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Leaf and root production, decomposition and fluxes of carbon and nitrogen during stand development in tropical moist forests, northeast India
In the tropics, secondary forests are increasing in coverage because of over-exploitation of natural forests for timber, fuel wood, mining and shifting cultivation. However, understanding of how secondary succession affects carbon (C) and nitrogen (N) cycling in these forests is limited. Litter fall, root biomass turnover and decomposition critically affect the functioning of forest ecosystems. Thus, it is important to study production and decomposition of leaf, branch and roots in secondary tropical forests. In this study two moist secondary forests (5 and 15 years old; referred as RF-5 and RF-15) and a natural forest (NF) were selected in Northeast India. Total litter input (above and belowground) increased during vegetation succession (7.6, 10.9 and 13.9 t ha-1 y-1 in RF-5, RF-15 and NF, respectively). The organic matter input through fine roots in RF-5 was 80% of NF, whereas in RF-15 it was equal to that of NF. While contribution of leaf litter to the total litter input increased during the course of vegetation succession (i.e. 63% in RF-5 and 68% in RF-15) with a maximum in NF (74%), the contribution of fine roots decreased. C and N input through root mortality were 60% and 90% in RF-5 and RF-15, respectively. Despite considerable changes in vegetation composition, litter chemistry and decomposition rates did not vary significantly among the forest sites. CO2-C efflux varied significantly (P<0.05) during the course of succession (i.e. 19 and 24 t ha-1y-1 in RF-5 and RF-15, respectively) and 31 t ha-1y-1 in NF. Concentrations of cellulose and N were significantly positively correlated with mass loss at an early stage of decomposition (r=0.54 to 0.65, P<0.05), whereas ratios of lignin/N and C/N were negatively correlated with the same at later stages. Percent mass remaining at the end of the study (15 months) was: 17-25% for leaf and fine root and 34-48% for coarse roots and branches. Litter decomposition and soil CO2 efflux were strongly influenced by monsoon season in all sites and 74–90% of the variability in mass loss rates was regulated by rainfall. C stock loss in decomposing litter was similar to that of mass loss, whereas N increased initially followed by its gradual release later. We conclude that total (above- and belowground) litter production and decomposition, especially belowground in RFs enhanced C and N fluxes through fine root mortality which facilitate rapid (<2 decades) recovery of secondary forests in this region. Further, recovering forests developed ecosystem level strategies like induced early leaf fall to reduce soil water loss through transpiration and conserve N through immobilization during decomposition. These results indicate a significant role of litter, particularly, fine roots in accelerating C and N cycling properties in young secondary forests so that they may potentially provide ecosystem services similar to that of NF in a short period of time (i.e. <2 decades).
SR16265 Accepted 10 November 2017
© CSIRO 2017