Sapwood proportion and nitrogen content in boreal and temperate tree species

The spatial and temporal variation in plant respiration is one of the largest unknowns in the global land carbon budget. While respiration rates are directly related to temperature, plant respiration of trees is also determined by their stem sapwood proportion, tissue nitrogen (N) contents and other factors. The sapwood proportion is related to the biomass fraction of respiring living cells in the tree stem. The respiratory costs that plants have to invest to maintain basic functions (maintenance respiration) are related to the vegetation N content, since maintenance respiration supports protein repair and replacement, and most plant organic N is in proteins.

Here we explore the variation and underlying drivers in these two plant traits (stem sapwood proportion, tissue N contents) and derive novel estimates of their spatial distribution in northern hemisphere boreal and temperate forests. For the first task, we make use of measurements of sapwood and total cross-sectional area in tree stems and of N contents per dry matter in stems, roots and leaves. Such data are collected from plant trait databases like TRY, the biomass and allometry database (BAAD) and extensive literature reviews covering the most common boreal and temperate tree species. For the second task, we apply the derived tree level relationships between these traits and the underlying drivers (species, climate, soil variables) in combination with satellite radar remote sensing based products of compartment (stem, branch, root and leaf) biomass and tree species distribution maps covering the entire northern boreal and temperate forests.

We find that both the proportion of sapwood to total stem biomass and the response of the N content to environmental conditions are fundamentally different among tree genera. For instance, the sapwood proportions are spanning from 20–30% in larch to > 70% in pine and birch forests. These findings highlight the need to consider genera-specific differences when estimating the response of plant respiration to changes in climate and forest management.