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8. Why minimum diameter cutting alone cannot fit with RIL objectives - Plinio Sist*, Jean-Guy Bertault** and Nicolas Picard***


* Cirad-Forêt, Convênio Cirad-Forêt-EMBRAPA, EMBRAPA Amazonia Oriental, Travessa Dr. Eneas Pinheiro, Belem-PA 66095-100, Tel:(+55) (91) 299 45 93, Fax: 276 98 45, E-mail: plinio@cpatu.embrapa.br

** Cirad-Forêt, Campus international de Baillarguet, TA/10C, 34 398 Montpellier Cedex 5, France, E-mail: bertault@cirad.fr

*** Cirad-Forêt, Cirad, BP 1813, Bamako, Mali, E-mail: picard@afribone.net.ml

INTRODUCTION

Since the late 1950s, due to increased use of heavy machinery for timber extraction, the impact of logging on tropical forests has attracted the attention of silviculturists and forest managers. The growing awareness of the need to protect forest ecosystem functions and to maintain biological diversity in production forests has promoted the introduction of reduced impact logging (RIL) in various regions and particularly in Southeast Asian countries (Pinard and Putz, 1996; Sist and Bertault, 1997). Since the mid-1990s, RIL guidelines were produced by forestry research organizations and forestry departments (Dykstra and Heinrich, 1996; Durrieu de Madron et al., 1998; Sist et al., 1998a; Elias, 1999; Vanuatu Department of Forests, 1997; Sabah Forestry Department, 1998). RIL is not only a tool to reduce damage to residual trees, soil disturbance and impacts on wildlife; it is also expected to maintain timber-yield capacity and biodiversity. More recently, modelling tools have led to growth and yield predictions in relation to logging intensity and logging damage (Ong and Kleine, 1995; Favrichon and Cheol, 1998; Favrichon et al., in Press; MacLeigh and Susanti, 2000).

The capacity of the forest to regenerate and maintain its ecological functions depends in the first place on the logging techniques, the logging intensity and the degree of damage. For this reason, logging is considered a most important silvicultural treatment. Because mixed dipterocarp forests exhibit high densities of timber trees, selective logging based on the minimum diameter cutting rule leads to high felling intensities from 10-20 trees/ha and high extracted volumes of 100-150 m3/ha (Pinard and Putz, 1996; Sist and Bertault, 1997).

This paper shows that, under such high logging intensities, RIL objectives cannot be achieved, in terms of damage reduction, yield sustainability and biodiversity. The first part is a synthesis of the main research results related to the impact of logging on forest structure and dynamic processes in mixed dipterocarp forests. Specific examples from two RIL experiments carried out in East Kalimantan (Bertault and Kadir, 1998; Wollenberg and Sist, 1996; Sist et al., submitted; Chabbert and Priyadi, 2000) are provided. The second part proposes complementary silvicultural rules to better reach the RIL objectives. The main objective of this paper is to discuss the principles of complementary logging regulations according to the main ecological characteristics of commercial species (Table 1).

Table 1. Principles and constraints of additional logging rules based on ecological features and behaviours of commercial species

Ecological features/behaviour

Principles

Constraints/limitations

Density

Rare species must not be felled Logging intensity must be density dependent

Definition unclear Complex

Structure

MDCL must be flexible according to the type of structure

Poor knowledge of the minimum diameter of adult trees and regeneration dynamics

Regeneration (dipterocarps)

Minimum spacing between harvested trees in areas with high density of timber to limit gap size < 600 m2 Trees with dbh > 120 cm should not be felled

Definition of spacing not based on reliable experience nor data Reluctance by loggers

Breeding systems

Minimum spacing distance between adult trees

Poor knowledge of pollination and seed dispersal distances

MAIN RESULTS OF RESEARCH RELATED TO RIL

RIL efficiency in reducing damage to forest structure

Until now, studies on RIL have focused mainly on the impact of RIL on forest structure and soils in comparison with unplanned and uncontrolled logging, also called conventional logging (CL) (Sist, 2000). These studies have shown that RIL can reduce skidding damage drastically (Pinard et al., 2000; Sist et al., submitted). Skidding operations are the main cause of tree mortality during logging (Sist et al., 1998b). The proportion of trees destroyed during logging is reduced significantly in RIL by 40 to 50 percent in comparison with CL techniques (Pinard and Putz, 1996; Sist and Bertault, 1997; Elias, 1999; Chabbert and Priyadi, 2000; Sist et al., submitted). In Bulungan (Indonesia), for a similar extracted volume (53 m3/ha in CL and 61 m3/ha in RIL), skid trail length per timber volume extracted was twice as long in CL blocks as in the RIL compartment (17 301 m3 vs 9 090 m3) (Chabbert and Priyadi, 2000; Sist et al., submitted). Skidding damage on residual trees decreased from 25 percent of the original stand in CL to 9.5 percent in RIL.

