ARE PERMANENT RUBBER AGROFORESTS AN ALTERNATIVE TO ROTATIONAL RUBBER CULTIVATION? AN AGRO-ECOLOGICAL PERSPECTIVE

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1 Forests, Trees and Livelihoods, 2011, Vol. 20, pp A B Academic Publishers Printed in Great Britain ARE PERMANENT RUBBER AGROFORESTS AN ALTERNATIVE TO ROTATIONAL RUBBER CULTIVATION? AN AGRO-ECOLOGICAL PERSPECTIVE GREGOIRE VINCENT 1,2*, FAUZAN AZHIMA 2, LAXMAN JOSHI 2,3 AND JOHN R. HEALEY 3 1 Institut de Recherche pour le Développement (IRD), UMR AMAP, F-34000, France 2 World Agroforestry Centre (ICRAF) SE-Asia, PO Box 161, Bogor 16001, Indonesia 3 School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd, LL57 2UW, UK ABSTRACT Many rubber smallholdings in Indonesia have developed into rubber agroforests as a result of the extensive management of the plantation. The resulting complex multi-species agroforests have a number of environmentally beneficial characteristics including a high level of natural biodiversity. Most environmental benefits would be significantly enhanced if these systems were not taken periodically through a new cycle of slash-and-burn, as normally happens when latex yield drops to uneconomic levels. This paper explores, from an agroecological perspective, the potential for such cyclical systems to be developed into permanent agroforests providing sustained latex yield over a longer time frame without a slash-and-burn intervention. Evidence is provided from direct observations, interviews with farmers and the results of specific agronomic experiments. Enrichment planting of seedling or grafted-clonal rubber plants into existing rubber agroforests resulted in low growth rates as a result of shading from canopy trees and probably below-ground competition. Below-ground competition also probably continued to limit rubber growth at the sapling and pole stage within agroforests. High investment has to be made in physical protection to prevent mortality of planted rubber in agroforests due to wild pig damage. However, direct grafting of clonal buds onto naturally regenerated rubber seedlings within agroforests provides a potential technical alternative. It is concluded that, though technically possible, such development towards permanent forest cover implies a significant change in management strategy and is unlikely to develop spontaneously on a wide scale in the study area in Jambi Province, Indonesia. Key words: Jungle rubber, Hevea brasiliensis, enrichment planting, slash and burn, tree competition, grafting, Indonesia INTRODUCTION Cases of introducing commercially valuable, often exotic, crops into forests through enrichment planting abound in the tropics (Michon et al. 2007). Natural vegetation is thinned to some degree and the new crop is planted without clear cutting. Documented cases of such enrichment planting strategies include coffee (Coffea canephora Pierre ex Fröhner) in Indonesia (Huitema 1935), *Author for correspondence: gregoire.vincent@ird.fr

2 86 VINCENT ET AL. cinnamon (Cinnamomum burmanii (Nees) Blume) in Indonesia (Burgers 2004) cocoa (Theobroma cacao L.) in Cameroon (Sonwa et al. 2007) and Miang tea (Camellia sinensis (L.) Kuntze) in northern Thailand (Preechapanya 1996). In Indonesia, rubber has been incorporated, as an enriched fallow, into a traditional crop-fallow system that normally involves slashing and burning at the end of each fallow period. Smallholdings are the dominant form of rubber plantation in Indonesia, representing an estimated 86% of the total rubber area in 1997 in Sumatra (Anonymous 1998). Rubber was quickly integrated in the traditional slash-and-burn agricultural system and has gradually evolved from a mere fallow enrichment to the major cash crop for most smallholders (Gouyon et al. 1993). A large proportion (over 75%) of smallholdings is managed quite extensively. More intensive management practices involving improved germplasm, use of chemical fertilizers and systematic control of weedy vegetation seem to be limited to smallholdings that are within or close to government rural development projects (Penot 2001). Traditional practices led to the development of extensive systems consisting of multi-species uneven-aged forests where the dominant tree species is rubber (Hevea brasiliensis (Willd. ex A. Juss.) Müll. Arg.), commonly referred to as rubber agroforest (de Foresta & Michon 1993; Gouyon et al. 1993). In such a system, rubber seedlings are initially planted after slashing and burning the pre-existing vegetation and are usually grown in association with rain-fed rice cultivation for one or two years. Fruit trees such as Durian (Durio zibethinus Murray), Pete (Parkia speciosa Hassk.), Duku (Lansium domesticum Corr. Serr.) and Jengkol (Archidendron jiringa (Jack) I. Nielsen) may also be planted while naturally regenerated valuable wild fruit and timber species are retained. After the final rice harvest the plantation is given little care until rubber trees are ready to be tapped when they are c cm diameter at breast height (DBH). Then, about 10 years after planting, a full scale weeding of the unwanted vegetation (which may include canopy trees) is usually carried out by slashing before harvesting of latex is started. During slashing naturally regenerating individuals of selected species may be spared. These would commonly include wild fruit and valuable timber species. Future weeding activity is usually restricted to clearing paths and to spot weeding to favour the growth of naturally regenerating trees of rubber or other valuable species. A secondary forest structure slowly develops over time. Despite its low productivity per unit area compared with pure plantations, such a system is attractive to farmers because it is less demanding of inputs and is more flexible: it requires very little effort in addition to the labour-intensive rubber harvesting, while tapping intensity can be adjusted to farmers needs and/or to market prices much more readily than in an intensive system where economic recovery from high periodic investment costs is a constraint (Vincent et al. 2010). To a certain extent the resulting situation is not very different from that of naturally occurring rubber trees tapped in the Amazon forest. From the point of view of rubber production, the major difference is the greater density of rubber

