Intercropping with banana to improve fractional interception and radiation-use efficiency of immature rubber plantations

Abstract

Intercropping provides an important means of raising not only productivity and land-use efficiency of smallholder rubber lands, but also income generation during the unproductive immature phase of the rubber tree. To evaluate current recommendations for intercropping rubber in Sri Lanka, we assessed the effects of a range of planting densities of banana, the most common companion crop of rubber on productivity and resource capture. In this paper, we test the hypothesis that rubber/banana intercropping, even at high densities of banana, results in an increase in biomass per unit land area and per crop plant due to an increase in both radiation capture and radiation-use efficiency. Five treatments were imposed: sole crop rubber (R); sole crop banana (B) and three intercrop treatments comprising an additive series of one (BR), two (BBR) and three (BBBR) rows of banana to one row of rubber. Dry matter production in the rubber-based treatments was directly related to planting density, being least in the sole rubber and greatest in BBBR intercrop. A more than four-fold increase in dry matter across treatments derived from an increase not only in light capture (270%) but also radiation-use efficiency (RUE, 230%). Neither R nor BR treatment, which is currently recommended for intercropping in Sri Lanka, achieved full ground cover with fractional interception remaining below 40 and 50%, respectively. Fractional interception was greatest in BBBR treatment and by the end of the measurement period, total intercepted radiation was 23 and 73% greater than that in the BBR and BR intercrops, respectively. Shade did not limit either photosynthesis or growth of component crops in the intercrops, even when planting density of banana was increased three-fold. In fact, intercropping increased growth of both rubber and banana components suggesting that shade associated with the denser intercrop canopies, moderated the microclimate and alleviated plant stress. These results highlight the potential gains that can be made by intercropping and optimising planting density for improved resource capture in immature rubber plantations.