In the Indonesian experiments, RIL clearly failed to reduce felling damage on the forest stand significantly (Sist et al., 1998b; Sist et al., submitted). Although vine cutting prior to logging is regarded as a promising technique to reduce felling damage, its effectiveness is still very controversial (Cedergen, 1996). At present, there are no available felling techniques to reduce overall felling damage in tropical forests. Directional felling, commonly applied in RIL, aims essentially to place the logs in position to facilitate ground-skidding extraction and, if possible, to limit damage to potential crop trees. However, this technique cannot reduce the overall damage on the stand, which mainly depends on the height of the tree, the size of its crown and topography (Cedergen, 1996).

In the two experiments in Indonesia (Berau and Bulungan), the proportion of trees destroyed and damaged in RIL plots under the high felling intensity (n >8 trees/ha in Berau, n >9 trees/ha in Bulungan) was similar to that recorded in CL; in both techniques, this affected about 50 percent of the original stand (Sist et al., 1998b; Sist et al., submitted).

Impact of logging techniques and intensity on forest dynamics

Minimizing the effect of logging damage is insufficient to achieve sustainable forest management. It is equally important to define logging intensities compatible with forest-yield and regeneration capabilities. The main impacts of logging techniques and intensity on the processes of forest dynamics are reviewed.

In Southeast Asia, and particularly in Indonesia, studies based on reliable growth and yield data and on how logging damage intensity affects forest dynamics processes, are still limited (Manokaran and Kochumen, 1987; Ong and Kleine, 1995). In Berau, forest dynamics processes were monitored over a six-year period including a period of four years after logging in 12 4-ha plots (200 m x 200 m), each divided into four 1-ha squares or subplots. To assess the effect of logging damage intensity on forest dynamics processes, regarding the logging techniques, the 48 resulting subplots were divided into 4 groups according to the proportion of remaining basal area (BA) (Table 2).

Table 2. Main structural characteristics of STREK[5] subplots before and after logging according to three main groups of logging damage intensity and control plots (G1 = remaining BA ³ 80 % of the original one, G2 = remaining BA 70- 80 %, G3 = remaining BA < 70 %, G0 = control plots, no damage)


G1
Low damage

G2
Moderate damage

G3
High damage

G0
Control

Total number of subplots

11

14

11

11

Mean felling intensity (harvested trees/ha)

5.8 ± 2.2

8.3 ± 3.3

13.9 ± 3.0

-

Mean % of damaged trees

15.3 ± 5.6

21.5 ± 6.0

25.4 ± 5.5

-

Mean % of trees destroyed

14.8 ± 4.6

22.0 ± 4.9

33.0 ± 6.7

-

Mean % of BA remaining after logging

86.2 ± 4.6

75.4 ± 19.7

58.6 ± 7.7

-

Mean density before logging (1990)

557.7 ± 71.9

540.2 ± 80.3

481.9 ± 55.6

527.9 ± 56.9

Mean BA before logging (1990)

32.8 ± 4.9

31.8 ± 5.5

29.5 ± 1.7

30.7 ± 3.1

Mean density of dipterocarps before logging

139 ± 43.0

113.1 ± 35.1

104.5 + 28.1

109.3 ± 23.0

Mean BA of dipterocarps before logging

15.5 ± 3.6

14.8 ± 3.6

15.7 + 3.1

14.5 ± 2.9

Mean density after logging

486.4 ± 80.3

429.4 ± 65.4

331.0 + 64.2

524.1 ± 54.7

Mean BA after logging

28.3 ± 4.9

23.9 ± 3.6

17.3 ± 2.7

30.7 ± 3.2

Mean density of dipterocarps after logging

117.9 ± 34.9

85.8 + 26.1

63.6 + 20.8

108.1 + 23.0

Mean BA of dipterocarps after logging

12.4 ± 3.3

9.3 + 2.2

6.4 + 2.3

14.5 + 3.1

Based on a six-year monitoring period (1990-1996), we built a matrix model to assess the impact of logging on long-term forest dynamics processes. The model was used to estimate the return time on each subplot applying the mean logging intensity and damage characteristics of each group of damage (G1, G2 and G3, Table 2). Return time was defined as the time required after logging to reach 90 percent of the steady-state density of harvestable dipterocarps (dbh[6] > 60cm). We used the steady-state density rather than the initial density because the time required to reach the initial state was generally too long. A very small variation in the initial density drastically increased the return time sensu stricto. The model also allowed us to assess the rotation length in each damage group.