3 AN AGRO-ECOLOGICAL PERSPECTIVE 87 trees in the agroforests. Maintaining an acceptable density of rubber trees is indeed crucial for smallholder farmers. Hence, when the number of productive trees drops below a certain threshold and the latex yield decreases to an uneconomic level, the agroforest will normally be subjected to slash-and-burn, followed by a new planting cycle. An emerging alternative strategy used by smallholders relies on enrichment planting: rubber seedlings are inter-planted within a productive agroforest in order to maintain a sufficient number of tappable trees to guarantee sustained latex yields for a longer period. This practice, locally called sisipan, has recently been recognized as being widespread amongst farms in Jambi, Sumatra (Joshi et al. 2003). The practice extends the economic life span of the agroforest by delaying the next slash-and-burn event. Initiating a new cycle with slash-and-burn deprives the smallholder of income from the plot of land for five years (in case of intensive management practices) to ten years (in the traditional extensive system) and also entails a considerable investment in terms of labour (often requiring cooperation with neighbours) although some annual crops may be cultivated in the first two to three years. Rejuvenation of a rubber plot through slash-and-burn also involves significant risks for the farmer. The cleared land may be invaded by Imperata cylindrica (L.) P. Beauv., an aggressive light-demanding weed that very much delays the growth of young rubber trees through competition and may even threaten their survival through its combustibility in the dry season (Purnomosidhi & Rahayu 2002). Young rubber plantations are also very susceptible to damage by wild pigs (Sus scrofa L.) which are abundant in certain areas and may require the plot to be guarded intensively during the unproductive years of rubber establishment (Williams et al. 2001). Where slopes are steep, complete clearing of the vegetation also has the disadvantage of exposing the highly susceptible soils to erosion (Young 1997). From a global perspective, reducing the frequency of slash-and-burn will provide a number of environmental benefits. It will reduce the smoke hazards that have in the recent past been so serious as to lead to public health problems and international disputes, even though smallholders may not be the most significant contributors to land clearing by fire (Tomich & Fagi 1998). It will also increase the time-averaged carbon stock of the system (Palm et al. 1999). In addition, it can enhance the potential of rubber agroforests to preserve biodiversity by allowing late-successional species to regenerate and become fully established and mature. This may be essential for the maintenance of populations of those species such as certain epiphytes or forest understorey herbs (Beukema et al that are only able to become established under a mature forest canopy. It also enhances the populations of species that only reach reproductive maturity after a long time period and will fail to set seed if the frequency of slashand-burn disturbance is too high (the relationship between tolerance of frequent disturbance and rapid maturation in plant species has been widely recognised in other ecological contexts (Grubb & Hopkins 1986). In turn, this increased plant species richness will contribute to sustaining a more diverse fauna.

4 88 VINCENT ET AL. In the following sections, we review current evidence that can help assess the productive potential of permanent rubber agroforests and the feasibility of enrichment planting practice under an existing canopy. The first section focuses on direct observations made on growth and survival of rubber trees in agroforests. The second section brings together data from a series of experiments in order to investigate the important agro-ecological constraints associated with replacement of individual rubber trees within a mature rubber agroforest. Finally, in the third section we examine the potential of direct grafting as an alternative strategy to incorporate high yielding germplasm in an existing rubber agroforest and its likely consequences. DIRECT OBSERVATIONS Natural regeneration of rubber in agroforests and the long term maintenance of rubber tree density Latex production over long time-spans in rubber agroforests is strongly influenced by the intensity and the quality of tapping, which will determine the length of the economic life of a tree, and eventually by the progressive replacement of the initial rubber tree population. In rubber estate monoculture plantations, latex production has been observed to increase gradually in the early years, then plateau and finally decrease once exploitation of second generation renewal bark starts. The more intensive the tapping, the shorter the economic life of a rubber tree as excessive bark consumption will not allow for proper bark renewal (Webster & Paardekooper 1989). Wounds may also be sustained to the deeper cambium layer that will prevent proper regeneration of the bark and the associated laticiferous tissue. After many years of being tapped, the surviving trees show a thin and scarred bark, ultimately making further tapping impractical. A decrease in the latex yield of a tree over time may also be expected due to ageing. Gomez et al. (1972) have reported that untapped trees of 25 years age show a fall-off in latex vessel ring production. In any case, low intensity and careful tapping will only delay the inevitable decrease in individual tree production. In addition, tree mortality (due to root diseases, wind damage etc.), will gradually decrease the overall number of mature rubber trees in a plot and, ultimately, long-term sustained production will have to rely on rubber tree replacement. In rubber plantation estates in Indonesia, the reduction in production after c years of exploitation makes replanting the favoured option (D. Boutin, pers. comm., 2001). In the rubber agroforests of Jambi, the rejuvenation of the population of productive trees is largely based on natural regeneration of rubber seedlings in the agroforest, and is actively promoted by farmers. Naturally occurring seedlings may be left in place and their growth simply favoured (by local weeding). Alternatively seedlings can be transplanted to other places deemed more suitable within the plot. Seedlings may be directly transplanted or, more

