Consequences of tropical rainforest conversion to tree plantations on fine root dynamics and functional traits

Despite the crucial role of fine roots for water and nutrient uptake, soil biological activity and ecosystem carbon cycling, the response of root dynamics to rapidly advancing land-use change in the tropics is still poorly understood. To address this uncertainty, we investigated the consequences of tropical forest conversion to intensively managed tree plantations for a suite of functional fine root traits. We analysed fine root morphology (diameter, specific root length (SRL), tissue density) and chemistry, as well as root lifespans in four prevalent land-use systems in the lowlands of Sumatra (Indonesia), namely natural forest, jungle rubber, rubber and oil palm monocultures. Fine root production was estimated using three independent approaches (sequential coring, ingrowth cores, mini-rhizotrons). Contradicting the expected trend from more conservative to more acquisitive fine root traits with increasing land-use intensity, we found that SRL and tissue density were significantly higher in forest trees, while fine root diameter was largest in rubber trees and root N content lowest in the oil palm system. Median fine root longevity in the top soil was 11% higher in rubber plantations (238 days) than in jungle rubber (211 days), and more than 50% greater than in the forest (140 days) and oil palm plantations (125 days). Fine root production was higher in the forest and oil palm plantations (ranging between 2 and 9 Mg ha⁻¹ year⁻¹) than the rubber stands, but annual totals varied depending on the methodological approach. Conversion of tropical lowland forest to agricultural systems significantly altered community-level fine root morphology, dynamics and longevity, with likely consequences for soil carbon cycling and soil biological activity. However, land-use intensification did not consistently lead to more acquisitive fine root systems; rather, differences in root morphology and dynamics were driven by species-specific root trait syndromes especially of rubber and oil palm.