Carbon fractions in wood for estimating embodied carbon in the built environment.

Determining wood carbon (C) fractions (CFs)-or the concentration of elemental C in wood on a per unit mass basis-in harvested wood products (HWP) is vital for accurately accounting embodied C in the built environment. Most estimates of embodied C assume that all wood-based building material is comprised of 50 % C on a per mass basis: an erroneous assumption that emerges from the literature on tree- and forest-scale C estimation, which has been shown to lead to substantial errors in C accounting. Here, we use published wood CF data from live trees, alongside laboratory analyses of sawn lumber, to quantify generalizable wood CFs for HWPs. Wood CFs in lumber average 51.7 %, deviating significantly from a 50 % default wood CF, as well as from CFs in live wood globally (which average 47.6 % across all species, and 47.1 % in tree species not typically employed in construction). Additionally, the volatile CF in lumber-i.e., the quantity of C lost upon heating of wood samples, but often overlooked in C accounting-is lower than the volatile CF in live wood, but significantly >0 % suggesting that industrial lumber drying processes remove some, but not all, of volatile C-based compounds. Our results demonstrate that empirically-supported wood CFs for construction material can correct meaningful systematic biases when estimating C storage in the built environment.

Carbon concentration in the world's trees across climatic gradients.

Wood carbon (C) concentration is a key wood trait that varies widely among tree species, but our understanding of the factors governing this trait is limited, despite reason to hypothesize that wood C varies systematically across environmental gradients. We compiled a novel database of 1145 geo-referenced wood C observations from 415 species, to elucidate climate correlates of wood C concentrations, and test if these relationships differ across tissue types and major taxonomic divisions (i.e. angiosperms vs gymnosperms). Climate variables, including mean annual temperature (MAT) and precipitation and temperature seasonality, are significantly correlated with wood C concentrations. Relationships between wood C and these variables differ across tissue types and taxonomic divisions, yet there is a negative relationship between wood C and MAT that exists across all tissues and species groups. Wood C concentrations in trees are influenced by climate, with experimental evidence (albeit scant) indicating that climate-driven changes in lignin concentrations likely govern these relationships. Our study presents among the first lines of evidence indicating that wood C concentrations are correlated with environmental conditions, thereby enhancing our understanding of the potential adaptive significance of wood C variation in trees.

Carbon fractions in the world's dead wood.

A key uncertainty in quantifying dead wood carbon (C) stocks-which comprise ~8% of total forest C pools globally-is a lack of accurate dead wood C fractions (CFs) that are employed to convert dead woody biomass into C. Most C estimation protocols utilize a default dead wood CF of 50%, but live tree studies suggest this value is an over-estimate. Here, we compile and analyze a global database of dead wood CFs in trees, showing that dead wood CFs average 48.5% across forests, deviating significantly from 50%, and varying systematically among biomes, taxonomic divisions, tissue types, and decay classes. Utilizing data-driven dead wood CFs in tropical forests alone may correct systematic overestimates in dead wood C stocks of ~3.0 Pg C: an estimate approaching nearly the entire dead wood C pool in the temperate forest biome. We provide for the first time, robust empirical dead wood CFs to inform global forest C estimation.