5 AN AGRO-ECOLOGICAL PERSPECTIVE 89 commonly, their leaves and roots are trimmed and the seedlings are stored in running water for 2 to 3 weeks (until the terminal bud starts swelling, indicating a new flush of leaves) before being replanted. Seedlings may also be brought in from outside the plot, thus reducing the dependency on local regeneration (Joshi et al. 2003). Once the density of productive rubber trees falls below a certain threshold level, which depends on each farmer s perception, enrichment planting of rubber seedlings is no longer the preferred choice of farmers for rejuvenating their old jungle rubber agroforests; instead they would then opt for slash-andburn (Wibawa, pers. comm.). They believe that enrichment planting does not guarantee successful replacement of old and unproductive rubber trees with new plants. Farmers have observed that, although the rubber seedlings become established in the first few years, the saplings seldom grow to tappable size and form without active management to reduce competition from surrounding vegetation (Joshi, unpublished). Vines such as Mikania micrantha H.B. & K., which develops preferentially in gaps, and climbing ferns such as Dicranopteris linearis (Burm.f.) Underw. are cited by farmers as the most detrimental weeds for growth of rubber saplings. How dependable is the natural regeneration of rubber for long-term maintenance of the density of latex producing trees in agroforests? To address this question, data collected between 1999 and 2001 from 86 rubber agroforest plots in Muara Bungo District (Jambi Province) were combined. The information on age and management regime in those plots was obtained through farmer interviews. Ages of these agroforests were estimated to range from less than 10 years to approximately 70 years. There is a strong negative relationship between age of plot and density of rubber trees over 10 cm DBH (Figure 1). The large scatter of rubber density found for a given age nevertheless indicates that density of rubber tree population varies greatly with general site conditions and/or management regime. Overall, active enrichment planting appeared to be either only a recent practice, or not to have been practised at all. In most plots, some level of selective weeding to support natural regeneration had taken place. The strong negative relationship between plot age and tree density arises despite the fact that natural regeneration of rubber seedlings is abundant in young agroforests and is not affected by canopy closure (Kartika 2001). Rather, it was found that seedling abundance was positively correlated with mature tree density. This implies that the critical step is not germination and initial seedling establishment but seedling and sapling survival and growth to a tappable size (which is examined in the following section). Survival of seedlings and young saplings is mostly related to the incidence of wild pig damage. Wild pigs are a major pest of rubber seedlings and saplings in Sumatra, and this has significant implications for permanent agroforests. For instance, a recent survey (ICRAF, unpublished) of 40 farmers in five villages practising rejuvenation by active planting of rubber within their agroforests revealed that the overall survival rate for transplanted seedlings was less than

6 90 VINCENT ET AL. Figure 1. Relationship between age of plot and rubber tree density in 86 agroforest plots in Jambi Province, Indonesia. The fitted line is based on the Distance Weighted Least Square algorithm in Systat 9. Sources: (+) ICRAF, unpublished; (x) Philippe, 2000; ) Kartika, 2001; ( ) Hardiwinoto et al., % one year after planting, mostly due to pig damage (interview-based figures). This occurred despite the deployment of a range of protection strategies (individual fencing, planting large seedlings and trapping pigs seemed to be the most effective techniques). One probable reason why the wild pigs damage seedlings and saplings by uprooting or breaking them is that the availability of rubber seeds, which form an important part of their diet, is limited for around half of the year (Sibuea & Tular 2000) and Figure 2). Farmers also cite deer, monkeys and termites as occasional but important pests, but they cause a much lower incidence of damage than wild pigs. The wild pig problem which is common to all forms of rubber cultivation seems to have become more acute in the last decade as forest conversion to agriculture has intensified (Sibuea & Tular 2000). The effect of the decrease of the wild pigs natural habitat and natural predators (mostly tigers) is compounded by cultural restrictions on hunting pigs, as Muslims form the majority of the population in the Muara Bungo area. However, farmers in some villages are making efforts to control the wild pig population by inviting non-muslim people, including Suku Anak Dalam people (indigenous forest dwellers), to hunt in the rubber agroforests.

7 AN AGRO-ECOLOGICAL PERSPECTIVE 91 Figure 2. Phenology of rubber trees in agroforests and rainfall pattern in the Muara Bungo area (Jambi Province, Indonesia). Data on phenology obtained through farmer interviews. Muara Bungo average rainfall (Holmes 1998a). AGRONOMIC EXPERIMENTS Relative importance of competition for light, water and nutrients Even though regeneration may be abundant in rubber agroforests, the proportion of rubber seedlings that reach a tappable size is low. Apart from direct pest damage, death and very slow growth may result from both above- and belowground competition, both likely to be intense in an agroforest environment. Experiments show that growth under an existing canopy is slow and affected by canopy closure. However, light may not be the single or even the most important factor limiting growth, as below-ground competition for nutrients and water is also likely to be intense, even though the area immediately surrounding the rubber sapling is cleared of other vegetation. Direct evidence of belowground competition in an agroforest environment is lacking because it is difficult to quantify. Similarly, alleviating nutrient shortage of seedlings in a forest environment by use of fertilizers is likely to increase below-ground competition by favouring significant root ingrowth from surrounding trees to the area of soil around the seedling (Cuevas & Medina 1988; Dalling & Tanner 1995); Combining fertilizer application with root trenching is one possible way of overcoming this problem, but is not without difficulties such as the invasion of the area by deep exogenous roots growing below the trench. The potential

