Contribution of aboveground and belowground biomass of Robinia pseudoacacia trees to total plant carbon stocks in a young agroforestry system

Allocation to aboveground and belowground compartments of tree biomass in Mediterranean agroforestry systems (AFS) is poorly documented, especially for young trees. This work aimed at (i) characterizing the effects of land use (Agroforestry, AF vs. Tree Plantation, TP) on 5-year-old black locust tree growth, tree biomass allocation, and tree C stocks at plot scale, and (ii) assessing the effect of land use on total carbon stocks (AF vs. TP vs. Crop Monoculture). Allometric equations were built for upscaling tree biomass at the plot scale. Biomass of understory vegetation and crops were estimated at the plot scale in the three land uses. Tree diameter was 19 % higher in the AF than in TP, likely due to different light microclimate, while tree height did not vary significantly between land uses. Tree biomass allocation to aboveground and belowground compartments (70 % and 30 % of total tree biomass, respectively) did not vary between land uses. Higher efficiency in building tree carbon stock was shown in agroforestry than in tree plantation per area unit. Trees accounted for 39 % and 66 % of total carbon stocks in biomass in AF and TP, respectively. Understory vegetation accounted for 8 % and 34 % of total carbon stocks in biomass in AF and TP, respectively. Land equivalent ratio values, which involved tree, crop and understory vegetation carbon stocks, did not yet indicate a synergetic effect on accumulation of plant carbon compared to sole crop or tree plantations. This study provides new reference values of carbon stocks in biomass in a young AFS.