Variations in temperate forest stem biomass ratio along three environmental gradients are dominated by interspecific differences in wood density

Stem biomass ratio (SBR, kg m−3) is a forest state variable that converts forest volume of growing stock into biomass. However, huge intraspecific variation in wood density (WD) driven by biotic and abiotic environments of tree growth remains ignored in C budgets. The aims of this study were (i) to identify variations in SBR along water, soil nutrition and elevation gradients, (ii) to test for differences between broadleaved and conifer tree species in SBR variations, and (iii) to weight the contributions of interspecific and intraspecific diversity in SBR variations. Analyses were based on massive X-ray WD measurements performed on 54,700 tree cores collected in 2016 and 2017 on the spatially systematic plot sampling design of the French national forest inventory (NFI) programme. Stem biomass ratio (SBR) variations along the three gradients were found to be significant, with differences between botanical classes found on the water gradient only. SBR hence decreased by 73 kg m−3 (conifers) and 126 kg m−3 (broadleaves) along a 180 mm gradient of soil water holding capacity (SWHC), increased by 153 kg m−3 along a full gradient of soil basicity index (SBI), and decreased by 155 kg m−3 from 200 to 2000 m of elevation asl. Species distribution, not intraspecific variation in WD, was found to be the main cause of SBR variation along these gradients. SBR was found to be efficient for analysing large scale vegetation changes along environmental gradients, with interspecific variability being well explained by the three gradients. Mean WD species values were found to suffice to assess these SBR variations at the scale of the French forest resources.