Mean return times in G1 (low damage), G2 (moderate damage) and G3 (high damage) are respectively 66, 96 and 106 years and statistically different (ANOVA[7], F = 3.75, df = 2, P = 0.03). Hence, there is a strong impact of logging damage intensity on forest dynamics: the higher the damage the longer the time of forest recovery.

After logging, density of pioneers increases proportionally with the amount of damage; the most damaged stands show the highest density of pioneers (Figure 1). In the three groups, pioneers reach their highest density 20 years after logging and their maximum BA at 30 years (Figure 2). These results suggest that 30 years after logging, pioneers enter a phase of senescence to reach their original density recorded before logging at about 80 to 100 years after logging (Figures 1 and 2).

Figure 1. Simulation of pioneer density dynamics in the three groups of logging damage in Berau (G1: diamonds, squares, G3: triangles)

Figure 2. Simulation of pioneer BA dynamics in the three groups of logging damage in Berau (G1: diamonds, G2: squares, G3: triangles)

In the three groups, dipterocarps reach their maximum density at t = 50 years (Figure 3). The mean densities in the groups at t = 50 years were similar (G1= 122/ha, G2 = 124/ha, G3 = 128/ha) but statistically different (ANOVA, F = 15.7, df = 35, P <0.01). However, at t = 50 years, G1 shows the highest BA (13.9 m2/ha, 94.5 percent of the original BA) followed by G2 (12.7 m2/ha, 86.4 percent of the original BA) and G3 (11.7 m2/ha, 79.6 percent of the original BA; ANOVA, F = 16.04, df = 35, P <0.01, Figure 4). The time required for dipterocarps to reach 90 percent of their original BA varies significantly among the groups (ANOVA, F = 7.58, df = 35, P <0.001) from 45 years in G1 to 65 in G2 and 85 in G3 (Figure 4).

We simulated six felling cycles under constant time t and constant extracted number of trees and applied several rotation lengths varying from 20 to 100 years with the following intermediary values: 24, 28, 32, 36, 40, 50, 60, 70, 80 and 90 years. The extracted number of trees was the mean number of felled trees in the three groups of damage: 6 stems/ha for G1, 8 trees/ha for G2 and 14 trees/ha for G3. The extracted volumes (G1 = 44 m3/ha, G2 = 78.5 m3/ha, G3 = 130 m3/ha) were calculated based on the average volume of dipterocarps in each dbh class tabulated in Favrichon and Cheol (1998). In G1, G2 and G3, the shortest sustainable rotations were respectively 23, 41 and 92 years, and significantly different (one-sided Wilcoxon significant at the 5 percent level). These rotation cycles give mean harvesting volumes of 2.4 m3/ha/yr, 2.0 m3/ha/yr and 1.4 m3/ha/yr respectively in G1, G2 and G3.

Figure 3. Simulation of dipterocarp density dynamics in the three groups of logging damage in Berau (G1: diamonds, G2: squares, G3: triangles)

Figure 4. Simulation of dipterocarp BA in the three groups of damage in Berau (G1: diamonds, G2: squares, G3: triangles)

DEFINITION OF COMPLEMENTARY RULES FOR SELECTIVE LOGGING

Under high-felling intensity, RIL techniques do not reduce damage on forest stands effectively. The long-term consequence of high-damage rates is an increase of the rotation cycle (>80 years). Moreover, besides the yield consideration, RIL aims also to limit impacts on biodiversity (Putz et al., 2000). Considering the diversity of tree species only, considerable damage to forest structure and large felling gaps are likely to favour light-demanding and fast-growing species, whereas shade-tolerant species are more vulnerable to such severe disturbance (Oldeman and van Dijk, 1990). Therefore, heavy logging associated with high damage can modify forest-floristic composition substantially. Commercial species face drastic shifts in their density and structure. Their capability to adapt to altered environmental conditions created by logging and to maintain populations in the ecosystem after logging will depend mainly on the following features: original population density, population structure, regeneration dynamics and breeding systems. If complementary rules to the MDCL must be found to fit with RIL objectives, they should be defined according to these main features.

