Commercial eucalyptus plantations with Taungya system in Lao PDR: analysis of tree root biomass

The increasing demand for wood, fibre and pulp coupled with efforts to mitigate greenhouse gas emission has put immense importance on the development of forest plantations. The rapidly growing human population will also face shortage of food, particularly in developing world where agricultural productivity is generally low. The taungya system – an age-old agroforestry practice involving intercropping of crops with trees on the same unit of land – is opined as a win-win strategy to meet the needs of wood products and food at the same time. In recent years, the taungya system has gained increasing attention by big forest companies as a tool and an opportunity to contribute to the social well-being of the local community. Stora Enso Lao Co, Ltd is one of these forest companies that adopted the taungya system in its commercial Eucalyptus plantations in Laos. The tree spacing commonly used is 9 × 1 m, and 5 × 2 m on a trial basis. Eucalyptus camaldulensis was intercropped with rice and cassava, and stands were established in 2007 (7-yr old) and in 2012 (2-yr old). The main objective of this thesis was to examine whether the intercropping has an effect on the root system of trees; thereby generating knowledge that supports evidence-based plantation management decisions involving taungya system. The key research questions were: (a) Does intercropping of rice/cassava with Eucalyptus influence the horizontal and vertical distributions of root biomass of trees compared with monoculture plantation of different spacing? (b) Does spacing (5 × 2 m versus 9 × 1 m) influence the horizontal and vertical distributions of root biomass of trees intercropped with rice? (c) Does root biomass differ between young (2-yr old) and mature (7-yr old) intercropped stands with 9 × 1 m spacing compared with monoculture plantation? If so, does the effect vary in relation to distance from the tree and soil depth? To characterize the root system architecture, trenches were made on six young trees in both pure monoculture and intercropped stands. To quantitatively estimate root biomass, a total of 324 soil cores (6 stands × 6 trees × 3 distances × 3 soil depths) was collected, roots sorted and dried to constant mass in an oven at 60°C for 48 hours. The root dry mass data were subjected to analysis of variance to examine significant effects of intercropping, spacing and stand age. The results show that (1) the root system of E. camaldulensis is mainly confined to shallow depth but well elongated horizontally in both pure and intercropped stands with 4-6 thick lateral roots; (2) Intercropping of rice/cassava with Eucalyptus has no effect on total root dry mass of the tree component (p > 0.05) irrespective of the plantation models (5 × 2 m or 9 × 1 m); however root biomass decreases with increasing horizontal distance from the tree base and in deeper soil layer; particularly for trees in young stands; (3) The effects of spacing design, narrow (5 × 2 m) versus wide (9 × 1 m), on root dry mass are dependent on horizontal and vertical distribution of the root system; and root biomass appears to be higher at 40 cm soil depth for the stand with narrow spacing between trees and crops than stands with wider spacing; (4) Root biomass was larger for older than younger trees in both monoculture and intercropped stands; suggesting lack of carry-over effect of intercropping on root biomass. As root biomass varies with horizontal distribution, further research is recommended to test buffer zones between trees and crops other than 1 m (which is currently used)