Uncertainty of biomass stocks in Spanish forests: a comprehensive comparison of allometric equations

Biomass and carbon content are essential indicators for monitoring forest ecosystems and their role in climate action, but their estimation is not straightforward. A typical approach to solve these limitations has been the estimation of tree or stand biomass based on forest inventory data, using either allometric equations or biomass expansion factors. Many allometric equations exist, but very few studies have assessed how the calculation methods used may impact outcomes and how this impact depends on genera, functional group, climate or forest structural attributes. In this study we evaluate the differences in biomass estimates yielded by the most widely used biomass equations in Spain. We first quantify the discrepancies at tree level and among the main forest tree species. We observed that the divergences in carbon estimations between different equations increased with tree size, especially in the case of hardwoods and for diameters beyond the range used to calibrate the equations. At the plot level, we found considerable differences between the biomass values predicted using different methods (above 25% in one out of three plots), which constitutes a warning against the uncritical choice of equations to determine biomass or carbon values. The spatial representation of the differences revealed geographical patterns related to the dominance of fast-growing species such as Eucalyptus or Pinus pinaster, with a minor effect of forest structure, and almost no effect of climate. Finally, we observed that differences were mostly due to the data source rather than the modelling approach or equation used. Based on our results, BEF equations seem a valid and unbiased option to provide nation-level estimations of carbon balance, although local equations should preferably be used if they are available for the target area.This work was funded by the LIFE CLIMARK project (LIFE16 CCM/ES/000065) and by the Spanish State Research Agency through the UMBRACLIM project (PID2019-111781RB-I00). The authors would like to thank Lluis Coll for helpful discussions during the conceptualization of the study and Pere Gelabert for his help in map design and creation. Special thanks to Miquel de Caceres, Jordi Vayreda and Jordi Martinez-Vilalta for the creation of the Catalan Forestry Lab and the gathering and release of the IEFC data

Birch and conifer deadwood favour early establishment and shade tolerance in yellow birch juveniles growing on sugar maple dominated stands

Small-seeded tree species such as yellow birch (YB, Betula alleghaniensis Britt.) require deadwood or mineral soil for their establishment. Although much research has been done comparing YB germination on leaf litter vs. exposed mineral soil, less is known about deadwood as a seedbed and how different seedbeds affect YB early growth along light availability and size gradients. We examine how three common seedbeds (deadwood, moss cover on deadwood, and mineral soil) affected establishment and growth, biomass partitioning, and morphological traits of YB juveniles growing in the understory of temperate mixed deciduous and coniferous forests in southern Quebec. A total of 274 YB were sampled in four sugar maple (Acer saccharum Marsh.) dominated northern hardwood stands where selective cuts had been applied 6 and 15 years prior to sampling. Over 75% of the YB found on deadwood were on material of birch and conifer origin, although these species made less than 40% of the basal area. YB juveniles growing on deadwood showed traits that improve survival in shade such as reduced height growth for tall plants, higher efficiency in resource capture, and multilayered crowns. Our results demonstrate the importance of deadwood of birch and conifer origin in maintaining an abundant, natural, spatially well-distributed, and multistoried regeneration of YB.The authors are particularly grateful to I. Pratte, D. Schönig, A. Paquin, N. Bergeron, M. Messier, E. Messier, S. Carpentier, Y. Gauthier, and E. Mauri for their valuable help with the field and lab work. They also thank M. Mazerolle and S. Daigle for their assistance with statistical models. This work was funded by a MVRMF grant from the Ministry of Natural Resources of Quebec (MRNQ). J.-B. Lambert was also awarded with a Fonds de recherche du Québec — Nature et technologies (FRQNT) scholarship