8 92 VINCENT ET AL. limitation to the lateral extension of the root system of the rubber seedling caused by the trench is also of concern, particularly if the major limiting factor is not nutrients but water (availability of water may be particularly limited by the available rooting area). Trenching is also a technique quite disruptive to the soil around the targeted seedling. Foliar application of fertilizer could be an alternative way to explore nutrient limitation: however it has many complications and is most appropriate for micronutrient studies (Marschner 1995). This study presents results comparing the reduction of growth associated with increasing canopy closure in rubber agroforests with results obtained under artificial shading (with limited competition for water and nutrients), to test the hypothesis that light is the overriding factor limiting the growth of rubber seedlings in an agroforest. MATERIALS AND METHODS A summary of soil chemical characteristics and texture for the five sites is given in Table 1. Soils from Sumatra (three sites in Jambi and one site in Lampung) are typically acidic soils with low Cation Exchange Capacity and low exchangeable K. Soils from Jambi also show a high Al 3+ saturation rate. The long-term annual rainfall pattern recorded in Muara Bungo shows a relatively dry period between June and September (Figure 2). Rubber survival and growth under natural shade enrichment planting experiments To test the tolerance of rubber plants to low light levels and high betweenplant competition like that encountered with enrichment planting, two similar experiments were set up. Rubber plants of four different clones were interplanted in two existing rubber agroforests in order to explore: a) the sensitivity of rubber plant growth to canopy openness, and b) the performance of various genetic materials in agroforest conditions. Enrichment Planting 1: This experiment was set up in a 35-year-old productive agroforest, with 16 m 2 basal area and c.150 tappable rubber trees per hectare, in Muarakuamang village territory (S 01 o E 102 o ). Four clones (PB260, RRIC 100, RRIM 600, BPM1) and illegitimate GT1 seedlings (F1 progeny of GT1 mother trees) were compared. Rubber clones are actually grafted plants, a clonal scion being grafted onto a non-clonal rootstock. GT1 is the most abundant improved genetic material in the area and it is commonly used as a rootstock for the grafting of clonal buds. Spontaneously regenerating seedlings in the agroforest (of unknown origin, thereafter named wildlings) were also included in the analysis as a control. The planting area was pre-stratified into three levels of canopy openness using a canopy-scope (Brown et al. 2000). To characterise light availability, hemispherical photographs were taken above each seedling three times during the 20 months of monitoring.

9 AN AGRO-ECOLOGICAL PERSPECTIVE 93 TABLE 1 Soil characteristics in the different experimental sites: two agroforestry enrichment planting sites in Jambi, Sumatra; nurseries at the Biological Management of Soil Fertility (BMSF) sites in Lampung, Sumatra and Cianjur, West Java; fertilization trial site in Jambi, Sumatra. Site Soil property Agroforest (1) Agroforest (2) Lampung Cianjur Plantation Enrichment planting Enrichment planting Nursey Nursery Fertilization trial depth (cm) sand (%) silt (%) clay (%) ph H ph KCl C (%) N (%) C/N P HCl (mg/kg) K 2 0 HCl (mg/kg) P Bray (mg/kg)) K 2 0 Morgan (cmol/kg) Ca (cmol/kg) Mg (cmol/kg) K (cmol/kg) Na (cmol/kg) Al 3+ (cmol/kg) H + (cmol/kg) CEC (cmol/kg) BS (%) ECEC (cmol/kg) Al saturation (%) Soil Type (USDA) Inceptisols a Inceptisols a Ultisol b Oxisol c Inceptisols a a (Anonymous 1990); b (van der Heide et al. 1992);. c (Anonymous 1979)

