Sist, P., Brown, N. 2004. Silvicultural intensification for tropical forest conservation: a response to Fredericksen and Putz. Biodiversity and Conservation 13 : 2381-2385
Biodiversity and Conservation 13 : 2381-2385
Silvicultural intensification for tropical forest conservation: a response to Fredericksen and Putz
Introduction
In their recent paper Fredericksen and Putz (2003) (henceforth F&P) recognize the necessity to minimize unnecessary logging perturbations but claim that Reduced Impact Logging (RIL) techniques create insufficient disturbance to permit regeneration of some commercially valuable timber species. They argue that in tropical forests where a valuable species requires substantial disturbance for seedling establishment, intensive silviculture consisting mainly in increasing canopy gaps, is necessary in order to maintain a sustainable flow of timber. We argue that tropical forest silviculture for the sustainable management of its resources is much more complex than the increment of gap as claimed by F&P. We take issue with a number of claims made by F&P.
Most tropical forests are not shaped by substantial disturbances
F&P appear to have confused a simple model of forest regeneration dynamics with the claim made by some environmental groups that natural forests are fragile, pristine environments that should be protected from any disturbance. Aubréville’s (1938) mosaic theory of regeneration proposed that rather than consisting of stable plant associations, tropical rainforest was composed of a patchwork of seral stages. This patchwork is created by a sequence of disturbances that vary in their magnitude. The resultant gaps of different sizes and stages of regrowth are an important source of species diversity of tropical rainforests (Brokaw 1985). F&P’s claim that this paradigm has recently been superseded by a type of ecological catastrophism is fallacious. The role of past major disturbance in contributing to that mosaic has been appreciated for decades (e.g. Stevenson 1927, Jones 1956). Large disturbances are known to play an important part in structuring forests in the Caribbean, parts of Central America and Pacific islands (Whitmore 1989, Brokaw and Walker 1991, Tanner et al. 1991). Furthermore, there is evidence that many (if not most) tropical rainforests experience infrequent large disturbances (Whitmore and Burslem 1998). However, it is wrong to imply that this is the predominant scale of disturbance determining the structure and composition of most tropical rain forests. Their low frequency and rapid forest recovery (e.g. Nelson et al. 1994) mean that their impact is transitory (Burslem et al. 2000).
RIL creates adequate disturbance for the regeneration of many valuable timber species
RIL techniques do not mimic natural disturbance regimes: they are imposed on top of them. The proportion of trees killed by harvesting operations under RIL techniques vary between 7 % in the Amazon to 15% of the original stand in South East Asia (Sist 2000). Logged forests also suffer a much higher natural mortality during the 2-5 years following RIL than primary forest (5% p.a. vs 1% p.a. respectively, Silva et al. 1995, Sist and Nguyen-Thé 2002). In South East Asian dipterocarp forest the disturbance created by RIL (8 harvested trees/ha but 75-80% of the original basal area remaining) was enough to stimulate rapid dipterocarp seedling growth for several years after logging (Sist and Nguyen-Thé 2002). In contrast, in intensively logged and damaged stands (33 % of the original tree population killed and less than 75 % of the original basal area remaining) dipterocarp regeneration was much poorer (Sist and Nguyen-Thé 2002). Numerous field experiments have shown that gaps should be limited to ≤500-650 m2 to favour dipterocarp regeneration and to limit pioneer invasion (Brown and Whitmore 1992, Kuusipalo et al. 1996, Tuomela et al. 1996, Van Gardingen et al. 1998). Forest dynamics modelling suggested that RIL of moderate intensity would also permit a sustainable harvesting rotation of 40 years (volume of about 60 m3/ha at each harvest) while RIL under higher extraction rates and damage, rotation increased to more than 60 years with a substantial risk of favouring pioneer species (Sist and Nguyen-Thé 2002, Sist et al. 2003c).
