indies (Tectona) some 60–70 years ago. Protagonists of the ‘sponge’ theory (Steup, 1927; Oosterling, 1927) vigorously opposed the ‘infiltration theory’ (which stated that baseflow is governed predominantly by geological substrate rather than by the presence or absence of a forest cover; Roessel, 1927, Roessel, 1939; Zwart, 1927). Others (De Haan, 1933; Coster, 1938; Heringa, 1939) took an intermediate position, emphasising the positive influence of forests with respect to the prevention of soil erosion and floods rather than on dry season flows (see Chin A Tam, 1993 for the English abstracts of these papers which were originally written in Dutch). A paired catchment experiment was set up in West Java in 1931 to study the long-term effects of forest clearing for rainfed agriculture on the flows of water and sediment and so settle the debate (De Haan, 1933) but most of the experimental results were lost during World War II, thus illustrating how the ‘environmental issue cycle’ may remain stuck in the debating phase for many years.
In a classic contribution which may be seen as the beginning of a new and more ‘scientific’ view of tropical forest functioning, Hamilton and King (1983) considered that ‘roots may be more appropriately labelled a pump rather than a sponge’ and that ‘roots certainly do not release water in the dry season but rather remove it from the soil in order that the trees may transpire and grow’. Similarly, they considered that ‘major floods occur because too much rain falls in too short a time, or over too long a time. In either case, the rainfall exceeds the capacity of the soil mantle to store it and the stream channel to convey it’. Because of the paucity of hard quantitative evidence from the tropics proper at the time, many of Hamilton’s statements had to be based on research results from the temperate zone (notably the US, New Zealand, Australia, and South Africa) and professional judgement. Nevertheless, his contentions were confirmed later by a series of in-depth reviews of various aspects of the tropical literature by the present author (Bruijnzeel, 1986, Bruijnzeel, 1989, Bruijnzeel, 1990, Bruijnzeel, 1992, Bruijnzeel, 1997, Bruijnzeel, 1998, Bruijnzeel, 2002a, Bruijnzeel and Proctor, 1995, Bruijnzeel and Veneklaas, 1998). The early reviews by Hamilton and King (1983) and Bruijnzeel (1986) were criticised, in turn, by some who were afraid that these views would lead to a ‘selling out to the enemy’ (Smiet, 1987, Nooteboom, 1987). However, as pointed out by Hamilton (1987b), the ‘attackers’ of the traditional line of thought merely aimed at greater accuracy and realism. Ironically, and perhaps partly as a result of the sometimes rather provocative style used by the early messengers of the new line of thought, there seems to be a growing tendency nowadays to emphasise the more ‘negative’ aspects of forests, such as their higher water use and their inability to prevent extreme floods rather than their protective values (enhanced water quality, moderation of most peak flows, carbon sequestration) (Forsyth, 1996, Calder, 1999, Calder, 2002; cf. Van Noordwijk et al., this volume). As will be discussed in more detail in the section on stormflows and floods it is important to distinguish between the effects of land cover (‘vegetation’) per se and those of soil water storage capacity.
The aim of the present paper is to review the available evidence with respect to the influence of the presence or absence of a good forest cover on rainfall, streamflow totals and seasonal distribution (peak flows, low flows), as well as on erosion and catchment sediment yields in the humid tropics. Although what follows below draws to a fair extent on two earlier literature reviews by the author (Bruijnzeel, 1993, Bruijnzeel, 1996), an effort has been made to update these publications and to highlight soil and geological aspects. Furthermore, particular attention is paid to southeast Asia, in keeping with the regional focus of the Chiangmai workshop and to answer some (though not all) questions raised in the introductory paper by Tomich et al. (this volume). The paper concludes with various suggestions as to what the author perceives as the most pressing research needs with respect to the hydrological role of forests in southeast Asia and elsewhere in the humid tropics.
Section snippets
Tropical forest and precipitation
Although the higher evapotranspiration and greater aerodynamic roughness of forests compared to pasture and agricultural crops will lead to increased atmospheric humidity and moisture convergence, and thus to higher probabilities of cloud formation and rainfall generation (Andre et al., 1989, Pielke et al., 1998), early reviewers of the subject of forests and rainfall concluded that there was no significant effect. Any observations of enhanced rainfall in forested areas were attributed either
‘Contradictory’ results
As indicated in Section 1, a common notion about the hydrological role of forests is that the complex of forest soil, roots and litter acts as a ‘sponge’ soaking up water during rainy spells and releasing it evenly during dry periods. Upon clearing, the ‘sponge effect’ is lost through the rapid oxidation of soil organic matter, compaction by machinery or grazing, etc. (Lal, 1987), with diminished water yield as a result. Indeed, accounts of springs and streams drying up during the dry season
Dry season flow
In areas with seasonal rainfall, the distribution of streamflow throughout the year is often of greater importance than total annual water yield. As indicated earlier, reports of greatly diminished streamflows during the dry season after tropical forest clearance are numerous. At first sight, this seems to contradict the evidence presented earlier, that forest removal leads to higher overall water yields and wetter soils (Fig. 2, Fig. 3, Fig. 4), even more so because the bulk of the increase in
Hydrological effects of (re)forestation
In response to the widely observed degradation of formerly forested land and the rising demands for paper pulp, industrial wood and fuelwood, the need for large-scale reforestation programmes has been expressed repeatedly (e.g. FAO, 1986b, Postel and Heise, 1988, Valdiya and Bartarya, 1989; cf. Brown et al., 1997). It is of great interest, therefore, to examine to what extent plantations and other conservation measures aimed at promoting infiltration are indeed capable of restoring the original
General considerations
Findings such as that overall streamflow amounts from non-forested areas are higher than those associated with forested areas (Fig. 2, Fig. 9), or that changes in stormflow volumes (‘floods’) are determined less by the presence or absence of a forest cover as rainfall becomes more extreme, should not be taken to imply that forest removal could not have serious adverse consequences (Smiet, 1987). On the contrary, surface erosion and catchment sediment yield normally show dramatic increases in
Research needs
Having reviewed the various hydrological impacts of tropical forest conversion in the preceding sections, what are the chief remaining gaps in knowledge hindering the sound management of soil and water resources? Answering this question is perhaps not as straightforward as it would seem at first sight as answers coming from different interest groups are likely to differ (cf. Tomich et al., this volume). There are those who believe that ‘greater emphasis on biophysical research strategies
Conclusions
Available evidence indicates effects of forest disturbance and conversion on rainfall will be smaller in southeast Asia than the average decrease of 8% predicted for complete conversion to grassland because the radiative properties of secondary regrowth quickly resemble those of original forest. And, under ‘maritime’ climatic conditions, effects of land-cover change on climate will likely be less pronounced than those of changes in sea-surface temperatures.
Total annual water yield appears to
Acknowledgements
This paper would not have been written were it not for the convincing powers of Dr. Tom Tomich. I thank him for the opportunity to participate in the Chiangmai meeting as well as for his leniency in accommodating subsequent additions to the original manuscript. The bulk of this paper was written while the author was a guest at the Training Centre for Land Rehabilitation and Agroforestry, Cilampuyang, West Java. I am grateful to Dr. Edi Purwanto and his family, Albert van Dijk, Sigit Enggarnoko
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