Population density

Dipterocarp forests and rain forests in general, are characterized by high species diversity. Usually, tree species are represented by only a few individuals. Dipterocarps can show a wide range of density from 0.05 adult trees/ha for rare species (e.g. Shorea ochracea in Berau) to two adult trees/ha (dbh > 50 cm) for the most common species (e.g. Dipterocarpus acutangulus and Shorea parvifolia in Berau; Nguyen-Thé and Sist 1998; Sist and Saridan, 1999). In tropical forests, even the most common species exhibit low densities. In Berau, in a 12-ha inventory of primary forest, Elateriospermum tapos, with only 16 trees/ha (dbh >10cm), representing only 3 percent of the total stem density, was the most common species (Sist and Saridan, 1999). The definition or concept of rare species is, therefore, very difficult and often associated with the concept of endangered species. In their tentative definition of ecological criteria of vulnerability to population declines after logging, Pinard et al. (1999) considered rare species to be those with less than one adult (dbh > 20 cm)/ha. In a RIL project in the Brazilian Amazon, the CIKEL company defined rare species (and therefore protected from logging) as those with less than seven adult trees in a 100-ha block. Similarly, in Cameroon, species with less than five stems (dbh >20 cm) in a 100-ha block are not harvested (Eric Forni Pers. Comm.). Because of the variability of species density among regions and even within the same concession area, it is difficult to formulate specific rules to regulate felling intensity for each timber species. However, it is critical to recognize that RIL guidelines should include specific and practical recommendations regarding these species. It is important to address the issue of rare species in RIL guidelines and define rules according to the data provided by forest inventories. Moreover, density is clearly not the single criterion to consider as population structure and regeneration dynamics are other important features for species maintenance.

Population structure

The impact of logging on the ecology and maintenance of a given species is linked to the original structure of the species. In tropical mixed dipterocarp forests, three main types of population structure can be recognized (Rollet, 1974, Figure 5). Dipterocarps mainly show the structure of type I (Appanah and Weinland, 1993). For this type, and in the case of dipterocarps, the MDCL can be generally applied, as the growth of potential crop trees will be favoured by the removal of adult trees. In type II (e.g. Agathis borneensis in Bulungan), adult trees are the main component of the population and juveniles are only few. Thus, logging intensities based on the MDCL will reduce the adult population density radically with dramatic consequences on reproductive ecology and regeneration success. Populations with type II structure are likely to be light-demanding trees, whose seeds are unable to germinate under low-light conditions. However, following logging, seeds dispersed in gaps will find favourable conditions to germinate and saplings will have optimal light intensities to develop and grow. For these species, regeneration success after logging will depend mainly on the maintenance of their breeding and dispersal systems. For this type of structure, it is suggested to increase the minimum diameter limit to leave sufficient numbers of adults able to ensure reproduction. In the case of Agathis borneensis, in Bulungan, fixing the MDCL to 80 cm will represent a mean of 3 trees/ha vs 5 trees/ha for an MDCL of 60 cm.

Type III is rare in mixed dipterocarp forests. In this type, applying the MDCL will result in a very low extraction density of this species, whereas the overall population density is relatively stable. It is likely that in these populations, trees reach their sexual maturity at a dbh lower than the fixed MDCL. For such populations, it should be possible to decrease the MDCL to about 45 cm (Figure 5).

Figure 5. The three main types of tree population structure in mixed dipterocarp forest (type I = dipterocarps; type II = Agathis borneensis; type III = rare mixed dipterocarp forest; arrows show the suggested MDCL for each structure)