10 94 VINCENT ET AL. In February 2000, 6 7 plants of each of the five selected genotypes, growing in plastic polybags, were randomly allocated to a planting spot within each of the three light strata. Plants had one or two fully developed leaf whorls, and an average stem diameter of 0.7 cm, measured 5 cm above grafting point (clones) or at 10 cm height (seedlings). Twenty-three wildlings of a similar diameter to the 92 transplanted rubber plants were selected within the natural regrowth to encompass a similar range of canopy openness. All 115 plants were individually fenced using bamboo shafts and wire to minimize pig damage. The Global Site Factor (GSF), i.e. the fraction of the total radiation above the canopy that reaches the measurement point below the canopy (Becker et al. 1989) was determined from the hemispherical photographs using HemiView software v 2.1 (Delta-T Devices). Estimates of the percentage of diffuse light and the atmospheric transmittivity which are needed to calculate the GSF were obtained from quantum sensor measurements taken at the same site both above and below the canopy (Azhima 2001). Enrichment planting 2: The agroforest used for this second experiment, using only the four clones of Experiment 1, is located in a different village (Sepunggur, S 01 o ; E 102 o ) and showed a similar structure (basal area was 15 m 2 ha 1 ) and similar age. In this experiment, forty locations were selected to encompass a similar range of canopy openness as in the first experiment. In February 2000, in each selected location, four plants, one of each clone, were planted in a 2 2 m square. Light availability was again assessed by taking hemispherical photographs (in the middle of each group of four plants) three times during the monitoring period that lasted 17 months. Average GSF was then calculated using the same estimates of the percentage of diffuse light and the atmospheric transmittivity as in the first experiment. In addition to survival, in both experiments the following variables were measured for each plant every three months: stem diameter, total height, total number of leaf whorls and total number of green leaves. Additional observations included scoring of the incidence of an unidentified leaf blight disease. Artificial shading experiments Two artificial shading experiments were set up in nurseries to explore the response to different light levels. Shading intensities of c. 55%, 80% and 91% were obtained by using neutral shading net. Young plants grown in plastic pots were transplanted at a low density (1 1.5 m) into plots under the shading net and were clean weeded during the whole month study period. At planting time rubber plants had at least one fully developed leaf whorl and an average stem diameter of 0.5 cm. Daily watering was provided during the first month following transplantation to ensure quick establishment. No fertilizer was applied. One experiment was established in the nursery of the Biological Management of Soil Fertility experimental site in Lampung, Sumatra (S 4 31, E ) and used GT1 seedlings (planted in April 1999 and monitored for 16 months).

11 AN AGRO-ECOLOGICAL PERSPECTIVE 95 The second experiment was located in a nursery at Cianjur, Java (S 6 21, E ) and used clonal GT1 plants (planted in March 1999 and monitored for 14 months). Long-term mean annual rainfall at both sites is c mm with a moderate dry season during June to September in Lampung and June to August in Cianjur, during which monthly rainfall is below 150 mm (Holmes 1998b). Fertilization trial in Simpang Babeko, Kabupaten Bungo, Jambi In this plot, located close to the enrichment planting plots (S ; E ), two rubber germplasm sources (GT1 seedlings and clonal BP260 plants) were planted together at the standard density of 660 trees per hectare, after slash and burn of an old rubber agroforest. Four fertilization levels crossed with two weeding regimes on the two germplasm sources were tested: a slight but significant effect of fertilization was observed for the first eighteen months of growth whereas weeding and germplasm effects were not significant (see (Akiefnawati & Noordwijk 1997) for results after 18 months). Data from the second and third year of growth are presented below in a comparison with growth of poles measured in an agroforest. Rubber agroforest permanent sample plot A one-hectare permanent sample plot (PSP) was set-up in 1999 in a rubber agroforest in Muarakuamang (the same location as the first enrichment planting experiment). This 35-year-old agroforest shows an average rubber tree density and tree species diversity for the area (Hardiwinoto et al. 1999). RESULTS Rubber survival and growth under natural shade enrichment planting experiments Across both enrichment planting experiments, overall annual survival rate (96%) was much higher than that reported by farmers, probably due to the careful individual fencing of rubber plants. Nevertheless, as many as a quarter of the plants had had their stem broken within a year, the damage mostly caused by monkeys feeding on the young leaves. The statistical analysis of diameter increment (reported below) included all plants surviving for at least six months regardless of whether or not their main stem had been broken, as such damage was found not to significantly affect growth rate in a preliminary analysis. Data from both experiments were pooled and analysis of variance performed using the GLM procedure in Systat v.9.0. Leaf disease incidence and GSF were used as covariates and site and germplasm as categorical variables in the analysis

12 96 VINCENT ET AL. of variance. No significant interactions were found between the predictors (Table 2). The GSF ranged from 0.09 to 0.51 (average 0.28) across sites. It is worth noting that the various genetic types showed statistically significant differences in their sensitivity to leaf blight disease, the least sensitive genotypes being clonal PB260 and seedling GT1. Canopy openness did not significantly affect the intensity of leaf blight attack. In both experiments the negative effect of light limitation under increasing shade on rubber growth was extremely clear (P < 0.001, Table 2). Differences in mean diameter increments between genotypes were also highly significant (P < 0.001) with only one clone (BPM1) being clearly outperformed by both seedlings and wildlings. No significant interaction effect was detected between genotype and light level. Overall, diameter growth was slow; the average uncorrected diameter increment was only 0.4 cm yr 1 with a median increment of 0.3 cm yr 1 (Table 3). The next section explores some of the possible reasons for this slow growth. TABLE 2 ANOVA table for annual stem diameter increment of rubber plants planted under an existing canopy in two sites. No significant interaction was found between any of the predictors. Leaf disease: score of incidence of leaf blight disease; Germplasm: germplasm of planted rubber; GSF: Global site factor. Source Sum-of-Squares df Mean-Square F-ratio P Site Leaf disease < Germplasm < GSF < Error TABLE 3 Adjusted Least Square Means of stem diameter increment of rubber plants of different germplasms planted within an existing rubber agroforest (summary for two locations). Values followed by the same letter are not significantly different (Bonferroni multiple comparison test, alpha=0.05) Mean stem diameter increment Number of Germplasm (cm yr 1 ) SE observations GT1 seedling 0.56a PB a Wildling 0.46a,b RRIM a,b RRIC a,b BPM b

