|dc.description.abstract||Informal agroforestry is a traditional practice in Solomon Islands, mostly intercropping food crops with fruit trees. Agroforestry systems that intercrop food crops with rows of hardwood species for the purposes of timber production have not been practiced. In response to the common problem that growers of valuable hardwood species such as teak (Tectona grandis) are reluctant to thin their trees, the Australian Centre for International Agricultural Research (ACIAR) established project FST/2007/020 to develop novel silvicultural systems to overcome this problem. Flueggea flexuosa (flueggea) is a local hardwood species that is widely utilized for housing and fencing applications. The project team established several agroforestry trials testing the hypothesis that teak and flueggea could be successfully grown together with the local tree being progressively harvested for local use, effectively thinning the plantation and allowing the teak to develop through to harvestable size. The trees could be intercropped with food crops allowing for multiple land use and income generating opportunities whilst the trees grow through to maturity.
The model was based on the assumption that trees and crops, when properly managed, complement each other rather than compete in the capture and efficient use of available growth resources. This system is a hybrid of the informal agroforestry traditionally practiced in Solomon Islands and the silvicultural management techniques applied to production forestry. However, teak and flueggea have not been previously grown together under an agroforestry regime and little is known of the species interaction or of the effect of growing different food and cash crops in the interrow. Traditional practices for growing food crops in Solomon Islands begin with forest clearance and burning of debris, a slash and burn system. Mounds are made using man-made hoes or picks to give crops better growing spot away from competing weeds and surface water flow during rainy seasons. Most root crops and vegetables are planted in their own blocks and kept separate from other crops. Mixing of food crops is not a usual practice in most parts of the Solomon Islands. Other crops and trees are usually planted at the boundaries of each crop’s patch or along the boundary. Most food crops reach harvesting age after 3 months. When crops are harvested, the whole plant is uprooted, plant parts and debris are piled at a location and are burnt when dry. Where the area available for gardening area is large, the harvested plot is left to fallow but if the available area is small, crops are replanted straight after harvest, leaving no time for the soil to be replenished with nutrients through decomposition of plant materials.
Amongst several silvicultural trials established in Solomon Islands, this study focuses on trials established at Ringgi and Poitete which are located at the southern and northern parts of Kolombangara Island in Western Province. These silvicultural trials were established for the purpose of examining the effect of stocking rate and species mix on teak and flueggea growth and on their potential influence over the hybrid agroforestry system. Two mixed species spacing trials and one hybrid agroforestry trial of teak and flueggea were established to test the hybrid model. The two main mixed species spacing trials have 5 treatments with 4 replicates each, treatments are a combination of stocking rate and species mix. The five different treatments include teak being grown as a monoculture (Treatment 1); and then in rows interspersed with 2 rows of flueggea at different stocking rates (Treatments 2, 3, and 4); and alternating rows of teak and flueggea at standard spacing (Treatment 5). The hybrid agroforestry trial was based on the standard 4 x 3 m spacing (Treatment 5) and further intercropped with food crops. Only Treatment 3 (4 x4m) and Treatment 4 (4 x 6m) have wider planting spaces. Standard stocking is 833 stems per hectare for Treatments 1, 2 and 5, and 625 and 416 stems per hectare for Treatments 3 and 4.
This research examined the interactions occurring between teak and flueggea, and between teak, flueggea and food crops grown in the inter-row with respect to competition for nitrogen (N), light and water, resource access, changes in system interactions with the development of the canopy, nutrient loss and issues of sustainability related to harvesting of food crops, biogeochemical cycling of carbon (C) and N, root architecture and growth and yield. Total carbon (TC), total nitrogen (TN) and stable isotope δ13C and δ15N, and 15N-labelled tracer were analyzed using field sampled soil, foliage, branch, stem, root and litterfall from the stands to examine soil nutrient uptake, biomass content and cycling as a result of the intraspecific and interspecific interactions with relation to tree growth and productivity of the hybrid system over time and space. Root architecture, tree mean total height (THt) and diameter at breast height (DBH) were measured and assessed over the study period.
We investigated the competition between teak and flueggea for N using a 15N-labelled tracer in a field trial in a 2 year old and a 4 year old mixed species stand. The study also reports the acquisition and allocation of TC and C isotope composition (δ13C) in different tree components of teak and flueggea. Seven pairs of trees, one of each species, were isolated using an impermeable membrane 60 cm deep and 15N-labelled tracer was applied to the soil surface. The first four plots were sampled for a period of 18 months and the age of the trees at final excavation was 4 years. The final three plots were sampled for 12 months and the age of the trees at final excavation was 2 years. Each tree was measured, felled and roots excavated, divided into the components: roots, stem, branch and foliage, and then weighed for biomass, samples of each component were oven dried at 60° C to constant weight, ground to a fine powder and analysed for TN, TC, 15N enrichment, and δ13C. There was no significant difference in component 15N enrichment between teak and flueggea at both ages, suggesting that there could be equal uptake of added 15N-labelled tracer by both species.