Regeneration dynamics

Although dipterocarps are climax species (Swaine and Whitmore, 1988), growth of seedlings and saplings is stimulated by canopy openings (Ashton, 1998). However, even the most early light-demanding dipterocarps reach a maximum growth rate under moderate light intensity, which occurs in rather small gaps, while pioneers require a much larger canopy opening to germinate and grow (Clearwater et al. 1999; Sist and Nguyen-Thé submitted). Based on these observations, several authors have recommended that gaps created by logging should be limited to 500-650 m2 to favour advanced regeneration growth for dipterocarps and to limit pioneer invasion (Kuusipalo et al., 1996; Tuomela et al., 1996; van Gardingen et al., 1998). However, gap size resulting from logging is determined mainly by the density and the spatial distribution of felled trees. In mixed dipterocarp forests, and in tropical forests in general, many commercial timber trees exhibit a cluster distribution pattern. Hence, felling in patches with a high-timber density, while applying the MDCL, will lead to excessively large gaps. The complementary rule that we suggest is to define minimum spacing between harvested trees and to apply directional felling in such a way that trees are felled in different directions to create small and single gaps. The minimum spacing distance has not been studied in tropical forests. This rule has been applied recently in Central Africa with a minimum spacing between harvested trees of 30 m (Durrieu de Madron et al., 1999). The main objective was to reduce the number of large gaps created by the felling of several trees in a small area. The efficiency of this rule to reduce the number of large gaps has not been demonstrated clearly and more experiments are needed, particularly in the context of mixed dipterocarp forests. The felling of big trees (dbh >120 cm) usually with extensive crowns, causes significant damage to the residual stand and creates rather large gaps. It is suggested to retain these trees as seed bearers to avoid excessive damage.

Breeding systems and genetic diversity

Selective logging removes only the biggest adult trees of commercial interest, which are especially important for ensuring the reproduction of the species. For many tropical tree species, including dipterocarps, outcrossing is the usual mode of reproduction. However, the self-incompatibility systems developed by a wide range of dipterocarps are relatively weak (Bawa, 1998). Although dipterocarp flowers are visited by a wide range of insects, thrips and bees are recognized as the main pollinators of dipterocarp flowers (Ashton, 1982; Appanah, 1990; Bawa, 1998). After logging, distances between adult trees may be increased to the extent that the inter-tree movement of pollinators is reduced considerably and pollen dispersal becomes inefficient. Because of the weakness of the self-incompatibility systems of dipterocarps, logging favours inbreeding. This can have important consequences on the reproduction and the population’s genetic structure. Fruits set in self-pollinated flowers suffer from higher abortion rates than fruits from cross-pollinated flowers (Bawa, 1998) and increased inbreeding after logging leads to a reduction of the genetic diversity within a population, whereas genetic differentiation among populations increases. To keep breeding systems unchanged after logging, the most suitable rule would be to define, for each species, a minimum spacing between adult trees that should not exceed the mean pollination and seed-dispersal distances. Unfortunately, our knowledge of the breeding systems of dipterocarps is still very limited (Bawa, 1998). Because seed dispersal is another important factor of gene flux, information on seed-dispersal distances is also important.

CONCLUSIONS

The main objective of this paper was to discuss the principles of complementary logging regulations according to the main ecological characteristics of commercial species. Practical and detailed rules could not be defined because these must be based on data from pre-harvest inventories, which, so far, have been used mainly in RIL for skid-trail planning and delineation of protection areas. However, stand inventories that generally include all commercial species from at least 40 cm dbh or below (Sist et al., 1998), provide valuable information about density, structure and the spatial distribution of commercial species. Geographic information systems offer an opportunity to analyse spatial distribution of species according to different factors (soil, topography canopy disturbance). This area of research should be developed in the future to assist foresters in defining more sophisticated logging rules than the MDCL. More fundamental research on regeneration dynamics and breeding systems is needed to improve logging practices. Minimum-spacing rules are still very difficult to apply because our knowledge of the mean-pollination and seed-dispersal distance is limited. Regeneration dynamics of commercial species provide valuable and practical information as several studies have demonstrated (Kuusipalo et al., 1996; Tuomela et al., 1996; van Gardingen et al., 1998; Clearwater et al., 1999). This area of research must be encouraged in the future.

The additional logging rules proposed here are based primarily on the ecology of commercial species. However, the impacts of logging on wildlife are also potentially important in defining rules to improve logging practices.

ACKNOWLEDGEMENTS

This paper was inspired by two research projects in East Kalimantan - the STREK project (1989-1996), which was a cooperative research and development initiative involving Cirad-Forêt and the Forest Research and Development Agency (FORDA) of the Ministry of Forestry of Indonesia; and the Forest, Science and Sustainability: Bulungan Model Forest Project funded by the International Tropical Timber Organization (ITTO) and implemented by the Center for International Forestry Research (CIFOR) and FORDA.

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Wollenberg, E. & Sist, P. 1996. The Bulungan research forest. 1996 Annual Report. Bogor, Center for International Forestry Research. pp. 10-11.


[5] STREK is the acronym for Silvicultural Techniques for the Regeneration of logged-over forests in East Kalimantan.
[6] dbh = diameter at breast height.
[7] ANOVA = Analysis of variation

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