13 Nursery shading experiments AN AGRO-ECOLOGICAL PERSPECTIVE 97 In both experiments maximum growth was achieved under full light and 55% shading (Table 4). Greater shade significantly reduced diameter increment. Maximum diameter increment reached c. 2 cm yr 1 while average increment under 91% shade was 1.2 cm yr 1. In summary, rubber plants showed a strong positive response to light only up to no more than 45% of full sunlight; and under dense shade they still showed a growth increment of more than 50% of the maximum value. TABLE 4 Stem diameter increment of young rubber plants grown under artificial shading in two experiments. The effect of shading intensity is highly significant. Numbers followed by the same letter within one column are not significantly different (Bonferroni test, P = 0.01). Lampung (Sumatra) Cianjur (West Java) Shade Mean increment SE Number of Mean increment SE Number of (cm yr 1 ) observations (cm yr 1 ) observations a a % 1.8 a a % 1.3 b b % 1.2 b b Comparison of seedling growth in nursery shading experiments and in agroforests Data from the above studies (first year of growth only) were used to compare the growth of young plants in agroforest and nursery. All the rubber plants were of similar size and age. Growth rates from the agroforest were first corrected for leaf disease incidence, which was found to be a significant factor reducing growth in both experiments, by calculating the expected growth in the absence of leaf disease. The corrected adjusted values of monthly diameter increment of the GT1 seedlings and the best performing clone, BP260, (cf. Table 3) from one agroforest (EP1) are plotted together with the data from the two nursery experiments, expressed in the same units, against fraction of full sunlight (Figure 3). The rubber plants in the agroforests had much slower growth than those in the artificial shading experiments under the entire range of overlapping light levels (Figure 3). Seedlings grown under the artificial shading probably experienced less competition as the planting density in the nursery was low (in relation to the final size of the plants) and the plots were clean weeded. This suggests that below-ground competition contributes significantly to the slower growth observed in young rubber plants growing under an existing agroforest canopy. However, in interpreting these differences in rubber growth rates, the potential impact of differences between the experiments in soil fertility (Table 1) as well as in light quality are considered below.

14 98 VINCENT ET AL. Light quality Figure 3. Average monthly diameter increment (cm) of rubber seedlings, grown: (a) within existing rubber agroforests and experiencing natural shade (empty circles), and (b) in nursery under artificial shade (crosses) during the first year of growth. Fraction of full sunlight omits light transmitted by leaves in the agroforest. Regression lines are based on Distance Weighted Least Square (Systat v. 9.0). There are important differences in light quality between artificially shaded experiments and in situ planting in agroforests. The two situations have different temporal distribution of light, different variability in light intensity, and different wavelength distributions. A constant fraction of 20% of above-canopy PAR radiation (artificial shading) is generally more effective for plant growth than a 20% GSF under a forest canopy. In the latter case, sudden high-energy sunflecks which are not readily usable (due to induction time and saturation rate of photosynthesis) follow periods of very low light intensity. Brief episodes of high light may even induce photo-inhibition (Pearcy & Sims 1994). In the agroforest even in a medium-sized canopy gap (GSF of 23%), PFD was less than 0.2 of that above canopy for 75% of the time during five days of measurement (Figure 4). Using photosynthetic light response curves established under artificial shading (Ruhiyana et al. 2000) daily net carbon uptake at leaf level was compared in leaves receiving the same total daily PAR but either artificially shaded or growing below a natural rubber canopy. Predicted total C-uptake was about 10% higher under the more buffered artificial shading (with 20% GSF). The actual difference in daily C-uptake between light regimes is probably slightly greater because neither the delay in induction to suddenly high irradiance nor the possible photo-inhibition effect were considered in the computations. However,

15 AN AGRO-ECOLOGICAL PERSPECTIVE 99 Figure 4. Distribution of the photosynthetically active Photon Flux Density (PFD) measured below the canopy of a rubber agroforest in a medium size gap (Global Site Factor = 23%) expressed as a proportion of above-canopy PFD, measured every five minutes for eight consecutive days. even though biologically significant, this difference in light distribution between artificial and natural shading cannot account for all of the measured difference in average growth, which was more than twice as high in the nurseries than in the agroforest (Figure 3). This therefore suggests that the unaccounted-for differences were due to below-ground factors. Soil fertility Soil analyses (Table 1) suggest that differences in fertility between the sites may have played some role in the different growth rates observed. The ph was generally lower in the agroforests than the nurseries, however the Effective Cation Exchange Capacity in the agroforest sites was within the range of that encountered in the nurseries. Exchangeable K values at all four sites were low (Dierolf et al. 2001; Watson 1989b) but the lowest values were in the Cianjur nursery, not the agroforests. The two agroforest sites did show high Al-saturation rates compared with the two nursery sites but these values are not likely to be seriously limiting for rubber. More significantly, however, available P (P Bray) values were lower in the agroforest sites than in the one nursery measured and total P was low in one of the agroforest sites. Exchangeable Ca and Mg were also low, particularly in the second agroforest site where concentrations of both P and Mg were likely to be deficient (Dierolf et al. 2001). Leaf nutrient analyses were conducted for seedlings growing in both agroforests and revealed low P and K concentrations but high Mg.

