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Shifting, or swidden, cultivation remains an important land use across Southeast Asia and other parts of the tropics, albeit under pressures from other land uses. The swidden cycle of cultivation and regrowth creates mosaic landscapes with regrowing fallows of various age interspersed with active fields and patches of mature forest. Quantifying tree biomass in these landscapes is limited by the availability of reliable allometric models, hindering accurate carbon stock estimation and thus quantification of GHG emission associated with land use transitions. We therefore developed new allometric models for the prediction of both above- and below-ground woody biomass in swidden systems based on a destructive harvest of 150 trees in Luang Prabang Province, Laos People’s Democratic Republic (PDR). This study is the first to develop allometric models of root biomass for swidden landscapes in this region, which we hypothesised would be a major carbon pool given that resprouting, and associated high root biomass, is a common physiological/morphological trait in regularly disturbed ecosystems. We found that a general model including tree diameter (DBH, cm) and height (H, m) was best for estimating aboveground biomass (AGB, kg) for both resprouts and trees growing from seed (AGB = 1.09 + 0.027D2H) though the inclusion of H only resulted in a marginal increase in model performance meaning a DBH-only model is acceptable to use in the absence of height data (AGB = 0.1286DBH2.134). Tree height was less important for estimating root biomass, with models including only DBH performing best. Re-sprouting trees exhibited greater root biomass (BGB = 0.355DBH1.732) compared to those growing from seed (0.016DBH2.597) meaning different root allometric models were developed for each tree type. Thus, we suggest that field efforts should be directed towards checking resprouting status over the estimation of tree height. We also found that models fit using non-linear regression provided equally good fits to the data compared to the traditional approach of log-transforming the data. Our models were subsequently applied to 12 nearby plots spanning a chronosequence of fallows to examine the impact of re-sprouting allometry on biomass estimation. Root biomass stocks were on average 58% higher after accounting for the allometry of resprouting trees, resulting in an average 9% increase in total biomass stocks, highlighting the importance of choosing root allometrics based on growth form in swidden fallows. If re-sprouting status is unknown, a stand-level root:shoot ratio of 0.32 can be applied. Our analysis suggests that using our models will substantially improve the accuracy of estimates of tree biomass and its distribution among different pools in swidden fallows. This information is crucial in better quantifying carbon stock changes resulting from the conversion of swidden land to other land uses.