Regeneration is about more than just gap size
One important development that Fredericksen and Putz (2003) have ignored is the recognition that gap size is only one of many influences on patterns of rainforest regeneration. The gap-phase regeneration paradigm assumed that the most competitive plant was the one that had the greatest relative growth rate in response to the ambient light environment (Denslow 1980). Field experiments have shown that tall plants are able to capture more light and consequently grow faster and cast shade on the shorter plants beneath them. As a consequence, regeneration is often dominated by the tallest plants in a gap regardless of their species (Brown and Whitmore 1992). When the seedling bank and all advance regeneration is destroyed by a disturbance the first plants to re-colonise a gap will often pre-empt the light and delay or inhibit further colonisation by other species. In large gaps these are typically pioneer species that have widely dispersed seeds and a persistent soil seed bank. For silvicultural systems that depend on natural regeneration it is therefore crucial that the forest is already well stocked with abundant seedlings and advance regeneration of desirable speciesprior to logging. It is also important that logging does not destroy those seedlings. F&P argue that many important commercial species such as Swietenia macrophylla, Entandrophragma spp. and Shorea leprosula require big gaps or even catastrophic disturbance to regenerate. However, a number of experimental studies have shown that these species have poorer germination and establishment in very open conditions than in partial shade (Nicholson 1960, Nussbaum et al. 1995, Kyereh et al. 1999, Morris et al. 2000, Hall et al. 2003) making it difficult for them to establish naturally in large felling gaps.
Past experience suggests that substantial opening of the forest canopy typically triggers vine and pioneer infestations that are very costly to control. There are numerous reports of failed attempts to stimulate the regeneration of light-demanding species using substantial canopy opening (Lancaster 1961, Britwum 1976, Lowe 1978, Wyatt-Smith 1988, Bruenig 1996). Moreover, large canopy openings significantly increase forest flammability, particularly during long periods of drought such as those that occur in South East Asia during El Niño events (Dennis 1999).
Another factor which F&P have failed to take account of is that commercially valuable timber species occur in forests across a range of climate types and their response to disturbance varies accordingly. For example, Entandrophragma utile regenerates in large gaps in high forest in the south of Ghana but in the drier north is restricted to moist shaded areas away from direct sunlight (W. Hawthorne pers comm.). Swietenia macrophylla is a species that regenerates profusely in the more open conditions found in the transition zone from open deciduous forest to evergreen rain forest (Brown et al. 2003). In these forests, small-scale disturbances created by controlled logging will be enough to stimulate natural regeneration (Brown et al. 2003). In high forest S. macrophylla persists as a relic population in the process of competitive exclusion. In these high forests as those of the Brazilian Amazon, populations of mahogany are typically represented by very few adults and seedlings and saplings are rare (Grogan et al. 2002, Brown et al. 2003). Under such circumstances large disturbances may be necessary for natural regeneration. However, in this type of population, it is also absolutely essential to leave sufficient adult trees to ensure reproduction and therefore seed production (Jennings et al. 2001). Indeed, the creation of a favourable microclimate for regeneration is useless if reproduction processes are not preserved, an essential point that F&P did not address at all.
Conclusions
Whilst there is evidence that many humid tropical forests have experienced large disturbances in the past, very few commercially important species require cataclysmic disturbance for regeneration. Recent research has shown that RIL techniques are necessary but not sufficient for sustainable harvesting (especially when based solely on minimum diameter cutting limits) (Sist et al. 1998, Sist et al. 2003a, b). Intensive silviculture (meaning the rigorous application of skilled forest management as opposed to substantial opening of the forest canopy) is urgently needed to stimulate the regeneration and growth of many valuable commercial species, not just the most light -demanding ones. This is a complex task which cannot be reduced simply to the manipulation of felling gap sizes. Large gaps may promote fast growth by a small number of light demanding species but they have other ecological ramifications which may not be beneficial for sustainable production. Threshold levels of damage that can be sustained in the long-term have only been quantified for a small number of tropical forests (e.g. a harvesting threshold of 8 trees/ha, and maximum gap size of 600 m2 for mixed dipterocarp forest, Sist et al 2003a, b; or one third of the original basal area in French Guiana, Gourlet-Fleury pers. comm.). But without a good understanding of these thresholds, general recommendations such as F&P’s proposal to increase gap sizes in order to favour a few light demanding timber species, may have disastrous and irreversible effects on forest recovery after logging.
Most experiences over the last century in tropical rain forests around the World show that intensive silviculture was neither effective nor sustainable. Tropical forestry has been dominated by western silvicultural concepts that mainly aimed to favour a limited number of species while eliminating non-commercial ones. In contrast, we believe that modern silviculture should maintain and promote the extreme high diversity of tropical forest as it represents an important biological and potential economical value for the future.
Acknowledgments
We are grateful to Sylvie Gourlet-Fleury, Eric Forni, Robert Nasi and Peter Savill for their comments and inputs in an earlier draft.
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[1] Forestry Department of CIRAD. Convênio Cirad-Embrapa, Belém, Brasil, plinio@cpatu.embrapa.br
[2] Department of Plant Sciences, Oxford University, UK