16 100 VINCENT ET AL. However, because most leaves exhibited leaf blight symptoms, these foliar nutrient results should be interpreted with care. Overall, the soil in the two agroforest plots appeared slightly less fertile than in the nurseries. Lowest fertility was in the second agroforest where young rubber plants showed a slightly lower growth rate. However, given rubber s general tolerance of low soil fertility (Watson 1989b) it is unlikely that these differences in soil fertility can explain the large differences in rubber growth between the two sets of experiments; therefore below-ground competition remains the most likely primary cause. Factors affecting rubber tree growth at the pole and later stages in agroforests Rubber trees are usually not tapped before they reach cm diameter. Therefore, it is important to know how long this unproductive period will last for trees within an existing agroforest where they are competing with the established canopy trees, and what major factors affect their growth at the pole stage. To answer these questions, we used data collected at one year interval ( ) in the Muarakuamang one-hectare sample plot where the first enrichment planting took place. The total number of stems tallied greater than 5 cm DBH was 858, 296 of which were rubber trees, the rest belonging to 122 other species. The diameter of these stems was measured at three different heights to increase precision and the mean diameter increment used in the analyses presented. The heights of measurement were 1.1m, 1.3m and 1.5m for non-rubber trees and 1.8m, 2.0m and 2.2m for rubber trees (to avoid the deformation associated with the lower areas of tapped bark). In addition to diameter, Dawkins crown position and crown form indices (Dawkins 1958) were recorded for each individual rubber tree. Both are recorded on a scale ranging from 1 to 5 (Vincent et al. 2002). The Crown Form Index is a measure, based on crown shape, of its development quality. The Crown Position Index is based on the exposure of the crown to direct sunlight (only trees with scores 4 and 5 receive full overhead light). In addition, at four different dates during the year of observation, a record was made for each tree of whether it was currently being tapped or not. Rubber trees were then grouped into three categories: not tapped (if never recorded as being tapped), continuously tapped (if recorded as being tapped on all four dates), and irregularly tapped (if recorded as being tapped on some of the four dates). A general linear model was used to test the significance of the association between rubber diameter increment and the two crown indices and tapping frequency (Figure 5). Greater tapping frequency was significantly negatively associated with diameter increment (P < 0.05). Higher crown form and crown position scores (indicating better developed and more exposed crowns) were significantly positively associated with diameter increment (P < 0.001). The asymptotic relationship between Crown Position Index and the adjusted mean square diameter increment (Figure 5b) indicates that only lower light levels are associated with severe limitation of rubber growth, supporting the above conclusion of the experimental studies of the seedling/sapling stages.

17 AN AGRO-ECOLOGICAL PERSPECTIVE 101 Figure 5. Relationship between adjusted least square mean of rubber tree diameter increment and indices of tree crown form, position and management: a) Crown Form Index (from 1, least satisfactory, to 4, most satisfactory); b) Crown Position Index (from 1, no direct light, to 4, full overhead light); and c) tapping regime (1, not tapped; 2, irregularly tapped; 3, continuously tapped). Based on the general linear model estimates of the factors, the expected growth of rubber trees in agroforests was computed for a crown position score of 4 (full overhead light) and a crown form score of 4 (well-developed but slightly unbalanced crown). Those values of the crown indices are thought to be representative of the status of most trees grown in a pure stand under standard planting density, whose crowns are in the main canopy. Growth was further corrected for the reduction in growth associated with tapping by recalculating expected growth of each tree under no-tapping. Following this standardization, the tree diameter increments for agroforest rubber trees were compared with growth data obtained from another plot where non-clonal rubber plants were grown in a pure stand at the standard density of 660 trees per hectare. The management of this fertilization trial plot at Simpang Babeko Kabupaten Bungo was described in the methods section and its soil analyses are given in Table 1. Clonal rubber growth data from that plot was not used in this comparative analysis. The data from the pure stand were left uncorrected as fertilization and weeding effect was no longer significant after year 3. No correction needed to be made for tapping either, as the plot was not yet being tapped, or crown form and crown position, as these were assumed to be equal to or greater than 4. In the range of diameters common to both plots (5 15 cm DBH), mean diameter increment is significantly lower (almost by 50%) in the agroforest (once corrected for local crown crowding and shading effects) than in the pure plantation (1.1 mm*mo 1 (agroforest) vs 2.1 mm*mo 1 (plantation) P<0.001, two-sample t test). Therefore, this indicates that the remaining growth deficit in the agroforest plot is most probably a consequence of below-ground factors. Both plots have deep sandy acidic soils, with low Effective Cation Exchange Capacity and exchangeable Mg, K and Ca (Table 1). Values of exchangeable Mg and K were lower in the soil of the mature agroforest plot than the plantation plot and these differences in soil fertility cannot be ruled out as contributory factors, along with below-ground competition, to the observed difference in