The 15N -labelled tracer concentration was greater in the foliage followed by the root, stem and branch for teak and flueggea. However, stem had significantly greater biomass and therefore had greater 15N enrichment mass (kg) than other components of teak in the 2 years trial and with teak and flueggea at 4 years trial. Approximately 55 % of added 15N tracer was recovered in the 4 years trial and 43 % was recovered in the 2 years trial, suggesting that higher uptake is possible with well-established root structure with age. Although teak had significant growth, 15N tracer uptake and enrichment were not statistically different to those of flueggea which may mean that competition in growth resources was still at minimum stage and growth rates were species specific. TN was not significantly different between teak and flueggea components at age 2 and 4 years and may indicate equal access to available N belowground and with similar allocations. TC was not significantly different between components of teak and flueggea in either age and may indicate equal access to atmospheric C and similar allocations of photosynthates. Higher δ13C in teak components than those of flueggea indicated that teak has higher water use efficiency per kg of tree and does not discriminate against 13C as strongly as flueggea during photosynthesis. Similar 13C values in tree components within the species may be the result of subsequent partitioning of the photosynthates synthesized during photosynthesis.
The litter production and C and N cycling in both teak monoculture and teak and flueggea mixed species plantings in the two trials were studied over 18 months period. Leaf litter samples were collected monthly from the five treatments. Monthly litterfall production ranged from 250.51 to 541.61 kg ha-1 depending on treatment and trial. Treatment 1 produced significantly higher total litter than Treatment 4 at Ringgi but this difference will have been due to stocking rates. When based on individual tree productivity, teak in Treatment 4 at both trials produced significantly higher litter per tree than the teak in Treatments 3, 2, 5 and 1 while there was no significant difference with flueggea productivity. Although teak and flueggea TC and TN, and δ13C and δ15N varied over the study period, their mean values were not statistically different except for teak in T4 having significantly lower values at Ringgi. Teak and flueggea C/N ratios were not statistically different at both trials except for flueggea in Treatment 2 at Ringgi which was significantly higher. The highest annual TC and TN returned to the soil from total litterfall were observed in Treatment 1 followed by Treatments 3, 5, 2 and 4 for Ringgi. The highest at Poitete was Treatment 5 followed by Treatments 1, 3, 2 and 4. When comparing each treatment and using individual tree productivity, Treatment 4 produced and returned the significantly highest litter and nutrient than Treatments 3, 2, 5 and 1. Overall, individual tree productivity demonstrated that mixed species stands have significant potential for cycling higher rates of C and N than monoculture teak stand, therefore establishment of mixed species stands especially using the stocking rates of Treatment 3 and Treatment 4 is recommended as a practical measure in forest rehabilitation and agroforestry systems to realize sustainable development of community forestry in the Solomon Islands.
The spatial distribution of the root systems of teak and flueggea were examined by excavating pairs of trees of each species that had been grown in isolation plots for 2 (3 pairs) and 4 (4 pairs) years. Additional trees grown without a barrier were partly excavated to ensure that the effect of the barrier on root architecture was not significant. The root architecture of both species had similar patterns of development but showed a different topology and distribution. Teak had extensive horizontal and vertical roots and occupied a larger portion of the soil volume than flueggea. Both species had similar root biomass increment of 87 % between 2 and 4 years and roots made up 20-22 % of total tree biomass at both ages. Teak and flueggea roots occupied different depths within the soil volume, which would promote nutrient uptake efficiency and therefore minimize competition.
The study evaluated the effects of stocking rate and species mix on early growth of teak in a mixed species system. Intercropping with flueggea promoted diameter, height and form of teak. Teak diameter and basal area growth significantly increased with wider planting spacing though height was not statistically different to teak in single-species stands. Intercropping with flueggea resulted in teak developing smaller branches which facilitated a self-pruning habit that promoted clear wood production. Differences in teak height between all treatments were not significant though it is interesting to note that sixty months after planting, teak in T1 at Ringgi and teak in T5 at Poitete had the greatest height as had Flueggea in T5 at Ringgi though again differences in height of flueggea was not significantly throughout the treatments. Diameter and basal area were greatest at the lower stocking than at the higher stocking for teak and flueggea. Teak of T4 had the significant diameter and basal area growth than other treatments at age 60 months. Teak form was best at the pure and mixed species stands due to self-pruning while larger crown and big branches occurred at lower stocking rates. While this can be corrected with timely silviculture, a 4 x 3 m spacing would seem to optimise the benefits of higher stocking and lower maintenance.
Overall, mixed species and agroforestry systems promoted reduction and delay of competition for growth resources in the early phase of the systems compared to monocultures. Both single and mixed species systems promoted similar C and N cycling in the plantation establishment phase. Growth in basal area was significantly higher at the mixed species stands at the lowest stocking rate, which also enable longer period of intercropping of food crops. However, as the present investigation was confined to the first 5 years, which is considered as establishment phase for teak, more studies are needed as the systems mature to fully understand the systems development and interactions to maturity.||