A tree's root system generally goes unnoticed by industry professionals and forest owners alike. Most often, a visual impression of a tree's root system, its structure, and size can be gained in the forest by observing windthrown trees. To simplify, one could say that roots have three essential tasks: they anchor the tree in the soil, they absorb necessary water and nutrients from the soil, and they store nutrients needed by the tree within their cells. The most dynamic part of the root system, playing a significant role in the cycle of carbon and nitrogen in forest soil, consists of the smallest fine roots, which do not exceed 2 millimetres in diameter. Fine roots possess high adaptability to edaphic (soil), climatic, and other environmental conditions. The lifespan of these roots is short, and as they die off, they replenish the soil with carbon. How significant is this replenishment? This can be understood from specific examples.

When assessing 14-year-old planted Scots pine stands, it was found that the carbon stock had returned to the same volume as it was before the clear-cut and site preparation. Therefore, fast-growing young trees provided a rapid replenishment of carbon stock both through roots and needle litter. It is precisely good tree growth that is the key to this correlation. This can be seen from another example. Foresters know that by allowing tree roots to breathe – for example, by digging a ditch in a marshy forest – spruces will seed and begin to grow well. Assessing sample plots in old (average 150 years) Scots pine stands in a marshy forest and the situation in a drained peatland forest reflecting conditions after ditching, we see that the more these spruces have 'entered', the greater the volume of fine roots (See the graphic).

Graphic: Biomass of fine roots of Norway spruce depending on the growing stock of Norway spruce (m3 ha-1) in a marshy forest (Nd) and a drained peatland forest (Ks).

Thus, after a major disturbance (fire or storm, or a clear-cut), the carbon stock in the soil decreases temporarily, but tree roots and leaf litter ensure its rapid recovery. But what happens next? The simple answer would be – nothing. That is, the total storage does not change significantly as the stand age increases further. In reality, many processes take place: as roots grow and decompose, carbon compounds from leaf litter are washed in and out; however, all these processes together are in a dynamic equilibrium and the total volume of carbon does not change. We can assess this both by collecting and analysing root and litter data, and by comparing carbon storage in stands of different ages. For example, there is no significant difference in carbon accumulation in the soil in Vaccinio-myrtillosa and Oxalidosa forest types in 58–69-year-old stands compared to those twice as old on average (112–131 years).

As with every proper rule, there are exceptions, which apply to forest types close to peatland ecosystems. However, this does not change the overall correlation: the carbon stock in the soil recovers quickly and subsequently remains in dynamic equilibrium as the stand age increases. Thus, the story of carbon in the forest is a story about trees, their growth, and what we do with trees (wood products).

But how can all of this be known?

Obtaining the data requires no small amount of patient work – collecting samples of precise volume from the soil (and the roots within it) in forest stands, washing and sorting them – which roots are not from trees, which are (and of what tree species), which are dead and which are alive (See photo). Then – weighing, determining carbon content, and looking for links between stand and root parameters.

Photo: Sorted fine root samples

We have carried out this work in a study funded by the European Regional Development Fund: “Development of a decision support tool by integrating information from old semi-natural forest stands for more accurate carbon balance assessment” (No. 1.1.1.1/19/A/130)