18 102 VINCENT ET AL. rubber growth rate between the plots. However, given rubber s tolerance of low fertility acid soils (Watson 1989a), the overall results of this comparison after the light factor has been accounted for are certainly compatible with belowground competition (e.g. for water) being the major factor explaining observed difference in growth between the two plots. The potential for incorporation of improved germplasm within existing agroforests Breeding is believed to be the single most important technical improvement in rubber domestication and cultivation, which has contributed to a fourfold increase in the initial average latex yield in estate plantations since the beginning of the twentieth century (Simmonds 1989). However, in Indonesia most smallholders outside government sponsored development projects still rely on non-clonal rubber of uncertain origin. This situation can be attributed to the low availability of high quality germplasm and the low cost of unselected nonclonal rubber. It has been hypothesized that the introduction of clonal rubber into smallholder agroforests would have the potential to double or even triple their annual latex yield, provided that these genotypes could tolerate the more competitive environment of the agroforest (Penot 2001). The results of the enrichment planting experiments presented in the previous section show that, in terms of growth and survival, some rubber clones compare well with naturally regenerated seedlings, at least during the first year and a half after planting in mature rubber agroforest. Therefore, for farmers practising enrichment planting it seems an attractive alternative to gradually incorporate high latex-yielding clonal material into these rubber agroforest systems. Directly grafting clonal buds onto naturally occurring seedlings in situ may offer a way of cheaply upgrading an existing agroforest with clonal rubber germplasm, provided that natural regeneration is sufficient to provide ample rootstock material. Therefore, the potential for grafting buds of high potential clonal genetic material onto local seedlings was assessed in a multi-location experiment in Jambi Province, Indonesia. Factors examined were germplasm, which had two levels (PB260 and RRIC100), and canopy closure rated using a canopy scope (Brown et al. 2000), which had three levels (dense, medium and no canopy). In five mature rubber agroforests a total of 303 rubber wildlings, including both transplanted and undisturbed seedlings within the stem diameter range 2 4 cm, were selected. Due to difficulty in finding enough appropriate seedlings, the number of seedlings under each treatment varied. At the beginning of the rainy season in September 1999, budstock of one of two commonly recommended clones (PB260 and RRIC100) was grafted onto each seedling. This was accomplished in a day by using three professional nurserymen. A number of seedlings in all plots were randomly allocated to the control or nografting treatment. Survival of budded grafts was monitored monthly. The first measurement of bud growth (stem height, diameter at 10 cm above the graft,

19 AN AGRO-ECOLOGICAL PERSPECTIVE 103 TABLE 5 Performance of budded grafts eight months after grafting: a) percentage surviving; and b) height growth of new shoots from grafted bud (cm). a) percentage surviving Clone PB260 RRIC100 Mean Canopy density Dense Light None (open field) Mean b) height growth (cm), mean (SE) Clone PB260 RRIC100 Mean Dense (16.7) ( ) (13.6) Light (10.2) (14.9) (8.5) None (open field) (14.9) (9.3) (8.8) Mean (8.8) (7.9) (6.2) and branching) was taken five months after grafting. The final observations were made eight months after grafting. Overall the grafting success was 35.6% (seedlings with surviving budstock eight months after grafting) with significantly higher survival (Table 5a) and growth rates (Table 5b) under no overhead canopy than under the two levels of shade. ANOVA of height growth rate indicated a highly significant difference (P = 0.003) between canopy densities, while both clone and the interaction between clone and canopy density were non-significant (P > 0.1). The number of leaf whorls produced is a good indicator of rubber shoot growth because of the regularity of the growth pattern of rubber. Overall whorl count at the last observation (eight months after grafting) was higher in RRIC100 (3.1 ± 0.1 SE) than in PB260 (2.4 ± 0.1 SE), although the difference was statistically insignificant (P > 0.1). The difference in number of whorls between canopy treatments was significant (P = 0.003), with more whorls under a more open canopy in both clones (Figure 6); there was no significant interaction between clone and canopy density. There was no variation in grafting success between the three grafters and no pest damage was observed in the grafted plants. The low survival and slow growth of the budstock overall is believed to have resulted from the combined effect of competition with ground-level vegetation and the physiological unpreparedness of the rootstock, as well as shading by mature trees. Although canopy shade clearly significantly reduces growth, even in the open-grown (no overhead canopy) seedlings, survival of grafted budstock was lower than 50%. This preliminary experiment confirms the technical feasibility of the direct grafting technique within an existing agroforest, but it demonstrates also the

20 104 VINCENT ET AL. 5 Mean number of whorls produced Clone PB260 RRIC100 1 Dense Light Open Overstorey Figure 6. Growth of budstock on rubber seedlings 8 months after being grafted in situ with two different clonal genotypes under different overstorey densities in a multilocation experiment in Jambi Province, Indonesia. need for improvement in the technique before it could be applied widely. It also requires technical know-how for grafting and access to clonal buds. Furthermore, a consequence of the incorporation of clonal material within the system (be it through direct grafting or enrichment planting of nursery-grafted plants) will be largely to preclude the further use of naturally regenerating plants, adding a constraint to the management of the system. Indeed, rubber trees arising from seeds of clones are likely to have a lower latex yield potential, due to the genetic disjunction occurring through seeding which will imply a higher variability in yield productivity in the F1 progeny, some of which will be low producers. In the case of direct grafting, compatibility issues may arise as not all budstock scions can be grafted onto any rootstock. Introducing grafted clonal material may therefore ultimately accelerate the slash-and-burn rotation by preventing the cultivation of a second generation of interplanted rubber trees before the site is cleared. Monoclonal populations might also prove more fragile in the long run, particularly in relation to susceptibility to fungal diseases (Johnston 1989). Therefore mixed-clone populations or alternatively small plots of different clones would be a lower-risk option (Libby 1982; Roberds et al. 1990) that resourcepoor farmers should be encouraged to consider.