Sapwood biomass carbon in northern boreal and temperate forests

Martin Thurner,Stefano Manzoni,Daniel Falster,Anatoly Prokushkin,Ernst‐Detlef Schulze,Christian Beer,Thomas Crowther,Thomas Gillespie

doi:10.1111/geb.12883

Abstract

AbstractAimInformation on the amount of carbon stored in the living tissue of tree stems (sapwood) is crucial for carbon and water cycle applications. Here, we aim to investigate sapwood‐to‐stem proportions and differences therein between tree genera and derive a sapwood biomass map.LocationNorthern Hemisphere boreal and temperate forests.Time period2010.Major taxa studiedTwenty‐five common tree genera.MethodsFirst, we develop a theoretical framework to estimate sapwood biomass for a given stem biomass by applying relationships between sapwood cross‐sectional area (CSA) and stem CSA and between stem CSA and stem biomass. These measurements are extracted from a biomass and allometry database (BAAD), an extensive literature review and our own studies. The established allometric relationships are applied to a remote sensing‐based stem biomass product in order to derive a spatially continuous sapwood biomass map. The application of new products on the distribution of stand density and tree genera facilitates the synergy of satellite and forest inventory data.ResultsSapwood‐to‐stem CSA relationships can be modelled with moderate to very high modelling efficiency for different genera. The total estimated sapwood biomass equals 12.87 ± 6.56 petagrams of carbon (PgC) in boreal (mean carbon density: 1.13 ± 0.58 kgC m−2) and 15.80 ± 9.10 PgC in temperate (2.03 ± 1.17 kgC m−2) forests. Spatial patterns of sapwood‐to‐stem biomass proportions are crucially driven by the distribution of genera (spanning from 20–30% in Larix to > 70% in Pinus and Betula forests).Main conclusionsThe presented sapwood biomass map will be the basis for large‐scale estimates of plant respiration and transpiration. The enormous spatial differences in sapwood biomass proportions reveal the need to consider the functionally more important sapwood instead of the entire stem biomass in global carbon and water cycle studies. Alterations in tree species distribution, induced by forest management or climate change, can strongly affect the available sapwood biomass even if stem biomass remains unchanged.

Highlights

In addition to the pith, cambium, phloem and bark, tree stems con‐ sist most importantly of heartwood and sapwood
We aim to investi‐ gate sapwood‐to‐stem proportions and differences therein between tree genera and derive a sapwood biomass map
First, we develop a theoretical framework to estimate sapwood biomass for a given stem biomass by applying relationships between sapwood cross‐sectional area (CSA) and stem CSA and between stem CSA and stem biomass

Summary

Introduction

In addition to the pith, cambium, phloem and bark, tree stems con‐ sist most importantly of heartwood and sapwood. Total forest biomass has been used for model evaluation (Beer, Lucht, Schmullius, & Shvidenko, 2006; Thurner et al, 2017; Yang et al, 2018), an assessment of sapwood mass per ground area at large spatial scales is still lacking. In addition to their biogeochemi‐ cal importance regarding the carbon, water and nutrient cycles, sap‐ wood‐to‐heartwood ratios are relevant variables in wood industry, owing to the different chemical composition, mechanical properties and often colour of heartwood compared with sapwood

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